Microstructural features governing the strength of immiscible Fe–Mg joints formed by eutectic melt-induced liquid metal dealloying

Joining steel to magnesium (Mg) alloy contributes to weight reduction in automobiles, but it is challenging to achieve strong joints because the main elements of these alloys, iron (Fe) and Mg, are immiscible. Here, eutectic melt-induced liquid metal dealloying (EMI-LMD) was used to join pure Fe and Mg and the microstructural features affecting the Fe–Mg joint strength were examined. Nickel (Ni) is essential as a sacrificial element in EMI-LMD, and thus an Fe–Ni layer was prepared in advance by annealing Ni-plated Fe. EMI-LMD between Fe–Ni and Mg formed a composite layer consisting of α-Fe and Mg phases with a continuously changing interlocking structure at the Fe–Mg interface. The compositional profile of the Fe–Ni layer affected the thickness and structure of the composite layer. The microstructure at the dealloying back, i.e., the interface between the composite layer and Mg, was the most important factor affecting joint strength. Formation of a composite layer with an Mg area fraction exceeding ~50% at the dealloying back increased joint strength because stress concentration in Mg parts of the composite was distributed effectively. Ideal joint strength was achieved when the composite layer thickness was ~20 times that of the α-Fe ligaments at the dealloying back.