Executive Summary : | Automobile battery packs for electric vehicles (EVs) consist of a large cluster of individual cells with different joining configurations. The configuration comes in three hierarchical assembly structures at the cell level, at the module level (consisting of several cells) and pack level (comprising of several modules). Out of these, the cell level joining and the pack level joining can be easily achieved. This is because the electrodes are very thin at the cell level and mechanical joints are preferred, like fasteners or clips, for pack level joints. However, the module level assembly forms the bottleneck of the battery manufacturing process as complex assembly designs are required to meet the power requirements at the application end of EVs. The designs require combining several cells in parallel and/or series to achieve the desired power output resulting in a complex network of joints. The requirement is to robustly join the module electrodes with good mechanical and electrical performance; otherwise, it severely degrades the battery performance. Joining these modules throw several challenges like joining dissimilar materials, highly conductive materials joining, multiple sheets joining and varying thickness joints. Therefore, the module level is the most critical assembly operation in the automotive battery pack manufacturing process.
Prismatic Lithium-ion cell packages are preferred for automotive applications. These cells have Copper (anode) and Aluminium (cathode) tabs (0.3 mm to 1 mm) connected to Aluminium or Copper bus bars (1.5 mm to 9.5 mm) at the module level in different configurations like tab-to-tab and tab-to-bar. Attempts have been made using joining processes like Laser welding, resistance welding, brazing, Ultrasonic welding, however, face performance setbacks. This proposal is about a methodical study on the joining and characterisation of Aluminium and Copper plates using the Friction Stir Welding (FSW) process. The project is planned to be executed in two parts. The first part consists of preliminary joining experiments and their joint characterisation to obtain optimised FSW operational parameters aiming at a defect-free welding process. The plan includes a set of experiments with operating parameter sweeps and joint characterisation for microstructure analysis and mechanical defects such as fractures and tearing. The second part is to perform Al-Cu joining in various configurations like lap joints, butt joints, multiple sheet joints and joining plates of different thicknesses. Following is a thorough characterisation of these joints for their mechanical strength, endurance and conductivity. A detailed exploration of the data collected during the experimentation is planned to find correlations between the FSW process variables, the evolved microstructures and joint performances. The study is expected to evaluate FSW as a prospective technology for the automotive battery assembly process. |