Study On Microstructure And Properties Of Laser-arc Hybrid Welding Joint Of A High-strength Bainitic Steel

With the further acceleration of high-speed trains,the research on the lightweighting of high-speed trains has received extensive attention.As the important traveling mechanism and bearing mechanism of high-speed trains,bogies propose high demands on the strength and fatigue performance of materials.In recent years,research on high-strength bainite steel has become a hot spot.This steel with excellent strength and fatigue properties points a way to the lightweight of high-speed train bogies:improving the strength of the material without sacrificing the fatigue properties,thereby reducing the amount of material used in the bogie manufacturing process to achieve the purpose of the lightweight of high-speed train bogies.In this paper,the laser-arc hybrid welding process of a new high-strength bainitic steel with a thickness of 10 mm was studied.The microstructure and hardness,tensile,impact and low frequency fatigue crack growth behavior of welding joints welded by the optimized two-pass and single-pass welding parameters were studied,to reflect the influence of the laser-arc hybrid welding process on the microstructures and properties of the bainitic steel hybrid welding joints.In the study of the laser-arc hybrid single-pass welding process of bainitic steel,a single-pass welding joint with a good surface morphology was gained by changing welding parameters such as laser power,wire feeding speed,welding speed and gap size of the blunt edge.However,coarse lath structure apprears in the microstructure of the single-pass weld zone,while the microstructure of two-pass weld zone is relatively finer.The full thickness tensile tests were performed on the welded joints.The single-pass welding joints fractured at the weld zone and the tensile strength is only about 790 MPa.As a contrast,the two welded joints were broken in the base metal.Besides,the impact tests performed on the wele zone and heat affected zone show that the impact toughness of the single-pass welding jionts is somewhat worse than that of the two-pass welding joints.In addition,there are many welding defects such as pores on the tensile and impact fractures of the single-pass welding joints.It can be seen that under the optimized welding parameters used in this single-pass welding process,the internal quality of the single-pass weld joint is poor,although the weld surface morphology is formed well.The research on the thermal efficiency of laser-arc hybrid welding process shows that the thermal efficiency of the welding process decreases with the increases of laser power,increases with the increases of wire feeding speed,and decrease with the welding speed increase on the condiction that no observe burnout appears.In this paper,the fatigue crack growth test of base metals with different thicknesses,full thickness joints obtained by different welding processes,and layered joints in various micro-areas were carried out.The fatigue crack growth tests performed on the base metals with different thicknesses show that,due to the difference of the crack tip stress field of different thickness samples in different crack propagation stages,the 4 mm thick plate base metal samples have a better capacity to resist the crack propagation and a lower crack growth rate at the low stress intensity factor amplitude?K than the 10 mm thick plate base metal samples.And when the?K>76.3 MPa?m^0.5,the crack growth rate of the 4 mm thick plate base metal sample is larger.The fatigue crack propagation tests tested on the full-thickness welding joint show that the hard and brittle microstructure on the weld zone has a strong ability to resist crack initiation,but the ability to resist crack propagation after crack initiation is poor.The fatigue crack propagation tests of the micro-zones of the layered joints show that the severe microstructure inhomogeneity in the heat affected zone force the crack deflects when crack extends through this zone.And the stress concentration at the place where the crack deflects makes these regions become high-incidence zones of secondary cracks.As a result,the crack propagation paths of the fatigue crack growth specimens sampling on the heat affected zone are much more tortuous than the crack propagation paths of the specimens sampling on the other micro-zone specimens,and the secondary crack is more in the former path.