# 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 Forming by Applying a Magnetic Field to Control the Flow of the Molten Pool  
The welding arc is a high-temperature plasma, and external magnetic fields have a significant impact on its behavior. By applying an external magnetic field, an electromagnetic force is generated on the arc or molten pool, opposingthe force of gravity. The shape of the arc and the force state of the molten pool can be altered, thereby improving weld seam quality. The improvement of molten pool behavior by electromagnetic forces can be categorized into two types: the first involves using electromagnetic forces to control the rotation or deflection of the arc at a certain angle, which serves to stir or support the molten pool; the second involves the direct action of the magnetic field on the molten pool, utilizing the electromagnetic force to counteract the gravitational force on the molten pool to improve its force state. [PERSON] et al. [49] generated a longitudinal magnetic field parallel to the welding torch axis using a magnetic generator installed on the welding torch. Under the influence of the Lorentz force \(F_{X}\) and the magnetic field \(B\), charged particles in the arc undergo rotation (as shown in Figure 11), causing the arc to rotate at high speed, which increases the heated area and thereby controls the flow of the molten pool. Similarly, applying a transverse magnetic field uses the Lorentz force to deflect the arc, thereby changing the heat distribution on both sides of the groove and enhancing the quality of the weld seam formation [50]. To achieve sufficient sidewall fusion without significantly increasing the heat input, [PERSON] et al. [51] found that by applying an alternating magnetic field to assist arc welding, an alternating magnetic field can be formed in the arc burning zone,periodically deflecting the welding arc within the pulse current cycle, which can provide more arc heat. However, the effectiveness of this method relies on precise adjustment of magnetic field parameters, which can be challenging to achieve in actual operation. [PERSON] et al. [52] established a mathematical computational model for narrow-gap tungsten inert gas welding to study the behavior of the welding arc and molten pool under the influence of an alternating longitudinal magnetic field. This research enhances the understanding of how external magnetic fields can be effectively utilized to control the welding process and improve weld seam quality. However, the model does not fully take into account the various complex conditions and interactions among variables that may occur during actual welding processes. Most existing studies focus on the impact of a single variable (such as magnetic field strength or frequency) on the welding process while neglecting the interactions among multiple factors and their overall effect on the welding outcome. For example, [PERSON] et al. [53] successfully achieved the joining of low-carbon steel Q235B using plasma-enhanced pulsed GMAW with an external magnetic field. Their research found that as the magnetic field current increased, the coupling of the dual arcs also increased. However, when the magnetic field current exceeded 1A, the changes in arc shape tended to level off. Changing the frequency of the applied alternating magnetic field can alter the shape of the welding arc, reducing the spatter rate and improving the appearance of the weld seam. [PERSON] et al. [49] analyzed the effect of magnetic field frequency on GMAW arcs using high-speed camera techniques. Their analysis revealed that the shape and movement of the arc change with variations in the external magnetic field frequency. Below 60 Hz, the arc shape expands, leading to a deterioration in the weld bead appearance and mechanical properties. Conversely, above 60 Hz, the weld bead appearance and mechanical properties are significantly improved.