Process-structure-property correlation in friction stir welded WAAM 316L stainless steel: Effects of welding parameters on flexural response and microhardness

This study investigates the process–structure–property relationships in wire arc additively manufactured (WAAM) 316L stainless steel subjected to friction stir welding (FSW), with particular emphasis on flexural performance and microhardness evolution. WAAM-fabricated walls, characterized by a heterogeneous columnar dendritic austenitic matrix containing δ-ferrite, were successfully butt-joined via FSW within an optimized process window. The combined influence of tool rotational speed (400–630 rpm) and traverse speed (50–100 mm/min) was systematically evaluated using a relative heat input index (RHI). The results reveal that FSW effectively transforms the as-deposited WAAM microstructure into a refined, equiaxed grain structure within the stir zone (SZ) through dynamic recrystallization (DRX). Lower heat-input conditions (low ω/high V) promote enhanced grain refinement and microstructural homogeneity, leading to a pronounced hardness gradient following the trend SZ > TMAZ > HAZ ≈ BM. The maximum SZ hardness (∼216 HV0.5) represents an increase of approximately 30% compared to the base material. Three-point bending tests demonstrate a strong dependence of flexural response on FSW parameters. The optimum condition (400 rpm, 100 mm/min) yields the highest flexural strength (∼1243 MPa), approaching that of the WAAM base material and surpassing its anisotropic weaker orientation. This improvement is attributed to the combined effects of DRX-induced grain refinement, strain hardening in the TMAZ, and a more homogeneous distribution of δ-ferrite. Overall, the WAAM–FSW hybrid approach is shown to be an effective strategy for simultaneous joining and localized microstructural homogenization, significantly enhancing hardness uniformity and flexural performance. This study provides a parameter-dependent process–structure–property framework for understanding and optimizing the hybrid WAAM–FSW processing of 316L stainless steel wall structures.