On the nature and causes of spatter redistribution in laser powder bed fusion

Abstract

State-of-the-art laser beam-powder bed fusion (PBF-LB) metal additive manufacturing (AM) systems are capable of producing dense workpieces without systematically occurring defects. Nonetheless, stochastically occurring defects with significant impacts on material quality levels persist. One possible cause is the fallout effects of spatter, matter ejected from the process zone, which may redistribute back onto the powder bed. The understanding of spatter redistribution and its impact on the PBF-LB process is still developing. This work provides insight on the nature and causes of spatter redistribution through a study of process variables and responses. Additionally, fundamental gaps in process-monitoring technology are addressed through a multi-modal sensing approach to spatter measurement. High-resolution optical layer-wise imaging was utilized to capture spatter concentration over the powder bed as a function of carrier gas flow direction, spatter generating workpiece location, distance from said workpiece, gas flow condition, and laser scan direction. These data were compared to ultra-high spatial resolution optical imaging as well as topographical measurements of the powder bed, which served to benchmark data and quantitatively capture powder bed quality measures. Spatter particles themselves were also characterized and correlated to powder bed location. It was concluded that even nominal carrier gas flow conditions fail to evacuate large spatter particles, which may be up to five times as large as feedstock powder. Additionally, the vast majority of large spatter particles tended to redistribute within 0–10 mm of the spatter generating workpiece. This observation contradicts commonplace assumptions that spatter may travel the full distance of the powder bed before landing on a workpiece. It also suggests that workpieces self-contaminate with spatter, this possibly being a significant contributor to lack-of-fusion porosity formation.