## 3 Results
### Results of Metallographic Tests  
There are 6 samples selected to analyze the fusion zone, heat-affected zone, and base metal of the weld seam. The microstructure of the heat-affected zone is observed under a magnification of 50 times. The Figure 5a-f is corresponded to the Sample 1-6 respectively. The matrix is ferrite, while the strip and block structures are austenite. The microstructure of the heat-affected zone has irregular strip and block-like features, and the characteristics of two-phase are interpenetrated distribution. In Sample 1, a heat-affected zone of 200-1600 \(\upmu\)m width is shown in Figure 5a, while the heat-affected zone with 150-1000 \(\upmu\)m width of Sample 2 as shown in Figure 5b is narrower than that of the heat-affected zone in Sample 1. In Sample 3, a heat-affected zone of 200-1200 \(\upmu\)m width is shown in Figure 5c, while the heat-affected zone with 300-800 \(\upmu\)m width of Sample 4 as shown in Figure 5d is narrower than that of the heat-affected zone in Sample 3. In Sample 5, a heat-affected zone of 500-600 \(\upmu\)m width is shown in Figure 5e, while the heat-affected zone with 150-400 \(\upmu\)m width of Sample 6 as shown in Figure 5f is narrower than that of the heat-affected zone in Sample 5. This is because the fluidity of the molten pool metal in the presence of additional shielding gas is better than that without additional gas due to the action of external gas flow. The better fluidity provides more driving force for the diffusion of  
Figure 4: X-ray NDT photograph: (**a**) X-ray NDT for weld seam in Sample 1, (**b**) X-ray NDT for weld seam in Sample 2. Figure 3: Morphology of the weld seam: (**a**) Penetration depth and width/depth, (**b**) The cross-section of joint of Sample 1, (**c**) The cross-section of joint of Sample 2, (**d**) The cross-section of joint of Sample 3, (**e**) The cross-section of joint of Sample 4, (**f**) The cross-section of joint of Sample 5, (**g**) The cross-section of joint of Sample 6, (**h**) the comparison before and after the introduction of additional shielding gas. the metal and a shorter residence time in the liquid state, and so the extent of its heat-affected zone is smaller than that of the weld seam without additional shielding gas. The microstructure of the fusion zone is observed under a microscope at a magnification of 500 times. The fusion zone exhibits an as-cast dendritic crystal structure, which is mainly austenite. It can be seen that a large number of austenite segments precipitate with the feather and dendritic shapes distributed in the grain boundaries and crystals of ferrite and form a relatively obvious boundary with the base metal 2205 as shown in Figure 5g. The coarse and large Widmanstatten structure appears in the fusion area, indicating that the hardness and toughness of the metal become poorer while the brittleness of the weld pool metal increases. The grain size in the weld fusion zone in Sample 1 is obviously coarser and larger than that of the Sample 2, as shown in Figure 5g,h. The grain size in the weld fusion zone in Sample 3 is obviously coarser and larger than that of the Sample 4, as shown in Figure 5i,j. The grain size in the weld fusion zone in Sample 5 is obviously coarser and larger than that of the Sample 6, as shown in Figure 5k,l. This is because there is no additional shielding gas, the metal has poorer fluidity and the liquid metal has a longer residence time. As a result, the austenite in the fusion zone continues to increase, which increases the brittleness of the weld pool metal, thereby indicating that the hardness and toughness obtained in Sample 1 are less favorable. The Rockwell hardness test results of Samples 1-6 are shown in Figure 6. It can be observed that the hardness of the heat affected zone (HAZ) metal is higher than that of base metal (BM) and weld metal (WM), and the hardness of weld metal is higher than that of base metal.