Weak penetration strategy combined with post-weld aging treatment to optimize interface joining for dissimilar friction stir lap welded aluminum and titanium alloys

In order to address the problems caused by excessive tool penetration into lower hard sheet during the friction stir lap welding (FSLW) of aluminum and titanium alloys, a weak penetration strategy combined with a post-weld low-temperature aging treatment is proposed and investigated for the dissimilar FSLW of 6061-O aluminum alloy and Ti-6Al-4V titanium alloy in the present research. The results indicate that post-weld aging is capable of the elimination of cracks and voids formed at the hook structure during Al/Ti FSLW at relatively low rotation speed. Additionally, the low-temperature aging renders an improvement in bonding strength of lap surface, leading to the maximum joint efficiency of 95.5 % relative to the Al matrix. Microstructural analyses on weld interface reveal that the further recrystallization caused by aging induces approximately 90° rotation and obvious refinement for the stir zone grains. Furthermore, the smoother and more flattened Al/Ti weld interface characterized by a significant reduction in fragments and intermetallic compound clusters is produced through Gibbs-Thomson effect and Ostwald ripening principle during post-weld aging. The both factors represent the core mechanism through which low-temperature aging optimizes the interface structure and improves the joint performance. This research provides an effective technological approach for the interface optimization of Al/Ti lap joint, achieving a controllable balance between weld formation and mechanical property for significant practical implications.