Synergistic improvement of surface quality and bulk properties in laser powder bed fusion 316L stainless steel via sequential deep cryogenic treatment and high-energy shot peening

Laser powder bed fusion (L-PBF) fabricated 316L stainless steel exhibits excellent mechanical properties but suffers from poor surface quality, which deteriorates in-service performance. To address these limitations, this study proposes a sequential deep cryogenic treatment (DCT) and high-energy shot peening (HESP) hybrid treatment (termed DH) to synergistically enhance surface quality and bulk mechanical performance. Experimental results demonstrate that DCT pretreatment introduces nanotwins and increases the interplanar spacing, thereby intensifying HESP-induced severe plastic deformation. This achieves a 63.7 % reduction in surface roughness (S_a = 3.94 ± 0.25 μm) and triples the severe plastic deformation layer thickness from 28 μm (HESP-only) to 91 μm (DH). Tensile testing reveals remarkable strength-ductility synergy: yield strength increases by 405.8 MPa (794.3 MPa compared to 388.5 MPa in the as-fabricated state), ultimate tensile strength rises to 906.9 MPa, and ductility remains at 26.4 % elongation to fracture. Microstructural analyses attribute this enhancement to hierarchical strengthening mechanisms, including grain boundary strengthening via the Hall-Petch effect, dislocation-nanotwin synergy for strain hardening, and gradient nanostructure for deformation compatibility. These findings establish DH as an effective post-processing approach for additively manufactured alloys, delivering concurrent improvements in both surface characteristics and mechanical properties.