Powder Bed Layer Characteristics: The Overseen First-Order Process Input
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R Bed Additive Manufacturing is a form of Additive Manufacturing (AM) that deposits very thin layers of metal powder (microns). A heat source (laser or electron beam) melts the metallic powder in certain areas of the powder bed. These areas then solidify to become a section of the final build. An additional powder layer is added, and the process is repeated. At the end of the build process, unprocessed powder is removed to reveal the final product. There are a large number of control parameters that interact in a complex manner affecting the final product quality.[1] A large amount of research has been reported investigating the energy absorption of the powder bed,[2,3] melt pool characteristics,[4–8] thermal evolution of the build, residual stresses, and final work-piece distortion.[6, 9–17] In spite of the general acceptance that the powder quality is a key factor in the overall process and the final product quality,[18,19] very little attention had been paid to the powder coating process and the characteristics of the powder bed. When referencing powder layer thickness, we distinguish three different values:
H.W. MINDT, Senior Developer CFD and Multiphysics CoE, and M. MEGAHED, Manager CFD and Multiphysics CoE, are with ESI Group, Essen, Germany. Contact e-mail: [email protected] N.P. LAVERY, Director of MACH1, M.A. HOLMES, Researcher, and S.G.R. BROWN, Director of Research and Deputy Head, are with Swansea University, Swansea, UK Manuscript submitted November 18, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS A
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Processing table displacement is the vertical motion of the processing table prior to the application of a new powder layer. It is chosen/set by the machine operator at the beginning of the build processes. The processing table displacement is often wrongly used to quantify the powder layer thickness, as will be discussed below. Powder layer thickness is the thickness of the newly coating powder layer. It corresponds to the minimum depth the heat source must penetrate to achieve bonding of the new layer with the base material. Consolidated layer thickness is the height of the processed powder material (deposited material).
We relate the different layer thicknesses to one another via the powder bed packing density. When a newly coated layer is processed, the thickness will decrease proportionally to the packing density as the material melts and solidifies again: dc ¼ dp qp ;
½1
where dc is the consolidated powder layer thickness after laser processing, dp is the fresh powder layer thickness, and qp is the packing density of the fresh powder layer. The volume remaining after material consolidation and prior to displacing the processing table again leads to a larger powder layer thickness. The table downward displacement dt is added to the free height above the processed material to obtain the new powder layer thickness: dpjnþ1 ¼ dtjnþ1 þ dpjn ð1 qpjn Þ;
½2
where n and n + 1 denote the previous and new layer, respectively. Assuming a uniform powder packing density of 50 pct and that the final bu
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