Study On Microstructure And Mechanical Properties Of Resistance Spot Welded Joints Of 05CuPCrNi/301L Dissimilar Steels

There is a welded structure of weathering resistant steel and stainless steel in the connection between body of the railway passenger trains and sleeper beam,traction beam and other parts,while MAG welding is currently mainly used.Resistance spot welding was widely used in the connection of various structures of rail vehicle due to advantages of energy saving,environmental friendly,safety and efficiency,but resistance spot welding of dissimilar steel was seldom used.In order to improve the capability of rail vehicle equipment manufacturing,this paper conducted a resistance spot welding test on the 05Cu PCr Ni/301L-DLT welded structure with three plate thickness combinations of2.0+2.0,4.0+1.5 and 4.0+4.0（mm）commonly used in rail vehicle structures,research was conducted on the microstructure,static tensile and fatigue properties of spot welded joints to explore the feasibility of resistance spot welding of dissimilar steel for rail vehicle welding.The nugget of the 05Cu PCr Ni/301L resistance spot welded joint presented an asymmetric structure,and the nugget diameter in 301L-DLT stainless steel plate was larger than that of 05Cu PCr Ni weathering resistant steel plate.The nugget microstructure was austenite,lath martensite and Cr₇C₃ compound.Due to the uneven mixing of the nugget components,the nugget hardness in the stainless steel plate was slightly higher,with an average hardness higher than 403 HV.The size of heat-affected zone of weathering resistant steel was much larger than that of stainless steel.Its microstructure was ferrite and martensite in different sizes and shapes.The hardness was lower than the nugget,but higher than 165 HV of the base material.The heat-affected zone of the stainless steel plate was annealed austenite and a small amount ofδ-ferrite,and the hardness was close to base material.With the increase of plate thickness,the tensile strength of 05Cu PCr Ni/301L resistance spot welding joint also increased,the maximum tensile load of the thick plate4.0+4.0 spot welding joint was two times more than that of 2.0+2.0,indicating that the weldability of the thick plate was not lower than thin plate;the fracture displacement of4.0+1.5 spot welded joint was the largest due to the obvious tensile deformation of 1.5mm 301L sheet.The fracture modes of the three groups of resistance spot welded joints were pull-out fractures.The fracture positions of 2.0+2.0 and 4.0+1.5 spot welded joint was at the edge of the nugget of stainless steel plates,while the fracture positions of 4.0+4.0 spot welded joint was at the junction of nugget and heat-affected zone of the weathering resistant steel plate.With the increase of plate thickness,the fatigue strength of 05Cu PCr Ni/301L resistance spot welding joints also increased.The median fatigue limits of 2.0+2.0,4.0+1.5 and 4.0+4.0 spot welding joints were 2.60 k N,2.97 k N and 5.85 k N,respectively.The low-cycle fatigue failure modes of 2.0+2.0 and 4.0+4.0 spot welded joints were double-plate eyebrow fractures,while the high-cycle fatigue failure modes were single-plate eyebrow fracture of stainless steel plate（2.0+2.0）and weathering resistant steel plate（4.0+4.0）;the fatigue failure modes of 4.0+1.5 spot welded joints were all single-plate eyebrow fractures of stainless steel.The fatigue fracture of the stainless steel plate occurs at the edge of the nugget,and the fatigue fracture of the weathering resistant steel plate occurs in the heat-affected zone.With the effect of fatigue load,severe stress concentration and high plastic strain occur in these areas,which was the main reason for fatigue crack initiation.The median fatigue limit load was substituted into the spot welded joint FEA model to calculate the high-cycle fatigue resistance in the fatigue fracture area.The maximum Mises stress of high-cycle fatigue fracture in 2.0+2.0 and 4.0+1.5 spot welded joints was located at the edge of stainless steel nugget,323 MPa and 308 MPa respectively;while the maximum Mises stress of 4.0+4.0 spot welded joint in high-cycle fatigue fracture was 401MPa,located at the heat-affected zone of weathering resistant steel.The maximum Mises stress could be used as a reference for the allowable fatigue stress in the fatigue design of 05Cu PCr Ni/301L spot welding structure.