**2. Weldability of Mg alloys to steel**

Some of the inherent properties of Mg include high thermal conductivities and coefficients of thermal expansion, large solidification temperature ranges, strong tendency to oxidize, low viscosity and surface tensions, high solidification shrinkage, low melting and boiling temperatures, a tendency to form low melting point constituents and high solubility for hydrogen in the liquid state [8, 30, 49]. It is obvious that the properties of Mg differ significantly from those of Fe. For instance, the melting points of magnesium and iron are 649°C and 1536°C, respectively. This wide discrepancy in melting points makes it very difficult to melt the base materials at the same time as might be required in fusion-welding process [21].

In addition, the crystal structure of Iron at room temperature is body-centered cubic (BCC), whereas that of magnesium is close-packed hexagonal (HCP). Crystallographic analysis has shown that the lattice mismatch of Fe and Mg is very large [34, 50, 51]. Although the welding process itself is a non-equilibrium process, phase diagram has always been an effective tool to predict the reactions formed during welding process and serves as a reference to examine the feasibility of achieving a metallurgical bonding between the metals. According to the Mg/Fe binary phase diagram, the maximum solid solubility of iron in magnesium is 0.00043 wt.% while that of magnesium in iron is nil, and the Mg concentration at the eutectic point is estimated to be less than 0.008 at.% [34, 52–54]. Therefore, magnesium and steel are immiscible (neither the formation IMC nor atomic diffusion occurs between them after solidification), thereby presenting difficulty in joining them together.
