Fatigue life improvement of fillet welded joints by friction stir processing

Abstract Friction Stir Processing (FSP) is an advanced solid-state surface treatment technique that achieves significant microstructural refinement through frictional heat generation and intense plastic deformation. This study proposes a novel methodology to enhance the fatigue performance of fillet-welded T-joints made of SM490A steel, employing Tilted Friction Stir Processing (TFSP). A total of thirteen fillet-welded specimens, fabricated using SM490A steel base metal and Z3313 T49J0T1-1CA-U weld metal, were examined, with six of them treated by TFSP. A tungsten carbide (WC) tool tilted at an angle of 9° was utilized, rotating at 600 rpm and advancing at a speed of 150 mm/min along the weld toes. Detailed microstructural analyses revealed substantial grain refinement and a 54.8% increase in Vickers hardness within the stir zone (SZ) compared to untreated regions. Four-point bending fatigue tests at a stress ratio R = 0.1 were performed, complemented by finite element method (FEM) analysis to elucidate stress distributions and crack initiation behaviors. Results demonstrated that TFSP significantly prolongs fatigue life, primarily due to a synergistic combination of microstructural refinement, hardness enhancement, and mitigation of stress concentration at the weld toe. Although TFSP introduced localized changes in residual stress, these had negligible adverse effects on fatigue performance. The findings underscore the efficacy of TFSP as a practical technique for fatigue improvement in welded steel structures.