# 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
Applying an external magnetic field to generate an upward electromagnetic force that counteracts part of the gravitational force on the molten pool can also control the flow of the molten pool in all-position welding. [PERSON] et al. [54] proposed applying a high-frequency alternating magnetic field to the molten pool during downward vertical MAG welding (as shown in Figure 12). By applying a high-frequency alternating current to coil 0, coil 1, or coil 2, the generated magnetic field acts on the molten pool. The eddy current effect produced by this magnetic field generates an eddy current force that counteracts the gravitational component of the molten metal, overcoming the adverse effects of gravity on the formation of the molten pool. This method can be extended to vertical welding positions to enhance the versatility and flexibility of the welding process. Based on numerical simulations and experiments, [PERSON] et al. [55] investigated the electromagnetic support effect on the molten pool during laser beam welding of austenitic stainless steel. Their research demonstrated that this electromagnetic support can prevent the sagging of the molten pool and detachment due to gravity. [PERSON] et al. [56]  
Figure 11: Motion and force of a charged particle in a longitudinal magnetic field (\(B\), \(F_{X}\) and \(V_{X}\) indicate the longitudinal magnetic field intensity, Lorentz force and radial motion component.). Reprinted with permission from Ref. [49]. Copyright 2019, IOP Publishing. proposed a method of applying an external electromagnetic field to generate an upward electromagnetic force to control the flow of the molten pool (as shown in Figure 13). The interaction between strong currents \(I\) and the magnetic field \(B_{X}\) produces an electromagnetic force in the opposite direction of gravity, thus suppressing the downward flow of the molten metal. This approach not only enhances the stability of the molten pool but also improves the quality and consistency of the weld seam. [PERSON] et al. [57] developed an approach based on GMAW, wherein they controlled the fluid flow and its velocity distribution within the molten pool by applying an external magnetic field. They proposed that combining an appropriate electromagnetic field with a rear-tilted welding torch can yield high-quality welds without humping. [PERSON] et al. [58] applied an external magnetic field to the molten pool of low-carbon steel plates to generate a forward electromagnetic force, which breaks the reverse flow of molten metal. As a result, they achieved welds without humping. Given that low-carbon steel is a ferromagnetic material whose permeability changes significantly with increasing temperature, the behavior of the molten pool and its surrounding high-temperature regions under the influence of an external magnetic field becomes complex and difficult to predict. Consequently, research on utilizing external magnetic fields to control the dynamic behavior of the molten pool during the welding of low-carbon steel is relatively limited. Despite the significant improvement in weld bead quality that can be achieved by controlling arc shape and molten pool behavior through an externally applied magnetic field, this approach faces multiple challenges in practical application. Firstly, magnetic field parameters (such as intensity, frequency, etc.) need to be precisely regulated according to specific welding conditions, which pose higher demands on existing control systems. Secondly, different materials respond variably to magnetic fields, especially ferromagnetic materials whose permeability changes significantly with temperature, adding complexity  
Figure 12: Principle diagram of the structure of the external high-frequency alternating electromagnetic field. Reprinted with permission from Ref. [54]. Copyright 2013, Journal of Mechanical Engineering.