## 1 Introduction  
The growing demand for lightweight design in the automotive industry arises a strong interest in the joining of aluminium alloy to steel. It is difficult to obtain a sound joining of aluminium to steel because of the large differences in the melting points, the coefficient of the thermal conductivity, and thermal expansion, especially the formation of intermetallic compound (IMC) at the Fe/Al interface. According to the Al-Fe phase diagram, the formation of IMC was inevitable during welding, due to the poor solubility between Fe and Al [1; 2; 3]. However, an excessive formation of IMC resulted in the brittleness in the joints. The published results showed that the thickness of IMC should be below 10 \(\upmu\)m, which was the critical value in order to obtain good mechanical properties in Fe/Al dissimilar metal welding [4; 5]. The total thickness of the IMC was primarily dominated by thermal diffusion and was related to the thermal cycle (i.e., peak temperature and duration) of the welding process [6]. The arc welding-brazing process combined the advantages of welding and brazing. Therefore, it was a preferable method to join aluminium alloy and steel, as it could suppress the growth of IMC and obtain good mechanical properties of Al/Fe joint. In this process, part of the aluminium alloy was melted, but the steel was in a solid state. The melted filler wire and aluminium alloy wetted the solid steel surface. Meanwhile, the Fe atoms diffused into the melted metal, and then IMC was formed at the interface of the steel/brazing metal. Except for the thickness of IMC, the wettability and width of the wetting length were also critical factors influencing the mechanical properties of the joints in industrial manufacturing [7]. However, the wettability of liquid aluminium on a steel surface was fairly poor, due to the presence of an oxide layer on the surface of base material [8]. Usually, the oxide layer was removed by flux or a cathodic cleaning method so as to promote the wetting [7]. Moreover, the wettability can be improved by the zinc coatings and the preheating treatment on the steel sheet [9; 10; 11]. Therefore, galvanized steel was often used as a base material. The Zn coating is electro-galvanized or hot-dip galvanized. The thickness of coating is about 5-11 \(\upmu\)m [7]. The spreadability of the Al-based droplets on the steel with preheating was greatly improved, which indicated that the preheating treatment process might be beneficial for obtaining a sound Al/Fe welding-brazed joint. In the metal inert gas (MIG) welding-brazing of Al/Fe with preheating treatment, [PERSON] [10] and [PERSON] [11] found that the preheating treatment process can increase the spreadablility of the weld seam and obtain a higher strength with a suitable preheat temperature. Cold transfer metal (CMT) is a modified process by MIG welding. In the CMT process, the filler wire is intentionally retracted instantaneously, which assists the droplet detachment during the short circuit. The short circuit current is kept small, as shown in Figure 1. Compared with the conventional welding processes, the melted metal can be transferred into the welding pool without the aid of an electromagnetic force. It is characterized by a low heat input and no-spatter welding, which is a preferable process to join Al and Fe, as reported by [PERSON] [12] and [PERSON] [13]. From the above, the low heat input CMT welding-brazing process or preheating treatment can be used to increase the strength of the joint. Thus, the aim of this study was to investigate the feasibility and mechanism of joining aluminium and steel. The 7075-T651 aluminium alloy with a high strength (more than 538 MPa) and low density (2.81 g/cm\({}^{3}\)) is used, which is suitable for structures and components that are subjected to static and dynamic loads. Bead-on-plate CMT brazing was used to better understand the wetting characteristics of the brazing metal on steel.