Microstructural evolution and mechanical properties of laser-powder bed fusion processed 316L stainless steel with an ultrasonic-nanocrystalline surface modification

Because the surface roughness of additively manufactured (AM) metallic alloys affect their mechanical properties, surface post-treatment has become an attractive strategy for improving the performance of AM parts. Although several studies have revealed that surface treatment improves the mechanical properties of AM parts, changes in their surface morphologies and deformation mechanisms have not been well investigated. To solve this problem, in this study, the role of ultrasonic nanocrystalline surface modification (UNSM) treatment on the plastic deformation behavior of AM 316L stainless steel was investigated. The shear strain from the UNSM treatment induced severe grain refinement and crumbled traces at the surface of the as-built samples, causing a strength-ductility combination improvement compared to the non-treated sample. Moreover, the UNSM treatment removed the unmelted particles that relieved strain localization at the surface of the as-built sample during the plastic deformation process. Meanwhile, UNSM treatment in the machined sample reduced the surface quality, such as microcracks, which causes strength-ductility combination degradation. This result indicates that the post-surface mechanical treatment of the as-built metallic parts effectively improves the mechanical properties. This result was accomplished by inducing a gradient microstructure and a crumbled trace, which contribute to the strength improvement and the crack growth inhibition, respectively.