Weld Propagation in Ultrasonic Welding of Multi-Layered Dissimilar Metal Sheets

Abstract

Growing concerns over the environmental impact of carbon emissions have led to increased interest in electric vehicles (EVs) and hybrid electric vehicles. A reliable and efficient battery system is the key technological element to the development of practical EVs. In a typical EV battery pack, a large number of battery cells are assembled together to provide sufficient voltage and energy capacity to the vehicle. The battery assembly requires good quality joints to ensure system reliability. The challenge in battery joining for EVs is that joints involve multi-layered thin metal sheets of similar and dissimilar materials that have high thermal and electrical conductivity, such as Al and Cu. Ultrasonic welding (USW), as a solid-state process, is a promising method to efficiently produce good-quality welds in battery assembly. However, the process parameter window for making joints in multi-layered USW is narrow and the identification of the parameter window has been based on trial-and-error methods. In addition, it is difficult to make consistent weld quality at different interfaces in the multi-layered joint. To improve process robustness and the efficiency of process development, a deeper understanding of the underlying physics of USW and advanced techniques for improving the weldability need to be developed. In this dissertation, three topics are addressed: 1. Understanding weld formation in multi-layered similar and dissimilar metal sheets: The weld formation mechanisms of two frequently used weld configurations in battery assembly were experimentally studied. The two configurations were 3 layered thin Ni-coated Cu tabs to 1 thick Ni-coated Cu bus bar (3CC) and 3 layered thin Al tabs to 1 thick bare Cu bus bar (3AC). By tracking the evolution of microstructures and the corresponding weld quality subjected to the weld energy, the dominant bonding mechanism for similar material is found to be metallic adhesion accompanied with dynamic recrystallization and interfacial undulations, while the bonding between dissimilar materials is shown to be driven by diffusion. The phenomenological observations also imply parametric dependent weld formation in multi-layered USW. 2. Influence of interfacial undulations on weld formation and performance: The influence of interfacial undulations on weld formation were experimentally studied using two anvils with different knurl shapes. Several key weld attributes related to the weld formation were characterized and compared by cross-sectioned weld samples using microscopy. Finite element models were developed to predict the mechanical behavior of samples with different bonding strength and undulation degrees in lap-shear test. The empirical results reveal that interfacial undulations retard the formation of the weld and prevent excessive thinning of the workpiece. Meanwhile, numerical study helps provide insight on influence of interfacial undulations on weld performance with same bonding strength and possible influence on multi-layered USW. 3. Weld robustness enhancement by preheating: Localized preheating was investigated as an approach to enhance the weldability in multi-layered USW of four layered Ni-coated Cu. The influence of preheating temperature on mechanical performance of samples welded at different weld energy was investigated. The process robustness is found to be enhanced by increasing weld strength at the preheated interface while keeping performance at the non-preheated interface insignificantly affected. 2D finite element process models were built to understand thermal-mechanical behavior under different thermal conditions. This dissertation provides in-depth understanding of joint formation and propagation in multi-layered ultrasonic welding in battery pack manufacturing and develops a practical means to enhance process robustness through pre-heating.