The effect of high strain rate deformation on intermetallic reaction during ultrasonic welding aluminium to magnesium

High power ultrasonic spot welding (USW) is a low heat input solid-state joining process that may offer a solution for welding difficult dissimilar-material couples, like magnesium (Mg) to aluminium (Al) for automotive body applications. However, the high strain rate dynamic deformation in USW has been claimed to accelerate inter-diffusion rates in dissimilar joints. The interfacial reaction between Al, AA6111, and Mg AZ31 alloys has been studied as a function of welding energy. For the optimum welding condition of 600J (0.4s) the reaction layer thickness was already ∼5μm thick. Intermetallic reaction centres were found to nucleate within microwelds at the interface at very short welding times and spread and grow rapidly to form a continuous layer, composed of two sub-layers of Al12Mg17 and Al3Mg2. Interface liquation was also found for longer welding times at temperatures below the recognised lowest eutectic reaction temperature in the Al–Mg binary system. Modelling has been used to show that the solid state reaction kinetics were over twice the rate expected from parabolic growth predictions made using rate constants obtained under static test conditions. The reasons for this discrepancy and the depressed melting reaction are discussed.