## 2 Experimental Methods and Materials
### Experimental Materials and Processing  
The base metal is a 2205 duplex stainless steel plate, with dimensions of 250 \(\times\) 100 \(\times\) 3 mm. ER2009 weld wire with a diameter of 1.2 mm is adopted. Their chemical compositions are shown in Table 1. The peak and base current of the leading and trailing wire are 350 A and 120 A. The weld frequency is 3 Hz, and the other weld parameters such as the flow rate of additional shielding gas, and weld speed are shown in Table 2.  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline
**Materials** & **C** & **Mn** & **Si** & **Cr** & **Ni** & **Mo** & **N** & **S** & **P** \\ \hline
2205 & 0.024 & \(\leq\)2.0 & \(\leq\)1.0 & 22–23 & 4.5–6.5 & 3.0–3.5 & 0.15–0.2 & \(\leq\)0.02 & \(\leq\)0.03 \\ ER20209 & 0.025 & 1.6 & 0.3 & 22.5 & 9.5 & 3.1 & 0.16 & 0.01 & 0.025 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Chemical compositions of 2205 and ER2209.  
Figure 1: Schematic diagram of the twin wire pulsed MIG welding under additional shielding gas.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Sample Number** & **Flowrate of Additional Shielding Gas** & **Weld Speed** \\  & **L/min** & **cm/min** \\ \hline
1 & 0 & 160 \\
2 & 12 & 160 \\
3 & 0 & 180 \\
4 & 12 & 180 \\
5 & 0 & 200 \\
6 & 12 & 200 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Main processing parameters of weld process.  
In this paper, flat butt welding is used, and the gap between two plates is 1 mm. The draft of plate and all dimensions are shown in Figure 2a. The additional shielding gas is argon (Ar) with a purity of 99.99% and a flow rate of 12 L/min. Before the welding test, the surface of the sample is first grounded with sandpaper to remove the oxide deposits, and the surface of the sample is wiped clean with alcohol. The welding starts after the surface is dried. The self-developed wavelet analyzer is used to acquire voltage and current signals during the welding process. The waveforms of current and voltage are statistically analyzed. After welding, a spark discharge wire cutter is used to remove 20 mm from both ends of the weld, and samples for the tensile test, metallographic, and impact samples are cut from the weld as vertical sections. The dimensions of the test samples are shown in Figure 2b,c. The metallographic test specimens are prepared using standard metallographic procedures. The microstructure of the welded joint is observed and analyzed using an Olympus optical microscope (OM) (Olympus, Shibuya, Japan) and scanning electron microscopy (SEM) (Hitachi, Tokyo, Japan). The Rockwell hardness test is performed on the polished and etched specimens by the Rockwell hardness test machine (Buehler, Lake Bluff, IL, USA). For each test point, the test time is 15 s, with a load of 150 kg and a step size of 1 mm. The tensile test is conducted on the test machine (INSTRON, Boston, MA, USA) at room temperature. Tensile test specimens are produced following ASTM E8 at the crosshead speed 3 mm/min, and 3 specimens are tested for each welded joint. In order to evaluate the impact toughness of the welded joint, the Charpy impact test (with a test specimen thickness of 3 mm) is carried out, and the Chary impact test samples are prepared in accordance with ISO 5173:2000. After the tensile test and the impact test, the fracture morphology of the welded joint is observed by SEM.