## 2 Materials and Methods
### Method  
#### 2.2.1 Selecting Input and Response Parameters  
Figure 4 shows the welding position on the positioner. Based on the presented figure, 0\({}^{\circ}\), 90\({}^{\circ}\), and 180\({}^{\circ}\) denote the flat welding, vertical down welding, and overhead welding positions, respectively. The welding direction is downward around the pipeline.  
During welding from 0\({}^{\circ}\) to 180\({}^{\circ}\), gravity has an opposite force on the molten pool, and the weld bead shows a different morphology. Figure 5 shows the typical weld bead morphology at different positions. It is observed that at the flat position, the cross-section of the weld bead is semioval, and the top of the weld bead is slightly concave. Moreover, Figure 5 shows that the weld pool is uniformly solidified in the weld groove. At the vertical welding position, Figure 5 shows that the weld-bead morphology changes greatly; the concave degree at the top of the weld bead increases sharply, and the bottom morphology of the weld bead changes from elliptic to conical form. Compared to the center of the weld bead, the molten pool is mainly solidified on both sides of the bevel. At the overhead position, the gravity is opposite to the solidification of the molten pool, which results in the convex shape of the welding bead, as shown in Figure 5.  
Figure 4: Schematic of NG-GMAW in different positions.  
Figure 3: Overall process flowchart.  
However, the concave and convex morphology, as shown in Figure 5b,c, are prone to forming defective weld beads in subsequent welding. Therefore, many researchers optimize welding process parameters to obtain as flat a weld bead as possible. [PERSON] et al. [2] and [PERSON] et al. [7] showed that in all-position NG-GMAW, wire feeding speed (\(W_{f}\)), welding speed (\(W_{s}\)), and welding position (\(\alpha\)) have obvious influences on the welding bead formation. [PERSON] et al. [36] showed that the welding voltage (\(V\)) affects the arc shape, thereby affecting the welding pass formation. As the vertical oscillation arc was used in this study, the effect of the oscillation width (\(O_{W}\)) on the weld bead formation should be considered in the calculations; Accordingly, wire feeding speed (\(W_{f}\)), welding speed (\(W_{s}\)), welding voltage (\(V\)), welding position (\(\alpha\)), and oscillation width (\(O_{W}\)) were selected as the input parameters. The geometric dimensions of the weld bead, including the sidewall penetration \(S_{p}\), welding penetration \(D_{p}\), welding height \(W_{h}\), and the flatness degree of the weld bead surface \(C\) were introduced to the model as the response parameters. Measurement metrics of concave and convex weld beads are presented in Figure 6a,b, respectively. \(C\) is the flatness degree of the weld bead, and negative and positive values of \(C\) reflect concave and convex weld beads, respectively.  
Figure 5: Transection of typical weld-bead morphology at (**a**) Flat position (\(0^{\circ}\)), (**b**) Vertical position (\(90^{\circ}\)), and (**c**) Overhead position (\(180^{\circ}\)).  
Figure 6: Definition measurement metrics of (**a**) Concave weld bead and (**b**) Convex weld bead.