2018 年 36 巻 4 号 p. 1WL-4WL
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) was applied to the weld toe of high-strength low-alloy steel joints using a newly developed tool with a conical shoulder. With use of FSP, the weld toe geometry and microstructure were successfully modified without defect formation in the stir zone (SZ). Hardness was increased due to significant grain refinement and compressive residual stress was produced on the weld surface. Fatigue strength and life of the FSP-modified welded specimens was improved, though not largely, since a new stress concentration region with reduced plate thickness and serrated surface was produced by the shoulder edge of the FSP tool, just beside the base metal. Fatigue cracks initiated there and propagated in the SZ, thermo-mechanically affected zone, and base metal for the FSP-modified welded specimens. The fatigue strength could be further increased by prevention of the stress concentration at the SZ edge.