Abstract
Abstract The perceived advantages of laser powder bed fusion (PBF) at reduced pressure include a more stable melt pool and reduced porosity. In this study, high-speed imaging was used to investigate the interaction of the laser beam with the powder bed at sub-atmospheric pressures. At atmospheric pressure, the laser plume produces a flow in the ambient atmosphere that entrains particles toward the melt pool. As the pressure decreases, this hydrodynamic entrainment increases but eventually the expansion of the laser plume prevents the particles reaching the melt pool: profiles and cross-sections of the track reveal a drastic reduction in its cross-sectional area. As the pressure decreases further, into the molecular flow regime, particles are only repelled by the plume away from the melt pool. The regime between 1 bar and ∼50 mbar (the threshold pressure at which the penetration depth no longer increases) could provide a window for successful processing but might require a pre-sinter to maintain the integrity of the powder bed. Lower pressures would definitely require a pre-sinter, for which the additional processing time and increase in process complexity might be justified for porosity-critical applications.
Highlights
Metal powder bed fusion (PBF) is an additive manufacture process in which thermal energy selectively fuses regions of a powder bed [1]
As the pressure decreases further, into the molecular flow regime, particles are only repelled by the plume away from the melt pool
The stated advantages of sub-atmospheric pressure include reduced porosity and surface roughness in the fabricated part, similar to that achieved with laser welding: any pores that do remain would not be filled with shield gas and could be removed more effectively by hot isostatic pressing
Summary
Metal powder bed fusion (PBF) is an additive manufacture process in which thermal energy selectively fuses regions of a powder bed [1]. The process is sometimes referred to by manufacturers' names, for example selective laser melting (SLM) and direct metal laser sintering (DMLS). Production components can be manufactured by commercial PBF systems, but generally require part-specific process settings to be determined in order to control thermally-induced residual stresses and defects. The stated advantages of sub-atmospheric pressure include reduced porosity and surface roughness in the fabricated part, similar to that achieved with laser welding: any pores that do remain would not be filled with shield gas and could be removed more effectively by hot isostatic pressing. Other potential advantages stated by these authors include reduced oxidation and the control of crystal orientation. These papers describe initial studies on single powder layers: the pressure and laser settings required for a successful process have not been established and no multi-layer builds were undertaken. The process settings reported in these papers are somewhat contradictory as described
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