Impact of External Magnetic Field on Weld Quality of Resistance Spot Welding

Electromagnetic stirring (EMS) has been demonstrated to have significant effect on molten metal in terms of crystal orientation, grain refinement and macro appearance of solidified structures by making use of Lorentz force. In the present study, resistance spot welding (RSW) process of 1.25 mm thick dual-phase steel DP780 with and without the external magnetic field applied has been experimentally investigated. Impacts of the EMS method on nugget appearance, quasi-static performance, fatigue life, and fracture morphology have been systematically discussed. Results of the metallographic tests showed that, compared with the traditional resistance spot weld (RSW weld), the weld under the EMS effect (EMS-RSW weld) was wider and thinner with an obvious increase in nugget diameter. Besides, within the EMS-RSW weld, crystal orientation along the faying surface of workpieces was less directional and the grains were refined. Slightly higher uniformity in the fusion zone and more notable softening in the heat affected zone of the EMS-RSW weld were observed by microhardness tests. With regard to the mechanical properties, both tensile-shear and cross-tension samples of the EMS-RSW welds exhibited higher ultimate failure loads and longer elongations at the failure points than that of the traditional RSW welds. The EMS-RSW welds also showed longer fatigue life under dynamic tensile-shear loads, especially in high cycle conditions. Furthermore, the EMS-RSW welds exhibited a higher frequency of button-pullout fractures under the welding current close to the minimum current that the traditional RSW welds required to prevent weld interfacial fractures under quasi-static tensile-shear loads. Even if both types of the welds exhibited interfacial fractures under a relatively weak welding current, more dimples were found in the fracture surfaces of the EMS-RSW welds than that of the traditional RSW welds. It can be concluded that the external magnetic field during RSW process could improve weld performance of DP780 by enhancing weld strength and plasticity. EMS could be an effective method to improve the weldability in RSW of advanced high strength steel, ultra high strength steel, and even light metals.