Strengthening strategy for high-performance friction stir lap welded joints of 5083 aluminum alloy

During aircraft, ship, and automobile manufacturing, lap structures are frequently produced between aluminum alloy skins, wall panels, and stiffeners. However, the occurrence of lap defects severely decreases mechanical properties during friction stir lap welding (FSLW). This study focuses on investigating the effects of rotation rate, multi-pass welding, and cooling methods on lap defect formation, microstructure evolution, and mechanical properties. It was discovered that the hook defects were eliminated by decreasing the welding speed, applying 2-passes FLSW with a small welding tool, and introducing the additional water cooling, leading to a remarkable increase in effective sheet thickness and lap width. This strategy leads to a defect-free joint with an ultrafine-grained microstructure, elevating the tensile shear force from 298 N/m to 551 N/mm. The fracture behavior of FSLW was systematically studied, and a fracture factor of the lap joint was proposed to predict the fracture mode of the FSLW joint. Through the implementation of decreasing rotation rate, 2-passes welding, and the additional water cooling strategies, the enlarged, strengthened, and defect-free lap zone with refined ultrafine grains was achieved, comparable to the quality of lap welds for 7xxx Al alloys. Importantly, this study provides a valuable welding method for FSLW to eliminate hook defects and improve joint performance.