Balling phenomena in direct laser sintering of stainless steel powder: Metallurgical mechanisms and control methods

Balling effect, as an unfavorable defect associated with direct metal laser sintering (DMLS), is a complex physical metallurgical process. In this work, two kinds of balling phenomena during DMLS of 316L stainless steel powder were investigated and the metallurgical mechanisms of balling were elucidated. It was found that using a low laser power gave rise to the first kind of balling characterized by highly coarsened balls possessing an interrupted dendritic structure in the surface layer of balls. A limited amount of liquid formation and a low undercooling degree of the melt due to a low laser input was responsible for its initiation. The second kind of balling featured by a large amount of micrometer-scaled (∼10 μm) balls on laser sintered surface occurred at a high scan speed. Its formation was ascribed to laser-induced melt splashes caused by a high capillary instability of the melt. Feasible control methods were proposed to alleviate balling phenomena. It showed that increasing the volumetric density of energy input, which was realized by increasing laser power, lowering scan speed, or decreasing powder layer thickness, decreased the tendency of balling. The addition of a trace amount of deoxidant (H3BO3 and KBF4) in the powder yielded a smooth laser sintered surface free of balling.