## 2 Materials and Methods
### Effect of the Oscillation Parameters on the Formation of the Weld  
The arc oscillation parameters of the NG-GMAW overhead welding position mainly include the oscillation width, oscillation frequency, and dwell time on the sidewall. These three relationships are as follows:  
\[\frac{1}{f}=2\bigg{(}\frac{W}{v_{x}}+t_{dwell}\bigg{)} \tag{1}\]  
where \(f\) is the oscillation frequency, \(W\) is the oscillation width, \(t_{dwell}\) is the dwell time on the sidewall (abbreviated as dwell time), and \(v_{x}\) is the oscillation speed of the motor  
Figure 4: Weld formation of NG-GMAW overhead welding: (**a**) Forces acting on the molten pool. (**b**) High-speed camera image of the arc on the right sidewall. (**c**) High-speed camera image of the arc on the left sidewall. (**d**) Weld section morphology. perpendicularly to the welding direction, which remains unchanged. If the oscillation width and sidewall dwell time increase simultaneously, the oscillation frequency decreases, If the oscillation width and sidewall dwell time decrease simultaneously, the oscillation frequency increases. Therefore, this paper only discusses the effects of oscillation width and sidewall dwell time on weld formation. In subsequent experiments, the welding parameters were as follows: wire feeding speed of \(v_{w}\) = 8.7 m/min; welding speed of \(v_{y}\) = 353 mm/min; oscillation speed of the motor perpendicularly to the welding direction of \(v_{x}\) = 1413 mm/min; \(t_{well}\) = 70 ms; average current of \(I\) = 162 A; and voltage of \(U\) = 21.4 V.  
Figure 5 shows the morphology of the weld cross-section obtained at different oscillation width for the NG-GMAW overhead welding position, and Figure 6 shows the effect of the oscillation width for the NG-GMAW overhead welding position on the size of the weld cross-section. When the oscillation width is 0 mm, that is when the arc does not oscillate, the weld penetration is large and finger-shaped, with an apparent bump height in the middle of the weld, a small penetration width, and almost no sidewall penetration. As the oscillation width of the welding arc gradually increases, the welding penetration gradually decreases, and the finger-shaped penetration gradually disappears. At the same time, the welding penetration continuously increases, while the bump height in the middle of the weld first decreases and then slightly increases in the increase of the oscillation width, and the side wall penetration slightly increases. It can be observed that the oscillation type of the welding arc has a certain effect on reducing the bump height in the middle of the overhead welding seam, but it is not as good as a larger oscillation width, because narrow gap welding is affected by both sides of the wall. When the oscillation width is too large, the distance from the welding wire tip to the side wall is smaller than the distance from the welding wire tip to the bottom of the groove. According to the minimum voltage principle [35], the arc is prone to arcing on the side wall, causing undercut defects. Therefore, to obtain an overhead weld with a small bump height in the middle, the maximum oscillation width should be used to avoid the phenomenon of the arc \"jumping over the wall\". Therefore, the oscillation width of 2.66 mm was more appropriate in this experiment. Figure 5: Weld cross-sections with different oscillation widths: (**a**) 0 mm; (**b**) 1.66 mm; (**c**) 2.66 mm; (**d**) 3.66 mm; and (**e**) 4.66 mm. Figure 7 shows the effect of the sidewall dwell time of the NG-GMAW overhead welding position on the cross-sectional size of the weld. The sidewall dwell times were 0 ms, 35 ms, 70 ms, 105 ms, and 140 ms. It can be observed that with the increase in the sidewall dwell time, the sidewall penetration increases to a certain extent and remains unchanged, the weld fusion width increases, the degree of the bump height in the middle of the weld remains basically unchanged, and the weld penetration continues to decrease.