## 3 Results and Discussion  
### Wettability Characteristics of Bead-on-Plate Brazing  
Metallographic cross sections of different bead-on-plate brazing specimens are shown in Figure 4. The width of the wetting length (L) and wetting angle (\(\theta\)) at different preheating temperatures are given in Figure 5. It was found that all of the wetting angles were below 90\({}^{\circ}\). Compared with the joint without preheating, the width of the wetting length was longer and the wetting angle was lower with the preheating treatment. The width of the wetting length increased and the wetting angle decreased with the increasing preheating temperature. The minimum wetting angle (30\({}^{\circ}\)) and the maximum width of wetting length (15 mm) were obtained at a 200 \({}^{\circ}\)C preheating temperature. This indicated that the liquid AlSi5 filler wire had a good wettability on the galvanized steel. The preheating treatment could increase the spreading of the AlSi5 filler wire. Figure 3: Geometry of the specimen for tensile test. Influence of Preheating Temperature on the Overlap Welding-Brazing of Aluminium Alloy and Galvanized Steel  
The cross sections of the overlap welding-brazing specimens are shown in Figure 6. The width of the wetting length (L) and wetting angle (\(\theta\)) at different preheating temperatures are given in Figure 7. As can be observed in Figures 6 and 7, the wetting angles were higher and the widths of the wetting length were lower than those for the bead-on-plate brazing at the same preheating temperatures (see Figure 5). This was because the width of the wetting length or the wetting angle was influenced not only by the heat input of arc and preheating treatment, but also by the heat transfer of steel and  
Figure 4: Metallographic cross sections for bead-on-plate brazing at different preheating temperatures. Figure 5: Width of wetting length and wetting angle at different preheating temperatures. aluminium. Compared with the bead-on-plate brazing, the extra and fast heat transfer by the 7075 aluminium alloy led to heat loss. Meanwhile, the variation tendency of the width of the wetting length and wetting angle with different preheating temperature was the same as bead-on-plate brazing. The variation trend was the same as the MIG welding-brazing reported in the literature [10]. Figure 8 shows the typical microstructures of the CMT welding-brazing joint (without preheating). Table 3 gives the average chemical compositions of each zone. According to the microstructure features,  
Figure 6: Metallographic cross sections for overlap welding–brazing at different preheating temperatures. (**a**) Without preheating; (**b**) 100 \({}^{\circ}\)C; (**c**) 200 \({}^{\circ}\)C.  
Figure 7: Width of wetting length and wetting angle at different preheating temperatures. the microstructures of the joint were divided into four different zones, namely: the interfacial layer, weld metal zone, Zn-rich zone, and heat affected zone (HAZ). An irregular interfacial layer (A) occurred at the interface of the steel and brazing filler. From the EDS result, the interfacial layer mainly contained Al and Fe. No Zn was found in the interfacial layer. According to the phase diagram of Fe-Al [14], it was probably Fe-Al IMC. The thickness of the IMC layer was about 1.5 \(\upmu\)m, as shown in Figure 8a. During the welding-brazing process, the arc heat melted AlSi5 filler wire and increased the temperature of the steel. The Zn coating within the region of the heavy heating was easy to evaporate, because of the low evaporating temperature of Zn. At the same time, a clean surface of the steel was obtained, which was beneficial to wetting. This was the main reason that the galvanized steel with a thin zinc layer on the surface was well wetted without flux. Figure 8b shows the microstructure of the weld metal zone (B). B mainly consisted of Al and C was composed of 76.90 at.% Al and 20.03 at.% Si. Because the original filler wire was Al-5 wt.% Si. The microstructure of the weld metal was like the phases of the original filler wire.