## 2 Materials and Methods  
Based on previous research, a pipeline all-position oscillating arc NG-GMAW welding system has been independently built, as shown in Figure 1. The system adopted the structure of soft band + welding vehicle, carrying a welding torch with an oscillating mechanism and crawling on the soft band laid on the surface of the pipeline to achieve the all-position welding of the pipeline. The model of the welding power supply was Artsen_Plus 500 (MEGMEET, China). The wire feeder was a supporting equipment for the welding power supply. The welding torch is a water cooled welding torch. The water-cooled machine model was CT-20 (B). The remote controller and control box were independently designed and processed. The entire welding system used a motion control card as the control unit. The step angle of the stepping motor is 1.8\({}^{\circ}\), and the angular displacement accuracy of 4.5 \(\times\) 10\({}^{-4\circ}\) can be realized with 4000 subdivision of the stepping motor driver. The ball spline adopts THK LT16X imported from Japan, and the lead accuracy is 0.011 mm. Therefore, the theoretical control accuracy of the system can reach 0.01 mm, and the actual displacement accuracy of the system can achieve 0.5 mm by measuring with the laser rangefinder. Before welding, the motion control card receives the welding parameters issued by the process parameter management software of the upper computer through Ethernet communication. During the welding process, the operator can adjust the welding parameters in real time through a pendant, and the welding parametersare displayed and can be recorded through the man-machine interface and the upper computer for the subsequent analysis and optimization of the process parameters. During the welding process, the arc and droplet behavior were collected and observed using a Phantom VEO1310 high speed camera. The high-speed camera had a shooting frequency of 5000 frames/s with an exposure time of 1 us, and a resolution of 560 \(\times\) 504. The background light source was a diode high-frequency pulsed laser with a power of 400 W and a wavelength of 640 nm. The model was CAVILUX_ Smart. The welding base material was X80 steel. In the actual welding process, the outer diameter of the pipe was 1420 mm, and the wall thickness was 25.7 mm. The pipe groove was a double V groove, formed by splicing two pipes, with a pairing gap of 0 mm. It was processed by a hydraulic beveling machine. After the root-layer welding of the pipeline was completed by the internal welding machine, it was used for the subsequent welding experiments, as shown in Figure 2a. This article focuses on the influence of oscillation parameters on the weld formation of the filler layer f3 in the overhead welding position of narrow gap welding. Therefore, the groove form during the test was simplified as shown in Figure 2b, and the size of the test piece was 300 mm \(\times\) 160 mm \(\times\) 25.7 mm. The welding wire was Lincoln 80 Ni1 with a diameter of 1.0 mm. The chemical compositions of the base metal and welding wire are shown in Table 1. The GMAW welding method was adopted, with a shielding gas of 80% argon + 20% carbon dioxide and a gas flow rate of 30 L/min. The arc oscillation mode was straight, as shown in Figure 3a. The ball spline converts the rotary motion of the stepper motor into the horizontal reciprocating motion of the welding torch, causing the welding torch to move at \(v_{x}\) speed in the \(x\)-axis direction. The welding torch was directly connected to the ball spline, and the welding torch and control mechanism were all installed on the welding vehicle. In the actual welding process, the welding vehicle moves on the soft band with the welding torch oscillating mechanism, causing the welding torch to move at \(v_{y}\) speed in the \(y\)-axis direction. The final trapezoidal welding trajectory was formed, as shown in Figure 3b.