## 3 Results  
### Formation of the Weld Seam  
It can be seen from Table 3 that at the same weld speed and weld current, the weld seam obtained with additional shielding gas has larger width and deeper penetration. With the increase in welding speed, both the penetration depth and weld width decrease. In this paper, D is weld depth, W is weld width, D/W is the ratio of weld depth and weld width. The lower D/W ratio than the weld formed without the additional shielding gas as shown in Figure 3a. It can be seen from Figure 3b,c that Sample 1 has obvious unfused defects and Sample 2 is completely penetrated. This comparison indicates that without changing the welding current and welding speed, adding only one branch for the additional shielding gas can effectively improve and diminish the unfused defects of the weld. The shape of the obtained welds varies greatly and the morphologies of welds formed with additional shielding gas are obviously superior to those without shielding gas. Similarly, Sample 3 has obvious unfused defects as shown in Figure 3d, and Sample 4 is completely penetrated in Figure 3e. In contrast to Figure 3b,d, the penetration depth and weld width decreases with the increase of welding speed. It can be seen from Figure 3f,g that Sample 6 has slightly larger width and deeper penetration than Sample 5, but both samples have unfused defects. As the welding speed increases, the heat transferred will  
Figure 2: Dimensions of test specimens (unit: mm): (**a**) Draft of plate and all dimensions, (**b**) Specim for tensile test, (**c**) Specim for Charpy impact test.  
decrease synchronously, and the heat input will decrease significantly if the effective power remains unchanged. As shown in Figure 3g, when the welding speed increases to 2 m/min, the welding heat input decreases, resulting in narrow weld width and insufficient penetration. Although additional gas can improve part of the weld formation and stir pool, it cannot completely solve the problem of unfused defects caused by insufficient heat input. It can be concluded from Figure 3h that the part of the weld bead without additional shielding gas is unevenly curved formation, and the weld is narrow and the penetration depth is insufficient, indicating poor formation of the weld bead. After the additional shielding gas is introduced, the surface of the weld seam is relatively smooth and uniform. The weld surface exhibits an obvious fish scale-like weld bead, with larger width and deeper penetration. This is attributed to the two actions of the additional shielding gas: (1) the impact force generated by the gas flow can significantly change the original natural solidification mode of the weld pool metal, resulting in the improvement of the weld formation and stirring of the molten pool; (2) the gas flow hood formed during the gas impact can effectively extend the shielding time for the surface of the weld seam and feed the heat back to the molten pool directionally.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**No.** & **Weld Shape** & **Section of Joints** \\ \hline \hline
1 & & \\
2 & & \\
3 & & \\
4 & & \\
5 & & \\
6 & & \\ \hline \hline \end{tabular}
\end{table}
Table 3: The weld shape and section of joints.  
Figure 3: _Cont._  
After welding, X-ray NDTs are employed to examine the defects in the weld. As the weld shapes are similar when the samples are welded at 160, 180 and 200 cm/min, we only carry out X-ray NDT at the weld speed of 160 cm/min. The results of X-ray NDT testing of Sample 1 and Sample 2 are shown in Figure 4. When the welding speed is 1.6 m/min, no obvious gas pores are found in the weld seams with and without additional shielding gas, indicating that both welds have good compactness.