# Research Status and Progress of All-Position Narrow-Gap GMAW for Pipelines
## 2 Current Status of Research on Narrow-Gap All-Position GMAW
### Arc Behavior in Narrow-Gap GMAW  
Welding arc is an electrically neutral, high-temperature plasma whose shape and stability have a crucial impact on weld seam formation and droplet transfer. The constraint of the narrow-gap groove on the arc differs significantly from that of V-groove and T-groove, thus exhibiting many unique characteristics. Studying the special arc features during the narrow-gap welding process and uncovering the mechanisms behind their formation is of great significance for improving the quality of narrow-gap welding and overcoming welding defects [20; 21].  
[PERSON] et al. [22] discovered the phenomenon of arc \"jumping\" during the narrow-gap P-GMAW process and developed a weld seam tracking arc sensor based on this observation. They found that the arc interruption phenomenon occurring during the narrow-gap P-GMAW process follows a clear pattern, explaining the intrinsic mechanisms that cause arc interruption. The special constraints imposed by the narrow gap lead to changes in arc behavior and droplet transfer paths, which accumulate the necessary conditions for arc interruption. Their findings provide theoretical guidance for avoiding arc interruptions, contributing to the improvement of welding quality and reliability in narrow-gap welding applications. Additionally, when performing GMAW in narrow-gap, the arc appearance is significantly compressed. Without measures to alter the arc shape, there is a higher likelihood of generating numerous welding defects such as lack of fusion, porosity, and slag inclusion. However, by employing the method of laser thermal stabilization of the arc, although it can improve arc stability to some extent, the limitation of laser power becomes a concern. As the welding speed increases, inadequate heat distribution from the laser beam results in poor spreading of the molten metal and insufficient wetting at the weld toe [23], affecting the weld quality.  
[PERSON] et al. [13] investigated the influence of different oscillation amplitudes on arc behavior during the overhead welding process of narrow-gap P-GMAW. The arc takes on a trumpet-like shape, and as the oscillation amplitude increases, the angle at which the arc deflects toward the sidewall gradually increases (as shown in Figure 6). This leads to greater heat input to the sidewall, which is beneficial for ensuring sidewall penetration; however, excessive oscillation amplitude causes the arc pressure to push the molten metal in the molten pool back toward the center of the groove, forming a convex weld bead under the influence of gravity. Regarding the issue of sidewall lack of fusion, many scholars have conducted analyses from the perspective of arc behavior. [PERSON] et al. [24] explained the process of arc climbing along the sidewall based on the self-regulating characteristics of the arc and the principle of minimum arc voltage. They found that the arc tends to climb onto the sidewall in narrow gaps, causing changes in the arc's electrical conductivity path, which leads to various droplet transfer modes and consequently affects weld seam quality. [PERSON] et al. [25] studied the lack of fusion defects in narrow-gap GMAW upward vertical welding with oscillating arcs, noting that sidewall lack of fusion typically occurs on the opposite sidewall after severe arc climbing. Within one oscillation cycle, if the arc climbs prematurely to the right sidewall, the temperature distribution and the state of the molten pool are affected, leading to a lack of fusion on the left sidewall.