Weld toe modification using spherical-tip WC tool FSP in fatigue strength improvement of high-strength low-alloy steel joints

Highlights: • FSP using a spherical tip tool was applied to the weld toe of HSLA steel joints. • The weld toe geometry and microstructure were successfully modified without defect formation. • Tool wear on the surface induced compressive residual stress. • Fatigue strength was improved by FSP, while was influenced by surface roughness related to tool travel speed. Fatigue strength of fusion-welded joints is lower than that of the base metal due to stress concentration, tensile residual stress, and microstructural degradation at the weld toe. To improve these issues, friction stir processing (FSP) using a spherical tip tool was directly applied to the weld toe of high-strength low-alloy steel joints. The toe geometry was successfully modified without defect formation, resulting in 25% reduction in fatigue notch factor in comparison to the as-welded joints. Significant grain refinement due to FSP increased hardness beneath the toe surface. In addition, compressive residual stress related to tool wear was produced on the FSP surface. Bending fatigue strength was improved by these benefits, with dependence on the tool travel speed. Fatigue crack initiation occurred at the toe surface for all the joints. Although high travel speed FSP produced serrated surface resulting in degradation of FSP improvement in fatigue strength, low travel speed FSP contributed to reduction of surface roughness leading to maximum FSP improvement of 50% in fatigue strength. The results obtained in this study suggest the encouraging prospect of direct application of FSP to weld toes as new post-weld treatment for steel joints.