## 5 Conclusions  
In the present study, a statistical model was proposed for all-position NG-GMAW with a vertical oscillation arc. The performed analyses revealed that the proposed model could  
Figure 12: Transection macrographs of a single-layer with optimized parameters. (**a**) Flat position (0\({}^{\circ}\)). (**b**) Vertical position (90\({}^{\circ}\)). (**c**) Overhead position (180\({}^{\circ}\)).  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \(W_{f}\) & \(V\) & \(W_{s}\) & \(O_{W}\) & \(\alpha\) \\ (mm/min) & (**v**) & (mm/min) & (**mm**) & (**\({}^{\circ}\)**) \\ \hline
10.4 & 24.0 & 424 & 2.0 & 0 \\
10.2 & 23.7 & 448 & 2.1 & 90 \\
8.7 & 21.4 & 353 & 2.6 & 180 \\ \hline \hline \end{tabular}
\end{table}
Table 9: Optimal welding parameters.  
Figure 13: Transection macrograph of multi-layer with optimized parameters. (**a**) Flat position (0\({}^{\circ}\)). (**b**) Vertical position (90\({}^{\circ}\)). (**c**) Overhead position (180\({}^{\circ}\)).  
be applied to optimize the transection morphology of weld beads. The main achievements of this article can be summarized as follows:  
1. A statistical model was established to find the correlation between the welding parameters and the weld-bead geometry with the CCD-based RMS. The ANOVA was applied to evaluate the significance and accuracy of the model. The obtained results indicate that the model can be applied to optimize the all-position narrow gap weld-bead geometry.
2. Welding voltage and oscillation width significantly positively affect the sidewall penetration, while the welding speed has the opposite effect. Therefore, it is necessary to reduce the welding speed and increase the oscillation width to ensure sidewall penetration after vertical welding. In addition, due to the influence of gravity, the groove surface gradually changes from concave to convex. The groove surface convexity can be suppressed by decreasing the welding speed and increasing the wire feeding speed. Moreover, the oscillation width and the wire feeding rate significantly affect the weld height. In order to ensure consistency of the weld height at all positions, the oscillation width should be appropriately reduced before vertical welding, while the oscillation width should be appropriately increased after vertical welding. Meanwhile, it is necessary to optimize the welding speed to ensure the penetration depth of the sidewall.
3. When the optimized parameters were used in automatic all-position NG-GMAW in pipeline welding, the macroscopic metallography of the weld bead in flat, vertical, and overhead positions shows that there is no incomplete fusion in sidewall and interlayers, demonstrating that the weld bead has deep sidewall penetration.  
Conceptualization, H.L. and J.Z.; methodology, R.X.; software, H.L.; validation, H.L., J.Z. and Y.B.; formal analysis, Y.B.; investigation, H.L. and R.X.; resources, H.L. and R.X.; data curation, Y.B.; writing--original draft preparation, H.L.; writing--review and editing, H.L. and Y.B.; visualization, H.L. supervision, J.Z. and Y.X.; project administration, R.X.; funding acquisition, J.Z., R.X. and Y.B. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the National Natural Science Foundation of Xinjiang, grant number 2022D01C391, the Science and Technology Innovations Project of the Outstanding Doctor of Xinjiang University grant number XJUBSCX-201906, and the Technology Innovation Team for Robots and Intelligent Equipment, grant number 2022D14002.  
Not applicable.  
We sincerely thank York Technology Co., Ltd. (Shenzhen, China) for providing the high-speed cameras and laser light source equipment for this study free of charge.  
The authors declare no conflict of interest.