# Parameter Optimization of Orbital TIG Welding on Stainless Steel Pipe
## 2 Experimental Material and Method
### Experimental Method  
The tubes were cut into pieces with a length of 100 mm and tacked together with a total length of 200 mm. The type of welding head equipment was open arc, and the wire feed type was continuous filler wire. The type of metal transfer was controlled under touching transfer. The electrode was a thorium electrode of type EWTh-2 with a diameter of 2.4 mm and a tip electrode angle of 45\({}^{\circ}\). The shielding gas was argon with a gas flow rate of 12 LPM (liter per minute).  
After some initial tests to survey the welding process, the Taguchi method was used to design parameters with an L9 matrix. Three factors--filler wire diameter (D\({}_{w}\)), current (I), and filler wire speed (V\({}_{t}\))--were examined at three levels, as presented in Table 2.  
The heat input value is calculated according to Formula (1) [21]:  
\[\mathrm{HI}=\eta\times\frac{\mathrm{U}(V)\times\mathrm{I}(\mathrm{A})}{\mathrm{ V_{s}(mm/s)}} \tag{1}\]  
where HI is heat input (J/mm) and \(\eta\) is the weld thermal efficiency, which is often 0.6 for the GTAW method, U is arc voltage (V), V\({}_{s}\) is travel speed (mm/s), and I is current (A).  
Other welding parameters were fixed, i.e., a voltage of 20 V, an arc length of 2 mm, a travel speed of 5.5 mm/s, a cup diameter of 9 mm, and shielding gas argon with a flow rate of 12 LPM. The Taguchi design table and HI values are shown in Table 3.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline
**Grade** & **C** & **Mn** & **Si** & **P** & **S** & **Cr** & **Ni** & **N** \\ \hline SUS 304 & \(\leq\)0.07 & \(\leq\)2.00 & \(\leq\)0.75 & \(\leq\)0.045 & \(\leq\)0.030 & 17.5–19.5 & 8.0–10.5 & \(\leq\)0.10 \\ \hline \end{tabular}
\end{table}
Table 1: Nominal composition of SUS304 austenitic stainless steel [20].  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Levels**} & \multicolumn{3}{c}{**Factors**} \\ \cline{2-5}  & \multicolumn{1}{c}{**D\({}_{w}\) (mm)**} & **I (A)** & **V\({}_{t}\) (mm/s)** \\ \hline Low & 1 & 0.8 & 90 & 3.8 \\ \hline Medium & 2 & 1.0 & 100 & 4.5 \\ \hline High & 3 & 1.2 & 110 & 5.6 \\ \hline \end{tabular}
\end{table}
Table 2: Levels and factors of experimental parameters.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **D\({}_{w}\) (mm)** & **I (A)** & **V\({}_{t}\) (mm/s)** & **HI (k/mm)** \\ \hline S1 & 0.8 & 90 & 3.8 & 0.26 \\ \hline S2 & 0.8 & 100 & 4.5 & 0.29 \\ \hline S3 & 0.8 & 110 & 5.6 & 0.32 \\ \hline S4 & 1.0 & 90 & 4.5 & 0.26 \\ \hline S5 & 1.0 & 100 & 5.6 & 0.29 \\ \hline S6 & 1.0 & 110 & 3.8 & 0.32 \\ \hline S7 & 1.2 & 90 & 5.6 & 0.26 \\ \hline S8 & 1.2 & 100 & 3.8 & 0.29 \\ \hline S9 & 1.2 & 110 & 4.5 & 0.32 \\ \hline \end{tabular}
\end{table}
Table 3: Experimental parameters according to Taguchi design and the calculated HI values.  
After welding, the samples were tested by visual assessment according to AWS D18.1 [22]. The welding samples were also assessed using a tensile test according to ISO 6892-1 [23]. The tensile test shape is presented in Figure 2. The test position was selected following the ASME IX standard [24]. Finally, the microstructures of the samples were also observed after cutting, molding, grinding, polishing, and etching, as shown in Figure 3.