Microstructure and mechanical properties of Mg-to-Al dissimilar welded joints with an Ag interlayer using ultrasonic spot welding

Lightweight ZEK100-O Mg alloy and Al6022-T43 Al alloy with an Ag interlayer were joined via ultrasonic spot welding (USW), focusing on the microstructural change and tensile lap shear strength of the welded joints in relation to welding energy. Mg/Al interface was superseded by Mg/Ag and Al/Ag interfaces, and unfavorable Mg17Al12 intermetallic compound was eliminated. Ag foil was observed to be intact in the nugget center, while it was broken or dissolved at the nugget edge at high welding energy levels. The diffusion layer at the Mg/Ag interface consisted of two distinctive sub-layers: Mg3Ag intermetallic compound adjoining Ag foil, and Mg3Ag + Mg eutectic structure adjacent to Mg. Only a thin diffusion layer consisting mainly of Ag3Al occurred at the Al/Ag interface. The tensile lap shear strength first increased, reached its peak value, and then decreased with increasing welding energy. The shear strength achieved in the present study was similar to 31% higher than that of the joint without interlayer. Interfacial failure occurred at all energy levels, with Ag foil particles or fragments being stuck on both Mg and Al sides due to its intense interaction with Mg and Al via accelerated diffusion during USW. The results obtained pave the way for the challenging dissimilar welding between Mg and Al alloys. (C) 2020 Published by Elsevier B.V. on behalf of Chongqing University.