Insights of material flow and recrystallization in bi-metallic Al-Cu joint with friction stir welding

The electric vehicle transportation industry relies on advanced battery systems with high-voltage wiring harnesses composed of copper and aluminum conductors. Joining these dissimilar metals using conventional fusion welding is challenging due to the formation of brittle intermetallic compounds and differences in their thermal and metallurgical properties. Friction stir welding (FSW) has emerged as an effective technique for manufacturing Al–Cu joints in electric vehicle battery assemblies. In this study, dissimilar lap joints between AA6063 aluminum alloy (top) and commercially pure copper (bottom) were FSWed. A carbide-coated crown probe with a scroll-featured shoulder was used to promote material flow and suppress tunnel defects. Macrostructural, microstructural, and mechanical characterizations were conducted using optical microscopy, SEM–EDS- EBSD, and microhardness testing. Distinct particle-rich zones containing dispersed Cu particles within an Al matrix were observed along upwards flow tracks indicative of complex material transport within the stir zone. The welded interface contained primarily θ-Al₂Cu and γ-Al₄Cu₉ intermetallics (IMCs). Microhardness mapping showed pronounced softening in the aluminum stir zone (∼100 HV0.1) and heat-affected zone (∼60 HV0.1), contrasted with localized hardening near Cu-rich intermetallic compounds (∼130 HV0.1), relative to base material hardness values (∼70 HV0.1 for Al and ∼80 HV0.1 for Cu). EBSD analysis focused on the hook in the AS confirmed significant grain refinement due to dynamic recrystallization in addition to a partially-retained texture. Overall, the findings provide insights into the mechanisms involved in the material flow during the Cu hook formation in Al–Cu bi-metallic FSW joints, providing guidance for reliable bi-metallic connections in electric vehicle battery systems.