Evaluation of Zn-rich coatings for IMC reaction control in aluminum-magnesium dissimilar welds

The dissimilar welding of Al to Mg is very challenging owing to the rapid formation of an intermetallic (IMC) reaction layer at the joint interface, typically comprised of the binary phases γ-Al12Mg17 and β-Al3Mg2. A potential metallurgical solution has been explored that aimed to change the reaction path by using Al-Zn alloy coatings to promote alternative IMCs with a slower growth rate. To determine the optimum composition for the coating, combined CALPHAD thermodynamic calculations and diffusion simulations were used to estimate the minimum Zn concentration (20 at.%) required to suppress the formation of the faster growing Al3Mg2 phase. This prediction was verified by both static diffusion and Refill™ friction stir spot welding (FSSW) experiments. In each case the selected coating alloy was effective in changing the Al-Mg reaction path by forming the τ-(Al,Zn)49Mg32 phase as a substitute for β-Al3Mg2. However, in static heat treatments there was no reduction in the IMC reaction layer thickness due to the new phase having similar growth kinetics to the β-Al3Mg2 phase it replaced. Nevertheless, Refill™ friction stir spot welds prepared with the new AL-Zn coating alloy exhibited a thinner reaction layer and an increase in lap shear strength. This benefit was attributed to the Zn addition reducing the joint interface liquation temperature, which resulted in a lower peak weld temperature and less IMC reaction; although an increase in local eutectic melting limited the improvement observed.