Abstract: Existing studies focus on the microstructure and mechanical properties of joints, but there are few reports on the interfacial stress distribution and forming mechanism during welding process in the field of Mg/Al ultrasonic spot welding (USW). Mg/Al dissimilar metal lap joints are prepared by USW technology. The effects of different welding energies on the interfacial stress distribution, interfacial peak temperature, interfacial connection state, microstructure characteristics and mechanical properties of joints are investigated by means of FEA, OM, SEM, EDS, XRD and shear tests. It is shown that the Mg/Al interfacial stress distribution is uniform during the welding process, and the connected interface is relatively straight with no obvious whirlpool-like plastic flow and mechanical occlusion. The interfacial stress level and the peak temperature increase with the increase of welding energy, resulting in the metallurgical bonding area at the interface gradually increases. Based on the finite element simulation, the interface forming mechanism is systematically analyzed to guide the selection of appropriate heat input, in order to provide the necessary technical and data support for the actual production application of Mg/Al ultrasonic spot welding. When the welding energy is 500 J, the Al₁₂Mg₁₇ intermetallic compound (IMC) is formed at the interface and the intermetallic compound layer gradually thickens as the welding energy increasing, leading to the shearing force of joints increase first, reach to a peak value and then decrease with the further increase of welding energy. The moderate thickness of the IMC layer plays an important role in improving the mechanical strength of the joint.

Key words: interfacial stress distribution, intermetallic compound, Mg/Al dissimilar metal, ultrasonic welding, welding energy