# Research Status and Progress of All-Position Narrow-Gap GMAW for Pipelines
## 3 Methods to Improve Weld Bead Formation in Narrow-Gap GMAW
### Changing Process Parameters to Control Melt Pool Flow to Improve Weld Forming  
Adjusting the angle of the welding torch or the workpiece can significantly influence arc force acting on the molten pool, thereby improving the weld bead formation. [PERSON] et al. [43] studied the effects of different torch tilting angles on the depth and width of the molten pool in vertical position metal active gas (MAG) welding of pipelines based on a dual-ellipsoid heat source model. Compared with the perpendicular and backward tilt torch postures, the forward-tilted torch resulted in a wider and shallower molten pool in welding (as shown in Figure 10). [PERSON] et al. [44] investigated the suppression of molten pool flow in laser-arc hybrid welding by altering the welding torch angle and utilizing the force of the shielding gas and the arc force acting on the molten pool. [PERSON] et al. [45] studied the effect of workpiece inclination angle on the morphology of the welding molten pool in pulsed MAG welding. During uphill welding, increasing the workpiece inclination angle leads to a longer molten pool but a decreased width and surface area, leading to an accumulation of more metal at the rear of the molten pool and the formation of a depression at the front. When the inclination angle exceeds 60\({}^{\circ}\), the weld seam essentially fails to form properly. In contrast, during downhill welding, as the workpiece inclination angle increases, the length of the molten pool decreases while the width increases. The molten metal flows forward due to gravity, resulting in a shape characteristic where the front of the molten pool is higher than the rear. However, when the inclination angle exceeds 75\({}^{\circ}\), the welding process becomes unstable, and the spatter increases. An appropriate welding torch oscillation amplitude can helps balance the surface tension and heat input of the molten pool, leading to good weld seam formation. [PERSON] et al. [46] employed the Taguchi method to investigate the influence of welding current, arc voltage, and welding speed on the side wall melt depth. They found that the primary cause of the formation of side wall unfused defects is the significant effect of temperature and oxides on the surface tension of the molten pool. Furthermore, they concluded that arc oscillation enhances heat transfer to the side walls, serving as an effective measure to eliminate side wall unfused defects. [PERSON] et al. [13] investigated the impact of different oscillation amplitudes on the molten metal in the molten pool during overhead welding. Excessive oscillation amplitude can lead to a reduction in surface tension, making  
Figure 10: Schematic diagram of torch angle in vertical welding position. the molten pool more prone to flow. Additionally, the arc pressure drives the molten metal in the molten pool back toward the center of the weld seam, and under the influence of gravity, a convex weld seam forms. On the other hand, an insufficient oscillation amplitude results in relatively higher surface tension, poor molten pool fluidity, and the formation of a deeper and narrower weld seam. [PERSON] et al. [33] used computational fluid dynamics simulation to investigate the effect of arc oscillation on the molten pool behavior during narrow-gap GMAW. The results showed that changes in the arc oscillation frequency and amplitude significantly altered the fluid flow patterns within the molten pool. A larger oscillation amplitude promoted the rise of gas bubbles within the molten pool, improving side wall fusion. Although adjusting the oscillation amplitude can help to better fill the weld seam and reduce defects, dynamically adjusting the oscillation pattern to adapt to the changing real-time welding environment remains an unresolved problem, especially under complex working conditions. The wire feeding speed also has an impact on the molten pool. In vertical and overhead welding positions, the molten pool is prone to flow due to gravity.