## 2 Materials and Methods  
The experiments utilized 6061-T6 aluminum alloy sheets measuring \(\mathrm{I00\,mm}\times\mathrm{\SIUnitSymbolown}\times\mathrm{\SIUnitSymbolown} \times\mathrm{\SIUnitSymbolown}\). Figure 1 shows the fiber-like morphology of the aluminum alloy, while the mechanical properties of 6061-T6 were characterized by ultimate tensile strength (UTS): \(\mathrm{\SIUnitSymbolown}\) = 308 MPa, elongation (EL): 16.7%, and microhardness (HV0.5): 113 (the above data are measured by a universal testing machine and a hardness tester, and the specific experimental details are provided in the following text). The ER5356 aluminum-magnesium welding wire with a diameter of 1.2 mm was employed, and Table 1 presents the elemental composition of 6061-T6 and ER5356. Before welding, the sheets were meticulously polished with a stainless steel brush to eliminate surface oxides and cleaned with acetone. The chosen welding method was DC double-pulse MIG (DP-MIG) butt welding (electric welding machine, DP500, DAIHEN Corporation, Osaka, Japan), wherein the current waveform comprised two components, i.e., a low-frequency pulse and a high-frequency pulse, supplemented by varying current amplitudes. This approach facilitated the agitation of the molten pool within the FZ, reducing porosity and refining grain size. The welding parameters included a current of 80 A, as detailed in Table 2, wire feed speed adjusted to match the welding current, welding speed of 800 mm/min, and the use of 99.99% Ar shielding gas at a flow rate of 16 L/min, with a 13 mm wire extension. Using a rolling machine, the TWBs with weld excess metal underwent CR experiments at room temperature. The rolling direction aligned with the welding direction, and the roll gap was set at 2.0 mm, equivalent to the thickness of the BM, controlled via an operating platform. For the SHT, the temperature was maintained at 520 \(\pm\) 5 \({}^{\circ}\)C for 1.5 h, with a quenching transfer time reduced to less than 10 s to prevent decomposition of the supersaturated solid solution (SSSS) during quenching. The SHT is the same as the solution heat treatment in the T6 treatment of the original BM. The four post-weld treatment processes are shown in Table 3.  
The polarized optical microscope (POM, Zeiss HAL-100, Carl Zeiss AG, Jena, Germany) examined grain morphology. Metallographic samples underwent mechanical polishing \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Materials** & **Mg** & **Si** & **Fe** & **Cu** & **Cr** & **Mn** & **Zn** & **Al** \\ \hline
6061-T6 & 1.10 & 0.62 & 0.42 & 0.26 & 0.19 & 0.08 & 0.01 & Bal \\ ER5356 & 4.90 & 0.05 & 0.12 & 0.01 & 0.12 & 0.12 & 0.01 & Bal \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Elemental composition of materials (%wt). Figure 1: The microstructure of 6061-T6. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Welding** & **Base Current** & **Feak Current (\(\mathbf{f_{p}/A}\))** & **Base Time (\(\mathbf{t_{p}/ms}\))** & **Peak Time (\(\mathbf{t_{p}/ms}\))** \\ \cline{2-7}
**Current (U/A)** & **(\(\mathbf{t_{p}/A}\))** & **Low Pulse** & **High Pulse** & **Low Pulse** & **High Pulse** & **Low Pulse** & **High Pulse** \\ \hline
80 & 28 & 310 & 336 & 0.8 & 1.6 & 1.0 & 0.8 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Current waveform parameters. using 400#-2400# SiC sandpaper sequentially. Subsequently, electrochemical polishing was performed with a solution of HClO4:C2H5 OH in a 1:9 ratio for 10 s. Anodic oxidation was then carried out for 100 s, utilizing a solution comprising HBF4:H2O in a 1:40 ratio. The final POM metallographic samples were prepared for analysis. The scanning electron microscope (SEM, ZEISS-SUPRA55, Carl Zeiss AG, Jena, Germany) with an energy-dispersive spectrometer (EDS, Oxford Instruments, Abingdon-on-Thames, UK) was utilized for elemental and second-phase distribution analysis.