Roles of anisotropy and mutual transformation of the phases β/γ in multiphase-field simulation of IMCs in friction stir welding of Al/Mg alloys

In multiphase-field modeling the formation and growth of intermetallic compounds (IMCs) during the friction stir welding (FSW) process of Al/Mg dissimilar materials, the effects of anisotropy and the mutual transformation of IMCs like phase beta (Al3Mg2) and phase gamma (Al12Mg17) have been considered. It indicates that Al3Mg2 grains exhibit growth not only within the Al matrix but also within Al12Mg17, leading to an increase in the thickness of Al3Mg2 and a reduction in the thickness of Al12Mg17. By taking into account the mutual transformation of IMCs, the model can accurately predict the thickness of IMCs at both the bottom and middle positions of the weld with a high level of accuracy. Upon taking the anisotropic growth of IMCs into consideration, the morphology of IMC grains underwent a transformation from fan-shaped columnar crystals to square-shaped crystals. Experimental validation reveals that when accounting for the mutual transformation and anisotropic growth of IMCs, the predicted results of the model match more closely with the experimental ones. Upon evaluating the factors that contribute to the growth of IMCs, it has been found that both chemical energy and elastic energy are the driving forces for the transformation of Al3Mg2 into Al12Mg17. The three-dimensional model demonstrates that the two IMCs phases (Al3Mg2 into Al12Mg17) initially form sheet-like structures after nucleation. Subsequently, their growth primarily occurs in the direction of thickness after growing into layers in the transverse plane. The final grain structure consists of polygonal prismatic grains.