Microstructure and mechanical properties of ultrasonic spot welded copper-to-magnesium alloy joints

• Copper-to-AZ31B magnesium alloy is successfully welded via USW.
• Accelerated diffusion occurs between Cu and Mg alloy during USW.
• A characteristic outburst-like diffusion pattern forms at higher welding energy.
• A peak lap shear tensile strength is achieved at a welding energy of 1500 J.
• Cohesive failure mode in the interface diffusion interlayer is observed.

High-power ultrasonic spot welding (USW) was used to join copper-to-AZ31B magnesium alloy at different welding energy levels, focusing on the interfacial microstructure and strength of the dissimilar joints. The enhanced diffusion during USW led to the presence of an interface diffusion layer mainly consisting of a eutectic structure of Mg and Mg2Cu. The thickness of the interface diffusion layer increased with increasing welding energy or temperature at the joint interface. A unique diffusion pattern formed at high levels of welding energy of 2000 and 2500 J was attributed to the outburst of near-eutectic liquid at localized hot spots under internal pressure, which was explained in four stages. The tensile lap shear strength of the joints was observed to increase initially, reach a peak value, and then decrease with increasing welding energy. The failure of the joints made with the optimum welding parameters of 1500 J and 0.75 s occurred in the mode of cohesive failure in the eutectic structure of the interface diffusion interlayer.

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