Toward defect-less and minimized work-hardening loss implementation of Al alloy/high-purity Cu dissimilar lap joints by refill friction stir spot welding for battery tab-to-busbar applications

The refill-friction stir spot welding (RFSSW) process on the Al5052–H32 alloy joined with high-purity Cu film resulted in notable changes in the work-hardened Al alloy. The stirring action caused dynamic recrystallization in the weld zone, leading to the loss of work-hardening. Surprisingly, the plot of grain size-hardness in Al alloy after RFSSW did not follow the Hall-Petch relationship. Obtained through combined analysis with a transmission electron microscope, X-ray diffraction, and thermodynamic calculation, at the interface of Al and Cu, diffusion in nanoscale and microscale between the metals induced the formation of secondary phases (e.g., CuxAly intermetallic compounds, θ′ or θ'' phases), positively contributing to increased mechanical properties. The near-surface region in contact with the tool experienced significant compressive stresses, resulting in maximized compressive residual stresses. This, in turn, sustained or even enhanced the initial hardness of the material. Our study systematically investigates whether creating a lap joint with Cu in work-hardened Al alloy with RFSSW can yield a robust joint without defects in Al and without an overall loss of work-hardening.