Enhanced Weld Properties of Aluminum-Copper Joints for Battery Pack Applications: From Single-to Double-Step Micro Friction Stir Spot Welding

Abstract

Abstract. In electric vehicles (EVs), reliable performance during charging and discharging hinges on robust mechanical connections and superior electrical properties at both the cell and pack levels. Therefore, the connector material, joint design, and joining method emerge as critical factors. This paper presents a solid-state joining technique for submillimeter-thick Al and Cu using micro friction stir spot welding (μFSSW), targeting battery pack applications. A double-step plunging method for μFSSW using a pinless tool is proposed, and the resulting joints are evaluated against conventional pinned and pinless single-step methods in terms of mechanical and electrical performance. The intermediate dwell time in double-step plunging enabled uniform softening of the top Cu sheet, preventing rupture and Al exposure, and produced a larger joint interface that improved joint strength. The results showed a 32.40% increase in lap-shear load and a 26.99% increase in T-peel load over joints made by single-step plunging. At optimum joint strength, electrical resistance decreased by 34.39%, with a corresponding 29.17% reduction in Ohmic temperature rise, compared to pinned tool μFSSWed joints under a reasonably high (150 A) current conduction. By linking joint interface characteristics to mechanical and electrical properties across process conditions, this study outlines an efficient joining method for battery pack applications.