Impact of ultrasonic welding on multi-layered Al–Cu joint for electric vehicle battery applications: A layer-wise microstructural analysis

Abstract

Multi-layered aluminium (Al) tabs to copper (Cu) busbar joints are increasingly being used for electric vehicle (EV) battery applications. Being a solid-state joining process, ultrasonic welding (USW) offers several benefits including less intermetallic or no porosity formation and larger weld area compared to fusion type welding, especially for highly conductive/reflective multi-layered dissimilar materials welding. In spite of being a suitable joining process, the impact of ultrasonic process parameters needs in-depth analysis for multi-layered stack-up where the process parameters play a pivotal role to join the layers of weldments. In this study, three layers of 0.3 mm Al tabs were welded to 1.0 mm single Cu busbar for the investigations of multi-layered Al–Cu dissimilar joints. Joint macro and microstructures, welding mechanism, layer-wise micro-hardness and grain formation were studied to understand the flow of material, the formation of grains and mixing of the Al and Cu for under-weld, good-weld and over-weld categories. The effects of amplitude of ultrasonic vibration, welding pressure and welding time were investigated to produce the satisfactory tab-to-busbar connection. The layer-wise microstructural study revealed the welding mechanism, propagation of micro-bonds and flow of material. The micro-hardness study unveiled different weld zones indicating the area of material mixing and the affected region whereas the crystallographic orientation maps disclosed the grain formation and recrystallization after the welding. The results showed that interfacial material mixing, wave-like material flow and interfacial micro-bonds formation were the prominent reasons for the satisfactory ultrasonic weld.