## 1 Introduction  
Long distance pipelines are the main means of long-distance oil and gas transportation. With the rapid development of the energy industry, the construction of long-distance pipeline projects has developed rapidly. Welding is the main process technology for long-distance pipeline construction. In the welding process of these large thick-walled parts, due to their large volume, complex structure, and the difficult displacement of the pipeline, the all-position welding process emerged as required. Narrow-gap gas metal arc welding (NG-GMAW) has the advantages [1] of high efficiency, low heat input, and use of less filler metal. It has been applied to the connection of thick-walled components in the fields of oil and gas pipelines, nuclear power, and ships. During the production process of the all-position NG-GMAW welding of actual pipelines, 80% of welding defects were found at the overhead welding position, seriously affecting the safety of pipeline use. At the overhead welding position, the flow and spreading area of the molten pool metal is reduced by the action of gravity, and the molten metal is more prone to flowing downward, even dripping, which can easily cause welding defects [2] such as lack of fusion of the weld sidewall and surface protrusion, which seriously affect the molding quality of the welded joint. In order to avoid surface protrusion and insufficient penetration of sidewalls in narrow gap welding and obtain high-quality welded joints, domestic and foreign scholars have conducted a series of studies on narrow gap welding. [PERSON] et al. [3] studied the narrow-gap GMAW with a swing arc system in the horizontal position. They found that the swing arc can effectively control the formation of a horizontal welding seam, and increasing the swing frequency and amplitude can effectively suppress the outer convex weld formation,which has a certain inhibitory effect on the flow of the molten pool. [PERSON] and [PERSON] et al. [4; 5] used a self-made oscillating arc narrow-gap welding torch to study in depth the influence of the oscillation parameters [6] of the GMAW weld formation process for narrow gap vertical welding on the weld formation and the center convex height, and also studied the influence of the parameters of the weld formation process on the droplet transfer [7] of the oscillating arc narrow gap GMAW. The basic principle of the oscillating arc is the use of methods such as the oscillation of the entire welding torch [8], the bending of the welding wire [9], and of the contact tip [10], and alternating magnetic fields [11] to oscillate the arc back and forth on both sides of the groove to ensure sidewall fusion. The narrow-gap welding torch with a high-speed rotation arc was developed first by Japanese scholars [12; 13]. [PERSON] et al. [14; 15] improved the rotating arc narrow gap MAG welding system, and based on this, focused on studying the influence of welding parameters and oscillation parameters on the weld formation at the flat position of narrow-gap welds. [PERSON] and [PERSON] et al. [16; 17] optimized the narrow-gap welding torch with a rotating arc. [PERSON] et al. [18; 19; 20] designed a narrow-gap gas shielded three-wired indirect arc welding torch, greatly improving the welding efficiency. After the narrow-gap three-wired indirect arc, a tungsten electrode arc was used, making the convex shape of the weld surface change into a concave shape. The method of electromagnetic controlled pool welding (ECMP) was proposed by the University of Ryukyu in Japan [21] to obtain an ideal joint in the flat position; [PERSON] [22], [PERSON] [23], [PERSON] [24] and others [25] have studied the process method of laser arc hybrid welding in the flat position. For narrow-gap welding, research on specific methods, such as optimizing the parameters of the welding process, oscillating arc, rotating arc, multi wire welding, and composite welding [26; 27] has greatly optimized weld formation and improved welding efficiency.