# Parameter Optimization of Orbital TIG Welding on Stainless Steel Pipe
## 3 Results and Discussion
### Tension Test Results  
The tensile test results are presented in Table 6. The UTS values ranged from 72 MPa to 562 MPa. The lowest UTS value was 72 MPa, for the S1 sample. This low value is the \"failed\" result based on visual assessment according to the AWS D18.1 standard, as shown in Table 5. On the contrary, sample S6 reached the highest UTS value of 562 MPa, much higher than the other samples. The relationships between HI and D\({}_{\rm w}\) on tensile strength are illustrated in Figure 4. The average UTS values were 137 MPa, 251 MPa, and 334 MPa, corresponding to HI values of 0.26 kJ/mm, 0.29 kJ/mm, and 0.32 kJ/mm. The reason for this phenomenon is that a higher HI leads to a better melting rate of the base metal. Therefore, the penetration depth is greater. This once again demonstrates that HI plays a crucial role in increasing the depth of penetration of the weld. Greater penetration depth leads to a better UTS value of the weld joints [26]. Moreover, the average UTS of welds \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Samples**} & \multicolumn{4}{c}{**Critical Positions**} \\ \cline{2-5}  & **(A)** & **(B)** & **(C)** & **(D)** \\  & **Max. Misalignment** & **Max. Concavity** & **Max. Convexity** & **Min. Face Width** \\ \hline S1 & P & F & P & P \\ \hline S2 & P & F & F & P \\ \hline S3 & P & P & P & P \\ \hline S4 & P & P & P & P \\ \hline S5 & P & P & P & P \\ \hline S6 & P & P & P & P \\ \hline S7 & P & P & P & P \\ \hline S8 & P & F & P & P \\ \hline S9 & P & P & P & P \\ \hline \hline \end{tabular} Note: F. failed; P. passed.  
\end{table} 
Table 5: Visual assessment according to AWS D18.1 standard. using D\({}_{\rm w}\) of 0.8 mm, 1 mm, and 1.2 mm were 237 MPa, 321 MPa, and 162 MPa, respectively. Therefore, high UTS values are usually found in samples with HI of 0.29 kJ/mm and 0.32 kJ/mm and D\({}_{\rm w}\) of 0.8 mm and 1 mm. Specifically, the highest UTS value at HI of 0.29 kJ/mm is 385 MPa with D\({}_{\rm w}\) of 0.8 mm, and 562 MPa is the highest UTS value at HI of 0.32 kJ/mm with D\({}_{\rm w}\) of 1 mm. The results of this study also indicate that D\({}_{\rm w}\) of 1.2 mm seems unsuitable for the HI values and base material thickness surveyed. Increasing the filler diameter can significantly impact the final weld quality. For example, the larger filler diameter could lead to a higher deposition rate, which means more materials are deposited. Moreover, a larger filler diameter can increase the HI rate that transfers to the base metal. Filler diameter also can impact the penetration depth and weld bead geometry, where larger filler diameters might provide deeper penetration and a wider bead. With a suitable HI of 0.32 kJ/mm, the tensile strength reached its highest value of 562 MPa when using a filler diameter of 1.0 mm, indicating that 1.0 mm is a suitable diameter selection. However, the UTS value needs further analysis via the Taguchi method. In addition, other factors, such as wire feed speed, also impact the welding strength. Figure 5 shows the relationship between HI and V\({}_{\rm f}\) for tensile strength. From 0.26 kJ/mm to 0.29 kJ/mm, increases in the wire feed speed led to an improvement in the UTS value of the weld. However, at 0.32 kJ/mm, only the 5.6 mm/s case showed a trend of improvement. The other wire feeding speed declined. In other words, the HI rate and wire feeding speed strongly impact the tensile strength of orbital 304 steel pipe [25; 26; 27]. The results will be further investigated via the Taguchi method. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Sample** & **S1** & **S2** & **S3** & **S4** & **S5** & **S6** & **S7** & **S8** & **S9** \\ \hline UTS (MPa) & 72 & 385 & 255 & 206 & 196 & 562 & 132 & 172 & 184 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Average tensile strength results. Figure 4: The relationship between HI and D\({}_{\rm w}\) for ultimate tensile strength. Figure 6 demonstrates the relationship between \(\rm D_{w}\) and \(\rm V_{f}\) for tensile strength.