Patent Publication Number: US-2005133527-A1

Title: Powder feeder for material deposition systems

Description:
CROSS-REFERENCES TO RELATED PATENT APPLICATIONS &amp; CLAIMS FOR PRIORITY  
      This application is a continuation application of U.S. patent application Ser. No. 10/128,658, entitled “Forming Structures from CAD Solid Models”, filed on Apr. 22, 2002, which is a continuation-in-part application of U.S. patent application Ser. No. 09/568,207, now U.S. Pat. No. 6,391,251, entitled Forming Structures from CAD Solid Models, filed on May 9, 2000, which claims the benefit of the filing of U.S. Provisional Patent Application Ser. No. 60/143,142, entitled “Manufacturable Geometries for Thermal Management of Complex Three-Dimensional Shapes”, filed on Jul. 7, 1999. The specifications and claims of all of the above references are hereby incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION  
      The present invention relates to the field of direct material deposition processes which allow complex structures to be fabricated efficiently in small lots to meet stringent requirements of a rapidly changing manufacturing environment. More particularly, the invention pertains to the fabrication of three-dimensional metal parts directly from a computer-aided design (CAD) electronic “solid” model. The invention also provides methods which use existing industry-standard computer file formats to create unique material structures including those having thermal characteristics embedded within them. The invention addresses methods to control direct material deposition processes to achieve a net-shaped or near net-shaped article, and to fabricate metal articles having exceptional material properties and dimensional repeatability.  
     BACKGROUND OF THE INVENTION  
      Manufacturing techniques or technologies generally known as “layered manufacturing” have emerged over the last decade. For metals, the usual shaping process forms a part by removing metal from a solid bar or ingot until the final shape is achieved. With the new technique, parts are made by building them up on a layer-by-layer basis. This is essentially the reverse of conventional machining. According to the paper appearing at the Internet site of Helsinki University of Technology, the first commercial process was presented in 1987. The process then was very inaccurate, and the choice of materials was limited. The parts were considered, therefore, prototypes and the process was called rapid prototyping technology (RPT). The prior art has advanced, however, to a point where it has been favorably compared too conventionally numerically controlled (NC) milling techniques. Considerable savings in time, and therefore cost, have been achieved over conventional machining methods. Moreover, there is a potential for making very complex parts of either solid, hollow or latticed construction.  
      Stereolithography technique (SLT), sometimes known as solid freeform fabrication (SFF), is one example of several techniques used to fabricate three-dimensional objects. This process is described in the Helsinki University of Technology paper. A support platform, capable of moving up and down is located at a distance below the surface of a liquid photo polymer. The distance is equal to the thickness of a first layer of a part to be fabricated. A laser is focused on the surface of the liquid and scanned over the surface following the contours of a slice taken through a model of the part. When exposed to the laser beam, the photo polymer solidifies or is cured. The platform is moved downwards the distance of another slice thickness and a subsequent layer is produced analogously. The steps are repeated until the layers, which bind to each other, form the desired object. A He—Cd laser may be used to cure the liquid polymer. The paper also describes a process of “selective laser sintering.” Instead of a liquid polymer, powders of different materials are spread over a platform by a roller. A laser sinters selected areas causing the particles to melt and solidify. In sintering, there are two phase transitions, unlike the liquid polymer technique in which the material undergoes but one phase transition: from solid to liquid and again to solid. Materials used in this process included plastics, wax metals and coated ceramics. A number of Patents and other disclosures have preceded and followed these processes, including the following:  
      U.S. Pat. No. 4,323,756, issued on Apr. 6, 1982 to Clyde O. Brown, et al., entitled Method for Fabricating Articles by Sequential Layer Deposition, discloses a method for the production of bulk rapidly solidified metallic objects of near-net shape, by depositing multiple thin layers of feedstock using an energy beam to fuse each layer onto a substrate. The feedstock may be in the form of metal powder or wire. A net shaped or near-net shaped article is one which approximates all of the desired features of its contemplated design so that little or no finishing work is required.  
      In his U.S. Pat. No. 4,724,299, dated Feb. 9, 1988, Albert W. Hammeke describes a laser spray nozzle in which a beam passageway between the end portions permits a laser beam to pass through. A housing surrounds a second end portion and forms an annular passage, coaxial with the beam passageway. A cladding powder supply system is connected with the annular passage so that the powder exits the coaxial opening with the beam. The laser beam melts the powder which is deposited on a target substrate. The powder distribution system is contained within the nozzle assembly.  
      A laser spray nozzle assembly is a part of the Axial Flow Laser Plasma Spraying apparatus disclosed by Eric J. Whitney et al. in their August 1991 U.S. Pat. No. 5,043,548. The apparatus for depositing a feed material onto a substrate, has a plasma confinement chamber into which a laser beam is focused, the focal point being at a distance sufficiently far from the substrate that the substrate, is not melted. Finely divided feed material in a carrier gas flow is fed axially into the confinement chamber along the direction of the laser beam and melted into the plasma formed in the interaction of the laser beam, the feed material and the gas at the focal point. The feed material is then directed to deposit onto the substrate while the plasma energy is largely confined within the apparatus by the confinement chamber and constriction of the flow path upstream of the chamber.  
      A Rapid Prototyping System is disclosed by Joshua E. Rabinovich in U.S. Pat. No. 5,578,227, issued Nov. 26, 1996. The system involves a model making method and apparatus which projects a laser beam, circular polarizes the beam and directs the circular polarized beam for fusing a rectangular wire to a substrate or a previously fused wire on a target stage. The disclosure is differentiated by fusing the deposited feedstock to bond to a previously deposited layer without substantially altering the cross-section of the newly deposited material.  
      Such a deposition process would seem to have substantial problems of warping and distorting the deposited layers because of incomplete melting of feedstock material. Unlike Rabinovich&#39;s disclosed process, a powder deposition completely consumes the feedstock material in the three-dimensional net shape. The powder&#39;s cross-section and material properties are significantly altered. Rabinovitch does not disclose how the properties of the deposited material are controlled in his invention.  
      U.S. Pat. No. 5,697,046, dated Dec. 9, 1997 and entitled Composite Cermet Articles and Method of Making was issued to Edward V. Conley. It discloses methods for making and using and articles comprising ferromagnetic cermets, preferably carbides and more preferably tungsten carbide having at least two regions exhibiting at least one property that differs. The cermets are manufactured by juxtaposing and densifying at least two powder blends having different properties. The methods described are very specific to cermets and do not employ solid models and automated processes.  
      U.S. Pat. No. 5,705,117 dated Jan. 6, 1998 discloses a Method of Combining Metal and Ceramic Inserts Into Stereolithography Components. Kurt Francis O&#39;Connor et al. describe a stereolithography process for developing a prototype part in which inserts of non-photo polymer material are included in the resulting part so as to develop a functioning prototype part. In order to allow the inserts to be placed within the developing prototype part, a series of STL files are defined for forming the part in individual sections. The method is very specific to metal-ceramic composite structures for PC boards. It is not a direct fabrication method for three-dimensional objects with graded or multiple material structures.  
      Direct fabrication of three-dimensional metal parts by irradiating a thin layer of metal powder mixture is described in U.S. Pat. No. 5,393,613, entitled Composition for Three-Dimensional Metal fabrication Using a Laser, and issued Feb. 28, 1995. Colin A. MacKay uses a temperature equalization and unification vehicle in the mixture which is melted by a laser, selectively applied to form a solid metal film. The vehicle protects the molten metal from oxidation. The metal powder can contain an elemental metal or several metals. The material has a lower melting temperature because of the vehicle, which is essentially a flux. The method does not create structures of gradient material.  
      U.S. Pat. No. 5,707,715, issued to L. Pierre deRochemont et al. on Jan. 13, 1998, presents a disclosure of metal-ceramic composite comprising a metal member bonded to a ceramic oxide member through a covalent bond formed at temperatures less than 880 degrees Centigrade. Metal-ceramic composites are also described that are so constructed to control internal stress or increase crack resistance within the ceramic member under applied thermal or mechanical loads. The disclosure does not reveal a direct fabrication method for three-dimensional objects with graded or multiple material structures.  
      U.S. Pat. No. 5,126,102, entitled Fabricating Method of Composite Material, was granted to Masashi Takahashi on Jun. 30, 1992, and describes a method of preparing a composite material, excellent in joint strength and heat conductivity. More specifically, it describes a method of preparing a composite material composed of high melting temperature tungsten (W) material and low melting temperature copper (Cu) material by forming pores in the tungsten to obtain a substrate with distributed porosity. The method forms a high-porosity surface in at least one region of the substrate, the porosity gradually decreasing outward from the region. A second step impregnates the tungsten material with the copper material in the porous surface forming a gradient material of tungsten and copper. The patent describes the advantages of gradient materials, however, it does not discuss the use of solid models to achieve the shape of the gradient article. Direct material deposition processes produce three-dimensional parts by sequential layer deposition of feedstock material in powder or wire form.  
      Robert A. Sterett et al., in their aptly named U.S. Pat. No. 5,746,844, issued on May 5, 1998, disclose a Method and Apparatus for Creating a Free-Form Three-Dimensional Article Using A Layer-By-Layer Deposition of molten Metal and Using Stress-Reducing Annealing Process On the Deposited Metal. A supply of substantially uniform droplets of desired material having a positive or negative charge, is focused into a narrow stream through an alignment means which repels each droplet toward an axis through the alignment means. The droplets are deposited in a predetermined pattern at a predetermined rate onto a target to form the three-dimensional article without use of a mold of the shape of the article. The disclosure reveals means for reducing stress by annealing portions of the deposited droplets which newly form a surface of the 3-D article. Melting of the metal is not done by laser and molten metal. Metal powder is carried from a liquid supply to the target surface. The invention produces “fully dense” article of one metal or an alloy material having uniform density, no voids and no porosity. The method allows creation of part overhangs without using supports, by relying on the surface tension properties of the deposition metal.  
      U.S. Pat. No. 5,837,960 to Gary K. Lewis, of Los Alamos National Laboratory, et al. was filed on Nov. 30, 1995 and issued on Nov. 17, 1998. Its title is Laser Production of Articles from Powders. A method and apparatus are disclosed for forming articles from materials in particulate form in which the materials are melted by a laser beam and deposited at points along a tool path to form an article of desired shape and dimensions. Preferably, the tool path and other parameters of the deposition process are established using computer-aided design (CAD) and computer-aided manufacturing (CAM) techniques. A controller consisting of a digital computer directs movement of a deposition zone along the tool path and provides control signals to adjust the apparatus functions, such as the speed at which a deposition head which delivers the laser beam and powder to the deposition zone moves along the tool path. The article is designed using a commercially available CAD program to create a design file. A “cutter location file” (CL) is created from the design file and an adapted, commercially available CAM program. User-defined functions are established for creating object features in the adapted CAM program. The functions are created by passing an “electronic plane” through the object feature. A planar figure created in the first plane at the intersection with the feature is a first portion of the tool path. A second plane is passed through the feature parallel to the first plane. The second plane defines a second tool path. The end of the tool path in the first plane is joined to the beginning of the tool path in the second plane by a movement command. The process is continued until the tool path required to make the feature is complete.  
      Lewis et al. describe certain methods of preheating an article support (substrate) to overcome the fact that without it, an article support will be cold when the deposition is started in comparison to the material on which deposition is later done in the fabrication process. Computer modeling of heat flow into, through and out of an article and the data generated from such modeling imported into the CAM program is suggested. The fabrication of articles of two different materials is addressed by forming a joint between dissimilar metals by changing powder compositions as the joint is fabricated. As an example, one could introduce a third material as an interlayer between mild steel and 304 stainless steel. The interlayer material might be a Ni—Cr—Mo alloy such as Hastelloy S.  
      U.S. Pat. No. 5,993,554 to David M. Keicher et al., dated Nov. 30, 1999 and entitled Multiple Beams and Nozzles to Increase Deposition Rate, describes an apparatus and method to exploit desirable material and process characteristics provided by a lower power laser material deposition system. The invention overcomes the lower material deposition rate imposed by the same process. An application of the invention is direct fabrication of functional, solid objects from a CAD solid model. A software interpreter electronically slices the CAD model into thin horizontal layers that are subsequently used to drive the deposition apparatus. A single laser beam outlines the features of the solid object and a series of equally spaced laser beams quickly fill the featureless regions. Using a lower power laser provides the ability to create a part that is very accurate, with material properties that meet or exceed that of a conventionally processed and annealed specimen of similar composition. At the same time, using multiple laser beams to fill in featureless areas allows the fabrication process time to be significantly reduced.  
      In an article entitled The Direct Metal-Deposition of H13 Tool Steel for 3-D Components by J. Mazurnder et al., the authors state that the rapid prototyping process has reached the stage of rapid manufacturing via direct metal deposition (DMD) technique. Further, the DMD process is capable of producing three-dimensional components from many of the commercial alloys of choice. H13 is a material of choice for the tool and die industry. The paper reviews the state of the art of DMD and describes the microstructure and mechanical properties of H13 alloy deposited by DMD.  
      The problem of providing a method and apparatus for optimum control of fabrication of articles having a fully dense, complex shape, made from gradient or compound materials from a CAD solid model, is a major challenge to the manufacturing industry. Creating complex objects with desirable material properties, cheaply, accurately and rapidly has been a continuing problem for designers. Producing such objects in high-strength stainless steel and nickel-based super alloys, tool steels, copper and titanium has been even more difficult and costly. Having the ability to use qualified materials with significantly increased strength and ductility will provide manufacturers with exciting opportunities. Solving these problems would constitute a major technological advance and would satisfy a long felt need in commercial manufacturing.  
     SUMMARY OF THE INVENTION  
      The present invention pertains generally to a class of material deposition processes that use a laser to heat and, subsequently, fuse powder materials into solid layers. Since these layers can be deposited in sequential fashion to ultimately form a solid object, the ability to alter the material properties in a very localized fashion has far reaching implications.  
      The present invention comprises apparatus and method for fabrication of metallic hardware with exceptional material properties and good dimensional repeatability. The invention provides a method for controlling material composition, and thus material characteristics, within a structure made from a plurality of materials, directly from computer renderings of solid models of the desired component. Both industry-accepted stereolithography (STL) file format as well as solid model file format are usable.  
      One embodiment of the invention is used to form embedded features in a three-dimensional structure. A plurality of separate material feedstock are fed into a directed material deposition (DMD) process which places a line of molten material onto a substrate. The depositions are repeated in a layer-by-layer pattern, defined by solid models which describe the structure, to create an article having complex geometric details. The bulk properties of the deposition are controlled by adjusting the ratio of laser irradiance to laser velocity along the line of deposition.  
      In addition to external contours, the solid-model computer files describe regions of each separate material, regions of a composite of the materials and regions of voids in each layer or “slice.” The depositions are repeated in each of the “slices” of the solid models to create the geometric details within the three-dimensional structure.  
      Heating the substrate and the deposition produces parts with accurate dimensions by eliminating warping of the substrate and deposition. A prescribed temperature profile is used for processing tempered material. A temperature profile for heat treating may be used to enhance the mechanical properties of the part by ensuring the correct material microstructure during processing.  
      Although the prior use of DMD processes has produced solid structures, the use of this technology to embed features for thermal management of solid structures is novel. Embedding voids and/or composite material regions, enables thermal management engineering techniques for solid structures that are not available through conventional processing techniques. In one embodiment of the present invention, a method is provided to construct a solid structure with integral means to control its thermal properties.  
      Active thermal control is provided by forming passages and chambers for a coolant medium. The cross-section area and length of individual embedded structures are made approximately equal to provide uniform flow characteristics and pressure in the three-dimensional structure. Passive thermal control is provided by embedding materials having diverse thermal indexes.  
      Another embodiment of the present invention provides methods to locally control the thermal history of a three dimensional structure. Thermal history is the temperature variation in the part as a function of time. A part made with high thermal conductivity material in one region and a low thermal conductivity material in another region, will have a different thermal variation with time in each region.  
      In a further embodiment of the present invention, high-efficiency heat transfer is obtained within a three dimensional structure by incorporating regions of other materials within the article. For example, in parts having varying cross-sections, heating and cooling in selected regions is controlled to prevent thermal stresses.  
      In yet another embodiment of the present invention, three dimensional components are formed in which thermal characteristics such as heating and cooling rates are engineered into the component.  
      Embedding multi-material structures within a normally solid component produces articles with diverse mechanical properties. Articles having complex internal and external contours such as heat exchangers and turbine blades are easily produced with the methods and apparatus disclosed.  
      To enhance the deposition process for manufacture of three-dimensional, multi-material structures with interior cavities either hollow or filled with diverse material, new apparatus, methods of deposition and material delivery are disclosed. These include: 
          1. Engineering properties such as tensile strength, toughness, ductility, etc. into the material layers by reference to a laser-exposure factor (E) which includes variables of laser power (p), relative velocity of the deposition (v) and material constants (a).     2. A fast-acting diverter valve for regulating feedstock flow allows precision depositions of gradient materials. The diverter valve controls the flow of a stream of a carrier gas and powder material to the deposition head. The valve comprises one diverter for a stream of gas only and another for a stream of gas and powder. The diverters are proportionately controlled so that the total volumetric flow rate of the powder and gas is constant, but the mass flow rate of powder to the deposition head can be quickly varied from no powder to the maximum available. Waste gas with powder is re-circulated and waste gas is reclaimed.     3. A self-contained, volumetric, low-friction powder feed unit which allows a user to use extremely low flow rates with a variety of powder materials; the powder feeder design is a marked improvement over current disk-style powder feeders in which the disk typically is buried in powder. In the present invention, powder flow from a reservoir to a transfer chamber is limited by the angle of repose of the powder feedstock, preventing the disk from being overwhelmed and clogged with powder. The present invention is insensitive to variations in flow rate of the gas which transports the powder to the deposition head. The spacing between the feed disk and the wipers which remove powder from the disk can be greater than in prior art designs without losing control of powder metering. This promotes much less wear on the wipers and substantially improves the life of the powder feed unit.     4. A multi-axis deposition head, including the powder delivery system and optical fiber, laser beam delivery system, moveable about a plurality of translational and rotational axes; the relative directions of the powder stream in the deposition process ( 123 ) being coordinated with a control computer ( 129 ) in a plurality of coordinate axes (x, y, z, u, v).     5. “Smart” substrates which are useful for construction of articles with internal spaces, unreachable from the surface, but serve as a starting point for conventional shaping methods.     6. Protection for the fiber optic which delivers a laser beam to the work piece to prevent catastrophic failure of the fiber because of beam reflections from the deposition surface, using a folding mirror, offset from 45 degrees by a small angle, to image a reflected laser beam at a distance from the fiber optic face, and water cooling of the fiber optic face.     7. A laser beam shutter with a liquid-cooled beam “dump” to aid testing and adjustment of the fiber optic, laser beam delivery system.     8. Using the surface tension property of melted materials to creating structures having unsupported overhanging edges.     9. Using a rotated plane of deposition or rotating a multi-axis deposition head to build unsupported overhanging edges.     10. Particle beam focusing to reduce material waste.        

      An appreciation of other aims and objectives of the present invention may be achieved by studying the following description of preferred and alternate embodiments, and by referring to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematic representation of the prior art, Laser Engineered Net Shaping (LENS.™.) process showing a vertically-movable laser beam, powder delivery nozzle and a substrate situated upon X-Y positioning stages.  
       FIG. 2  is a plan view of a sample object used in dimensional repeatability of metal deposition experiments.  
       FIG. 3  is a chart showing deposition layer thickness as a function of the volumetric exposure (laser irradiance/velocity) of the metal deposition measured in several samples.  
       FIG. 4  depicts a graph of surface finish as a function of powder mesh size and laser power used in creating deposited metal samples.  
       FIG. 4   a  is a graph that demonstrates how material strength of 316 stainless steel varies with exposure parameter E.  
       FIG. 5  is an elevation-view comparison of deformation of three substrate samples exposed to metal depositions using laser beam melting, the most deformed substrate having no preheating, the least deformed substrate preheated to 200 degrees centigrade and the intermediate substrate preheated to 100 degrees centigrade.  
       FIG. 6  schematically depicts in side-view, a metal deposition apparatus according to the present invention in which heat is applied to a substrate by a radiant source, the heating being measured over time by a monitoring source.  
       FIG. 7  is a side view of the metal deposition apparatus depicting substrate heating by a platen having internal heating elements, temperature monitoring being accomplished by sensors in the platen and on the substrate.  
       FIG. 8  presents a graph of one profile of heating a substrate during deposition. Controlling the temperature to a profile insures the correct material microstructure during processing.  
       FIG. 9  is a side-elevation view comparing two substrates, showing warping in the upper one which was not heated during deposition processing and no warping of the lower one which was heated during deposition processing.  
       FIG. 10  shows yet another profile of thermal heating applied to a part during fabrication by directed material deposition, the added steps being applied to further improve the properties of the deposited material.  
       FIG. 11  is a schematic view showing the components of a directed material deposition system for fabricating objects from two different metal powders, according to the present invention.  
       FIG. 12  is a perspective sketch illustrating the concept of capturing a solid model made of one material within a solid model made of a different material by means of the present invention.  
       FIG. 13  is a plan view representing a plane section A-A taken through the solid models of  FIG. 12 , revealing the outer solid model of one material, the inner solid model of a second material and the region composed of both materials, graded from the first material to the second material. Composite cross-hatching illustrates the intersecting solid models and the region composed of two or more materials.  
       FIG. 14  is a plan view of a thin slice of the solid models at section A-A wherein a solid model representing a region of a first material is combined with a solid model representing a region of a second material to create a central core of one material, an outer region of the first material and an intermediate region of graded, composite material. To aid visualization of the process which combines two solid models, the solid model representing the region containing the first material is exploded from the solid model of the region containing the second material.  
       FIG. 14   a  is a cutaway view of a fast-acting diverter valve used to regulate powder flow to the work area.  
       FIG. 14   b  is a schematic diagram of the operation of the diverter valve of  FIG. 14   a , illustrating how the volumetric flow of powder in a gas carrier is maintained constant.  
       FIG. 15  is a perspective view of a low-rate powder feed unit.  
       FIG. 16  is a perspective view of the low-friction, volumetric powder feed unit seen along view B-B of  FIG. 15 .  
       FIG. 16   a  is a perspective view of the powder feed disk and wiper assembly, alone, revealing the feed holes, disposed circumferentially in the disk, which pick up powder from the supply pile.  
       FIG. 16   b  is a graph of average flow rate for 316 stainless steel powder versus feed disk RPM for three test conditions, showing the nearly linear performance of the powder feeder.  
       FIG. 16   c  shows an elevation view of a directed material deposition, revealing “buttering” layers of a first and second transitional material, deposited between two dissimilar metals to provide metallurgical compatibility between them.  
       FIG. 17  is a perspective view of a cross-sectioned mold insert with a mold cavity, showing the detail of the conformal cooling passages integrated into the mold during manufacture using DMD methods.  
       FIG. 18  is a perspective view of the whole mold insert showing the internal geometries as hidden lines, with inlet and outlet ports for coolant media, fabricated using invention methods disclosed herein.  
       FIG. 19  is a cross sectional view of a solid, rectangular DMD article, showing the internal cooling passages and an inlet and exit made integral with the article.  
       FIG. 20  is a cross-sectional view of a cylindrical article fabricated by the deposition method of the present invention, illustrating integral cooling passages.  
       FIG. 21  is a cross-sectional view of a cylindrical object with complex geometries of separate cooling passages fabricated into the component, made by DMD.  
       FIG. 22  is a perspective view of a solid, curved object made by directed material deposition, having the cooling passages following the contour of the outer shape.  
       FIG. 23  is a perspective view of an airfoil shaped DMD article, such as a turbine blade, with cooling channels fabricated integrally within the airfoil.  
       FIG. 24  represents a perspective view of a partially constructed “smart” substrate which can be made by DMD methods or by conventional machining.  
       FIG. 25  is another view of the “smart” substrate in process shown in  FIG. 24  to which has been added additional deposited material.  
       FIG. 26  is a perspective view of the finished “smart substrate” article shown in process in  FIGS. 24 and 25 .  
       FIG. 26   a  is a perspective view of a latticed “smart” substrate, depicting tubular cooling channels which support the substrate bearing surfaces.  
       FIG. 26   b  is a perspective view of a plastic injection mold having embedded cooling channels of circular cross-section.  
       FIG. 26   c  is view C-C of the plastic injection mold revealed in  FIG. 26   b.    
       FIG. 27  reveals a side-view schematic of a method of manufacturing overhanging structures using 3-axis positioning of the deposition head in respect of the work piece.  
       FIG. 28  is a closer look at view B of  FIG. 27 , showing how surface tension aids in maintaining the deposited material bead at the edge of a part.  
       FIG. 28   a  is another look at view B of  FIG. 27  illustrating how additional beads of material may be attached to an existing overhanging surface. Additional deposition contours are added serially and Δx is kept small with respect to the bead diameter.  
       FIG. 29  shows a method of making an overhanging structure by rotating the work piece relatively in respect of the deposition head so the focused laser beam is parallel to a tangent to the surface being built. The deposition head can be rotated in multiple axes to implement the relative movement.  
       FIG. 30  is an enlarged view C of  FIG. 29  showing the relationship of the laser beam-powder interaction area to the edge of the part which is being built.  
       FIG. 31  is a side-view schematic of the work piece which is the target of the deposition, showing previously deposited material beads at the edges of the layer to be constructed which act as dams to contain fill material.  
       FIG. 32  is a side-view schematic of the deposition head using a standard fill process for filling in the deposition layer behind material beads which have been placed at the edges as dams, as depicted in  FIG. 31 .  
       FIG. 33  is a schematic diagram of an optical fiber laser beam delivery system.  
       FIG. 34  is a perspective view of a laser beam shutter assembly, having a liquid-cooled laser beam “dump.” 
       FIG. 35  is a perspective view of the laser beam shutter assembly depicted in  FIG. 34  with a top cooling plate removed to reveal the laser beam “dump” chamber and reflecting beam absorbers.  
      FIGS.  36  to  40  are side and front elevations and perspective views of a multi-axis deposition head. The head includes an integral powder delivery system.  
       FIG. 40   a  presents a perspective view of the multi-axis deposition head, illustrating deposition of three-dimensional structure having a curved surface. In this example, the head is positioned in three translational and two rotational axes.  
       FIG. 41  depicts one of a plurality of powder delivery nozzles of the prior art, which are disposed in a deposition head.  
       FIG. 42  shows an improved powder delivery nozzle used in the present invention.  
       FIG. 43  reveals a still further improved powder delivery nozzle alternatively used in the present invention.  
       FIG. 44  depicts schematically the operation of a coaxial gas flow sheath which acts as a boundary layer barrier to the entrained powder stream in a powder delivery nozzle. The velocity of the coaxial gas flow and the entrained powder stream are approximately the same.  
       FIG. 45  is another schematic representation of powder delivery nozzle having a coaxial gas sheath and entrained powder stream. In this illustration, the velocity of the coaxial gas stream is much greater than that of the entrained powder stream resulting in mixing of the two streams. Large de-focusing of the powder stream causes powder to be scattered widely over the deposition surface. 
    
    
     A DETAILED DESCRIPTION OF PREFERRED &amp; ALTERNATIVE EMBODIMENTS  
      1. Forming Structures Directly from a CAD Solid Model  
      The present invention comprises apparatus and methods for fabricating metallic hardware with exceptional material properties and good dimensional repeatability. The term “net shape” refers to an article fabricated to the approximate desired size and features, solid or latticed, by a process which requires little or no machining. The prior art in this technology has focused on methods to enable the deposition process. However, little work has been done on how to best control the process to achieve a desired outcome in a solid structure.  
      The present invention uses the laser-based process to provide users with the ability to create a net shape or near-net shape, fully dense, metallic object directly from a computer aided design (CAD) solid model. The shapes are created a layer at time. In this Specification and in the claims that follow, the invention is referred to as a directed material deposition (DMD) process. This DMD process has the potential to revolutionize the approach to designing hardware. Presently, designers must often make compromises in materials selections, and, as a result, achieve a less than optimum solution to a problem. The layer approach used in the material deposition process provides the freedom to vary material composition within a single structure. This ability enables components to be engineered a layer at a time to satisfy conflicting material requirements. Currently, the process is capable of producing metallic objects using stainless steel and nickel-based super alloys that have nearly a two to three fold increase in strength and with improved ductility in comparison to conventionally processed materials. Other materials that have been processed include tool steels, copper and titanium.  
      The material deposition process of the present invention is functionally similar to many of the existing rapid prototyping technology (RPT) methods in that it utilizes a computer rendition of a solid model of an article to build an object a layer at a time. Conventional stereolithography (STL) file format may be used. The file is sliced electronically into a series of layers that are subsequently used to generate the motion of the apparatus which deposits each layer of material. The layers are deposited in a sequential fashion to build an entire part.  
      A schematic representation of the prior art, laser engineered net shaping process apparatus, is shown in  FIG. 1 . To begin the fabrication process, a metal substrate  19  is used as a base onto which new material  15  is deposited. A high power laser  12  is focused by lens  13  onto the substrate  19  to create a molten puddle  17  and metal powder  20  is injected into the puddle  17 . The substrate  19  is moved relative to the laser beam  12  in a controlled fashion to deposit thin metallic lines of a finite width and height. A stage  16  provides relative motion between the work piece and the deposition head  11  in orthogonal directions and the focusing lens  13  is moved in the z-axis as the material grows in height. Lines of material are deposited side by side in the desired regions to create the pattern for each layer. In this fashion, each layer is built up line by line and the entire object evolves, layer by layer.  
      Testing was done to prove that a prior technology called LENS.™. processing (“laser engineered net shaping,” a Trade Mark used by the Sandia National Laboratories) was viable for direct fabrication applications. Mechanical testing data from tensile specimens prepared in 316 stainless steel and Inconel 625 are given in Table 1:  
               TABLE 1                          Mechanical test data from LENS ™ manufactured tensile specimens.                             Plane Orientation   Ultimate   Yield           with Respect to   Strength   Strength   Elongation       Tensile Direction (mesh size)   (MPa)   (MPa)   (% in 2.54 cm)               316 SS Perpendicular (−325)   0.793   0.448   66       316 SS Perpendicular   0.793   0.448   51       316 SS Parallel (−325)   0.807   0.593   33       316 SS Anneal bar (Standard)   0.586   0.241   50       625 Parallel (100/325)   0.931   0.634   38       625 Perpendicular (100/325)   0.931   0.517   37                  
 
      The ultimate tensile strength and yield strengths of the DMD samples are given in mega-Pascals (Mpa). As can be seen from these data, the specimens produced using the metal deposition process exhibited very good material properties and, in fact, in all cases the measured yield strengths of these samples were significantly better than typical annealed wrought material. Additionally, the ductility of these specimens was as good or better than the annealed wrought material with only one exception. This improvement in material properties occurred for both the 316 stainless steel and Inconel 625 alloys. Transmission electron microscopy analysis of the 316 stainless steel specimens has shown that the grain size within the DMD fabricated structures is on the order of five to ten micrometers (μm) whereas the grain size for the annealed  316  stainless steel is typically around 60 μm. This difference in grain size is believed to be the primary cause of the improved material properties for the DMD fabricated structures. In addition, the simultaneous increase in strength and ductility would suggest that although there is undoubtedly residual stress within the DMD fabricated structures, it is not sufficiently large to result in degraded material properties.  
      Another problem for many RPT processes is the inability to produce accurate parts directly from a CAD solid model. Studies were performed that characterize the DMD process in this area as well. The component geometry used in these studies is shown in  FIG. 2 . The part  28  shown in  FIG. 2  represents a simple half-mold that was fabricated for molding plastic. Measurements made of several areas of this part  28  are included in Table 2.  
               TABLE 2                       Measured physical dimensions of mold halves made using LENS ™       process along with statistical results for error and repeatability.                  Measurements of Features for NSF Phase I SBIR Parts                         Thunderbird Dimensions (mm)                             Outside Dimensions (mm)   Part                                             Part Number   Length   Width   Number   Wing Span   Tail-to-Head               1   76.619   45.009   1   14.681   14.478       2   76.670   45.034   2   14.732   14.529       3   76.645   45.034   3   14.656   14.427       Avg. Value   76.645   45.027       14.68   914.478       Std.   0.025   0.015       0.038   0.051       Deviation                         Alignment Hole Dimensions                                 Upper Right Hole   Upper Left Hole   Lower Center Hole                                         Part Number   X Dim.   Y Dim.   X Dim.   Y Dim.   X Dim.   Y Dim.               1   4.826   4.877   4.890   5.004   3.543   3.607       2   4.826   4.826   4.953   4.902   3.632   3.645       3   4.724   4.686   4.775   4.724   3.505   3.556       Avg. Value   4.793   4.796   4.872   4.877   3.607   3.602       Std. Dev.   0.058   0.099   0.089   0.142   0.066   0.046                  
 
      In Table 2, the standard deviation of measurements over several parts is less than 0.142 mm, suggesting that dimensional repeatability of the DMD process is very good in the deposition plane. Modification of control software to account for the finite laser beam width allows this process to very accurately produce parts. A process capable of fabricating hardware within ±0.127 mm. directly, in metal, will satisfy many current needs for direct fabrication applications. It is reasonable to expect these numbers to approach machine tool accuracy. At this time the dimensional repeatability of the DMD process in the growth or vertical direction is approximately ±0.381 mm, which is not as good as in the deposition plane.  
      DMD technology is clearly valuable for tooling applications. This process holds the promise of significantly impacting many other manufacturing areas. Although work to date has focused on producing fully dense metallic structures, modification of existing process parameters allows porous structures to be produced. Both step-function and gradient-transition interface characteristics between differing materials is described below.  
      Impacted immediately by the DMD technology, are applications where high strength-to-weight ratio materials are required. For many applications, a tenuous qualification process must be performed prior to substitution of one material for a second material. Even after qualification, designers are often reluctant to make the transition to a new material. Using DMD technology, composite materials can easily be fabricated for testing and evaluation.  
      In developing the DMD process, statistical data from experiments have been used extensively by the inventors. These experiments have caused controlling relationships between process variables and response variables to be identified and defined. From the experimental results, response surface models were developed to optimize the process. One critical relationship identified through these experiments was the deposition layer thickness as a function of certain process parameters. Using the deposition layer thickness as a response variable, both laser irradiance and the velocity of the deposition were identified as key process parameters.  
       FIG. 3  shows a graph  30  of deposition layer thickness  32  in a z-plane vs. laser irradiance (J/sec-cm 2 ) divided by the deposition velocity in an x-y plane (cm/sec)  34 . Inspection of this graph  30 , shows that the deposition layer thickness  32  varies approximately linearly with J/cm 3    34 .  
      The relationship of surface roughness  42  with powder particle size  44  is displayed in the chart  40 , shown as  FIG. 4 . For this set of experiments, the average roughness (am) of the surface finish of the deposited material was measured as a response variable while a variety of process variables including laser power, particle size, particle size distribution, etc. were considered as process variables. The chart  40  shows that at a given laser power, the surface finish roughness  42  is a function of the particle size  44 , as one might expect. Closer analysis of the graph  40  shows that there is a strong functional dependence of the surface finish on laser power. In fact, a statistical analysis indicates a power level of approximately 350 watts achieves the best finish independent of the particle size. These relationships have been used for DMD process optimization in the present invention.  
      To develop a single system to produce finished parts directly from a CAD solid model, other laser techniques have been evaluated to enhance the DMD process. As an example, laser glazing of a previously deposited layer has yielded significant improvements in surface finish. The results suggest that this method can be applied to achieve the surface finish required for tooling and other precision applications. The measured surface finish for laser glazing tests is given in Table 3:  
               TABLE 3                          Measured surface finish for laser-remelted directed material deposition                             Surface Condition   Surface Finish (am)                                         As Deposited   10.97           Remelt Condition 1   2.46           Remelt Condition 2   1.88                      
 
      The measured surface finish for the DMD fabricated part without additional processing is 10.97 am. Applying different processing conditions for the second and third sample demonstrates that surface finish can be dramatically improved using a laser glazing technique. In fact, without significant optimization, a surface finish of approximately 1.88 am was obtained. Laser glazing is programmable into the control files, so that the process is not interrupted by removing a partially completed article from the work flow.  
      1a. Using a Finished Part as a Substrate  
      In another embodiment of the invention, the substrate may be a generally or substantially finished part which requires another finish, feature or modification. In one specific application, this method may be employed to seal up cavities. In another application, this additional deposition step may be used to provide a hard surface on top of a softer material. In general, this extra deposition can offer a high value-added manufacturing process.  
      1b. Finished Part is Segmented into Different Features  
      In another embodiment of the invention, the component being fabricated is segmented into different features that are built in a sequential fashion by determining the optimum build direction for each segment prior to building the complete component.  
      2. Controlling the Microstructure of Materials Formed by Directed Material Deposition  
      Referring again to  FIG. 1 , it shows the directed material deposition apparatus  10  of the prior art. A collimated laser beam  12  is focused onto a substrate  19  and powdered material  14  is then injected into the deposition spot. The powder  14  streams come into the deposition area  20  and are melted by the laser beam which is focused by lens  13 . Initially the deposition begins at the surface of the substrate at the deposition spot on the substrate  19 . As the fabrication progresses, the deposited material layers  15  are built into the desired shape.  
       FIG. 1   a  shows a layer of material  15 , having a thickness Δt, deposited on top of a substrate  19  or substructure. Material properties such as tensile strength, toughness, ductility, etc. may be engineered into the material layer  15  using a laser-exposure factor (E), defined as:  
             E   =     a   ⁢       p   v     .               Equation   ⁢           ⁢     (   1   )                 
      Constant a includes the focused laser spot diameter and material constants. Variable p is the laser power in Joules per second, and v is the velocity in centimeters per second, of the deposition  15  relative to the surface of the substrate  19  or substructure. The exposure parameter E is a measure of energy input and thus has an effect on the solidification or quench rate of the deposited material  15 . Thickness Δt of the layer is critical to the control of material microstructure. It affects the quench rate, but it also affects the thermal gradient created in the deposited structure. If thickness Δt is precisely controlled along with solidification rate, then the material microstructure within a DMD structure can be controlled. Knowing the thermal gradient and how to vary it allows one of ordinary skill in the art to precisely control the microstructure of the deposited material. Production of articles having directional solidification and even single-crystal structure is enabled. See the discussion below in Sections 5 in respect of forming structures from multiple materials.  
      Substrate temperature biasing helps when one wishes to make parts having single-crystal growth. This technique is described in more detail below.  
       FIG. 4   a  depicts a tensile stress versus exposure graph  48  that demonstrates how material strength varies with the exposure parameter E. In the graph, 0.2% yield strength  47  of 316 stainless steel is plotted against values of the laser-exposure factor  49 . A regression line drawn through the data points shows that the 0.2% yield strength  47  of the test material, declined approximately 0.030 kilopounds (Kips) per square inch, per unit of laser-exposure factor  49 .  
      3. Substrate Heating for Producing Parts Having Accurate Dimensions  
       FIG. 5  reveals a side view of three different substrate pieces  50 ,  52 ,  54  that were exposed, in an actual experiment on H13 tool steel, to the laser beam  12 . Different biasing temperatures were applied to the substrates  50 ,  52 ,  54 . The first substrate  50  was not preheated before the laser beam impinged on its upper surface  51 . Because the upper and lower surfaces cooled differentially, the substrate  50  has deformed  56 . A second substrate  52  was preheated to 100 degrees centigrade (° C.) before the laser beam melted the upper surface  53 . The substrate  52  also suffered deformation  56   a , but considerably less than in the first case. A third substrate  54  was preheated to 200° C. No deformation is seen, even though the upper surface  55  was subjected to the same melting conditions as the other two substrates  50 ,  52 .  
      The substrates  50 ,  52 ,  54  were first ground flat. On the upper surface  51 ,  53 ,  55  of each substrate  50 ,  52 ,  54  was deposited two one inch by four inch patterns of material. Each of the substrates  50 ,  52 ,  54  was measured for flatness prior to beginning the tests. The first pattern deposited was one layer thick and the second pattern was 10 layers thick. For substrate  50  which was at room temperature when the deposition was made, a distortion  56  or change of flatness of 0.012 inches was observed. For substrate  52 , preheated to 100° C., a distortion  56   a  or change of flatness of 0.008″ was observed. For substrate  52 , preheated to 200° C., no measurable change in flatness was detected. An additional test was made preheating a substrate to 300° C. bias temperature. No measurable distortion was observed.  
       FIG. 6  shows an embodiment  70  of the directed material deposition apparatus in which heating is applied to the substrate  19  and deposition  15 . A heat lamp  72  or other radiant source such as a laser directs radiant energy  74  to the work area  15 ,  19 . A thermal monitoring device  76  such as an optical pyrometer is utilized to control the temperature of the work area  15 ,  19 .  
       FIG. 7  reveals another heating method  80  in which a heated platen  81  is part of the x-y axis movable stage. The platen  81  provides heat to the substrate  19  and deposition  15  during processing. A temperature sensor  86  is attached to the substrate  15 . Heating elements  82  are built into the platen  81 . A platen temperature sensor  84  monitors platen temperature.  
      Of course, the heating methods described above are only examples. Other methods of heating the work are possible, such as an inductive heating source or a furnace surrounding the work.  
       FIG. 8  is a chart of temperature versus time  90  and depicts a profile of temperature applied to a substrate  19 . The temperature of the substrate  19  is controlled before, during and after the build sequence to insure that the optimum material properties are obtained in the deposited material. The thermal profile  90  shown in  FIG. 8  begins at room temperature. The temperature is then raised in a controlled ramp  95  up to the processing temperature. A constant temperature  96  is maintained during material deposition, and a controlled ramp  97  down in temperature is programmed during the time the material is cooling. This insures the correct microstructure of the material is achieved when the article cools.  
       FIG. 9  depicts the difference between a material deposition with heating  15  applied during processing and a material deposition without heating  15   a . In the non-heating case, the top surface of the deposit  15   a  is flat but the substrate  19  is distorted. Because of the distortion of the substrate  19  and deposition  15  without heating, it is very difficult to control the dimensions of the deposition  15  both horizontally and vertically. However, when heating by use of apparatus shown in  FIG. 7  or  8 , the substrate  19  and deposition  15  have no detectable distortion and the dimensions of the deposited article  15  are closely controlled.  
      An alternate profile  110  of heating applied to the deposition  15  during fabrication is depicted in  FIG. 10 . As in the earlier-described profile  90 , the cycle begins from room temperature with a controlled ramp  112  up before deposition and a steady soak  114  during deposition and a controlled ramp  117  down in temperature after deposition. In this profile  110 , steps  118 ,  120 ,  122  are added to further improve the properties of the deposited material  15 . The part is not allowed to cool to room temperature prior to completing the entire thermal cycle 110.  
      4. Depositions Using Several Materials  
       FIG. 11  shows schematically the directed material deposition system  123  for fabrication with at least two different materials, and having means to preheat the substrate  19  and the material layers  15  thereon. The laser  124  projects a beam  125  through the powder deposition head  11  onto the substrate  19  and subsequently the material layers  15 . The substrate  19  is mounted on an x-y axis positioning stage  16  which contains heating and heat control apparatus. The positioning stage  16  moves the substrate  19  in a plane under the focused laser beam  125   a . Two different powder feed units  126 ,  127  supply the powder deposition head  11 . A z-axis positioning stage  18  raises or lowers the focal point of the focused laser beam  125   a  as the deposition grows. An enclosure  128  controls the atmosphere in the process area. The atmosphere is usually desired to be inert, but could be a reducing or oxidizing atmosphere.  
      A computer  129  and monitor  129   a  control the deposition process from stored data and CAD control files.  
      5. Forming Structures from Multiple Materials  
      Adaptation of the DMD apparatus  123  and methods have been applied to the problem of creating articles comprised of multiple materials in order to take advantage of the properties of each material. Multiple-material structures have been made by other processes, however, in the prior art there is no useful method of fabricating these structures directly from a computer rendering of an object. Prior art CAD systems and associated software only describe an article by the surfaces bounding the object. Thus, they are not effective to define the regions of gradient materials directly from computer files in those CAD systems.  
      In creating these structures using DMD techniques, several technical hurdles were overcome. These include: material compatibility; transitions from one material to another material; and definition of the multiple-material structure so that a simple computer controlled machine may automatically produce such a structure.  
      Instant change of feedstock materials, delivered from the powder feed units  126 ,  127  in a controlled manner, is another key requirement for the production of three-dimensional, gradient material structures. Known powder feed systems do not meet this requirement.  
      The invention includes hardware to control powder flow with little hesitation. It also provides a method for controlling the material composition, and thus the material characteristics, within a multiple material structure directly from computer renderings of solid models of the desired component. This method functions both with the industry accepted stereolithography (STL) file format as well as with other solid model file formats. The concept allows designers to create multiple material structures that are functionally graded, have abrupt transitions, or both. In addition, this invention provides a method to create these structures using the current solid model renderings that only define the surfaces of a model.  
      The development of solid free form (SFF) technologies, such as stereolithography, has created an increasing interest in creating functionally graded materials directly from a computer-rendered object. Once an object&#39;s shape is defined and the regions identified within the object where different materials are to be deposited, the object can then be broken down into a series of solid models that represent each of the different material regions.  
       FIG. 12  is a perspective sketch illustrating the concept  130  of capturing a solid model made of one material within a solid model made of a different material by means of the present invention.  FIG. 12  presents a simple case for illustration purposes in which a block  132  of a first material is located within a second larger block  134  composed of a second material. The larger block  134  contains a cavity at its center which is the desired shape of the second block  132 .  
       FIG. 13  is a cross-sectional view of the composite, two-materials structure  130  seen along section A-A of  FIG. 12 . In this structure  130 , the outer block  134  is shown as being formed by layered depositions  138  made horizontally. The inner block  132  is seen as formed by layered depositions following the hatching  142  along a 45 degree angle. There is a region  136  between the inner block  132  and the outer block  134  which is to composed of both materials and is graded from outer block  134  material beginning at a surface interface  140  in the large block  134  and continuing to a surface interface  144  of the inner block  132 .  
      In this example, the multiple material structure is defined from two solid models.  FIGS. 13 and 14  illustrate these two solid models  141 ,  146  in cross-sectional view looking along section A-A of  FIG. 12 . The solid model  141  representing a first material is bounded by the exterior outline  143  and interior outline  144  of outer block  134 . In  FIG. 14 , the solid model  146  representing a second material is “exploded” for easier visualization of the two separate models  141 ,  146 . The solid model is bounded by the outline  140  and forms the inner block  132  and the composite, graded material zone  136 . The regions defined by each of the solid models include the region  136  where the composite of the two materials is graded. By defining each of the solid models  141 ,  146  as containing the region where the desired amount of first material and second material will exist, the DMD apparatus  123  is programmed to deposit each of the materials in the correct proportion.  
      Using conventional methods, each of the solid models  141 ,  146  can be electronically sliced into layers, from which programming the solid object is fabricated. For a typical solid free-form method, a series of contours  140 ,  143  and hatch-fill lines  138 ,  142  are used to deposit the structure a layer at a time. The contour information is used to define the boundaries  140 ,  143 ,  144  of the object and the hatch-fill lines  138 ,  142  are used to fill the region within the bounding surfaces. It is only necessary now to define how the material is to be graded within the overlap region. This is input to the computer as a function of the coordinate axes, f(x,y,z). If it is assumed that the solid model slices are taken in steps along the z principal axes, then the grading becomes a function of the x and y coordinates on any given layer.  
      A preferred method of implementing this strategy is to define each of the solid models  141 ,  146  as independent entities and to electronically slice each of these models  141 ,  146  into layers as is typical for a solid free form method. When dimensions of the first solid model  146  and the second solid model  141  allow, these two “sliced” objects are recombined on a layer-by-layer basis. The slice information can be compared in the computer in order to define the single-material boundaries as well as the hatching information for the graded material region. The combined-slice files can then be used to directly drive a DMD apparatus  123  where the composition can be varied directly by the computer  129 .  
      Referring again to  FIG. 11 , the directed material deposition process is carried out inside a sealed chamber  128 , although this is not strictly required. The laser  124  generates a beam  125  which is focused to heat simultaneously a deposition substrate  19  and powder feedstock material  126 ,  127  that is supplied to the beam/powder interaction region  20 . The laser beam  125  is focused  125   a  to provide an area of high irradiance  17  at or near the surface on which the deposition is to occur. The area including the focused laser beam  125   a  and initially, the deposition substrate  19  surface comprises the deposition region. The deposition region changes with time, thus it is not necessary for the deposition to always correspond to the surface of the deposition substrate  19 . As the deposited material layers  15  build up, the deposition region can be moved far away from the original deposition substrate  19  surface. At or near the deposition region, the powder feedstock material  126 ,  127  intersects the focused laser beam  125   a  and becomes molten to create a new layer of material  15  on an existing substrate  19 .  
      As additional new material is supplied to the deposition region, the substrate  19  on which the deposition  15  is occurring is scanned in a fashion predetermined by computer programming such that a specific pattern is created. This pattern defines the region where the material is deposited to create one layer of an object that is comprised of a series of lines. The relative position between the focused laser beam  125   a  and the powder feedstock material  126 ,  127  is fixed with respect to each other during the deposition process. However, relative motion between the deposition substrate  19 , which rests on the orthogonal, x-y positioning stages  16 , and the beam/powder interaction zone  20  is provided to allow desired patterns of materials to be deposited. Through this motion, materials are deposited to form solid objects a layer at a time, to provide a surface-coating layer for enhanced surface properties, and to deposit certain materials in a specific pattern to produce the object configurations described above and below. Computer controlled motion of the x-y stages provides one means for controlling the relative motion between the deposition substrate  19  and the beam/powder interaction zone  20 . The computer control method is preferred to control this motion since the process is driven directly by the solid model data contained within the CAD files. Persons skilled in the art will appreciate that alternatively, the stage  16  can be stationary and the deposition head  11  moved in relation thereto. Movement of the deposition head in multiple axes, for example up to five axes, offers advantages of flexibility over the conventional x-y plane positioning, for producing overhangs and other shapes.  
      The present invention offers a deposition process that uses more than three axes of motion such that the part build axis can be varied during the process to allow unsupported overhangs to be built. In an alternative deposition process, the additional axes of motion may be used to fabricate outer surfaces that are unsupported by directing the deposition beam such that it is substantially tangent to the overhang surface. In one embodiment of the invention, these additional axes of motion are provided by a multi-axes deposition head  480 .  
      5a. Feedstock Rapid-Action Powder Metering Valve  
      Rapid-action metering of powder feedstock flow is controlled by a spool valve assembly  149  such as shown in schematic form in  FIG. 14   a . The process of proportional powder flow control implemented with the use of the valve assembly  149  is depicted schematically in  FIG. 14   b . Rapid response to changing mass-flow requirements for powder material delivery is accomplished by using a plenum to mix powder-rich and powder-free gas streams. No stagnant flow condition can be permitted in the system once powder is “fluidized” in a carrier gas stream. Downstream of the mixing plenum a flow diverter  149  directs part of the powder-rich stream Gp and part of the powder-free stream G into a powder material delivery path  154 . The volumetric flow rate of the carrier gas into each inlet  150 ,  151  is separately controlled and maintained. The flow diverters  158  are controlled proportionally so that the total volumetric flow rate of powder material is constant. The powder mass flow rate in the delivery path  154  to the work piece can be varied quickly from no powder to the total mass flow available in the powder-rich stream Gp. Waste gas and powder  150   a  are re-circulated. Waste gas  151   b  is reclaimed.  
      A flow of powder, entrained in a gas vehicle Gp such as argon or helium, is introduced into a valve body  152 . A flow of gas only G enters the valve body  152  through inlet  151 . A plunger  156  in which diverter passages  158  are formed, slides in and out of the body  152 . With the plunger  156  in the position shown, the gas with entrained powder PG is separated into two flows  150   a ,  150   b  through the diverter passages  158 . The gas G entering through inlet  151  is also separated into two flows  151   a ,  151   b . The flow through each of the diverter passages  158  is proportional to the cross-sectional area of each passage  158  which is presented to the inlets  150 ,  151 . Therefore, depending on valve position, a proportional amount of powder and gas  150   b ,  151   a  flows to the work through the diverter passages  158  and a first valve outlet  154 . Waste powder and gas  150   a  flow from a second valve outlet  153 . Remaining gas  151   b  flows from a third valve outlet  155 . The residual gas flow  151   b  from the third outlet  155  is combined with the waste powder and gas flow  150   a  downstream of the valve. This ensures a constant flow of gas through the system while the valve is in any open position, but varies the flow of powder to the work according to plunger  156  position. Powder and gas  150   b ,  151   a  are delivered to the deposition apparatus. Waste powder and gas  150   a ,  151   b  are returned to storage.  
      Rapid variation of the flow of powder and gas Gp occurs when the plunger  156  is partially withdrawn from the valve body  152  and the diverter passages  158  are no longer fully presented to the to the inlets  150 ,  151 . The flow paths  153 ,  154 ,  155  are quickly altered without stopping the motion of the powder particles. The plunger  156  is positioned under computer control in accordance with the CAD files used to control the deposition  15 . A mass flow sensor  159  measures powder flow rate in real time. The sensor  159  output is used for closed-loop control of powder flow  150   b ,  151   a . As variations in powder flow occur, the sensor signals for the powder required for the process.  
      One embodiment of the invention utilizes a fast acting valve for power flow control comprising at least two inlet ports and three outlet ports. A powder and a gas flow into the one outlet  150 , and impinge onto the separating unit  156  where the powder and gas stream are separated into two streams  150   a , and  150   b . The separated streams are then directed out of the valve into tubes  153 ,  154 . Gas input in tube  151  is also simultaneously separated into two streams  151   a  and  151   b , and directed into tubes  154  and  155  such that it combines with the two powder streams to provide additional gas flow. This feature prevents the powder streams from slowing down this additional gas is required to maintain the minimum powder velocity to avoid having powder settle out of the gas stream.  
      Another embodiment of the invention, a spool valve for controlling powder flow rate may be employed. The spool valve comprises a gas and powder inlet  150 ; a separator  156  and two outlet tubes. The second gas inlet is provided to make up for the flow reduced caused by the separator  156 .  
      5b. Volumetric Powder Feed Unit  
       FIG. 15  is a perspective view of a volumetric powder feed unit  170 .  FIG. 16  is a perspective view of the same unit, seen in the direction of view B-B of  FIG. 15 . The unit  170  allows a user to achieve extremely low flow rates with a variety of powder materials  185 .  FIG. 16   a  is a perspective view of the powder feed disk  179  and wiper assembly  184 , alone (i.e., removed from the powder transfer chamber  178 ), revealing a series of powder feed receptacles  181 , disposed circumferentially around the face of the powder feed disk  179 , which pick up powder from the supply pile  185 . The powder feed receptacles  181  are formed by piercing the powder feed disk  179 , typically by drilling, at a radial distance from the axis of disk rotation  183 .  
      The powder feeder design is a marked improvement over current disk-style powder feeders in which the disk typically is buried in powder. In the present invention, powder flow from a reservoir  172  to a transfer chamber  178  is limited by the angle of repose of the powder feedstock  185 , preventing the disk  179  from being overwhelmed and clogged with powder  185 . The present invention is insensitive to variations in flow rate of the gas  187  which transports the powder  185  to the deposition head. The spacing between the feed disk  179  and wipers  184  which remove powder from the disk can be greater than in prior art designs without losing control of powder metering. This promotes much less wear on the wipers  184  and substantially improves the life of the powder feed unit  170 .  
      During powder feeder  170  operation, powder feedstock  185  from the powder reservoir  172  enters the powder transfer chamber  178  through feed tube  190 . The powder  185  necessarily forms a heap that limits flow into the powder transfer chamber  178  but presents a constant source of powder  185  to the feed disk  179 . The powder  185  partially covers the powder feed disk  179  which is disposed perpendicular to the axis of rotation of the feed disk  179  so a portion of the disk  179  and a portion of powder receptacles  181  are immersed in the feedstock powder  185 . The powder feed disk  179 , is driven by a motor  180  and motor controller  182 . The series of feed receptacles  181  in the face of the disk  179  bring a controlled volume of powder  185  to the wiper assembly  184 . Gas  187  entering under pressure through a gas inlet  186  clears the powder receptacles  181  of powder  185  by blowing it into a powder-and-gas outlet  188 . From there, the powder  185 , entrained in gas  187  is transported to the deposition zone  15 .  
      To facilitate the transport of powder  185  from the powder mound to the wiper assembly  184 , the powder feed disk  179  is partially immersed in the powder mound as it is rotated by the motor  180 . The receptacles  181  in the disk  179  fill with powder  185 . As the disk  179  rotates, only the powder in the disk receptacles  181  remains with the disk  179  as it exits the powder  185  mound. When the disk holes pass the wiper assembly  184 , powder transport gas  187  “fluidizes” the powder  185  and entrains it in a gas stream  174  that is carried to the deposition area for use in the directed material deposition process. The transport gas is typically an inert gas such as argon or helium, although other gases such as nitrogen can be used in order to obtain special properties in the deposited material  15 .  
      Another optional feature employs a tube on the bottom of the hopper which extends into the horizontal chamber  178 . The powder passing through the extended tube can form a powder heap in the horizontal chamber  175  that partially covers the vertical powder feed wheel.  
      The powder feed wheel may be configured to rotate through the powder heap so that the holes in the powder feed wheel fill with powder that is carried past the gas  187  inlet or outlets. This arrangement renders the powder. This feature of the powder feed unit allows close tolerances between the powder feed wheel and the gas inlet and outlet wipers to be maintained, while keeping the powder largely away from these surfaces. As a result, these surfaces are not covered with powder continuously, and so the reliability is increased substantially.  
      The graph of  FIG. 16   b  plots average flow rate for 316 stainless steel powder versus powder feed disk rotational velocity (RPM) for three test conditions, showing the performance of the powder feeder. The flow rate  200  can be varied approximately linearly from about 0.1 grams per minute to about 30 grams per minute depending directly on the rotational speed of the powder feed disk  179 . The powder feeder  170  is a needed improvement to facilitate fabrication of gradient material structures.  
      5c. Joining Dissimilar Metals in DMD Process  
      When joining dissimilar metals in a DMD process, it is often necessary to place a “buttering” layer of one or more materials between the two dissimilar metals being joined. Buttering is a method that deposits metallurgically compatible metal on one more surfaces of the dissimilar metals to be joined. The buttering layers prevent coalescence of the dissimilar metals and provide a transitional region between them, because of, among other things, material incompatibility. An example of one preferred method of this process is shown in  FIG. 16   c.    
      In  FIG. 16   c , a substrate  210  is first manufactured or deposited from a “base” material. Buttering layers  212  &amp;  214  of a first and second transitional material are next deposited over the first base material  210 . When the transitional layers  212  &amp;  214  are completed, a second base material  216  is deposited on top of the transitional layers  212  &amp;  214 . It should be appreciated that one or more buttering layers  212  &amp;  214  maybe required depending on the properties of the dissimilar metals to be joined. As examples, some practical combinations are nickel as a buttering layer between copper alloys and steel, 309 or 310 stainless steel as a buttering layer when joining stainless steel to a carbon or low alloy steel, and 309 stainless steel as a buttering layer between a ferritic and austenitic stainless steel.  
       FIG. 16   c  depicts flat material layers, but it should be appreciated that these layers may also be contoured in several directions.  FIGS. 17 through 23 , discussed below, illustrate the invention&#39;s ability to form surfaces having complex contours.  
      6. Forming Cooling Channels for Thermal Control of Three-Dimensional Articles  
      Directed material deposition processes allow complex components to be fabricated efficiently in small lot sizes to meet the stringent requirements of the rapidly changing manufacturing environment. The present invention creates within a solid article, internal features using direct material deposition techniques coupled with a layer-by-layer manufacturing. These internal features provide thermal control of complex shapes, in ways not previously available. One important use for this invention is providing high efficiency cooling for injection mold tooling. The technology provides the ability to create an isothermal surface as well as produce thermal gradients within the part for controlled cooling.  
      The following discussion discloses features that are obtainable in an article by using direct material deposition manufacturing techniques including material sintering techniques. The development of precise material deposition processes provides the ability to create structures and material combinations that were previously not capable of being manufactured easily. Traditional methods cannot be used easily for manufacturing these internal geometries and multiple material structures that are completely enclosed in a solid body. Embedded structures forming conformal cooling channels support rapid and uniform cooling of many complex shapes. The shapes may have irregular internal or external geometry.  
      7. Thermal Management within Solid Structures  
      There are often compromises that must be made to work within the constraints of the physical environment. Compromises in the thermal management within solid structures have often been required. For example, in tooling there are often conflicting requirements for long-lifetime tool and one with efficient cooling properties. For these applications, designers will typically use a form of tool steel which can be hardened and which will provide a very good wear surface. However, the thermal conductivity of tool steels in general is relatively poor. Therefore, the cooling cycle time is compromised in favor of long tool life. The invention described herein allows these normally conflicting requirements to be simultaneously satisfied. In addition, methods of the present invention provide the ability to fashion the structures beneath the surface of a component to tailor the thermal characteristics of the structure. Thermal characteristics within a structure can be manipulated to control the rate at which a component is heated and cooled.  
      The opportunity to embed features such as passages, chambers and multiple material structures is provided with the present invention. As an example, structures are shown in  FIGS. 17 through 23  in which passages and chambers are integrally formed. The passages and chambers can be empty, or filled with a circulating coolant liquid. They may also be filled with another material that performs a function such as increasing or decreasing the rate of cooling or heating in the structure. The passages and chambers may be interconnected to provide uniform thermal control or several passages or chambers can exist within a component that are not interconnected to provide localized thermal management. The structures may be actively or passively temperature controlled. Active control is accomplished by flowing a fluid coolant medium through the passages and chambers. Passive temperature control is achieved by combining the basic component material with other materials that locally affect the thermal gradients in particular regions of the component.  
      A schematic diagram of one preferred embodiment of this invention is given in  FIG. 17 . Cooling passages  252  which conform to the shape of a mold cavity  254  are integral with the mold block  256 . For clarity, the mold block  256  has been cross-sectioned through a mid-plane  258 , exposing the internal cooling passages  252  and support fin structures  259 . The arbitrarily shaped injection mold block  256 , is the base that houses the mold cavity  254  and the conformed cooling passages  252 . The passages  252  follow the contour of the surface they lie beneath at a prescribed distance beneath the surface. The conformal cooling passages  252  are designed to follow the surface of the mold cavity at prescribed distances, determined by the desired cooling balance of the mold cavity.  
      The present invention produces injection molds having rapid, uniform cooling. The conformal cooling systems are integrated into the mold inserts  256  fabricated by directed material deposition techniques. Cooling passages  252  are fabricated using a DMD system  123 . When using DMD techniques, passage width can be chosen such that no support material is needed and the passages will remain open cavities that are completely enclosed in the mold base  256 . The conformal cooling channels  252  provide uniform support for the mold cavity  254  as well as increase the surface area of the cooling channel surfaces  259 .  
      The embedded features  252 ,  259  are produced in the three-dimensional mold insert structure  256  by feeding one or more separate material feedstock  126 ,  127  into the directed material deposition process  123  and depositing the melted feedstock  126 ,  127  onto a substrate  19 . The deposition is made in a manner depicted in  FIGS. 12 through 14  and described above, according to computerized files of solid models of the elements of the completed article. In addition to external contours, the solid-model computer files describe regions of each separate material, regions of a composite of the materials and regions of voids in each layer or “slice.” The steps are repeated a sufficient number of times in layer-by-layer patterns, defined by “slices” of the solid models, to create the three dimensional structure  156  having the geometric details depicted in  FIGS. 17 and 18 . It should be appreciated that these steps can produce nearly any other shape that can be imagined.  
      A finned structure  252  as shown in  FIG. 17  provides several advantages over structures that can be produced using existing methods. Typically, cooling passages are drilled into a structure. However, the circular cross-section of the drilled passages present a minimum surface area in contact with the thermally conductive medium. Finned structures  252  can provide an order of magnitude increase in surface area. A finned structure  252  provides support for the exposed surface  254 . This is critical for applications such as injection molding of plastic parts where the pressure can be on the order of 5000 pounds per square inch. One of the factors that influence the heat transfer rate of a structure is thermal conductivity of the material. A second is the efficiency at which the energy is transferred to the heat conducting medium. Uniformly distributing fins  259  beneath the surface of the component compensates for poor thermal conductivity of its materials.  
       FIG. 18  is a perspective view of a full mold block  256 . Shown here are inlet port  260  and outlet port  262  for the flow of a coolant medium through the mold conformal cooling passages  252 . In this application, cooling passages  252  that conform to the shape of the mold cavity are precisely located. The cooling passages  252  can be connected to other passages through the ports  260 ,  262 . The passages  252  can be designed for equal and uniform flow of coolant, or whatever flow is optimum in the circumstances.  
      These structures offer another advantage in thermal management of fabricated articles. They can be designed to create a constant pressure and uniform flow of the coolant medium across the entire structure.  FIG. 19  is a cross-sectional view of a solid, rectangular article  270 , showing the internal cooling passages  276  and inlets  272 ,  274  made integral with the article  270 . Although coolant inlet and outlet ports  272 ,  274  can be introduced into the part from almost any location, their respective location to the inlet and outlet of the cooling channels  276  plays a significant role in obtaining uniform cooling in these structures. The cooling channels  276  are terminated in reservoir-like features  278 . The inlet  272  to a first reservoir  278  is at one end and the outlet  274  is at the end of a second reservoir  279 . A constant pressure drop and uniform flow through the structure is thus provided. This is similar to the structure of a cross-flow style radiator used in an automobile. Of course, other structures used for flow control can also be formed by DMD processes.  
      The DMD processes provide the unique ability to deposit a plurality of materials within a single build layer. This provides yet another advantage of fabricating structures with integral thermal management features. In many structures, the control of the temperature by active means is not possible. There may be no way to embed cooling passages  252  within a low thermal conductivity material structure  256  to facilitate heat transfer. In that case, the structure is fabricated such that the region beneath the surface is composed of a high thermal conductivity material. A technique similar to that depicted in  FIGS. 12 through 14  and described above is used. High thermal conductivity material deposited in the cavities of lower thermal conductivity material provides a solid structure that acts as a heat pipe. The high thermal conductivity material is placed in contact with a heat exchange medium which provides a means to quickly cool adjacent lower thermal conductivity material.  
       FIG. 20  is a schematic of an alternate embodiment. It is a cross-sectional view of a cylindrical article  280  of random length having integral cooling passages  282 . The structure&#39;s geometry increases cooling surface area by a significant amount. The internal cooling structure can vary in cross section and direction.  
       FIG. 21  is a cross-sectional view of a cylindrical object with complex geometries of separate cooling passages fabricated into the component. The view depicts an cylindrical shape  286  with multiple independent loops of cooling passages  288  and a plurality of cooling channels  289  having a common reservoir. A plurality of cooling passages  288 ,  289  can be incorporated for separate cooling media.  
       FIG. 22  is a perspective view of a solid, curved object  290  having the cooling passages  292  following the contour of the outer shape of the object  290 . A person of ordinary skill in the art will appreciate that cooling passages  292  of very complex geometry can be incorporated into an arbitrarily curved shape  290 . The method of the present invention, however, is not limited to heat exchanger technology in solid bodies.  
       FIG. 23  is a perspective view of an airfoil shape  300 , such as a turbine blade, with cooling channels fabricated integrally within the airfoil. The figure illustrates incorporation of cooling channels  302  into an irregular, arbitrary shape having length, twist and curvature. The advantage of fabrication with the present invention over extruded shapes which can only have a constant cross-section should be clear.  
      8. Smart Substrates for Reduced Fabrication Time  
      The present invention is clearly useful for construction of articles with internal spaces which cannot be reached easily from the surfaces of the article for machining. Of course, there is no point in fabricating a portion of an article which can be made by using conventional means effectively. But certain “smart” substrates can be made by deposition, used as a starting point for manufacturing the whole article and can become part of the final structure.  FIG. 24  is a perspective view of such a substrate  310   a  in which the outside envelope  312  and inside cavities  314  have been partially constructed by deposition using methods already described above. In  FIG. 25 , the upper surfaces  316  of the substrate  310   b  have been approximately three-quarters deposited.  FIG. 26  reveals the completed substrate  310 , before any additional, conventional machining.  
      Yet another embodiment of “smart” substrate is revealed in the thermal management structure of  FIG. 26   a . The lattice structure  318  is an embedded structure in which the volume of deposited material is minimized but the design offers sufficient support for many different applications. This structure allows the tubular structure  319  between the surfaces  320  to be flooded with a liquid or gas medium providing good energy-transfer efficiency between the surface  320  and the tubular structure  319 . Such a device for providing thermal management of surfaces  320  allows an end user to control temperatures of the structure at a surface  320 .  
      In  FIGS. 26   b  and  26   c , another structure  322  is illustrated in which thermal management structures are embedded beneath the surface  323  using the DMD process. This structure  322 , shows particularly a tool for plastic injection molding in which the cooling channels  324  within the structure  322  conform to the shape of the molding surface  325 . Unlike the finned structures shown in  FIGS. 17 through 23 , the channels here have a circular cross-section. Further enhancing the ability to control the temperature of a structure at its surface, cooling structures can alternatively comprise embedded materials of different thermal conductivity from the surface material. For example, copper can be used as an embedded material of high thermal conductivity.  
      9. Fabricating Unsupported Structures  
      A combination of methods is used to build three-dimensional, graded material structures. A problem of construction is creating overhanging edges which may occur in cavities within a structure.  FIGS. 27 and 28  illustrate one preferred method of producing an unsupported overhang  346  in a structure  15  using three-axis positioning. The focused laser beam  340  is moved a distance Δx over the edge of a previously deposited surface  15  and a bead of material  344  is deposited. The distance Δx is typically less than ½ of the focused laser beam diameter  17 . At this distance Δx, surface tension of the melted material  342  aids in maintaining the edge, thus allowing a slight overhang  346 . By repeating this deposition several times in one layer  348 , an angle of the overhang  346  of approximately 60 degrees can be achieved. After the over hanging edge  346  bead  344  and other edge beads  344  are deposited, material is filled in to complete the layer  348 .  
       FIG. 28   a  shows how additional beads of material may be attached to an existing overhanging surface  346 . By defining the overhanging surface  346  as a series of contours that incrementally move outward, away from a solid structure  15 , several beads  345  of material may be added to a structure to extend the build over an unsupported region. A second bead of material  345  is deposited to the first edge bead  344  using a multiple contouring method. The overhanging surface is extended into a region where there is no underlying support for the bead. The method provides a “virtual” support for the overhanging build.  
      In an alternative embodiment, the multi-axis capability of the invention is used to deposit the overhanging surfaces  344 , and then the filled regions are filled  348  by the deposition beam, which is directed towards the build surface in a direction normal to the substrate surface.  
      In another alternative embodiment, the plane of deposition is rotated in respect of the work piece  15  as shown in  FIGS. 29 and 30  so the focused laser beam  340  is parallel to a tangent  343  to the surface which is being built. When the edge beads  344  have been deposited as in  FIG. 31 , the part can be reoriented with the deposition layer  348  normal to the laser beam  340  axis as seen in  FIG. 32 . The layer  348  is filled in, as before.  
      Note that either the part  15  or the laser deposition head  14  can be adjusted to accomplish parallelism of the laser beam  340  axis with the tangent  343  to the surface of the deposition  15 . In fabricating certain configurations of structures, it is easier to tilt and rotate the deposition head axes than those of the part. The present invention, therefore, includes a deposition head which deposits materials in directions other than downward along the z-axis.  
      10. Protecting the Fiber Optic which Delivers Laser Power to the Work  
      Work with known systems  10  in the field has shown that catastrophic failure of a fiber optic used to deliver laser energy to the deposition surface  15  can occur because of the effect of reflected laser energy on the optical fiber. The present invention includes a laser beam delivery system which eliminates this problem by imaging both specular and diffuse reflections from a laser beam emanating from the work area  17  on an area of surface that is a distance from the fiber optic face.  
      The laser beam delivery system  420 , depicted in  FIG. 33 , provides a laser beam  436  from a preferred Neodymium YAG laser. The beam  436  emerges from an optical fiber  430  and is focused on a spot  17  on the surface of the work piece  15 . The beam  436  is reflected to the work piece  15  at an approximate right angle by a folding mirror  438 . After the diverging laser beam  436  leaves the optical fiber  430 , it is collimated by lens  433 . The collimated beam  436  is then focused by a convex lens  434  to achieve the high power density required to melt material at the work piece surface  15 .  
      In prior fiber delivery systems, off-axis reflections result when rays of an unfocused laser beam reflect from a folding mirror used in the optical system, at an angle other than 45°. In the present invention, because the beam is focused before it strikes the mirror  438 , the off axis reflections do not occur. While the reflected beam  439  has a small aberration, it only serves to spread out the beam energy at the beam image  17  on the deposition surface  15 .  
      Typically, the folding mirror is positioned at  450  to the axis  440  of the beam  436  and reflects the focused beam  436   a  normal to the work piece surface  15 . When the laser beam  436   a  is sharply focused on the deposition surface, any reflected light travels along the reverse path. A reflected beam  439  is incident on the folding mirror  438  and is directed through the focusing lens  434  in a reverse direction. The focusing lens  434  now collimates the reflected laser energy and the collimating lens  433  focuses the reflected beam  439  onto the optical fiber  430 . Since there is generally some tolerance associated with the mirror  438  mounting, the beam  436  may not always be coupled directly back normal to the optical fiber face. If coupling should occur, some of the reflected laser light  439  leaks out of the fiber  430  and for a short time no serious heating results. During the powder deposition process, however, the operating time is long enough that the optical fiber  430  can be damaged by the additional heat of the reflected laser beam  439 .  
      To solve this problem, the reflected laser beam  439  is deliberately imaged elsewhere than on the optical fiber  430 . By tilting the folding mirror slightly from 45° to the beam axis  440 , an angular deviation of the optical system is introduced. For example, if the folding mirror is tilted at a 2° angle away from 45°, a sufficient offset is introduced into the beam  439  to prevent the reflected beam  439  from being imaged back onto the fiber optic  430 . When specular reflection of the focused laser beam  436   a  occurs at the work piece surface  17 , the beam  436   a  is reflected away from the surface  17  at an angle equal to the angle of incidence. The reflected beam  439  propagates back towards the folding mirror  438  at an angle of 2° with respect to the normal to the work piece surface  17 . When the reflected beam strikes the folding mirror  438 , a second 2° offset is added to its direction of propagation with respect to the optical axis  440  of the emergent beam  436 . That is, the reflected beam  439  is now directed  40  away from the axis of the optical fiber  430 . In a preferred embodiment, the reflected beam  439  is imaged harmlessly on the water-cooled optical fiber holder  431   a  distance away from the optical fiber  430  itself. This small angular deviation introduces a small displacement of the focused spot  17  from a point normal to the deposition surface  17 . Through proper design, negative effects due to the different trajectory angle of the reflected beam  439  through the powder stream intersection region  20  are negligible.  
      The focused beam  436   a  is incident onto the surface  17  of the work  15  at 20 from normal. The beam  436   a  passes through the powder stream intersection region  20  at this angle also. If it is assumed that the deposition occurs in a 0.100 inches long region of the powder stream intersection zone, that is along the work piece surface  17 , the “pointing” error of the beam  436   a  in the deposition plane is as about 0.0035 inches. This error is negligible.  
      Zemax™, a commercially available optical design package, was used to determine the offset as the beam  436 ,  439  was propagated through the collimating and focusing lenses  433 ,  434 . The prescription data and details used to model the lens are not included here. However, the predicted location of the final specular-reflected beam image on the fiber holder  431  was displaced from the center of the optical fiber  430  by approximately 0.310 inches. An image due to diffuse reflections should be offset by at least half of this amount.  
      Although the offset image of the reflected beam  439  prevents the reflected laser energy from damaging the optical fiber  430 , there is also an issue of direct fiber heating by the laser beam  436  as it is transmitted through the optical fiber cable  430 . To mitigate this effect, the output end of the fiber  430  is mounted in a water-cooled copper block  431 . The copper block  431  has an output aperture diameter of about 0.2 inches. The diameter was chosen to be sufficiently large to accommodate the diverging output beam  436  from the fiber  430  without blocking the beam  436 . At the edge of the aperture, the surface of the copper block  431  is beveled at 45° to reflect any light incident onto this surface outward away from the center line  440  of the fiber mount. The inner, rear surface of the block  431  traps the reflected light  439  so that the laser energy can be absorbed in the fiber holder  431  where the heat can be subsequently carried away by cooling water.  
      The above system of laser beam  436  delivery has been performed while operating the laser at 900 watts and scanning the focused beam  436   a  on a copper substrate  15  for approximately one hour. The copper substrate  15  has a reflectivity of approximately 98% at the laser wavelength of 1.064 μm. Essentially, all of the laser power was reflected back to the water-cooled surface of the fiber holder  431 . There was no degradation of the optical fiber  430  at its output.  
      11. Laser Beam Shutter  
      To cut-off the laser beam  125  while re-positioning the deposition head  11  from place-to-place on the work piece  15 , a laser beam shutter assembly  450  has been created for the DMD process such as outlined below.  FIGS. 34 and 35  are perspective views of the laser beam shutter assembly  450 .  FIG. 35  shows the assembly of  FIG. 34  with the cover and a section of the liquid cooled beam “dump”  452  removed. The design of the beam “dump”  452  for this shutter assembly  450  is unique. The beam dump  452  is a liquid-cooled metal block  453  on which the laser beam  436  is focused by the laser beam shutter mechanism  462 . To allow operation at high power, it is important to be able to spread the laser energy out over a large surface to avoid damage to any of the beam dump surfaces. Liquid is circulated through tubes  454  to cool the whole beam dump block  453 .  
      Probably the most important reason for avoiding damage to the beam dump  450  is danger of generating vapor which will degrade optical surfaces near a damaged dump surface. As with any optical surface, once some damage has occurred, the surface quickly degrades to a point of uselessness.  
       FIG. 35   a  is a schematic sketch of the operation of the laser beam shutter mechanism  462  with the cooling caps  451  removed. The laser beam  125  is interrupted by a mirror  465  which redirects the laser beam  125  into the laser beam absorption chamber  466  through laser dump aperture  468 . The beam  125  falls on a first reflective, diverging surface  469 . The divergent beam is reflected onto a second reflective surface  470  and then onto surface  471  where it is absorbed. Creation of the divergent beam may be by alternative means such as a lens, concave or convex reflective surface.  
      12. Multi-Axis Deposition Head  
       FIGS. 36 through 40   a  reveal a multi-axis deposition head  480  which is designed to deposit materials in other directions in addition to the z-axis. The head  480  contains the powder delivery system integrally. When coupled with a three-axis stage which positions the deposition head  480  in the x-y-z orthogonal axes, the deposition head  480  provides rotation  482  about a fourth axis u and rotation  484  about a fifth axis v. Of course, the work piece can also be moved in the x-y-z orthogonal axes and the deposition head  480  held stationary.  
       FIG. 40   a  shows how the deposition head  480  is continually positioned to produce a three-dimensional, curved object  490 . It is the relative motion of the deposition head  480  and the work piece which creates the lines of material deposition, as has already been seen. Applying the multi-axis feature of the deposition head  480  enables three-dimensional structures of virtually every kind to be fabricated directly from a CAD solid model. In addition to the multi-axis head  480 , robotic arms and tilting, rotating stages for the work piece are usable for fabrication of many three-dimensional structures. These features also facilitate use of transformations to various coordinate systems which accommodate specific geometric configurations such as cylinders and spheres.  
      The multi-axis deposition head  480  includes the powder delivery system  170  and optical fiber, laser beam delivery system  420  described above.  FIG. 40   a  illustrates how the multi-axis deposition head  480  is positioned in order to produce a three dimensional, curved structure  490 . Controlled translation in three axes x, y and z and controlled rotation about two axes u an v are used to position the deposition head  480  with respect to the work piece  490 . Note that the translation of the head in the x, y and z axes can be used in place of or in combination with the translation of stage  16 .  
      13. Particle Beam Focusing to Reduce Material Waste  
       FIG. 41  depicts one of a plurality of powder delivery nozzles  14  of the prior art, which are disposed in a deposition head  11 . In this configuration, a stream of gas-entrained powder  502  exits a powder tube  500  and tends to disperse away from the axis of the stream  502  because of expansion and deceleration of the gas.  
       FIG. 42  shows an improved powder delivery nozzle  504  used in the present invention. A coaxial flow tube  506  surrounds the powder tube  500  and is coextensive with it. The bore of the coaxial flow tube is slightly larger than the outside diameter of the powder tube  500 . Gas  508  is forced to flow through the coaxial flow tube  506 , between the outside diameter of the powder tube  500  and the inner bore of the coaxial flow tube  506 . The gas  508  forms a sheath-like column  510  surrounding the entrained powder  502  as it leaves the powder tube  500 . The gas column  510  provides a barrier to the entrained powder  502  and as a result, the powder  502  is projected from the powder tube  500  in a coaxial stream, and remains so for an extended distance and time period.  
      The improved nozzle  504  projects a smaller, constant-diameter powder stream  502  for a longer distance than the prior art nozzle  14 . As a result, the powder delivery nozzle  504  can be located farther away from the deposition  15  surface with much less waste of material. Material utilization efficiency depends on the ratio of area of the laser-created molten pool  17  to that of the powder stream  502  footprint on the deposition  15  surface.  
       FIG. 43  reveals a still further improved powder delivery nozzle  515  which increases the efficiency of directed material depositions with the present invention. A coaxial flow tube  520  which surrounds the powder delivery tube  500  is constricted at the outlet  526  so the coaxial gas column  508  is directed inward toward the entrained powder stream  502  as the powder stream  502  leaves the powder tube  500 . Turbulence in the coaxial gas column  528  concentrates the powder stream  502  and focuses it to a small footprint on the deposition surface. This innovation provides an even higher concentration of powder at the deposition  15  surface than powder delivery nozzle  504 , the least waste and therefore the best powder utilization efficiency. The outlet orifice depicted in  FIG. 43  is as approximately square edged, which is easily manufactured. A more precision, converging-diverging nozzle shape is an alternative embodiment to the square-edged outlet  526 .  
      The operation of the sheath-like column  510  which forms a “no-slip” fluid boundary layer may be better understood by referring to  FIGS. 44 and 45 .  FIG. 45  reveals a flow of powder entrained in gas  502  which is moving at a velocity of V a . The coaxial gas flow has a velocity of V b . The gas surrounding the powder tube  500  and coaxial flow tube  506  in the environmentally controlled chamber  128  in which the deposition takes place has a velocity of V c . Control of the velocities V a  and V b  is essential to the operation of the coaxial gas sheath  510 . Flow rate conditions considering V a  and V b  and V c  are: 
          1. V a ≈V b ; V c ≈0     2. V b &lt;&lt;V a ; V c ≈0     3. V b &gt;&gt;V a ; V c ≈0        

       FIG. 44  illustrates a flow rate condition where V a ≈V b  and V c ≈0. For this first condition, there is no significant change in the direction of powder stream  502  as it leaves the powder tube  500 . V b  will decrease at the edge of the coaxial gas stream  510  because the velocity V c  of the gas in the environmentally controlled chamber  128  is approximately zero. But the sheath formed by the coaxial gas flow  510  maintains the focus of the entrained powder stream  502  until it strikes the deposition surface  15 .  
      However, if as in condition 2, V b  is much less than V a , then V b  “peels back” the entrained powder stream  502 , de-focusing it and causing the powder to spread out at the deposition surface  15 .  
      In condition 3, depicted in  FIG. 45 , where V b  is much greater than V a , an adverse situation develops in which the coaxial gas stream mixes  532  with the entrained powder stream and the powder spreads out unacceptably at the deposition surface  15 .  
      In respect of the improved nozzle  515  shown in  FIG. 43 , control of the gas velocities V a  and V b  is still important even though the localized turbulence caused by the orifice  526  helps to focus the entrained gas flow  502 .  
     CONCLUSION  
      Although the present invention has been described in detail with reference to particular preferred and alternative embodiments, persons possessing ordinary skill in the art to which this invention pertains will appreciate that various modifications and enhancements may be made without departing from the spirit and scope of the claims that follow. The various hardware and software configurations that have been disclosed above are intended to educate the reader about preferred and alternative embodiments, and are not intended to constrain the limits of the invention or the scope of the claims. The List of Reference Characters which follows is intended to provide the reader with a convenient means of identifying elements of the invention in the Specification and Drawings. This list is not intended to delineate or narrow the scope of the claims.  
     List of Reference Characters  
     
       FIGS. 1 &amp; 1 
       a  
     
     
         
           10  LENS™ apparatus, prior art  
           11  Deposition head  
           12  Laser beam  
           13  Focusing lens  
           14  Powder delivery nozzle  
           15  Deposited material  
           16  X-Y positioning stages  
           17  Molten metal pool  
           18  Z-axis positioning stage  
           19  Substrate  
           20  Laser beam-material powder interaction region  
          Δt Deposition layer thickness 
 
  FIG. 2  
 
           28  Sample object 
   FIG. 3   
           30  Chart of Deposition Layer Thickness v. Laser Irradiance/Velocity  
           32  Deposition Layer Thickness  
           34  Laser Irradiance/Velocity 
   FIG. 4   
           40  Graph of Average Surface Roughness vs. Material Particle Size  
           42  Average Roughness  
           44  Particle Size  
           46  Legend: Average Roughness and Laser Power 
   FIG. 4   a    
           47  0.2% yield strength  
           49  Laser-exposure factor  
           48  Tensile Strength vs. Exposure graph 
   FIG. 5   
           50  Unheated substrate  
           51  Upper surface of unheated substrate  
           52  Pre-heated substrate (100° C.)  
           53  Upper surface of preheated substrate (100° C.)  
           54  Preheated substrate (200° C.)  
           55  Upper surface of preheated substrate (200° C.)  
           56  Deformation of first substrate  
           56   a  Deformation of second substrate 
   FIGS. 6 &amp; 7   
           70  Directed material Deposition (DMD) apparatus with heated substrate  
           12  Laser beam  
           13  Beam focusing lens  
           14  Powder delivery nozzle  
           15  Material deposition  
           16  x-y axis position stages  
           18  z-axis positioning stage  
           17  Molten metal pool  
           19  Substrate  
           72  Radiant heating source  
           74  Radiant heat  
           76  Temperature sensor/pyrometer  
           80  Directed material deposition apparatus with heated platen  
           81  Heated platen and x-y positioning stages  
           82  Heating element  
           84  Platen temperature sensor  
           86  Substrate temperature sensor 
   FIG. 8   
           90  Temperature profile chart for DMD processing  
           92  Temperature  
           94  Time  
           95  Temperature cycle: controlled temperature increase  
           96  Temperature cycle: steady temperature maintained  
           97  Temperature cycle: controlled temperature decrease 
   FIG. 9   
           100  Comparison deformation of deposition for heated and unheated substrates  
           15  Deposition on heated substrate  
           15   a  Deposition on unheated substrate  
           19  Heated substrate  
           19   a  Unheated substrate 
   FIG. 10   
           110  Temperature profile chart for DMD processing  
           92  Temperature  
           94  Time  
           112  Temperature ramp-up  
           114  Steady state temperature  
           116  Temperature decrease to above room temperature  
           117  Steady state, elevated temperature  
           118  Second cycle: Temperature ramp-up  
           120  Steady state, high temperature  
           122  Temperature ramp-down to room temperature 
   FIG. 11   
           123  Directed Material Deposition apparatus  
           11  Deposition head with focusing lens  
           15  Deposited material  
           16  x-y axis positioning stages  
           18  z-axis positioning stage  
           19  Substrate  
           20  Laser beam-material powder interaction region  
           124  Laser  
           125  Emitted laser beam  
           125   a  Focused laser beam  
           126  First material storage  
           127  Second material storage  
           128  Environmentally controlled chamber  
           129  Computer, controller  
           129   a  Computer monitor  
           129   b  Computer signals to positioning stages 
 
 FIGS.  12  Through  14   
           130  Solid model of a first material captured within a solid model of a second material  
           132  Inner block made of a first material  
           134  Outer block made of second material  
           136  Region of overlapping solid models and composite material  
           138  Hatch-fill lines of deposition of second material  
           140  Boundary of composite material  
           141  Cross-section of solid model of second material  
           142  Hatch-fill lines of deposition of first material  
           144  Outer boundary of block of first material; inner boundary of composite material region  
           146  Cross-section of solid model of first material 
   FIGS. 14   a  &amp;  14   b    
           149  Rapid-acting metering valve  
           150  Gas and powder inlet port  
           150   a  Gas and powder waste  
           150   b  Gas and powder to delivery path (to work piece)  
           151  Gas only inlet port  
           151   a  Gas to reclamation  
           151   b  Gas to powder delivery path  
           152  Valve body  
           153  Outlet port, powder delivery to work piece  
           154  Waste powder outlet port  
           155  Gas flow to powder delivery path, outlet port  
           156  Diverter plunger  
           158  Diverter passages  
           159  Powder flow rate sensor  
          Gp Gas and powder input flow  
          G Gas input flow 
 
  FIGS. 15, 16 ,  16   a  
 
           170  Powder feed unit  
           172  Powder reservoir  
           174  Gas and powder flow to deposition head  
           175  View ports  
           176  Reservoir lid  
           178  Transfer chamber  
           179  Powder feed disk  
           180  Motor  
           181  Powder receptacles  
           182  Motor controller  
           183  Rotational axis  
           184  Wiper assembly  
           185  Powder mound  
           186  Gas inlet  
           187  Powder and gas stream to work piece  
           188  Gas and powder outlet  
           189  Mounting bracket  
           190  Powder feed tube 
   FIG. 16   b    
           200  Flow rate axis  
           202  RPM axis 
   FIG. 16   c    
           210  First dissimilar material  
           212  First transitional material deposition  
           214  Second transitional material deposition  
           216  Second dissimilar material 
   FIGS. 17 &amp; 18   
           250  Cut-away view of injection mold insert with integral cooling passages  
           252  Cooling passages  
           254  Mold cavity  
           256  Mold block  
           258  Cross-sectioned face of mold block  
           259  Finned structure separating cooling passages  
           260  Cooling medium inlet  
           262  Cooling medium outlet 
   FIG. 19   
           270  Cross-section of solid rectangular article with uniform-flow cooling passages  
           272  Cooling medium inlet  
           274  Cooling medium outlet  
           276  Cooling passage  
           278  Cooling medium inlet reservoir  
           279  Cooling medium outlet reservoir 
   FIG. 20   
           280  Cross-section of a cylindrical article of random length with integral cooling passages  
           282  Cooling passages 
   FIG. 21   
           286  Cross-section of a cylindrical shape with multiple independent loops of cooling passages and a plurality of cooling channels  189  having a common reservoir  
           288  Independent cooling passages  
           289  Cooling channels with a common reservoir 
   FIG. 22   
           290  Solid, curved object having integral cooling passages which follow the contour of the outer shape of the object  
           292  Cooling passages 
   FIG. 23   
           300  Airfoil-shaped article having length, curvature and twist, with integral cooling passages  
           302  Cooling passages 
 
 FIGS.  24  Through  26   c    
           310  Completed substrate  
           310   a  Partially completed substrate  
           310   b  Partially completed substrate with partially completed upper surface  
           312  External surfaces  
           314  Internal cavities  
           316  Partially completed upper surface  
           318  Latticed substrate  
           319  Tubular cooling channels structure  
           320  Latticed substrate support surface  
           322  Injection mold substrate with embedded cooling channels  
           323  Upper surface of mold  
           324  Cooling channels  
           325  Molding surface 
 
 FIGS.  27  Through  32   
           14  Deposition head  
           15  Material deposition  
           20  Laser beam-powder interaction zone  
           340  Focused laser beam  
           342  Powder stream  
           344  Material bead deposition at the part edges  
           346  Overhanging structure  
           348  Deposition layer  
          θ Work piece rotation  
          Δx Material bead overhang dimension 
 
  FIG. 33  
 
           15  work piece deposition  
           17  molten pool, deposition plane  
           420  laser delivery system  
           430  optical fiber  
           431  water-cooled fiber holder  
           433  collimating lens  
           432  laser beam center line  
           434  focusing lens  
           436  deposition laser beam  
           436   a  focused deposition laser beam  
           438  folding mirror  
           439  reflected laser beam  
           440  reflected laser beam image  
           441  lens housing 
   FIGS. 34, 35  &amp;  35   a    
           450  laser beam shutter “dump” assembly  
           451  cooling caps  
           452  laser beam “dump” 
           453  “dump” block  
           454  cooling fluid tubes  
           455  shutter aperture  
           460  cut-away view of laser beam shutter “dump” assembly  
           461  light path diagram  
           462  shutter mechanism  
           464  shutter actuator  
           465  mirror  
           466  laser beam absorption chamber  
           468  aperture, beam “dump” 
           469  diverging first surface  
           470  reflective second surface  
           471  absorbent surfaces 
 
 FIGS.  36  Through  40   a    
           16  stage  
           125   a  focused laser beam  
           480  multi-axis deposition head  
           482  rotation about u-axis  
           484  rotation about v-axis  
          x, y, z orthogonal translation axes 
 
 FIGS.  41  Through  45  
 
           14  powder delivery nozzle of prior art  
           15  deposition  
           500  powder tube  
           502  entrained powder stream  
           504  improved powder delivery nozzle with axial-flow gas tube  
           506  coaxial gas flow tube  
           508  coaxial gas flow  
           510  coaxial gas column and turbulence  
           515  improved powder delivery nozzle with axial-flow gas tube restrictor.  
           520  coaxial flow gas tube with restrictor  
           526  gas tube restricted outlet  
           528  restricted gas column and turbulence  
           530  deposition footprint of powder stream  
           532  coaxial gas flow and entrained powder stream mixing  
          V a  velocity of entrained powder stream  
          V b  velocity of coaxial gas stream  
          V c  velocity of gas in environmentally controlled chamber ( 128 )