Mechanical properties and microstructure evolution of dissimilar Mg and Al alloys welded using ultrasonic spot welding

Magnesium and aluminum alloys are used in the development of lightweight materials for the automobile industry. Ultrasonic metal welding is a solid-state bonding technology widely used in lightweight metal welding that has unique advantages for welding dissimilar Mg-Al alloys. In this work, the mechanical properties and interfacial reaction behavior of ultrasonically spot-welded joints of dissimilar Mg-Al alloys were studied using a universal tensile testing machine, scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD). The results showed that the joint strength first increased and then decreased as the welding energy increased. At a welding energy of 980 J, the maximum peeling force of the joint was 0.107 kN, and the failure mode of the weld occurred via a 'local Al alloy nugget pull-out'. At a welding energy of 1540 J, a continuous ∼20 μm thick intermetallic compound (IMC) layer composed of Al₁₂Mg₁₇ and Al₃Mg₂ phases formed at the welding interface, which deteriorated the mechanical properties of the joint. As the interfacial temperature increased along with the welding energy, mixed particles formed, and element diffusion accelerated at the interface, which promoted growth of the IMC layer. Dynamic recrystallization occurred near the weld interface under shear deformation as well as deformation heating, which refined the grain size and changed the texture. (paper)