## 5 Conclusions  
This paper presented the formation characteristics of a weld and the effects of arc oscillation parameters, such as the oscillation width and sidewall dwell time, on the formation of a narrow-gap overhead welding seam. This research avoided welding defects,such as weld seam bump and lack of sidewall fusion, during the overhead welding of narrow-gap thick-walled X80 pipes, and improved the welding quality of thick-walled pipes. The conclusions are as follows: (1) In the overhead welding position, the effect of gravity on the weld pool is apparent. The oscillation arc NG-GMAW welding process in the overhead position has an obvious weld formation feature with a bump height in the middle, which can easily cause specific defects, such as a poor interlayer fusion and incomplete sidewall fusion. (2) With the increase in oscillation width, the weld fusion width increases, while the bump height in the middle of the weld decreases firstly and then increases. If the oscillation width is too large, undercut defects can easily occur. Increasing the dwell time increases the weld penetration width and has a relatively small impact on the bump height in the middle of the weld, but it can increase the sidewall penetration to a certain extent, further reducing the lack of sidewall fusion. (3) In the process of oscillation arc NG-GMAW welding, E\({}_{1}\) < E\({}_{2}\). E\({}_{2}\) affects the penetration of the sidewall, while E\({}_{1}\) mainly affects the height of the weld bump in the middle. 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 wire oscillation speed and a relatively low welding speed should be used. This study laid a theoretical foundation for the further optimization of the overhead welding process of NG-GMAW thick-walled pipelines and has important implications for the actual production.  
The purpose of this article was to research the formation of an oscillating arc NG-GMAW overhead welding seam. However, the correlation between the coupling effect of the arc, droplet, and molten pool and the formation of the weld seam was not studied. In order to fundamentally improve and better understand the process of oscillation arc NG-GMAW overhead welding, future work will aim to provide a multi-scale coupled model and user-defined functions combined with simulation software to evaluate the impact of arc-droplet-melt pool on the formation of an oscillating arc NG-GMAW overhead welding seam.  
Conceptualization, Y.B. and J.Z.; Methodology, R.X.; Software, H.L.; Validation, Y.B. and Y.X.; Formal analysis, Y.X.; Investigation, Y.B.; Resources, R.X.; Data curation, Y.B.; Writing--original draft preparation, Y.B.; Writing--review and editing, R.X.; Visualization, J.Z.; Supervision, Y.X.; Project administration, H.L.; Funding acquisition, J.Z. All authors have read and agreed to the published version of the manuscript.  
This research was funded by \"The Science and Technology Innovations Project of the Outstanding Doctor of Xinjiang University, grant number XUIBSCX-201906 \", \"Technology Innovation Team for Robots and Intelligent Equipment, grant number 2022D14002\" and \"General Fund Program of Xinjiang, grant number 2022D01C391\".  
Not applicable.  
We sincerely thank York Technology Co., Ltd. for providing the high-speed cameras and laser light source equipment for this study free of charge.  
The authors declare no conflict of interest.