| With the rapid development of rail vehicles in the direction of high speed,light weight,etc,the body of rail vehicles in the city and high-speed vehicles less than 200km/h are made of austenitic stainless steel sheet.Meanwhile,some components are also made of stainless steel and carbon steel sheet joint structure.The method of spot-welded of stainless steel and carbon steel dissimilar materials can not only comprehensively utilize the advantages of the two,but also overcome the problem of reduced joint strength caused by the large deformation of traditional thin plate arc welding.So the main application of the body of stainless steel is resistance spot-welded.But because of the distinctions of microstructure,chemical components and physical property,the process of resistance spot-welded dissimilar steel joints is more complex.As a result,the mechanical,fatigue properties and fracture mechanism of resistance spot-welded dissimilar steel joints are completely different from same steel.For the above problem,the resistance spot-welded dissimilar(SUS301-DLT and Q235B)steel joints of seven kinds of combinations of different thickness(1mm+1mm,1.5mm+1.5mm,2mm+2mm,3mm+3mm,4mm+4mm,1.5mm+5mm,2mm+5mm)are investigated.The structure was studied by macroscopic,microscopic,microhardness,tensile-shear and tensile-shear fatigue.The relationship between mechanical properties and microstructures,overload fracture and fatigue fracture mechanisms were analyzed.Based on the analysis of the macroscopical appearance of seven kinds of combinations of different thickness stainless steel/carbon steel resistance spot-welded joints,it was found that the penetration rate met the standards.Due to the good thermal conductivity of the carbon steel,the size of heat affected zone of the carbon steel side were larger than the stainless steel side.When both sides of the plate thickness of the resistance spot-welded dissimilar(SUS301-DLT and Q235B)steel joints of seven kinds of combinations of different thickness were equal,due to poor thermal conductivity of stainless steel,the nugget was offset toward the stainless steel side.When both sides of the plate thickness were not equal,due to the larger influence of thermal conductivity,the nugget of 1.5mm+5mm was offset toward the stainless steel side.However,with the increase of the thickness of stainless steel and the welding time,the influence of plate thickness was larger.So the nugget of 2mm+5mm was offset toward the carbon steel side.The microstructure of the nugget mainly consisted of lath-shaped martensite.The heat affected zone of the stainless steel had a single austenite structure.The carbon steel side heat affected zone was a mixture of many kinds of microstructures.Because of martensite formation,the microhardness of nugget was maximum.The morphologies of tensile-shear fracture were all dimple ductile fracture.Two distinct failure modes were observed during the tensile shear test:interfacial,pullout failure modes.The tensile-shear strength of the specimen was positively related to the nugget diameter.The 3mm+3mm plate thickness combination had a maximum tensile-shear strength of 34.43kN.The failure locations of tensile-shear fatigue specimens were all near the fusion line of stainless steel side.The morphologies of terminal fracture area were all shear dimple.The 4mm+4mm plate thickness combination had a maximum tensile-shear fatigue strength of 5.875kN at a specified life of 2×106 cycles. |
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