Microstructure and mechanical properties of Mg/Al joints welded by ultrasonic spot welding with Zn interlayer

Abstract Mg/Al dissimilar metals were joined together utilizing ultrasonic spot welding (USW) with a Zn interlayer. The interfacial layer forming, microstructure, mechanical properties and fracture mechanisms of the Mg/Zn/Al ultrasonic spot welded (USWed) joints were studied systematically. Results showed that four typical regions with different forming morphologies emerged in the interface owing to the uneven distribution of stress and temperature. These four regions were the vortex-like plastic deformation region on the outside of the sonotrode tip teeth, the thinner flat region directly under the sonotrode tip teeth, the network-like diffusion region inside of the sonotrode tip teeth, and the thicker flat region between the two sonotrode tip teeth. The interfacial layer of Mg/Zn/Al USWed joints was mainly composed of a Mg-Zn liquid phase layer of α-Mg + (α-Mg + MgZn) + MgZn2 + (Mg2Zn11 + β-Zn), a residual Zn interlayer, and a η-Zn + α-Al solid solution layer. The addition of Zn interlayer blocked the inter-diffusion of Mg and Al atoms successfully and avoided the generation of Mg-Al IMCs effectively. The formed Mg-Zn system structure and Zn-Al solid solution have relatively good performances and lower brittleness. In comparison, Mg/Al joints welded with a Zn interlayer in-between displayed the maximum shearing strength about 89.6% greater than those of joints without a Zn interlayer.