Metallurgical and mechanical properties of friction stir-welded AISI 316L stainless steel

AISI 316 L austenitic stainless steel is known for its superior high-temperature strength, excellent mechanical properties, resistance to extreme environments, and good weldability. It is second only to AISI 304 in terms of application criticality, yet it has been comparatively less investigated. Fusion welding is widely used for joining AISI 316 L; however, it requires careful process control to avoid microstructural degradation. Friction stir welding (FSW) represents an attractive solid-state joining technique, but its application to AISI 316 L has not been thoroughly explored. In the present study, AISI 316 L stainless steel plates in the 1/8-hardened condition were friction stir welded using a cylindrical, pinless tungsten carbide (WC) tool. A bead-on-plate technique was employed to avoid welding defects. The use of a pinless tool is advantageous in this context because conventional pin-based tools suffer from rapid pin wear; nevertheless, this approach has received very limited attention in the literature. This paper investigates the mechanical properties of the welds, specifically tensile strength and microhardness, along with their microstructural evolution. Detailed microstructural characterization was carried out using optical microscopy, electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM), complemented by a fractographic analysis. Furthermore, a dynamic recrystallization (DRX)-based model is proposed, in which the entire weld seam is treated as a thermomechanically affected zone (TMAZ). The results indicate that dynamic recrystallization occurs during welding and is responsible for significant grain refinement within the TMAZ, without compromising mechanical performance. In addition – the parent material seems to be unstable and to undergo DRX as well. The findings of this study demonstrate the technological potential of friction stir welding of AISI 316 L stainless steel using a pinless tool.