Study On Technology And Microstructure And Mechanical Properties Of Laser-MIG Hybrid Welded T-joints For SMA490BW Weathering Steel

For high speed train bogie thick steel plate T-joint,traditional welding adopts gas shielded welding with large angle groove such as MAG.There are many defects such as multiple welding passes,large heat input,low production efficiency and large deformation after welding.The laser and arc double heat sources are coupled to the molten pool.It can effectively improve weld microstructure and improve its performance,reduce welding distortion and improve welding production efficiency.In this paper,8mm thick SMA490 BW weathering steel is used as the test material,T-joint welding is carried out by means of laser-MIG hybrid welding.The influence of welding parameters such as laser power and laser defocus on droplet transfer behavior was studied.The effects of welding parameters on the quality and forming of T-joint were also studied.Weld 8mm SMA490 BW weathering steel with optimized welding parameters.The microstructure and mechanical properties of the composite welded joint were tested and analyzed.The droplet transfer behavior of T-joint laser-MIG hybrid welding was captured and captured by high-speed photography equipment.The effects of laser power,welding current and etc.on the droplet transfer behavior were studied.In the process of T-joint laser-MIG hybrid welding,the addition of laser makes the MIG welding change from the original droplet transition and the short circuiting transition to the droplet transition.The frequency of droplet transition becomes faster,the process is more stable and the weld shape is good.When the laser power increases,the transition mode from droplet transition to droplet transition occurs.The droplet transfer time increases first and then decreases,and the droplet size increases first and then decreases.When the defocus quantity changes from 0mm to +10mm,the droplet transfer form changes from large droplet transition to droplet transition,the welding process becomes stable and the weld is well formed.When the defocus volume continues to increase to +20mm,the effect of plasma flow force and surface tension becomes larger,and the droplet transfer mode becomes short circuiting transition.The transition form of droplet transition is changed from short circuiting to ejection and jet transition with the wire spacing increases.The welding process becomes stable,the spatter decreases and the weld forming is good.With the introduction of laser,the weld width and penetration depth are significantly improved.Welding appearance of arc-leading is better than the laser-leading.The laser-MIG hybrid welding is carried out under the incident angle of 8° and the incident height of 0.5mm.The thickness and depth of the weld seam are obtained,and the weld is formed well.The good lap of the two weld seam welded by T-joint is more easily realized.The larger the laser power is,the more obvious the laser penetration is,the larger the weld thickness and penetration depth.When the welding speed is 0.9m/min,the energy density of the filler wire and the heat input will be increased,which is beneficial to weld formation.When the defocus amount is +20mm,the single side fillet weld is well formed,and the weld foot,penetration depth and weld thickness meet the requirements.The best process parameters of SMA490 BW weathering steel for T-joint laser-MIG hybrid welding 8mm are as follows: laser power is 5kW,defocusing is +20mm,welding current is 230 A,welding speed is 0.9m/min,the angle of incidence is 8°,and the incident height is 0.5mm at the front of arc.The application range of the welding process is 0~0.5mm.The laser-MIG hybrid welding of 8mm thick SMA490 BW was carried out by using the optimized welding parameters,the microstructure and mechanical properties of the joint were tested.The results show that the weld area is mainly composed of ferrite,pearlite and some bainite.The overheated zone is mainly composed of bar ferrite,pearlite and upper bainite.Recrystallization in normalizing zone is mainly composed of fine and uniformly distributed ferrite and pearlite.Only partially recrystallization occurs in incomplete normalizing zone,so its microstructure is equivalent to the mixture of parent material and normalizing zone.The hardness of the joint is the smallest and the hardness of the heat affected zone is the largest.From the base metal to the weld seam,the hardness distribution increases first and then decreases after incomplete normalizing,normalizing and overheating,and reaches the highest in the superheated zone.Compared with MIG welding,laser-MIG hybrid welding improves the welding speed by 180% and the welding deformation of the bottom plate is reduced by 35%.laser-MIG hybrid welding improves the welding efficiency.