## 3 Results
### Morphology of High-Temperature Oxidation (HTO) Film  
Figure 3a depicts the surface macroscopic morphology of the HTO film of the weld joint. It can be seen from the figure that the 304 stainless steel side is yellow, and no metallic luster can be seen on the side of the weld and 6061 aluminum alloy. Three locations were selected as shown in Figure 3a to analyze the microstructure of the HTO film by SEM, as shown in Figure 3b-d, in which the upper right corners are partially enlarged views. Figure 3b shows that the surface of the IMC side is smooth without an obvious oxidation film, the metal surface of 304 is smooth at a magnification of 2000\(\times\) and there is an obvious discontinuous oxidation film at a magnification of 10,000\(\times\). Some oxide convex and concave areas can be clearly observed on the surface of the 6061 side at 2000\(\times\) or 10,000\(\times\). EDS surface scanning analysis was carried out, as shown in Figure 3c-e, and the analysis results are shown in Figure 4. It was found that there were oxygen-containing compounds in these three surface scans (IMC, 304 and 6061) or in the three EDS point scans, indicating that the sample substrate was covered with the high-temperature oxidation film, and their atomic percentages of oxygen from the surface scan results were 4.12%, 7.65% and 20.51%. Fe and Cr oxides were mainly formed on the side of 304 as well as Al oxides on the side of 6061 during the oxidation process, as shown in Table 3. According to the EDS surface scanning analysis (Table 3), more oxygen elements were found on the 6061 aluminum alloy side of the welded joint compared with the side of IMC and 304 which underwent high-temperature oxidation treatment at a temperature of 500 \({}^{\circ}\)C because at the same temperature, the Gibbs free energy of the Al\({}_{2}\)O\({}_{3}\) is lower than the Cr\({}_{2}\)O\({}_{3}\), and the ability of aluminum to reduce oxygen is stronger. In short, the driving force for the formation of Al\({}_{2}\)O\({}_{3}\) is greater than that of Cr\({}_{2}\)O\({}_{3}\), as demonstrated in the diagrams of \(\Delta\)Gv\(\ominus\)of the formation of oxides [12]. At the same time, [PERSON] et al. [13] found that the oxidation initiates at 140 \({}^{\circ}\)C for aluminum, and the oxidation resistance of austenite stainless steel is still very good at 900 \({}^{\circ}\)C, according to the oxide thickness scale for oxidation of 304 [14]. Figure 3: Surface image of the HTO-treated specimen: macromorphology for the image (**a**), microstructure for the remaining images, (**b–d**) partial enlarged images of the interface, stainless steel and aluminum alloy, respectively(the upper right corner is a partially enlarged view), (**e**) partial enlarged images of the IMCs. Figure 5a depicts the cross-section SEM image of the HTO film of the weld joint. Three locations were selected as shown in Figure 5a to analyze the microstructure of the weld joint, as shown in Figure 5b-d. Figure 5e is the cross-section SEM image of the 6061 side of the weld joint, which is the same sample as in Figure 5a. Figure 5f is the cross-section SEM image of the 6061 side of the weld joint without HTO treatment. The thicknesses of the HTO film on the 304 and 6061 sides are 449.35 nm and 2401 nm, respectively, while the thickness of the 6061 side of the weld joint without HTO treatment is 181.1 nm. That is, the film thickness of the sample increases after HTO treatment. At the same time, we found that there were cracks in some parts of the oxide film on the 6061 side of the weld joint, as shown in the red circle in Figure 5a,e.