Effects Of External Alternating Magnetic Field On Spot Welding Quality Of Ultra-fine Grained Steel

As representative of high-performance steel material of new generation, ultra-fine grained steel realize, for its super-fine grain, realize the performance strengthening and toughening. But in welding processes, the welded joints of ultra-fine grained steel will be grain growth, softening and strength decline, which will reduce the product quality, reliability and service life. For this issue, the method, which is the quality improving of spot welding of ultra-fine grained steel in the external alternating magnetic field, will be discussed by this article. Taking 400MPa ultra-fine grained steel for an example, this paper systematically analyzes the effect of external alternating magnetic field on the quality of spot welded joint.This paper analyzes the macro-features and crystal habit of spot welded joint first. Under the selection of welding parameter, the section of normal spot welded joint is close to rectangular, but which is ellipse in external magnetic field. And it has a greater nugget diameter, a smaller joint height and penetration rate in alternating magnetic field. The nugget of ultra-fine grained steel is made up of single and coarse columnar crystals under general spot weld process, which has strong orientation. When it is applied external magnetic field, the orientation of columnar in nugget is weakened and there are variable amounts of equiaxed crystals in the center and fringe of the nugget. The research indicates that, as the same magnetic current, the effect of reverse magnetic field is superior to that of the same magnetic field; as the same magnetic direction, the effect of 150A magnetic current is better than others. Second, the microstructure and mechanical properties are researched. The microstructure of spot welded joint of ultra-fine grained steel of made up of pearlite, martensite and little of ferrite. When it is applied external magnetic field, the structure distribution is more fine and uniform and the average hardness in the nugget increases. The result of tensile shear experiment shows that, compared to the normal spot welding, the load against tensile shear improved the 1.90% when applied reverse magnetic field and 150A magnetic current. Finally, the inner defect of spot welded joint is researched. When the electrode force is below, there are cracks and shrinkage in the nugget. When it is applied magnetic field, the defects in the nugget are improved to same degree, similarly, the improvement effect is the best for150A reverse magnetic.It is obvious that external alternating magnetic field can modify the direction of crystallization, improve primary and secondary solidification structure, and increase the mechanical properties of spot welded joints. So this method has unexceptionable prospect in spot welding quality control.