## 3 Results
### Second-Phase Particle Precipitation  
Figure 11 shows the SEM images of the second-phase particles of the three Al-Mg-Si alloy thin-walled profiles in different aging states. Tables 7 and 8, respectively, show the chemical composition and area fraction of second-phase particles of the different alloy thin-walled profiles. There are some undissolved second-phase particles in all three types of Al-Mg-Si alloy thin-walled profiles, of which the coarser second-phase particles mainly comprise elements such as Fe and Si. In addition, the alloy composition and aging process have almost no effect on the area fraction of the second phase, and the area fraction of the second phase of the three profiles with different aging processes are similar, with the 6 xxx-2(1.33 Mg/Si) profile having a higher number of primary alloying elements having a slightly higher area fraction of the second phase. Therefore, it can be concluded that differences in the properties of the three profiles are independent of second-phase particles. Different aging processes of the profiles result in the precipitation of different types, sizes, quantities, and distributions of strengthening phases. Usually, two types of precipitates can be observed in the Al-Mg-Si alloy matrix after near-peak aging treatment: (1) a fine needle-shaped precipitate, which is distributed across in the matrix and (2) a point-like precipitate. Dislocations require a higher amount of energy to bypass the fine precipitation phases, thereby increasing the strength of profiles in the near-peak-aged state. Compared with profiles in the near-peak-aged state, precipitation phases of profiles in the over-aged state undergo significant coarsening, and dislocations require less energy to bypass the coarsened strengthening phases than finer strengthening phases, reducing the profile strength. Figure 12 shows the TEM images of intragranular precipitation of the three Al-Mg-Si alloy thin-walled profiles in different aging states. Numerous spherical and needle-shaped precipitates are distributed within the grains of all three profiles, most of which are the \(\beta^{\prime\prime}\) phase. The needle-shaped phase is the primary strengthening phase of the Al-Mg-Si alloys; it improves the profile strength by hindering dislocation motions and has the best strengthening effects, while the coarse spherical precipitation phase is the Q phase, and its strengthening effect is weak. \begin{table}
\begin{tabular}{c c c c c} \hline \multirow{2}{*}{**Profile Number**} & \multirow{2}{*}{**Aging Process**} & \multirow{2}{*}{**Specimen Thickness**} & **Measured Bending** & **2 mm Equivalent** \\  & & & **\(d_{1}\) (mm)** & **Angle \(\kappa_{1}\) (\({}^{\circ}\))** & **Bending Angle \(\kappa_{2}\) (\({}^{\circ}\))** \\ \hline \multirow{3}{*}{6 xxx-1(1.6 Mg/Si)} & 175 \({}^{\circ}\)C/10 h & 2.66 & 96 & 81 \\  & 175 \({}^{\circ}\)C/16 h & 2.66 & 91 & 77 \\  & 205 \({}^{\circ}\)C/6 h & 2.65 & 83 & 102 \\ \hline \multirow{3}{*}{6 xxx-2(1.33 Mg/Si)} & 175 \({}^{\circ}\)C/10 h & 2.65 & 51 & 59 \\  & 175 \({}^{\circ}\)C/16 h & 2.65 & 36 & 45 \\  & 205 \({}^{\circ}\)C/6 h & 2.66 & 60 & 69 \\ \hline \multirow{3}{*}{6 xxx-3(1.14 Mg/Si)} & 175 \({}^{\circ}\)C/10 h & 2.64 & 46 & 57 \\  & 175 \({}^{\circ}\)C/16 h & 2.64 & 47 & 54 \\ \cline{1-1}  & 205 \({}^{\circ}\)C/6 h & 2.66 & 48 & 60 \\ \hline \end{tabular}
\end{table} 
Table 6: Bending properties of the three Al-Mg-Si alloy thin-walled profiles in different aging states. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Placement** & **Al** & **Mg** & **Cu** & **Fe** & **Mn** & **Si** \\ \hline a1 & 88.03 & 0.69 & 0.49 & 6.66 & 1.23 & 2.88 \\ a2 & 86.50 & 0.69 & 0.38 & 7.07 & 1.96 & 3.40 \\ d1 & 67.82 & – & – & 21.83 & 4.22 & 6.12 \\ d2 & 83.09 & 0.43 & – & 11.38 & 1.29 & 3.81 \\ f1 & 65.07 & – & 1.01 & 19.70 & 7.22 & 6.99 \\ f2 & 74.35 & 0.36 & 0.63 & 14.17 & 5.08 & 5.41 \\ g1 & 76.29 & – & – & 20.23 & 0.91 & 6.47 \\ g2 & 76.51 & – & – & 16.52 & 0.86 & 5.74 \\ \hline \hline \end{tabular}
\end{table}