Effect of gas flow velocity and multi-laser scanning strategy on surface quality and mechanical properties of 316L parts printed by multi-laser powder bed fusion

During multi-laser powder bed fusion (ML-PBF) process, spatters increase with the number of lasers employed in the printing process. The flow velocity of shielding gas plays a crucial role in determining part quality, and thus warrants further investigation into its interaction with multi-laser scanning strategies. In this work, two multi-laser scanning strategies, i.e., parallel scanning (single laser operating) and simultaneous scanning (two lasers operating simultaneously) along the gas flow direction, within a range of flow velocity from 0 to 3 m/s, were employed to print 316L parts. The effect of the flow velocity and multi-laser scanning strategy on the surface quality, internal defects, and microhardness of 316L printed via ML-PBF were investigated. Both multi-laser scanning strategies obtained minimum surface roughness at an upstream area within the flow velocity range of 2–3 m/s, due to the elimination of balling phenomenon and the improvement of large spatters attachment. The printed 316L alloy achieved a density of over 99 % and an average microhardness exceeding 180 HV when the flow velocity exceeded 2 m/s in simultaneous scanning and 1 m/s in parallel scanning. The superior mechanical properties were attributed to the decrease in both the number and size of lack-of-fusion defects. Additionally, spatters generated at the upstream areas had a detrimental impact on the sample quality at the downstream areas, resulting in inferior surface quality and density. These findings provide valuable insights into the importance of considering flow velocity as a crucial process parameter, in addition to optimizing multi-laser scanning strategies.