# 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
Under the same welding wire diameter, the wire feeding speed (WFS) determines the magnitude of the welding current, and thus the impact of WFS on droplet transfer reflects the influence of welding current on droplet transfer. [PERSON] et al. [12] observed the droplet transfer modes in GMAW at different wire feeding speeds. They found that when the WFS was between 3 and 8 m/min and the arc voltage was moderate, the droplet transfer was in a stable short-circuit transfer mode (as shown in Figure 2), leading to good weld seam formation; as the WFS increased, the droplet transfer became unstable, with more spatter; when the WFS exceeded 10 m/min, the droplet transfer mode shifted to spray transfer (as shown in Figure 3), resulting in poorer weld seam formation. [PERSON] et al. [7] analyzed the impact of WFS on droplet transfer at the overhead welding position. They found that increasing the WFS helps to obtain smaller droplets and a higher transfer frequency, making it easier for droplets to reach the molten pool. However, under the effect of gravity, the molten pool struggles to flow towards the sides, easily forming a raised weld seam in the middle, which increases the amount of post-weld grinding work. Figure 1: Schematic diagram of the effect of gravity on the transition of the melt drop for different welding positions (The red and blue arrows represent the direction of gravity and droplet transition direction). Figure 2: Short-circuit transfer (WFS = 6 m/min). Reprinted with permission from Ref. [12]. Copyright 2021, Elsevier. The effect of oscillation amplitude on droplet transfer essentially reflects the impact of sidewall constraints on the transfer mode. [PERSON] et al. [13] recorded the droplet transfer during the overhead welding process of narrow-gap P-GMAW at different oscillation amplitudes using high-speed photography. As the oscillation amplitude increased, the droplet transfer mode changed from one droplet per pulse to multiple droplets per pulse (as shown in Figure 4), affecting the stability of the molten pool and leading to defects such as sidewall lack of fusion and undercutting. [PERSON] et al. [14] studied the influence of oscillating arc on the droplet transition. During upward welding, the effect of gravity on droplet transfer is reduced. When the welding wire oscillates toward the groove sidewall, the transfer frequency increases, manifesting as spray transfer; when the welding wire oscillates toward the center of the groove, droplet transfer becomes difficult, and as the droplet size increases, large droplet transfer eventually occurs; when the welding wire oscillates to the other side of the groove, the droplet transfer frequency increases and the droplet size decreases, reverting to spray transfer, as illustrated in Figure 5. During downward vertical welding, due to the downward flow of molten metal under the influence of gravity, the arc burns on the surface of the molten pool and the groove sidewall, and droplets are likely to contact the molten pool before completely detaching from the tip of the welding wire, leading to short-circuit transfer, which can cause spatter. Figure 4: Droplet transport behavior of the right wall when the welding torch oscillated with 4.6 mm (The red circles indicates the droplet). Reprinted with permission from Ref. [13]. Copyright 2023, Authors. Figure 5: Droplet transfer in oscillating arc narrow gap welding: (**a**) left sidewall of the groove; (**b**) center of the groove; (**c**) right sidewall of the groove. Reprinted with permission from Ref. [14]. Copyright 2014, China Welding. Figure 3: Spray transfer (WFS = 13 m/min). Reprinted with permission from Ref. [12]. Copyright 2021, Elsevier. Furthermore, the assembly gap significantly influences droplet transfer behavior [15]. As the gap size increases (within the range of 0 to 4 mm), the droplet transfer mode in GMAW upward vertical welding evolves from short-circuit transfer to a special droplet transfer.