Laser Engineered Net Shaping, also known as LENSTM, is an advanced manufacturing technique used to
fabricate near-net shaped, fully dense metal components directly from computer solid models without the
use oftraditional machining processes. The LENSTM process uses a high powered laser to create a molten
pool into which powdered metal is injected and solidified. Like many SFF techniques, LENSTM parts are
made through a layer additive process. In the current system, for any given layer, the laser is held
stationary, while the part and its associated substrate is moved, allowing for the each layer's geometry to
be formed. Individual layers are generated by tracing out the desired border, followed by filling in the
remaining volume. Recent research into LENSTM has highlighted the sensitivity ofthe processes to
multiple software controllable parameters such as substrate travel velocity, border representation, and fill
patterns. This research is aimed at determining optimal border outlines and fill patterns for LENSTM and
at developing the associated software necessary for automating the creation ofthe desired motion control.Mechanical Engineerin

Ensz, M. T.

Griffith, M. L.

Harrwell, L. D.

English

Laser Engineered Net Shaping, also known as LENS{trademark}, is an advanced manufacturing technique used to fabricate near-net shaped, fully dense metal components directly from computer solid models without the use of traditional machining processes. The LENS{trademark} process uses a high powered laser to create a molten pool into which powdered metal is injected and solidified. Like many SFF techniques, LENS{trademark} parts are made through a layer additive process. In the current system, for any given layer, the laser is held stationary, while the part and its associated substrate is moved, allowing for the each layer`s geometry to be formed. Individual layers are generated by tracing out the desired border, followed by filling in the remaining volume. Recent research into LENS{trademark} has highlighted the sensitivity of the processes to multiple software controllable parameters such as substrate travel velocity, border representation, and fill patterns. This research is aimed at determining optimal border outlines and fill patterns for LENS{trademark} and at developing the associated software necessary for automating the creation of the desired motion control

Harwell, L. D.

UNT Digital Library

I Software Development for Laser Engineered Net Shaping M. T. Ensz, M. L. Griffith, L. D. Harwell Sandia National Laboratories Albuquerque, NM 87185 CON F- 4 B 0 g 26 - - Abstract Laser Engineered Net Shaping, also known as LENSTM, is an advanced manufacturing technique used to fabricate near-net shaped, fully dense metal components directly from computer solid models without the use of traditional machining processes. The LENSTM process uses a high powered laser to create a molten pool into which powdered metal is injected and solidified. Like many SFF techniques, LENSTM parts are made through a layer additive process. In the current system, for any given layer, the laser is held stationary, while the part and its associated substrate is moved, allowing for the each layer’s geometry to be formed. Individual layers are generated by tracing out the desired border, followed by filling in the remaining volume. Recent research into LENSTM has highlighted the sensitivity of the processes to multiple software controllable parameters such as substrate travel velocity, border representation, and fill patterns. This research is aimed at determining optimal border outlines and fill patterns for LENSTM and at developing the associated software necessary for automating the creation of the desired motion control. Introduction During the past few years, the capabilities of solid freeform fabrication have progressed enough to allow for the direct fabrication of fully dense metallic components using computer aided design (CAD) models [ 1-41. One such technique, being developed at Sandia National Laboratories to fabricate high strength, near net shape metallic components, is Laser Engineered Net Shaping (LENSTM). In the past several years, a variety of components have been fabricated using LENSTM for applications ranging from part prototypes to tooling for injection molding [5-61. The basic LENSTM system consists of a high power Nd:YAG laser, a 3-axis computer controlled positioning system, and multiple powder feed units. The positioning stages are mounted inside an argon- filled glove box (nominal oxygen level of 2-3 ppm), while the laser beam enters the glove box through a top mounted window. A powder delivery nozzle is used to inject a metal powder stream directly into the focused laser beam. The lens and powder delivery nozzle move as an integral unit in the z-axis, while the part, positioned under the laser beam, is transitioned in x and y .  To create a part, a CAD solid model is sliced into a sequence of cross-sections that are then translated into a series of tool path patterns to build each layer. The laser beam is focused onto a substrate (or previous layer of the part) to create a weld pool into which powder is simultaneously injected to buildup each layer. The substrate is translated beneath the laser beam to deposit the desired geometry for the current layer. After completing a layer, the powder delivery nozzle and focusing lens assembly is incremented in the z-direction, and the process begins again. After determining the basic LENSTM parameters (i.e. laser power, powder feed rate, traverse velocities, layer thicknesses and hatch spacings) for a chosen material or materials, extrusion or solid geometries may be fabricated. With a full understanding of the LENSTM parameters and with proper computer DISTR This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or use- fulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any spe- cific commercial product, process, or service by trade name, trademark, manufac- turer, or otherwise does not nccessarily constitute or imply its endorsement, m m -  mend.ation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or refiect those of the United S t a t a  Government or any agency thereof. DISCLAIMER Portions of this document may be illegible electronic image products. Images are produced from the best available original document. Figure 1. Intersection of a triangular mesh with a plane typically results in a 2D contour with redundant vertices. control of the LENSTM process, fully dense parts may be repeatedly built with errors less than +0.005” in the x and y-axes, and less than 0.0 15” in the z-axis. Recent research into LENSTM has highlighted the sensitivity of the processes to multiple software controllable parameters such as substrate travel velocity, border representation, and fill patterns. This research is aimed at determining optimal border outlines and fill patterns for LENSTM and at developing the associated software necessary for automating the creation of the desired motion control. Overall Software Flow For any layered manufacturing technology, the overall goal of the control software is to take a CAD representation of the part, slice the model into appropriate thickness layers, and create path instructions for the manufacturing equipment. For LENSTM, the path planning process begins with an object defined using the STL format. This triangulated mesh is intersected with appropriately defined planes in order to create 2D contour information for each layer to be formed. These contours are then refined to remove any vertex redundancies and to improve part quality. After cleanup, if creating a solid part, the contours are used as a basis for creating the necessary fill pattern. The final result of the path planning process is a tool path program (G-code) that is used to drive the three axes of the LENSTM machine. File Format Used by LENS As previously stated, the LENSTM process starts with an object defined through the RP industry standard STL format. While the deficiencies of this format are well understood, the use of STL meshes for object definition does allow a large variety of CAD packages to be used to develop parts for use with LENSTM. Further, the simple triangular format used by STL allows for relatively easy manipulation of the part files. While the STL format is typically not a precise representation of the original CAD model (curved surfaces are faceted), meshes can be readily cut which are well within the tolerances of the LENSTM process. The only true difficulty posed by the use of the STL format is that the triangular representation, when sliced, often results in vertex redundancies (straight edges defined by more than two vertices, arcs being “over defined”, etc.). As will be discussed, these redundancies will often have adverse affects on the build process. To minimize these affects, the contours require significant refinement. Con tour Ref i neme n t Due to the nature of triangular meshes, when intersecting a mesh with a plane to create a 2D contour, the vertices formed hom the intersection process will inevitably contain numerous “redundant” vertices - vertices that do not add information about the contour’s geometry or topology. Even for a simple cube, as (4 (b) Figure 2. (a) Using too many edges to define contours results in excessive buildup. (b) Reducing the number of edges used to define the contour produces a more desirable part. shown in Figure 1, a straightforward intersection of any plane with the triangular mesh will create line segments which contain more than the minimum two vertices. While these redundancies may not pose problems for other layered manufacturing processes, they may significantly affect the quality of parts produced through the LENSTM process. Primarily, each line segment stadstop reduces the physical machine speed as the stages are forced to decelerate and accelerate through each vertex. Since the LENSTM build rate is related to laser energy per unit time, decreasing the stages’ velocities results in a significant alteration of the metal deposition. Another obstacle, resulting from the use of a triangulated mesh to define the object, is a possible “over refinement” of arcs and circles. As with over-defined straight segments, the accelerations and decelerations through each vertex of an over defined arc results in a decrease in the travel speed and a corresponding increase in the metal deposition rate (Figure 2(a)). The contour refinement process is relatively straightforward. First, any duplicate vertices are removed from the contour. (Duplicate vertices are considered as those that are coincident to each other within some epsilon.) Next, any redundant vertices are located and removed from line segments. To locate the redundant vertices, the included angle of the two line segments that share each vertex is computed. Any vertex whose included angle is greater than a user defined value (typically around 179”) is removed. It should be noted that this process also eliminates excessive vertices in arcs or other curve segments. It is also possible at this stage to’locate any arcs or circles in the contour representation and replace the segmented representation with appropriate second order formulations. As shown in Figure 2(b), dramatic improvements in part quality may be obtained through proper contour refinement. Finally, any desired beam offset calculations are applied to the contour. It should be noted that for LENSTM, the beam offset serves several purposes. First, beam offset may be utilized in its traditional fashion, correcting for the fact that the weld pool created by the laser has a finite width by providing an (4 (b) Figure 3. (a) With no beam offset applied, the weld beads drawn for the borders at the tip of the channel overlap, resulting in an uneven surface. (b) Applying beam offset eliminates the overlap, resulting in a flatter top face. Border subject to gas flow disturbances overlap Figure 4. For even basic geometry, many problems may be encountered during a till pass. Since the width of the weld pool will not always produce weld bead edges aligned with the border geometry, regions of large overlap, and regions with voids may be formed. Further, gas currents blowing back from the part may affect the powder stream, especially in tight areas. If the same till pattern is repeated, the small disturbances produced by these flaws will build upon themselves, resulting in a poor final product. offset compensation to account for this width. This compensation not only provides for improved dimensional accuracy in the ny-plane, but also assists in creating flat, even layers by preventing weld beads from overlapping each other (Figure 3). Since LENSTM is focusing on creating near-net shapes, rather than net shapes, beam offset may also be used to control the amount of excessive material left on a part, providing for the proper amount of stock required for the final finishing process. Fill Pattern For solid parts, an appropriate technique must be utilized to create the necessary fill pattern. The choices for this fill technique may be broken down into one of two primary methods: rasters and c'conformal'' contours. Regardless of method, the fill technique must meet one primary requirement. Since the metal for material buildup is injected into the LENSTM part as a powder stream, effects of the localized geometry on the powder stream must be controlled. Further, it must be recognized that the weld beads have finite width; widths that may not always "fit" evenly into a layer's border geometry, causing both regions of excessive overlap and possible voids (Figure 4). While neither overlaps nor voids are in and of I (4 (b) (c> Figure 5. (a) Created using conformal contours. (b) Created using a O", 90" raster hatch pattern. (c) Created using a 0", 105", 220", ... hatch pattern. Figure 6. (a) Shallow angles of attack between border vectors and the fill rasters produce buildup along borders. (b) Eliminating these shallow attack angles produces improved characteristics. themselves detrimental to the build process (overlaps will produce a slight increase in thickness, while voids will result in a slight decrease), the errors caused by these localized flaws can, if compounded across enough layers, will result in a poor part. Attacking the geometry from different directions during the fill process can alleviate the localized affects. To accomplish the different angles of attack, the fill pattern must be designed such that a "randomness" is introduced into the build process. Conformal contours, which follow the outline of the part in decreasing size, were studied as one possible fill technique. However, basic experiments with conformal contours revealed that even for simple cylindrical parts, the.required randomness is not introduced, resulting in extremely poor builds (Figure 5(a)). The use of raster fills has proven an effective method of introducing the necessary variations between the fill vectors on each layer. However, even here, care must be exercised when determining the raster angles on a per layer basis. Early work on LENSTM utilized rasters that were always oriented at either 0" or 90". While a vast improvement over the use of conformal contours, these static angles were still the source of localized build errors, due to the repetition of the fill angles. As a result, regions aligned with the x and y-axes would often experience unevenness in the build (Figure 5(b)). The first attempt at introducing more variation into the hatch angles was to use randomly generated values for the hatch orientations. However, this allowed for angles that are just a few degrees off of the "standard" angles of 0", 30", 45", 60", and 90". When filling near these standard angles with a shallow angle of attack (e.g. filling along a 30" border with a 3 1" hatch angle), the width of the weld bead repeatedly overlaps the border, producing a significant buildup, again resulting in an uneven build, as demonstrated in Figure 6. Furthermore, retracting the hatch to prevent the overlap was undesirable due to the likelihood of forming voids in the part. After several iterations, the angle pattern converged on using a value of 105" between successive layers. This value provided several key benefits. First, the fiII angles across two layers are nearly orthogonal, allowing for a large differentiation of attack angle to minimize the localized geometry effects. Also, at 105", it takes 12 layers before any raster angle is repeated in a part, which further introduces pseudo-randomness into the process. Finally, 105" causes the standard angles to be either hit exactly, or at an angle greater than or equal to 15", preventing the shallow angle of attack problems previously discussed. As shown in Figure 5(c), the results obtained through the 105" hatch algorithm are significantly improved over both 0", 90" hatching and conformal contours. Future Work Even though the current three-axis LENSTM system is “officially” a two- 1/2D process, the weld pool characteristics, combined with the high weld pool fi-eezing rates do allow for the creation of slight overhung surfaces. To date, parts with overhangs up to 15” (for 0.01 5” thick layers) have been created. However, the creations of overhangs is still not fully understood in terms of the best speeds and feeds required to consistently create even, non-distorted layers. Future work will study what software alterations can be used to aid in the process of creating overhung surfaces. Another near-term task being studied is what software controls will be required for creating complex, multi-material geometries. While previous work has looked at creating layered objects (where the material choice for each layer is user defined on a layer-by-layer basis), future work will look at the software controls needed to identify and control material composition throughout any given layer. Recent experiments with using implicit blending functions, combined with surface-distance functions, have proved promising [7]. Conclusions Recent experience in LENSTM has demonstrated the strong reliance that the metal deposition process has to parameters such as border representations and fill patterns. This research was aimed at understanding and developing optimized software solutions for the generation of LENSTM parts. Specifically, this work studied the affects of the border representation schemes, identified difficulties associated with over- defined representations, and developed methods of refining the contour data to improve the build process. Further, this research was directed at identifying an “ideal” fill pattern for creating solid parts. The final result of the fill pattern studies produced pseudo-random raster fill technique which introduces the necessary randomness into the build process to minimize localized geometry affects, while preventing problems with shallow angles of attack along standard design surfaces. Acknowledgements This work is supported by the U.S. Department of Energy under contract DE-AC04-94ALS50000. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy. Ref e re n ces [I] M.L. Griffith, D.M. Keicher, C.L.Atwood, J.A. Romero, J.E. Smugeresky, L.D. Harwell, D.L. Greene, Free Form Fabrication of Metallic Components using Laser Engineered Net Shaping (LENS Ty, Proceedings of the Solid Freeform Fabrication Symposium, August 12-14, 1996, Austin, TX, p. 125. [2] J.R. Fessler, R. Merz, A.H. Nickel, F.B. Prim, L.E. Weiss, Laser Deposition of Metals for Shape Deposition Manufacturing, Proceedings of the Solid Freeform Fabrication Symposium, August 12- 14, 1996, Austin, TX, p. 117. [3]  F.Klocke, H. Wirtz, W. Meiners, Direct Manufacturing of Metal Prototypes and Prototype Tools, Proceedings of the Solid Freeform Fabrication Symposium, August 12-14, 1996, Austin, TX, p. 141. [4] J. Mazumder, K. Nagarathnam, J.Choi, J. Koch, D. Hetzner, The Direct Metal Deposition of HI 3 Tool Steel for 3-0  Components, Journal of Materials, Vol. 49, Number 5, May 1997, p. 55.  . . .  . .  [5] D.M. Keicher, J.L. Jellison, L.P. Schanwald, J.A. Romero, D.H. Abbott, Towards a Reliable Laser Powder Deposition System through Process Characterization, 27' International SAMPE Technical Conference, Vol. 27, Diversity into the Next Century, proceedings of SAMPE '95, Albuquerque, NM, Oct. 12-14, 1 9 9 5 , ~ .  1029. [6] M. L. Griffith, C.L. Atwood, J.E. Smugeresky, L.D. Harwell, D.L. Greene, E. Schlienger, Using Laser Engineered Net Shaping (LENS Ty to Fabricate Metal Components, proceedings of the Rapid Prototyping and Manufacturing Conference, April 22-24, 1997, Dearborn, MI. [7] C.T. Lim, M.T. Ensz, M.A. Ganter, D.W. Storti, Object Reconstruction9om Layered Data using Implicit Solid Modeling, Journal of Manufacturing Systems, Vol. 16, Number 4, 1997, p. 260. 

Software development for Laser Engineered Net Shaping

Texas ScholarWorks

Software Development forLaser Engineered Net ShapingM. T. Ensz, M. L. Griffith, L. D.llarwellSandia National LaboratoriesAlbuquerque, NM 87185AbstractLaser Engineered Net Shaping, also known as LENSTM, is an advanced manufacturing technique used tofabricate near-net shaped, fully dense metal components directly from computer solid models without theuse of traditional machining processes. The LENSTM process uses a high powered laser to create a moltenpool into which powdered metal is injected and solidified. Like many SFF techniques, LENSTM parts aremade through a layer additive process. In the current system, for any given layer, the laser is heldstationary, while the part and its associated substrate is moved, allowing for the each layer's geometry tobe formed. Individual layers are generated by tracing out the desired border, followed by filling in theremaining volume. Recent research into LENSTM has highlighted the sensitivity of the processes tomultiple software controllable parameters such as substrate travel velocity, border representation, and fillpatterns. This research is aimed at determining optimal border outlines and fill patterns for LENSTM andat developing the associated software necessary for automating the creation of the desired motion control.IntroductionDuring the past few years, the capabilities of solid freeform fabrication have progressed enough to allowfor the direct fabrication of fully dense metallic components using computer aided design (CAD) models[1-4]. One such technique, being developed at Sandia National Laboratories to fabricate high strength,near net shape metallic components, is Laser Engineered Net Shaping (LENSTM). In the past severalyears, a variety ofcomponents have been fabricated using LENSTM for applications ranging from partprototypes to tooling for injection molding [5-6].The basic LENSTM system consists ofa high power Nd:YAG laser, a 3-axis computer controlledpositioning system, and multiple powder feed units. The positioning stages are mounted inside an argon-filled glove box (nominal oxygen level of2-3 ppm), while the laser beam enters the glove box through atop mounted window. A powder delivery nozzle is used to inject a metal powder stream directly into thefocused laser beam. The lens and powder delivery nozzle move as an integral unit in the z-axis, while thepart, positioned under the laser beam, is transitioned in x and y. .To create a part, a CAD solid model is sliced into a sequence of cross-sections that are then translatedinto a series of tool path patterns to build each layer. The laser beam is focused onto a substrate (orprevious layer of the part) to create a weld pool into which powder is simultaneously injected to buildupeach layer. The substrate is translated beneath the laser beam to deposit the desired geometry for thecurrent layer. After completing a layer, the powder delivery nozzle and focusing lens assembly isincremented in the z-direction, and the process begins again.After determining the basic LENSTM parameters (Le. laser power, powder feed rate, traverse velocities,layer thicknesses and hatch spacings) for a chosen material or materials, extrusion or solid geometriesmay be fabricated. With a full understanding of the LENSTM parameters and with proper computer359Figure i'lntersection.ofa triangular me~h with a planetypically results in a 2D contour with redundant vertices.control ofthe LENSTM process, fully dense parts may be repeatedly built with errors less than +0.005" inthe x andy-axes, an~ less than 0.015" in the z-axis.Rec~ntresearchintoLENSTMhashighlightedthe sensitivity ofthe processes to multiple softwarecontrollal>leiparam~terssuchassul>stratetravel velocity, border representation, and fill patterns. Thisresearch is aimed at determining optimal border outlines and fill patterns for LENSTM and at developingthe associated software necessary for automating the creation of the desired motion control.Ovel"allSoft'WareFlowFor any layered manufacturing technology, the overall goal of the control software is to take a CADrepresentation of the part, slice the model into appropriate thickness layers, and create path instructionsfor the manufacturing equipment. Forl.,ENSTM, the path planning process begins with an object defmedusing the STLformat. Thistriangulated.m~shis.intersectedwith.appropriately defined planesiin order tocreate 2P contour informationfor each layer to •. be formed. These contours are then refined to removeany vertexredllndancies and to improve part quality. After cleanup, if creating.a solid part, the contoursare used as a basis for creatiIlg the necessary fill pattern. The final resultofthe path planning process is atool path program (G-code) that is used to drive the three axes of the LENSTM machine.File Format Used by LENSAs previously stated, the LE~"STM process starts with an object defmed through the RP industry standardSTL format. While th.e deficiencies of this format are well understood, the use of STL meshes for objectdefinition does allow a large variety of CAD packages to b~ used to develop parts for use with LENSTM.Further, the simple triangular format used by STL allows for relatively easy manipulation of the part files.While the STL format is typically not a precise representation of the original CAD model (curvedsurfaces are faceted), meshes can be readily cut which are well within the tolerances of the LENSTMprocess. The only true difficulty.posed by the use oithe STL format is that the triangular representation,when sliced, often results in vert~x redundancies (straight edges defined by more than two vertices, .arcsbeing "over defined", etc.). As will be. discussed, these redundancies will often have adverse affects onthe build process. To minimize these affects, the contours require significant refinement.Contour RefinementDue to the nature of triangular meshes, when intersecting a mesh with a plane to create a 2D contour, thevertices formed from the intersection process will inevitably contain numerous "redundant" vertices-vertices that do not add information about the contour's geometry or topology. Even for a simple cube, as360Figure 2. (a) Using too many edges to define contours results in excessive bUildup. (b) Reducing the number ofedges used to define the contour produces a more desirable part.shown in Figure 1, a straightforward intersection of any plane with the triangular mesh will create linesegments which contain more than the minimum two vertices. While these redundancies may not poseproblems for other layered manufacturing processes, they may significantly affect the quality of partsproduced through theLENSTMprocess. Primarily, each line segment start/stop reduces the physicalmachine speed as the stages are forced to decelerate and accelerate through each vertex. Since theLENSTM build rate is related to laser energy per unit time,decreasing the stages' velocities results in asignificantalteration of the metal deposition. Another obstacle, resultingfrom the use of a triangulatedmesh to define the object, is a possible "over refinement" ofarcs and circles. As with over-definedstraightsegments, the accelerations and decelerations through each vertex of an over defined arc results ina decrease in the travel speed and a corresponding. increase in the metal deposition rate (Figure 2(a)).The contour refinement process is relatively straightforward. First, any duplicate vertices are removedfrom the contour.· (Duplicate vertices are considered as those that arecpincidentto each other withinsome epsilon.) •Next, any redundant vertices are located and removed from line segments. To locate theredundant vertices, .the included angle of the two line segments that share each vertex is computed. Anyvertexwhose included angle is greater than a user definedvalue (typically around 179°) is removed. Itshould be noted th~tthisiprocess also .eliminates excessive vertices in arcs or other curve segt11ents. It isalso possible at this stage to locate any arcs or circles in the contourrepresentation andreplace theseglllentedrepresentation with appropriate second order formulations .• As shown in Figure 2(b), dramaticimprovements in part quality may be obtained through proper contour refinement.Finally,any desired beam offset calculations are aPJ)ue:C1LENSTM, the beam offset serves several "",n1»rtnc"""fashion, correctingfofthe fact that theFigure 3. (a) With no beam offset applied, the weld beads drawn for the borders at the tip of the channel overlap,resulting in an uneven surface. (b) Applying beam offset eliminates the overlap, resulting in a flatter top face.361Figure 4. For even basic geometry, many problems may be encountered during a fill pass. Since the width of theweld pool will not always produce weld bead edges aligned with the border geometry, regions of large overlap, andregions with voids may be formed. Further, gas currents blowing back from the part may affect the powder stream,especially in tight areas. If the same fill pattern is repeated, the small disturbances produced by these flaws will buildupon themselves, resulting in a poor final product.offset compensation to account for this width. This compensation not only provides for improveddimensional accuracy in the xy-plane, but also assists in 'creating flat, even layers by preventing weldbeads from overlapping each other (Figure 3). Since LENSTM is focusing on creating near-net shapes,rather than net shapes, beam offset may also be used to control the amount of excessive material left on apart, providing for the proper amount of stock required for the final finishing process.Fill PatternFor solid parts, an appropriate technique must be utilized to create the necessary fill pattern. The choicesfor this fill technique may be broken down into one of two primary methods: rasters and "conformal"contours. Regardless of method, the fill technique must meet one primary requirement. Since the metalfor material buildup is injected into the LENSTM part as a powder stream, effects of the localized geometryon the powder stream must be controlled. Further, it must be recognized that the weld beads have finitewidth; widths that may not always "fit" evenly into a layer's border geometry, causing both regions ofexcessive overlap and possible voids (Figure 4). While neither overlaps nor voids are in and ofFigure 5. (a) Created using conformal contours. (b) Created using a 0°,90° raster hatch pattern. (c) Created using a0°, 105°,220°, '" hatch pattern.362Figure 6. (a) Shallow angles of attack between border vectors and the fill rasters produce buildup along borders.(b) Eliminating these shallow attack angles produces improved characteristics.themselves detrimental to the build process (overlaps will produce a slight increase in thickness, whilevoids will result a slight decrease), the errors caused by these localized flaws can, if compoundedacross enough layers, will result in a poor part. Attacking the geometry from different directions duringthe fill process can alleviate the localized affects. To accomplish the different angles of attack, the fillpattern must be designed such that a "randomness" is introduced into the build process.Conformal contours, which follow the outline of the part in decreasing size, were studied as onepossible fill technique. However, basic experiments with conformal contours revealed that even forsimple cylindrical parts, the required randomness is not introduced, resulting in extremely poor builds(Figure 5(a)). .The use of raster fills has proven an effective method of introducing the necessary variations betweenthe fill vectors on each layer. However, even here, care must be exercised when determining the rasterangles on a per layer basis. Early work on LENSTM utilized rasters that were always oriented at either 0°or 90°. While a vast improvement over the use of conformal contours, these static angles were still thesource of localized build errors, due to the repetition of the fill angles. As a result, regions aligned withthe x and y-axes would often experience unevenness in the build (Figure 5(b)).The first attempt at introducing more variation into the hatch angles was to use randomly generatedvalues for the hatch orientations. However, this allowed for angles that are just a few degrees off of the"standard" angles of 0°, 30°, 45°,60°, and 90°. When filling near these standard angles with a shallowangle ofattack (e.g. filling along a 30° border with a 31° hatch angle), the width of the weld beadrepeatedly overlaps the border, producing a significant buildup, again resulting in an uneven build, asdemonstrated in Figure 6. Furthermore, retracting the hatch to prevent the overlap was undesirable due tothe likelihood of forming voids in the part.After several iterations, the angle pattern converged on using a value of 105° between successivelayers. This value provided several key benefits. First, the fill angles across two layers are nearlyorthogonal, allowing for a large differentiation of attack angle to minimize the localized geometry effects.Also, at 105°, it takes 12 layers before any raster angle is repeated in a part, which further introducespseudo-randomness into the-process. Finally, 105° causes the standard angles to be either hit exactly, orat an angle greater than or equal to 15°, preventing the shallow angle of attack problems previouslydiscussed. As shown in Figure 5(c), the results obtained through the 105° hatch algorithm aresignificantly improved over both 0°, 90° hatching and conformal contours.363Future WorkEven though the current three-axis LENSTM system is "officially" a two-l/2D process, the weld poolcharacteristics, combined with the high weld pool freezing rates do allow for the creation of slightoverhung surfaces. To date, parts with overhangs up to 15° (for 0.015" thick layers) have been created.However, the creations of overhangs is still not fully understood in terms of the best speeds and feedsrequired to consistently create even, non-distorted layers. Future work will study what softwarealterations can be used to aid in the process of creating overhung surfaces.Another near-term task being studied is what software controls will be required for creating complex,multi-material geometries. While previous work has looked at creating layered objects (where thematerial choice for each layer is user defined on a layer-by-Iayer basis), future work will look at thesoftware controls needed to identify and control material composition throughout any given layer. Recentexperiments with using implicit blending functions, combined with surface-distance functions, haveproved promising [7].ConclusIonsRec.entexperience inLENS™hasdemonstratedthe strong reliance that the metal deposition process hasto parameters such asborderrepresentations and fill patterns. This research was aimed at understandingand deveIRpipgoptimizedsoftware.solutions.forthe generation ofLENSTM parts. Specifically, this workstudied the affects· ofthe border representation schemes, identified difficulties associated with·over-defined representations, and developed methods of refining the contour data to· improve the build process.Further,this research was directed atidentifying an "ideal" fill pattern for creating solid parts. The finalresult ofthe fill pattern studies produced pseudo-random raster fill technique which introduces thenecessary randomness into the build process to minimize localized geometry affects, while preventingproblems with shallow angles of attack along standard design surfaces.AcknowledgementsThis workis supported by the U.S. Department of Energy under contract DE-AC04-94AL850000.Sandiais a multiprpgram laboratory operated by Sandia Corporation, a Lockheed Martin Company, forthe United States Department of Energy.References[1] M.L. Griffith, D.M. Keicher, C.L.Atwood, lA. Romero, J.E. Smugeresky, L.D. Harwell,D.L.Greene, Free Form Fabrication ofMetallic Components using Laser Engineered Net Shaping (LENS TAJ,Proceedings of the Solid Freeform Fabrication Symposium, August 12-14, 1996, Austin, TX, p. 125.[2] J.R. Fessler, R. Merz, A.H. Nickel,F.B. Prinz, L.E.Weiss, Laser Deposition ofMetals for ShapeDepositionManufacturing, Proceedings of the Solid Freeform Fabrication Symposium, August 12-14,1996, Austin, TX, p. 117.[3] F.Klocke, H. Wirtz, W. Meiners, DirectManufacturing ofMetal Prototypes andPrototype Tools,Proceedings of the Solid Freeform Fabrication Symposium, August 12-14, 1996, Austin, TX, p. 141.[4] J. Mazumder, K. Nagarathnam, lChoi, J. Koch, D. Hetzner, The Direct Metal Deposition ofHi3 ToolSteelfor 3-D Components, Journal of Materials, Vol. 49, Number 5, May 1997, p. 55.364[5] D.M. Keicher, J.L. Jellison, L.P. Schanwald, J.A. Romero, D.H. Abbott, Towards a Reliable LaserPowder Deposition System through Process Characterization, 27th International SAMPE TechnicalConference, Vol. 27, Diversity into the Ne:x.t Century, proceedings of SAMPE '95,Albuquerque, NM,Oct. 12-14, 1995, p. 1029.[6] M. L. Griffith, C.L. Atwood, J.E. Smugeresky, L.D. Harwell, D.L. Greene, E. Schlienger, Using LaserEngineered Net Shaping (LENS T~ to Fabricate Metal Components, proceedings of the Rapid Prototypingand Manufacturing Conference, April 22-24, 1997, Dearborn, MI.[7] C.T. Lim, M.T. Ensz, M.A. Ganter, D.W. Storti, Object Reconstructionfrom Layered Data usingImplicit Solid Modeling, Journal ofManufactuting Systems, Vol. 16, Number 4, 1997, p. 260.365366

Software Development for Laser Engineered Net Shaping

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Software Development for Laser Engineered Net Shaping

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