## 2 Materials and Methods  
The workpiece in current study was AA6061-T6 aluminum alloy sheet with dimensions of \(300\:\mathrm{mm}\times 60\:\mathrm{mm}\times 3\:\mathrm{mm}\). Its tensile strength was 327 MPa. A butt joint was prepared. The ER4043 and pure argon were, respectively, selected as the filler wire and the shielding gas for the DP-GMAW process. The self-developed digital welding machine, named Power Pulse(r) II 350, was adopted as the welding power source to carry out the DP-GMAW by programming. Prior to welding, the surfaces of the aluminum alloy sheet were scratched by a steel brush and cleaned by acetone to prevent the effect of oil, oxides, and water. Table 1 shows the chemical compositions of base metal and filler wire. Table 2 shows the main welding parameters of DP-GMAW. Electrical signals  
Figure 1: The current waveform of double pulsed gas metal arc welding (DP-GMAW).  
throughout the experiment were collected and were disposed for offline analysis by the arc dynamic waveform wavelet analyzer [20]. The experimental setup of the current research is shown in Figure 2.  
After the DP-GMAW, metallographic specimens, tensile specimens, microhardness specimens, impact specimens, bend specimens, and corrosion specimens were obtained from each butt welded sheet by the electric cutting machine STDX600. The cutting of all the specimens was perpendicular to the weld, except for the specimens that were used for the inspection of porosity. The dimensions of specimens are shown in Figure 3. The metallographic specimens were prepared by standard metallographic procedures. The microstructure character of the welded joints was analyzed by the optical microscope (OM) (Carl Zeiss AG, Heidenheim, Germany), the scanning electron microscope (SEM) (Hitachi, Tokyo, Japan), and the energy dispersive spectrometer (EDS) (Krates, Manchester, UK). Vickers microhardness tests were carried out on polished and etched specimens with a period of 15 s, a load of 200 g, and a step size of 1 mm. The room temperature tensile and bend tests were carried out on an AG-IC tensile machine (SHIMADZU, Kyoto, Japan). Charpy impact specimens (thickness of 3 mm) were prepared to evaluate the impact toughness of the welded joints. When the tensile tests and impact tests were finished, the fracture morphology of welded joints was observed using SEM.  
Tensile test specimens for the AA6061-T6 sheet and the welded joint were produced following ASTM E8. Bent tests were conducted in accordance with ASTM E290, and Charpy impact test samples were prepared in accordance with ISO 5173:2000.  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Element** & **Si** & **Fe** & **Cu** & **Mg** & **Cr** & **Zn** & **Ti** & **Al** \\ \hline AA6061-T6 & 0.52 & 0.25 & 0.012 & 0.96 & 0.26 & 0.021 & 0.012 & Bal. \\ ER4043 & 4.50-6.00 & 0.80 & 0.30 & 0.05 & – & 0.01 & 0.20 & Bal. \\ \hline \hline \end{tabular}
\end{table}
Table 1: The chemical compositions of base metal and filler wire (mass fraction, %).  
Figure 3: Dimensions of test specimens (unit: mm): (**a**) Tensile test specimen; (**b**) Charpy impact test specimen; and, (**c**) Bend test specimen.  
Figure 2: The experimental setup of DP-GMAW.