Elucidating the morphology difference of intermetallic compounds in Mg-to-Al weld nugget zone via multiphase-field simulation

Abstract

Friction stir welding (FSW) of dissimilar Mg/Al alloys enables lightweight hybrid structures, yet joint performance is strongly governed by the thickness and morphology of intermetallic compounds (IMCs) formed in the weld nugget zone (WNZ). The coupling between dynamic recrystallization (DRX) and IMCs nucleation/growth is quantified using experimental characterization and a multiphase-field model. The results reveal an Al-based supersaturated solid solution at the Al/Al₃Mg₂ interface, providing sufficient driving force for Al₃Mg₂ nucleation, whereas the driving force for Al₁₂Mg₁₇ nucleation at the Mg/Al₁₂Mg₁₇ interface is much smaller, leading to equiaxed Al₃Mg₂ but predominantly columnar Al₁₂Mg₁₇. The observed morphological difference arises from the combined effects of the disparate diffusion rates of Al and Mg atoms and the distinct composition ranges of the two IMCs on the phase diagram. Compared with the bottom region, the mid-WNZ experiences stronger DRX, generating more grain-boundary nucleation sites. Incorporating DRX also delays vacancy-mediated diffusion such that the peak diffusion period becomes synchronized with the most intense DRX stage, promoting the formation of new IMC layers. Consequently, the peak diffusion coefficient increases at the mid-WNZ but decreases at the bottom region. Owing to the higher temperature in mid-WNZ and enhanced nucleation kinetics, equiaxed IMCs are more likely to form there. This work provides a mechanistic basis for tailoring IMC thickness and morphology in dissimilar Mg/Al FSW.