Microstructure Evolution, Tensile/Nanoindentation Response, and Work-Hardening Behaviour of Prestrained and Subsequently Annealed LPBF 316L Stainless Steel

Additive manufacturing is increasingly used to produce metallic biomaterials, and post-processing is gaining increasing attention for improving the properties of as-built components. This study investigates the effect of work hardening followed by recrystallisation annealing on the tensile and nanoindentation behaviour of laser powder bed-fused (LPBF) 316L stainless steel, with the aim of optimising its mechanical properties. As-built and thermally stabilised (at 900 °C) specimens were prestrained in a uniaxially tensile manner at room temperature (0.12 plastic strain, ~75% of maximum work hardening) and subsequently annealed (at 900 °C or 1050 °C for 1 h). The microstructure and mechanical properties were then characterised by optical microscopy, SEM, EBSD, XRD, nanoindentation, and tensile testing. It was found that prestraining increased yield tensile strength (YTS) 1.2-1.7 times (to 690-699 MPa) and ultimate tensile strength (UTS) ~1.2 times (to 762-770 MPa), but decreased ductility 1.5 times. Annealing led to recovery and partial static recrystallisation, decreasing YTS (to 403-427 MPa), restoring ductility, and increasing the strain hardening rate; UTS and indentation hardness were less affected. Notably, the post-LPBF thermal stabilisation hindered recrystallisation and increased its onset temperature. Mechanical property changes under prestraining and annealing are discussed with respect to microstructure and crystalline features (microstrain, crystal size, dislocation density). All specimens exhibited ductile fractures with fine/ultra-fine dimples consistent with the as-built cellular structure. The combined treatment enhanced tensile strength whilst preserving sufficient ductility, achieving a strength-ductility product of 40.3 GPa·%. This offers a promising approach for tailoring LPBF 316L for engineering applications.