**1. Introduction**

Ultrasonic welding can produce welds faster and with less material loss than other common welding methods of dissimilar lap joints, such as laser welding (LW) [1,2] and friction stir welding (FSW) [3,4]. It also requires lower energy input than resistance spot welding (RSW) [5]. These advantages make this method suitable for welding dissimilar metals, such as copper and aluminum, which have high thermal and electrical conductivity and are widely used in the aerospace industry and battery packs [6,7]. Recently, the dissimilar copper and aluminum joint, welded by high-power ultrasonic welding (HPUSW) technique, is becoming more attractive due to its ability to join thicker sheets [8]. However, a major challenge of this technique is the thick continuous intermetallic compound (IMC) layer that is formed at the Cu/Al interface, which reduces the mechanical properties of the joint and leads to poor welding quality [9,10]. This issue inhibits the wider commercialization of this technique. Recently, interlayer metals, such as Al and Zn, were placed on the Cu/Al interface and were used to enhance the ultrasonic welding quality of the joint [11,12]. However, the metallurgical reaction at the specimen/interlayer interface is very complicated, making it difficult to understand the effects of the interlayer on the mechanical properties of the joints [13]. In addition, during the ultrasonic metal welding process, the metal interlayer will widen the heat-affected zone [14].

In order to improve the joint quality, several types of energy sources have been used to assist the ultrasonic welding, such as laser beam and resistance heat. Dehelean et al. [15] carried out hybrid ultrasonic-resistance welding of advanced materials and found that the resistance heat results in low weld strength. In our previous work, resistance heat-assisted low-power ultrasonic welding was proposed and it was found that the additional resistance heat can significantly increase the peak power

of ultrasonic vibration and the welding strength [16]. Unfortunately, this welding method is only suitable for joining thin sheets. The finite element method was used to study the influence of additional resistance heat on the interface temperature and plastic deformation of the material, during ultrasonic welding [17]. However, the mechanical properties and microstructure of joints prepared by resistance heat-assisted high-power ultrasonic welding (RUSW) have not yet been reported. In this paper, the mechanical properties and microstructure of Cu/Al joints prepared by RUSW are studied, in order to obtain high welding quality.
