Effect of Ultrasonic Nanocrystal Surface Modification on Microhardness and Tensile Properties of Laser Powder Bed Fusion 316L Steel

Laser Powder Bed Fusion (LPBF) enables the fabrication of complex 316L stainless steel components, valued for their corrosion resistance and mechanical properties in aerospace, biomedical, and other sectors. However, LPBF 316L steel exhibits surface imperfections, limiting its high-demand applications. This study investigates the effects of Ultrasonic Nanocrystal Surface Modification (UNSM), an impact-based severe plastic deformation technique, on the microstructure, microhardness, and tensile properties of LPBF-manufactured 316L steel. Both as-built and post-LPBF annealed 316L samples were subjected to UNSM using a static load of 30 N, a frequency of 20 kHz, and a vibration amplitude of 30 m. UNSM leads to texturing of the as-built cellular structure, accompanied by crystalline refinement, lattice defect accumulation, and deformation-induced martensite transformation, resulting in a surface hardness of 500-550 HV10. However, UNSM only slightly improves tensile strength while substantially reducing ductility due to intense work hardening and earlier surface cracking under tensile testing. Post-UNSM recrystallization annealing (at 900C for 1 hour) promotes the formation of an ultrafine-grained microstructure (1-5 m, average grain size 2.48 m) in the near-surface layer affected by UNSM. This treatment restores ductility (total elongation of 61-63%) while maintaining elevated surface hardness (~400 HV10). For as-built specimens, the combination of UNSM and recrystallization annealing results in a superior strength-ductility balance, as reflected by an increased product of strength and elongation (PSE index), thereby enhancing both surface integrity and mechanical performance. In softer post-LPBF annealed samples, UNSM leads to deeper plastic deformation and a less steep hardness gradient. However, it also induces surface cracking, indicating the need for further optimization of UNSM parameters to accommodate the initial material hardness.