Abstract:
A method has been developed to exploit the desirable material and process characteristics provided by a low powered laser material deposition system, while overcoming the low material deposition rate imposed by the same process. One application of particular importance for this invention is direct fabrication of functional, solid objects from a CAD solid model This method of fabrication uses a software interpreter to electronically slice the CAD model into thin horizontal layers that are subsequently used to drive the deposition apparatus. The apparatus uses a single laser beam to outline the features of the solid object and then uses a series of equally spaced laser beams to quickly fill in the featureless regions. Using the lower powered 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 the multiple laser beams to fill in the featureless areas allows the fabrication process time to be significantly reduced.

Description:
[0001]    This is a division of Ser. No. 09/010,673, Filed Jan. 22, 1998. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of Invention  
           [0003]    This invention relates to the use of multiple beams and nozzles in direct material deposition (DMD) processes in order to increase the deposition rate without compromising material properties or dimensional accuracy.  
           [0004]    2. Description of Prior Art  
           [0005]    Direct material deposition processes allow complex components to be efficiently fabricated in small lot sizes to meet the stringent requirements of the rapidly changing manufacturing environment. This process produces three-dimensional parts directly from a computer aided design (CAD) solid model. U.S. Pat. No. 4,323,756 teaches that complex, net-shaped objects can be built by sequential layer deposition of feedstock material in powder or wire form, whereby the material is directed into a focused laser beam, melted, and deposited onto a deposition substrate to generate solid objects of varying three-dimensional complexity in a layer-wise manner. Other prior art using this method includes “Using the Laser Engineered Net Shaping (LENS™) Process to Produce Complex Components from a CAD Solid Model” by D. M. Keicher et al. in SPIE Conference, San Jose, Calif., January 1997. This method of direct material fabrication uses a single nozzle or powder delivery system that introduces a converging stream of powdered material into the laser beam at or near the beam&#39;s minimum diameter (i.e. focus or focal plane). The stream is at an angle off-normal to a deposition surface whereby uniform geometries of three-dimensional objects can be produced by providing computer controlled motion of the deposition surface relative to the laser beam. Experience has shown, however, that this nozzle design does not provide uniform flow independent of the translation direction. In addition, to achieve goal material properties, the deposition rate is sufficiently low, such that fabrication times required for even intermediate volume objects (100 cubic inches) are prohibitive.  
           [0006]    U.S. Pat. No. 5,043,548 discloses a laser plasma spraying nozzle and method that permits high deposition rates and efficiencies of finely divided particles of a wide range of feed materials. This system uses powdered materials that are carried to the interaction regions via a carrier gas and lasers to melt these particles. However, this system relies solely on the use of a plasma to melt the particles before they are ever introduced to the deposition region. In fact, the carrier gas is often a mixture which promotes ionization, and, as such, the formation of a plasma. The plasma serves to melt the powder particles before they ever come into contact with the deposition substrate. In addition, the beam is diverging such that when it does impact the deposition substrate, the beam irradiance is sufficiently low so that no melting of the deposition substrate occurs. A great distance between the focal point of the laser and the central portion of the plasma is maintained to prevent the substrate from melting. This distance, ranging from 1-6 inches, is a characteristic of this apparatus. The materials are deposited in either a liquid or gaseous state. This design provides a unique method for coating parts; however, it has never been intended for fabrication of multilayered parts. Due to the diverging nature of the powder material, this plasma technique fails to provide the feature definition necessary for fabricating complex, net-shaped objects.  
           [0007]    Another nozzle design is shown in U.S. Pat. No. 4,724,299. This nozzle design requires the powder to be delivered from an annular source that is coaxial with a single laser beam. This design provides a uniform feed of powder to the cladding region, a laser used as an energy source to melt the powder that is to be deposited, and a powder distribution system. However, this system requires that the powder distribution system be contained within the nozzle assembly.  
           [0008]    This nozzle design is very specific to the laser cladding application. The term laser cladding is used specifically to imply surface modification and not the direct fabrication method. More importantly, the design relies on having an annular powder distribution channel to deliver the powder to the deposition region. The annular powder distribution region provides powder to the focused laser beam from all directions and does not concentrate the powder for a tightly focused powder stream. For a single laser beam that is coaxial to the powder flow, this nozzle should work well to provide a uniform layer; however, there is concern that the powder distribution at the deposition surface is greatest at the center of the deposition region, causing it to diminish radially away from the center of the deposition spot. With this change in powder volume uniformity, the inclusion of multiple beams will certainly result in varying line size for parallel deposited lines.  
           [0009]    U.S. Pat No. 4,323,756 also covers the direct metal deposition (DMD) process. This technique uses both wires and powders as filler material. It also uses a single laser beam to deposit the various materials. This patent teaches that the volume of the feedstock material must be less than that of the melted substrate material. However, this reduces the rate of deposition and increases the time to produce parts. The requirement to limit the volume of the feedstock material to be less than that of the melted substrate material was driven by the desire to remove impurities and obtain epitaxial growth. Instead of removing impurities by continuously remelting the previously deposited materials, impurities can be efficiently eliminated by performing the deposition in a controlled atmosphere environment, such as a glove box. Furthermore, expitaxial growth is not desired in most three-dimensional parts, since it may result in anisotropic material characteristics. For most general applications, uniform material properties are desired that do not limit the feedstock volume to be less than that of the deposition substrate melted region.  
           [0010]    The above single laser nozzle&#39;s side design lack the ability to increase the deposition rate of powder without sacrificing vital process conditions, including reduced residual stress, enhanced material properties, process time, and good dimensional repeatability, as well as feature definition. Also, this nozzle design does not provide uniform flow independent of the translation direction. Therefore, such nozzle designs are not suitable for mass 3-D net-shape production.  
           [0011]    U.S. Pat. No. 5,578,227 contains similarities to the present invention, such as the use of a positioning system to direct the location of deposition, and the use of a laser to deposit the feedstock material. However, this patent only uses a single laser beam for the deposition process, which uses wire as the feedstock material. This patent also claims that the laser causes the feedstock material to bond to the previously deposited layer without substantially altering the cross-section of the newly deposited material. Such a continuous form of material would appear to be prone to substantial problems of warpage and distortion of the deposited layers due to an incomplete melting of the feedstock material. For the powder deposition processes, the feedstock material is completely consumed within the 3-D net shape, with the powder&#39;s cross-section being substantially altered.  
           [0012]    A need exists for improved material deposition nozzles for the laser-assisted process.  
         Objects and Advantages  
         [0013]    Accordingly, several objects and advantages of the present invention are:  
           [0014]    (a) to provide multiple beams and nozzles within a single system which will increase the deposition rate without compromising process conditions or time;  
           [0015]    (b) to provide multiple beams and nozzles within a single system which will increase the volume usage rate of feedstock;  
           [0016]    (c) to provide multiple beams and nozzles within a single system which will reduce residual stress within parts by using more lower powered, finely focused laser beams in a single system;  
           [0017]    (d) to provide multiple beams and nozzles within a single system which will achieve good dimensional repeatability and feature definition by using more lower powered, finely focused laser beams in a single system;  
           [0018]    (e) to provide multiple beams and nozzles within a single system which can create uniform material properties in parts that do not limit the feedstock volume to be less than that of the deposition substrate melted region.  
           [0019]    (f) to provide multiple beams and nozzles within a single system which can form streams of particles that are partially or completely melted and can then be consumed to become part of a solid structure.  
           [0020]    Further objects and advantages are to provide multiple beams and nozzles within a single system that allow for the use of either a single laser beam or any combination of laser beams during any part of the process. The powder filler nozzles can be used in a combination of one or more pairs, depending on the needs of the design. Still further objects and advantages will become apparent from a consideration of the ensuing description and drawings. 
       
    
    
     DRAWING FIGURES  
       [0021]    [0021]FIG. 1 schematically illustrates the position of the present invention within a material deposition system.  
         [0022]    [0022]FIG. 2A shows a three-dimensional front view of the multi beam apparatus without the manifold, and a substrate.  
         [0023]    [0023]FIG. 2B is an enlarged front view of the powder nozzles.  
         [0024]    [0024]FIG. 3 is a three-dimensional front view of the deposition head and the deposition substrate.  
         [0025]    [0025]FIG. 4A is three-dimensional close-up of the powder nozzles and deposition substrate.  
         [0026]    [0026]FIG. 4B is an enlarged side view of the powder nozzles.  
         [0027]    [0027]FIG. 5 is a close-up frontal view of the multi beam fill deposition approach, showing the two outline powder nozzles.  
         [0028]    [0028]FIG. 6 is a three-dimensional front view of the multi beam apparatus with the manifold.  
         [0029]    [0029]FIG. 7A is a three-dimensional isometric bottom view of the manifold system connected to the deposition head.  
         [0030]    [0030]FIG. 7B is a three-dimensional isometric top view of the manifold system connected to the deposition head.  
                                                 Reference Numerals in Drawings                                10a   filler powder feeding   10b   outline powder feeding           apparatus       apparatus       12   laser   14   vertical stage       15   focused laser spot   16   prescribed distance       17   spacing between focused   18   focus plane           laser spots       20   sealed chamber   22   convergence plane       24   fill region   26   orthogonal positioning                   stages       32   computer   34   software       36   deposition head   38   beam delivery fibers       40   divergent laser beams   42   collimating lens       44   collimated laser beams   46   focusing lens       48a-h   filler powder nozzles   48i-j   outline powder nozzles       49   powder convergence points   50   focused laser beams       51a-h   stream of powder   52   powder       54   line deposits   56   interaction zone       58   deposition substrate   60a   external deposited feature       60b   internal deposited feature   62   manifold       64a   filler powder inlet tube   64b   outline powder inlet tube       66a-b   filler powder separation   66c   outline powder           channel       separation channel       68a   filler powder layer-   68b-e   outline powder layer-           connecting orifice       connecting orifice       70a   filler powder delivery tube   70b   outline powder delivery                   tube       72   front view of powder   74   side view of powder           nozzles       nozzles                  
 
       SUMMARY OF THE INVENTION  
       [0031]    The present invention provides an apparatus for the use of multi beams and nozzles in the deposition of materials to form three-dimensional parts whereby the deposition rate increases. The deposition head assembly consists of the following features: an array of output powder nozzles for creating a converging flow of powder to the deposition region, and a central orifice which allows the multiple beams to be focused onto the deposition substrate. In particular, the invention includes a manifold system designed into the deposition head assembly that can use more than one laser beam simultaneously for the deposition process.  
         [0032]    The use of multiple beams and nozzles within a single system allows an increase in the deposition rate without compromising the process conditions or time. The present invention will increase the volume of feedstock used in a given time. The use of more than one laser beam allows for lower powered, finely focused laser beams to be used in order to minimize residual stress within parts. This characteristic also achieves good dimensional repeatability and feature definition, while providing excellent material properties.  
         [0033]    The present invention also provides a means for using either a single laser beam or any combination of laser beams during any part of the process. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0034]    A schematic of the preferred embodiment of this invention is given in FIG. 1, showing the position of the invention within a direct material deposition system. The system includes: a powder feeding apparatus  10 , to deliver a uniform flow of powder to the deposition region; a laser  12 , to cause heating and subsequent melting of the powder feed particles; and a deposition head/heads  36 . The system also includes a set of orthogonal positioning stages  26 , which are computer driven to direct the location of deposition; a vertical stage  14 ; a computer  32 , on which software  34  is used to slice the CAD solid models and generate a motion control program to control deposition processing sequence based on CAD file data; and a sealed chamber  20 , to contain the powder particles during processing and provide an inert environment.  
         [0035]    Referring to FIGS. 2A and 2 B, the multi beam deposition apparatus begins with four beam delivery fibers  38 , that are equally spaced apart at a prescribed distance  16 , which are used to transport the laser beams to the deposition apparatus. The beams out of the fiber are divergent laser beams  40  and transmitted through the spherically shaped collimating lens  42 . The laser beams then leave the collimating lens  42  as collimated laser beams  44  located above the deposition head  36 . These collimated laser beams  44  are then transmitted through a second spherically shaped focusing lens  46  to be focused onto the deposition substrate  58 . The focused laser beams  50  create a linear array of focused laser spots  15  on the deposition substrate  58 . The focus plane  18  of the focused laser spots is located at/or near the deposition substrate, which is also the same as the deposition substrate  58 . This spacing between the focused laser spots  17  is dictated by the prescribed distance  16  between the beam delivery fibers  38  and the magnification provided by the imaging system created by the combination of the collimating lens  42  and the focusing lens  46 .  
         [0036]    For this configuration, the deposition head is conical in shape. An array of powder nozzles  48  project from underneath the deposition head  36 . Streams of powder  51   a - d  exit from the powder nozzles  48  and interact with the focused laser beams  50  to form line deposits  54  on a deposition substrate  58 .  
         [0037]    [0037]FIG. 3 shows the relationship between the focused laser beams  50 , the powder  52 , and the deposition substrate  58 . The focused laser beams  50  are shown both above and below the deposition head  36 . The powder  52  and focused laser beams  50  then interact below the deposition head  36  in order to form features  60  on the deposition substrate and to deposit material in the region between the features to form a uniform solid layer of material. For this process, the internal deposited features  60   b  and external deposited features  60  a on the deposition substrate  58  serve to define the geometry of the component being fabricated as well as provide boundaries for the fill region  24 .  
         [0038]    Referring to FIGS. 4A and 4B, the multi beam deposition apparatus includes eight filler powder nozzles  48   a - h  and two outline powder nozzles  48   i - j,  of which only one is shown  48   i , that project from the deposition head  36 . Streams of powder  51   a - h  are ejected from the powder nozzles  48   a - h . The eight filler powder nozzles  48   a - h  are positioned within the deposition head  36  such that the stream of powder  51   a - d  from one filler powder nozzle  48   a - d  intersects with the stream of powder  51   e - h  from a second filler powder nozzle  48   e - h  from an opposing direction. In this fashion, the streams of powder  51  from two of the eight powder filler nozzles  48  interact to form a pair of filler powder nozzles whose powder streams form a powder convergence point  49  on the deposition substrate  58 . The convergence plane  22 , which cuts through the intersection of the powder streams from the nozzles and is orthogonal to the propagation direction of the laser beams, is located at/or near the deposition plane, which parallels the top surface of the orthogonal positioning stages  26 , as shown in FIG. 1. From these eight filler powder nozzles  48   a - h,  four pairs of filler powder nozzles interact to form four powder convergence points  49 . The powder convergence points  49  can be described as the location where the area circumscribed by one powder nozzle overlays the area circumscribed by the second powder nozzle forming one pair of interacting powder nozzles. The four powder convergence points form a linear pattern on the substrate and are separated by a distance equal to the spacing of the focused laser spots  17 , as shown in FIG. 2B. These powder convergence points  49  are located so that the elliptical pattern circumscribed by the powder convergence point  49  is concentric with one of the focus laser spots  15 .  
         [0039]    All powder nozzles, regardless of whether they are outline powder nozzles  48   i - j  or filler powder nozzles  48   a - h,  join the focused laser beams  50  in the interaction zone  56 . However, not all types of powder nozzles are always working at the same time. Outline powder nozzles  48   i - j  interact with one pair of the filler powder nozzles  48   a - h  to provide powder that is used to form external deposited features  60  a and internal deposited features  60   b.  Filler powder nozzles  48   a - h  then operate independently of the outline powder nozzles  48   i  to provide streams of powder  51   a - d  that form the uniform deposited solid material layer between the boundaries created by various combinations of external deposited features  60   a  and internal deposited features  60   b.  Each of the focused laser beams  50  has independent on-off control to allow the length of each of the layer fill lines to be varied independently. This feature permits the solid material layers to be formed within regions whose external deposited features  60  a or internal deposited features  60   b  do not conform to the linear front or the width of the deposition region formed by the linear array of focused laser spots  15 . Each of the line deposits  54  can be sequenced on or off as the various line deposits intersect external deposited features  60   a  or internal deposited features  60   b,  allowing material to be deposited in desired regions only.  
         [0040]    [0040]FIG. 5 shows a close-up frontal view of the multi beams and nozzles in contrast to FIG. 4B. In this diagram, the outline powder nozzles  48   i - j  are shown to be directed to interact with only one pair of the filler powder nozzles  48   a - h.  This geometry allows uniform deposition of the external deposited features and internal deposited features independent of the direction of motion of the deposition substrate  58  relative to the deposition head. The powder to the outline nozzles  48   i - j  is directed at the deposition surface only when the external deposited features and internal deposited features are being formed. Otherwise, powder is not flowing from the outline powder nozzles  48   i - j.    
         [0041]    [0041]FIG. 6 shows the multi beam deposition apparatus with a manifold  62  and powder inlet tubes  64   a - b.  For this apparatus, the powder is delivered to the deposition head  36  via a carrier gas. This gas can be inert or reactive depending on the specific application. Once the powder is entrained in the gas stream, it is introduced into the deposition head  36  through the powder inlet tubes  64   a - b.  The powder then exits the deposition head  36  through the filler powder delivery tubes  70   a  and the outline powder delivery tubes  70   b,  as shown in FIGS. 7A and 7B.  
         [0042]    The internal structure of the manifold system  62  is shown in FIGS. 7A and 7B. The powder is injected into the filler powder inlet tube  64  a and enters into a manifold system  62  that separates the powder into eight approximately equal parts. To separate the powder into the eight approximately equal parts, two series of separation channels  66   a - b  and an orifice  68   a  are used. The powder first enters the deposition head  36  through the filler powder inlet tube  64   a  and is separated into two approximately equal parts. One half of the powder is then carried through the first filler powder separation channel  66   a.  At the end of this filler powder separation channel  66   a,  the powder is again separated into two approximately equal parts and directed through the filler powder layer-connecting orifice  68   a.  After passing through the orifice  68   a,  the powder is directed down through the deposition head  36  to exit out of the deposition head through the filler powder delivery tubes  70   a  and finally out the filler powder nozzles. The path for the outline powder is similar to that for the filler powder. A second powder feed unit is used to meter the powder for the outline powder nozzles. The powder used for the outline powder nozzles first enters the deposition head  36  through the outline powder inlet tube  64   b  and is separated into two approximately equal parts. One half of the powder is then carried through the outline powder separation channel  66   c.  At the end of this channel  66   c,  the powder is directed downward through a series of outline powder layer-connecting orifices  68   b - e,  exiting the deposition head  36  through the outline powder delivery tubes  70   b,  and finally out of the outline powder nozzles, onto the top surface of the deposition substrate. The powder flow rate from the second powder feed unit is metered to be approximately equal to one fourth of the total quantity of powder delivered to the filler powder nozzles. This method of powder separation relies principally on the path lengths for each of the nozzles to be approximately equal with an equal pressure drop at each nozzle.  
       OPERATION OF THE PREFERRED EMBODIMENT  
       [0043]    This invention can be explained in terms of the multiple nozzle deposition apparatus and the multiple laser beam deposition apparatus. The multiple nozzle deposition apparatus has been developed for the application of powdered materials onto a substrate to create a uniform fused layer in a desired pattern with exceptional material properties, very good dimensional accuracy, and at a rate that is practical for the direct fabrication of solid objects. In the preferred embodiment, four beam delivery fibers  38  transmit approximately equally powered laser beams that become four focused laser beams  50  that are used as heat sources to melt the powder supplied to the deposition region, where it is fused to the deposition substrate  58 , as shown in FIGS. 2A, 2B, and  6 . For this invention, the number of focused laser beams  50  is not nearly as important as the concept of using multiple beams to achieve normally conflicting requirements of a high deposition rate coupled with exceptional material properties and very good dimensional accuracy. The trend among current research groups has been to focus on using a single laser beam with increased power to achieve the higher deposition rates. Although the high-powered laser beam does allow these groups to achieve the high deposition rates, the results reported for material properties and dimensional accuracy are poor. Conversely, using a lower powered laser beam of several hundred watts has allowed solid parts to be made with increased material strength and ductility with very good accuracy. This is the cornerstone on which this invention builds upon.  
         [0044]    For the multiple beam deposition apparatus, one or more powder feeding apparatus  10   a - b  are used to supply powdered materials to the deposition head  36 , as shown in FIG. 1. The powdered material from these sources can be either gravity fed or, preferably, carried to the deposition region via a carrier gas. The carrier gas can be inert or reactive with the powder material. The powder is entrained in a gas stream and brought into the multiple beam deposition apparatus through either of the two powder inlet tubes  64   a - b,  as shown in FIGS. 6, 7A, and  7 B. The two different powder feeding apparatus  10   a - b  supply powder for either the filler powder nozzles  48   a - h  or the outline powder nozzles  48   i - j . The manifold  62  serves to separate the powder input to the deposition apparatus into approximately equal parts for the deposition process. For the filler powder, the powder enters the powder separation manifold  62  through the filler powder inlet tube  64   a  and is directed onto the wall of the powder passage. The collisions of the powder with the passage wall along with the length of each passage being approximately equivalent for each of the nozzles serves to separate the powder into approximately equal parts for the deposition process. After being separated into several parts, the gas entrained powder material exits from the multiple beam deposition apparatus from a series of nozzles located in the base of the multiple beam deposition apparatus. In a similar fashion, the powdered material from the second powder feeding apparatus  10   b  is separated into two approximately equal parts and directed out of the multiple beam deposition apparatus from the outline powder nozzles  48   i - j.  The volume of powder output from the filler powder nozzles and the outline powder nozzles is controlled to be equivalent by monitoring the volumetric powder flow out of the two powder feeding apparatus  10   a - b.    
         [0045]    Referring to FIGS. 4A and 4B, each of the filler powder nozzles  48   a - h  is located within the multiple beam deposition apparatus, such that the powder stream out of one filler powder nozzle interacts with the powder stream from a second filler powder nozzle to create a plane of convergence  22 . This plane of convergence  22  is similar to the focal plane of one of the focused laser beams  50  from the multiple beam deposition apparatus. Deposition of the powdered material occurs in a plane ±0.5 inches above or below this plane of convergence  22 . Referring to the multiple beam deposition apparatus in the description section, each of the filler powder nozzles  48   a - d  interacts with only one other filler powder nozzle  48   e - h  to create four pairs of filler powder nozzles whose powder convergence planes are located at a similar position from the multiple beam deposition apparatus. In a similar fashion, the outline powder nozzles  48   i - j  are located within the multiple beam deposition apparatus such that the powder stream from each of these nozzles interacts to form a convergence plane  22 . This convergence plane  22  is located at a similar distance from the deposition apparatus as is the convergence plane formed by the filler powder nozzles  48   a - h.  It should be noted that the convergence plane of the intersecting powder streams from the outline powder nozzles is similar to that for the filler powder nozzle pairs, and the convergence point of the intersecting powder streams from the outline powder nozzles is co-located with the convergence point created by the powder from one pair of filler powder nozzles. During the outline process, the center pair of filler powder nozzles interact with the outline nozzles to provide uniform flow from all four directions, simulating a Cartesian coordinate. The convergence plane created by the interacting outline powder nozzles  48   i - j  forms the plane at/or near the plane where the material deposition will occur.  
         [0046]    In the convergence plane, each pair of the filler powder nozzles  48   a - h  interacts to form an elliptical spot that has the maximum powder density in its central region. One of the focused laser beams is directed through this location of maximum powder density such that the focused laser beam is centered within the elliptical spot formed by the two powder nozzle streams. This is similar for each of the filler powder nozzle pairs, each pair having one laser beam. Each of the laser beams interacts with both powdered materials and the deposition substrate  58  to cause the powdered material to become molten and be fused/bonded to the surface of the deposition substrate  58 . The elliptical spot created by the interacting outline powder nozzles  48   i - j  is located to be coincident with the elliptical spot created by one set of the filler powder nozzles and the focused laser beam that passes through this spot. In this fashion, a single deposition head  36  can be used to both outline features and then fill in the featureless regions in an expeditious fashion. The powder flowing to the outline powder nozzles  48   i - j  is only flowing to the deposition region where feature outlining will occur. During the feature outline deposition process, only the focused laser beam that interacts with the filler powder nozzles aligned with the outline powder nozzles is transmitted to the deposition surface. In this fashion, the uniform fill layer thickness can be preserved and the outline layer thickness can be made uniform to the fill layer thickness by control of the traverse rate of the deposition substrate  58  relative to the focused laser beam. Although the embodiment described uses a single deposition head for the material deposition application, this operation can also be performed using multiple deposition heads  36  with one or more powder feeding apparatus.  
         [0047]    This invention can be used for material cladding, surface modification, or other processes where the addition of a layer or region of material is required. One application of particular importance in using the multiple beam deposition apparatus involves building a solid object from the powdered material one layer at a time. This direct fabrication application will be used to describe the general operation of the deposition system that incorporates the multiple beam deposition apparatus. For the direct fabrication application, a computer aided design (CAD) solid model of the component to be fabricated is used. The CAD solid model, which represents the component in its entirety, is first sectioned into thin layers within the computer. This operation is generally referred to as a slicing operation. Each of these layers represents a cross-section of the component at a given distance away from the base of the component, including a hatching pattern that defines the regions to be filled during the deposition process. The hatching pattern includes a series of equally spaced parallel lines that are used to define the line deposition regions within the areas to be filled.  
         [0048]    Referring to FIGS. 2A and 2B, the deposition process is similar to a printing process in which a thick printed layer is deposited. The computer transforms the layer information into a series of control commands for the deposition process. Typically, the outline of the feature is deposited first using the outline powder nozzles  48   i - j  and the filler powder nozzles  48   a - h  that coincide. The deposition substrate  58  is located beneath the multiple beam deposition apparatus and the outline process is initiated by turning on the outlining beam and moving the part relative to the beam to outline both the internal and external features required for each layer. The outline layers can have a varying thickness in comparison to the fill layers. Then the flow of powder to the outline powder nozzles  48   i - j  is stopped and the filling process occurs. The filler powder nozzles  48   a - h  interact with the focused laser beams  50  to create lines that are equally spaced apart such that an integer number of lines will exactly fill the regions between the lines. The first series of lines is deposited. Next, the substrate  58  is translated relative to the deposition head  36  such that a second set of lines can be deposited alongside the first set of lines. The operation continues until the space between the first set of equally spaced parallel lines is completely filled with material, creating a flat layer with a uniform thickness. After this region is filled, the substrate  58  is again translated relative to the deposition head  36  to begin to fill the region adjacent to the first fill region. The fill process is repeated until the space between the first set of equally spaced parallel lines in the second region is filled, once again creating a flat layer with a uniform thickness, which is also equal to the thickness of a line deposit  54 . This process is repeated across the layer until the entire area requiring material has been filled. Once this layer is deposited, the multiple beam deposition apparatus is translated away from the deposition plane, a distance equal to the thickness of the flat layer. This entire process is repeated to deposit subsequent layers in a sequential manner, until the finished object is completed.  
         [0049]    It is also possible to have different scan patterns for alternating layers, while maintaining the process time advantage of multiple beams and nozzles. This is achieved by rotating the orientation of the deposition surface relative to the deposition head  36  about the axis, which is normal to the deposition surface. For example, if a Cartesian coordinate system is used to define the three-dimensional space above the substrate, where the deposition substrate lies in the X,Y plane, then the rotation would occur about the Z-axis. This is desired in some parts because it creates a uniform heating within the part that results in uniform stress throughout the part.  
         [0050]    Independent control of the focused beams  50  transmitted to the surface of the deposition substrate  58  allows each beam to be modulated on and off independent of the other beams. During the filling operation for each layer, each of the beams is turned on and off as the deposition lines intersect the external deposited feature  60   a  and internal deposited features  60   b.  In this fashion, the filling process can be controlled to fill the areas where material is required and leave void those areas that do not require material. One alternative to leaving certain regions void is to apply a second material which is sacrificial and used only as a support structure material.  
         [0051]    While this invention has been described as having a preferred embodiment, it is understood that it is capable of further modifications, uses and/or adaptations of the invention, following in general the principle of the invention and including such departures of the present disclosure as come within known or customary practice in the art to which the invention pertains, and as may be applied to the central features hereinbefore set forth, and fall within the scope of the invention of the limits of the appended claims.