## 3 Results and Discussion
### Wettability Characteristics of Bead-on-Plate Brazing
So B was \(\upalpha\)-Al solid solution. C was Al-Si eutectic phase. The Zn-rich zone (D) contained 70.38 at.% Al and 18.39 at.% Zn. According to the Al-Zn phase diagram [14], it was most likely an Al-Zn solid solution. The Zn-rich zone existed at the weld toe. Figure 8: Microstructures of bead-on-plate brazing (without preheating): (**a**) interfacial layer; (**b**) weld metal zone; (**c**) Zn-rich zone; (**d**) heat affected zone (HAZ) zone. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Location** & **Fe** & **Al** & **Zn** & **Si** & **Possible Phase** \\ \hline A & 18.56 & 71.34 & 0.5 & 9.60 & Fe–Al intermetallic \\ B & 0.78 & 96.68 & 0.64 & 1.9 & \(\upalpha\)-Al \\ C & 0.88 & 74.66 & 0.76 & 23.7 & Al–Si eutectic phase \\ D & 0.93 & 71.39 & 19.39 & 09.29 & Al–Zn solid solution \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Energy dispersive spectrometer (EDS) energy spectrum analysis. Based on the characteristics of the arc, the temperature in the center of the arc was higher than the vaporization temperature of Zn. Little Zn was residual after the welding-brazing. However, in the edge of the arc, the temperature was probably lower than the vaporization temperature of Zn, and consequently, the Zn dissolved into the melted Al-based filler metal, forming Zn-rich phase (Figure 8c). During the welding-brazing process, the microstructure of the base material near the weld metal zone was affected and changed by the heat conducted to the base material, as shown in Figure 8d. The phenomenon of the grain boundary melting occurred in the HAZ zone nearest to the weld metal. The hardness of the fusion line (about 116 HV\({}_{0.1}\)) and HAZ (about 146 HV\({}_{0.1}\)) is higher than that in the weld metal zone (79 HV\({}_{0.1}\)), as shown in Figure 8d. The minimum hardness is obtained in the weld metal zone, then the hardness increased rapidly to a higher level in the HAZ adjacent to the fusion line. The peak temperature of the HAZ nearest to the fusion line exceeds the fully dissolved temperature of the \(\eta\) and \(\eta^{\prime}\) phases. The \(\eta\) and \(\eta^{\prime}\) phases dissolve during welding and dissolve out in the cooling stage, thus the hardness of the HAZ nearest to the fusion line is about 146 HV\({}_{0.1}\), which is lower than that of the base material (about 180 HV\({}_{0.1}\)). The fusion line is between the weld metal zone and the HAZ. The microstructure of the fusion line is very heterogeneous, thus it is one of the weaker regions of the joint. Figure 9 gives the microstructures of the interfacial layer obtained at different preheating temperatures. Compared with the joint without preheating, the thickness of the interfacial layer slightly increased with the preheating temperature at 100 \({}^{\circ}\)C. By further increasing the preheating temperature at 200 \({}^{\circ}\)C, the thickness of the interfacial layer can be increased to 3.5 \(\upmu\)m, which is lower than the thickness of the total layer (about 6 mm) at the 200 \({}^{\circ}\)C preheating temperature in the literature [10]. The mechanical properties, fracture location, and fracture surface at different preheating temperatures are shown in Figures 10 and 11. The tensile load of the welding-brazed joint without preheating was 8580 N, and the tensile load obtained with a 100 \({}^{\circ}\)C preheating temperature was 9730 N. Both of the joints were fractured at the fusion line. This was because the thickness of the IMC was thin, the tensile force of the fusion line on the Al side was sufficient to cause the fracture before the tensile shearing force of the interfacial layer on steel side would cause fracture. Moreover, because of the preheating of the base materials to 100 \({}^{\circ}\)C, the cooling speed and welding stress of the joint decreased, which led to higher strength of the fusion line. But when the base materials were preheated to 200 \({}^{\circ}\)C, the tensile load decreased to 5636 N. The cracks occurred at the interfacial layer.