Shape Deposition Manufacturing (SDM) is one of several layered manufacturing (LM)

methods highlighted in this issue. It is a process that incrementally builds up complex parts by systematically combining material additive processes with material removal processes. The advantages of each type of process are thus combined such that

novel structures can be fabricated with SDM that could not be practically fabricated

with either material additive or material removal processes alone.
Three/1998 1

Naval Research Reviews publishes articles about research conducted by the laboratories and contractors of the

Office of Naval Research and describes important naval experimental activities. Manuscripts submitted for

publication, correspondence concerning prospective articles, and changes of address, should be directed to Code

00PA, Office Naval Research, Arlington, VA 22217-5660. Requests for subscriptions should be directed to the

Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20403. Naval Research

Reviews is published from appropriated funds by authority of the Office of Naval Research in accordance with

Navy Publications and Printing Regulations. NAVSOP-35.

CHIEF OF NAVAL RESEARCH

RADM Paul G. Gaffney, II, USN

DEPUTY CHIEF OF NAVAL RESEARCH

TECHNICAL DIRECTOR

Dr. Fred Saalfeld

CHIEF WRITER/EDITOR

William J. Lescure

SCIENTIFIC EDITORS

Dr. George Spanos

Dr. Steve Fishman

Dr. Ralph Wachter

MANAGING EDITOR

Norma Gerbozy

ART DIRECTION

Cynthia Nishikawa

Jorge Scientific Corporation

Office of Naval Research

Three/1998

Vol L

Articles

2

Foreword

G. Spanos, S. Fishman, and R. Wachter

5

Potential Navy Applications

for Selective Laser Sintering

(SLS) Technology

P. Bergan

9

Fabricating Metal Tooling

and Metal Parts by 3D

Printing

E. Sachs, S. Allen, M. Cima, X. Xu,

J. Baños, J. Serdy, D. Brancazio, and

H. Guo

19

Novel Applications and

Implementations of Shape

Deposition Manufacturing

F. B. Prinz and L. E. Weiss

27

Solid Freeform Fabrication

(SFF) of Functional

Advanced Ceramic

Components

S. C. Danforth, A. Safari, M. Jafari, and

N. Langrana

About the cover

This figure shows four parts produced at Rutgers University by one particular method of layered

manufacturing known as Fused Deposition. From top to bottom the parts are: (1) a fluidic control

device, (2) a turbine blade, (3) a nozzle shape for a rocket engine, and (4) a thin gear. (1) – (3)

are made of silicon nitride, while (4) is made of stainless steel.

39

Solid Freeform Fabrication

of Ceramics via

Stereolithography

G. A. Brady and J. W. Halloran

44

Direct Photo Shaping of

Ceramic Components

S. Ventura and S. Narang

51

Gas Phase Solid Freeform

Fabrication and Joining of

Ceramics

K. J. Jakubenas, J. E. Crocker, S.

Harrison, L. Sun, L. Shaw, and H. L. Marcus

58

Process Planning Issues in

the Layered Manufacture of

Heterogeneous Objects

D. Dutta, P. Kulkarni, and V. Kumar

Departments

4

Profiles in Science
2 Naval Research Reviews

Foreword

Dr. George Spanos, Guest Editor, Materials Science

and Technology Division, Naval Research Laboratory

Dr. Steve Fishman, Guest Editor, Materials Science

and Technology Division, Office of Naval Research

Dr. Ralph Wachter, Guest Editor, Manufacturing Technology Division, Office of Naval Research

A challenge in manufacturing is to invent new processes for the rapid production of complex functional and

structural components. One very promising class of manufacturing processes, descriptively called Layered Manufacturing (LM), has been made possible through recent advances

in several fields of science and technology including materials science, mechanical engineering, and information technology. LM offers unique capabilities, important for agile

manufacturing and logistics, to create rapidly on-demand

manufactured components of complex design.

The essential characteristic of a LM process is its successive building up of a part layer-by-layer by precise computer-controlled deposition of materials. This process is

roughly analogous to stacking masonry bricks to form different brick wall designs. Since LM is a material additive

process, it differs from traditional manufacturing approaches

of machining, which involve removal of material. LM processes also differ from traditional laminate composite manufacturing where, for instance, layers of composite tape are

shaped and bonded together.

The Office of Naval Research in cooperation with the

Defense Advanced Research Projects Agency and the National Science Foundation has been advancing the science

and technology for LM. Through joint efforts, the range of

materials has been expanded considerably from, e.g., plastics, to metals and ceramics, and towards functionally graded

materials. Methods for selective precise deposition of materials in LM are now quite varied and include, e.g., high intensity light sources, hot extrusion, and metal droplet

deposition; a few hybrid approaches are even combining LM

with machining and cutting. These processes have motivated new materials science research into solidification,

bonding between layers, and evolution of microstructure and

residual stress in LM components.

This issue of Naval Research Reviews provides an

overview of the science, technology and applications of LM.

Our first article, by Mr. Patrick Bergan of the Naval Undersea Warfare Center, highlights Naval applications of LM and,

in particular, their use of selective laser sintering technology

from DTM Corporation, a spin-off company from the University of Texas at Austin. The article describes where LM

technology can significantly affect affordability.

Professors Emanuel Sachs and Michael Cima at the

Massachusetts Institute of Technology (MIT) developed a

method for forming parts from metal powders by an ink-jet

like process, which they aptly named “3D Printing.” The

article discusses advantages of the process for making parts

with highly complex external and internal geometries such

as internal cooling channels for injection mold tooling. Their

research has been licensed by MIT to the company, Soligen

Technologies, Inc.

Professor Fritz Prinz at Stanford University and Dr.

Lee Weiss at Carnegie Mellon University describe a novel

approach for making complex metal, ceramic and plastic

parts. Known as Shape Deposition Manufacturing, it involves

combining techniques of material deposition with material

removal, e.g., using 5-axis machining for precisely shaping

individual layers to a specified tolerance during the layering

process. They discuss several applications relevant to the

Department of Defense (DoD) including a composite steel/

copper injection molding tool, a miniature turbine wheel assembly, and a rugged casing with embedded wiring for a

wearable computer for underwater use.

A multidisciplinary research team at Rutgers University, lead by Professor Steven Danforth of the Ceramic and

Materials Engineering Department, present a LM method

known as Fused Deposition of Ceramics (FDC) in which

hot extruded ceramic-loaded organic filaments are used to

deposit the layers. Their paper surveys FDC for structural

ceramics, bio-ceramics, and piezo-electric transducers.

Dr. G. Allen Brady and Professor John Halloran at the

University of Michigan describe a technique for producing

ceramic parts, based on conventional stereolithography,

which uses a laser to scan an outline of a shape on a layer of

photosensitive polymer. They developed new ultravioletcurable polymers with a suspension of ceramic powder to

produce ceramic parts. The authors show examples of fully

dense ceramic parts with excellent microstructures.

Dr. Susanna Ventura and her collaborators in the Electrochemical and Polymer Technology Center at SRI International have developed an innovative technique for producing

ceramic parts using digital, pixelated visible light projected

on a photocurable ceramic dispersion. Their process, known

as “Direct Photoshaping,” uses digital micromirror technology, developed by Texas Instruments, Inc., to expose and

cure specific regions of each layer all at once to rapidly build

up a part with embedded ceramic powder. The polymer can

subsequently be burned off and the resulting “green” ceramic

object sintered to final density. A silicon nitride turbine vane

for a gas turbine engine has been produced by this method.

Professor Harris Marcus of the Institute of Materials

Science at the University of Connecticut and his colleagues

describe a very different LM process that uses lasers to decompose locally reactant gases in a controlled manner. Their

process is known as selective area laser deposition. The authors show how to build ceramic parts and how to join separate ceramic pieces together with this process.

Professor Debasish Dutta and his colleagues at the

University of Michigan are investigating digital representations of parts with graded materials and microstructures, for
Three/1998 3

LM computer aided design. A distinct advantage of LM is

the conceptualization of a part as a stack of layers, which

actually simplifies computation, design, and manufacturing

processes.

The articles in this issue of the Naval Research Reviews provide insight into tomorrow’s manufacturing capabilities. These capabilities have important implications for

production and productivity. LM will advance affordable

manufacturing technologies without the need for conventional or specialized tooling and without the need for part

fixturing. LM will replace some traditional manufacturing

processes but not all. LM will complement some conventional high-volume manufacturing processes by enabling the

rapid production of machine tooling, e.g., from durable, erosion-resistant, high temperature materials.

The capability of LM to produce nearly arbitrary shapes

lends itself to rapid, flexible, customized production.1

 LM

technology is often capable of producing more kinds of parts

with less lead time, setup time, production time, assembly,

and effort than, e.g., by comparable machining. Moreover,

as greater precision is achieved with LM, part size and not

geometric complexity will be the dominant factor in part cost

and time whereas in comparable conventional machining both

part size and complexity are significant factors. And with

lower costs of small-lots and reduced production time,

customization of commodity parts on-demand with LM will

become an alternative to traditional mass production of standard machined parts and stockpiles of inventories.

Layered Manufacturing also has important implications

for repair and replacement of parts for which engineering

designs are no longer available or where designs never existed. The capability to reverse engineer, e.g., using x-ray

tomography or coordinate measurement machines, can provide geometric information necessary for replicating a part

with LM. Moreover, complex part designs stored on a computer can be electronically transferred to sites such as shipyards and potentially directly on-board ships or submarines,

where LM would be used. With these capabilities, LM will

be well suited for engineering service bureaus engaged in

online/Internet commerce.

Industry is adopting today’s commercial LM technologies but mostly for use in early product design models and

for nonfunctional prototyping of parts. This will change as

commercial LM technologies improve, are certified, and can

manufacture complex parts with more materials and better

dimensional accuracy than with the plastics that are widely

and commercially available today. Traditional machine shops

with their hazards and noise may be replaced by LM technologies in quiet environments with less exposure to hazardous materials and less waste. LM will move rapid

prototyping to rapid manufacturing.

We expect future LM research to achieve part features

down to micron scales with specified microstructural and

materials properties. This capability will enable fabrication

of complex components in one operation with minimal

postprocessing and assembly. And someday, we anticipate

LM will find its way into the office as your desktop “3D”

fax machine.

1

For these reasons, LM is also referred to as “solid freeform fabrication (SFF)” for its ability to produce complex parts

and as “rapid prototyping” because often LM can produce a part more quickly than other means.
4 Naval Research Reviews

Profiles

in Science

Dr. Joseph J. Beaman is the Andersen Consulting Endowed Professor in Manufacturing Systems Engineering at

the University of Texas, Austin, where he has been on the

faculty since 1979. His particular interest in manufacturing

is Solid Freeform Fabrication, which is also known as desktop manufacturing. It is a technology that produces freeform

solid objects directly from a computer model of the object

without part-specific tooling or human intervention. Professor Beaman has been a principal investigator of the Office of Naval Research.

One of his successful desktop manufacturing approaches is selective laser sintering, where components are

built by material addition rather than by material removal.

In selective laser sintering, a directed laser beam is used to

consolidate individual powder particles in selected regions.

Compared to manual manufacturing methods, selective laser sintering is inherently fast. In addition, this process has

the potential to produce accurate, structurally sound threedimensional renditions of objects designed in a computer

and to make such objects available to the user in minutes or

hours. The benefits of this new process include greatly reduced prototyping cost and design time, and the capacity to

achieve, in one operation, shapes that would otherwise require multiple operations or in some cases shapes impossible to manufacture with standard techniques

Professor Beaman is also a founder of DTM Corporation, which specializes in selective laser sintering. This technology is a freeform fabrication technology developed in

Beaman’s laboratory at the University of Texas at Austin and

is the first private commercial venture with the University

of Texas’ Center for Development, Technology and Transfer.

Joseph J. Beaman
Three/1998 5

Abstract

Maintaining Navy weapons systems and support equipment is a continuous process, requiring periodic repair and

replacement of parts. Procurement of spare parts is becoming increasingly difficult as fewer of the traditional defense

suppliers exist. This problem is especially prevalent in the

procurement of low quantity, high value, complex metal parts

which are traditionally machined, forged or cast. These types

of parts have long supplier lead times and high unit costs

because of the need to produce complex tooling, forming

dies, casting molds and perform Numerical Control (NC)

programming. To help maintain equipment and improve

lifecycle support in Navy ship and shore facilities, new technologies are needed to reduce the cost and lead-time associated with producing high value metal parts. Selective Laser

Sintering (SLS) is a cutting edge, rapid manufacturing technology intended to address this problem. SLS uses a high

power laser to fuse metal powder in a layer-by-layer buildup

process to produce a part on demand. Potential applications

include production of metal spare parts, fabrication of metal

molds for plastic parts and creation of complex tooling. SLS

technology is still in its infancy, but initial results have been

promising and the process clearly has a large number of potential Navy applications.

Introduction

The mission of the Naval Undersea Warfare Center

(NUWC)-Keyport is to test and maintain weapons for a variety of undersea warfare programs. As a part of this function, NUWC depots must repair or replace components from

weapons systems damaged by corrosion, wear and other problems. Due to the complexity of the parts and difficulties in

finding sources of manufacture, procurement of new replacement parts is often a time consuming and costly process.

To help meet this challenge, NUWC-Keyport is constantly looking at new technologies and processes to improve

Potential Navy

Applications for

Selective Laser

Sintering (SLS)

Technology

Patrick Bergan, Naval Undersea Warfare Center - Division Keyport, Keyport, WA
6 Naval Research Reviews

the repair and fabrication of weapons components. One innovative technology that has potential for helping with this

problem is the Selective Laser Sintering (SLS) process. SLS

is a new manufacturing technology which uses rapid

prototyping techniques, in combination with conventional

Hot Isostatic Pressing (HIP), to produce actual metal parts,

not just plastic prototypes. The goal of the SLS project is to

develop a rapid manufacturing process for creating high

value, metal parts - traditionally produced by conventional

casting, forging or machining processes.

The SLS project team contacted NUWC in 1997 to

look at the possibility of applying the SLS process to weapons components and more conventional shipboard parts.

Initial metallurgical results produced in the first phase of the

project showed that the basic SLS process had good potential. These results, in combination with the inherently rapid

build-up process, showed that SLS technology holds promise as a method to rapidly produce replacement parts.

SLS Background

SLS uses a revolutionary process, patented by the

University of Texas, for solidifying layers of metal powder

into an actual part using a high power laser. The SLS technique builds the part layer by layer in a process similar to

that used by Stereolithography equipment. A layer of powdered metal is deposited on a work surface. A high power

laser fuses the powder in the proper shape for each “slice” of

the part. Successive layers are produced in this manner until

the part is complete.

Normally if a metal powder is completely melted, the

surface tension of the liquid metal causes the material to

“pool” together, destroying the geometry of the cross-section. The SLS process uses an innovative technique for preventing this from happening. This problem is avoided by

using a laser to totally fuse only a thin outside layer of each

slice. The interior of each slice is only partially fused- allowing the cross section to maintain the proper geometry.

After SLS processing, the part is not yet complete. At

this point, the part consists of a porous interior encapsulated

by an impermeable shell (Figure 1). This shell acts as a selfcontained “can” for the follow-on Hot Isostatic Pressing

(HIP) operation. Using a conventional HIP process, the porous portion of the part is compressed to full density using

hot pressurized gas. The impermeable shell acts as a mold

or forming die for the HIP operation.

Unlike conventional Stereolithography, which produces a prototype facsimile of a part, the end product of SLS

is an actual metal part. Not all part features can be produced

in this manner. Features such as tight tolerance holes, threads

and fine surface finishes may require subsequent machining. However, the SLS process would be beneficial for producing parts with complex contours, normally requiring

castings, forgings or complex machining.

Potential Navy

Applications of SLS

The SLS process is appealing to the Navy because of

the potential to quickly produce replacement parts. During

regular maintenance on torpedoes and other weapons, NUWC

depots constantly use replacement parts from component

inventories. As parts are consumed, spare parts inventories

need to be constantly restocked. Often, these parts are weapons components with unusual shapes and features to meet

tight packaging and functional requirements. Additionally,

these parts often use exotic materials to minimize corrosion

and meet high structural or temperature requirements.

With these types of requirements, replacement parts

are often difficult to procure. Sources of supply and manufacture are becoming increasingly difficult to locate. Parts

often require complex tooling, forming dies or casting molds

for conventional manufacturing processes, adding significant cost and production lead-times to the part acquisition.

This problem is compounded when emergency replacements

are required for out of stock spare parts. In this situation,

procurement of parts in small quantities often results in very

high unit cost and unsatisfactory lead times. With smaller

weapons inventories, this problem becomes more prevalent

as smaller lot sizes of parts are procured.

Repair and replenishment problems are not limited to

just weapons components, but also occur in ancillary support equipment and more common shipboard components.

Many of these systems can contain high value metal parts

requiring periodic repair or replacement. In many cases, spare

parts inventories may not exist because of the unique nature

of the part or an infrequent need for replacements. When

fully developed, rapid manufacturing techniques, like SLS,

Figure 1

Image of “Canned” SLS Part.
Three/1998 7

could significantly help reduce equipment downtime by producing parts needed for unexpected repair work.

SLS could also be applied to the creation of tooling to

support conventional manufacturing processes. Molds

needed to produce complex rubber or plastic components

can be expensive and time consuming to produce. Undersea

weapons and support equipment utilize a considerable number of custom, unique cable connectors and other plastic parts.

Similarly, shipboard equipment can contain a number of different types of plastic or rubber parts, which may not be on

hand for emergency replacement. If the part is out of stock

and the original manufacturer is not available, molds may

need to be produced for emergency replacements. In these

situations, the need to produce molds can make for extremely

high unit costs and long replacement lead times.

Tooling is usually a significant cost for parts produced

conventionally by Computer Numerical Control (CNC) machining. By their nature, parts produced using CNC equipment typically can contain complex features and geometry.

Similarly, the “workholding” tooling and fixtures needed to

support the part during machining can be complex. For one

time replacement of parts or small lot sizes, the expense and

time needed to produce support tooling often extends the

replacement part cost and schedule. Applying a rapid manufacturing process like SLS to the creation of CNC tooling

could benefit another problem area in producing replacement

parts.

Typical Part Applications

SLS would provide the greatest benefit in producing

high value parts with complex features or difficult tooling

requirements. In the undersea warfare arena, there are many

examples of this type of part. Components within a torpedo

must meet many difficult requirements. Parts must endure

high stresses due to pressure at depth and a corrosive environment from seawater and torpedo fuel. Many parts must

withstand very high temperatures during test runs. To meet

all functional requirements and fit in a very physically confining package, many of the parts have unusual geometries.

As a result of this, conventional machining or casting of these

parts typically requires a considerable investment in tooling, setup time and machine time.

An example of this type of part is the Rotary Valve

utilized in the Lightweight (MK 46) Torpedo (shown in Figure 2). During a torpedo run, this part functions as a timing

valve to direct torpedo fuel in the proper sequence to the

torpedo cylinder barrel. The part must withstand high temperatures, high fuel pressures and function in a corrosive

environment. The Rotary Valve is currently fabricated out

of Molybdenum in a powder metallurgy process. Critical

areas on the valve are then subsequently machined to tolerance.

Conventionally, this part would require an expensive

forming die to be produced. With the free-form fabrication

abilities in SLS, the raw shape of the part could be produced

without tooling with some minor subsequent machining operations. The time spent in sintering, subsequent HIP processing and follow on machining could possibly be

accomplished in days rather than weeks.

A similar example of a high value, complex shaped

part is the Radius Boot (shown in Figure 3). This part is

used on submarines to pay out wire attached to Towed Array

equipment. The Radius Boot must withstand a corrosive sea

water environment and exhibit excellent wear resistance.

Currently, this part is fabricated out of Titanium in a

CNC machining process using custom-built cutting tools.

This is another example of a part which would be difficult to

produce quickly in an emergency replacement scenario. In

this case, it would clearly be a benefit if a rapid manufacturing process, like SLS, could produce this type of part without the need for custom cutting tools or workholding fixtures.

SLS technology would not only be beneficial in directly producing parts. Another potential use of SLS would

be in the production of general tooling for CNC machining

or molds for plastic parts. Undersea weapons programs use

many types of custom plastic parts produced by plastic injection molding. As is the case with other torpedo parts,

these parts typically have unusual geometries and tight tolerances.

The ADCAP A-Cable (shown in Figure 4) is just one

example of a part used with the peripheral equipment of the

Figure 2

MK 46 Rotary Valve.
8 Naval Research Reviews

torpedo. The connector end of the A-cable is plastic injection molded around a “bed” of connector pins and other electrical components. The molds to produce this part are

expensive because the geometry of the connector body is

complex and the tooling must hold the internal connector

components precisely during the molding process.

The A-Cable mold was machined conventionally requiring a great deal of cost and a long lead-time. If this part

could be produced in an SLS process, considerable time and

expense could be avoided. This would be especially beneficial in instances where replacement parts which do not have

any tooling available must be made.

Closing

The Rotary Valve, Radius Boot and A-Cable connector are typical examples of parts needing periodic replacement or repair. Within all the systems on board ships,

submarines and on shore stations, there are certainly numerous quantities of high value, complex parts contained in Navy

equipment. In an era of declining defense budgets and fleet

reductions, maintaining weapons systems, support equipment

and stocking spare parts inventories will be more challenging in the future. Provisioning of replacement parts becomes

more of a problem as fewer of the traditional commercial

sources exist to produce replacement parts. New technologies and processes are needed to enable high value parts to

be produced on demand.

The ability to make actual parts or tooling from a variety of materials in days, rather than weeks or months, makes

SLS a potentially valuable tool. While the SLS technology

is still in its infancy, this type of rapid manufacturing technology clearly has potential to address some maintainability

problems for the Navy.

Currently, the SLS project is in the midst of Phase I.

The goal of Phase I is to demonstrate and test the metallurgical qualities of the process and refine the basic technology.

Initial testing performed on Titanium, Inconel 625 and Mild

Steel/Nickel alloys have had promising results. Development work is proceeding on refining surface finishes and

improving dimensional control.

Phase II of the project, scheduled to begin in Fiscal

Year 1999, will involve the production of a higher power

laser sintering station to use in an actual test-bed setting. In

this phase of the project, NUWC-Keyport will function as

the DOD test-bed for the project. The SLS process will be

used to develop process parameters and techniques that can

be applied to production of actual components for Navy use.

Biography

Patrick Bergan is a Mechanical Engineer specializing

in the fields of manufacturing technology and mechanical

design at the Naval Undersea Warfare Center – Division

Keyport. Mr. Bergan initially functioned as the Lead Engineer in the Robotics and Technology Group which successfully implemented several robotic systems and Computer

Integrated Manufacturing (CIM) initiatives. Mr. Bergan is

currently the Lead Manufacturing Technologist in the Industrial Technology and Operations Department at NUWCKeyport. In this position, Mr. Bergan is involved in a number

of advanced manufacturing technology projects including

Selective Laser Sintering, Laser Cladding for Torpedo Repair and a number of mechanical design/manufacturability

analysis projects.

Figure 4

A-Cable Connector Mold.

Figure 3

Radius Boot (Section View).
Three/1998 9

Abstract

Three Dimensional Printing is being used to create

metal parts directly by repetitively spreading a layer of metal

powder and then selectively joining the powder in that layer

by ink-jet printing of a binder material. The printed green

part is then removed from the powderbed and furnace postprocessed to yield a fully dense metal part which can be used

as a tool for subsequent forming operations or can serve as

an end-use metal part. This paper focuses on the application

to injection molding tooling.

The in situ fabrication of cooling channels within an

injection mold which are conformal to the molding surface

has been shown to result in 15% shorter cycle time while

yielding parts with 10% less distortion as compared to a conventional production tool. A “Conformal Cooling Criterion”

has been developed to facilitate the design of such cooling

channels. Printed textures on the interior of cooling channels have been further shown to be capable of an 8-fold increase in heat transfer coefficients as compared to smooth

channels.

With the present materials system, dimensional variability is introduced during the furnace processing and data

taken on a set of 20 tools shows that the current tolerance is

approximately ±.25%. Surface finish improvements due to

improved drop placement accuracy have been demonstrated,

allowing for hand finishing of tools. Materials systems are

under development with the goals of hardness higher than

the current system (30 HRC) and lower shrinkage (better

dimensional control).

Introduction

Three Dimensional Printing

Three Dimensional Printing (3DP) is a process for the

rapid fabrication of three dimensional parts directly from

Fabricating Metal

Tooling and Metal

Parts by 3D Printing

Emanuel Sachs, Samuel Allen, Michael Cima, Xiaorong Xu, Javier Baños, James Serdy, David Brancazio, and

Honglin Guo, Massachusetts Institute of Technology, Cambridge, MA
10 Naval Research Reviews

computer models [1,2]. A solid object is created by printing

a sequence of two-dimensional layers. The creation of each

layer involves the spreading of a thin layer of powdered

material followed by the selective joining of powder in the

layer by ink-jet printing of a binder material. The powder

bed is lowered at the completion of each layer by lowering

the bottom of the rectangular cylinder which contains the

bed. Figure 1 depicts the steps involved in creating a part.

Unbound powder temporarily supports unconnected portions

of the component, allowing overhangs, undercuts and internal volumes to be created. The unbound powder is removed

upon process completion, leaving the finished part.

3D Printing offers three key features which define its

competitive advantages:

• Geometric Flexibility. Any geometry can be created with a minimum feature size of 100 microns. The resolution of edge location is approximately 10 microns.

Overhangs and undercuts can be created because the powder acts as a support medium for the part under construction.

• Material Flexibility. A part can be constructed out

of any material which can be obtained as a powder, including ceramics, metals and polymers. The composition of a

part may be controlled on a local basis by printing different

materials through different nozzles, resulting in the ability

to control composition on a 100 micron length scale.

• Production Rate. Ink-jet technology may be scaled

up using multiple nozzle technology.

3D Printing is being exploited for a wide range of applications including many applications that are being commercialized. Soligen Incorporated of Northridge, CA,

provides metal castings made using 3D Printed ceramic

molds and cores [3, 4]. Z Corp. of Somerville, MA, provides 3D Printing machines for the office modeling application used to produce models to verify geometry. Therics,

Incorporated of Princeton, NJ, is developing various medical applications based on 3D Printing technology. Specific

Surfaces of Franklin, MA, is using 3D Printing to create ceramic filters for hot gas filtration. ExtrudeHone Corporation of Irwin, PA, provides machines and services for the

fabrication of metal tooling and metal parts by 3D Printing

[5]. Several other application areas are under development

including fine resolution metal parts and ceramic parts for

electronic and structural applications. Military applications

under development include: i) ceramic cores for lost-wax

casting, ii) dies for injection of ceramic cores, iii) tools for

resin transfer molding, iv) die casting tools with conformal

cooling, and v) structural ceramic parts for high performance

engines.

Tooling by 3D Printing

This paper focuses on the application of 3D Printing

to the fabrication of tooling and specifically to the fabrication of tooling for injection molding. Direct printing of tooling involves the following steps (see Figure 2):

1. Print a polymeric binder into stainless or tool-steel

powder. This step defines a green part within the powder

bed.

2. Remove the loose powder, thereby revealing the

green part (60% dense).

3. Burn out the polymeric binder in a furnace and

lightly sinter the part.

4. Infiltrate the part with a copper alloy in a second

furnace operation, typically performed at 1100o

C. At this

point, the part is fully dense.

5. Finish the tooling to achieve desired surface finish

and dimensions as required.

Figures 3a and 3b show the scalability of the process.

Figure 3a shows an 8-jet printhead and Figure 3b shows the

printed layer which is created by raster scanning the 8-jet

printhead over the surface of the powder. Figure 3c shows a

set of tooling inserts emerging from the powder bed. Figure

3d shows a tooling insert after sintering. Figure 3e shows

the same core, together with its mating cavity after finishFigure 1

A schematic diagram of the 3D Printing process.

Figure 2

Process sequence for creating tooling directly by 3D Printing.
Three/1998 11

Figures 3

a) An 8-jet multiple nozzle printhead. b) Printing a layer of binder with a raster-scanned 8-jet printhead. c) Printed tools

emerging from the powder bed. d) A core after printing and sintering. e) A finished tool set and a glass filled nylon part

injected in it. f) A set of tooling inserts printed for various industrial applications. All tools have been infiltrated and approximately

half of the tools have been finished and used to inject parts. For scale, the tool in the lower left hand corner is 150 mm long.

(a) (b)

(c) (d)

(e) (f)
12 Naval Research Reviews

ing, together with a glass filled nylon part injected into this

cavity.

Periodically, a set of tools is fabricated for our industrial sponsors to serve as a benchmark for our process. The

project is currently engaged in the fabrication of the third

such set of tools. The results presented in this paper are

derived from the second set of tools made. Figure 3f shows

a “family portrait” of 14 of 22 tooling inserts made in the

second set of tooling. The tool inserts in the foreground

have been finished and used to inject plastic parts in materials including glass filled nylon and polycarbonate. Each tool

set was designed by a different company and is typically

relevant to a different industrial sector. Each company chose

to finish the tool set in a different manner ranging from hand

polishing to EDM. The tools toward the back of the photo

are in earlier stages of processing and some are in the as

infiltrated condition. A number of the tool inserts in this

photo have conformal cooling channels within them (see

below). All the tools were printed with 420 stainless steel

powder and a bronze (90 copper, 10 tin) infiltrant. The hardness of these tool inserts is in the range of 25-30 HRC.

Conformal Cooling

Performance of Conformal Cooling

Control of internal geometry is one of the key capabilities in tooling made by 3D Printing and this capability

has been used to create cooling channels which are conformal to the molding cavity [6, 7]. Such cooling channels have

been shown to improve the control of mold temperature, reduce the cycle time and improve the dimensional control of

injected parts. Recent work by an industrial sponsor has

extended this approach to a high volume commercial product. Figure 4 shows the outline of the mold cavity with a

representation of the serpentine conformal cooling channel

printed in place (details are absent at the request of the sponsor). This cavity was run in controlled tests against the cavity used in production with the results summarized in bullet

form below.

• At one set of molding conditions, a 15% improvement in cycle time was obtained using the 3D Printed

cavity with conformal cooling SIMULTANEOUS with a

9% reduction in part distortion. It was further noted that

the factor limiting even further reduction in cycle time in the

cavity with conformal cooling was freezing of the sprue and

not the cavity itself, thus offering the potential of further

cycle time reduction with a runner system redesign.

• At a second set of molding conditions, a 37%

reduction in part distortion was obtained using the 3D

Printed cavity with conformal cooling with the same cycle

time as the production tool.

Design of Conformal Cooling

To take full advantage of the geometric flexibility of

3D Printing, the design of conformal cooling lines can be

quite complex and effort has gone into the codification of

this design process. This codification begins with a formal

definition of what is meant by conformal cooling lines and

follows with tools designed to address the trade-offs associated with the design of these cooling lines.

As the name implies, conformal cooling is used to signify cooling channels which conform to the surface of the

molding cavity. However, in this paper, the term conformal

cooling has a further significance which is related to the transient heat transfer within the mold. When a mold with conventional cooling channels is started up it takes some time

(and many injection cycles) before the mold reaches a steady

state operating temperature. With each injection cycle, a

heat pulse propagates in from the plastic and eventually the

steady sate is reached. However, if the cooling lines are

placed very close to the surface, the steady state condition is

reached very quickly. In fact, if the lines are close enough to

the surface, the steady state is reached after one injection

cycle. By our definition, cooling lines are conformal only if

they are close enough to the surface to allow the tool to reach

a steady state operating temperature within one injection

cycle. In the case where the heat transfer between the cooling line surface and the fluid within it is high, a simplified

expression for a “Conformal Cooling Criterion” can be obtained as follows:

Tool Time Constant Cycle Time = < ρcl

K

2

This condition states that the time constant for thermal equilibration of the tool is related to three thermophysical

properties of the mold (thermal conductivity, K, mass density, ρ, and specific heat, c) and to the square of the distance

from the mold surface to the cooling lines (l). When this

time constant is less than one injection cycle, steady state

will be reached after one injection cycle.

In addition to satisfying the proximity condition embodied in the “Conformal Cooling Criterion”, conformal

cooling lines must be designed so that: i) the flow be suffiFigure 4

A cavity with a serpentine cooling channel.
Three/1998 13

cient given a maximum allowed pressure drop, ii) the temperature drop along the cooling line not exceed a certain

value, say 2oC, iii) the surface temperature of the mold be

sufficiently uniform between cooling lines, and iv) the mold

have sufficient strength, and low enough deflection over the

cooling channels. These conditions may be expressed in

mathematical form and used to develop specific designs.

Figures 5a and 5b show the result of the application of these

methods to a complex injection molding core.

Heat Transfer Augmentation

In order for cooling channels to be effective either in

standard cooling geometries or in conformal cooling geometries, there must be effective heat transfer between the wall

of the cooling channel and the fluid within it. Conventional

wisdom has it that the flows within the cooling channels must

be turbulent so as to provide the high heat transfer coefficients required. However, turbulent flows require high flow

rates and significant pumping requirements. In order to reduce the pumping requirements, the use of heat transfer augmentation features was investigated. With guidance from

the literature on heat transfer augmentation, 14 different surface textures were designed and printed. These textures are

all based on ribs which are approximately 450 microns wide

and stand 250 microns from the surface. The textures may

be grouped into three categories: i) ribs perpendicular to the

direction of the flow, ii) ribs at an angle with respect to the

flow, iii) V-shaped ribs. Figure 6a shows a schematic of a

block made by 3D printing with 4 channels printed within it,

each of which has a different surface texture. Figures 6b -

6d show examples of each of the three classes of surface

texture. Two controls were also fabricated and tested. In

one case, the channel was fabricated by wire EDM so as to

produce a smooth surface. In the other case, a channel with

no surface texture features was tested in the as printed condition.

The steady state heat transfer performance of the two

controls and the 14 surface textures was measured using water

as the working fluid over a range of Reynolds numbers from

1,000 to 15,000. Figure 7 shows the measured heat transfer

coefficients for the two control channels and the two surface

textures with best performance as a function of Reynolds

number. As may be seen, the as printed surface roughness

itself provides some heat transfer augmentation as compared

to the smooth channel. Most significant, however, is that 2

of the V-shaped geometries provide almost an order or magnitude increase in heat transfer coefficient over the performance of the smooth channel. This improvement is observed

throughout the range of flow tested which encompasses both

laminar and low turbulent regimes (the transition between

these regimes is at approximately a Reynolds number of

3,000). The other 12 textures range in performance from

not much better than the smooth channel to approximately

half the performance observed in the best textured channel.

Figure 8 presents data on the pressure drop versus Reynolds

number for the same four channels as performance is depicted in Figure 7. As may be seen, there is little or no penalty in pumping requirement for the channels with surface

texture.

From this work, it seems that 3D Printed tools could

incorporate heat transfer augmentation features which would

substantially reduce the pumping requirement by allowing

for effective heat transfer even with laminar flow. Further,

the scalability of 3D Printing opens up the possibility of utilizing the process to make heat exchangers in small and

moderate quantity.

Dimensional Control

The dimensional control of tooling made by 3D Printing is governed by the furnace processing steps and in particular by uncertainty in the shrinkage during the sintering

operation. During the sintering operation there is some

shrinkage of the part which is associated with the formation

of a skeleton which can subsequently be infiltrated. The

CAD file is “prestretched” by the amount of anticipated

shrinkage. A green part is then printed and these green parts

have been found to conform quite closely to the

“prestretched” CAD dimensions. At the present time, the

Figure 5

a) Drawings of a complex cooling line design in a core. b) A 3D Printed core with conformal cooling lines (see Figure 6a).

(a) (b)
14 Naval Research Reviews

shrinkage during the sintering step is 1.8 ± .25 % (linear),

which represents a nominal shrinkage of 1.8% and an uncertainty in shrinkage of .25%. While the nominal shrinkage

can be anticipated by “prestretching” the CAD file, the uncertainty in shrinkage dictates a loss of dimensional control

which scales with part dimension.

Based on previous observations, a specification range

was created which contemplates a multiplicative error which

is expected to scale with part size and an additive error which

reflects uncertainty in the location of the edges of the part

which is not expected to scale with part size. This specification range is illustrated as the trapezoidal shaded region in

Figure 9 where the vertical axis is the error in linear dimension (deviation from desired dimension) of the infiltrated tool

and the horizontal axis is the length of a particular dimension. The vertical intercepts represent the additive error while

the slope of the top and bottom of the spec range represent

the multiplicative error.

Figure 10 shows data taken on 18 tools in both the fast

and slow axes within the print plane. As can be seen, 81 of

the 107 data points lie within the anticipated spec range.

Figure 11 shows analogous data for the vertical axis. In this

case less data is available but the ratio of 15 out of 21 points

(on 6 tools) within the specification range is roughly comparable to the in plane print data. While the results were not

quite as good as expected, it does seem that we have come

close to defining a proper specification range which anticipates the dimensional control of our tooling. Further, it can

be noted that most of the data of Figures 10 and 11 has a

positive bias, and so correction of this bias would further

improve the results. Nonetheless, the dimensional control

Figure 6

a) Schematic of a 3D Printed block with channels with surface textures. b) Perpendicular printed ribs on the interior of 5 mm

x 5 mm channels for heat transfer augmentation. c) Angled printed ribs on the interior of 5 mm x 5 mm channels for heat

transfer augmentation. d) V-shaped printed ribs on the interior of 5 mm x 5 mm channels for heat transfer augmentation.

(a) (b)

(c) (d)

Figure 7

Heat transfer coefficient as a function of Reynolds Number

for control and textured channels.
Three/1998 15

of our tooling is not sufficient for net shape tooling inserts

(perhaps by a full order of magnitude). Thus, materials systems with substantially improved dimensional control are a

major thrust for this project. The primary thrust is to develop systems which have less shrinkage, on the assumption

that the uncertainty in shrinkage will scale with the magnitude of the shrinkage.

Surface Finish

Figure 12b shows a detail area of the cavity geometry

of Figure 12a created with two different printing approaches.

The detail to the right illustrates a significant improvement

in surface finish which has been achieved by control of the

droplet landing position with 10 micron resolution in each

of the two in-layer axes (contrast this with 10 micron x

150 micron resolution in the detail at the left). This achievement represents a combination of new printhead hardware,

new software used to create the printing instructions from

an .STL model and the integration of on-line measurement

used to characterize and adapt to changes in the performance

of the printhead [8].

Harder Tooling

As noted earlier, the current tooling materials system

is 420 stainless powder, infiltrated with bronze, a system

Figure 8

Pressure drops as a function of Reynolds Number for control

and textured channels.

Figure 9

Illustration of the specification range for 3D Printed tooling.

Figure 10

Data on dimensional control of 3D Printed tooling within the

print plane.

Figure 11

Data on dimensional control of 3D Printed tooling in the vertical

axis.
16 Naval Research Reviews

which produces a hardness of 25-30 on the Rockwell C scale.

However, this hardness is actually the result of a composite

system which has particles which are quite hard (HRC 50+)

and infiltrant which is much softer (HRB scale).

A goal of the project is to develop materials systems

with higher hardness and with greater uniformity of hardness between powder and infiltrant though the use of

hardenable infiltrants. However, when the molten infiltrant

contacts the powder, mutual solubility and fast inter-diffusion at the infiltration temperatures can lead to changes in

the composition of both powder and infiltrant resulting in a

loss of hardenability of both (as well as possible distortion).

Thus, an important goal of the project is to understand the

interaction of powder and infiltrant, design materials systems which minimize this interaction, and manage any remaining interaction. Toward this end, computer-aided alloy

design has been used in the 3DP material system selection.

The computer modeling can simulate the thermodynamic

interaction of the multi-component 3DP system at any temperature. The simulation results show the mole fraction of

each phase and the composition of each phase at equilibrium state. In addition, the simulation can also show the

melting point of the infiltrant, the phase transformation temperature and the effect of an additive element on the thermodynamic equilibrium.Good agreement between the computer

calculation and experimental result has been obtained for

the interaction of the 420 stainless/bronze system. This tool

is now being used to design alloy systems.

Conclusions

Three Dimensional Printing is being applied to the

fabrication of tooling directly from a computer model using

metal powders. Stainless steel powder is spread and a polymeric binder is ink-jet printed to define a green part which is

removed from the powder bed. The binder is burned out and

the part lightly sintered. The porous preform is then infiltrated with a copper alloy to produce a fully dense part. This

paper reports on results attained on a set of approximately

20 tooling inserts, ranging in size from 5 cm to 20 cm in

length. The tools were fabricated using hardenable stainless

steel powder with a resultant tooling hardness of 25-30

Rockwell C. Significant improvements in surface finish were

obtained using improved printing technology with the result

that tools could be finished by hand and used to inject parts.

Dimensional control of tools conformed well to the expected

result of being dominated by control of shrinkage and being

predictable to within ± .25%.

Thermal management of the tool is a crucial component of injection molding, substantially determining the cycle

time and part quality for a given part design. The ability of

the technology to create internal geometry has been exploited

in the form of cooling channels which are conformal to the

molding cavity. Such channels have been shown to result in

substantial improvements of production rate and part quality when compared to conventional tooling. Methodologies

for the design of tools with complex conformal cooling lines

have been developed. Especially important is the development of a “Conformal Cooling Criterion” which provides an

upper limit to the spacing between the molding cavity and

the cooling line. Order of magnitude improvements in heat

transfer coefficients have been demonstrated through the use

of printed-in-place surface textures on the channels. In the

future, such heat transfer enhancement can result in substantially reduced pumping requirements.

Critical areas for the technology center around the deFigure 12

a) An infiltrated core and cavity set. b) A detailed view of the “k” in the tool of Figure 21 with low resolution (left) and high

resolution (right).

(a) (b)

50 mm 10 mm 10 mm
Three/1998 17

velopment of new materials systems. Work is under way on

the development of low shrinkage systems in order to improve dimensional control and on the development of alloy

systems with higher hardness. 3D Printing of tooling and

metal parts is being commercialized by ExtrudeHone Corporation of Irwin, PA.

Acknowledgments

Support for this work from the Technology Re-Investment Project, Cooperative Agreement (DMI - 9420964),

members of the Three Dimensional Printing Industrial Consortium and the DARPA/ONR Solid Freeform Fabrication

program under contract #N000149410832 is gratefully acknowledged.

Biographies

Emanuel Sachs is Professor of Mechanical Engineering and specializes in the design of manufacturing processes.

He is a co-inventor of the Three Dimensional Printing Process for the creation of 3D parts directly from a computer

model by a layered building process. Prior to joining the

faculty, Dr. Sachs spent seven years working in the field of

photovoltaics (solar cells), including 2 years at Mobil-Tyco

Solar Energy Corp, and 5 years at Arthur D. Little, Inc. Dr.

Sachs is the author or co-author of more than 80 technical

papers and is the inventor or co-inventor of more than 25

patents. Dr. Sachs was awarded the B.S., M.S. and Ph.D.

degrees, all in Mechanical Engineering and all from MIT in

1975, 1976, and 1983, respectively. Dr. Sachs was a Hertz

Fellow and earned the Hertz Foundation Doctoral Thesis

Prize in 1983. Together with co-workers, Dr. Sachs was

awarded an IR&D 100 award in 1994 for his work on 3D

Printing.

Samuel M. Allen is Professor of Physical Metallurgy

in the Department of Materials Science and Engineering at

M.I.T. He was educated at Stevens Institute of Technology

(B. Eng., 1970) and at M.I.T. (S.M., 1971; Ph.D., 1975). In

1976, Prof. Allen was Visiting Lecturer at the University of

California, Berkeley. After several years as a Research Associate at M.I.T., he joined the M.I.T. Faculty in 1979. He

worked for Nippon Steel Corporation in Kawasaki, Japan

for the 1987-88 academic year, and was Visiting Professor

at Hiroshima University in 1989. His research interests include phase transformations, solid/solid interfaces, structure/

property relations in high-temperature alloys, three-dimensional printing of metal tools for plastic injection molding,

and alloys for high-strain actuators. Prof. Allen is coauthoring with his colleague Ned Thomas an undergraduate

textbook “The Structure of Materials,” to be published by

John Wiley & Sons in December, 1998.

Michael Cima is a Professor of Materials Science and

Engineering at MIT. He earned a B.S. in chemistry in 1982

(phi beta kappa) and a Ph.D. in chemical engineering in 1986,

both from the University of California at Berkeley. He received the Norton chair at MIT in 1988. Prof. Cima became

Director of the Ceramics Processing Research Laboratory in

1989 and received a tenured appointment from MIT in 1992,

and was promoted to full Professor in 1995. He was elected

a Fellow of the American Ceramics Society in 1997 and has

recently been awarded the Sumitomo Electric Industries

Chair at MIT. His general research interests include: powder processing, ceramics processing, slurry and ink formulation, ceramic thin films, and ceramics manufacturing. He

is author or coauthor of over one hundred scientific publications and six patents. Prof. Cima is also a coinventor and

coprincipal investigator on MIT’s 3D printing process and

has recently applied this technology to drug delivery systems.

Xiaorong Xu received his B.S. degree in precision

engineering from the University of Science and Technology

of China in 1992 and M.S. degree in mechanical engineering from the State University of New York at Stony Brook

in 1995. He is currently a Ph.D candidate at the Lab for

Manufacturing and Productivity, Department of Mechanical

Engineering, Massachusetts Institute of Technology. His

research interests include precision instrumentation, polymer processing, process development and control, solid

freeform fabrication, CAD and Internet application.

Javier V. Baños has a Bachelor of Science and a Master of Science degree in Mechanical Engineering, both from

MIT. As a graduate student, he worked as a research assistant at MIT’s Three Dimensional Printing Laboratory, where

his work focused on the application of the 3D Printing technology for building heat transfer-enhancing cooling channels on metal surfaces. Mr. Banos was the recipient of a

National Science Foundation Graduate Fellowship, and currently works as an analyst for Arthur D. Little, Inc.

James Serdy, Sponsored Research Technical Staff with

the Laboratory for Manufacturing and Productivity, has been

working with the 3DP project for the past 7 years. Prior

professional experience includes production managment with

Sintered Metals Corp. and R & D in solar photovoltaics with

Arthur D. Little, Inc. and with Mobil/Tyco Laboratories.

David Brancazio received his B.S. and M.S. degrees

from MIT in ’89 and ’91, respectively, and since then has

held the position of Assistant Laboratory Director for the

Three Dimensional Printing Laboratory. During this period

he has designed and/or coordinated the design of major pieces

of equipment including the main 3D Printing machine, developed and maintained this equipment, conducted research

projects, coordinated lab and machine throughput, coordinated student activity and projects, and worked with sponsors and licensees.

Honglin Guo is currently a Yield Enhancement Engineer at Texas Instruments, working in the Semiconductor

Group. In December of 1997, he completed a Ph.D. in metallurgy on post processing and materials selection for 3-D
18 Naval Research Reviews

Printing, under the direction of Samuel Allen and Emanuel

Sachs at the 3DP Lab at M.I.T.

REFERENCES

1. Sachs, E., Haggerty, J., Cima, M., Williams, P.,

“Three Dimensional Printing Techniques”, US Patent

# 5,204,055, April 20, 1993.

2. Sachs, E., Cima, M., Williams, P., Brancazio, D., and

Cornie, J., “Three Dimensional Printing: Rapid

Tooling and Prototypes Directly From a CAD

Model”, Journal of Engineering for Industry, Vol.

114, No. 4, November 1992, pp. 481-488.

3. Sachs, E., Cima, M., Bredt, J., Curodeau, A., “CADCasting: The Direct Fabrication of Ceramic Shells

and Cores by Three Dimensional Printing”, Manufacturing Review, Vol. 5, No 2, June 1992, pp. 118-126.

4. Sachs, E., Curodeau, A., Gossard, D., Jee, H., Cima,

M., Caldarise, S., “Surface Texture by 3D Printing”,

Proceedings of 1994 Solid Freeform Fabrication

Symposium, Austin, TX, August 8-10, 1994, pg. 56-

64.

5. Michaels, S., Sachs, E., Cima, M., “Three Dimensional Printing of Metal and Cermet Parts”, Proceedings P/M2

TEC ’94 Conference - Advances in Powder

Metallurgy and Particulate Materials, May 8-11,

1994, Toronto, Canada.

6. Sachs et al., “Production of Injection Molding

Tooling with Conformal Cooling Channels using The

Three Dimensional Printing Process”, Solid Freeform

Fabrication Symposium Proceedings, August 7-9,

1995, pp. 448-467.

7. Sachs, E., Allen, S., Cima, M., Wylonis, E., and Guo,

H., “Production of Injection Molding Tooling with

Conformal Cooling Channels using The Three

Dimensional Printing Process”, to appear in Polymer

Engineering and Science.

8. Sachs, E., Brancazio, D., Milner, J., Bredt, J., Serdy,

J., Curodeau, A., “High Rate, High Quality 3D

Printing through Machine Design, On-line Measurement, and Control”, to appear in International

Journal of Machine Tools and Manufacturing.
Three/1998 19

Abstract

Shape Deposition Manufacturing (SDM) is a solid

freeform fabrication (SFF) process that incrementally builds

up complex parts by systematically combining material additive processes with material removal processes. The advantages of each type of process are thus combined such

that novel structures can be fabricated with SDM that could

not be practically fabricated with either material additive or

material removal processes alone. Examples of such structures, which are relevant to Navy/DOD applications, are

described in this paper including a waterproof wearable computer with embedded electronics, a composite steel/copper

injection mold tool, and a miniature turbine wheel assembly. In addition, this article presents a novel implementation

of a SDM system based upon the integration of deposition

apparatus (i.e., material additive process) with an existing

computer-numerically-controlled (CNC) milling machine

(i.e., a material removal process). Such an implementation

is a cost-effective way to create high-quality SFF machines.

1. Introduction

Most solid freeform fabrication (SFF) systems are

based upon a material additive, layered manufacturing

method. Computer-aided-design (CAD) models are first decomposed into thin cross-sectional layer representations, then

physical parts are built up in custom automated fabrication

machines, layer-by-layer, using material additive processes

(1). Layers of sacrificial structures are simultaneously built

up to fixture and support the growing shapes. While layered

manufacturing facilitates rapid prototyping (e.g., quickly

fabricating “models”, as opposed to production parts) of arbitrarily complex shapes, the resulting surface finish and

accuracy, which are critical factors for being able to fabricate functional parts, are compromised by the “stair-steps”

Novel Applications

and Implementations

of Shape Deposition

Manufacturing

Fritz B. Prinz, Stanford University, Stanford, CA

Lee E. Weiss, Carnegie Mellon University, Pittsburgh, PA
20 Naval Research Reviews

from layer-to-layer. High accuracy and quality surface finishes, required for such applications as custom tooling, precision assemblies, and structural ceramics, are best achieved

with material removal processes such as 3- and 5-axis computer-numerically-controlled (CNC) milling and electrical

discharge machining (EDM) machines.

Shape Deposition Manufacturing (SDM) is a SFF process for which the original goal was to combine the advantages of geometry decomposition and material addition with

the advantages of material removal processes (Figure 1). The

basic SDM fabrication methodology is to deposit individual

segments of a part, and of support material structure, as near

net shapes, then machine each to net-shape before depositing and shaping additional material (2). This method takes

advantage of the basic SDM decomposition strategy which

is to decompose shapes into segments or ‘compacts’, such

that undercut features need not be machined, but are formed

by depositing onto previously deposited and shaped segments. For example, undercut part features are formed by

depositing onto shaped support material compacts, and viceversa. In addition, the decomposition plan preserves the 3Dgeometry information of the outer surface of each compact

so that the desired shape of the CAD model can be accurately replicated when 5-axis machining is available. Each

compact in each layer is deposited as a near-net shape using

one of several available deposition processes that are described in subsequent sections. The thickness of each compact depends not only on the local part geometry, but also on

Figure 1

Shape Deposition Manufacturing.

Figure 2

Multi-material structures with embedded components. (a) Example of a heterogeneous structure. (b) Sequence for depositing

and shaping; each compact is deposited, then shaped before proceeding to next compact.

(a) (b)
Three/1998 21

deposition process constraints. After the entire part is built

up, the sacrificial support material is removed to reveal the

final part.

In addition to the rapid prototyping of complex shapes,

selective additive material processing enables the fabrication of multi-material structures and it also permits prefabricated components to be embedded within the growing shapes

as depicted in Figure 2a. Another goal of SDM research is to

investigate how the capability to fabricate such heterogeneous structures enables the manufacture of novel product

designs (3). An example of the compact splitting strategy

and sequence for depositing and shaping materials for a typical layer of a heterogeneous structure is depicted in Figure

2b. Note how depositing onto the machined surface of one

compact forms the undercut surface of another compact on

top of the first compact. Several examples of heterogeneous

designs are described in subsequent sections including a

waterproof wearable computer with embedded electronics,

and a composite steel/copper injection molding tool. In addition to heterogeneous designs, novel assemblies of parts

can also be directly built up with SDM by using sacrificial

material to separate the individual parts. An example of a

miniature metal turbine assembly is described in this paper.

Another key issue for our research is how to implement SDM in a cost-effective fashion. Until recently, SDM

operations have been executed by building up the parts on

pallets and transferring them to individual operating stations

using a robotic palletizing system (4). Robotic manipulation

was used in order to create a flexible system, for an R&D

environment, which could be easily modified in order to investigate alternative deposition, shaping or other intermediate processing operations. Such a system, however, is too

expensive and large for general dissemination of this technology. The next section describes a novel, cost effective

and compact implementation of SDM.

2. Integrated CNC

Shaping and Deposition

Machine

Commercialized SFF systems are customized machines, and high performance SFF apparatus can be relatively expensive. As an alternative to customization, or to

robotic automation, we are exploring implementing SDM

by simply adding deposition apparatus directly to existing

CNC milling machines such as are typically found in machine shops throughout the world. In addition to shaping

operations, the CNC milling machine provides the precision

Figure 3

Integrated CNC shaping/deposition machine for SDM. (a) Fadal VMC-15 CNC milling machine with integral extruder. (b) ACR

extruder.

(a) (b)
22 Naval Research Reviews

motion control required for deposition. When not being used

for SDM, such an integrated CNC deposition and shaping

machine can still be used as a conventional milling machine.

For one example, the integrated CNC deposition/shaping machine shown in Figure 3a is being used to investigate

the fabrication of ‘green’ ceramic parts using an extrusion

deposition process (5). Green materials, which are deposited by the extruder, are composed of ceramic powders

densely bound in polymeric binders. After the green part is

built up and removed from the machine, the binder is burned

out in a furnace. Then, the part is sintered to fuse the powder

to form the final ceramic part. The CNC machine is based

upon a commercially available Fadal VMC-15 3-axis mill

with an automatic tool changer carousel1

. An Advanced Ceramics Research extruder2

 is mounted on a pneumatically

actuated slide that is attached to the Z-axis spindle housing

of the CNC machine. The slide is retracted when the extruder is not in use (e.g., during machining operations) and

lowers the extruder into the workspace during deposition

operations. The extruder is used to deposit both support and

part materials; currently, we manually switch extrusion tubes/

nozzles preloaded with the different materials. Being able to

quickly build complex ceramic parts is important for many

military applications such as components for high-performance miniature turbine engines for drone aircraft.

An example of a ‘green’ ceramic part built on the integrated SDM machine is shown in Figure 4. The part material is silicon nitride, and the support material is ACR 200, a

proprietary non-ionic, water-soluble, machinable thermoplastic. While this particular shape could have been cut directly

from a block of ‘green’ ceramic stock, such conventional

machining would require re-orienting, re-fixturing, and registering the part after the top-side has been cut in order to cut

the bottom-side. Another advantage of SDM over conventional machining is that first depositing shapes in near-net,

before machining, reduces the waste of costly materials.

We have also explored the use of an integrated SDM

machine that incorporated conventional welding to directly

deposit steel and copper parts (6). We are currently creating

an SDM machine for fabricating wax molds for molding gelcast or thermoset materials (5).

3. Multi-Material

(Heterogeneous)

Structures

We believe that one of the most important roles for

SFF in the future will be to help manufacture heterogeneous

product designs. Several of these novel products that have

been built with SDM are described below. While conventional manufacturing methods could have been used to fabricate these products, these methods would have required

additional time-consuming operations, including the need for

custom fixturing and tooling, complex assembly operations,

and high-strength material joining processes.

3.1. Steel/Copper Tooling

Injection molding is the process of forming plastic parts

by first flowing heated plastic into the cavity of a custom

tool (i.e., the cavity is in the shape of the part), then allowing

the plastic to cool down and solidify, and finally opening the

tool to remove the part. Injection molding is used to massproduce plastic parts in quantities from hundreds to millions

of parts. SFF has been widely investigated for fabricating

injection mold tools with complex molding surfaces, as well

as with conformal internal cooling channels for thermal

management. With SDM, even more advanced tools can be

fabricated composed of multi-materials such as steel/copper

composites. While steel provides strength and wear-resistance, copper’s superior heat transfer properties provide quick

heat up and cool down of the tool as well as uniform heatFigure 4

Example of a part built with an integrated SDM CNC shaping/deposition machine. (a) Green Si3N4 part. (b) Part after

pressureless sintering in N2 at 1750°C.

(a) (b)

1

Fadal Engineering, Chatsworth, CA.

2

The extruder and the feedstock materials are manufactured by Advanced Ceramics Research Corporation, Tucson, AZ.
Three/1998 23

ing. For example, Figure 5a shows a composite 316L stainless steel injection molding tool produced by SDM using

robotic-controlled laser welding and microcasting deposition processes (7). One half of the tool, which is shown schematically in Figure 5b, has four internal copper deposits for

temperature equilibration. Both halves of the tool have a “U”-

shaped channel for water cooling during the molding process. The channels were formed by sacrificial copper, which

was removed by etching in nitric acid. Portions of the cavities contained small features that could not be cut with end

mills and these were finished with EDM.

In this tool, the steel was deposited with a laser welding process (Figure 5c). A 2.4 kW CW Nd:YAG laser scans

over the substrate and a melt pool forms into which metal

powder is injected (Figure 5c). The injected powder fuses

onto the substrate, leaving a bead of deposited material in its

wake. While this laser welding process is very precise, in

comparison with conventional welding methods, it cannot

effectively deposit copper due to copper’s high reflectively.

Therefore, microcasting was used to deposit the copper (Figure 5d). Microcasting is a non-transferred welding process

that deposits discrete droplets of super-heated molten metal

(6).

In addition to creating steel/copper structures, the laser system has also been used to deposit INVARTM, a low

coefficient-of-thermal-expansion (CTE) nickel alloy, onto

copper that was previously deposited onto steel. Such multimaterial structures will have significant advantages in a wide

variety of military applications. For example, in dies used

for forming composite airfoils (e.g., for airplane and boat

bodies), INVAR provides a closer match to the CTE of composite materials, thus resisting deforming the material during molding operations.

Ideally, the transition between different materials

should be functionally graded, e.g., having a gradual change

in material composition from one material to the next material. The laser system is particularly suitable for producing

functional-gradient, multi-material parts because different

materials can be continuously alloyed during the build process by simply mixing the powders which are fed to the melt

pool.

3.2. Embedded Electronics

Another example of a heterogeneous design is an embedded electronic device fabricated by building up a nonconductive housing package and simultaneously embedding

and interconnecting electronic components within the housing. With this approach it is feasible to relatively quickly

fabricate compact, rugged, customized computer modules

in small lot sizes. This capability is particularly well suited

for military applications, to manufacture mission-specific,

conformal shaped ‘smart’ devices such as wearable computers tailored for an individual soldier or a small military unit.

Figure 5

Multi-material metal structures built with SDM. (a) Multi-material injection mold tool. (b) Schematic of one half of tool.

(c) Laser welding process. (d) Microcasting apparatus.

(a) (b)

(c) (d)
24 Naval Research Reviews

These computers might store maps, equipment descriptions,

help to log data, or provide communication links.

For one example, the ‘Frogman’ shown in Figure 6a

and 6b is a waterproof computer that can store maps for navigational aids, or detailed assembly drawings for service,

maintenance, or field operations. The graphical information,

which is stored on Personal Computer Memory Card International Association (PCMCIA) cards, is displayed on a

heads-up display (Figure 6c). A conformal shaped rear surface was also required so that the unit could be comfortably

strapped to a diver’s leg. The device is built up in layers of

polyurethane (PU) and sacrificial wax. The PU is deposited

as a 2-part thermoset (left side of Figure 6d). The wax can be

extruded with a conventional hot-glue gun (right side of Figure 6d), or thick layers can be poured from a hot-melt pot.

The fabrication details, including component embedding and

interconnection are described in detail in (8). The important

points are that custom tooling was not required to manufacture the Frogman and that embedding facilitates waterproofing.

4. Integrated Assemblies

SDM has also been used to build up simple assemblies in a single operation. As an assembly is being built up,

its individual components are separated by and encased

within sacrificial support material. After the assembly structure has been completely built up, the sacrificial material is

removed, freeing the components to move with respect to

each other. For example, SDM was used to create the steel

crank mechanism shown in Figure 7. In this mechanism, a

piston is connected to a crankshaft with a connecting rod.

Turning the crank causes the piston to move back and forth

in its chamber. The mechanism components are 316L stainless steel, deposited with laser welding, and the sacrificial

support material was microcast copper.

The capability to create such integrated assemblies may

be particularly useful for producing miniature mechanisms

where discrete assembly is difficult, i.e., similar to the micro-electro-mechanical systems (MEMS) methodology. To

demonstrate the feasibility of SDM for the fabrication of

structures with feature sizes in the range of tens to hundreds

of and thousands of microns, several simple artifacts have

been built (9). This regime has been recently referred to as

the ‘mesoscopic regime’ which means that characteristic feature dimensions are bigger that those typically achieved using very large scale integration (VLSI) fabrication methods

(e.g., used to make integrated circuit chips), yet smaller than

parts produced using conventional processing techniques. We

believe that mesoscopic assemblies will be particularly important for enabling future DOD applications such as autonomous micro-vehicles and micro-flying machines.

In SDM, mesoscopic structures are built up using sputtering and electro-plating deposition processes, and shaped

Figure 6

Embedded Electronics. (a) 'Frogman' computer. (b) Waterproof application. (c) CAD model of 'Frogman'. (b) Deposition

apparatus.

(a) (b)

(c) (d)
Three/1998 25

with micro CNC or EDM machining. For one example, the

1.1mm high nickel structure shown in Figure 8a consists of

a wheel (5mm dia., 0.3mm thick) which is permanently

mounted on a nickel axle (1mm dia.). The scanning electron

microscopy (SEM) photograph in Figure 8b shows a crosssection of the wheel and axle before removal of the copper

support structure.

Additional examples of novel SDM mesoscopic integrated assembles are shown in Figure 9. Figure 9a is a nickel

substrate carrying nine mesoscopic wheels. This structure

suggests the possibility for building massively parallel miniature machinery. Figure 9b shows a 130mm thick

microturbine impeller that rotates at high speeds when air is

passed through the gas jets. This structure establishes the

feasibility of building assembled devices with clearances on

the order of less than 20 microns.

5. Discussion

SFF has been successfully used within the limited

realm of Rapid Prototyping. However, as SFF processes

improve and are able to build functional, engineering modFigure 7

Complete assemblies and mechanisms directly built up with SDM. (a) Steel crank and piston mechanism. (b) Schematic of

mechanism.

els, SFF will be used for mass customization, i.e., customers

able to order products in small-lots (as small as one) customized for their specific needs. Such ‘mass-customization’

will be attractive for not only consumer and commercial

markets, but also for defense industries as a “dual-use” technology for creating products tailored to individual soldiers

needs, as well as for cost effective tooling for manufacturing

defense systems. In addition, by opening up the design space,

novel designs, inaccessible with conventional manufacturing techniques, will be possible. One class of such novel

designs is heterogeneous structures, such as embedded electronics, and another class is integrated assemblies as described in this article. For heterogeneous structures to be

practical, however, streamlined CAD systems will be required

which enable concurrent representation and manipulation of

geometry, material and embedded components. Other possible novel designs will involve functional gradient structures such as ceramic to metal parts or graded metal to metal

structures allowing the transition from highly thermally conductive regions, inside, to tough surfaces on the outside of a

part. Beyond these novel applications of SFF, it is hard to

predict where exactly this will lead to when creative people

have access to SFF technology. Undoubtedly, the creation

(a) (b)

Figure 8

Mesoscopic nickel wheel on axle built with SDM. (a) Mesoscopic Ni wheel. (b) Cross-section of wheel and support.

(a) (b)
26 Naval Research Reviews

of products that no one has even conceived of as yet will be

one of the likely outcomes.

Acknowledgments

This work has been supported by the ONR, DARPA,

and NSF. The authors acknowledge the following individuals for their contributions in creating the systems and the

parts built with the systems described in this article: Gennady

Neplotnik, Larry Schultz, Robert Merz, Alexander Cooper,

John Fessler, JohnKeitzman, and John Lombardi.

Bibliographies

Dr. Weiss is a Principal Research Scientist at the Robotics Institute of Carnegie Mellon University where he directs the Shape Deposition Manufacturing Laboratory. Prior

to joining the Robotics Institute in 1979 he designed cardiac

pacemakers in industry. He received his Ph.D. in Electrical

and Computer Engineering and his Masters degree in

Bioengineering from Carnegie Mellon.

Dr. Prinz is the first holder of the Rodney H. Adams

professorship in the School of Engineering at Stanford University. Prior to joining Stanford in 1994 he taught Mechanical Engineering at Carnegie Mellon University for fourteen

years. He obtained his Ph.D. in Physics and Mathematics

from the University of Vienna, Austria.

REFERENCES

1. Weiss, L.E., Solid Freeform Fabrication Processes

Overview, in Japanese Technology Evaluation Center

and World Technology Evaluation Center Panel

Report on Rapid Prototyping in Japan and Europe,

Analytical Chapters, ed. G. M. Holdridge, SME, I, 5,

(1997).

2. Merz, R., Prinz, F.B., Ramaswami, K., Terk, M. and

Weiss, L.E., Shape Deposition Manufacturing,

Proceedings of the 1994 Solid Freeform Fabrication

Figure 9

Examples of mesoscopic assemblies. (a) Assembly of nine mesoscopic wheels. (b) Miniature turbine impeller.

Symposium, The University of Texas At Austin, eds.

Harris Marcus, J.J. Beaman, J.W. Barlow, D.L.

Bourell and R. Crawford, August, 1, (1994).

3. L.E. Weiss, R. Merz, F.B. Prinz, G. Neplotnik, P.

Padmanabhan, L. Schultz, K. Ramaswami, Shape

Deposition Manufacturing of Heterogeneous Structures, SME Journal of Manufacturing Systems, 16,

239, (1997).

4. Weiss, L.E., Hartman, K., Krishman, R., Merz, R.,

Neplotnik, G., Prinz, F.B., Schultz, L., and Terk M.,

Robotic-Assisted Shape Deposition Manufacturing,

in Proceedings of 1994 IEEE International Conference on Robotics and Automation, 2890, (1994).

5. Kietzman, J.W., Cooper, A.G., Weiss, L.E., Schultz,

L., Lombardi, J.L., and Prinz, F.B., Layered Manufacturing Material Issues for SDM of Polymers and

Ceramics, 1997 Solid Freeform Fabrication Symposium, The University of Texas At Austin, 133,

(1997).

6. R. Merz, “Shape Deposition Manufacturing”, PhD.

Dissertation, Institut für Allgemeine Elektrotechnik

und Elektronik, Technische Universität Wien, (1994).

7. Fessler, J.R., Merz, R., Nickel, A.H., Prinz, F.B., and

Weiss, L., Laser Deposition of Metals for Shape

Deposition Manufaturing, 1996 Solid Freeform

Fabrication Symposium, The University of Texas At

Austin, August, 117, (1996).

8. Weiss, L.E., Neplotnik, G., Prinz, F.B., Schultz, L.,

Padmanabhan, P, Krishnan, R., and Merz, R.., Shape

Deposition Manufacturing of Wearable Computers,

1996 Solid Freeform Fabrication Symposium, The

University of Texas At Austin, 31, (1996).

9. Merz, R. and Prinz, F.B., Rapid Prototyping of

Mesoscopic Structures, Proc. of the 7th International

Conference on Rapid Prototyping, ed. A. Lightman,

San Francisco, March, 261, (1997).

(a) (b)
Three/1998 27

Abstract

Solid Freeform Fabrication (SFF) technologies are now

being developed for functional ceramic components. SFF

offers unique manufacturing opportunities, free from constraints imposed by the part’s external geometric complexity, or the internal complexity resulting from the presence of

multiple phases, porosity, etc. This paper gives a brief overview of novel SFF techniques under development for: structural ceramics, bio-ceramics, and electroceramic components.

Parts were made directly by Fused Deposition of Ceramics

(FDC). Fused deposition modeling (FDMTM) and Sanders

Prototyping (SPI) methods were used to form a variety of

complex shaped molds (some with very intricate graded internal structures) for the indirect (or lost mold) method of

fabrication. Current research is directed at developing the

processing and manufacturing science needed for layered

manufacturing of multi-material, multi-functional ceramic

components for electromechanical as well as structural applications.

Introduction

Solid Freeform Fabrication (SFF) refers to methods

that fabricate freestanding solid objects. While many of these

techniques do not require the need for tooling or machining,

some do. SFF methods are computerized (CAD file driven)

material additive processes, originally applied to form and

fit polymer applications which allow dramatic reductions in

the time and cost of each component design iteration. [1-4]

Several SFF technologies are under development at Rutgers

University to make structural ceramic, bio-ceramic, as well

as electronic ceramic components and devices. The electronic

ceramic components under investigation are piezoelectric

ceramics and ceramic-polymer composites for use as transducers, actuators, vibration control devices, etc. All SFF techSolid Freeform

Fabrication (SFF) of

Functional Advanced

Ceramic Components

S. C. Danforth and A. Safari, Ceramic and Materials Engineering, M. Jafari, Industrial Engineering, N. Langrana,

Mechanical and Aerospace Engineering, Rutgers, The State University, Piscataway, NJ
28 Naval Research Reviews

nologies share the same general approach; they start with a

surface tessellated .stl file, that is then sliced mathematically

into discrete layers, and then sent to a fabricator which builds

the object in a layerwise additive fashion. Solid freeform

fabrication methods differ in the details of the materials, material deposition, etc. For SFF of ceramics and some SFF

techniques for metals, the techniques use powders as the raw

material, and therefore require post-SFF processing (binder

removal, sintering, or infiltration) of parts to achieve the

desired fully dense microstructure and desired properties.

Some SFF techniques are able to make fully dense metal

parts without the need for post-processing steps.

Direct & Indirect SFF

Processes

Two different approaches are under development for

manufacturing functional ceramic components; referred to

as the “direct” and the “indirect” technique, Fig.1. In the

direct method: Fused deposition of ceramics (FDC), based

on fused deposition modeling (FDMTM) developed by

Stratasys, Inc., uses ceramic (or metal) particle loaded thermoplastic filament wound on a spool as the material feedstock, fed via counter rotating rollers into a heated extruder

referred to as a liquefier.[5,6] The filament acts as the piston to extrude the molten ceramic loaded polymer material

out of a (250 micron to 635 micron diameter) nozzle onto a

z-stage platform, where the material cools rapidly and bonds

to adjacent layers, Fig. 2. The material deposition rate and

liquefier x-y position, etc., are controlled by the computer.

For ceramic (or powder metal) parts, the binder is subsequently removed followed by sintering to full density. In the

indirect method, Fig. 1, components are manufactured by

first making a polymer or wax mold, using either FDMTM or

Sanders Prototyping (SPI) techniques. The FDMTM or SPI

mold is then infiltrated with a ceramic (or metal) powder

suspension, dried (or gelled), followed by thermal or solvent mold removal, binder removal, and finally sintering.

Processing details, as well as further physical and electromechanical characterization of structural and electroceramic

components processed via these direct and indirect SFF

methods, are reported elsewhere.[7-12]

Structural Ceramics

 The FDC process was initially developed for

AlliedSignal’s GS-44 insitu-reinforced (ISR) silicon nitride

(GS-44 Si3N4). The process steps for FDC of GS-44 Si3N4

are: ball milling the powder with a surfactant, drying, grinding and sieving, compounding with the multi-component

binder system (RU9) in a high shear torque rheometer, followed by cooling and granulation. The material is then extruded into 1778 micron diameter filaments in a single screw

extruder fitted with a high shear tip screw and a breaker plate

and screen that insure removal of large agglomerates. Parts

are built using a liquefier temperature of 185°C in a chamber at 40°C. Typically, 381 micron diameter extrusion nozzles

are used, building parts with 254 micron layer thickness.

Ceramic part build times have ranged from 1 hr., for small

simple shapes, to greater than 16 hr. for large, or very complex shapes. Figures 3 a,b show complex shaped GS-44 Si3

N4

parts, made at Rutgers, by the FDC process. The typical densities of sintered FDC GS-44 Si3

N4 parts are >99 % of theoretical density.[10] The average measured four-point bend

Figure 2

Schematic of the fused Deposition of Ceramics (FDC) SFF

process.

Figure 1

Schematic of direct and indirect routes of layered

manufacturing.
Three/1998 29

fracture strength of FDC bars is >900 MPa, similar to values

for commercially produced GS-44 Si3N4 parts. The measured

Chevron notch fracture toughness is 8 MPa m0.5. While some

subtle microstructural features are present which relate to

layerwise building, there is little evidence, to date, of any

significant difference in the microstructure, fracture strength

or fracture toughness in samples where the cracks are driven

parallel or perpendicular to the build plane. At present, GS44 Si3

N4 samples fall into two categories: (1) those with average four-point fracture strengths > 900 MPa, with a Weibull

modulus of 15 (which fail from large grains or machining

damage), and (2) those which fail from process induced Fe

bearing defects and have strengths in the range of 500-700

MPa. The Weibull modulus, m, is a statistical parameter that

describes the width of the measured strength distribution.

Recent results indicate that the fracture toughness and fracture strength are independent of the crack direction relative

to the build plane direction. Feasibility for FDC has also been

shown for a wide range of other materials with average particle sizes ranging from 0.5 microns to over 50 microns, including to date: lead zirconate titanate (PZT), Al2

O3, SiO2,

WC-Co, and 17-PH stainless steel.

Porous Hydroxyapatite

Ceramics

Hydroxyapatite (HAp) and related calcium phosphate

(CP) materials have been widely used as bone implant materials for many years because of their close similarity in composition and high biocompatibility with natural bone. Many

researchers have emphasized the importance of controlling

the pore size in HAp ceramics, and have claimed that a minimum pore size of 100 microns is necessary for the porous

implant materials to function well, and pore sizes > 200 microns are essential for osteoconduction. Many traditional

techniques have been used to make porous bone scaffolds.

These techniques include the following: (1) the replamine

process, which involves coating a polymer foam with a 50

vol. % ceramic slurry and burning out the foam, and (2) pressing HAp powder with polymer beads and then evaporating

the spheres during heating, etc. These sintered structures

typically have poor strength, because of the discontinuous

pore network obtained using these methods. Also, it is nearly

impossible to control the pore sizes and shapes for use in

Figure 3

a) Various green ceramic (light) and metal (dark) parts built by FDC process. b) Complex shaped, sintered, machined

GS-44 Si3N4 component built by the FDC process.

(a) (b)

Figure 4

3-D Honeycomb structure with interconnected, regular,

cylindrical pore channels.
30 Naval Research Reviews

different applications. The FDMTM and SPI indirect SFF

methods have been used to make porous bone scaffolds with

a controlled porosity. First, the bone implant is designed using AutoCAD or ProEngineer, followed by fabrication of

the sacrificial polymer or wax mold, and infiltrating with a

HAp aqueous suspension, etc. A variety of porous structures have been fabricated, with porosity ranging from 30 to

70 vol. %. The pore sizes of the structures have also been

varied from 200 microns to 750 microns. Figure 4 shows a

3-D honeycomb structure made by the FDM indirect method

where the controlled pores are interconnected in three dimensions. This type of 3-D honeycomb structure has never

been made using any other method. Figure 5 shows HAp

structures with ~ 55 vol. % porosity made by these techniques.

High Authority

Actuators

In the last few years, the technology of using piezoelectric and electrostrictive actuators for applications requiring large displacements such as linear motors, cavity pumps,

switches, loudspeakers and noise-canceling devices, has

undergone significant development. Such applications require

very large displacements (>1000 microns) combined with a

moderate force (>10 kg). However, the induced strains in

piezoelectric and electrostrictive ceramic discs, regardless

of their size, are a few tenths of a micron. Higher displacements and generative force are achievable with these materials by employing strain-amplification techniques. As a

result, Multilayer, Bimorph, Moonie, and Rainbow actuators, with magnified output displacement, have been developed. [13-15]. A “Moonie” is an actuator consisting of a thin

ceramic disk and two thin metal plates bonded together. Each

of the metal plates has a narrow “moon”-shaped cavity in it.

A “Bimorph”, is an actuator consisting of two thin ceramic

plates, which are bonded onto each side of an elastic metal

shim. A “Rainbow” is a curved actuator, consisting of an

oxide ceramic disk which has been reduced and internally

biased.

These components are useful for Naval and other DoD

applications. These piezoelectric actuators can be used for

active and passive vibration suppression on the space shuttle,

missiles and other military as well as commercial vehicles.

They can also be used as components of a “smart skin” in a

system. For this application, the signal of projectile impact,

turbulent flow, or other undesirable vibration, is detected by

a transducer, and then fed back to an actuator (in the smart

skin), to change its shape, so as to negate, cancel, or minimize the deleterious effects of the event.

The focus of the layered manufacturing (LM) research

at Rutgers is on high authority actuators, such as a metalelectroceramic composite flex-tensional transducer termed

a “Moonie,” and dome shaped actuators, Figs. 6 & 7. Traditional Moonies consist of a monolithic piece of

Figure 5

Sintered, HAp bone structures made by indirect SFF

technique.

Figure 6

Schematic of the top half of a novel alternate poling design

for a Moonie flex-tensional high authority actuator.

Figure 7

Photograph of a PZT ceramic dome actuator fabricated by

the direct LM (FDC) process.
Three/1998 31

electroceramic (lead zirconate titanate, PZT, lead magnesium

niobate-lead titanate, PMN-PT) sandwiched between two

metal end caps, where the ceramic is poled in the thickness

direction, and upon actuation, extends in the thickness direction. There is an accompanying Poisson contraction in

the transverse direction that flexes the metal end caps resulting in an axial displacement, which couples with the axial

displacement of the ceramic, thus amplifying the axial motion and transmitting it to the metal end caps (see Fig. 6).

These transducers are characterized by values of displacement and force on the order of 20-30 microns and 3 N respectively. Two critical aspects to the successful development

of LM of Moonies are: (1) multiple material co-deposition

by extrusion, and (2) co-firing (in the range of 1200-1280

°C) of the PZT ceramic with Cu-Ni end caps, and any required electrode and insulator compositions, Fig. 6. This

novel Moonie design is based on an alternate poling concept. This was developed via finite element and kinematic

electromechanical performance modeling, that predicts twice

the displacement compared to a design which uses a monolithic PZT disk. [16,17]. A multi-ring ceramic element, based

on this design, has been fabricated at Rutgers via FDC, Fig.

8. PZT made by FDC has been successfully co-fired with Pt

at 1285°C in an excess lead atmosphere, and the electromechanical properties of the PZT/Pt structure are: a dielectric

constant, K, of 3400, a longitudinal piezoelectric charge

coefficient, d33, of 620 pC/N, and an electrical dissipation

factor, tan δ, of 2%, values which are in excellent agreement

with the literature, confirming that these LM techniques can

fabricate high quality PZT components. Current research is

also directed at development and optimization of co-firing

conditions for ceramic, electrode and metal end caps for

Moonies.

A new type of high authority, dome-shaped actuator

with different curvatures, diameters and thickness, is under

development via LM processing. Unlike the conventional

techniques used for fabrication of Rainbow actuators with a

curved structure, this method has the advantage of precise

control of curvature, thickness and geometry. A thermoplastic binder with moderate strength and flexibility, while maintaining low viscosity at the deposition temperature, was

developed for FDC with PZT based materials. The powder

and binder were compounded, using 60 vol. of PZT ceramic

powder (TRS Ceramics, Inc., State College, PA), in a torque

rheometer at 135°C and 100 rpm for one hour until a stabilized torque was reached. The compounded mix was granuFigure 8

Photograph of green PZT multi-ring ceramic actuator element

fabricated by direct LM (FDC) process.

Figure 9

SEM micrograph of a sintered PZT dome actuator element

fabricated by direct LM (FDC) process.

Figure 10

SEM micrograph of a cut cross section of a sintered PZT

dome actuator element fabricated by direct LM (FDC) process.
32 Naval Research Reviews

lated and sieved to use as the feed material for filament extrusion. A capillary rheometer was used for extrusion of short

lengths (~ 30 cm) of green PZT filaments. The FDC process

was used to manufacture dome shaped parts at a liquefier

temperature of 145-170°C, while the surrounding environment temperature was maintained in a range of 35-40°C.

After binder removal and sintering a density of up to 94 %

theoretical density was reached. Figures 7 and 9 show exterior and interior views of a dome-shaped actuator, fabricated

by the FDC process. Examination of sintered parts using

electron microscopy techniques showed no sign of delamination for parts made by layered manufacturing techniques,

Fig. 10. The samples were thermally etched and their microstructure compared well with that of samples prepared

by conventional processing methods, Fig. 11 a and b.

Similar to Rainbows, the dome-shaped actuators have

shown a bending mode resonance at about 17 kHz, in addition to radial and thickness mode resonance frequencies. The

piezoelectric coefficient, d33, of these samples was measured

when the samples were unclamped, using a Berlincourt Piezometer. Even though there was a large variation of the d33

coefficient across the diameter of these structures, with a

minimum d33 of 550 pC/N at the center of the dome, a large

d33 of up to 1270 pC/N was measured at the edges of these

samples, using pointed fixtures, Fig. 12. These measured

values are comparable with values reported in the literature

for Rainbow actuators. These samples also have shown a

low dielectric loss (~3.5%) compared to Rainbow samples

of the same size (8.5%). It is believed that the higher loss in

Rainbows is due to the presence of carbon in the structure as

a result of the reduction process.

Piezoelectric

Transducers and

Sensors

Piezoelectric transducers and sensors with novel structures and designs have been manufactured by both the direct

(FDC) and indirect (FDMTM and SPI) techniques. Figure 13

shows an SEM micrograph of an oriented PZT fiber structure made by the FDC process using a 55-vol. % PZT powder loaded filament as the feedstock, and a 406 micron nozzle

in the liquefier. This geometry generates very large piezoelectric charge coefficients, due to the contribution of the d33

and d15 coefficients. Many other fine scale PZT structures,

including volume fraction gradient and a novel radial com-

(a) (b)

Figure 11

a) SEM micrograph of a polished and etched, sintered PZT dome actuator element fabricated by direct LM. b) SEM micrograph

of a polished and etched, sintered PZT element fabricated by conventional means.

Figure 12

Distribution of d33 across the LM dome actuator.
Three/1998 33

posite design, have been fabricated for use in ultrasonic

medical imaging and towed array transducers by the SPI indirect method, Figs. 14 a and b.

LM Computer Aided

Design (CAD)

These and other novel SFF processes are under development for high authority actuator designs that are able to

take advantage of the flexibility provided by SFF processes.

In one Office of Naval Research Multi-Disciplinary University Research Initiative program at Rutgers (with The University of Delaware and the University of Illinois at Chicago),

a new Layered Manufacturing (LM) system is under development. The overall objective of the MURI program is to

establish the processing and manufacturing science required

for the development of a novel, intelligent LM system for

the cost effective fabrication of multiple material, net shape,

complex shaped electromechanical components/devices and

arrays. The specific objectives of the program are: (1) Develop the necessary machine control systems, and material

delivery systems for the deposition of one or more ceramic

phases, one metal phase and a fugitive organic support phase

for a given component, (2) Develop the science base for accurate process modeling and to achieve a predictive capability both for LM and performance of the complex components,

and (3) Develop a system for real time control and a LM

CAD system for advanced computer simulation of the entire

layered manufacturing process. Figures 15 and 16 show a

highly conceptual schematic of a single build layer of a fictional component that could be built by this new type of intelligent SFF process and a conceptual schematic of the

multiple material LM system under development.

In currently available FDC based systems (as with most

SFF systems), the CAD-manufacturing process is open loop.

That is, users cannot predict the quality of the part, and the

system itself cannot verify or improve the quality of the part.

Therefore, several designs of experiments are needed to improve part quality. [5,9]. Multiple material LM refers to the

process of fabricating a part from a three-dimensional CAD

model, layer by layer, using more than one material per layer.

This complexity increases the chances that the open loop

Figure 13

SEM micrograph of an oriented PZT fiber structure fabricated

by direct LM (FDC) process.

(a) (b)

Figure 14

a) SEM micrograph of PZT structure with volume fraction gradient fabricated by indirect LM (SPI) process. b) Polymer

mold, PZT part, and PZT-polymer radial composite structures fabricated by the indirect LM (SPI) process.
34 Naval Research Reviews

processes will not fabricate high quality parts. A multi-material CAD model consists of several different material blocks

connected by interphase boundaries. The LM system will be

able to generate CAD models, sliced files, and toolpaths, as

well as fabricate a multi-material part in the manufacturing

process. Achievement of this goal requires the development

of a CAD system that will perform the virtual simulation of

the multi-material LM process. SGI OpenGL is used to run a

LM simulation, as it provides the necessary flexibility, viewing and animation tools for the designer to verify the LM

manufacturability of the component CAD file. The multimaterial CAD model is created using commercially available software such as I-DEAS and ProE. Therefore, one .stl

and one .sml file per material is generated. The terms .stl

and .sml refer to the surface tessellated object model, and

the SFF machine tool path files, respectively. This information is then used as the input to the simulation algorithm,

which systematically, layer by layer, integrates all .sml files

into one tool path build file. This algorithm is written in C++

computer language. The results of the multi-material tool

path are graphically visualized. The part is stacked layer by

layer, and there can be several domains with different materials in each layer. Any defects or voids in each layer can be

visualized immediately. The results of actual physically fabricated parts match with our virtual LM simulations, which

demonstrates that the simulations provide sufficiently accurate and detailed information. Virtual tool path simulations

of multi-material electro-mechanical components have also

been created, Figs. 17-19. The LM simulation reduces the

time and materials that would otherwise be wasted in manufacturing inaccurate parts.

LM Machine System

Control

As noted above, current FD based technology only

supports open loop positioning and deposition control systems. Thus, the first key step in developing the next generation of LM technology is to close the control loop at the

Figure 15

Schematic of multi-material LM manufacturing system.

Figure 16

Schematic of a single, multi-material, multi-functional

component built by LM.

Figure 17

Virtual LM simulation of a 2-material stacked cymbal actuator

structure.

Figure 18

Virtual LM simulation of a multi-material structure.
Three/1998 35

local machine level, so that on-line feedback on the position

and velocity of the various axes can be used to minimize

potential position relayed part defects, such as internal voids

or surface roughness. In terms of trajectory planning (i.e.,

knowledge of the position and velocity of the deposition system as a function of time) for the positioning system, one

should compensate for the fact that only linear interpolation

is used with current technology. These inaccuracies in geometric representation contribute to tracking and contour errors in the positioning system, which can translate to

over-filling or under-filling at any point in a given layer.

Bouhal, et. al. formulated the problem of trajectory planning

in Layered Manufacturing, subject to the constraint that the

liquefier head must move with constant velocity as much as

possible.[18] The main reason for this constraint is that the

dynamics of the extrusion deposition system are not sufficiently known at this time. Any lack of synchronization between the deposition and the positioning systems could also

result in voids.

The second step in improving the current technology

is to provide an on-line process monitoring system so that

defects are first detected and then repaired on-line in each

given layer, prior to depositing the next layer. Current focus

is on a machine vision based, on-line process monitoring

and feedback system. This system is envisioned to work as

follows. Upon the completion of the fabrication of a given

layer, the cameras that are positioned along the same axes as

the liquefier head take images of the layer, which is then

analyzed through various image-processing techniques to

detect the size, nature and position of the defect. Finally, the

defects will be eliminated and then the machine would advance to the next layer.

Here, we will briefly discuss the issues that involve

the image processing aspects of the above system. Figure 20

illustrates an interior region of a layer of a sample of ceramic part built by the FDC process. On the image we note

a specific pattern of two thick lines (or edges) with a thinner

line in between. Figure 21 illustrates the interior region of a

second (different ceramic and binder) sample. In this image,

the pattern is different with a flat surface embedded within

boundaries that are sometimes cavities. At first glance, the

flat surface seems to contain only a single road (material

deposited by one pass of the extruder), but by more careful

examination, we have determined that it actually contains

two adjacent roads. This confirms our belief that the analysis of the image alone would be misleading, and more detailed knowledge of the process, from the parameter settings

at the CAD level to material properties, thermal effects and

material bonding are needed to assist the image analysis stage.

Various elements of the LM process are being modeled that

Figure 19

Multi-material structure made by direct LM (FDC) technique.

Figure 20

Optical image of the surface of a silicon nitride sample built

by the direct LM (FDC) process.

Figure 21

Optical image of the surface of a PZT sample built by the

direct LM (FDC) process.
36 Naval Research Reviews

will yield the input data and intelligence tools to assist in

image signature determination.[19,20] In particular, one

should be able to define an Ideal Process Signature that is

uniquely defined by a specific LM process and material. At

the same time, one can also define image process signatures

from the image. Figure 22 illustrates the process signature

for the sample in Fig. 20. The peaks in Fig. 22 signify the

three-edge pattern, that we noted in Fig. 20. For a given layer,

there may be several process signatures corresponding to the

various uniform segments in that layer. In a perfect FDC

process the ideal and the image process signatures must be

the same within a given tolerance. Any significant difference between these two sets of signatures will signify potential defects in a given layer, requiring repair.

Summary & Conclusions

This paper presents a brief summary of recent progress

at Rutgers in the development of novel SFF techniques for

functional quality advanced structural, bio-ceramic, and electronic ceramic components. For direct fabrication of ceramic

components, green ceramic filaments of Si3N4 or PZT powder loaded thermoplastic binder were used for fabrication of

complex ceramic structures by the FDC process, followed

by binder removal and sintering. Parts made by FDC exhibit

properties that are the same as those of components made by

traditional manufacturing methods. Fused Deposition of

Ceramics has several advantages as a prototyping and manufacturing technique: (1) the technique can be used with nearly

any ceramic, metal or composite powder, (2) the technique

produces parts with high density and functional properties,

etc. (3) FDC can make complex shaped components with

fine features and complicated internal structures, and (4) the

technique can be used with multiple materials. Fused deposition modeling (FDMTM) and Sanders Prototyping (SPI)

methods were used to form a variety of complex shaped

molds (some with very intricate graded internal structures)

for the indirect (or lost mold) method of fabrication. Once

built, the wax molds were infiltrated with a PZT, or hydroxyapatite ceramic slurry, followed by drying, mold and binder

removal and sintering. One can readily see the advantages

of manufacturing multi-material, multifunctional components, such as Moonie and Dome actuators, where one can

now design and manufacture both the component as well as

the internal macro- and microstructure for optimized performance. These actuators have great potential as transducers

(sonar, vibration sensing) and actuators (motion and vibration control, and smart skin) for Naval and other DoD platforms and systems. Current research is directed at developing

the processing and manufacturing science needed for layered manufacturing of multi-material, multi-functional components for electromechanical, as well as structural

applications. These novel ceramic manufacturing methods

show tremendous potential for affordable manufacturing of

functional quality structural ceramics and piezoelectric ceramics (and composites) and bio-ceramics with freedom from

the traditional design and manufacturing constraints.

Acknowledgments

The authors would like to thank DARPA, Dr. W. S.

Coblenz, and the Office of Naval Research (ONR) Drs. S.

Fishman, W. Smith, R. Wachter, and G. Spanos, for the financial support for this work under contract numbers:

N00014-93-1-1059, N00014-94-C-0115, N00014-94-1-

0588, N00014-96-1-0959, and MURI Grant No. N00014-

96-1-1175. In addition, the authors wish to thank the

following for technical and financial support of this research:

Dr. P. Whalen and Dr. C. Gasdaska, AlliedSignal, Inc., Dr.

L. Bowen, Material Systems, Inc., and Dr. V. F. Janas,

Johnson and Johnson, Inc.

Biographies

Stephen C. Danforth, Ph.D. is a Professor of Ceramic

and Materials Engineering, at Rutgers, The State University

of New Jersey. His principal fields of interest include: processing of ceramic powders, nanostructured materials, and

advanced manufacturing methods for ceramics and metals,

specifically solid freeform and layered manufacturing techniques. He is one of the inventors of the Fused Deposition of

Ceramics process. He is a Fellow of the American Ceramic

Society, and holder of their Karl Schwartzwalder Professional

Achievement in Ceramic Engineering (PACE) Award. He

has published over 150 papers and given over 100 invited

talks at professional meetings around the world.

Ahmad Safari, Ph.D. is a distinguished Professor of

the Department of Ceramic and Materials Engineering and a

member of the Center for Ceramic Research at Rutgers, The

Figure 22

Process signature of the image from Figure 20, of silicon

nitride sample made by direct LM (FDC) process.
Three/1998 37

State University of New Jersey. His main fields of interest

include: Layered manufacturing of functional ceramics,

electroceramic materials for dielectric, piezoelectric and ferroelectric applications, ferroelectric thin films for non-volatile memories, electroceramic and ceramic - polymer

composites for transducers, sensors, and actuators. Professor Safari is a Fellow of American Ceramic Society (1993),

a Centennial Fellow of The College of Earth and Mineral

Science, The Pennsylvania State University (1996), and honorary professor of Harbin Institute of Technology, Harbin,

China (1997). He chaired the US-Japan Meeting on Ferroelectric and Dielectric Materials (1995) and the 10th International Symposium on the Applications of Ferroelectrics

(1996). Dr. Safari is the author/co-author of over 190 papers and holds eleven patents. He is one of the inventors of

the Fused Deposition of Ceramics process. He has given over

250 lectures on his research at national and international

meeting and industries.

Mohsen Jafari received his Ph.D. in Industrial Engineering and an M.S. in Computer Science from Syracuse

University. He taught in Syracuse University from 1985-

1987, and from 1987 to the present he has been an Associate

Professor in the Department of Industrial Engineering at

Rutgers, The State University. He has been Principal or CoPrinicipal Investigator to over $3M research projects funded

by various government agencies. He has published over 25

journal articles and has given many invited and contributed

talks in international conferences. Seven Ph.D. and over ten

M.S. thesis have already been completed or are in progress

under his supervision.

Noshir A. Langrana, Ph.D. P.E. is a Professor in Mechanical and Aerospace Engineering, at Rutgers, The State

University of New Jersey. His principal fields of interest include: computer-aided design, solid modeling, virtual reality simulation, optimization and finite element analysis,

computer controlled compression molding manufacturing

process, rapid prototyping technology (SLA, SFF and FDC),

mechanical testing of composites, and mechanical behavior

in high temperature environment. He is a Fellow of ASME

and AIMBE. He is a recipient of Greatest Advances of 1996

(one of 26 major scientific advances in American Universities) recognition by Science Coalition Washington, DC 1997.

He is a joint inventor on five patents and has published extensively.

REFERENCES

1. Proceedings of the Solid Freeform Fabrication

Symposium, Vol. 1-8, Ed. Edited by J. J. Beamen, H.

L. Marcus, D L. Bourell, and J. W. Barlow, Univ. of

Texas at Austin, Austin, TX, 1990-1997.

2. M, B. Hsieh, “Laminated Object Manufacturing

(LOM): A simple Process”, 123-130, Proc. Solid

Freeform Fabrication Proceedings (Austin, TX,

August 1991). Edited by H. Marcus, J. J. Beamen, J.

W. Barlow, D L. Bourell, and R. H. Crawford,

University of Texas at Austin, Austin, TX, 1991.

3. Griffin, J. Daufenbach, S. McMillin, “Desktop

Manufacturing: LOM vs. Pressing,” Am. Ceram. Soc.

Bull., [73], 8, 1994.

4. Cawley, P. Wei, Z. E. Liu, W. S. Newman, B. B.

Mathewson, A. Heuer, “Alumina Ceramics Made by

CAM-LEM (Computer Aided Manufacturing of

Laminated Engineered Materials Technology”, 9-16,

Solid Freeform Fabrication Proceedings (Austin, TX,

August 1995). Edited by H. L. Marcus, J. J. Beamen,

D L. Bourell, J. W. Barlow, and R. H. Crawford,

University of Texas at Austin, Austin, TX, 1995.

5. M. K. Agarwala, A. Bandyopadhyay, R. van Weeren,

V. Jamalabad, P. Whalen, N. A. Langrana, A. Safari,,

S. C. Danforth, “Fused Deposition of Ceramics” J.

Rapid Prototyping, Vol. 2, 4, 4-19, (1996).

6. M. K. Agarwala, A. Bandyopadhyay, R. van Weeren,

V. Jamalabad, P. Whalen, N. A. Langrana, S. C.

Danforth and A. Safari “Fused Deposition of Ceramics: Rapid Fabrication of Structural Ceramic Components”, Bull. Am. Cer. Soc., 75, 11, 60-5, (1996).

7. S. Rangarajan, G. Qi, A. Bandyopadhyay, C. Dai, J.

W. Han, P. Bhargava, S. Wu, A. Safari, S. C.

Danforth, “The Role of Processing Variables in the

FDC Process,” Proceedings of Solid Freeform

Fabrication Symposium, University of Texas at

Austin, 431-40, 1997.

8. G. Qi, C. Dai, S. Rangarajan, S. Wu, S. Danforth,

and A. Safari, “Properties of RU955 Si3N4 Filament

for FDC”, Proceedings of Solid Freeform Fabrication

Symposium, University of Texas at Austin, Austin,

Texas, 421-29, 1997.

9. C. Dai, G. Qi, S. Rangarajan, N. Langrana, A. Safari,

and S. Danforth, “High Quality, Fully Dense Ceramic

Components Manufactured Using Fused Deposition

of Ceramics”, Proceedings of the Solid Freeform

Fabrication Symposium, The University of Texas at

Austin, Austin, TX, August 1997.

10. R. Clancy, V. Jamalabad, P. Whalen, ,. Bhargava, C

Dai, G. Qi, S. Rangarajan, S. Wu, S. Danforth,

Langrana, and A. Safari, “Fused Deposition of

Ceramics: Progress Towards a Robust and Controlled

Process for Commercialization”, Proceedings of

Solid Freeform Fabrication Symposium, University

of Texas at Austin, 185-193, 1997.

11. “Processing of Piezocomposites by Fused Deposition

Technique,” A. Bandyopadhyay, R. K. Panda, V. F.

Janas, M. K. Agarwala, S. C. Danforth and A. Safari,

J. Am. Cer. Soc., 80, 6, 1366-72, (1997).

12. S. C. Danforth and A. Safari “Solid Freeform

Fabrication: Novel Manufacturing Opportunities for

Electronic Ceramics”, Proceedings of 10th Interna-
38 Naval Research Reviews

tional Symposium on Applications of Ferroelectrics,

IEEE 96CH35948, 183-188 (1997).

13. R.E. Newnham, Q.C. Xu, “Metal-Elecroactive

Ceramic Composite Actuators,” U.S. Patent #

5,276,657, Jan. 4, (1994).

14. A. Dogan, J.F. Fernandez, K. Uchino, R.E.

Newnham, “ New Piezoelectric composite Actuator

Designs for Displacement Amplification,” Materials

for Adaptive Structural Acoustic Control (Penn StateAnnual Report), Vol. IV, Appendix 40, (1995).

15. K. Onitsuka, A. Dogan, J.F. Tressler, Q.C. Xu, S.

Yoshikawa and R.E. Newnham, “Metal-Ceramic

Composite Transducer, The “Moonie”,” Journal of

Intelligent Material Systems and Structures, Vol. 6,

pp. 447-455, (1995).

16. Ruan, S.C. Danforth, A. Safari, and T-W. Chou, “

Design and Analysis of Electromechanical Devices,”

submitted to Journal of the American Ceramic

Society, June 1997

17. Ruan, S.C. Danforth, A. Safari, and T-W. Chou, “

Analysis of the Electromechanical Coupling Effect in

Linear Piezoelectric Materials,” submitted to Journal

of Solids and Structures, November, 1997.

18. A. Bouhal, M. A. Jafari, W. B. Han, and T. Fang,

“Tracking Control and Trajectory Planning in

Layered Manufacturing Applications,” August 1997,

under review for IEEE Transactions in Industrial

Electronics.

19. S. I. Guceri, A. Yardimci, “Process Analysis of Fused

Deposition of Ceramics”, Final Technical Report

under DARPA/ONR Contract Number N00014-94-

C-0115, Manufacturing Processes Laboratory,

University of Illinois at Chicago, May, 1997.

20. “Part Quality Prediction Tools for Fused Deposition

Processing”, M. A. Yardimci, S. I. Guceri, S. C.

Danforth and M. Agarwala, Proceedings of the 7th

Solid Freeform Fabrication Symposium, Edited by H.

L. Marcus, J. J. Beamen, D. L. Bourell, J. W. Barlow

and R. H. Crawford, University of Texas at Austin,

539-548, 1996.
Three/1998 39

Abstract

Solid freeform fabrication of ceramics can be accomplished with stereolithography by using a stereolithography

apparatus (SLA) to build a ceramic-loaded polymer object

from an ultraviolet (UV) curable suspension of ceramic powders in an acrylate monomer. Objects are later sintered in a

separate furnace to remove the polymer and densify the ceramic. Fully dense ceramics with excellent microstructure

are achieved. Like conventionally fabricated ceramics, dimensional accuracy is limited by sintering shrinkage, which

is anisotropic but repeatable within about 0.7%. Higher refractive index materials, which scatter the UV radiation more

strongly, require longer build times. Examples are drawn

mostly from aluminum oxide, but are related to other ceramics.

This research was supported by the US Office of Naval Research under grant N00014-95-1-0527, through Drs.

S. Fishman and R. Wachter.

Introduction

Solid freeform fabrication (SFF) techniques provide

two significant benefits to the engineering design cycle. First,

layer-wise fabrication allows unique and otherwise cumbersome and uneconomical designs to be fabricated easily. Second, these designs can be fabricated directly from the

computer-aided design (CAD) or other data file, even at remote sites, reducing design time and other costs. The time

and cost savings using these processes in the design and

manufacturing cycles are evident in the growth of the rapid

prototype industry.

Fabrication of ceramics via solid freeform fabrication

methods has been an area of active research in the past several years. The ability to directly fabricate ceramic bodies

into complex shapes without the time and expense of tooling (which can take months to procure) would be a unique

and important advance in solid freeform fabrication techniques. Specifically, the investment casting design cycle is

Solid Freeform

Fabrication of

Ceramics via

Stereolithography

G. Allen Brady and J. W. Halloran, Department of Materials Science, University of Michigan, Ann Arbor, MI
40 Naval Research Reviews

usually longer and more costly than other manufacturing

methods. Direct fabrication of complex molds for investment casting items such as blades for turbine aircraft engines by ceramic SLA without tooling can lead to significant

cost savings. In addition, adapting stereolithography (SL) to

produce ceramics offers access to a large, experienced market as a potential user base. To this end, this work aims to

provide an overview of the advances made in directly producing ceramics in a commercially available

stereolithography apparatus (SLA 250/40, Systems, Valencia,

CA).

The stereolithography process is based upon

photopolymerization of a liquid monomer mixture which,

when irradiated with an ultraviolet laser, rapidly polymerizes to form a solid polymer gel. The first step is curing a

cross section by drawing the cross section on the surface of

the resin with the laser (the part is attached to a platform

beneath the surface of the resin by supports). Second, the

elevator platform dips into the liquid resin a finite distance

allowing the liquid resin to flow over the cured portion of

the part. A doctor blade sweeps over the part, leaving a layer

of fresh resin that becomes the next cured layer as the laser

draws the next cross section. Repeating these steps builds

up the three-dimensional part.

Formulating a photocurable ceramic suspension for use

in the SLA extends the stereolithography technique. The ceramic suspension consists of fine, alumina powder (Reynolds

RC-HC DBM, Malakoff Ind., Malakoff, TX; 0.5 μm mean

particle size) suspended in a mixture of acrylate monomers.

Addition of a dispersant allows a stable colloidal suspension

of at least 50 vol% alumina to be formulated. This suspension has a viscosity of about 200 mPa*s at a shear rate of

100 s-1. The low viscosity allows the resin to self-level during fabrication, keeping the top surface very smooth. The

suspension is rendered photocurable by addition of a

photoinitiator. As the laser traces the part in the resin, the

monomer polymerizes and forms a gel that becomes a polymer binder holding the ceramic particles together. After the

part is completed in the SLA, the green ceramic body is

slowly heated to 500°C to remove the polymerized binder

and then sintered to full density at 1580°C [1,2,3]. The binder

removal and sintering steps are traditional methods for powder-processed ceramics.

Fabrication of Ceramics

Several ceramic parts have been fabricated in the SLA

using the alumina resin. These parts are shown together in

Figure 1. They represent a wide range of complexity and

size and serve to illustrate how the SL process can create

various shapes in ceramics. These parts are: 1) an 8.5X life

size replica of a human trabecular bone (Cerbone), 2) a

uniquely designed 3D object displaying “UofM” on one face

and “ONR” on the opposite face (UM-ONR), 3) a block letter M (BlockM) and 4) a mechanical testing specimen

(Theta). The Theta part and Block M are quite simple, and

used to determine the accuracy of the process. Because of its

complex shape, the UM-ONR object cannot be produced by

conventional molding techniques. The trabecular bone model

is another excellent example of an object that could not otherwise be fabricated, as it is a complex random structure with

interconnected solid and void phases. Table 1 lists relevant

data for each part regarding size, build time and sintering

data.

The surface finish of the as-built and sintered parts is

excellent. As seen in a close-up view of the UM-ONR part

in Figure 2, the top surface has a shiny appearance and the

side edges have a scalloped appearance. The scalloping is

due to layer by layer build up. Close inspection of the center

rod in Figure 3 shows more evidence of the stair-stepping

that occurs as the cylinder is approximated by a series of

slabs of finite thickness.

On average, the parts exhibited sintering shrinkage of

16.5% in the build-plane and 21.8% in the build direction.

Figure 1

Collection of various ceramic parts fabricated via

Stereolithography. The larger copies are as-built and the

smaller copies are sintered.

Table 1

Data on various ceramic parts fabricated by Stereolithography.
Three/1998 41

The anisotropy reflects the layered processing route and is

similar to what is seen in tape-cast materials. The weight

loss is within +0.2% of the expected value according to the

suspension composition. Thermogravimetric data also confirms this result.

The alumina parts were sintered to about 95% theoretical density. Bonding between the layers was observed to

be excellent and no trace of layering could be found in fracture surfaces. However, imperfect laminations were localized near the edges emanating from the surface scalloping.

Another important property of the finished parts is the

smoothness of the upfacing surfaces. This smoothness is due

to the fact that the final surface is as smooth as the resin can

self level. Profilometry scans shown in Figure 4 illustrate

the difference in roughness between the surface and sides.

The calculated RMS roughness of the upfacing surface is

4.0 μm and the roughness of the scalloped side surfaces is

27.5 μm.

Scattering

As the laser scans across the surface of the resin, a

region of resin is polymerized. The cure depth of this region

is given by the equation [4]

C D E

E

d p

o

c

= 





 ln (1)

where resin parameters Dp and Ec are depth of penetration

and critical exposure dose respectively and Eo is the average

exposure dose supplied by the laser. This equation was developed assuming the Beer-Lambert behavior for absorption.

For typical stereolithography resins, attenuation of the UV

radiation intensity occurs almost entirely by absorption. In

the photocurable ceramic suspensions used in this work, scattering contributes significantly to the UV radiation intensity

attenuation in addition to absorption. It turns out equation 1

also describes the cure depth-exposure dose relationship for

photocurable ceramic suspensions [1,3]. Semi-log plots

showing the cure depth as a function of exposure dose for a

variety of suspensions with different depths of penetrations

are shown in Figure 5.

While the alumina particles are transparent to UV and

therefore do not increase absorption over that of the

photocurable liquid medium, they do serve as scattering centers. The effect of this scattering is reflected in the resin parameter Dp

 and is given by [5]

D d

Q

n

n p

o

= 







2

3

2 ˆ

 ̃ φ ∆ (2)

Figure 2

Close-up top view of as-built UM-ONR part. Note the smooth

upfacing surface and scalloped sides.

Figure 3

Close-up oblique side view of as-built UM-ONR part. Note

the stairstepping effect on the center rod.

Figure 4

Profilometer scan of side and upfacing surface of an as-built

part. Solid line: side scan. Dotted line: upfacing surface scan.
42 Naval Research Reviews

where ˆ

d is the average particle size, Q is the scattering efficiency, φ is the volume fraction of solids, n0

 is the index of

refraction of the liquid medium and ∆n = (n0-n1) where n1 is

the index of refraction of the ceramic particles. As a result,

the difference in index of refraction between the alumina

particles (n1=1.7) and the monomer mixture (n0=1.45 to 1.51)

strongly influences Dp, and consequently, the cure depth at a

given exposure dose. Figure 7 confirms the relationship by

showing the linear proportionality between Dp and 1/∆n 2

(data from Figure 6).

Build Time

Although the parts shown in Figure 1 are relatively

small, the build times are much higher than they would be

for typical SL parts. This is mostly due to the low Dp and

high Ec of the ceramic resin compared to traditional SL epoxy resin. Identification and quantification of the factors that

influence the build time for a given part are essential to reduce the build time. Combining equation 1 and other machine parameters [2], the time to build a given shape can be

approximated [4] as

t n

W E

Ph

V

z

C

D b

oc s d

p

≈ 



 

















 



 π2 2

2 exp (3)

where n is the number of layers in the part, W0

 is the beam

radius, P is the laser power, h is the hatch spacing, Vs is the

volume of the part and z is the height of the part in the build

direction. This equation clearly illustrates the influence of

part parameters (n, Vs

, z), machine parameters (W0, P), build

style parameters (Cd, h) and resin parameters (Dp, Ec) on the

time to build a given part.

This calculation was used to estimate the build times

for each part. The results are compared to a more rigorous

build time estimation program[5] that takes information from

actual prepared and sliced build files to simulate the build

time. The results shown in Figure 7 compare the calculated

and simulated build times. The agreement is quite good for

all parts, despite the variety of size and complexity. These

results indicate Equation 3 is accurate enough to estimate

the time to build a single part.

As an illustration, the build time for the Cerbone part

was calculated as a function of laser power. This data is shown

in Figure 8. The nominal laser power of 25-30 mW was used

to build the parts in this study. However, there are other, more

powerful UV lasers for use in stereolithography, which have

Figure 6

Plot of penetration depth for resins with different scattering

efficiencies (1/∆n2).

Figure 7

Comparison between actual, calculated (equation 3) and

simulated (Ref. 5) build times for various ceramic parts built

by SLA.

Figure 5

Plot of cure depth as a function of exposure dose for resins

with different scattering efficiencies (1/∆n2).
Three/1998 43

powers of up to and exceeding 200 mW. So, if a 200 mW

laser was used, the build time for the Cerbone part would be

reduced from 40 hours to about 10 hours.

Conclusions

Highly loaded ceramic suspensions have been developed to produce ceramic parts via stereolithography. A variety of alumina parts have been successfully fabricated

possessing excellent microstructures and very smooth surfaces. The ceramic particles in suspension serve to scatter

the UV radiation. The effects of scattering can be understood

by a modified Beer-Lambert law and are incorporated into

the resin parameters, Dp and Ec. Furthermore, the effects of

the different resin parameters on the build time for a part can

also be accurately quantified.

Biographies

G. Allen Brady is a research engineer at Allison Engine Company. He competed his PhD thesis on

stereolithography at the University of Michigan in 1998. He

holds a MS degree from Michigan in Materials Science and

a BS degree in Ceramic Engineering from the University of

of Missouri-Rolla.

John Halloran is a professor of Materials Science and

Engineering at the University of Michigan. He recieved a

BS degree from the Univerisity of Missouri-Rolla in Ceramic

Engineering in 1973, and a PhD in Materials Science from

M.I.T. in 1977. Before coming to Michigan in 1990, he was

with an entrepreneurial company (CPS Superconductors and

Ceramic Process Systems Corp.) in the Boston area. His previous faculty positions have been at Penn State (1976-1980)

and Case Western Reserve University (1980-1985). His current research focuses on ceramic processing, structural ceramics, free form fabrication and novel microfabrication

methods.

Figure 8

Build time for “Cerbone” part as a function of laser power.

Dotted line: equation 3. Solid line: Build Time Estimator

Simulation (Ref.5).

REFERENCES

1. Michelle L. Griffith and J. W. Halloran, “Scattering

of Ultraviolet Radiation in Turbid Ceramic Suspensions”, J. Applied Physics, 81 2538-2546 (1997).

2. G. Allen Brady, Ph.D. Dissertation, “Solid Freeform

Fabrication of Ceramics via Stereolithography”,

University of Michigan, in progress.

3. G. Allen Brady and J. W. Halloran,

“Stereolithography of Ceramics”, Rapid Prototyping

Journal, 3 2 61-65 (1997)

4. Paul F. Jacobs, Rapid Prototyping & ManufacturingFundamentals of Stereolithography, 1st ed., Society

of Manufacturing Engineers, Dearborn, MI (1992).

5. Joel McClurkin, RPMI Build Time Estimator, v0.1,

October 1996, http://risky.marc.gatech.edu/joelm/

joel_home.html.
44 Naval Research Reviews

Abstract

This paper describes a new multilayer solid freeform

fabrication process (“Direct Photo Shaping”) where visible

digital light projection is used as a maskless tool to build

images on photocurable ceramic dispersions (ceramic powders in photopolymerizable liquid monomers) by flood exposure. For each layer the projected image is changed

according to the CAD data describing the object being built

and solidification takes place by photocuring of the exposed

areas. Multiple layers are dispensed and photocured to fabricate the object of interest. A final rinse with a suitable solvent allows the removal of any uncured ceramic dispersion.

The porous free formed “green” ceramic object can then be

fired and sintered into a highly dense ceramic part. This

ceramic forming process is based on visible-light photo

gelcasting. The photocurable ceramic dispersion is prepared

from a mixture of ceramic powder and organic monomers

containing a visible light photoinitiator. Upon photoexposure

the monomers solidify into a polymer thus forming a gelcast

green body. Digital Light ProcessingTM technology (developed by Texas Instruments) enables us to project digital, high

resolution, high brightness, high contrast visible light to

photocure and form components with a good degree of accuracy. In this paper Direct Photo Shaping is applied to the

fabrication of ceramic gas turbine components for military

and commercial applications. Silicon nitride specimens with

flexural strengths in excess of 1 GPa were fabricated by Direct Photo Shaping. A first-stage silicon nitride turbine vane

for the Allison Model 501K industrial gas turbine was fabricated by Direct Photo Shaping and tested in a gas-burner

test rig at 1204°C.

Introduction

The main advantage of desktop manufacturing is the

ability to move directly from the design stage on the workstation screen to a physical model, without having to wait

Direct Photo Shaping

of Ceramic

Components

Susanna Ventura and Subhash Narang, SRI International, Menlo Park, California
Three/1998 45

for a machine shop to set up a machine tool to custom-build

the model; thus, this approach gives unprecedented power

and flexibility to system level designers. By allowing the

system level designer to produce an instant functional prototype, desktop manufacturing promises to slash the design

cycle time for many applications, thus helping to bring a

product from design to production to marketplace faster and

more cheaply. In addition, the ability to generate models

easily will enable designers to experiment, trying out a variety of design options and comparing them side by side.

Rapid prototyping was initially applied to the fabrication of nonstructural materials by means of computer-aided

design/computer-aided manufacturing (CAD/CAM) technology, wherein computer files descriptive of the object were

used to create parts made from materials such as UV-curable

polymers. The components fabricated in this fashion were

considered nonfunctional, and their main application was for

iterative design evaluation. During the past few years there

has been an increasing demand to extend rapid prototyping

technology to the fabrication of functional components with

engineering properties and dimensional tolerances comparable to those of conventionally produced components. Today the promise is that advanced solid freeform fabrication

(SFF) manufacturing technology will allow the fabrication

of functional prototypes from advanced ceramic, metallic,

and multiphase materials for structural and electronic applications. The SFF approach to net shape forming holds great

promise for rapid prototyping of ceramic components through

simplification of the processing cycle, including the elimination of the time consuming steps of pattern making and

mold fabrication. SFF of ceramics will allow fast turnaround

design validation of new advanced ceramic components for

gas turbine engines. In comparison with conventional spark

ignition engines, gas turbine engines offer greater fuel efficiency, lower particulate levels, and multiple fuel capability.

Turbine engine technology is critical not only to maintaining US military superiority but also to continuing commercial prominence in the aviation, marine and industrial sectors.

Selective laser sintering [1], 3-D ink jet printing [2],

laser stereolithography [3], fused deposition [4] and layer

object manufacturing [5] are examples of rapid prototyping

processes applied so far to the fabrication of ceramic components.

Direct Photo Shaping

At SRI we have developed a new multilayer fabrication process called Direct Photo Shaping [6]. The process is

based on the layer-by-layer photocuring of polymerizable

compositions curable by visible light. Each layer is selectively photoimaged by digital light projection via a Digital

Micromirror Device (DMD) array that performs the function of an electronic maskless tool. While Direct Photo Shaping promises to be generally applied to the fabrication of

polymer, ceramic or metal components, the focus of this paper

is to describe its application to ceramics.

We refer to the Direct Photo Shaping ceramic forming

process as photogelcasting, since the “green body”1 is formed

by photocuring of a slurry of the ceramic powder in a solution of photopolymerizable monomers. Photogelcasting

closely relates to the gelcasting process [7] which is used for

forming ceramics from molds and has been shown to produce complex-shaped, near-net-shape parts with high reliability. While in gelcasting a thermal initiator is used to

promote polymerization upon heating, in photogelcasting the

polymerizable ceramic slurries are cured by exposure to radiation in the presence of a photoinitiator. As in the

gelcasting process, the formed green bodies are generally

strong and machinable, and after sintering, highly dense ceramic parts are obtained. The potential advantages and enhancements offered by Direct Photo Shaping, relative to other

rapid prototyping processes, are described in the next section.

Process Description

The Direct Photo Shaping process is described in Figure 1. CAD slice images are projected on the photocurable

composition which is dispensed and leveled on a build platform. Each layer is photoimaged by digital light projection

through a digital micromirror array which modulates the

image with a switching time of less than 1 msec. After each

exposure, a new layer of photopolymerizable dispersion is

applied on the build platform. When the fabrication is complete, the formed ceramic green part is removed from the

platform and rinsed with a suitable solvent to dislodge any

Figure 1

Schematic representation of the Direct Photo Shaping

process.

1

Green body refers to the formed porous ceramic part before densification.
46 Naval Research Reviews

uncured material. The final ceramic part is obtained after

binder removal and sintering.

The machine operates in a three-step cycle described

as follows (see Fig.1).

Step 1: Apply Slurry. The ceramic slurry is dispensed

on the build table by means of a peristaltic pump and uniformly applied as a thin layer by a doctor blade. Typically, a

layer thickness of 2 mil with a control of +/- 0.5 mil is used.

Thicker layers may be fabricated and the layer thickness can

be adaptively controlled.

Step 2: Photoexpose. The layer is shaped into the

desired cross section profile by digital light projection of

visible light (the light source is a 270 watt metal halide lamp).

The exposure time, which is material dependent, is typically

15 seconds or lower.

Step 3: Lower Platform. The photoexposed portion

of the layer is cured into a solid film. The platform is then

moved to a lower position and a new layer fabrication cycle

starts.

The size of the working area is determined by the light

projection settings, thus the light can be projected on smaller

or larger areas as desired. To optimize resolution, light projection on a small area is desirable. Our current system typically operates on a projected area of 6 x 8 inches down to

2 x 3 inches.

The process makes use of compositions photocurable

by visible light. This allows us to achieve good depth of

cure especially for highly filled compositions such as ceramic slurries [8]. Other advantages of Direct Photo Shaping include:

• Fast build time because each layer is shaped by flood

exposure, thus curing the entire profile at once (unlike the

case with a scanning laser); typically for making silicon nitride components, each layer is dispensed and imaged in less

than 45 seconds. On the other hand the dispensing and imaging of other ceramics, such as alumina, may take as low as

ten seconds.

• Minimum number of steps (no postprocessing after the fabrication of each layer is needed).

• Low cost.

• High resolution (each pixel is a 16-μm-square mirror and DMD arrays with resolution of 1280x1024 and a fill

factor of about 90% are available [9]).

Moreover, the use of digital light projection to

photocure the ceramic layer offers additional capabilities not

available with the traditional laser scanning systems. The

DMD-array can be used to project gray images, and the intensity of the gray can be modulated to control light transmission. We expect that the control of the gray scale (i.e,

controlled radiation dose) may be used to improve accuracy

and surface finish, for example by generating curved layers.

Positional control of the photocuring rate allows one to

Figure 2

(a) DMD optical switching principle. (b) Operating DMD arrays.

(a) (b)

16μm
Three/1998 47

change physical properties in a positional manner and to

minimize stresses developed during curing, as well as to

generate a robust self-support structure by partial cure of the

slurry surrounding the part being built.

DMD Array - A Maskless

Imaging Tool

Digital Light Processing by means of digital

micromirror devices is a new light projection technology

developed by Texas Instruments [9]. Digital micromirror

arrays are constituted by digital light switches, aluminum

mirrors, 16 μm square, that precisely control and modulate

light. The DMD array is interfaced with a suitable light

source and optics, and each pixel mirror is electronically

controlled to reflect the incident light in or out of the projection area. Figures 2a and 2b illustrate DMD arrays and their

optical switching mode. Because DMD is a reflective digital light switch, its optical efficiency - 62% - is more than

ten times higher than that of a liquid crystal display light

transmitting pixel (about 5%). This makes DMD arrays a

superior tool for maskless imaging of photocurable compositions. Previous attempts of using a LCD programmable

mask in combination with UV light to photocure polymers

were not successful because of the poor stability of liquid

crystal polymers to UV radiation [10].

Photocurable Composition

We have developed ceramic photocurable compositions that are photoactive in the visible region. This allows

us to readily interface the chemistry with the DMD technology and to obtain better depth of curing for the ceramic filled

compositions.

Our photocurable compositions contain photoactive

monomers, a suitable photoinitiator, a dispersant and the

ceramic powder. The ceramic content typically varies from

45 % to 55% on a volume basis. Ceramic slurries are prepared by ball-milling the silicon nitride powder in the

photocurable monomers, after addition of a suitable dispersant, a solvent/plasticizer and the photoinitiator. Ceramic slurries with viscosity of 10,000 cps or less are generally used.

The ceramic slurry composition is optimized to improve interlayer adhesion and eliminate any possible delamination during debinderization (see below). This is achieved

by reducing the layer stiffness and by using additives to produce a tacky top layer.

Fabrication and Testing

of Silicon Nitride

Specimens

Silicon nitride ceramics are of great commercial and

military interest because of their excellent mechanical properties, good oxidation resistance and thermal shock behavior both at room and high temperatures.

Silicon nitride specimens of a simple tile shape

(3” x 3”x 0.25”) were formed from NCX-51022 silicon nitride according to three different methods to compare their

mechanical properties and to validate the Direct Photo Shaping process. Tile specimens were prepared by (1) Direct Photo

Shaping, (2) gel casting in a mold, and (3) powder dry pressing.

2

 NCX-5102 refers to a high performance silicon nitride powder processed by Saint-Gobain Industrial Ceramics (Northboro, MA)

Figure 3

Representative microstructures of silicon nitride samples fabricated by (a) Direct Photo Shaping, and (2) dry powder uniaxial

pressing.

(a) (b)
48 Naval Research Reviews

After binder removal, the samples were initially

presintered to improve their handling strength and then densified by HIPing using glass encapsulation. After HIPing

the samples were >98% dense, as shown in Table 1, and no

evidence of the original layers was observed by SEM examination of cross-sectional surfaces of the SFF tiles.

Samples for microstructural evaluation were prepared by

polishing fracture surfaces and then plasma etching using a

95% CF4

/5% O2 carrier gas [11]. This etching technique

removes the Si3N4 and effectively marks the Si3N4 grain

boundaries. Figure 3 shows representative etched surfaces

for a sample prepared by the SFF method and for a tile formed

by dry pressing. Both samples show a pronounced bimodal

distribution of grain sizes and as is typical of in situ reinforced Si3

N4, show a pronounced acicular microstructure.

Table 1 summarizes the mechanical property data for

tiles formed according to the three different methods. Specimens for flexure testing were machined either to 1.5 mm x

2 mm x 25 mm or 3 mm x 4 mm x 50 mm size. Flexure

strength was measured by four-point bending in accordance

with ASTM method C1161 [12]. Hardness was measured

with a Vickers diamond indenter at a load of 0.5 kg [12].

Fracture toughness was measured by both the indentation

method and the controlled flaw method [12]. Sintered densities were measured in deionized water using the Archimedes

method [12].

The mold gelcast samples showed lower densities and

strengths than the dry pressed and layered samples; this was

attributed to slight differences in the HIPing cycles which

were used for these samples. The sample formed by Direct

Photo Shaping showed similar properties to the dry pressed

samples with modulus of rupture (MOR) values slightly in

excess of 1 GPa. The flexure strengths for the layered tiles

were not dependent on the orientation relative to the casting

direction. Fracture toughness measured by Vickers indentation was 4.0 MPa m1/2 for the dry pressed samples and

4.5 MPa m1/2 for the solid freeform sample. Fracture toughness as measured by controlled indentation was slightly

higher, as is typically observed, and was comparable to values previously reported on optimized slip cast material of

the same composition [7].

Table 1

Summary of Mechanical Property Data for Tiles Formed by Uniaxial Pressing, Thermal Gelation and Direct Photo Shaping

Notes:

1. Specimens e and f were cut from the same sample. Specimens e were cut so that the casting planes were parallel to specimen

faces; specimens f were cut so that the casting planes were perpendicular to the specimen faces. In both cases the lamination

direction was parallel to the long dimension of the specimen.

2. Some sample properties were not determined because of insufficient number of specimens.
Three/1998 49

Fabrication of Complex

Shape Silicon Nitride

Components

A first-stage silicon nitride turbine vane for the Allison

Model 501K industrial gas turbine was fabricated by Direct

Photo Shaping. The component, which is approximately

40 mm high with a chord length of 34 mm, was chosen to

demonstrate the feasibility of the Direct Photo Shaping process to build complex geometry components. Three vanes

were built at the same time on our build platform. The vanes

were fabricated from 523 layers, each 50 μm thick. This

layer thickness was found optimal to ensure good interlayer

adhesion and to generate smooth curved layers. The total

time required for the fabrication of the three green vanes

was about seven hours, thus just over two hours per vane.

After binder removal, the silicon nitride vanes were

densified to more than 98% theoretical density by HIPing.

The vanes were approximately 20% linearly undersized due

to the shrinkage during the densification process. Figure 4

illustrates a green silicon nitride turbine vane sample next to

a sintered one.

One silicon nitride vane was evaluated in a gas-fired

burner test rig and subjected to a total of 20 thermal cycles at

an average gas temperature of 1204°C. Nondestructive inspection of the component after testing was performed by

fluorescent penetrant inspection and real-time microfocus

X-radiography. As indicated in Figure 5, no cracks or defects were found on the part after the test.

Conclusions and

Outlook

We have presented a new multilayer solid freeform

fabrication process, Direct Photo Shaping, where visible digital light projection is used as a maskless tool to photocure

cross sections from CAD data. We have described the application of this process to the fabrication of silicon nitride ceramic components. Tile specimens (3”x3”x0.25”) were found

to be more than 98% dense and to have flexure strengths

slightly in excess of 1 GPa and comparable mechanical properties to those of tiles formed by conventional powder processing techniques. Silicon nitride turbine vanes were

fabricated and validated for their mechanical properties by

subjecting them to repeated thermal cycling at 1204°C. While

Direct Photo Shaping has been here described for the fabrication of silicon nitride components, the process has been

shown to be feasible for the fabrication of other ceramic (e.g,

alumina), polymer and metal parts.

Direct Photo Shaping is a low cost solid freeform fabrication process based on inexpensive visible light projection through a digital micromirror array used as a maskless

tool. Because the photocuring takes place by flood exposure, the build time for each layer is fast, a minimum number of steps are required and good resolution is achievable.

Direct Photo Shaping is a flexible fabrication tool

which allows us to dispense materials with positional and

compositional control and to fabricate functionally gradient

materials, microcellular materials, and in general materials

with controlled microstructure. The added feature of projecting gray scale light by Direct Photo Shaping further allows us to control the radiation dose in a positional fashion

for improved accuracy and surface finish. We are currently

applying Direct Photo Shaping to the fabrication of meso

and micro mechanical and electromechanical devices containing multiple ceramic/polymer/metal interfaces.

Figure 5

Microfocus x-ray photographs of silicon nitride vane after

testing at 1204°C.

Figure 4

“Green” (left) and sintered (right) silicon nitride turbine vanes.
50 Naval Research Reviews

Acknowledgments

This work was supported by the Defense Advanced

Research Projects Agency (DARPA) and the Office of Naval Research (ONR). We thank Dr. William Coblenz and

Dr. Steven Fishman for their continuous support and valuable suggestions. We thank our colleagues and collaborators for their essential contributions to this work: John Stotts

and Philippe Guerit at SRI for the fabrication of ceramic

green parts, Anne Hardy and Mike Mangaudis at Saint

Gobain Industrial Ceramics for supplying the silicon nitride

powder and for performing the post processing steps, and

Lance Groseclose at Allison Engine Company for the design and testing of the silicon nitride turbine vane.

Biographies

Dr. Susanna C. Ventura received her Doctorate in

Chemistry from the University of Padova, Italy, in 1981.

From 1982 to 1984, she worked as postdoctoral fellow at

Politecnico in Milan where she studied synthetic applications of free radical reaction. In 1986 she joined the Polymer and Technology Department in SRI International where

she is currently responsible for the solid freeform manufacturing program and polymer development.

Dr. Subhash C. Narang received his M.S. in chemistry

in 1970 from Panjab University, India, and his Ph.D. in chemistry in 1975 from Flinders University, Australia. From 1975

to 1976 he was postdoctoral fellow with Professor Sir Derek

Barton at Imperial College in London. From 1977 to 1981

he was postdoctoral fellow with Professor George Olah at

University of Southern California. Before joining SRI in

1985, he was assistant professor at Polytechnic Institute of

New York, Brooklyn. Present research interests include

polymer synthesis, battery technology, rapid prototyping and

material science.

REFERENCES

1. J.V. Tompkins, B.R. Birminghan, and H.L. Marcus,

“Solid Freeform Fabrication Using Gas precursors”;

Ceramic Transactions, proceedings of the First

International Symposium on Advanced Synthesis and

Processing, Cocoa Beach, FL, January 1995.

2. J. Grau, J. Moon, S. Uhland, M. Cima, and E. Sachs,

“High Green Density Ceramic Components Fabricated by the Slurry-Based 3DP Process”; Proceedings

of the Solid Freeform Fabrication Symposium

(Austin, TX, August, 1997), pp.371-8.

3. M.L. Griffith and J.W. Halloran, “Freeform Fabrication of Ceramics via Stereolithography”; J. Am.

Ceram. Soc., 79(10), 2601-8 (1996); T. Himmer, T.

Nakagawa and H. Naguchi, “Stereolithography of

Ceramics”, pp. 363-370.

4. R. Clancy, V. Jamalabad, P. Whalen, P. Bhargava, C.

Dai, R. Rangarajan, W. Wu, S. Danforth, N.

Langrana and A. Safari, “Fused Deposition of

Ceramics: Progress Towards a Robust and Controlled

Process for Commercialization”; Proceedings of the

Solid Freeform Fabrication Symposium (Austin, TX,

August, 1997), Proceedings of the Solid Freeform

Fabrication Symposium (Austin, TX, August, 1997),

pp. 185-194.

5. D. Klosterman, R. Chartoff, N. Osborne, G. Groves,

A. Lightman and G. Han, “Laminated Object

Manufacturing of Advanced Ceramics and Composites”; Proceeding of the Seventh International

Conference on Rapid Prototyping, San Francisco,

CA, April 1997, pp.43-50.

6. S. Ventura, S. Narang, S. Sharma, J. Stotts, D.

Annavajula, L.Ho, S. Lombardo, A. Hardy, M.

Mangaudis, and L. Groseclose “Direct Photo

Shaping: A new SFF Process for Ceramic Components”; Proceeding of the Seventh International

Conference on Rapid Prototyping, San Francisco,

CA, April 1997, pp.271-8.

7. O.O. Omatete, M.A. Janney, and R.A. Strehlow, “Gel

casting - A New Ceramic Forming Process”, Ceram.

Bull. 70(10), 1991.

8. G.A. Senich and R.E. Florin, J. Macro. Sci., Rev.

Macromol. Chem. Phys., C24(2), pp. 239-324

(1984).

9. L.J. Hornbeck, “Digital Light Processing for High

Brightness, High-Resolution Applications”, Presented at “Electronic Imaging, EI ‘97”, 10-12

February 1997, San Jose’, California; L.J. Hornbeck,

“Digital Light Processing and MEMS: Timely

Convergence for a Bright Future”; Presented at

Micromachining and Microfabrication ‘95, 23-24

October 1995, Austin, Texas.

10. M. Burns, “Automated Fabrication: Improving

Productivity in Manufacturing”; Prentice Hall,

Englewood Cliffs, NJ, 1993.

11. K. N. Seibein and W. M. Lovington, “Plasma

Etching of Si3

N4”; pp. 319-329 in “Microstructural

Science”, vol. 16. Edited by H.J. Cialoni, M.E.

Blum, G.W.E. Johnson, and G.F. VanderVoort, ASM

International, Metals`Park, OH, 1985.

12. “Ceramics and Glasses”, Vol. 4, Engineered Materials Handbook.
Three/1998 51

Abstract

Structural ceramic materials are finding growing use

in a wide range of applications, particularly advanced power

generation and propulsion systems. Yet, applications of ceramics are being limited by the very properties that make

them desirable, high temperature stability and high hardness.

Conventional forming techniques are severely tested by

materials that do not melt, are harder than tool bits, and are

not ductile. Even techniques specifically devised for forming ceramics, such as hot isostatic pressing, have strong limitations. Gas phase Solid Freeform Fabrication (SFF)

technologies offer the potential to form complex shapes from

ceramics and other materials without the restrictions of conventional forming techniques.

Two similar yet distinct gas phase SFF techniques are

being investigated as well as a related spinoff technology

that complements both SFF and conventional forming techniques. The techniques are Selective Area Laser Deposition

(SALD), SALD-Vapor Infiltration (SALDVI), and SALD

Joining. The common thread linking the techniques is the

use of a laser to create a localized reaction zone which decomposes a reactant gas to deposit material in a spatially

controlled manner.

Development and control of gas phase SFF techniques,

including shape, composition, and material properties, requires an understanding of the governing physical processes.

This paper begins with a description of the underlying physical processes of thermodynamics, kinetics, and heat / mass

transport and then shows the experimental effects of these

processes. The goals and continuing development of gas

phase SFF technology are discussed in light of the governing physical process and experimental results.

Introduction

The advantages of structural ceramic materials in high

temperature, high wear, corroding environments such as gas

Gas Phase Solid

Freeform Fabrication

and Joining of

Ceramics

K.J. Jakubenas, J.E. Crocker, S. Harrison, L. Sun, L. Shaw, H.L. Marcus, Institute of Materials Science, University of

Connecticut, Storrs, CT
52 Naval Research Reviews

turbines and jet engines in warships and aircraft have been

widely reported1-2. Application of the high temperature stability and high hardness properties of ceramics have often

been limited by the difficulty these properties present in forming operations. Gas phase Solid Free-Form Fabrication (SFF)

techniques are being developed as alternative shape forming

technologies for ceramics3-10. These approaches are also relevant to metals.

SFF techniques in general view complex three-dimensional shapes as being built of thin, two-dimensional layers.

The shape forming strategy of SFF is to add a thin layer of

defined contour to previously formed layers of the same or

different contour, thus building up a fully three-dimensional

shape. A wide range of techniques has been developed following this method. Several of these techniques are proving

commercially successful, especially for complex shapes made

from polymers. Significant work has also been done by many

groups on SFF of metals and ceramics11. Most of the approaches to ceramics have been targeted at making “green”

shapes, that is parts with high porosity that are densified in

post-SFF processing by firing and/or pressing as is done in

conventional ceramic forming. Unfortunately, such postprocessing may void many of the advantages of SFF because

of problems with shape retention due to non-uniform shrinkage during densification.

Gas phase SFF techniques seek to avoid most, if not

all, of the post processing steps currently necessary for forming ceramics. Gas phase SFF approaches differ from most

other techniques in that layers are formed as a result of a

chemical reaction rather than a phase change such as solidification. The governing processes involved are therefore

different, creating significant flexibility. For example, typical post processing for silicon carbide requires firing temperatures of ~1500o

C for several hours, while deposition of

bulk amounts of dense silicon carbide from a decomposition

reaction is possible at ~1000oC. Thus gas phase approaches

offer unique routes to fabricate difficult to form materials.

Three gas phase SFF techniques are described in this

paper: Selective Area Laser Deposition (SALD), SALDVapor Infiltration (SALDVI), and SALD Joining. In all three

techniques, material is deposited in a localized, laser heated

reaction zone in which decomposition of a reactant gas occurs. Figure 1 explains each of the three techniques. Initial

efforts were carried out at the University of Texas at Austin3-6. More recent research has been done at the University

of Connecticut.

Physical and Chemical

Factors in SALD

SALD techniques are controlled by a number of physical and chemical factors that are heavily interrelated. In order to understand SALD, not only must each contributing

factor be understood by itself, but the connections among

the factors must also be understood. The properties of SALD

deposits and the associated determining factors are listed in

Table 1.

For the purposes of discussion in this paper, the factors have been grouped into three areas: heat transport, mass

Figure 1

SALD, SALDVI, and SALD Joining. a) SALD - A scanning

laser beam creates a localized reaction zone on a substrate.

Within the reaction zone, a reactant gas decomposes to

produce a solid deposit. The part is built up by repeated

scanning of the beam in a controlled pattern. b) SALDVI -

Similar to SALD except that the substrate is a porous powder

bed into which material is deposited to create a dense layer.

The part is built up by spreading a fresh layer of powder and

repeating the scanning. c) SALD Joining - Similar to SALD

except that the substrates are the two parts to be joined. The

material deposited from decomposition of the reactant gas

serves as the filler material.

(a)

(b)

(c)
Three/1998 53

transport, and chemical reactions. This grouping scheme is

far from ideal because of the high level of interconnectedness

among groups, but the scheme does provide an overview of

the salient factors. The groupings are also useful in evaluating the effects of changes in the controllable process parameters as summarized in Table 2. Each of the groups is

described briefly in the following sub-sections.

Heat Transport in SALD

The effects of the temperature distribution caused by

a laser beam directed incident to a substrate are key to understanding SALD. The temperature distribution underlies

the occurrence and extent of chemical reactions and controls convection in the reactant gases. As a result, the composition, morphology, microstructure, and deposition rate of

SALD deposits are directly linked to heat transport.

In many conventional thermal processing operations,

the part being manufactured is isothermal for time periods

of hours. In SALD, temperatures can vary from more than

1500oC to room temperature over fractions of a millimeter

and change over that range in seconds. This variation in

temperature is inherent and essential to the process for spatial selectivity. Therefore it is clear that an understanding of

the determining factors of the temperature distribution is

necessary. Existing models of laser induced temperature

distributions are being enhanced using a finite element approach for use in SALD process planning, incorporating substrate and reactant gas properties.

Mass Transport in SALD

Mass transport in SALD involves the motion of reactant gas molecules into the localized reaction zone created

by the laser and the motion out of that zone of by-product

gases. Diffusion in and out of a small localized reaction

zone has been modeled using a range of techniques12. All of

the techniques emphasize the difference between one-dimensional mass transport seen in conventional wide area chemical vapor deposition (CVD) and the three-dimensional mass

transport occurring in highly localized processes such as

SALD. In conventional CVD, reactions occur throughout

an entire plane of deposition. This means that mass transport that is uniform over the plane can only occur perpendicular to the plane and thus in only one dimension. SALD

reactions occur only in a small, localized area, so mass transport can occur over a fully 3-D hemisphere.

The three-dimensional effect occurs when the reaction

zone size is of the same order as the characteristic diffusion

distance of the reactants. A 1000-fold increase in deposition

rates is seen for localized processes such as SALD in comparison to conventional chemical vapor deposition. This

phenomenon is attributed to the difference between the 3-D

mass transport in localized deposition and the 1-D, widearea mass transport of conventional CVD.

Chemical Reactions in SALD

Chemical reactions in SALD are viewed in two terms,

reaction kinetics and thermodynamics. Kinetics describe the

rate at which various reactions occur to deposit material from

the reactant gas. Kinetic calculations are applicable over a

wide range of SALD conditions. Unfortunately, reliable kiTable 1

Relationship between SALD Properties and Physical and

Chemical Processing Factors

Table 2

Externally Controllable SALD Parameters and the Associated

Affected Physical and Chemical Factors
54 Naval Research Reviews

netic data is not widely available, especially for the reactant

gases used in SALD. Kinetic data can also be difficult to

obtain experimentally. For that reason, thermodynamic equilibrium calculations are often used in modeling SALD reactions. Thermodynamic data is much more widely available

than kinetic data, but thermodynamic calculations are limited by an equilibrium assumption and do not convey deposition rates. Equilibrium may be reached in all, part, or none

of the SALD reaction zone depending on the temperature

distribution. Therefore, thermodynamic equilibrium calculations can only provide a limit on the potential reactions

occurring.

Equilibrium calculations for SALD are carried out to

create “deposition maps” which show potential solid phases

as a function of the externally controllable parameters of temperature and gas mixture. Figure 2 shows an example deposition map. The deposition maps are useful in planning

strategies for depositing various materials such as pure materials and composites. Their usefulness has been verified

in reducing excess carbon in silicon carbide deposits by the

addition of hydrogen and in varying the fraction of silicon

nitride in silicon nitride-silicon carbide composites.

Experimental Results

SALD Results

Deposition by SALD has been demonstrated for a number of oxide and non-oxide ceramics as shown in Table 3.

Both pure compounds and composites have been formed.

SALD processes are not limited to the compounds listed in

Table 3. Other SALD researchers have reported deposition

of additional ceramics and metals13-15. Additionally, approximately 80 elements and compounds spanning the periodic

table have been identified as potential SALD/SALDVI materials16-17. The governing factors of SALD discussed above

are applicable to all of these compounds.

One of the most interesting capabilities of SALD is

the ability to mix reactant gases to modify the material deposited. This is especially useful in forming composites. In

order to understand composite deposition from gas mixtures,

the interrelationships outlined in Tables 1 and 2 must be understood. One example of this is the microstructure that develops from SALD of a mixture of TiCl4

, SiCl4, and O2

reactant gases. The deposit is a layered mixture of silicon

oxide and titanium oxide as depicted schematically in Figure 3. The layers are the result of both the temperature distribution created by a scanning laser beam and the chemical

reaction occurring over a range of temperatures. The leading and trailing edges of the beam create a temperature sufficiently high to form silicon oxide but insufficient to deposit

titanium oxide. At the center of the beam, the temperature is

sufficient to deposit both oxides resulting in a composite layer

sandwiched between the silicon oxide layers. A three-layered microstructure is therefore created by a single pass of

the laser beam18. The layer thickness can be controlled by

varying the beam scan speed, reactant gas ratios, and laser

Figure 2

Deposition Map for Tetramethylsilane (TMS) (Si(CH3

)4) -

Ammonia (NH3) System showing the regions for which various

deposits are expected. Calculated using CET8912.

Table 3

SALD Materials

Figure 3

Multi-layered titanium oxide - silicon oxide composite resulting

from a mixture of reactant gases and the temperature

distribution caused by a scanning laser beam.
Three/1998 55

power. Although the titanium oxide-silicon oxide system is

of limited engineering interest, the basic principle is applicable to any SALD mixed gas system.

SALDVI Results

As with SALD, SALDVI has been demonstrated with

a number of materials systems as shown in Table 4. As with

SALD, the listed systems in no way exhaust the potential

materials list. The same range of compounds possible for

SALD are also applicable to SALDVI.

The greatest amount of work has been done on the

silicon carbide - silicon carbide system. A number of shapes

have been made in this system including those shown in Figure 4. Much effort has gone into optimizing the microstructure of the deposit with particular attention to achieving high

density. A cut and polished cross-section showing the level

of infiltration so far achieved is shown in Figure 4b. The

density has been markedly improved recently through a

greater understanding of the role of reactant gas mixture and

pressure and the temperature distribution used. For example,

the deposition must be carried out within a temperature range

that is sufficiently high to decompose the reactant gas and

deposit material throughout a powder layer depth of

~100-250 μm, but low enough that excessively rapid deposition does not create a sealed surface, thus preventing infiltration. A different effect is seen with varying pressure of

the reactant gas; at reactant gas pressures less than 5 torr,

infiltration is hampered by a lack of deposited material while

at pressures greater than 50 torr, temperature gradient driven

gas motion is sufficient to macroscopically disrupt the powder bed19. The interrelated roles of heat/mass transport and

chemical reactions have been shown to drive the final results of SALDVI.

 SALD Joining Results

Since SALD Joining relies on SALD for the material

to form the joint, the same materials possibilities are available. This is particularly advantageous for matching joint

material to the objects to be joined to create a monolithic

joints. Gradient joints are possible to connect two objects of

different materials, thus minimizing problems such as thermal expansion mismatch20.

Analysis of SALD joining experiments with silicon

carbide objects joined using silicon carbide as the filler have

shown that high deposit density and good deposit-to-substrate bonding has been achieved. Figure 5 shows the high

density of a deposit and the high quality of bonding possible. Further microstructural control is necessary, however,

because excess carbon present in the joints has led to unsatisfactorily low joint strengths. When robust joints can be

made repeatedly, SALD joining will promote the utilization

of structural ceramics in all settings. For instance, if a ceramic component fails on a ship at sea, the repair process

will be as straightforward as a typical welding procedure on

a metal structure.

Application of SALD

Techniques - Embedded

Devices

Several applications for SALD techniques are under

development. SALD, SALDVI, and SALD Joining have

great potential for forming shapes from ceramic materials,

(a)

(b)

Figure 4

Silicon Carbide SALDVI. a) SALDVI Silicon Carbide “ONR”

Letters. b) Microstructure of SALDVI Silicon Carbide

Table 4

SALDVI Materials
56 Naval Research Reviews

but the most exciting applications are the ones that take advantage of the flexibility of SALD techniques in depositing

materials with microstructural control.

The in-situ fabrication of embedded sensors is an example of how SALD processes can be combined to form

complex, functional structures. The process begins with

SALDVI of a portion of the part composed of several layers.

At the desired location for the embedded sensor, a thin, electrically insulating layer is deposited by SALD. The sensor,

for example a thermocouple, is then deposited by SALD followed by another SALD insulating layer. The rest of the

part is then completed by SALDVI. All of this is done in

one processing chamber without need to remove the part

between steps. Figure 6 shows a schematic of the process, a

silicon carbide piece with a functioning silicon carbide-carbon thermocouple embedded, and the response of the embedded thermocouple to a modulated external heat source.

Potential uses for these in-situ thermocouples involve monitoring the internal temperatures of high temperature application parts, such as a jet’s engine components or wing panels.

Summary

SALD techniques offer a set of gas-based SFF techniques for ceramic materials. The two primary advantages

of gas phase approaches are that the processes minimize or

even completely avoid post-processing and have great flexibility in materials and applications. Research on these techniques has focused on developing a clear understanding of

the dominant influences of heat transport, mass transport,

and chemical reactions. The roles of each of these has been

clarified by a wide range of experimentation. The additional

research and development needed to bring the approaches to

engineering reality is ongoing.

Figure 5

Interface between SALD Joining deposit and a substrate

object being joined.

Figure 6

Embedded thermocouple formed by SALD/SALDVI. a)

Schematic of Embedded Thermocouple Process. b) SiC-C

thermocouple embedded in SiC. c) Temperature response

of the embedded thermocouple shown in b)

(a)

(b)

(c)
Three/1998 57

Acknowledgements

The University of Connecticut research reported here

was supported by the Office of Naval Research Contract

#N00014-95-1-0978 and ONR/DARPA Contract #N00014-

96-1-1299.

Biographies

Shay Harrison is a graduate student in the Metallurgy

and Materials Engineering program at the University of Connecticut. With a BS degree in materials science and engineering from Rice University, he expects to finish his Ph.D.

studies in gas-phase ceramic joining in early 1999.

James Crocker is a graduate student in the Metallurgy

and Materials Engineering department at the University of

Connecticut. Having completed Masters Degree requirements in 1997, he is currently pursuing Ph.D. studies.

Kevin Jakubenas is a University Post-Doctoral Fellow at the University of Connecticut. He received his Ph.D.

in Materials Science and Engineering from the University

of Texas at Austin.

Harris L. Marcus is Director, Institute of Materials

Science and Professor of Metallurgy and Materials Engineering at the University of Connecticut. His research areas are

in fatigue and fracture of structural materials, materials science of surfaces and interfaces, materials processing and solid

freeform fabrication.

Leon L. Shaw earned his Ph.D. in Materials Science

and Engineering at the University of Florida in 1992. He is

currently an assistant professor in the Department of Metallurgy and Materials Engineering at the University of Connecticut.

Lianchao Sun is a graduate student at the University

of Connecticut working towards his doctoral degree in ceramics and ceramic processing.

REFERENCES

1. D.W. Richerson, Modern Ceramic Engineering, 2nd

Edition Revised and Expanded, New York: Marcel

Dekker, Inc., 1992.

2. Carbide, Nitride, and Boride Materials Synthesis and

Processing, A.W. Weimer, ed. New York: Chapman

& Hall, 1997.

3. G.S. Zong, Solid Freeform Fabrication Using Gas

Phase Selective Area Laser Deposition, Ph.D.

Dissertation, The University of Texas at Austin,

August, 1991.

4. W.R. Thissell, Processing and Control of Selective

Area Laser Deposition from Methane and Hydrogen,

Ph.D. Dissertation, The University of Texas at

Austin, 1994.

5. J.V. Tompkins, R. Laabi, B.R. Birmingham, and H.L.

Marcus, Solid Freeform Fabrication Symposium

Proceedings, The University of Texas at Austin,

1994, pp. 412-421.

6. B.R. Birmingham, Laser Based Solid Freeform

Fabrication Techniques for the Direct Production of

Ceramic and Metal/Ceramic Shapes, Ph.D. Dissertation, The University of Texas at Austin, 1995.

7. S. Harrison, J.E. Crocker, T. Manzur, and H.L.

Marcus, SFF Symposium Proceedings, UT-Austin,

1996, pp. 345-348.

8. J.L. Maxwell, J. Pegna, D. Messia, and D.

DeAngelis, SFF Symposium Proceedings, UT-Austin,

1996, pp. 227-237.

9. O. Lehmann and M. Stuke, Science, Vol. 270 (8 Dec.

1995), pp. 1644-1646.

10. F.T. Wallenberger, Science, Vol. 267(3 Mar 1995),

pp.1274-5.

11. J.J. Beaman, J.W. Barlow, D.L. Bourell, R.H.

Crawford, H.L. Marcus, and K.P. McAlea, Solid

Freeform Fabrication: A New Direction in Manufacturing, Kluwer Academic Publishers, Norwell,

Massachusetts, 1997.

12. S. Gordon and B.J. McBride, NASA Lewis Research

Center, NASA SP-273, March 1976.

13. D. Bauerle, Chemical Processing with Lasers,

Spring-Verlag, New York, 1986

14. J.L. Maxwell, J. Pegna, and A.G. Ostrogorsky, SFF

Symposium Proceedings, UT-Austin, 1993, pp. 253-

270

15. J.L. Maxwell, J. Pegna, and E. Hill, SFF Symposium

Proceedings, UT-Austin, 1995, pp. 143-150

16. K.J. Jakubenas, J.M. Sanchez, and H.L. Marcus,

Materials & Design, accepted and to be published in

1998.

17. K.J. Jakubenas, Precursor Selection for Selective

Area Laser Deposition, Ph.D. Dissertation, The

University of Texas at Austin, December 1996.

18. K.J. Jakubenas, Y.L. Lee, M.S. Shaarawi, H.L.

Marcus, and J.M. Sanchez, Rapid Prototyping

Journal, Vol. 3, No. 2, 1997, p. 66-70.

19. J.E. Crocker, Making Silicon Carbide Shapes by

Selective Area Laser Deposition Vapor Infiltration

(SALDVI), M.S. Thesis, University of Connecticut,

Dec. 1997.

20. J.V. Tompkins, B.R. Birmingham, and H.L.

Marcus,SFF Symposium Proceedings, UT-Austin,

1995, pp. 409-416.
58 Naval Research Reviews

Abstract

A novel feature of Layered Manufacturing, an emerging manufacturing technology, is that it enables fabrication

of heterogeneous objects – multi-material and functionally

graded interiors – directly from a CAD model. To fully exploit this capability, it is necessary to create CAD models

which capture both geometry and material information. We

describe our ongoing research in this area. Also, we consider process planning issues and provide details on adaptive slicing, a fundamental process planning task.

1.0 Introduction

Layered manufacturing (LM) is a new method of fabrication. Unlike the traditional manufacturing techniques, in

LM a part is fabricated directly from a computer model by

successively depositing raw material in layers. A distinct

advantage of creating a part layer-by-layer is that its geometric complexity has significantly less impact on the fabrication process. Furthermore, different materials, in varying

proportions, can be deposited to form a single layer. Such

heterogeneous objects, made from materials such as tough

ceramics, advanced metallized ceramic composites, and

materials with graded chemistries and microstructures, are

being designed to provide enhanced structural and mechanical properties in naval systems as well as in other commercial applications. Figure 1(a) shows the cross-section of a

model of a turbine blade reinforced with a hard material (in

gray) to improve the structural strength of the thin part of

the blade. Figure 1(b) shows a simplified model of a valve

seat composed of three regions – Aluminum (outer), Brass

(inner) and a graded composition in between. While commercial LM systems today are used for rapid prototyping

applications, fabrication of heterogeneous objects represents

the true potential of layered manufacturing.

Process Planning

Issues in the Layered

Manufacture of

Heterogeneous

Objects

Debasish Dutta, Prashant Kulkarni and Vinod Kumar, Department of Mechanical Engineering and Applied Mechanics, The University of Michigan, Ann Arbor, MI
Three/1998 59

1.1 Solid Modeling

The CAD industry has progressed from simple automated drafting/wireframe systems and complex surface

modelers to the current state-of-the-art solid modelers. From

CAD/solid models, information can be generated about part

functionality, manufacturing, assembly and inspection.

Hence, solid models have been the key to automation in

manufacturing process planning, assembly planning, engineering analysis, etc. Solid modeling has emerged as a key

area of research in computer science and several engineering disciplines.

Currently, the solid model of an object contains geometry information stored as data and topology information (e.g.

adjacency of faces, edges, vertices) incorporated in the data

structure. Typically, there is no information in the solid model

regarding the object interior (material, density or other heterogeneities). Without this information, fabrication of heterogeneous objects by LM cannot be fully automated.

1.2 Process Planning for

Layered Manufacturing

Conventional manufacturing process planning tasks

involve determining machining operations (turning, drilling,

etc.), parameters (feed, speed, depth of cut) and tools/fixtures necessary to convert a workpiece to the final part. In

LM, process planning tasks include defining the build orientation, determining supports, slicing the CAD model (to determine the layers) and the generation of toolpaths and other

process information. A block diagram of the key process planning tasks for LM is shown in Figure 2.

Conceptually, process planning tasks for layered manufacturing can be decomposed into tasks that are performed

in the model domain and ones that are performed in the layer

domain. In the model domain, the process planning tasks

(e.g., part orientation) require geometric information from

the CAD model and they output data pertaining to the whole

model. The process of slicing converts the model domain

into the layer domain. The process planning tasks pertaining

to the layer domain (e.g., path planning) are concerned with

generation of data for manufacture of individual layers.

Slicing is a fundamental process planning task since

all LM processes require it. It is a procedure in which parallel horizontal planes are intersected with the CAD model of

the part to determine contours on which material is to be

deposited. LM machines are being developed where additional degrees of freedom are being added to the machine,

by either pivoting the nozzle or pivoting the platform. In

such a case, the planes referred to above are no longer horizontal, but perpendicular to the build direction. Slicing affects the surface accuracy of the manufactured part and the

time to manufacture.

In this paper, we provide an overview of our research

focusing on the development of computational methods necessary for automated layered manufacturing. In particular,

we describe a method for modeling heterogeneous solids in

Section 2 and slicing such models in Section 3. We conclude

with a brief mention of our research in other process planning tasks.

2.0 Heterogeneous

Solid Modeling

In traditional geometric/solid modeling, techniques

have been developed to model (homogeneous) objects to

capture their geometry and topology [1][2]. Such solids can

be modeled as r-sets, a regular, compact, semi-analytic subset of the Euclidean space. It captures the notion of solidity

(rigidity) of an object having a deterministic boundary.

Figure 1

Heterogeneous objects.
60 Naval Research Reviews

However, in order to represent objects having heterogeneous material domains, the modeling space must be modified. We first consider the simple case where an integer

attribute is attached to the model to represent objects made

up of a finite number of (homogeneous) material regions.

We then extend this idea to model the geometry and material

information for a point inside a domain that contains continuously varying materials. This is achieved by considering

the volume fraction of each material at the point. A mathematical space of volume fractions is used to represent this

information. We extend the conventional r-set to rm-set (and

rm-object) to model a heterogeneous object whose geometry

is defined via r-set and the material defined by means of

volume fractions.

2.1 Theory

In order to model heterogeneous objects, the modeling space T must include a material dimension (M) apart

from the spatial dimension R3

 that captures its geometry and

topology. If we consider objects made of a finite number of

discrete materials [3], the simplest choice for the material

dimension M would be the set of integers Z. However, to

model more complicated objects with continuous material

distribution, the material space Z must be further expanded.

It should allow for the representation of a finite set of materials (referred to as primary materials) and also their combinations.

We consider the space T = R3×Rn, n being the number

of primary materials, as the suitable modeling space. In R3,

the geometry and topology of the object are defined. Rn is

the material space with each dimension representing one primary material. Since a point in the object S can be composed

of any of the primary materials or their combinations, the

composition of material at a point can be quantified by the

volume fractions of each of the primary materials. Noting

that these volume fractions must sum to 1, we can define the

space of volume fractions V as:

(1)

where vi (i-th component of v1) represents the volume fraction of material i. Thus, any point v ∈ Rn

 can represent a

material composition only if it lies on the subspace (or surface) V ⊂ Rn

. The porosity of a local region can also be

modeled by including void as one of the primary materials.

Thus, any subset B ⊆ V can describe the material composition of a heterogeneous object, S. For each point p ∈ S, the

geometrical point x is associated with its corresponding

material point v by a mapping from R3 to Rn:

(2)

Therefore, F is the material function. Additional constraints can be placed on the material function F when defining an appropriate model for S.

In summary, for the heterogeneous object S, each point

p ∈ S can be modeled as (x ∈ R3

, v ∈ V) in T, where x and v

(or equivalently, F(x)) are the geometrical and material points

respectively. The conventional r-set is now extended to a

material r-set which we denote rm-set.

Definition: rm-set – A subset (P, B) of T where P ⊂ R3

is an r-set and B ⊆ V assigns material to P.

To model multi-material objects and objects with

abrupt variations in material composition, we require a set

of rm-sets as a generic mathematical model. More precisely,

we define:

Definition: rm-object – A material object defined as a

1

An underline in v is used to denote a vector in the corresponding space, V.

Figure 2

Process Planning for Layered Manufacturing.
Three/1998 61

finite collection of rm-sets (Pj

, Bj) satisfying conditions C1

and C2:

• C1: The rm-sets are geometrically interior-disjoint

(i.e., the r-sets are interior-disjoint).

• C2: The rm-sets are minimal (i.e., there do not exist

two rm-sets whose C∞ material functions can be combined

into a single C∞ function).

Mathematically, an rm-object, S, can be defined as

(EQ-3):

(3)

Our method for modeling heterogeneous solids is based

on rm-objects. In addition, our research includes the development of new functionality and operations in CAD systems for the synthesis and manipulation of heterogeneous

objects. The methods described above have been implemented and details can be obtained from [3].

3.0 Slicing of

Heterogeneous Solid

Models

Processing of heterogeneous models for layered manufacturing involves considering the geometry as well as the

material distribution within the model. We first look at the

processing of geometry; the complications introduced as a

result of considering the material information are discussed

in section 3.2.

3.1 Adaptive Slicing

In slicing, a CAD model of the object is intersected

with parallel horizontal planes. Each plane generates contours, which we refer to as a slice of the model. Depositing

material on a slice creates a layer. Slicing can either be uniform, where the thickness of successive layers is constant,

or adaptive, where the layer thickness varies based on the

curvature of the bounding surfaces of the model. In industry,

uniform slicing is currently used. Layers generated by uniform slicing for a circular profile are depicted in Figure 3(a).

Note, this is a simple abstraction of the layer manufacturing

process and each layer is generated by extruding the slice

upwards. Depending on the LM process used, the vertical

walls of the layers in Figure 3 will be curved. In Figure 3(b),

BOR refers to the boundary of the original object (in dark

shading) and BLM is the boundary of the corresponding layer

manufactured object (in lighter shading). Depositing material in layers will naturally create cusps denoted δ1

, δ2, and

δ3 in Figure 3(c). The aim of adaptive slicing is twofold:

• Dimensional Control: Achieve user specified surface quality in the least build time by maintaining the cuspheight of each layer to be the same.

• Positional Control: Ensure that the cusps either

lie completely within the CAD model (deficient deposition,

Figure 3(b)) or the cusps lie completely outside the CAD

model (excess deposition, Figure 3(c)),

In [4], we presented methods for generating layers that

satisfy the above two criteria for homogeneous objects. The

basic steps in our slicing procedure include: (i) slice contour

generation, (ii) layer thickness determination, and (iii) layer

generation. We expand on each briefly.

Slice Generation

An orientation is chosen for the object (corresponding

to the build). A slice is generated by intersecting the CAD

model with a horizontal plane at a specified height. This is a

standard operation supported by every CAD system. A slice

can contain several contours depending on the complexity

Figure 3

Different Deposition Strategies.
62 Naval Research Reviews

of the object. For example, convex objects will always yield

a single contour while non-convex objects are likely to yield

multiple contours.

Layer Thickness Computation

Let us assume a slice contour has been generated. We

need to determine the increment for the next slice. This increment is the thickness of the layer of (material to be deposited on) the current slice contour. Recall, in adaptive

slicing, the layer thickness will vary based on the surface

curvature and the user specified cusp height, δ (i.e., the dimensional control parameter).

Let us assume we are at a point P on the current slice

contour. The layer thickness at P can be computed by taking

into account the curvature of the corresponding surface along

the build direction. We locally approximate the surface by

the circle of curvature along the vertical direction on the tangent plane. Based on this circular approximation and the user

specified allowable cusp height δ, we compute the layer thickness d at the point P.

In Figure 4, N denotes the surface normal at point P, θ

is the angle between N and the horizontal, ρ is the radius of

curvature of the circular approximation at P, δ is the cusp

height specified by the user and d is the layer thickness to be

computed. Let s(κ) denote the sign of the curvature, s(c) denote whether the point P lies on the upper or lower part of

the circle, and s(d) denote the choice of deposition in positional control (excess or deficient). The layer thickness d is

then given by (EQ-4).

(4)

There are eight possible forms of (EQ-4) using all combinations of values for the three parameters s(κ), s(c), and

s(d). Refer to [4] for a detailed description of these eight

combinations. Figure 4 illustrates the situation for excess

deposition, P in the upper semicircle and κ > 0.

In general, the CAD model of an object will contain

several surface patches joined together. Hence, each slice

contour is made up of multiple curves. We process each curve

individually in that different points on it would yield different layer thickness values. The minima of layer thickness

values computed over all points is the minimum layer thickness for that curve. This can be posed as an optimization

problem:

(5)

This optimization problem is solved for each curve

segment comprising slice contour. The minima of all the layer

thickness values is the minimum layer thickness for the slice.

If the layer thickness is less (greater) than the minimum

(maximum) manufacturable thickness, the minimum (maximum) manufacturable thickness value is used.

Layer Generation

The satisfaction of the user specified positional control (excess/deficient deposition) requires analysis of the layer

generation step. A layer can be generated by two different

approaches – bottom up or top down. In the bottom up approach, the lower slice (at the current z-level) is extruded

upwards. In the top down approach, the upper slice (at a distance d from the current z-level) is extruded downwards.

Under the assumption that the z-component of the surface

normal will either be positive or negative for the entire contour, the appropriate approach for a layer is determined by

the user’s choice of the positional control criterion (excess

or deficient). The assumption about the normal sign can be

easily verified by projecting one slice onto the other and

checking for the null intersection [4].

Figure 5 illustrates a simple situation in the layer generation process. Consider Figure 5(a) (middle) where the 3D

Figure 4

Circular approximation to determine layer thickness.
Three/1998 63

segment of an object (CAD model) has downward pointing

normals all around. That is, the upper slice of the 3D segment when projected onto the plane of the lower slice, will

properly contain the lower slice. To satisfy deficient (excess)

deposition criteria, the bottom-up (top-down) approach is

adopted and the lower (upper) contour is extruded up (down).

The upper (lower) slice in Figure 5(a) is used to generate

manufacturing information, such as nozzle paths etc., for

excess (deficient) deposition. A complementary situation with

upward pointing surface normals is illustrated in Figure 5(b).

3.2 Extension of Adaptive

Slicing to Heterogeneous Solid

Models

Whereas the CAD model of a homogeneous object

usually contains exterior surfaces (representing the object

boundary), the CAD model of a heterogeneous object also

contains “interior” surfaces. Such surfaces typically delineate distinct material regions in a multi-material object as

well as correspond to sharp changes in material/density/other

gradation.

To generate a heterogeneous slice at height z*, each

material domain in the heterogeneous object is intersected

with the plane z = z*. Using the notations from (EQ-3) and

denoting the slice by L(z) and the slicing plane by SP(z), we

have:

(6)

All surfaces, interior and exterior, of the material domains of the heterogeneous model are considered. Each slice

is now comprised of several material regions corresponding

to the different material domains. Within each region, the

material composition can be constant or continuously varying. The material distribution for each region of the slice is

evaluated from the material distribution function corresponding to that domain of the heterogeneous object.

The circular approximation of the vertical normal section is still used to calculate the layer thickness. However,

both the external and internal surfaces need to be considered. Several different strategies for dimensional and position control can be derived based on the application. In

addition to Exterior Dimensional Control, where only the

external surfaces are considered for computing the layer

thickness, the user can require Complete Dimensional Control. Here, all surfaces (both external and internal) are assumed to be critical for the application and are included in

the layer thickness computation. If the user specifies Exterior Positional Control, the internal edges/contours between

material regions in the slices are ignored and only the external contour of the slice is considered while generating the

layer. Refer to [5] for details on all positional/dimensional

control and methods to achieve them.

3.3 Other Process Planning

Tasks

As mentioned earlier, our research focus has been on

the development of computational tools for process planning tasks in layered manufacturing. A suite of tools for hoFigure 5

Positional control when normal sign assumption is satisfied.
64 Naval Research Reviews

mogeneous models have been developed. Their extension to

heterogeneous models is in progress.

A Solid Builder Module (SBM) has been developed

that generates a solid model of an object from information

contained in a 3D image [6]. There are numerous sources of

3D images that represent objects, such as CT and MRI scans,

and FEM output. In addition to a description of the boundaries of the object, the solid model is being extended to contain material information. This solid model can then be used

as an input for process planning.

Figure 6 displays a three dimensional model of a real

microstructure, manufactured for the Naval Research Laboratory using the Stratasys 3D Modeler. It depicts a model of

the cementite precipitates (the white solid) within an austenite grain (the austenite corresponds to the empty areas, but

there is a cementite “shell” coating the boundaries of the

austenite grain) in an Fe-1.3%C-13%Mn model steel alloy

isothermally heat treated at 650 o

C for 50 seconds [7]. A series of 128 images taken by optical microscopy from serial

sections at 0.2 micron depth increments were read into SBM.

Each image was used to generate one slice for the layer

manufacture of the model.

The capability to link layered manufacturing directly

to image data can be useful. For example, our SBM can enable the rapid creation of physical models of strategic terrains obtained from satellite images.

Our orientation module, ORM, determines possible

build orientations for parts to be fabricated by LM [8]. It

determines the orientation based on one of the following criteria: minimum build height, maximum support contact area,

maximum area of base, minimum volume of supports and

minimum average surface roughness. The user can select an

orientation that minimizes (or maximizes) any of the preceding objectives. The effect of choosing different deposition paths on a specific LM process (FDM) has also been

investigated [9]. The stiffness of parts manufactured using

the different strategies was experimentally determined. It was

then compared with an analytical model developed by using

laminate analysis. A good conformance of the models enables the laminate model to be used as a design aid in tailoring the deposition strategy to the stiffness requirements.

Finally, we are also developing methods to enable

interoperability of (layer manufacturing) data between various commercial machines. Once accomplished, it will allow

for a seamless integration of various LM machines which

can be operated using heterogeneous solid models and the

process planning tools.

4.0 Acknowledgments

Financial support for this work was received from

Office of Naval Research grants N00014-95-1-0767 and

N00014-97-1-0245 and is gratefully acknowledged. We also

acknowledge other members of our layered manufacturing

research group, namely, Paul Alexander, Seth Allen, Anne

Marsan and Xiaoping Qian, whose research was mentioned

in Section 3.3.

Biographies

Debasish Dutta received his PhD from Purdue University and is currently on the faculty of Mechanical Engineering & Applied Mechanics at the University of Michigan,

Ann Arbor. His current research interests are in heterogeneous solid modeling and layered manufacturing. He is an

Associate Editor of ASME J of Mechanical Design, SME J

of Manufacturing Systems, and J of Mechanics of Structures

& Machines. Dutta is the General Chair of the upcoming 5th

ACM Solid Modeling Symposium, in June 1999.

Prashant Kulkarni recently completed his Ph.D. from

the Mechanical Engineering and Applied Mechanics Department at the University of Michigan. His research interests

are in Layered Manufacturing, Computer Aided Design &

Manufacturing and Solid/Geometric Modeling.

Vinod Kumar is currently a Ph.D. candidate at the

University of Michigan, Ann Arbor. His primary interests

are in Solid/Geometric Modeling, Computer Aided Geometric Design and Layered Manufacturing.

REFERENCES

1. C. M. Hoffmann, Geometric & Solid Modeling,

Morgan Kaufmann Publishers, 1989.

2. A. Requicha, Representations for Rigid Solids:

Theory, Methods and Systems, Computing Surveys,

Vol. 12, No. 4, 1980.

3. V. Kumar and D. Dutta, An Approach to Modeling &

Representation of Heterogeneous Objects, ASME

Figure 6

Model of cementite precipitates and austenite grain

boundaries in a high carbon steel.
Three/1998 65

Journal of Mechanical Design, in review, available as

Technical Report UM-MEAM-97-05, Department of

Mechanical Engineering, University of Michigan,

Ann Arbor, 1997.

4. P. Kulkarni and D. Dutta, An Accurate Slicing

Procedure for Layered Manufacturing, Computer

Aided Design, Vol. 28, No. 9, pp. 683-697, 1996.

5. V. Kumar, P. Kulkarni and D. Dutta, Adaptive Slicing

of Heterogeneous Solid Models for Layered Manufacturing, Technical Report UM-MEAM-98-02,

Department of Mechanical Engineering, The University of Michigan, Ann Arbor, MI 48109.

6. A. Marsan and D. Dutta, Construction of a Surface

Model and Layered Manufacturing Data From 3D

Homogenization Output, Journal of Mechanical

Design, Vol. 118, No. 3, 1996, pp. 412-418.

7. M. V. Kral and G. Spanos, personal communication,

Naval Research Laboratory, January 1998.

8. S. W. Allen and D. Dutta, Determination and

evaluation of support structures in layered manufacturing, Journal of Design and Manufacturing, Vol. 5,

1995, pp. 153-162.

9. P. Kulkarni and D. Dutta, Deposition Strategies and

Part Stiffnesses in Layered Manufacturing, Proceedings of the 1997 ASME Design Automation Conference, Sacramento, CA, Sept. 1997.