As an ultra-high-strength steel, 6252 steel is widely applied in the manufacturing of critical structural components for high-end equipment in aviation, aerospace, and other equipment. Its welding quality is directly related to the reliability and safety of the entire equipment. Laser welding, characterized by high energy density and rapid cooling, is commonly employed to produce high-quality welded joints. Nevertheless, it can result in issues such as turbulent molten pool flow, keyhole instability, and thermal stress concentration, causing defects including porosity, cracks, and microstructural inhomogeneity, which limit the full exertion of the joint’s mechanical properties. Magnetic field-assisted welding, an environmentally friendly non-contact external physical field control technique, provides a novel method to precisely regulate the welding process.

A combined methodology of Taguchi experimental design and simulation software modeling was employed. Laser power, welding speed, and magnetic flux density were chosen as the key process parameters, and a three-factor, five-level experimental matrix was constructed. Using weld penetration depth and weld width as primary response objectives, the significant influence of each parameter on weld formation quality was evaluated through the larger-the-better characteristic of penetration depth and the smaller-the-better characteristic of weld width. The optimal parameter combination for achieving full penetration and the best weld formation was determined to be a laser power of 4.1 kW, a welding speed of 15 mm/s, and a magnetic flux density of 240 mT (Fig.6). Based on the determined optimal process parameters, a three-dimensional numerical model of laser welding was developed using Fluent software. This model coupled fluid flow, heat transfer, and magnetic field interactions, and focused on simulating and analyzing the changes in the flow velocity field and temperature field within the molten pool, as well as the dynamic behavior of the keyhole and the geometric evolution of the molten pool before and after the application of a longitudinal steady-state magnetic field.

The study revealed that welding speed exerted the most significant influence on weld width and penetration depth, while magnetic flux density had a relatively small effect on penetration depth. The Lorentz force generated by the introduction of the magnetic field effectively suppressed the violent flow of the melt, reducing the maximum flow velocity in the molten pool from 1.03 m/s to 0.84 m/s (Fig.8). Consequently, the keyhole morphology became more stable, with its width decreasing from 1.72 mm to 1.35 mm and its depth increasing from 2.24 mm to 2.71 mm (Fig.10). The thermal distribution in the molten pool became more concentrated, cooling rates accelerated, the upper surface width decreased from 2.51 mm to 2.04 mm, and the mid-section width decreased from 1.04 mm to 0.85 mm (Fig.11). After the application of the magnetic field, convection in the molten pool was suppressed, which improved the heat flow conditions and compositional distribution during solidification. As a result, the grains in the weld zone were significantly refined, and the martensitic microstructure distribution became more uniform and dense. Meanwhile, the number and size of defects such as porosity in the weld cross-section were significantly reduced, top collapse and undercut phenomena were effectively suppressed, and microstructural uniformity was significantly improved. Compared with the joints welded without the magnetic field, the tensile strength of the magnetic field-assisted welded joints increased by 4.65%, the elongation increased by 3.43% (Fig.15), and the impact toughness significantly increased by 11.2% (Fig.17). The improvements in mechanical properties were attributed to grain refinement, welding defect reduction, and optimized microstructure morphology resulting from the introduction of the magnetic field, leading to overall enhancement of the mechanical properties of the welded joint.

The study indicates that introducing a longitudinal steady-state magnetic field into the laser welding process of 6252 ultra-high-strength steel can effectively regulate molten pool flow via the Lorentz force, significantly improving the stability of the welding process and the quality of weld formation. Magnetic field-assisted welding technology offers an effective method for improving the microstructure and mechanical properties of laser-welded ultra-high-strength steel joints. These findings provide both theoretical support and practical guidance for the formulation of high-quality and high-efficiency welding processes for related materials.