Abstract: For friction stir lap welding (FSLW) process by the rotating pin greatly inserting into the bottom plate, the bending-down morphology of hook helps to obtain a high-strength lap joint, and can be more conducive to the joint strength when the tensile-fractured path is located in the top plate. In light of this, the reverse-flow FSLW (RF-FSLW) by a newly designed rotating tool with a right-left threaded X-shape pin (X-pin) was employed to weld 2024-T4 aluminum alloys with the same plate thickness, and the flow field simulation, in situ tensile test and EBSD analysis were utilized to understand the relations among the formation features, the fracture features and the joint strengths. The results indicated that under the integrated effects of the bending-down shape of hook, the bulging shape of nugget zone (NZ) and the bending-up shape of beginning part of cold lap near NZ outline, the RF-FSLW joint was tensile fractured in the top plate of joint. For the RF-FSLW joint, its maximum tensile strength was 412 MPa, and the corresponding joint efficiency (92.8%) was larger than that of reported friction stir welded joint of 2024 aluminum alloys in T temper condition. The RF-FSLW technology by the right-left threaded X-pin puts forward an extremely effective way for obtaining the superb-strength lap joint of aluminum alloys.

Key words: Reverse-flow friction stir lap welding, 2024-T4 aluminum alloy, Right-left threaded X-pin, Hook, Tensile strength