# Research Status and Progress of All-Position Narrow-Gap GMAW for Pipelines
## 2 Current Status of Research on Narrow-Gap All-Position GMAW
### Molten Pool Behavior in Narrow-Gap GMAW  
During the narrow-gap all-position GMAW process, the melt pool is susceptible to flow phenomena under the influence of gravity. Sincebehavior of the molten pool is closely related to weld seam formation, understanding the patterns and causes of molten pool behavior in all-position GMAW is crucial for controlling the formation of the weld seam. Gravity is a critical factor influencing the formation of all-position welds. [PERSON] et al. [26] used the P-GMAW process to weld in flat, vertical-down, and overhead positions, analyzing the behavior of the molten pool under the influence of gravity in different positions. As shown in Figure 7, during vertical-down welding, the molten pool flows downward due to gravity, accumulating at the center of the arc, which shortens the distance between the wire tip and the molten pool. When the droplet contacts the molten pool surface, short-circuiting transitions are likely to occur, leading to arc instability and spatter. In the overhead welding position, the combination of gravity and reverse flow causes the tail of the molten pool to experience strong reverse flow, leading to more molten metal accumulating at the tail of the molten pool (as shown in Figure 8). This increases the weld height and results in a weld shape with a central bulge. To mitigate the issue of molten pool flow, [PERSON] et al. [27; 28] studied the behavior of the molten pool in the overhead welding position under the influence of gravity, based on all-position tandem narrow-gap GMAW. Most molten metal flows backward due to gravity, resulting in less molten metal beneath the arc. By adjusting the wire feed speed, they achieved high-quality welds suitable for the overhead welding position. Compared to laser welding, laser-arc hybrid welding offers higher robustness against welding defects but also results in greater weld penetration and wider weld beads [29]. [PERSON] et al. [30] introduced a copper backing in laser-GMAW hybrid welding, which can accelerate the cooling and solidification process of the molten pool, particularly in vertical-up and overhead welding positions, thereby reducing the risk of molten pool downward flow. [PERSON] et al. [31] revealed the effect of the current waveform on the solidification process of the molten pool during P-GMAW welding and investigated the dynamic behavioral characteristics of the molten pool (shown in Figure 9). The front of the molten pool is compressed, forcing the molten metal to flow towards the rear of the molten pool due to the obstruction at the bottom. Upon reaching the boundary at the rear, the molten metal is reflected. Meanwhile, the metal flow on the front surface of the molten pool is driven backward under the combined effects of impact force and surface tension, converging with the other stream of molten metal at the rear of the molten pool, leading to the accumulation  
Figure 6: Arc characteristics for different oscillation widths: (**a**) 0 mm; (**b**) 2.6 mm; (**c**) 4.6 mm. Reprinted with permission from Ref. [13]. Copyright 2023, Authors. of molten metal. [PERSON] et al. [32] utilized high-speed photography and tracer particle technology to anlyze the characteristics of molten metal recirculation under different welding speeds and magnetic field excitation currents. Based on their findings, they proposed methods to suppress the formation of raised weld humping. [PERSON] and colleagues [33] also investigated the effect of oscillating arcs on the behavior of the molten pool. An oscillating arc brings more heat to the side walls, causing the molten pool to oscillate with a large amplitude. This facilitates the rise of bubbles and helps reduce the formation of incomplete fusion defects. Numerical computation techniques have played a significant role in predicting molten pool flow and weld seam formation. [PERSON] et al.