Abstract
TRIP980 high strength steel and SPCC low carbon steel of 1.5 mm thickness are connected by resistance spot welding. As joint shear tensile load for evaluation indexes, the results are analyzed by the range analysis and variance methods and the welding parameters are optimized by using orthogonal test. As adding the preheating before welding conditions, the shear tensile load of spot welding joint with the preheating and without preheating are comparatively analyzes under parameters of the same welding current, welding time and electrode pressure. Using electronic tensile testing machine, metallographic microscope and microhardness meter, the performance and microstructure of spot welding joint are tested and analyzed under the optimal welding parameters with the preheating and without preheating conditions. The results show that using preheating before welding could get spot welding joint of good performance and under the same welding parameters, shear tensile load of spot welding joint with preheating is more 5.5% than that of without preheating. Under the optimal welding parameters, spot welding joint without preheating is brittle fracture, and the joint with preheating is ductile-brittle mixed fracture. The spot welding joint organization with preheating is denser than that without preheating. Hardness of molten nuclear and the parent metal for spot welding joint with preheating is less than that without preheating, and the hardness of the fusion line is transition smooth. The main mechanism is that preheating makes molten nuclear surrounding of spot welding joint priority have a heat, and with the heat affected zone expands, the molten nuclear toughness will enhance and the stress of molten nuclear surrounding centrally relieve, thus improving the strength of the spot welding joint.