Rapid intermetallic growth under high strain rate deformation during high power ultrasonic spot welding of aluminium to steel

High power ultrasonic spot welding is an alternative manufacturing process which recently has been developed for joining automotive bodies. This technique is a very low energy process, forming effective welds in less than a second particularly for difficult dissimilar material combinations such as aluminium to steel joint. However, in dissimilar joint the interdiffusion and thus intermetallic formation was accelerated due to high strain rate dynamic deformation in ultrasonic spot welding which deteriorates mechanical performance. The interfacial reaction between aluminium 6111-T4 and DC04 uncoated steel has been investigated as a function of welding time. For the optimum welding time of 1.5 s under a 1.4 kN axial pressure, the intermetallic layer thickness could have reached ∼1.0 μm. Intermetallic islands were seen to nucleate across the microbonds at the interface within short welding times which spread and grow rapidly forming a continuous layer which mainly contains FeAl3 and Fe2Al5 phases. The interfacial reaction occurred in solid state even for a long welding time. Modelling was carried out with the aim of showing that the rate of interfacial reaction was over 6 times greater than the rate observed in diffusion couple using rate constants achieved from the static heat treatment condition. This confirms that deformation-induced vacancies during the thermomechanical welding process accelerates formation of intermetallic layer at the interface.