Effect of External Magnetic Field on Microstructure and Mechanical Properties of GMW2/H1000 Resistance Spot-Welded Joints

This study investigated the effect of magnet working distance on the external magnetic-field distribution, weld-nugget geometry, and mechanical response of GMW2/H1000 dissimilar steel joints produced by magnetic-field-assisted resistance spot welding (MA-RSW). Three-dimensional magnetostatic simulations were performed for working distances of 34, 28, and 22 mm, together with experimental characterization of weld morphology, tensile–shear behavior, microstructure, and microhardness. The simulated magnetic flux density at the geometric weld center increased from 23.88 mT at H = 34 mm to 41.63 mT at H = 28 mm and 82.39 mT at H = 22 mm. The mean nugget diameter increased from 5.14 ± 0.14 mm for conventional RSW to 5.67 ± 0.14 mm at H = 22 mm, while the mean peak tensile–shear load increased from 5738.6 ± 177.5 N to 6604.1 ± 211.1 N, corresponding to a 15.1% increase. A first-order geometrical analysis showed that the equivalent fusion area increased by approximately 21.7%, indicating that nugget enlargement is an important contributor to the improved load-bearing capacity. Optical microscopy revealed a finer and less directionally developed dendritic morphology under MA-RSW. The results are consistent with an additional electromagnetic influence, although molten-metal flow was not directly resolved by the present magnetostatic model.