Study on the Influence of Zn Interlayer on the Microstructure Evolution of AZ91D/T2 Dissimilar Metal Ultrasonic Spot Welding Joints

In this study, ultrasonic spot welding (USW) is performed on AZ91D Mg alloy and T2 pure Cu. The fracture surfaces, microstructures, and mechanical properties of the joint are analyzed. In addition, the performance of the joint is improved by adding a Zn foil interlayer at the interface of the Mg/Cu joints. The studies show that with the increase of welding energy, the thickness of the diffusion layer also increases, while the shear properties of the joint exhibit a trend of first increasing and then decreasing. The addition of the Zn interlayer not only prevents the formation of Mg-Cu intermetallic compounds (IMCs), but also forms Mg-Zn eutectic structure at the interface, thus improving the joint performance. The maximum tensile shear strength of the joint with the Zn interlayer is increased by 34.67% compared to that without the interlayer, and the fracture mode changes from brittle fracture to mixed fracture.