This work deeply explored differences between single-phase AC resistance spot welding (RSW) and three phase medium frequency DC RSW systems. The main difference between the two types of RSW systems was that they used different electrical structures, which led to different energy delivery modes. The RSW operation with former type had a simple but low cost structure, while the latter type had opposite features in these two aspects. In this work, a two-dimensional (2D) finite element (FE) model, which combined characteristics of thermal, electrical, and mechanical fields, was established, and then three modes of input welding currents, which were DC input mode, standard sinusoidal input mode and standard single-phase AC RSW input mode, were delivered into the welding system in the FE model. The three modes of welding currents had strictly the same effective values, and effected the same welding time. By means of iterative calculations between thermal-electrical and mechanical fields, the information of temperature increasing trend, dynamic resistance and electrode displacement under the three input modes were obtained. Some important characteristics about the energy delivery and absorptions, and mechanical properties variations under different welding current input modes which cannot be accurately obtained by actual welding operations had been obtained in this work. Also, other analyses about actual data processing and applications, and mechanical variations of the facilities during the welding process were provided. The work can supple valuable references for actual welding applications.

Ultrasonic welding is characterized by its energy-saving and environmentally friendly nature. Compared to conventional molten welding technology, the intermetallic compounds formed by diffusion during ultrasonic welding are thinner, and material deformation is reduced. This process has become a primary welding technique for assembling lithium batteries in electric vehicles. Aluminum and copper ultrasonic welding has increasingly gained attention as a research hotspot. The research on aluminum and copper ultrasonic welding primarily focuses on the interfacial microstructure evolution, mechanical performance during the welding process, and numerical simulations to investigate macro- and micro-scale physical phenomena. Given the aluminum and copper multi-layer structures used in lithium battery packaging, numerous studies have been conducted on aluminum and copper multi-layer ultrasonic welding. For Al/Cu joints, advancements in understanding the microstructure evolution, joint performance, and finite element modeling of the welding process have been systematically reviewed and summarized. Moreover, significant progress has been made in molecular dynamics simulations of Al/Cu ultrasonic welding and hybrid welding techniques based on Al/Cu ultrasonic welding. Finally, several new research directions for Al/Cu ultrasonic welding and joining have been proposed to guide further in-depth studies.

Continuous innovations in welding and additive manufacturing (AM) technologies have introduced challenges related to process stability and uncertainties in ensuring high quality. Given the high complexity and transient nature of these processes, effective online acoustic monitoring is crucial for ensuring manufacturing quality and improving processing efficiency. This paper first elucidates the principles of acoustic signal generation in welding and additive manufacturing. It then provides a comprehensive review of the application of acoustic methods for quality monitoring in these processes, covering both structural acoustic emission (AE) and airborne acoustic monitoring techniques. Finally, it summarizes the current challenges and issues faced by acoustic monitoring technologies in welding and additive manufacturing and outlines potential future development directions.

The Cu/Al dissimilar joint, welded by high-power ultrasonic welding technology, is still facing challenges despite the significant research attention it has attracted. In this work, the microstructure and mechanical properties of resistance heat-assisted high-power ultrasonic welding of Cu/Al are investigated, in order to obtain high-quality joints. The intermetallic compound (IMC) at the interface of hybrid welding is primarily composed of Al2Cu, and the additional resistance of heat reduces the thickness of this brittle IMC layer. The average shear stress for the joint prepared by hybrid welding is ~97 MPa, which is higher compared to the joint strength without resistance heat (90 MPa). Moreover, the duration of the hybrid welding process is shorter. Finally, the fracture of the hybrid weld is found to be a brittle–ductile hybrid mode.