Study On Effects Of Welding Process Factors On Microstructures And Properties Of Laser Welded Stainless Steel Joints For Railway Vehicles

With the development of society and the advancement of science and technology,rail transit has shown a gratifying development trend.Due to its large passenger capacity,fast speed and high safety,railway vehicles have gradually become one of the main ways for people to travel.Stainless steel railway vehicles have many advantages such as high strength,light weight,corrosion resistance and low cost,so they have become the first choice for people in China.This paper is aimed at the structural characteristics of stainless steel railway vehicles body and the problems existing in laser welding formation.The influences of welding process factors on the microstructure and performance of laser welded joints of stainless steel railway vehicles were systematically studied.Its purpose is to provide the necessary theoretical basis and experimental data for improving the quality of stainless steel railway vehicles,which has important theoretical significance and practical value.In this paper,the microstructure and mechanical behavior of SUS301 L austenitic stainless steel lap laser welded joints were studied.The effects of laser welding parameters(laser power,welding speed)and assembly gap on the microstructure and properties of welded joints with different thickness combinations(1.0mm+0.6mm,1.5mm+0.6mm)were studied.The results show that the laser welded joint of stainless steel was composed of weld zone(WM),heat affected zone(HAZ)and base material zone(BM).The stainless steel base material was mainly composed of austenite and a small amount of ferrite and martensite.The weld was well formed without defects such as cracks,pores,and incomplete penetration,and it was mainly composed of austenite and a small amount of ferrite.The weld had the characteristics of epitaxial solidification,and the cellular crystal was attached to the unmelted base material to nucleate and grow,and its growth direction is approximately perpendicular to the fusion line.As the crystal grew,the cellular crystal turned into the cellular dendrites,and the cellular dendrites turned into dendrites in the middle of the weld,which is mainly due to the increasing constitutional supercooling.The outstanding feature of the microstructure of the weld heat affected zone(HAZ)is grain coarsening.The microhardness distribution of the stainless steel laser welded joint is uneven.The base material has the highest average hardness value(300 HV),followed by the weld zone(240 HV),and the HAZ has the lowest hardness value of about 220 HV.Under the action of tensile shearing force,stainless steel laser welded joints mainly have two fracture modes,interface fracture and HAZ fracture.Therefore,the interface of the upper and lower plates of the laser welded joints and the HAZ of the lower plate are weak regions of the joints.The laser welding parameters(laser power,welding speed)had a significant effect on the morphology,microstructure and mechanical properties of the welded joints with 1.0mm+0.6mm and 1.5mm+0.6mm thickness combination.As the laser power increases,the weld width and depth increase,the cellular crystal size increases in the weld,and the HAZ width and grain size of the weld increase.This was mainly due to the increase in laser welding heat input.When the plate thickness combination was 1.0mm+0.6mm,the laser power increased from 1.1KW to 1.35 KW,the tensile shear of the joint increased from 0.1KN to 14.2KN,and the joint was the interface fracture mode;further increase of laser power led to the increase and then decrease of the tensile shear force of the joint,which is mainly in the HAZ fracture mode.When the plate thickness combination was 1.5mm+0.6mm,in the laser power of 1.53kW-1.88 kW the joint was the interface fracture mode;further increase of the laser power caused the joint tensile shear force to increase firstly and then reduce,and produced the HAZ fracture mode.With the increase of welding speed,weld width,weld depth and the size of cellular crystals in the weld decreased,and the weld HAZ width and grain size tended to decrease.This was mainly due to the reduction in laser welding heat input.When the welding speed was small,the joint was mainly the HAZ fracture mode;as the welding speed increased,it changed into the joint interface fracture mode.When the plate thickness combination was 1.0mm+0.6mm,the laser power of 1.45 KW and the welding speed of 2.54m/min were beneficial to improve the mechanical properties of the laser welded joint.The joint tensile shear force can reach 16.2kN;when the plate thickness combination was 1.5mm+0.6mm,the laser power of 1.98 KW and the welding speed of 2.54 m/min were beneficial to improve the mechanical properties of the joint.The joint tensile shear force was 15.1 kN.The assembly gap has a significant effect on weld depth,weld width,microstructure and mechanical properties of the stainless steel laser welded joint.As the assembly gap increases,the weld surface appears concave at different degrees,the back weld appeared discontinuous,the weld width increases first and then decreases,and the cellular crystal of the weld changed from transverse growth to upward growth from the bottom plate.The tensile test results of laser welded joint showed that when the thickness of the plate was 1.0mm+0.6mm,the tensile shear force of the joint was gradually reduced with the increase of the assembly gap.Combined with the joint fatigue performance test results,the control of assembly gap at ?0.2mm will not had an obvious impact on the mechanical properties of the laser weld joint.When the thickness of the plate was1.5mm+0.6mm,the tensile shear force of the joint increased firstly and then decreased with the increase of the assembly gap.the assembly gap controlled at ?0.3mm has not obvious influence on the tensile shear performance of laser welded joints.