Effect of laser polishing on the microstructure and mechanical properties of stainless steel 316L fabricated by laser powder bed fusion

Abstract

While metal additive manufacturing has seen significant growth in recent years, the surfaces produced often need post-processing to improve surface finish, mitigate residual stresses, and remove surface-connected porosity. Laser polishing, by means of remelting a thin layer of the surface, is one post-processing method being investigated for surface finish improvements and other surface enhancements. In this work, the surface morphology and microstructure of laser powder bed fused (L-PBF) stainless-steel 316 L (316 L) before and after laser polishing are characterized by optical microscopy (OM), scanning electron microscopy (SEM), electron back-scatter diffraction (EBSD), and transmission electron microscopy (TEM). In addition, the cross-sectional microhardness of the samples is measured and reported. Additionally, the as-built and laser-polished sample's tensile properties are characterized using uniaxial tension tests. The results indicate that the surface roughness of as-built 316 L (Sa = 4.84 μm) can be substantially reduced through laser polishing (Sa = 0.65 μm). After laser polishing, the average grain diameter is reduced and the proportion of low angle grain boundaries (2°~5°) is increased in the L-PBF 316 L. The maximum sub-surface hardness reaches 262 HV, and both the tensile strength and ductility of 316 L are increased after laser polishing. This enhancement is attributed to thermal cycling stresses, grain refinement, the elimination of surface defects, and dislocation strengthening after laser polishing.