## 2 Materials and Methodology  
### Material Selection and Preparation Process of Thin-Walled Profiles  
According to the different stoichiometric ratios of Mg and Si contents relative to Mg\({}_{2}\)Si, in this study, three Al-Mg-Si cast rods with different primary alloy compositions are designed: 6 xxx-1(1.6 Mg/Si), 6 xxx-2(1.33 Mg/Si), and 6 xxx-3(1.14 Mg/Si) cast rods. The Mg and Si contents of the 6 xxx-1(1.6 Mg/Si) cast rod approximately satisfy the stoichiometric ratio of Mg\({}_{2}\)Si, the Si content of 6 xxx-2(1.33 Mg/Si) cast rods exceeds the stoichiometric ratio of Mg\({}_{2}\)Si, and 6 xxx-3(1.14 Mg/Si) cast rod contents are comparable Mg and Si contents. The elemental contents of Fe, Mn, Cr, and Cu of the three cast rod alloys are approximated as Fe \(\approx\) 0.18%, Mn \(\approx\) 0.15%, Cr \(\approx\) 0.01%, and Cu \(\approx\) 0.20%, and the specific chemical compositions of the three cast rod alloys are shown in Table 1.  
The preparation process of the thin-walled profiles is as follows: first, the homogenized Al-Mg-Si cast rod is heated to 520 \({}^{\circ}\)C and then is extruded through an extruder into a double-cell rectangular thin-walled profile as shown in Figure 1, where the extrusion speed is 4 \(\pm\) 0.2 m/min and the extrusion outlet temperature is 550 \(\pm\) 5 \({}^{\circ}\)C. After completing the extrusion, the thin-walled profile is quenched by water at the outlet, after which the thin-walled profile is moved into the aging furnace for artificial aging at 175 \({}^{\circ}\)C/10 h (under-aged), 175 \({}^{\circ}\)C/16 h (near-peak-aged), and 205 \({}^{\circ}\)C/6 h (over-aged), respectively. The thin-walled profiles corresponding to the 6 xxx-1(1.6 Mg/Si), 6 xxx-2(1.33 Mg/Si), and 6 xxx-3(1.14 Mg/Si) alloy cast rods are designated as the 6 xxx-1(1.6 Mg/Si), 6 xxx-2(1.33 Mg/Si), and 6 xxx-3(1.14 Mg/Si) profiles, respectively.  
In aging treatment, the determination of heating time and heating temperature mainly depends on the following two aspects:  
1. A large number of engineering experiments. Our team has conducted a great number of engineering experiments on the 7 xxx series thin-walled aluminum alloys. It was found that with under 175 \({}^{\circ}\)C/10 h (under-aging), 175 \({}^{\circ}\)C/16 h (near-peak aging), and 205 \({}^{\circ}\)C/6 h (over-aging), the changes in crushing performance of 7 xxx thin-walled aluminum alloy were most obvious. Therefore, 6 xxx-series wall aluminum alloy also adopts the same failure system;
2. A large number of literature references [24; 25; 26]. [PERSON] et al. [11] studied AA6351 aluminum alloy and found that the fatigue strength of AA6351 aluminum alloy reaches its peak at 170 \({}^{\circ}\)C and an aging time of 6 h. [PERSON] et al. [27] artificially aged A350a aluminum alloy for 2 h at 175 \({}^{\circ}\)C and 205 \({}^{\circ}\)C, respectively. The results showed that artificial aging treatment at 175 \({}^{\circ}\)C can improve the fatigue life and performance of aluminum alloys.  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline
**Serial Number** & **Si** & **Fe** & **Cu** & **Mn** & **Mg** & **Cr** & **Al** & **Mg/Si** & **Mg + Si** \\ \hline
6 xxx-1(1.6 Mg/Si) & 0.55 & 0.17 & 0.20 & 0.14 & 0.90 & 0.01 & Bal. & 1.60 & 1.45 \\
6 xxx-2(1.33 Mg/Si) & 0.81 & 0.18 & 0.21 & 0.15 & 1.08 & 0.01 & Bal. & 1.33 & 1.85 \\
6 xxx-3(1.14 Mg/Si) & 0.59 & 0.19 & 0.20 & 0.15 & 0.67 & 0.01 & Bal. & 1.14 & 1.26 \\ \hline \end{tabular}
\end{table}
Table 1: Chemical compositions of the three cast rod alloys (wt. %).