| 301L austenitic stainless steel cold-rolled sheet series is the main material for manufacturing of intercity rail passenger car.There are some dissimilar austenitic stainless steel/mild steel welding joints in the vehicle body in addition to 301L stainless steel welding.The current arc welding and plug welding should be improved owing to its low efficiency and high cost,whose strength,stiffness and fatigue reliability.The resistance spot welding is used as the primary joining method to improve the mechanical properties of dissimilar steel welded structure and reduce production costs.There are widely differences of the electrical resistivity,thermal conductivity and expansion coefficient between stainless steel and carbon steel,which leading to more complicated metallurgy process of resistance spot welding and the chemical composition,in addition,hardness of various regions in the welded joints vary greatly,therefore welding is more difficult.Dissimilar steel resistance spot welding is also one of the central issue in the field of international welding.In this paper,the resistance spot welding of four kinds of equal thickness and unequal thickness 301L stainless steel and Q235B carbon steel sheet is the main research object.The influence law of spot welding process parameters on temperature field was studied using the numerical simulation of resistance spot welding temperature field,based on that,the process parameters of welding test were selected.Through the welding test,the microstructures of dissimilar steel spot welded joints were investigated.The accuracy and applicability of numerical simulation is verified comparing with the actual spot welded joints.Tensile tests were carried out to study the influence of the geometry of the dissimilar steel spot weld on the tensile properties and the fracture behavior.Finally,the reasonable resistance spot welding parameters and spot weld joint geometry of 301L/Q235 dissimilar steel were determined based on the static tensile properties.The results show that the weld spot nugget of dissimilar steel offset to the side of 301L stainless steel.The nugget geometry obtained by finite element simulation was in good agreement with the experimental results.The error of numerical analysis was lower than 10%when welding heat input was high.In addition,the law of simulated spot welding was basically the same as the experiment,so the model of resistance spot welding had a important reference value.When the sum of the thicknesses of two dissimilar steel plates of resistance spot welding didn't exceed 4mm,there were two kinds of fracture modes of the tensile shear specimen,such as interface failure and 301L nugget pull-out failure.When the Q235B nugget diameter was above 5?t,the spot weld was pulled out of the fracture,and the fracture location was in the nugget of 301L stainless steel.The fracture mode of 1.5mm-301L+4mm-Q235B unequal thickness plate combination was nugget partial pullout failure mode,and finally Q235B nugget was partially peeled off along the final solidification line.Compared with the tensile properties of the same 301L stainless steel with the same plate thickness combination,the 301L/Q235B dissimilar steel spot welded joint had higher breaking strength under the same interface nugget diameter and was more prone to occur pullout failure mode.In addition there was greater plastic deformation before fracture,so it had strong practicality. |
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