With the production of electric vehicles (EVs) on a gradual incline, high-conductivity materials such as aluminum, copper, and nickel have been applied as battery tabs and busbars. The battery of an EV consists of hundreds or thousands of cells, with battery cells electrically connected. For example, cell tabs in the pouch cell-based battery pack are to be connected to the busbar to make a successful electrical connection. Therefore, EV battery design and manufacture are some of the major key strategies, as the battery’s quality depends on the welded joints’ performance [1].
The battery cell is assembled through spot welding, micro-tig welding and laser welding. However, these conventional fusion welding processes face great challenges in dissimilar materials welding of copper and aluminum because of their remarkable differences in chemical and thermal properties (e.g. thermal conductivity, thermal expansion, heat capacity and melting point). Furthermore, another major issue associated with dissimilar joining is the formation of brittle intermetallic compounds (IMCs), which forms at the interface between aluminum and copper due to chemical reaction and interdiffusion. According to the Al-Cu equilibrium phase diagram, IMCs of Al4Cu9, Al3Cu4, AlCu and Al2Cu can be formed at a range of chemical compositions and temperatures in the Al-Cu system. These brittle IMCs at the joint interface cause a degrading of joint strength and electrical performance [2].
As a solution to these problems, Friction stir welding (FSW) has become a promising key for dissimilar joining in the solid-state joining process [3]. This process with relatively small heat input is characterized to provide the formation of limited IMCs. However, most previous studies have performed FSW lap joining of dissimilar materials, such as Al-Mg [4], Al-Steel [5], Al-Ti [6], and Fe-Mg [7]. Some researchers have studied the FSW of Al-Cu dissimilar joints, but their paper deals with high-strength aluminum alloys, such as aluminum 2XXX [8], 5XXX [9], and 6XXX [10] series.
Therefore, this study aims to assess the applicability of friction stir welded joints of battery tab and busbar dissimilar materials. Moreover, a lower electrical resistance is greatly desired for a high-quality battery assembly in addition to the performance requirement for battery tab and busbar joints, such as defect-free joint quality, mechanical joint strength, and chemical stability. A higher joint resistance in a battery assembly can cause excessive energy loss due to reducing battery power capability and efficiency [11]. Therefore, in the present study, the comprehensive effect of process parameters on mechanical and microstructure properties and electrical resistance characteristics has been extensively investigated.