Review on ultrasonic welding of composites: Processes, interface reconstruction strategies and long-term service performance

Composites, owing to their outstanding properties such as lightweight and high strength, have become a vital material foundation for the lightweight design of high-end equipment. However, their reliable connection, particularly in hybrid structures involving thermoplastic composites (TPCs), thermoset composites (TSCs), and metals, remains a key bottleneck limiting their engineering applications. Ultrasonic welding, offering advantages such as high efficiency and low thermal damage, represents a promising solution. Nevertheless, the technology still lacks systematic theoretical understanding regarding the synergistic mechanisms of multiple process parameters, the fundamental differences in interface reconstruction strategies, and the long-term service performance of welded joints. This review addresses these three core issues by summarizing recent research advances in the field of ultrasonic welding of composites. The results indicate that strong coupling interactions exist among process parameters, exerting nonlinear influences on joint performance. In terms of interface reconstruction, different material systems follow fundamentally different design logics: for TPC/TPC welding, the function of energy directors (EDs) relies on geometric design to alter the micromechanical boundary conditions at the interface, thereby achieving spatial redistribution of the heat source; for TPC/TSC welding, a coupling layer (CL) is essential to activate the originally non-fusible cured surface, transforming it into a weldable interface while suppressing thermal damage to the thermoset resin; whereas TPC/metal welding requires the synergistic combination of physical interlocking and chemical bonding. Moreover, interface reconstruction strategies directly determine the long-term reliability of joints in complex service environments. This review establishes a comprehensive knowledge framework encompassing process parameters, interface engineering, and service performance, identifies current research gaps, and provides a systematic reference for deepening the mechanistic understanding and promoting the engineering application of ultrasonic welding technology for composites.