## 3 Results  
### Microstructure of the Weld Joint  
Three locations (weld center, aluminum weld heat-affected zone and steel-aluminum interface) were selected to analyze the microstructure of the weld joint obtained by SEM, as shown in Figure 1, where Figure 1a-c represent the micromorphology of the different positions of the welding joint without high-temperature oxidation, and Figure 1d-f show the micromorphology of the different positions of the welding joint with high-temperature oxidation. The weld microstructure is mainly composed of \(\alpha\)-Al solid solution and Al-Si eutectic phases in Figure 1a,d. However, petal structures are sparsely distributed in the weld center after high-temperature oxidation treatment, as shown in the yellow circle in Figure 1d. Figure 2 and Table 2 show the surface scan and point scan results of the local  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Materials** & **Al** & **Fe** & **Si** & **Cr** & **Ni** & **Mg** & **Mn** \\ \hline SUS304 & - & Bal. & 0.57 & 17.51 & 8.02 & - & 1.53 \\
6061-T6 & Bal. & 0.7 & 0.4–0.8 & 0.04–0.35 & - & 0.8–1.2 & 0.15 \\ ER4043 & Bal. & \(\leq\)0.6 & 4.5–6.0 & - & - & \(\leq\)0.05 & \(\leq\)0.05 \\ \hline \hline \end{tabular}
\end{table}
Table 1: The main chemical composition of the base metals and filler wire (wt.%).  
magnified view in the upper right corner of Figure 1a,d, respectively. The EDS analysis of point 1 and point 4 showed that the composition in this area was mainly composed of aluminum substrate; point 2 indicates that the area is mainly Al-Si eutectic phases, and point 3 indicates that iron forms a petal-like compound with aluminum, which may be caused by the diffusion of iron in steel into the weld at high temperature. The results of weld surface scanning in Figure 2 show that iron diffuses to the central structure of the weld after high-temperature oxidation compared with that without high-temperature oxidation. The microstructure of the weld heat-affected zone is similar in Figure 1b,e, indicating that high-temperature oxidation has no obvious effect on this area. Figure 1c,f show the structure at the interface between steel and aluminum. There are intermetallic compounds (IMCs) due to the physical diffusion and chemical reaction between different elements at the interface [10]. The width of the intermetallic compounds at the interface of the sample after high-temperature oxidation treatment was wider (approximately 7-8 \(\upmu\)m) than that of the sample (2-3 \(\upmu\)m) without high-temperature oxidation treatment. This result indicated that the IMCs at the interface obtained heat energy and further grew during HTO treatment, which means that the thickness of the IMCs is related to the heat input (as reported by [PERSON] et al. [11]).  
Figure 1: Micrographs of the dissimilar weld-brazed joint. ((**a**,**d**) weld center, (**b**,**e**) aluminum weld heat-affected zone, (**c**,**f**) steel-aluminum interface).  
Figure 2: EDS surface scan of the partially enlarged view in Figure 1a,b: (**a**) without high-temperature oxidation, (**b**) with high-temperature oxidation.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Point No.**} & \multicolumn{5}{c}{**at.\%**} & \multirow{2}{*}{**Remarks**} \\ \cline{2-2} \cline{5-6}  & **A1** & **Fe** & **Si** & **Cr** & **O** & **O** \\ \hline
1 & 90.50 & - & 0.68 & - & 0.66 & spot scan \\
2 & 74.24 & 2.85 & 7.82 & 0.75 & 2.03 & spot scan \\
3 & 59.68 & 10.03 & 8.62 & 3.86 & 2.73 & spot scan \\
4 & 86.62 & 0.13 & - & - & 0.63 & spot scan \\
5 & 24.56 & 45.99 & 1.13 & 10.79 & 1.93 & spot scan \\
6 & 77.07 & - & 0.45 & - & 1.67 & spot scan \\
7 & 41.16 & 15.5 & 5.37 & 8.73 & 2.73 & spot scan \\
8 & 60.28 & 18.03 & 2.30 & 2.00 & - & spot scan \\
9 & 83.29 & 0.29 & - & - & 1.31 & spot scan \\ Figure 1a & 78.68 & 0.65 & 1.45 & 0.27 & 1.3 & surface scan \\ Figure 1d & 79.37 & 1.15 & 2.39 & 0.4 & 1.73 & surface scan \\ \hline \hline \end{tabular}
\end{table}
Table 2: EDS analysis in Figure 1.