Understanding surface roughness on vertical surfaces of 316 L stainless steel in laser powder bed fusion additive manufacturing

Laser powder bed fusion (L-PBF) offers high degrees of freedom to create complex geometries with fine features. Controlling surface roughness is one of the means of improving mechanical and functional properties. This is critical for those surfaces that post-treatments cannot process. This study investigated the effects of a wide range of processing parameters grouped by energy density on surface roughness, particularly at vertical surfaces in L-PBF. The surface arithmetic average roughness, Sa, was measured via confocal microscopy. Scanning electron microscope was used to examine the detailed characteristics and the solidification microstructure on and near surfaces. Increasing laser power and speed under the identical energy density or increasing energy density under the equal speed results in rough vertical surfaces. Solidified melt tracks and partially remelted particles dominate the vertical surface roughness in cases with low energy densities. In contrast, the dross formation contributes to the high vertical surface roughness in cases with high energy densities. By examining the melt pool morphology and the solidification microstructure near the surfaces, the mechanism grounded on a multi-layer melt pool instability is proposed to explain the dominant factors for vertical surfaces. The insights into the formation of key surface characteristics can assist in designing process parameters and generating innovative methods to improve surface roughness.