Microstructure, mechanical and functional properties of refill friction stir spot welds on multilayered aluminum foils for battery application

Abstract

The production of batteries often involves the joining of multilayered foils to a conductive tab. In the present study, a solid state spot welding method, the refill friction stir spot welding (refill FSSW), was employed for welding multilayered commercially pure aluminum (CP–Al) foils to 2024-T3 Al alloy sheets (tab). Defect-free welds with high lap shear strength (LSS) are obtained. In the stirred zone (SZ), 2024 Al alloy is only found within the shoulder refill region, which is greatly refined due to dynamic recrystallization. Unlike overlap joints in thicker sheets, a thermo-mechanically affected zone also forms above the SZ in the probe refill region. “Stop-action” experiments have been conducted to describe joint formation. The presence of the 2024 Al alloy tabs resulted in significant changes to material follow behavior and the formation of microstructural zones not previously observed in conventional refill FSSW overlap welds in thicker sheets. Most welds failed in SZ pull-out mode during LSS test. The cracks initiate at the interface between the upper sheet and the first layer of the multilayered foils and then propagate upward and circumferentially outside the SZ. Welds of multilayered Al foils produced by refill FSSW show low contact resistance. The decomposition of electrochemically active materials in batteries can be avoided by maintaining a minimum distance from the spot-weld. The produced welds with flat appearance, high mechanical properties, and potential to meet industry requirements imply that refill FSSW is a promising welding technique for battery production.