# Research Status and Progress of All-Position Narrow-Gap GMAW for Pipelines
## 2 Current Status of Research on Narrow-Gap All-Position GMAW
### Droplet Transition Behavior in Narrow-Gap GMAW  
The migration process droplets from the tip of the welding wire to the molten pool is referred to as droplet transfer. The droplet transfer behavior plays a crucial role in determining spatter, arc stability, and molten pool behavior, which directly impacts the stability of the welding process and the quality of the weld seam [3; 4]. In GMAW, there are three primary modes of droplet transfer: short-circuit transfer mode, globular transfer mode, and spray transfer mode [5]. Short-circuit transfer typically occurs at lower welding currents, where the molten droplet contacts the workpiece through the arc gap, forming a short circuit. This mode features low heat input and is suitable for thin plate welding. Globular transfer can be further divided into coarse droplet and fine droplet transfer. When the current is relatively small and the arc voltage is high, coarse droplet transfer forms, with the droplet diameter being larger than the wire diameter [6], resulting in significant spatter. Conversely, when the current is higher, fine droplet transfer occurs, characterized by minimal spatter and a stable arc, making it widely used in production. Spray transfer mode features high arc power, concentrated heat flow, strong penetration capability, and good weld bead formation quality. For pulsed gas metal arc welding (P-GMAW), there are three typical droplet transfer modes: one droplet multiple pulses (ODMP), one droplet per pulse (ODPP), and multiple droplets per pulse (MDPP). ODPP is considered the ideal droplet transfer mode by many scholars [7; 8]. Regardless of the welding form, if the droplet transfer is unstable, the weld seam is prone to defects such as porosity, slag inclusion, or poor fusion. For all-position welding, the effect of gravity on droplet transfer varies with different welding positions: in the flat welding position, gravity acts as a promoting force for droplet transfer, whereas in the overhead welding position, gravity acts as a hindering force, as shown in Figure 1. A stable droplet transfer process can be obtained by optimizing the process parameters, which in turn ensures the high-quality of weld formation. The effect of arc length on the droplet transition is reflected in three aspects: first, it determines the length of the droplet transition path; second, it affects the extent of magnetic bias blowing, which in turn affects the droplet force (especially in the position of the sidewall of the narrow level); and third, the arc length affects the magnitude of the welding current. [PERSON] et al. [9; 10] studied the characteristics of droplet transfer in GMAW at the flat welding position under different arc lengths. Their findings indicate that when the arc length is too short, a \"multiple droplets per pulse\" transfer mode tends to occur, leading to short circuits and spatter. Conversely, when the arc length is too long, it is more likely to exhibit a \"one droplet multiple pulses\" transfer mode, leading to large droplet transfer and causing disturbance in the molten pool. [PERSON], [PERSON] et al. [11] analyzed the effect of arc length correction on the transition of melt droplets in the narrow-gap P-GMAW at the overhead position and found that when the torch swings to the sidewall, the melt droplets will easily deviate from the wire axis to the other side of the wall transition, leading to a protrusion in the middle of the weld. Using a shorter arc is easier to control the arc shape, ensuring the droplets transition along the diameter of the wire to the molten pool. Since the wire axis is parallel to the sidewall of the narrow level, the longer the arc, the greaterthe effect of magnetic bias blowing, resulting more pronounced change in the arc morphology. As found in the literature [11], test results showed that when the arc length correction factor was 6, droplets even transferred to the right sidewall.