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http://ammtiac.alionscience.com The AMMTIAC Quarterly, Volume 3, Number 2 3

INTRODUCTION

In today’s unpredictable environment, weapon system readiness is

a vital issue. The necessity for rapid deployment of weapons as

needed throughout the world requires that these systems be readily available and that they remain operational when deployed.

The need for greater reliability and durability is reflected in excessive downtime for repair and maintenance, which in turn reflects

the difficulty in withstanding the harsh environments imposed by

operation in some of the world’s trouble spots. Some of the military systems experiencing exposure to high loading conditions as

well as abrasive and corrosive environments are Army aviation

transmission, drive and actuation systems. These components and

systems are consequently vulnerable to significant damage.

Within the last several years, a number of surface engineering

technologies have been experimentally demonstrated to improve

wear resistance characteristics of aerospace drive-train components. Although the results have been impressive, for example a

three-fold improvement in the life of precision gears, applications of the available technologies require individual case-based

development. Emerging technologies that hold promise for

improving the reliability and durability of military equipment

include superfinishing, laser cladding, nanotechnology, and thin

film solid coatings.

Recent studies have verified that various superfinishing technologies can help to maintain a full elastohydrodynamic lubricant

film that will prevent metal-to-metal contact in mating parts and

will virtually eliminate contact wear and pitting fatigue.

Additional benefits of superfinishing include an increase in load

bearing capacity, a decrease in noise, and a decrease in coefficient

of friction between mating surfaces.

Super-hard steel coatings, as applied by thermal spraying, are an

example of an emerging surface engineering technology that can

be applied to greatly enhance abrasion resistance. Furthermore,

recent advances in nanotechnology to develop nanoscale structures

in coatings produced by laser (thermal) deposition techniques

hold great potential for significantly improving wear, corrosion

resistance, toughness, and low friction characteristics.

Successful incorporation of these technologies into existing

material systems is expected to vastly extend lifetimes and reduce

total ownership costs for legacy and future systems. However, in

order to obtain these benefits, application engineering is required

to adapt them for military components.

The Engineered Surfaces for Weapons Systems Life Extension

project is being conducted under a partnership between the

University of North Dakota (UND) and Alion Science and

Technology. The program is being executed under the direction

of the Army Research Labs (Benet Labs), with supporting contributions from a Technical Advisory Board (TAB). The TAB consists of members from Army aviation prime contractors, local

North Dakota DoD manufacturers, as well as Alion and UND.

The project’s mission is to undertake fundamental and applied

research to develop surface engineering technologies with the

ability to extend operational life of vital weapon system components that are exposed to high contact loads, wear, and corrosive

and erosive environments.

RESEARCH EFFORTS

The first year of the program focused on superfinishing technology, as provided by REM Chemicals (REM), for Army

applications; a review of this previous work was reported in the

AMPTIAC Quarterly, Volume 7, Number 1, published in

2003.[1] In summary, the previous efforts led to a 300%

improvement in gear contact fatigue life with at least a 10%

increase in tooth bending fatigue life, without violating specified existing part geometry tolerances. Current work has built

upon these developments with the objective of establishing a

solid understanding of the process mechanisms and validating

the technology from a second-source supplier, Houghton

International, Inc. (Houghton).

Subsequent efforts have focused on the life limiting effects of

corrosion, erosion, wear, and fretting on specific candidate components. The intent of the R&D effort is to develop an understanding of a candidate component’s degradation or failure mechanisms

and find a surface engineering solution to mitigate or eliminate the

Juergen Fischer

D. Bryce Mitton

Engineered Surfaces Center

University of North Dakota

Grand Forks, ND

Benjamin Hoiland

John R. Rindt

Ranko Todorovic

Damian Wilmot

Jarrod M. Schell

Alion Science and Technology

Grand Forks, ND

Paul Niskanen

NiskanenConsulting

Glenview, IL
problem. The following discussion provides an overview of some

of the superfinishing experiments undertaken to date, as well as a

discussion of the center’s initial cold spray experiments.

Superfinishing

Although prior work had proven the REM process to be accurate,

efficient, and robust, a study of the process mechanisms was

beyond the scope of the work. Consequently, as mentioned above,

the goals of this follow-on work were to obtain a solid understanding of superfinishing process mechanisms and to validate technology from Houghton. It was believed that an understanding of the

process mechanisms would lead to the ability to tailor the process

chemicals and media used for application to specific alloy compositions, as well as to perhaps refine, optimize, or otherwise improve

the process. A number of experiments have been undertaken to

study the second supplier’s process. A discussion of some of the key

experiments, and their results, are summarized below.

Rolling/Sliding Contact

Fatigue Tests

Previous work with the REM

process used rolling/sliding

contract fatigue (R/SCF)

tests as screen before undertaking actual tests on aircraft

quality gears. These tests

showed that REM-processed

samples outperformed those

conventionally processed by

about 950%, on average.[1]

Consequently, the same type

of R/SCF tests were carried

out on samples superfinished

by the center using the

Houghton process. In both

cases, samples were made

from AISI 9310. In the tests

conducted on the Houghton-processed samples, the baseline

non-superfinished specimens averaged 3,595,568 lifecycles to

failure, whereas the superfinished specimens averaged 38,323,104

lifecycles. That is, the superfinished samples outperformed the

baseline samples by more than 10.5 times. It should be noted that

in actual gear tests, REM-processed gears showed a 300%

improvement compared to the baseline gears. This is obviously

less than the R/SCF tests indicated but still significant. Based on

the most recent R/SCF tests, it is expected that test results on

Houghton-processed gears will closely resemble the results

obtained with the REM process. Four-square gear tests are

planned for gears processed by the center using the Houghton

process for the superfinished samples.

Comparing Erosion, Corrosion, and the Combined Effect

Three experiments were conducted to compare the contributions

to the superfinishing process by erosion and corrosion mechanisms as well as their combined effects. Case-hardened, ground

gear test pieces made of Pyrowear 53TM and MicrosurfaceTM

5132, a pyrophosphoric acid based treatment solution for alloys,

was used in a 45.5 inch (1.16 m) diameter vibratory bowl. During

the acid treatment, single test pieces were taken out of the bowl,

the weight loss was established, and the average removed material

thickness was calculated (Figure 1). The removal rate at the beginning was around 150 micro-inches per hour (3.8 μm/hr) and then

it changed to a stable value of around 110 micro-inches per hour

(2.8 μm/hr). Also, the removal rate did not change after the

samples had been exposed to the acid treatment for 6 hours.

During the acid treatment, the surface of the samples becomes

coated with a black layer consisting of phosphorous containing

iron salt together with iron oxide/hydroxide. The rubbing ceramic media removes this soft coating and exposes fresh metal at the

peaks of the asperities. Further reaction occurs mostly in these

exposed areas. The asperities slowly get smaller. The conversion

layer in the valleys of the surface roughness reduces the reaction

rates in these areas. The reaction here is likely controlled by

diffusion. Figure 1 shows a curve labeled “Etching with

MicrosurfaceTM 5132 in the beaker without ceramic media”.

Here the acid etching was done without the action of the ceramic media in a beaker. Before

the part was weighed, the

coating was brushed away

with non-abrasive toothpaste. It is obvious that the

thicker the conversion layer

the slower the reaction with

the alloy, which is typical for

a diffusion controlled buildup of a conversion layer.

The results of two more

tests are also documented in

Figure 1. In one test, the

CAVSF process was completed with water instead of

acid. In the other, only the

abrasive action of the dry

ceramic media was investigated. The corrosion in

water gives a very slow reaction and a low material removal. The material removed during

treatment with dry media is 5 to 10 times lower than the material

removed with water. It was determined that about 2% of the metal

removed in the CAVSF process is taken away by only the abrasive

action of the ceramic media in air. Thus, it is fair to say that the

ceramic media only attacks the conversion layer built by the acid.

Material Removal and Roughness Changes

In this experiment, a closer look was taken at the material removal

and the roughness changes that occur during during CAVSF with

continuous flow of chemicals in a 45.5 inch (1.16 m) diameter

vibratory bowl. Twenty strip steel pieces (2 x 1 x 0.17 inches, 0.05

x 0.03 x 0.0043 meters) were ground to an average roughness of

around 19 micro-inches (0.48 μm). After the parts were weighed,

they were put into the 45.5 inch (1.16 m) vibrating bowl with a

continuous flow of chemicals, using MicrosurfaceTM 5132 as the

acid. After the water rinse, Cerfa-Kleen 5502 was used as a burnishing liquid. During the process, parts were taken out to monitor the removed material and the roughness changes. A second

test was performed with polished strip steel samples (arithmetical

mean roughness, Ra, around 0.86 micro-inches (0.022 μm)). The

results are shown in Figures 2 and 3.

4 The AMMTIAC Quarterly, Volume 3, Number 2

Figure 1. Material removal on Pyrowear 53TM during different processes

versus time.

Material removal (micro inches)

0 20 40 60 80 100 120 140 160

Time (minutes)

250

200

150

100

50

0 ◆ ◆

■

■ ■

■

▲

▲

x

x

x

x

x

x

◆

CAVSF with MicrosurfaceTM 5132 and ceramic media

Etching with MicrosurfaceTM 5132 in the beaker without rubbing ceramic media

“Superfinishing” with water

“Superfinishing” dry
After 45 minutes of the acid treatment, the end-roughness is

reached, and the slope of the removed material versus process

time curve changes; the removal rate slows down slightly. During

the 15-minute water rinse, the material removal continues, and

the roughness decreases from 5 – 6 micro-inches (0.13 – 0.15

μm) to the final level of 3.5 – 4 micro-inches (0.089 – 0.10 μm).

Burnishing removes the dark conversion layer from the surface

and gives an optically brighter appearance.

If polished samples (Ra around 0.86 micro-inches (0.022 μm))

are used initially, the acid

destroys the smooth surface

and reveals the material’s

microstructure. In general,

the chemically accelerated

vibratory surface finishing is

more or less a chemical

attack of the metal surface

controlled by the rubbing

action of the media.

However, it is important to

understand the thermodynamics and kinetics of the

etching.

The local etching rate

depends on the energy level

of the metal atoms involved

and the activation energy of

the reaction. This driving

force causes a substrate specific end-roughness if a certain CAVSF process is used.

The following situations

can lead to thermodynamic

differences in the local etching rate:

• In a metal some components are unevenly distributed - for example

carbon in steels and

inclusions in certain

alloys.

• Different crystal sizes in

the metal.

• The grain boundaries

are different than the

grain itself.

• Failures in the crystal

structure; tension in the lattice, which can be extreme on a

ground surface.

• The type of crystal structure (martensite, ferrite, retained

austenite, pearlite, bainite, cementite, ...) in the metal

causes different energy levels. For example, the free

energy of martensite is 1.7 kJ per mol higher than that

of ferrite [2].

• The grains are randomly oriented, and different lattice planes

get exposed during cutting (111 or 100 or 110...etc.), which

creates atoms surrounded by 6 atoms or only 4 atoms, etc.

• Atoms in tips and edges of crystals are only loosely connected to the bulk.

Differences in the local etching rate are also caused by the

kinetics of the etching. In the case of superfinishing, it is most

likely diffusion controlled.

• The conversion layer at the surface varies in thickness. The diffusion of the chemicals reacting with the metal surface and the

diffusion of reaction products away from the surface changes

if the conversion layer is thicker, as it is in roughness valleys.

• The chemical composition of thesolution at thesurface will be

very different from the acid bulk. The pH at the surface

could be much higher and

could change the chemical

reaction to the buildup of

product containing more

hydroxides instead of salt.

• The ceramic material rubbing at the surface of the

pieces with the conversion

layer create random lines

with freshly exposed metal

at the tops of the asperities.

The metal exposure is not

perfectly even. Some areas

get rubbed more often than

other areas, which results in

uneven etching and a contribution to a certain end

roughness.

Below are three practical

examples that highlight the

importance of the variables

discussed in the two preceding lists:

• A long hex-bar was superfinished in the 45.5 inch

(1.16 m) bowl to prove

that CAVSF is isotropic.

Because of the geometry of

both the bowl and the bar,

it was assumed that the

part would travel lengthwise and a pattern could

be discovered by lengthwise and widthwise roughness measurements. It was

found that the roughness

measured lengthwise was

always lower than the

roughness measured widthwise. It was shown under the

microscope that the etching revealed a pattern of the

stretched grains, which was the result of drawing during the

manufacturing process. After the stretched grains are

removed, the roughness after CAVSF was found to be equal

in both directions. This is important for the CAVSF of

drawn material (layers exhibiting a preferred orientation as

opposed to those with equiaxed grains) at the surface.

• AISI 9310, a common steel for gear manufacturers, was

superfinished to an end-roughness of 2 to 3 micro-inches

(0.051 to 0.076 μm). For another batch of AISI 9310, the

end-roughness was found to be 5 to 7 micro-inches (0.13

0 50 100 150 200 250

Process time (minutes)

20

18

16

14

12

10

8

6

4

2

0

Average material removal (micro-inches)

http://ammtiac.alionscience.com The AMMTIAC Quarterly, Volume 3, Number 2 5

Figure 2. Material removal of strip steel during the CAVSF process

in the 45.5 inch (1.16 m) vibratory bowl.

Figure 3. Average roughness of strip steel pieces versus CAVSF time.

0 50 100 150 200 250 300

Process time (minutes)

350

300

250

200

150

100

50

0

Average material removal (micro-inches)

▲

▲ ▲ ▲ ▲ ▲ ▲ ▲

▲

▲

▲ ▲

▲ ▲

▲

O

O O

O O O O

O O O O O O O

O O ▲ ▲ ▲ ▲ ▲ ▲

Acid time Burnishing time

Water rinse

◆

◆

◆

◆

◆

◆

◆

◆

◆ ◆

◆ ◆ ◆ ◆ ◆ ◆ ◆

◆

Acid time Burnishing time

Water rinse

◆
PRINCIPLE

The basic concept of cold spray is simple: when a particle-laden gas jet impinges on a solid surface, three different

phenomena can occur depending on the particle velocity (Vp). When Vp is low, the particles simply reflect (bounce)

off the surface. When Vp reaches moderate values, solid particle erosion of the surface occurs. When Vp exceeds a

critical value (typically in the range of 500-900 m/s but varies with particle and substrate materials, as shown in

Figure 7), particles plastically deform and adhere to the substrate or one another to form an overlay deposit. This

process is analogous to thermal spray coatings.

Russian scientists, carrying out supersonic wind tunnel tests, first observed this phenomenon [4]. They developed it

into a high deposition spray coating process in the mid-1980s and successfully deposited a wide variety of materials

including metals, alloys and composites [6].

Cold spraying has come to be understood, within the larger family of thermal spray processes, as a materials deposition

process in which relatively small particles (ranging in size from approximately 1 to 50 μm in diameter, as shown in Figure

7) in the solid state are accelerated to

high velocities (typically 300 to 1200

m/s or 980 to 3940 ft/s), and subsequently develop a coating or deposit on

an appropriate substrate by an impaction

process. Various terms—including kinetic energy metallization, kinetic spraying,

high-velocity powder deposition, and cold

gas-dynamic spray method—have been

applied to the general form of this technique. In most instances, deformable

powder particles are brought to high

velocities through introduction into a

nozzle, employing gas-dynamics principles of converging/diverging flows to

develop high-velocity gas streams [7].

Bonding

Upon impact on a substrate, the particles deform plastically and bond to the substrate to form a coating. Theoretical

investigations suggest that the bonding mechanism can be attributed to adiabatic shear instabilities due to an extensive localized deformation of the particles during impact on the substrate [8, 9]. Consequently thin surface films are

disrupted providing an intimate contact between the particles and the substrate, allowing bonding to occur. Based

to 0.18 μm). Both batches were prepared with the same

specified procedure. Microscopic examination of the

microstructure revealed that differences in the amount of

retained austenite in the case hardened layer were responsible for the differences in end-roughness. This shows that

even strict specifications of manufacturer procedures do

not ensure that the end-roughness is always the same for a

certain CAVSF process.

• The Pyrowear 53TM samples were superfinished. It was

found that the case hardened layer showed a smoother finish

(Ra = 3 micro-inches (0.076 μm)) than the side with the

exposed substrate (Ra = 7 micro-inches (0.18 μm)). Figure 4

shows the edge of the substrate side where both layers are

visible. The case hardened layer can be found at the edge

400 μm into the substrate. The microstructure of the surface

revealed by CAVSF is finer in the area with the case hardened

layer than in the substrate.

The End-Roughness of Different Carbon Steels

with Different Heat Treatments

The end-roughness of different carbon steels heat treated to

different hardness values was also investigated. Figure 5 shows the

test samples. After the first heat treatment, the test samples were

cooled using several methods: quenching with water, cooling

between wet paper towels or thick metal plates, letting the samples air cool, shutting off the oven, and finally conducting

a proper annealing procedure [3]. Sometimes after water quenching, a second heat treatment at 300 or 350°C was used to bring

the hardness of the coupons slightly below the hardness of the

water quenched samples.

6 The AMMTIAC Quarterly, Volume 3, Number 2

Figure 4. Cut Pyrowear 53TM

sample with superfinished

surface. The case hardened

layer is smoother (Ra = 3 microinches (0.076 μm )) than the

substrate (Ra = 7 micro-inches

(0.18 μm)). CAVSF reveals the

microstructure of the surface

through etching.

Figure 5. Carbon steel test

samples with different

carbon contents and

different heat treatments

to create different hardnesses and a variety of

grains (ferrite, cementite,

pearlite, bainite, martensite, retained austenite ...)

Figure 7. Calculated critical velocities for various spray materials [5].

Cold Spray

Deposition

900

800

700

600

500

400

300

200

100

0

Lead

Tin

Gold

Silver

Zinc

Copper

Tantalum

Niobnium

Tungsten

Aluminum

Zirkonium

Nickel

Iron

Steel 316L

Molybdenum

Magnesium

Titanium

Critical Velocity 25 μm particle,

Vcrit 25 μm (m/s)
on the microstructural examination of cold spray coatings, no evidence of particle melting has been reported [10, 11], despite some

localized melting predictions by theoretical models [8, 9]. The laminar grain structure observed in cold spray coatings is consistent

with the absence of melting and grain growth during the particle deformation process and subsequent bonding [10, 12]. Although

the particle-substrate and particle-particle bonding mechanisms are still not completely understood, it is generally accepted that the

particle velocity is the governing spraying factor for particle adhesion [8, 12, 13, 14].

The ability of aluminum to form a protective oxide layer upon exposure to atmospheric air makes it an attractive choice for surface

protection. When applied by the cold spray process, aluminum coatings have demonstrated an increased hardness [10, 15] and improved

corrosion resistance properties [15]. Several critical velocities for aluminum feedstock powder have been reported, depending on the

particle size examined; refer back to Figure 7 [8, 12, 16, 17, 18, 19].

With the recent installation of cold spray equipment, the Engineered Surfaces team has initiated experiments. The first is aimed at

developing a wear coating for an aerospace application using ceramic particles co-deposited in an aluminum matrix. Initial efforts

involved the use of silicon carbide (SiC), alumina (Al

2O3), and tungsten carbide (WC) particles. Coatings containing several different

ceramic particle loadings were laid down on a 6061-T6 substrate. The samples were sprayed with SiC and WC, and samples of a

WC/Al

2O3/aluminum coating were provided. Figures 8-10 show SEM images of the coatings at 500x. Those coatings containing the

highest percentage of hard particles were selected for the first set of abrasive wear screening tests to identify which of the three hard particle coatings to further develop. However, the initial test results indicated the temperature capability of the cold spray gun was inadequate to achieve a quality coating when ceramic particles were included. As a result, a higher temperature heater has been ordered to

increase the temperature capability of the gun. Samples will be rerun and retested after the new heater has been received and installed.

http://ammtiac.alionscience.com The AMMTIAC Quarterly, Volume 3, Number 2 7

The average end-roughness after the usual CAVSF

(with MicrosurfaceTM 5132

as the acid and Cerfa-Kleen

5502 as the burnishing liquid) is charted versus the

hardness of the steel in

Figure 6. Every curve in this

chart represents an average

curve from values of several

test runs. With annealed ferritic C-steel - hardnesses

around 100 on the Vicker’s

Hardness scale (HV) – the

CAVSF process produces an

average end-roughness of

around 5.3 micro-inches

(0.13 μm). These relatively

high roughness values are

probably caused by these

samples containing soft ferrite with some hard cementite crystals having slightly lower free

activation energy. If the carbon content is higher – around 0.8

weight% - and the hardness

around 200 HV, the endroughness of the annealed

material is about 4 microinches (0.10 μm). The

constituents are globular

cementite and ferrite. If the

hardness of the carbon steel

is between 400 and 700 HV,

the average end-roughness is

between 2 and 3 microinches (0.051 to 0.076 μm).

Here the microstructure is

very fine pearlite or bainite.

It seems that a fine

microstructure supports a

low end-roughness in the

CAVSF process. Carbon

steel with a carbon content

around 0.8 weight% has an

increased average endroughness if the hardness is higher than 700 HV. The constituents in these cases are tetragonal plate martensite and carbide.

Figure 10. CS by SST (WC/Al

2O3 Figure 8. CS 43 (85% Al/15% WC powder) 500x Figure 9. CS 46 (85% Al/15% SiC powder) 500x /Al matrix) 500x

Figure 6. Average end-roughness versus hardness for C-steels with

different carbon contents and heat treatments.

0 100 200 300 400 500 600 700 800 900

Hardness (Hardness Vickers)

6

5

4

3

2

1

0

Average end-roughness (micro-inches)

C 1075

C 1095

C 1020

C 1050

C 1040
http://ammtiac.alionscience.com The AMMTIAC Quarterly, Volume 3, Number 2 7-1

RESEARCH CAPABILITIES

The research facilities of the Engineered

Surfaces for Weapons Systems Life Extension

project are housed on the campus of the

University of North Dakota; they are described

in the following sections.

Superfinishing Cell

Superfinishing, or Chemically Accelerated

Vibratory Surface Finishing (CAVSF), is a

process (shown in Figure 11) in which an acid is

allowed to flow at a controlled rate through a

vibratory bowl that is filled with small pieces of

a ceramic media. This acid softens the outer

layer of the metal component that is being

superfinished, creating a salt layer or “conversion coating”. The microscopic peaks on the

surface of the metal are exposed to the media

first, and the conversion coating on those peaks

is sloughed off as the media vibrates past it. The

newly exposed substrate comes into contact

with the acid again, and the conversion coating

reforms on the new peaks. This process repeats

until all the peaks have been worn down and

only the valleys remain. After an amount of

time has elapsed (60-120 minutes), the bowl is

rinsed and a burnishing agent is introduced

into the bowl. This burnishing agent neutralizes the residual conversion coating and imparts

a luster to the surface of the material.

The superfinishing cell, which is shown in

Figure 12, occupies approximately 400 square

feet of an enclosed, secured laboratory space

dedicated to the process development of

chemically-assisted vibratory superfinishing.

Equipment supporting work done in this cell is

shown in Figures 13-16.

Cold Spray Cell

Thermal spray technology is used extensively in

defense, aerospace, and gas turbine industries.

Typical applications include fabrication of

components, preparation of protective surfaces,

and refurbishment of mis-machined and service damaged parts [20]. In recent years, an

innovative process known as cold spray has

been developed that is capable of producing

metal, alloy, and composite coatings with superior qualities [4]. The cold spray process uses

high velocity rather than high temperature to Figure 11. Superfinishing schematic. (Courtesy of REM Chemicals)

Figure 12. Superfinishing cell consisting of a vibratory bowl with

a 45 inch (1.16 m) inside diameter, a chemical control panel and

dosing station, and an effluent containment tank.

Figure 16. Rotating bending fatigue tester for determining the

ability of superfinishing to improve bending fatigue resistance.

Figure 13. Small vibratory bowl,

with an 11-inch (0.28m) inside

diameter.

Figure 14. Waste water evaporator system for dewatering

effluent and facilitating

subsequent disposal of solids.

Figure 15. Profilometer employed to assess superfinishing quality

and ultimately provide information on the spatial distribution of

superfinishing effectiveness for various media and fluid chemistries.

This profilometer provides 34 types of roughness and 24 types of

waviness parameters of a specimen surface up to 800μm. It has an

accuracy of 0.05 + 1.5 L/1000μm (L is measuring length).
7-2 The AMMTIAC Quarterly, Volume 3, Number 2

produce coatings, thus avoiding or minimizing many deleterious

high temperature reactions that are distinctive of typical thermal

sprayed coatings. Typical advantages of cold spray coatings

include compressive rather than tensile stresses, wrought-like

microstructure, near theoretical density, oxides, and other inclusion-free coatings. Moreover, the footprint of the cold spray beam

is very narrow, yielding a high density particle beam that results

in a high growth rate of coating thickness; this provides a rapid

lay down of material with better control over the shape of the

coating without masking requirements [4, 21, 22, 23]. These

advantages can be exploited to rapidly build up coatings for the

restoration of aluminum components in order to meet drawing

requirements. The cold spray cell is capable of applying a wide

range of coating materials including zinc, aluminum, copper, and

nickel. A photograph of this robotized cell is shown in Figure 17.

The gun unit is mounted on a robot arm allowing for CADcontrolled computer numerical control (CNC) input and offline

path programming.

Laser Cell

The laser cell, shown in Figure 8, is utilized for surface modification research and development (R&D) including coatings,

surface alloying, and surface hardening. It is also capable of

facilitating studies in welding, cutting, drilling, micro-machining, micro-fluidic channeling, and engraving.

Corrosion Testing Facilities

A potentiostat and frequency response analyzer (FRA) and a

portable electrochemical system are shown in Figure 19. Both

systems are employed to perform an electrochemical analysis

of samples within the Surface Engineering Center (SEC); however, the electrochemical system may be transported and used

Figure 18. Laser cell with 1 kilowatt fiber laser; rotation and

tilt controlled stage; and welding, cutting and coating nozzles

with wire and powder feeders.

Figure 17. Robotized cold

spray cell with powder feed

system and MotoSim software

for offline programming and

3D simulation with collision

detection.

Figure 19. Potentiostat

and portable electrochemical systems.
http://ammtiac.alionscience.com The AMMTIAC Quarterly, Volume 3, Number 2 7-3

Figure 23. (a, b) Metallographic equipment.

Figure 21. Scanning electron microscope with EDS. Figure 22. Confocal microscope.

Figure 20. Corrosive environment simulation cabinet.

to generate electrochemical data at various remote locations.

The center also has a corrosion simulation environmental

chamber, as shown in Figure 20.

Microscopy Facilities

A scanning electron microscope with energy dispersive system

(SEM/EDS), shown in Figure 21, allows samples to be viewed at

high magnification; it is used to analyze microstructural characteristics and fracture surfaces as well as the chemical composition

of constituent phases, inclusions, or other such features.

Another important piece of the center’s analytical equipment is

a laser scanning microscope (LSM), which is shown in Figure 22.

This confocal microscope system scans a specimen surface with

laser light, point-by-point and line-by-line, in order to acquire

an optical section. Many optical sections collected from different

z-planes form a 3D image stack, permitting analysis of the various surface features. This microscope is also used as an optical

microscope with halogen light. Bright field, dark field, and

differential interferometric contrast (DIC) are combined with

features to analyze shape and size of particles, automatically measure particle and grain sizes, and calculate volumetric percentages.

Other metallographic lab equipment includes a stereo zoom

microscope, a slow saw for cutting samples, four grinding and

polishing tables for further preparation of test specimens, and

three precision scales (0.2, 1, and 10 kg). Some of this equipment

is shown in Figure 23.

(a) (b)
8 The AMMTIAC Quarterly, Volume 3, Number 2

SUMMARY

The combined effect of corrosion (etching) and erosion in the

chemically accelerated vibratory surface finishing process with

continuous flow of chemicals produces a constant material

removal rate of around 120 micro-inches (3 μm) per hour with

a continuous flow of MicrosurfaceTM 5132. The chemical attack

of the surface reveals the microstructure of the substrate at the

surface. During superfinishing the average roughness reaches a

characteristic end-roughness, which is determined by the process,

the materials, and the microstructure of the surface. Low endroughnesses with carbon steels (CS 1020 – CS 1095) are produced if a very fine microstructure like bainite or a low carbon

lath martensite is present. Relatively high end-roughnesses are

reached if annealed carbon steel is superfinished. Water

quenched carbon steels with 0.75 to 0.95 weight% carbon consist of plate martensite and carbide, resulting in slightly increased

end-roughness.

ACKNOWLEDGEMENTS

This project is sponsored by the Defense Technical Information

Center, ATTN: DTIC-AI, 8723 John J. Kingman Rd., Ste 0944,

Fort Belvoir, VA 22060-6218. The work was made possible by

the contractual relationship between AMMTIAC and the DoD

to perform research and analysis of advanced materials. This

includes US Army ARDEC (Armament Research, Development

and Engineering Center) and Benet Labs.

Benet Labs, Alion Science and Technology, and the Engineered

Surfaces Center of the University of North Dakota are working

together to improve life and performance of materials used for

weapons systems.

The authors gratefully acknowledge the support of Douglas

Larson and Zahed Huq from the Engineered Surfaces Center,

University of North Dakota; and Danika V. Mathsen,

Christopher Howe, and Amanda Rindt from Alion Science and

Technology.

REFERENCES

[1] Niskanen, Paul, Ali Manesh, et al. (2003), “Reducing Wear with

Superfinish Technology,” AMPTIAC Quarterly 7(1): 3-9. http://

ammtiac.alionscience.com/pdf/AMPQ7_1ART01.pdf.

[2] Bhadeshia, H.K.D.H., and R.W.K. Honeycombe, Steels: Microstructure and Properties, Burlington, Massachusetts: Elsevier Ltd., 2006.

[3] Boyer, H.E., Practical Heat Treating, ASM International, ISBN 0-

87170-178-2.

[4] Alkhimov, P., A.N. Papyrin, V.P. Dosarev, N.J. Nesterovich, M.M.

Shuspanov, “Gas-Dynamic Spraying Method for Applying a Coating,”

US Patent 5,302,414, April 12, 1994.

[5] Schmidt, T., F. Gärtner and H. Kreye, “Development of a

Generalized Parameter Window for Cold Spray Deposition,” Acta

Mater., Vol. 54, 2006, pp. 729-742.

[6] Karthikeyan, J., “Cold Spray Technology: International Status and

USA Efforts,” ASB Industries, December 2004.

[7] Karthikeyan, J., R.C McCune, and M.F. Smith, “Cold Spray

Process,” Handbook of Thermal Spray Technology, pp. 77-84.

[8] H. Assadi, F. Gartner, T. Stoltenoff, and H. Kreye, “Bonding

Mechanism in Cold Gas Spraying,” Acta Mater., Vol. 51, 2003, pp.

4379-4394.

[9] Grujicic, M., C.L. Zhao, W.S. DeRosset, and D. Helfritch,

“Adiabatic Shear Instability Based Mechanism for Particles/Substrate

Bonding in the Cold-Gas Dynamic-Spray Process,” Mat. Des., Vol. 25,

2004, pp. 681-688.

[10] Ajdelsztajn, L., B. Jodoin, G.E. Kim, and J.M. Schoenung, “Cold

Spray Deposition of Nanocrystalline Aluminum Alloys,” Met. And Mat.

Trans. A, Vol. 36A, 2005, pp. 257-666.

[11] Dykhuizen, R.C., and M.F. Smith, “Gas Dynamic Principles of

Cold Spray,” J. Therm. Spray Technol., Vol. 7 (No. 2), 1998, pp. 205-212.

[12] Van Steenkiste, T.H., J.R. Smith, and R.E. Teets, “Aluminum

Coatings via Kinetic Spray with Relatively Large Powder Particles,” Surf.

Coat. Technol., Vol. 154, 2002, pp. 237-252.

[13] Gilmore, D.L., R.C. Dykhuizen, R.A. Neiser, T.J. Roemer, and

M.F. Smith, “Particle Velocity and Deposition Efficiency in the Cold

Spray Process,” J. Therm. Spray Technol., Vol. 8 (No.4), 1999, pp. 576-

582.

[14] Dykhuizen, R.C., M.F. Smith, D.L. Gilmore, R.A. Neiser, X. Jiang,

and S. Sampath, “Impact of High Velocity Cold Spray Particles,”

J. Therm. Spray Technol., Vol. 8 (No. 4), 1999, pp. 559-564.

[15] Balani, K., T. Laha, A. Agarwal, J. Karthikeyan, and N. Munroe,

“Effect of Carrier Gases on Microstructural and Electrochemical

Behavior of Cold-Sprayed 1100 Aluminum Coating,” Surf. Coat.

Technol., Vol. 195, 2005, pp. 272-279.

[16] Alkhimov, A.P., V.F. Kosarev, and A.N. Papyrin, “A Method of Cold

Gas-Dynamic Deposition,” Sov. Phys. Dokl., Vol. 35 (No. 12), 1990, pp.

1047-1049.

[17] Alkhimov, A.P., A.N. Papyrin, V.F. Kosarev, N.I. Nesterovich,

M.M. Shushpanov, “Gas-Dynamic Spraying Method for Applying a

Coating,” US Patent 5,302,414, 1994.

[18] Borchers, C., F. Gartner, T. Stoltenhoff, and H. Kreye,

“Microstructural Bonding Features of Cold Sprayed Face Centered

Cubic Metals, J. App. Phys., Vol. 96 (8), 2004, pp. 4288-4292.

[19] Sancoucy, E., B. Jodoin, P. Richer, and L. Ajdelsztajn, “Effect of

Spraying Parameters on the Microstructure and Bond Strength of Cold

Spray Aluminum Alloy Coatings,” Proc. of the 2006 International

Thermal Spray Conference, ASM International, 2006.

[20] Davis, J.R. (Ed), Handbook of Thermal Spray Technology, ASM

International, 2004.

[21] See for Instance Proceedings of Various Thermal Spray Conferences,

p. 288 of UTSC 1999, Düsseldorf; p. 255, 419, and 905 of ITSC 2000,

Montreal; pp. 399-446 of ITSC 2001, Singapore, 2001; pp. 366-389 of

ITSC 2002, Essen; pp. 1-122 of ITSC 2003, Orlando; ASM

International, 2003.

[22] Cold Spray CRADA, Project Task Statement No. 1589.01, Sandia

National Lab., Albuquerque, 2000-2003.

[23] Bhagat, R.B., M.F. Amataeau, A.N. Papyrin, J.C. Conway Jr, B.

Stutzman and B. Jones, “Deposition of Nickel Aluminum Bronze

Powder by Cold Gas Dynamic Spray Method on 2618 Al for Developing

Wear Resistant Coatings,” Thermal Spray: A United Forum for Scientific

and Technological Advances, ASM International, 1997, pp. 361-67.

To read more of this article (pages 7-1 through 7-3, which covers the research capabilities of the program),

download the online version which can be found at http://ammtiac.alionscience.com/pdf/AQV3N2_ART01.pdf.

Do you have comments or questions about this article?

Contact us at: http://ammtiac.alionscience.com/contact
INTRODUCTION

Thermography, also known as thermal inspection or infrared (IR)

imaging, is one of the most common nondestructive testing (NDT)

methods, and it is becoming more widely used as the technology

improves and users begin to realize the true potential of this testing

method. Thermography is characterized by the use of thermal and

infrared sensors to measure temperature variations in a variety of

test objects. This article will provide an overview of thermography,

including the history, physical principles, inspection requirements,

and advantages and disadvantages of the method. [1, 2]

HISTORY AND RECENT ADVANCEMENTS

Despite the long history of thermal inspection methods in noncontact NDT, the potential of thermography as a wide-area NDT

method is a fairly recent development. Prior to the development of

the infrared camera, thermography was primarily thought of as a

qualitative, companion testing method to the more established

techniques such as radiography and ultrasonic inspection.

Thermography was used as a secondary method to support the

primary testing techniques. However, as a result of significant

advancements in both IR cameras and software signal processing,

thermography has become a stand-alone, effective NDT method.

For example, after the Columbia Shuttle accident in 2003, NASA

turned to thermography in order to analyze the materials defects

that were present on the leading edge of the Space Shuttle (see

Figures 1, 2 and 3).[3, 4, 5]

Early Thermographic Techniques

In the 1960s, without the benefit of an adequate infrared camera to

remotely detect surface temperature changes, the process of thermographic testing was far more laborious than it is using current techniques. [4] Prior to the introduction of the infrared camera as the

primary component in thermal inspection, wide-area testing was

the commonly used thermographic technique. Wide-area testing

required the test object to be coated with a special temperaturesensitive material. This coating was necessary so that the object

needing evaluation could be photographed with a conventional

(visible light) camera to obtain a permanent record for analysis.

Also, devices such as the evaporograph had to be used to image radiation from the test object onto an oil-coated membrane. This membrane would generate interference patterns that corresponded to

temperature variations and could be observed as color changes.

Generally, these techniques had slow response times and limited

temperature sensitivity. In addition, to achieve maximum performance in a thermal spectral range the test objects had to be heated to

extreme temperatures. These factors, combined with the unreliable

performance of thermography as a NDT method at that time, were

the primary reasons that use of the method was limited. [3, 4]

Introduction of the IR Camera

Since its introduction in the late 1970s, the IR video camera has

been the primary sensor technology used in nearly all thermographic NDT systems. The basic approach used in modern thermographic NDT is to thermally excite the surface of a sample by heating (or

cooling) it and then monitor changes in the surface temperature

using an infrared camera. This approach is not much different from

earlier practices, but the image details that can be produced have

improved greatly. Along with improvements in the camera, the

adoption of the Thermographic Signal Reconstruction (TSR)

method as the primary means of processing and interpreting active

thermographic data has led to improved resolution, reliability and

quantification in measuring a variety of defects. [3, 4]

Brett J. Ingold

AMMTIAC

Rome, NY

http://ammtiac.alionscience.com The AMMTIAC Quarterly, Volume 3, Number 2 9

Selecting a Nondestructive Testing Method, Part VI: Thermal/Infrared Inspection Techniques – Thermography

techsolutions 8

Figure 1. NASA

employed

thermography

to analyze

defects found

on the Shuttle’s

leading edge.

[6]

Figure 2. Thermographic inspection of Space Shuttle leading edge. [7]

Figure 3. Delaminations on Shuttle

leading edge discovered using

thermography.

Wing Leading Edge

Processed Thermal Image

Delaminations
techsolutions 8

Thermographic Signal Reconstruction

Thermographic Signal Reconstruction became the commonly

accepted approach to processing and interpreting active thermography data in the early 2000s. TSR is a signal processing technique based on the physics of thermal diffusion. Each pixel in the

image detected by an infrared camera is treated as an individual

time history of cooling behavior at a specific point on the test

sample (see Figure 4). The pixel can be converted to an equation

that can be used to find specific characteristics at that local point

on the sample. This method is similar to an ultrasonic A-scan

and further illustrates how thermography has become a primary

testing method on the same level as ultrasonic testing and radiography. [3, 4, 5]

In the early stages, TSR was used to create compact, noise-free

sequence replicas of flash thermography data. Derivatives of these

replicas were used to obtain reference-free evaluation and quantification of results. To detect the presence of a sub-surface defect

and to measure defect depth, the derivative of a single pixel time

history is analyzed without reference to adjacent pixels. If no

defects are present, the derivative can also be used to measure

local thermal diffusivity and sample thickness. Modern flash

thermography systems are derived from TSR. [3, 4, 5]

Flash Thermography

The most widely used, active mode NDT method is flash

thermography. This method is characterized by flash heating

of the surface of a test piece by a light pulse that lasts a few

milliseconds. Under normal conditions, the part cools after this

flash heating and the heat dissipates throughout the part. The

heat flows towards the cooler interior at a rate that can be

monitored to determine if defects are present. Current flash thermography systems provide precise, highly repeatable measurements of sample or coating thickness and materials properties

such as thermal diffusivity or effusivity. Fully automated systems

that can be used to collect these measurements have become the

new standard in the current NDT environment. [3, 4, 5]

PHYSICAL PRINCIPLES

Thermographic NDT typically involves the evaluation of thermal

signatures of the test objects using imaging techniques. Optical

excitation techniques, including infrared, are the most common

excitation techniques used in thermography. Other excitation

methods include radiative heating (examples include: infrared,

light, microwaves, electromagnetic induction), mechanical stimulation (sonic/ultrasonic, convection, direct contact), chemical

(exothermic reaction of binary adhesive) and electrical (joule

heating). These thermal signatures are then used to evaluate various flaws, including leaks, cracks, debonding, corrosion, poor

electrical wiring and contacts, and delaminations. They also can

be used to assess overall thermal characteristics. The detection of

discontinuities in a material is possible because the uniform

diffusion of heat from the surface of a sample into its bulk is disrupted by anomalies in the surface properties of the material.

When the flow of heat is obstructed, it leads to a non-uniform

thermal signature on the sample’s surface. This type of heat flow

from a part and the corresponding thermal signature are observed

and analyzed using thermography (see Figure 5). [5, 6]

Despite the evolution of thermographic equipment and procedures, older methods for thermographic NDT data evaluation

are still being used. These practices require the user to watch the

data stream as a movie on a monitor or viewfinder and visually

identify anomalous areas called “hot spots.” This method may

prove effective for some applications or types of defects.

However, this type of testing methodology does not take full

advantage of all the technology available for thermographic

NDT. The visual assessment is entirely qualitative, relying on the

intuition and experience of the inspector. [3, 4]

In modern thermography systems, the sample is heated by a

precisely timed light pulse from a flash lamp discharge. The

infrared camera is used to observe surface temperature data as a

continuous stream of images that can be analyzed. This data is

processed using dedicated algorithms, which enable detection

and measurement of features that are not detectable by direct

viewing of the video data from the camera. [4, 5]

10 The AMMTIAC Quarterly, Volume 3, Number 2

Figure 5. Thermograph of power turbine blade showing wall

thickness. (Photo Courtesy of Thermal Wave Inc.)

Figure 4. Thermographic

evaluation by mapping local

sample points using TSR. [6]
Thermal Property Requirements

It is important to consider a variety of thermal properties in

order to determine how the surface of a sample will absorb or

emit thermal energy for thermographic NDT. Emissivity, for

example, is the ratio of the infrared energy radiated by an object

at a given temperature to the energy emitted by a perfect radiator (emissivity = 1), or blackbody, at the same temperature.

Therefore, emissivity provides an accurate measure of how

efficiently an object emits infrared energy. Since emissivity is a

surface property, it is possible to modify the surface of a test

object with a low emissivity value by using a high emissivity

coating in order to improve the optical absorption and infrared

emission of the surface and enable thermographic analysis. For

example, bare metals typically require some surface preparation,

while most polymer composites used in aerospace applications

have sufficiently high emissivity values to enable thermographic

testing with minimal preparation. [3, 4, 7]

Another important thermal property to consider when evaluating the applicability of thermographic NDT is diffusivity.

Thermal diffusivity is defined as the rate at which heat travels

through a material. In some materials that have high diffusivity

values, such as copper and aluminum, the heat propagates rapidly, which can be problematic for thermographic NDT analysis

because samples may require a higher speed infrared camera.

(There are several examples of IR NDT of aluminum in aerospace structures, without the use of high speed cameras. For

instance, inspection of the Boeing 737 fuselage and inspection of

military aircraft for corrosion have been performed without the

advantage of high speed cameras.) The ideal materials for thermographic analysis have more moderate diffusivity values and

thus are well suited for NDT because they require standard

infrared camera frame rates and conventional infrared radiation

sources. Polymer composites typically have thermal diffusivity

properties that are conducive to thermographic NDT. Some

materials, such as glass and rubber, have diffusivity values that

cause heat to propagate too slowly, thus making thermographic

NDT an untenable option. [3, 4]

INSPECTION REQUIREMENTS

Advances in TSR have improved the sensitivity and accuracy

of the NDT process and removed the reliance on subjective

interpretation of the images by operators. This decrease in

reliance on qualitative judgments has led to an improvement in

thermographic NDT results. Quantitative analysis of the

results has paved the way for new developments in the field:

thermographic measurement systems. These systems are able to

measure other variables, such as defect depth, wall thickness,

and thermal diffusivity, with an accuracy nearly equivalent to

the methods traditionally used to measure these variables. Such

progress is being recognized in the industry and further efforts

are ongoing to make thermography equipment capable of more

automated functions. As this progress continues, machines

capable of defect detection and internal quality control via

thermographic nondestructive testing will become more common in industrial applications. [4]

Training

As thermography continues to gain traction in the field of nondestructive testing, standards for inspection and operator training

are becoming more common. Currently, it is recommended that

inspection technicians be properly certified (Levels I, II, III).

PRACTICAL CONSIDERATIONS

There are many factors that determine whether thermography is

an appropriate test methodology for a particular application.

Furthermore, it should be determined which mode of thermography offers the best match for the given constraints of a particular application. Thermography is a well-established technique

that can be applied in one of two modes depending on the information or type of testing that is desired. The passive mode of

thermographic NDT is used for predictive and preventive maintenance, and the active mode is most useful when more detailed

information about the internal structure of a test object is

desired. Active thermographic NDT techniques are based on

analyzing a sample’s thermal response to an external excitation.

Passive NDT techniques are employed when the test specimen is

at steady state conditions. The active approach is much more

flexible and is generally characterized by the type of excitation

used on the component being evaluated. [5, 6]

Advantages

There are many factors that influence the use of thermography

over other nondestructive testing methods. Active thermography

is a flexible NDT method that can be applied to a variety of

materials or situations, and it can typically be carried out in a

matter of seconds after setup. Thermography is a noncontact,

noninvasive NDT method that can be used to inspect both

curved and flat surfaces (see figures 6 and 7). Also, thermography can be performed on critical parts where only single-side

access is available. This means that time and resources can be

saved if thermography is used to inspect a part without removing it from the original structure being evaluated. [3, 5, 7]

Disadvantages

Thermography has continually grown in popularity and effectiveness amongst the NDT community, however there are still areas

where another method may be more effective. Unlike other test

AMMTIAC

http://ammtiac.alionscience.com The AMMTIAC Quarterly, Volume 3, Number 2 11

A D V A N C E D M A T E R I A L S , M A N U F A C T U R I N G A N D T E S T I N G

Figure 7. Thermal inspection of

E2-C Hawkeye aircraft. [8]

Figure 6. Thermal inspection of

E2-C Hawkeye propeller. [8]
techsolutions 8

methods, thermography primarily relies on the diffusion of heat

throughout the test object. In this case, the energy injected into

the sample does not travel in a straight, easy-to-model path

throughout the sample as it does in other methods like radiography and ultrasonic testing. Due to this fact, a general guideline

for determining the minimum detectable defect size through

thermography has been developed. This rule states that the implication of the diffusion process on thermographic NDT is that the

size of the smallest detectable discontinuity increases as a function

of its depth below the surface. This rule is known as the aspect

ratio and a variety of guidelines exist for the exact limit of this

ratio. [5] The exact limit that is imposed will vary depending on

the excitation technique, the infrared camera performance, the

sample material, and the nature of the anomaly. This ratio is not

a final determinant of whether thermography can be utilized for

a certain application or test material, but it is an effective guide to

help users and inspectors make an accurate decision. [3, 4, 5]

Since older methods are still being used in thermographic

analysis, the interpretation of the results relies heavily on the

inspector. The results are therefore qualitative and can be inconsistent. [3, 4, 5]

CONCLUSION

The thermographic NDT method has continually improved

through application and technological developments over the

years, and it has become a reliable and commonly used NDT

method. As initial perceptions began to change, thermography

grew from a companion testing method that was considered less

reliable into one of the primary test methods. Thermography no

longer requires the subjective interpretation of results or the

qualitative judgements of the inspector to produce results. New

developments in the field are ongoing as the understanding of

what applications are needed for thermographic NDT continues to advance. Over the past few years the development of

thermographic signal reconstruction and the institution of automated machines into the thermographic NDT process have led

to further improvements and made it easier for the users and

inspectors to obtain useful results. From here, the future of

thermography looks promising as the once secondary NDT

method continues to grow and become more commonly used

for finding a variety of material defects. [4, 5, 7]

ACKNOWLEDGEMENT

Thermal Wave Imaging, Inc. (TWI), contributed many of the

photos and much of the referenced content in this article. In

particular, the author is grateful to Dr. Steven Shepard and

Mr. David Murdock for their extensive support in contributing

to and reviewing this article.

REFERENCES

[1] Forsyth, D., H. Yolken, G. Matzkanin, “TechSolutions 1: A Brief

Introduction to Nondestructive Testing,” AMMTIAC Quarterly, Vol. 1,

No. 2, 2006, pp. 7-10.

[2] Lane, R., “Material EASE: Sensors and Sensing Technologies for

Integrated Vehicle Health Monitoring Systems,” AMPTIAC Quarterly,

Vol. 8, No. 3, 2004, pp. 11-15.

[3] Shepard, S., “Understanding Flash Thermography,” Materials

Evaluation, May 2006, pp. 460-464.

[4] Shepard, S., “Thermography of Composites,” Materials Evaluation,

July 2007, pp. 690-696.

[5] Shepart, S., Y. Hou, T. Ahmed, and J. Lhota, “Reference-Free

Interpretation of Flash Thermography Data,” Insight, Vol. 45, No. 5,

May 2006, pp. 298-301.

[6] Shepard, S., “Flash Thermography The Final Frontier,” A Special

Section to Quality Magazine: NDT Nondestructive Testing Including

Materials Test, April 2007, pp. 10-12.

[7] Hardy, G., “Thermal Inspection,” ASM Metals Handbook, Ninth

Edition, Vol. 17, Nondestructive Evaluation and Quality Control, ASM

International, pp. 396-404.

[8] “Case Study: U.S. Navy – E2-C Hawkeye Propeller Inspection,”

Thermal Wave Imaging, Inc., 2006.

[9] “Case Study: NASA Return to Flight Program,” Thermal Wave

Imaging, Inc., 2006.

12 The AMMTIAC Quarterly, Volume 3, Number 2

Thermography Summary [1, 2, 3, 4, 5, 7, 9]

Discontinuity types • Cracks • Leaks

(e.g. what types the method can detect) • Delaminations • Thermal Profiles

• Poor Electrical Contacts • Adhesive Distribution

• Debonding • Porosity

• Corrosion • Wall / Coating Thickness

Size of discontinuities • Can be determined by aspect ratio

Limitations • Infrared camera performance • Aspect ratio of potential test objects

• Hot-spot method is less quantitative • Excitation technique used

Advantages • Can be used to detect defects • Can be used with limited access to the part

in a variety of materials • Rapid coverage of large areas

• Non-contact NDT method

Inspector training (level and/or availability) • Levels I, II & III

Inspector certification required • Understanding the proper way to interpret results is critically important

Equipment • Properly chosen infrared camera

• Automated systems

• Robotic gantry

Relative cost of inspection • Varies according to camera used and integrated equipment
The Army Research Laboratory (ARL) and Southern Methodist

University (SMU) recently introduced the acquisition of rapid

manufacturing equipment that can successfully build complex

prototypes and functional components using metal powders,

particularly Ti-alloys. The system uses an electron beam as a heat

source and operates in a vacuum environment.

In October of 2007 the US Army Research Office provided

SMU with a grant to develop the Advanced Precision Supply

Parts Manufacturing Program, which would benefit ARL’s

Weapons and Materials Research Directorate. As part of this

grant, SMU, in cooperation with ARL, proposed the acquisition

of the newest generation machines that perform solid free form

fabrication of fully dense components starting with metal powder

as the raw material. The machine acquired is based on a patented

technology called “CAD to Metal”®.

The operating principle of the electron beam melting (EBM)

process is similar to the operating principle of the selective laser

sintering or stereo-lithography process. A solid model of the component is sliced into thin layers. The shape of each layer is used

to control a focused electron beam during the sintering of a thin

layer of metal powder, which is spread across the powder bed. The

sintering process of the layer is done in two steps. First, the top

layer of powder is preheated with an electron beam of lower

power. Second, the layer of powder is melted using the electron

beam at maximum power. Once this sintering process is complete, a new layer of powder is spread over the just sintered layer

in order to continue the building of the 3D structure in a layered

fashion. Currently, a 4 kW electron beam is used as the heat

source, and the entire process is performed in a vacuum.[1]

Compared to laser-beam direct metal deposition, EBM has

many advantages, such as higher energy efficiency; easier manipulation; application on any electrically conductive materials,

including highly reflective ones; and higher quality buildups since

the process is performed in a vacuum. EBM can build solid,

porous or hybrid structures that have applications in different

industries such as aircraft, automotive, marine equipment, and

aerospace. Currently, there are three materials successfully

employed by the EBM system: Ti-6Al-4V, Ti-6Al-4V-ELI and a

cobalt-chrome alloy. There have been reports of successful usage

of other materials such as stainless steel, tool steel, and aluminum

alloys for building components by EBM.[1]

In January of 2008, the EBM apparatus was installed at SMU’s

Research Center for Advanced Manufacturing.[2] This system

has two interchangeable working chambers: one is tall and narrow

and the other is wide and shallow. The build tank of the taller

chamber has dimensions of 7.87 x 7.87 x 13.78 inches. The build

Radovan Kovacevic

Southern Methodist University

Dallas, TX

Pauline Smith

US Army Research Laboratory

Aberdeen Proving Ground, MD

Figure 1. Component with 3D cooling

channels built of Ti-6Al-4V powder by

the electron beam process. (Photo

provided courtesy of ARCAM AB) [1]

Figure 2. A complex housing built of

Ti-6Al-4V powder by the electron beam

process. (Photo provided courtesy of

ARCAM AB) [1]

Figure 3. A skull built of cobalt chromium

alloy by the electron beam process. (Photo

provided courtesy of ARCAM AB) [1]

http://ammtiac.alionscience.com The AMMTIAC Quarterly, Volume 3, Number 2 13
14 The AMMTIAC Quarterly, Volume 3, Number 2

tank of the wider chamber, which has the shape of a cylinder, has

a diameter of 11.81 in. and height of 7.87 in. This new apparatus, which was made available in May of 2007, has a number of

improvements with respect to the older system, such as a 75%

larger buildup tank, new software, better heat model and beam

quality control. The positioning accuracy of the electron beam

has been improved from +/- 0.002 in. to +/- 0.001 in. The thickness of each layer can range from 0.001 to 0.04 in., and the

buildup rate is 0.24 in. per hour. [1]

In order to further enhance the direct metal deposition based

on the electron beam process, it will be necessary to study the

interaction of the electron beam with different types of materials;

develop a procedure for qualifying the mechanical and physical

properties of the fabricated components; increase the deposition

speed; improve the surface quality; and increase the buildup size.

Figures 1 through 7 show a variety of components and structures

that have been built by the EBM machine.

NOTES & REFERENCES

* CAD to Metal is a registered trademark of the Arcam AB Company,

Sweden.

[1] www.arcam.com

[2] www.engr.smu.edu/rcam and www.engr.smu.edu/clam

Dr. Radovan Kovacevic is a Herman Brown Chair Professor of Mechanical engineering and Director of the Research Center for Advanced

Manufacturing and the Center for Laser-aided Manufacturing (www.engr.smu.edu/rcam and www.engr.smu.edu.clam) at Southern

Methodist University, Dallas, TX. He has to his credit seven US patents and he has authored or co-authored five books and more than

420 technical papers. His research has been supported by federal and state agencies and industry in the value of more than $10M.

Dr. Kovacevic is a Fellow of the Society of Manufacturing Engineers, the American Society of Mechanical Engineers, and the American

Welding Society. He is the recipient of a number of prestigious awards for his research and educational achievements.

Ms. Pauline Smith is a research chemist at the US Army Research Laboratory with concentration in coatings development and application.

Her efforts are focused on research activities and programs to develop and formulate enhanced coatings for DoD equipment with a

particular emphasis on US Army tactical vehicles and munitions. Material systems are developed for the direct support of our current armed

forces and the Army-After-Next. Prior to joining ARL in 1999, she worked at AKZO Nobel, as a Coatings Formulator and as an Analytical

Chemist. Ms. Smith has 17 years of diverse experience in the coatings industry.

Figure 4. 3D component built by the

electron beam process. (Photo provided

courtesy of ARCAM AB)

Figure 5. 3D component built of Ti-6Al-4V by the electron beam

process.

Figure 6. Lattice structures built of Ti-6Al-4V by the electron beam

process.

Figure 7. Application of electron beam system in building medical

models and implants.

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