# Research Status and Progress of All-Position Narrow-Gap GMAW for Pipelines
## 3 Methods to Improve Weld Bead Formation in Narrow-Gap GMAW
### Improvement of Weld Formation by Combined Laser and Arc Welding Methods  
Laser-arc hybrid welding is a welding technique that combines two heat sources: laser and arc. The high energy of the laser can rapidly heat the material, forming a deep and narrow molten pool, which significantly increases the welding speed [60]. The arc, on the other hand, serves as a compensatory element, enhancing the stability of the welding process. Additionally, during hybrid welding, the laser has the effect of attracting and stabilizing the arc [61]. When the arc and the laser are applied to the same molten pool, the welding process becomes more stable, effectively reducing issues such as side wall lack of fusion defects and thermal deformation of the weldment. Laser and arc welding can be simply divided into two methods. One of them is the laser-assisted arc welding process, which incorporates a low-power laser beam as an auxiliary heat source into traditional arc welding. This addition provides extra heating or preheating effects. However, the introduction of the low-power laser does not significantly impact the control of the weld penetration depth or changes in the welding speed [62]. Its application scope is limited. In contrast, arc-augmented laser welding uses high-power lasers as the primary heat source, complemented by an electric arc to broaden the molten pool width. This combination allows for achieving wide and deep weld beads at higher welding speeds, reducing the heat-affected zone and minimizing welding distortion [63; 64; 65]. The appropriate positioning of the laser beam relative to the arc (the light-wire spacing) can facilitate strong interactions and significantly increase the depth of penetration. When the laser beam irradiates the molten pool, the flow of the molten pool changes due to the influence of gravity, affecting the quality of the weld seam. A reasonable light-wire spacing can optimize the energy distribution of the laser on the molten pool [65], improving molten pool flow and reducing spatter. Research by [PERSON] et al. [66] shows that as the distance varies from 0 to 4 mm (as shown in Figure 14), the greater the distance, the stronger the preheating effect of the laser on the molten pool, which enlarges the area at the front end of the molten pool and makes the process of droplet incorporation into the molten pool smoother. Conversely, [PERSON] and [PERSON] et al. [67; 68] discovered that the laser has a compressing effect on the electric arc. The closer the laser's point of action is to the center of the arc, the stronger the interaction between the laser and the arc. This significantly compresses the arc, thus enhancing the penetration depth and stability of the molten pool. Most existing studies focus on the impact of a single variable (such as laser-wire distance, laser power, current, etc.) on the welding process while neglecting the interactions among multiple factors and their overall effect on the welding outcome. For instance, [PERSON] et al. [69] confirmed the weldability of hybrid laser-arc welding with a robot manipulator in the vertical-up position, but they did not fully consider the influence of gravity on the molten pool behavior under different welding pos  
Figure 14: Melt pool state at different laser-to-wire distances: (**a**) 0 mm; (**b**) 2 mm; (**c**) 4 mm. Reprinted with permission from Ref. [66]. Copyright 2023, Aeronautical Manufacturing Technology. lead to an incomplete understanding and optimization of welding processes across various conditions, where multiple factors interact simultaneously, affecting the final weld quality and efficiency. To address the effects of gravity on the molten pool behavior caused by changes in welding position, [PERSON] et al. [30] proposed laser-GMAW hybrid welding with a copper liner. This method supports the molten pool from the flat welding position to the vertical welding position, thereby preventing defects such as root sagging or burn-through at the root of the weld in this range.