**4. Summary and outlook**

As there is a desire in the aerospace, aircraft, and automotive industries to join magnesium alloys to steel in order to achieve lighter weight, versatile, and tailored properties in one composite part, and development of a welding technology for Mg alloys/steel with a strong metallurgical bond will expedite increased applications of magnesium alloys in these industries. The major challenge in welding of magnesium alloys to steel is the huge differences in physical properties and limited solubility that make the welding process difficult. Different joining processes have been used to join Mg alloys to steel sheets but metallurgical bonding can only be possible with insertion of intermediate interlayer elements or alloy or mutual diffusion of alloying elements from BM. The existence of intermetallic phases or solid solutions between Mg and the interlayer and also the interlayer and Fe is an indication that metallurgical bonding between Mg and Fe using the interlayer may be possible. Formation of thick, brittle intermetallic compounds along the interface between Mg and steel can cause significant deterioration of mechanical properties. Therefore, when choosing the interlayer and the joining process that will be used, minimization of the thickness of any brittle IMCs that might form at magnesium alloy-interlayer-steel joint interfaces and minimization of intermixing between the Mg and Fe in the liquid state are main factors that must be considered.

Solid-state bonding techniques based on FSW, USW, and diffusion and eutectic bonding have been used. Generally, the intimate contact between the dissimilar materials and the formation of IMC with insertion of the suitable interlayer played a significant role in controlling the joint performance of solid-state bonded magnesium alloys to steel.

Joining magnesium alloys to different grades of steels by fusion processes such as RSW, laser welding, and arc welding with and without insertion of interlayers were also reported. Despite the fast heating and cooling rate of the fusion process,
