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This paper investigates the mechanical properties and interface morphology of Mg/Al ultrasonic spot welding (USW) joint, adhesive bonding (AB) joint and ultrasonic spot weld bonding (USWB) joint. The peak load and fracture energy of USWB joint increased significantly compared to that of USW joint and AB joint. The USWB joint presented a hybrid fracture mode which was composed of the delamination failure at adhesive/Mg interface, cohesive failure within the adhesive and cleavage failure in the weld zone. The interface morphology suggested that USWB joint exhibited fewer defects in the cured adhesive matrix and elevated connection density of adhesive/metal interface, which improved joint strength and altered the fracture mode.

High-power ultrasonic welding technology, which has the excellent characteristics of lowenergy input and high efficiency, can effectively shorten the welding time, reduce the formation of intermetallic compounds (IMCs), and improve the strength of Mg/Al dissimilar welded joints in the future manufacturing industry. Mg/Al dissimilar metal ultrasonic welded joints with favourable mechanical properties were obtained through reasonable selection of sonotrode patterns and optimisation of welding parameters. The connection mechanisms of joints were discussed based on the analysis of weld interface morphology, microstructure evolution, and the composition and distribution of Mg-Al IMCs that varied with welding energy. The mechanical interlocking phenomenon and discontinuously distributed Mg 17 Al 12 with low thickness were observed at the weld interface, which helped to improve the joint performance.