Physics of large-area pulsed laser powder bed fusion

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The build rate of powder bed additive manufacturing could be significantly accelerated if consolidation of metal powders evolved from a serial process to a parallel process. In this work, the physics of Large-Area pulsed laser Powder Bed Fusion (LAPBF) in 316L stainless steel was studied through high speed imaging and high-fidelity physics simulations. Laser pulses were found to rapidly melt the metal powder, with subsequent fast coalescence of the melted particles into larger droplets. Conduction of heat from the molten droplets melted the substrate surface, and the molten droplets then spread out over roughly 100 μs. For the laser and metal powder parameters used in this study, layer thicknesses of greater than 40 µm resulted in uneven distribution of added material onto the substrate surface and thus an increase in porosity in multilayer prints. Simulations showed that pit features could be created (that can result in pores) from overlying powder particles shadowing the underlying substrate and blocking sufficient laser energy to deposit into the substrate. Simulations suggested that for these laser and powder parameters using thinner powder layers would reduce shadowing and allow the laser pulse to effectively heat the substrate thereby mitigating the defect formation. Implementing this change ultimately demonstrated > 99.5% density in the simulation, and > 99.8% density experimentally in 316L stainless steel prints. During the LABPF process very little material ejection was observed, a known impediment to laser powder bed fusion scaling to larger volume part production. This absence of eject a suggests that LAPBF may be able to produce material with high quality, suitable for critical applications, and scalable to high volume production.