**1. Introduction**

With the growing concerns on global warming and energy prices, the demand for environmentally friendly vehicles with better fuel efficiency is increasing [1–3]. These issues can be addressed by reducing car weight, lowering travel resistance, advancing drive-train efficiency, developing new sources of power, and so on. Vehicle weight reduction using advanced lightweight structural materials, such as Mg, Al, and Ti, is considered as one of the most promising strategies to address these issues [4–10]. Generally, for every 10% weight reduction, the specific fuel consumption could reduce by 3–7%, while maintaining the same functionality [11, 12]. Thus, the use of lightweight alloys as structural materials is considered as the factor for development

of aerospace and automotive manufacturing sectors in the future. To achieve lightweight, safety, and low cost, the multi-material structure using steels, Al, Mg alloys is considered to be efficient [9]. Therefore, effective dissimilar joining process of such light metals and steels is essential.

As the lightest structural material, Mg alloys receive great attention due to their high specific strength, sound damping capabilities, hot formability, good castability, recyclability among others [13–18]. Potential applications of Mg alloys in an automobile include seat components, bracket carrier, roof, bonnets, cylinder head, wheels, etc. [8, 19]. Steel is currently the automaker's material of choice, due to its inherent properties, including high strength and toughness, good ductility, and low cost [8, 20–22]. Recently, it has been demonstrated through the next-generation vehicle project that stainless steels are promising candidates for vehicle construction, and they can be used replaced carbon steels, especially in crash-relevant components such as door pillars.

Therefore, for practical applications in automotive industries, Mg alloys will have to be joined with existing steel parts. Recently, many automotive components have been produced using a combination of Mg alloys and steels, but the major issues arise from the joining techniques and corrosion of the joined parts [23–26]. Thus, attaining reliable Mg/steel hybrid joints is paramount for facilitating lightweight industrial fabrication and expanding the industrial applications of Mg alloys in automotive industries [21, 27–29].

Joining Mg alloys directly to steel is extremely difficult because of the huge differences in their physical and metallurgical properties, the lattice mismatch between Fe and Mg is very large and there is almost zero solubility between Mg and Fe [27, 29–35]. Hence, an appropriate technique that overcomes the aforementioned problems is very much desired.

Generally, a successful joint between Mg and steel can be achieved by inserting an intermediate material at the interface or diffusion of alloying elements from the BM. At present, several authors have focused on joining magnesium alloys to different grades of steel, using various welding technology, such as friction stir welding (FSW), ultrasonic spot welding (USW), diffusion and eutectic bonding, resistance spot welding, laser welding brazing, laser-TIG hybrid welding and gas metal arc weld-brazing. In these studies, various interlayer elements and alloys such as Zn, Ni, Cu, Cu-Zn, Sn, Al, and Ag have been explored. In contrast to direct joining of magnesium to steel, which is mainly a mechanical bonding, with insertion of the interlayer elements, formation of intermetallic phases or solid solutions between Mg and the interlayer and also the interlayer and Fe indicated that metallurgical bonding is achieved. However, the joint performance and the interfacial bonding achieved depend significantly on the IMC phase formed [31, 36–38]. To control the morphology and existence state of the intermediate phase, the selection of suitable interlayer material and joining techniques are essential. Generally, choosing the suitable interlayer for joining Mg alloys to steel largely depends on the interlayer composition that gives excellent wetting and bonding without generating thick layers of hard and brittle IMCs at the joint interface [31, 35, 39–41]. Moreover, when choosing the joining process that will be used, minimization of the thickness of any brittle intermetallic compounds along the interfaces of the magnesium alloy-interlayer-steel joint and minimization of intermixing between the Mg and Fe in the molten-state are the main factors that must be considered [8, 21, 42, 43].

Currently, a great deal of research has been conducted on the interface characteristics and mechanical performance of Mg alloys to steel joints, particularly under static loading. Under optimized processing conditions, excellent static strength has been achieved, even surpassing that of Mg alloy base metal with insertion of Ag, Cu, and Ni intermediate

### *Challenges and Advances in Welding and Joining Magnesium Alloy to Steel DOI: http://dx.doi.org/10.5772/intechopen.101862*

elements [38, 44]. However, few experiments have been carried out on the corrosion behavior of the jointed parts and the joints performance under dynamic loading [45–48].

With the continuously increasing usage of Mg alloys in industries and the large number of potential applications of Mg/steel hybrid structures, two of the specific areas of concerns for broader utilization of magnesium alloys are reliable joining techniques and corrosion behavior of the joined parts. To better understand and address these challenges, there is a need to comprehensively review the research conducted so far and provide the most efficient strategies to address the challenges. This paper presents a review on Mg alloys/steel joining techniques, with focus on the techniques used to control the morphology and existence state of intermetallic compound (IMC) and improving mechanical properties. The general motives behind this review are to obtain a better understanding on the weldability issues associated with joining magnesium alloys to steel. It would also establish global, state-of-the-art welding techniques of Mg alloys to steel.
