3 Welding Mechanism and Failure of Ultrasonic Welds

Ultrasonic metal welding has been used widely to join battery cell terminals, or tabs (either Al or Cu), with bus bars (Cu) to form assembled battery packs in battery electric vehicles. However, the mechanism of ultrasonic welding for Al/Cu is still not well understood. In this chapter, the microstructures of the ultrasonic welds between three layers of lithium-ion battery tabs (either Al or Cu) and bus bars were studied. From the microstructure analysis, the weld formation mechanism and failure modes were investigated. It was found that the metal inter-mix or interlocking is the main weld formation mechanism among Al tabs, while metallic bonding is the main mechanism for Cu-Cu or Al-Cu joining. Metallographs also indicated that the weld failure is a combination of the interfacial debonding between the innermost tab (either Cu or Al) and the Cu bus bar and the through-thickness fracture of the tabs. This chapter provides better understanding of weld formation and failure, and hence provides insight into ultrasonic welding process toward weld quality improvement. This understanding and insight can be used to develop science-based design guidelines toward selecting the most appropriate materials (including heat treatment and coating), and welding configurations (such as layers of tabs), and welding process parameters.