## 4 Discussion  
The narrow gap welding oscillating arc increases the welding path and changes the energy of the welding line. As shown in Figure 3b, the welding wire moves back and forth in a uniform linear motion along the \(x\)-axis at the speed of \(v_{x}\), and the dwell time when the welding wire moves to the side wall is \(t_{dwell}\). At the same time, the welding vehicle drives the welding torch uniformly in a straight line along the positive direction of the \(y\)-axis at a welding speed of \(v_{y}\). From this, it can be observed that:  
1. When the welding wire moves in the middle of the groove, the movement speed \(v_{1}\) of the welding wire is: \[v_{1}=\sqrt{{v_{x}}^{2}+{v_{y}}^{2}}\] (2) The energy of the welding line \(E_{1}\) is: \[E_{1}=\frac{UI}{v_{1}}\] (3)
2. When the welding wire dwells on the sidewall, the movement speed \(v_{2}\) of the welding wire is: \[v_{2}=v_{y}\] (4) At this moment in time, the energy of the welding line \(E_{2}\) is: \[E_{2}=\frac{UI}{v_{2}}\] (5)  
In the overhead welding position, the speed of the welding wire in the x-axis is \(v_{x}=1413\) mm\(/\)min, the welding speed in the y-axis is \(v_{y}=353\) mm\(/\)min, the voltage is \(U=21.4\) V, and the average current is \(I=162\) A. Then, \(\frac{E_{2}}{E_{1}}\approx 4\), that is, the linear energy E\({}_{2}\), when the arc dwells on the sidewall, is four times greater than the linear energy E\({}_{1}\) when the arc oscillates in the middle of the groove. In order to achieve a greater sidewall penetration and avoid a lack of sidewall fusion, it is necessary to distribute sufficient linear energy when the arc dwells on the sidewall. From Equation (3), it can be observed that E\({}_{1}\) has no relationship with the oscillation width W. However, when W is too small, the welding wire cannot oscillate to the side wall and dwells at the groove. Then, the linear energy of the arc is E\({}_{2}\), meaning that E\({}_{2}>\) E\({}_{1}\). Because the arc energy at the place where the welding wire dwells increases sharply, the height of the weld seam bumping in the middle becomes larger. When W exceeds a certain level, the welding wire is sufficient to oscillate to the side wall, but in this case, the increase in the oscillation width has no effect on the line energy in the middle of the groove, so W has almost no effect on the bump height in the middle of the weld. According to Equations (3) and (4), E\({}_{1}\), E\({}_{2}\), and the sidewall dwell times t\({}_{\text{dwell}}\) are irrelevant, so t\({}_{\text{dwell}}\) has almost no effect on the bump height in the middle of the weld. As E\({}_{1}\) is smaller when the arc oscillates in the middle of the groove, the bump height in the middle of the weld is smaller. E\({}_{2}\) is greater when the arc dwells on the side wall, and the penetration of the weld side wall is greater. E\({}_{1}\) is related to v\({}_{\text{x}}\) and v\({}_{\text{y}}\). When v\({}_{\text{x}}\) and v\({}_{\text{y}}\) increases, E\({}_{1}\) decreases. E\({}_{2}\) is related with v\({}_{\text{y}}\). When v\({}_{\text{y}}\) decreases, E\({}_{2}\) increases. Therefore, in order to obtain an oscillation arc NG-GMAW overhead weld with a small bump height in the middle and sufficient sidewall penetration, welding parameters with a relatively high v\({}_{\text{x}}\) and a relatively low v\({}_{\text{y}}\) should be used.