Understanding formation mechanisms of intermetallic compounds in dissimilar Al/steel joint processed by resistance spot welding

This paper confirmed the formation mechanism of intermetallic compounds (IMCs) in Al/steel resistance spot welds with transmission electron microscopy, electron backscatter diffraction, nanoindentation and thermodynamic calculations. In particular, the formation of AlFe with BCC_B2 structure, which is not commonly seen in welds, was identified. The formation mechanism of IMCs at the high welding energy region is described as follows. Al₁₃Fe₄ first nucleates from the Al side, followed by Al₅Fe₂ growth with Fe atoms accumulating at the Al₁₃Fe₄ grain boundaries. Then, Al₅Fe₂ grains grow continuously to coarse columnar grains, and AlFe forms at the interface of Al₅Fe₂ grains and the ferrite phase. Lastly, the needle-like Al₁₃Fe₄ forms in the cooling process. At the middle welding energy region, only equiaxed Al₅Fe₂ and small Al₁₃Fe₄ grains are formed at the interface because of the lower diffusion rates of Al and Fe, hence postponing the growth of Al₅Fe₂ and Al₁₃Fe₄. At the low welding energy region, only sporadic Al₅Fe₂ and Al₁₃Fe₄ grains are formed surrounded by the Al phase.