Due to its superior electrical conductivity, thermal conductivity, and ductility, the demand for T2 red copper components is increasing in aerospace, new energy vehicles, lithium copper foil, and electronics. With advancements in welding technology and quality control for T2 red copper weldments, traditional welding methods such as friction stir welding, brazing, and arc welding often result in a coarse microstructure in the fusion zone, leading to a reduction in the mechanical properties and electrical conductivity of the welded joints. Consequently, these methods struggle to meet the high-precision and high-performance connection requirements of modern automotive manufacturing [1,2]. In contrast, laser welding has shown greater potential for application in red copper components because of its advantages [3,4], including excellent flexibility, a large weld depth-to-width ratio, a narrow heat-affected zone, minimal post-weld deformation, high precision, and high efficiency [5]. Due to the high reflectivity and thermal conductivity of red copper, laser energy absorption is reduced, resulting in an unstable welding process prone to defects such as spattering, porosity, hump formation, and biting edges, which negatively impact weld quality [6]. Therefore, improving energy coupling efficiency and process stability in red copper laser welding is a key challenge. To address this, extensive studies have been conducted on enhancing weld quality.

In recent years, many scholars have focused on improving the laser absorption efficiency of copper through laser modulation, surface pretreatment, and the use of dual-beam lasers. For example, Liang et al. [7] demonstrated that circular laser oscillation welding significantly improves the quality of T2 red copper welds within a specific range of oscillation process parameters. Furthermore, the oscillation of the laser beam helps to increase the melt width of the bonding surface, thereby enhancing the mechanical properties of the welded joints. Andreas et al. [8] found that sinusoidal power modulation can significantly reduce the amount of melt ejection, which improves keyhole stability, thereby enhancing weld depth and surface morphology. However, since sinusoidal power modulation primarily controls the laser output power characteristics, its mode of action is more complex, which results in certain limitations for practical applications. Jiao et al. [9] used a nanosecond laser with a wavelength of 532 nm to pre-treat the surface of red copper, successfully reducing its reflectivity to near-infrared laser light from 95% to 15.5%, significantly improved the efficiency of laser energy utilization in the welding process. Furthermore, studies have demonstrated that surface pretreatment can significantly enhance the overall performance of materials [[10], [11], [12]]. Chen et al. [13] observed that surface treatments such as sandblasting, blackening, and nanocomposite addition enhanced welding efficiency in laser welding of high-reflectivity pure copper, with nanocomposites having the most significant effect. However, the formation of intermetallic compounds may adversely affect the mechanical properties of the weld. Wang et al. [14] pointed out that the use of a dual-wavelength laser beam to weld pure copper sheets can significantly improve the surface morphology and forming quality of the weld. However, this method complicates the control of energy distribution from the heat sources during welding.

Magnetic field-assisted laser welding, which improves weld quality through the electromagnetic effect, has received widespread attention and rapid development. This technology not only reduces laser energy loss in the molten pool and improves heat input, but also effectively inhibits porosity defects and enhances weld quality [15,16], providing a new approach for high-quality laser welding of red copper. Therefore, scholars have conducted numerous studies on magnetic field-assisted laser welding. For example, Liang et al. [17] integrated numerical simulation with experiments to reveal the Lorentz force generated by the interaction between the steady-state magnetic field and the molten pool and its effect on molten pool flow. Cao et al. [18] developed a three-dimensional numerical model to reveal the mechanism of the magnetic field's influence on the weld pool. The results showed that, under the influence of the magnetic field, the molten pool generates a Lorentz force opposite to the fluid flow, inhibiting fluid flow toward the edge, reducing heat transfer from the center to the edge of the molten pool, and lowering the temperature gradient. Chen et al. [19,20] developed a three-dimensional numerical model incorporating various physical mechanisms, including magnetohydrodynamics and electromagnetic effects, to analyze the molten pool behavior in magnetic field-assisted laser welding. The results indicate that the Lorentz force generated by the electromagnetic field facilitates uniform temperature distribution in the molten pool, reduces laser energy loss during welding, and improves weld quality. Hu et al. [21] used alternating magnetic field-assisted laser welding of red copper-steel dissimilar materials to study the effect of the alternating magnetic field on the microstructure, elemental distribution, and mechanical properties of the welded joints. Liu et al. [22] studied the effect of the applied alternating magnetic field on the microstructure and mechanical properties of 361 stainless steel laser-MIG hybrid welded joints. The results show that the stirring effect of the alternating magnetic field can effectively improve the solidification time of the molten pool, promote austenite grain refinement, and thus enhance the mechanical properties of the joint. Meng et al. [23] demonstrated that after applying an appropriate magnetic field strength, the grain structure in the upper part of the weld was significantly refined. The Lorentz force generated in the molten pool due to electromagnetic stirring positively affects the flow of molten metal and solidification. Zhu et al. [24] showed that the alternating magnetic field altered the morphology of the droplets via the Lorentz force and effectively suppressed spattering, thereby optimizing droplet deflection and improving welding quality and efficiency. Liu et al. [25] investigated the effect of different magnetic field waveforms on aluminium alloy welding. The results showed that all three waveforms improved the weld depth-to-width ratio and reduced porosity, attributed to enhanced keyhole stability, prolonged solidification time, and better gas bubble escape. Bachmann et al. [26] found that applying an alternating magnetic field with a certain strength during laser welding accelerates gas bubble escape in the molten pool, significantly reducing porosity. This effect is primarily due to the combined action of the electromagnetic Archimedean force and molten pool stirring.

Previous studies indicate that applying an alternating magnetic field with a specific strength during welding significantly improves weld quality. This is mainly due to electromagnetic stirring, which refines the grain structure, enhances mechanical properties, and reduces defects like porosity and cracking. However, current research on alternating magnetic field-assisted laser welding mainly focuses on metals like magnesium, aluminium, and steel, with limited studies on T2 red copper laser welding. To enhance the energy coupling efficiency and poor process stability in T2 red copper laser welding, this paper combines numerical simulation and experimental studies to investigate the impact of an alternating magnetic field on the melt pool behavior and weld quality in full-penetration (1 mm) and partial-penetration (2 mm) specimens. Firstly, an experimental study on T2 red copper laser welding assisted by an alternating magnetic field was conducted to analyze the effect of alternating magnetic field with different frequencies and intensities on the morphology of weld seam in both full- and partial-penetration states, with the aim of determining the optimal magnetic field parameters. Subsequently, a three-dimensional thermal-fluid coupling numerical model of T2 red copper laser welding was established based on the experimental parameters to compare the effects of alternating magnetic field strength on the temperature distribution, fluid flow, and keyhole morphology of the specimens in both full- and partial-penetration states. The melt pool morphology predicted by the model was validated by comparison with experimental results. Finally, the effects of the alternating magnetic field on the microstructure, mechanical properties, and electrical conductivity of the welded specimens in both full- and partial-penetration welds are analyzed. Through the above analysis, the mechanism by which the alternating magnetic field affects full- and partial-penetration welding is elucidated, providing both an experimental basis and theoretical support for optimizing the laser welding process of T2 red copper and its alloys.