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

• Multiphase-field model is developed to analyze the interplay between the DRX process and the IMCs formation/growth. • The DRX process has a significant impact on atomic diffusion and the number of nucleation sites of IMCs. • IMCs morphology depends on Al/Mg diffusion rates and composition ranges in the phase diagrams. • Supersaturated Al-based solid solution at the Al/Al 3 Mg 2 interface drives Al 3 Mg 2 equiaxed grain nucleation. • The reason why different morphology of IMCs at different positions in dissimilar weld nugget zone is firstly revealed. 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/Al 3 Mg 2 interface, providing sufficient driving force for Al 3 Mg 2 nucleation, whereas the driving force for Al₁₂Mg₁₇ nucleation at the Mg/Al₁₂Mg₁₇ interface is much smaller, leading to equiaxed Al 3 Mg 2 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.