A general strategy for achieving high-strength joining of 2024 aluminum alloys via impacting flow friction stir lap welding

Abstract

Plunging the pin of rotating tool into lower sheet is a common process in the aluminum alloys friction stir lap welding (FSLW) field for obtaining the discontinuously-distributed lap interface in weld nugget zone (NZ), but unavoidable up-bending morphology of hook outside NZ greatly reduces the bearing capacity of lap joint. In this study, the novel impacting flow FSLW (IF-FSLW) process was employed by the rotating tool with an X-shape reverse-threaded pin. 2024 aluminum alloys were selected as base material (BM), and three rotating tools with different junction points on the pin were developed to analyze how material concentrated zone (MCZ) influenced the formation and bearing capacity of lap joint. Results showed that the hook with the forky structure was formed under the horizontally-pushing effect of MCZ on the original lap interface, and the hook with the down-bending morphology was induced by the vertically-squeezing effect of MCZ above the original lap interface. The tensile fracture mode of IF-FSLW joint was obtained under the combined actions of the down-bending hook, the largely shortened cold lap and the substantially enlarged NZ, and the corresponding welded lap joint had an incredible tensile strength. The IF-FSLW joint with the maximum tensile strength of 410 MPa was obtained, and the joint efficiency reached 92 % with respect to the BM. The IF-FSLW technology assisted with the X-shape reverse-threaded pin puts forward an effective approach to make a lap joint with superb strength.