Powder Bed Fusion (PBF) systems are equipped with cross-flow nozzles that are designed to flow inert gas across the build plate and entrain the metallic powder particles that are ejected during the build. These particles need to be removed from the build chamber, lest they land on the powder bed, leading to degraded part quality, undesirable roughness, part porosity and fusion defects. Optimally, these cross-flow systems entrain and remove most of the ejected particles from the build chamber, but do not erode the freshly spread layer on the build surface. The onset of particle bed erosion can be characterized by the critical Shields number. Once the critical Shields number is known for the metal powders and system of interest, the flow of inert gas in the process chamber can be optimized to contribute to a clean and efficient build process.&nbsp;This work proposes a Shields number based method for obtaining design guidance for critical particle lift-off conditions. The design guidance for these gas flow sub-systems was developed using a combined experimental and computational fluid dynamics (CFD) study. The closed loop small footprint wind tunnel incorporates a 0.305 m x 0.305 m x 0.915 m test section, a variable geometry inlet nozzle, a variable height build plate upon which powder can be spread, and variable flow rate so that a variety of cross-flow configurations can be tested. Helium Filled Soap Bubble (HFSB) Particle Tracking Velocimetry (PTV)&nbsp;was used to characterize the single-phase flow at a number of these operating conditions / configurations. High speed videography was used to study particle liftoff and entrainment at the same conditions for various metal powders used in metal additive manufacturing, including aluminum alloy, nickel based alloy, cobalt chrome alloy, and stainless steel powders.&nbsp;In this paper we present details of the experimental techniques used, including hot wire anemometry, Helium Filled Soap Bubble (HFSB) Particle Tracking Velocimetry (PTV), and high speed videography. Experimental results are then compared to the results from CFD modeling. Using these experimental measurements and attendant CFD models, CFD predictions of friction velocity can be used to obtain critical particle liftoff Shields numbers for various metal powders used in metal additive manufacturing. Specifically, since the results of the CFD modeling show good agreement with measurements, predicted wall shear stress values are used to estimate Shields number, Sh versus particle Reynolds number, Reτ at flow rates where incipient particle lift-off is observed experimentally.

Experimental and Computational Studies on Saltation of Metal Powders Used in Laser Powder Bed Fusion Systems for Metal Additive Manufacturing

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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