## 4 Discussion
### Mechanical Property Consistency
Therefore, an investigation was conducted into the impact of the precipitated phases resulting from the PWSHT process on the mechanical properties within the FZ and the HAZ. Figure 15 shows the distribution of Mg, Si, and Fe elements within the FZ using the backscattering mode in the SEM. The upper part displays the element distribution in the FZ of the AW joints. The surface scan element diagrams for Mg and Si reveal an even distribution for most Mg and Si solute atoms. Some aggregation of Mg and Si solute atoms indicates the formation of a significant amount of Al-Mg eutectic compounds and a smaller quantity of Mg-Si precipitated phases. The lower part displays the FZ element distribution in the PWSHT joints. In this case, Mg and Si solute atoms are uniformly distributed with reduced element aggregation. Following an extended period of the SHT, most Mg and Si atoms dissolve into the Al SSSS and subsequently precipitate in clusters and the Gunier-Preston zone [29]. The type and quantity of the Mg-Si precipitated phase significantly impact the FZ strength [30]. Since the FZ filler metal mainly consists of Al and Mg elements, the Mg-Si precipitated phase content in the FZ is relatively tiny. As a result, the difference in mechanical properties is minimal, as shown in Figure 9. Regarding the Fe element, the surface scan element diagram on the right side of this figure indicates that the Fe content within the FZ is significantly lower than that of Mg and Si elements. Figure 16 shows the SEM images of the HAZ softening zone. Point analysis revealed that the white region corresponds to the Al-Fe-Si precipitated phase, while the black region represents the Mg-Si precipitated phase [31]. Figure 16a displays the SEM image of the AW joints. There is a considerable presence of massive Al-Fe-Si phases and point-like or flake-like Mg-Si phases distributed on the surface of the \(\alpha\)-Al matrix. Among these, the point-like or flake-like precipitated phase exhibits a Mg-Si ratio ranging between 6:5 and 2:1. Based on the existing literature [32], the precipitate phase consists of a small quantity of unstable \(\beta^{\prime}\) and a significant amount of stable \(\beta\) phases. These over-aging precipitate phases contribute to the HAZ softening, reducing mechanical properties in the joints. Figure 16b presents the SEM image of the PWSHT joints. The presence of black precipitated phases has notably decreased, indicating that the SHT has caused some of the Mg-Si precipitated phase to dissolve in the SSSS. The remaining precipitated phase is primarily identified as having a ratio close to between 5:6 and 6:5, corresponding to the unstable \(\beta^{\prime\prime}\) phase [33] and \(\beta^{\prime}\) phase. The ratio and content of the Al-Fe-Si precipitated phase remain largely unchanged, signifying that the SHT has minimal impact on the Al-Fe-Si precipitated phase. The SHT generally leads to re-solubilizing a significant portion of precipitated phases into \(\alpha\)-Al. This reduces the precipitation-strengthening effect and lowers the likelihood of crack nucleation [34]. Consequently, the SHT enhances the HAZ's ductility, as shown in Figure 10.  
In summary, the PWCR-SHT process leverages recrystallization behavior to enhance the ductility of the FZ and improves the HAZ ductility by reducing the content of precipitated phases. The hardness levels in the two regions, as shown in Figure 9, tend to align. Therefore, the PWCR-SHT process promotes greater uniformity in the mechanical properties of the FZ and the HAZ. Figure 16: The SEM images of the HAZ softening zone: (**a**) the AW joint and (**b**) the PWSHT joint. Figure 15: The distribution of Mg, Si, and Fe elements within the FZ using the backscattering mode in the SEM.