<p>The welding of 7075 aluminum alloys is highly susceptible to porosity, cracking, and grain coarsening, which severely impact joint integrity. To address these challenges, this study introduces a tri-coil composite magnetic field-assisted arc welding technique, designed to optimize weld morphology and mechanical properties. A multi-directional electromagnetic excitation device was developed to generate pulsed magnetic fields with a 180° phase difference, enabling precise control over arc oscillation and molten pool dynamics. The effects of varying excitation currents (0A, 2A, 5A, 8A) on weld formation, microstructural evolution, and mechanical performance were systematically investigated. The results indicate that under a 5A excitation current, the Lorentz forces effectively suppressed arc dragging, enhanced molten pool turbulence, refined the grain structure (reducing average grain size from 148.8 to 98.0&#xa0;μm), and significantly reduced porosity (pore area decreased from 1653.84 to 204.13 μm²). Consequently, the welded joints exhibited notable improvements, with yield strength, ultimate tensile strength, and post-fracture elongation increasing by 14.05%, 5.99%, and 7.67%, respectively. However, an excessive excitation current (8A) induced molten pool over-agitation, leading to weld bead deviation and porosity reformation. These findings demonstrate that the tri-coil composite magnetic field strategy effectively enhances weld quality by controlling electromagnetic-arc interactions, offering a novel approach for high-integrity welding of high-strength aluminum alloys.</p>