## 4 Discussion and Validation  
Studies show that incomplete fusion in the sidewall and interlay are very prone to form in NG-GMAW [2,7]. This is even more pronounced in vertical and overhead positions, where the same defect appears in the vertical oscillation arc NG-GMAW. As the welding position \(\alpha\) increases, the gravity force on the molten pool intensifies, so more molten filler metal flows to the rear of the weld pool and solidifies there. Subsequently, a convex shape forms in the middle of the weld bead. It is worth noting that the welding arc in the next layer cannot reach the lowest position of the convex weld, which results in incomplete fusion on the interlayer. Insufficient heat input on the sidewall of the groove will cause incomplete fusion on the sidewall. In this regard, the incomplete fusion in the sidewall and interlayer in NG-GMAW are shown in Figure 11a,b, respectively. To ensure a qualified NG-GMAW weld bead, the flatness degree of the weld-bead surface and the depth of the sidewall penetration should be controlled precisely. To this end, the flatness degree of the weld bead and the penetration depth of the sidewall were taken as the target parameters to be optimized. The flatness degree of the weld bead was set to zero, which means the desired weld bead surface is flat, and the suitable bead geometry is more conducive to the control of the layer height so that the same layer of the weld in different welding positions can obtain the same layer height, which is meaningful for the later all-position automatic welding. It should be indicated that the deeper the sidewall penetration, the better the achieved results, which is necessary to ensure obtaining a high-quality weld at an arbitrary position. In the present study, the flat position (0\({}^{\circ}\)), vertical position (90\({}^{\circ}\)), and overhead position (180\({}^{\circ}\)) are selected as the optimized target positions. The goal of each input factor and the importance of the input factor for each response are listed in Table 8.  
Figure 10: Effect of (**a**) Wire feed rate, (**b**) Welding speed, and (**c**) Welding position on the flatness degree. In step with the numerical optimization of optimization choices in software, after setting the optimization goal and the welding angle \(\alpha\), a list of the recommended welding parameter generated in the solution. When \(\alpha=0^{\circ}\), there are 24 sets of welding parameters in the solution, and the suggested selection are a wire feeding speed of 9.24 m/min, welding voltage of 22.12 V, welding speed of 362.2 mm/min, and oscillating width of 1.7 mm. However, in order to improve welding efficiency, the maximum wire feeding speed should be considered. The welding speed of group 4 and group 11 was 350 mm/min and 450 mm/min, respectively, and the oscillating width was 1.5 mm. In order to ensure better welding efficiency, a welding speed of 450 mm/min was selected for subsequent actual tests. In addition, in order to improve the penetration depth of the sidewall, an attempt was made to increase the oscillation width to 2.0 mm. The results show that the welding process is stable, and there is no fusion defect. Therefore, the final determined welding parameters at the flat welding position are shown in Table 9 with \(\alpha=0\). In the vertical welding position, among the recommended parameters, considering the highest welding efficiency and the largest sidewall penetration, the 38 th group of data is selected, which includes the 10.3 m/min for wire feeding speed, 24 v for welding voltage, and 450 mm/min and 2.07 mm respectively for welding speed and oscillating width. After the experimental test, the final determined parameters for the vertical welding position are shown in Table 9 with \(\alpha=90\). In the overhead welding position, the recommended parameters are 8.7m/min for wire feeding speed, 24 V for welding voltage, 350 mm/min for welding speed, and 2.6 mm for oscillating width.