## 3 Results
### Second-Phase Particle Precipitation

Table 7: Chemical composition of the second-phase particles (wt%) of the three Al-Mg-Si alloy thin-walled profiles. Figure 11: SEM images of second-phase particles of the three Al-Mg-Si alloy thin-walled profiles in different aging states: (**a–c**) 175 \({}^{\circ}\)C/10 h, 175 \({}^{\circ}\)C/16 h, 205 \({}^{\circ}\)C/6 h, 6 xxx-1(1.6 Mg/Si) profile; (**d–f**) 175 \({}^{\circ}\)C/10 h, 175 \({}^{\circ}\)C/16 h, 205 \({}^{\circ}\)C/6 h, 6 xxx-2(1.33 Mg/Si) profile; (**g–i**) 175 \({}^{\circ}\)C/10 h, 175 \({}^{\circ}\)C/16 h, 205 \({}^{\circ}\)C/6 h, 6 xxx-3(1.14 Mg/Si) profile. A two-by-two comparison of the three profiles reveals that profile 6 xxx-2(1.33 Mg/Si) has the largest number of \(\beta^{\prime\prime}\) phases, profile 6 xxx-1(1.6 Mg/Si) has the next largest number of \(\beta^{\prime\prime}\) phases, and profile 6 xxx-3(1.14 Mg/Si) has the lowest number of \(\beta^{\prime\prime}\) phases. Relevant literature shows that the \(\beta^{\prime\prime}\) phase is the best particle for hindering dislocations in Al-Mg-Si alloys [11,26]. Among the three profiles, the 6 xxx-2(1.33 Mg/Si) profile has the highest total Mg + Si content. Because the increase in total Mg + Si content promotes the precipitation of strengthening phases in the profile after aging treatment, the 6 xxx-2(1.33 Mg/Si) profile would have the largest number of \(\beta^{\prime\prime}\) phases and the highest strength, but the lowest elongation. By comparing the precipitation between the three aging states, it can be found that compared with the under-aged and over-aged states, the amount of \(\beta^{\prime\prime}\) phase precipitation of the three profiles in the near-peak-aged state is significantly higher; therefore, the profile strength is significantly improved, but with some reduction in ductility. Figure 13 shows the images of grain boundary precipitation of the 6 xxx-1(1.6 Mg/Si) profile in different aging states. The width of the precipitate free zone (PFZ) at the grain boundary in the near-peak-aged state is the widest at 128.8 nm, the PFZ in the under-aged state is the next widest at 118.6 nm, and the PFZ in the over-aged state is the narrowest at 111.9 nm. In the near-peak-aged state, large numbers of strengthening phases precipitated from the profile crystal, resulting in the solute atoms being largely consumed and reducing the solubility of solute atoms near the grain boundaries. This leads to the formation of a solute atom \"barren zone\" on both sides of the grain boundary, that is, the formation of a wider PFZ. Compared to the near-peak-aged state, the number of intragranular strengthening phases are lower for the over-aged and under-aged states, fewer solute atoms are consumed, and the solute atom solubility increases on both sides of the grain boundaries, thereby decreasing the PFZ width. Note that an increase in PFZ width weakens the grain  
Figure 12: TEM images of intragranular precipitation in of the three thin-walled profiles of Al-Mg-Si alloys in different aging states: (**a–c**) 175 \({}^{\circ}\)C/10 h, 175 \({}^{\circ}\)C/16 h, 205 \({}^{\circ}\)C/6 h, 6 xxx-1(1.6 Mg/Si) profile; (**d–f**) 175 \({}^{\circ}\)C/10 h, 175 \({}^{\circ}\)C/16 h, 205 \({}^{\circ}\)C/6 h, 6 xxx-2(1.33 Mg/Si) profile; (**g–i**) 175 \({}^{\circ}\)C/10 h, 175 \({}^{\circ}\)C/16 h, 205 \({}^{\circ}\)C/6 h, 6 xxx-3(1.14 Mg/Si) profile. boundary strength and increases the difference between intragranular strength and grain boundary strength, which in turn is unfavorable for ductility. In summary, the effects of profile composition and aging process on strength and crushing energy absorption are mainly attributed to differences in grain structure and precipitation phases. Coarse grain structures and wide grain boundary PFZs decrease the ductility of the profile. Simultaneously, the number of primary strengthening phases in the grain increases, thereby increasing the profile strength, which in turn results in the profile being unable to achieve a better strength-ductility match, reducing the crush performance.