Joining of dissimilar metals has always been a major challenge especially for immiscible systems (e.g. Mg/Fe, Mg/Ti). In this study, a set of design methodologies for dissimilar welding is proposed. Miedema Model and Toop model are applied to calculate alloy systems of up to four elements. In addition, finite element modelling (FEM) of temperature field and calculation of phase diagrams (CALPHAD) are combined to provide prerequisite information for thermodynamic modelling. This scheme can be applied to calculate the binary/ternary phase diagram, binary-quaternary formation enthalpy, and Gibbs free energy. As a test subject, laser welded lap configuration joints of AZ31B magnesium alloy to DP590 steel with a copper and nickel coating (Mg-Fe-Al-Cu and Mg-Fe-Al-Ni quaternary systems) were put into the design scheme. The interfacial elemental diffusion and formation of intermetallics (IMCs) along the interface during the welding process are predicted. It is shown that Al atoms will diffuse from the fusion zone (FZ) to the steel substrate and form FeAl phase. Cu atoms will take part in the metallurgical reactions and form Mg2Cu or Mg-Al-Cu LAVES phase. On the other hand, Ni atoms diffused and were accumulated near the interface due to the driving force of the chemical potential. Al atoms from the filler were attracted by the concentrated Ni atoms, which resulted in the formation of the FeAl phase.

The design scheme can be used to guide the selection of intermediate elements in dissimilar welding before the experiments, which contributes to improving efficiency and reducing cost. The effect of Cu/Ni coating thickness on laser welding-brazing of Mg/steel joints is studied. The interfacial microstructure, EDS analysis, Vickers hardness, and tensile properties are also investigated. The results of simulation and experients are combined to clarify the joining mechanism.