Mechanical properties and process optimization of dissimilar steel resistance plug weld

A new type of hybrid welding - resistance plug welding was used for welding of dissimilar steels between 1.5mm thick TRIP980 high strength steel and SPCC low carbon steel sheets. The orthogonal test was conducted to optimize the welding parameters. Then preheating before welding was carried out. The mechanical properties of the joints and the microstructure and hardness in different regions were compared under the same welding parameters with and without preheating before welding. The results show that the effect of welding parameters on the shear tensile load of the joint decreased in the order of is welding current, filler diameter, welding time and electrode pressure. When the end diameter electrode was 8mm and the filler diameter was 5.5 mm, the mechanical properties of the joints were better than those with other diameters. Under the same welding parameters, the tensile shear strength of the joints with preheating was improved 7% more than that without preheating before welding. With optimum welding parameters, weld nugget was larger with preheating than without preheating, the nugget offset was smaller with preheating, and the fusion zone with brittle martensite was also smaller. The fracture of joint with preheating was ductile, but that without preheating was brittle. The hardness of weld nugget and fusion zone with preheating decreased. The main mechanism is that preheating reduced the overheating tendency of the nugget zone in the plug weld, made the expansion in weld nugget uniform, and reduced the brittle and hard structure in the fusion zone, which was beneficial to improve the joint strength.