Study On Mechanical Properties Of 301L/Q235B Dissimilar Steels Resistance Spot Welded Joint In Railway Passenger Trains

The main welding structure of lightweight railway passenger train body made of 301L stainless steel plate is resistance spot welding.However,there is a welded structure of stainless steel and low carbon steel(weathering steel)in the part of train body.Among various alternative dissimilar steel welding methods,resistance spot welding is the best way to improve the mechanical properties of welding and reduce manufacturing costs.At present,dissimilar steel welding is difficult and there are many problems to be solved.It is of great significance to study the dissimilar steel spot welding technology to improve the vehicle equipment manufacturing level and enhance the service performance of the welded structure.In this thesis,the 2.0 mm+2.0 mm and 1.5 mm+4.0 mm 301L/Q235B resistance spot welding structures were studied.The static tensile,fatigue properties,and their influencing factors were analyzed.The tensile strength and fatigue limit were measured,the fracture mode was analyzed.And the finite element software was used to analyze the influence of the stress concentration of the weldment lap joint structure on the fracture behavior of the spot welded joints.The nugget of 301L/Q235B resistance spot welding is asymmetric structure.The diameter of the nugget in the 301L plate is larger than that of Q235B plate,and the hardness of the nugget is higher than that of the two base metals.The static tensile fracture modes of two sets of spot weld joints with different nugget sizes of 301L/Q235B have interface fracture and pull-out fracture.Spot welded joints that fail in the pull-out mode have higher fracture strength and fracture displacement.The critical conditions for the pull-out fracture of the 2.0 mm+2.0 mm,1.5 mm+4.0 mm spot welded joints are that the nugget diameter of the lap joint is 7.04 mm and 7.10 mm,respectively.The finite element analysis shows that the Mises stress value is the largest at the edge of the nugget at the loading end of the 301L plate,and the pull-out fracture driving force S11 of the spot welded joint reaches the critical value before the interface fracture driving force S12,causing the 301L side nugget to be pulled out.The fatigue life of the 2.0 mm+2.0 mm weldments increases with the increaseing of the nugget diameter.The fatigue limit of the 50%survival rate of the weldment measured by the up-and-down method is 2.58 kN,the fatigue limit is 11.2%of the maximum static tensile load,which accords with the general rule that the ratio of fatigue limit to static tensile strength of spot welded structure is 10%-15%.The fatigue fracture behavior is the"eyebrow-shape" fracture of the 301L/Q235B double-plate,which proves that the bearing capacity of the two plates is equivalent.The finite element stress analysis shows that the stress concentration at the edge of the nugget of the 301L plate with larger nugget size is much higher than that of the Q235B plate,which leads to the initiation of fatigue cracks in this part.The plastic strain in the heat affected zone of Q235B plate is the largest,plastic accumulation strain is generated in the local region as a result,and cracks are generated.Under the fatigue limit load level,the maximum equivalent Mises stress is 338.7 MPa at the edge of the 301L nugget,which is lower than the yield strength of the plate.The fatigue life of the 1.5 mm+4.0 mm weldments increases first and then decreases with the increaseing of the nugget diameter.The decreasing of the fatigue life is due to the increasing of the welding current,which causes the solidification crack of the nugget in the 301L plate.The fatigue limit of the 50%survival rate of the weldment is 2.77 kN,and the fatigue limit is 12.2%of the maximum static tensile load.The fatigue fracture is a "eyebrow-shape" fracture of the thin plate 301L.The finite element analysis shows that the maximum equivalent Mises stress is 306.4 MPa under the fatigue ultimate load,which is located at the edge of the 301L nugget.The stress of 301L thin plate reduces because of the high stiffness of 4.0 mm thick Q235B plate and the strong bending resistance.The fatigue life of a 1.5 mm+4.0 mm sheet depends on the fatigue life of the 301L sheet.