Study On Service Performance And Failure Behavior Of Traceless Laser Welded Joints On Side Walls Of Stainless Steel Railway Vehicles

Lightweight railway passenger cars made of cold-rolled 301 LN austenitic stainless steel have been widely used in subways and intercity express transportation systems because of their advantages of safety,energy saving and recyclability of material.Resistance spot welding with low heat input is usually used in the connection of the side walls of railway passenger cars,but this method generates a large number of visible indentations on the surface of the car body,which seriously affects the appearance quality and corrosion resistance of the car body.Non-penetration laser welding is an ideal method to replace resistance spot welding to solve the above problems.Therefore,the study of service performance and failure behavior of the non-penetration laser welded lap joint of the 301 LN stainless steel was conducted to provide a basis for the design and manufacture of stainless steel passenger cars.The specific research work includes the following aspects:The solidification mode,thermal cracking susceptibility and fracture strength of laser welds of 301 LN stainless steel were studied through microstructure characterization and static tensile tests.The result shows that the solidification of lap non-penetration laser weld of 301 LN steel was controlled by three-direction heat conduction of the plates on both sides and the bottom of the molten pool,and the cooling speed was fast.The main weld was solidified in FA mode,and the amount of ferrite was about 6.4% volume fraction;the bottom weld was solidified in mixed mode of FA and AF,and the amount of ferrite was about 3.2% volume fraction,the proportion of the mixed solidification zone increased with the cooling rate.301 LN austenitic stainless steel has excellent laser weldability and good resistance to welding thermal cracking,and laser weld metal has excellent mechanical properties not lower than cold-rolled sheet.The static tensile properties and failure behavior of lap non-penetration laser welded joints as well as the main factors affecting the static tensile properties were studied through static tensile tests.The result shows that the tensile failure mode of lap non-penetration laser welded joints is tensile-shear fracture of interface weld.For the specimen with a large weld rotation angle during the tensile process,the tensile stress ratio(decomposed normal stress to shear stress)in the weld fracture surface is greater than that of the specimen with a small weld rotation angle,resulting in higher fracture load and displacement.The fracture ductility of lap non-penetration laser welded joints increases linearly with the ratio of weld interface width to thinner plate thickness.In view of the low fracture ductility of the non-penetration laser welded lap joint of thick plate,a welding method in which the laser beam irradiates the tilt plate vertically was proposed.The prepared tilt weld has a regular geometry shape similar to that of a vertical weld,which effectively increases the weld interface width without increasing the penetration rate,thereby improving the fracture load and ductility.The weld tilt direction affects the static tensile property of the welded joint by changing the fracture mechanism of the interface weld.Compared with the positive tilt weld,the stress state of the fracture interface of the negative tilt weld changed from shear stress to tensile stress,and the fracture load and ductility increased by 11.4% and 58.9% respectively.The thermo-elastoplastic finite element model considering the contact constraints of the lap interface was developed,which can accurately predict the temperature field,residual stress,and welding angular deformation of the lap laser welded joint.The influence of penetration rate on the residual stress and welding angular deformation was studied.The result shows that the peak value of the transverse residual stress at the lap interface of lower plate of non-penetration welded joint is about 70 MPa higher than that of penetration welded joint due to the stronger constraint on the free expansion of weld metal,and the peak value of the transverse residual stress is located in the area about 2.5 mm away from the weld center,which is also the fatigue crack initiation zone.The difference in transverse shrinkage between the upper and lower surfaces of non-penetration laser welded joints is larger,resulting in the large welding angular deformation.Through fatigue test,fracture analysis and finite element analysis,the fatigue performance and failure behavior of lap laser welded joints were studied,and the influence of residual stress on fatigue performance and fracture behavior was analyzed.The fatigue design stress and fatigue life prediction method of laser welded lap joints was obtained.The results show that the fatigue fracture plate of the lap non-penetration laser welded joints changed from a thicker non-penetration lower plate to a thinner penetration upper plate with the increase of the ratio of lower plate thickness to upper plate thickness under high load amplitudes,and the fatigue fracture of welded joints is mainly affected by the local notch stress.The fatigue fractures of lap non-penetration laser welded joints occurred in the non-penetration lower plate under low load amplitudes,which is related to the higher transverse residual stress at the lap interface of the non-penetration lower plate.The fatigue limit of lap non-penetration laser welded joints increases significantly with the increase of the thickness of the non-penetration lower plate,while the increase of thickness of penetration upper plate has no obvious effect on improving the fatigue limit of welded joints.Under the influence of the higher transverse residual stress,the fatigue resistance of non-penetration lap welded joints for low load amplitudes is much lower than that of penetration joints,while the fatigue resistance for high load amplitudes is not much different.The fatigue resistance stresses at the fatigue crack initiation zone of the penetration and non-penetration laser welded joints calculated based on the fatigue limits are 408 MPa and 345 MPa,respectively,which can be used as reference stress for the fatigue design of laser welded structures.The fatigue life of the laser welded lap joint can be reasonably predicted using the equivalent structural strain method consider the influence of the welding angular deformation,and the error rate is within 30%.