# Parameter Optimization of Orbital TIG Welding on Stainless Steel Pipe
## 3 Results and Discussion
### Microstructure  
An overview of the microstructure of all samples is presented in Table 7. According to Figure 6 and Table 7, the samples with an HI of 0.26 kJ/mm are not fully penetrated. Therefore, this is also the cause of the low UTS of the samples in this group. In addition, for samples with an HI of 0.29 kJ/mm, samples S5 and S8 were not fully penetrated, but  
Figure 5: The relationship between HI and \(\rm V_{f}\) for tensile strength. Figure 6: The relationship between \(\rm D_{w}\) and \(\rm V_{f}\) for tensile strength. sample S2 was. The appearance of S2 was dark gray with lots of porosity on the surface, and the micrograph shows that the internal structure was also porous. The conclusion is that this sample was overheated. The reason is that the contact surface between the two pipes was not carefully cleaned, and the gas flow adjustment of the backing gas and gas entering the welding head was not carefully controlled, leading to more backing gas and reducing the amount of gas for the welding head. For samples with an HI of 0.32 kJ/mm, there was complete penetration in the S6 sample, and two samples, S3 and S9, were not fully penetrated. Observe the parameters of these three models and the maximum wire feed speed. According to [PERSON] et al. [31], the diameter of a molten metal bridge (d\({}_{8}\)) depends on the filler wire speed (V\({}_{\rm f}\)) and the metal transfer period (T) from the filler metal to the pool weld with the formula:  
\[\rm{d_{s}(mm)=(mm/s)\times T(s)} \tag{2}\]  
When V\({}_{\rm f}\) increases, the period and the metal supply will decrease. This phenomenon also makes the filler metals more inclined to penetrate the arc center before they are melted, which takes away the heat of the weld pool and prevents the arc from reaching the weld pool. In Figure 6, samples with a D\({}_{\rm w}\) of 1.2 mm are not fully penetrated, but possess an extensive breadth. The problem occurs when the models S7 and S9 with the largest V\({}_{\rm f}\) have a large width while the V\({}_{\rm f}\) of S8 is the smallest for a smaller width. The metal transfer period of S8 is the largest of the three samples such that the metal width will be more significant. The microscopic observation sampling location selection has the start or end of the metal transfer period, where the weld size is usually smaller. Possessing the largest D\({}_{\rm w}\) also increases the arc resistance and heat removal of the weld pool of these models. Samples with D\({}_{\rm w}\) of 0.8 mm and 1 mm have a smaller D\({}_{\rm w}\), but when combined with different V\({}_{\rm f}\) and HI levels, the quality of the weld is also affected. Only sample S6 combines maximum HI, minimum V\({}_{\rm f}\), and average D\({}_{\rm w}\), providing the best UTS. For more proof that S2 is overheating, the microstructure is further investigated in the following figure. Observing the microstructure of samples S7, S8, and S9, there was a significant misalignment between the weld root and the gap of the two pipe ends. The possible cause is the use of an unsuitable diameter of filler wire for the pipe thickness and HI that led to heat loss in the weld pool, which significantly reduces weld penetration. As a result, the UTS of these samples was below 200 MPa, lower than the UTS of sample S3 at 255 MPa, despite having nearly the same penetration depth, but the weld root in S3 was less misaligned compared to S7, S8, and S9. Similarly, the S1 weld also exhibited misalignment and low penetration, resulting in the lowest UTS value of 72 MPa. As for sample S4, due to misalignment during tack welding and the formation of a large gap, its penetration was also low, with a UTS of 206 MPa. These findings highlight the importance of selecting an appropriate filler wire for HI, as it directly impacts weld penetration, tensile strength, and the overall quality of the weld. Figure 7 shows optical micrographs of the microstructure of the weld zone and fusion zone of samples S2 and S6.