Achievement of superb-strength lap joint via opposite-directions flowing friction stir lap welding of 2024 aluminum alloys

It is common for the rotating pin largely plunging into lower sheet to break up lap interface of Friction Stir Lap Welding (FSLW) joint, but the unavoidable up-bending morphology of hook outside Nugget Zone (NZ) largely reduces the joint bearing ability. Based on the novel Opposite-directions Flowing FSLW (OF-FSLW) by the self-developed rotating tool with an X-shaped right-left thread pin, the 2024 aluminum alloys lap joint was successfully welded in this study. The migration law of lap interface during welding was investigated by the experimental and numerical methods, and then how the rotating pin and its rotating velocity affect the formation and strength of OF-FSLW joint was further analyzed. The results show that the Material Concentrated Zone (MCZ) which formed above the original lap interface made the hook bend downward, the NZ greatly enlarged and the beginning part of cold lap compressed and thickened, thereby heightening the joint bearing ability. For the OF-FSLW joint, its maximum tensile strength was 403 MPa, and the corresponding joint efficiency of 90.8% was an incredible and superb value for the 2000 series heat-treatment strengthened aluminum alloys friction stir welded joint. The OF-FSLW technology by the rotating tool with an X-shaped right-left thread pin is proven to be a greatly effective approach for manufacturing the aluminum alloys lap joint with superb strength.