# An Analysis of Mechanical Properties for Ultrasonically Welded Multiple C1220-Al1050 Layers
## 1 Introduction  
Ultrasonic welding is one of the representative welding technologies using plastic flow, with low electrical resistance as the oxide film and impurities are removed and welded through the diffusion of metal by vibration [1; 2; 3]. Ultrasonic welding also makes it possible to weld nonferrous metals or dissimilar metals that are difficult to bond using conventional methods, and it is mainly used for thin foils, because its effect on metal is relatively small compared to other welding methods due to the very short welding time. Research has found that ultrasonic metal welding is an excellent precision welding method, as it is very much superior in terms of process time and contact resistance [4].  
In addition, ultrasonic welding makes it easy to weld the same kind of metals or dissimilar metals which are difficult to weld using conventional methods; additionally, since the weld can be performedat a low temperature, it minimizes thermal damage due to low thermal expansion. It also has been capturing attention as a bonding technology for high-performance, multi-functional parts, because it is an eco-friendly method that does not use solder [5].  
The dissimilar welding of aluminum and copper uses non-fusion bonding technologies, such as ultrasonic welding, electromagnetic pulse welding, and friction stir welding [6; 7]. For fusion welding, a strong, brittle intermetallic compound (AlxCuy) is formed at the joint, which reduces the bonding performance [8; 9; 10]. In addition, these intermetallic compounds are reported to reduce the electrical conductivity. For these reasons, bonding methods that use mechanical bonding or plastic flow are used for these two metals, which have low melting properties [11; 12; 13; 14; 15].  
Ultrasonic welding has recently been applied to bonding nickel and aluminum in automobile battery cells or for the micro-wire bonding of electronic products [16; 17]. However, ultrasonic welding technology is now being applied even to bonding tabs and bus bars to modularize secondary battery packs for vehicles. The bonding of tabs and bus bars in secondary batteries is a leading example of aluminum and copper bonding [18; 19]. Lately, ultrasonic welding has been applied to connecting a terminal with CCAW (copper-clad aluminum wire), which has been developed to reduce the weight and improve the price-competitiveness of copper wires [20]. CCAW is composed of an inner aluminum core and outer copper cladding, processed by continuous rolling to within 0.01 mm, and attached to an aluminum or copper terminal [21]. This CCAW welding process is part of the dissimilar metal multi-layer bonding process.  
The purpose of this study is to analyze the strength characteristics of the multi-layer welding process of dissimilar metal (aluminum and copper) sheets and to determine the optimal welding conditions. To analyze the strength characteristics of the multi-layer welds, individual tensile tests were conducted for each layer and the effects of the welding conditions on the strength of a specific location were analyzed. In addition, the Taguchi experimental design method was used for optimization to increase the strength of each weld zone.