Optimized tilted friction stir processing for fatigue improvement of steel butt-welded joints

In welded joints of steel structures, stress concentrations caused by geometric discontinuities and tensile residual stresses strongly influence durability under cyclic loading. Friction stir processing (FSP) is increasingly recognized as an effective post‑weld treatment for enhancing joint integrity by refining microstructures and improving joint geometry. This study proposes a tilted friction stir processing (TFSP) configuration that uses a 9° tool tilt and places the advancing side near the weld toe to optimize microstructural refinement and fatigue performance in steel butt‑welded joints. TFSP was performed with a spherical‑tipped WC tool at 700 rpm and a traverse speed of 300 mm/min. Microstructural analysis, surface‑geometry characterization, Vickers hardness testing, and residual‑stress measurements were conducted. The results show that TFSP improved the microstructure at the weld toes, increased Vickers hardness in the heat‑affected zone (HAZ), and reduced residual stress. Four‑point bending fatigue tests, supported by finite element analysis (FEM), demonstrated that appropriately treated specimens achieved approximately twice the fatigue life of untreated welded specimens. These findings highlight TFSP as a practical and effective enhancement technique for structural steel weldments and clarify the mechanisms responsible for fatigue life improvement.