Study On Microstructures And Mechanical Properties Of Laser-arc Hybrid Welded S355J2H Steel Joints

The bogie frame is a key component of railway vehicles. It is very important to improvespeed, curving performance and comfort, and is also an important guarantee for the safetyand reliability of railway vehicles. The bogie forming mainly uses welding technology. Atpresent, the automatic or semi-automatic MAG welding is widely used in producing bogieframes. Although the welding process is mature, there are also some problems: themulti-layer welding and more preparation working before welding are needed for weldingmedium and thick plates, which results in low productivity; more deposition metal andlarger heat input increase the welding deformation; a large number of dust and harmfulgases are produced during welding process. The laser-arc hybrid welding is a new weldingtechnique. It not only inherits the advantages of both laser welding and MAG welding, butalso makes up the deficiencies and limitations of single welding method, which helps tosolve the problems for the bogie MAG welding.In this paper, the S355J2H steel was used as the base metal, and influences of thelaser-arc hybrid welding parameters (laser power, welding current, laser-wire spacing,defocus and welding speed) on the weld penetration, weld width, etc. were systematicallystudied, which provides necessary theoretical basis and experimental data for optimizingthe welding parameters and improving the welding quality. The laser-arc hybrid weldingwith three-layer welding process were adopted in beamed pipe welding tests, based on theinfluences of welding parameters and the pipe thickness (20mm). The characteristics ofmicrostructures and mechanical properties of laser-arc hybrid joints were studiedcompared with those of MAG welded joints. The results show that the morphology oflaser-arc hybrid weld is similar to the shape of "goblet". The weld arc zone mainlycontains side plate ferrite, fine needle ferrite and pearlite. The weld laser zone mainlycontains grain boundary ferrite and fine needle ferrite inside grain. Compared with the arc zone, the microstructure of laser zone is significantly refined. The microstructures oflaser-arc hybrid welded joints have a refining tendency compared with those of MAGwelded joints, which is mainly due to the lower heat input. The test results show that thehighest hardness of both laser-arc hybrid welding and MAG welding joints appears atoverheated zone, being299.6HV and236.5HV, respectively. The joint tensile strength andelongation are higher than the S355J2H base metal’s lowest value. With the joint lateralbending180o, the average crack length of hybrid welding and MAG welding joints is2.0mm and0mm, respectively. The impact property of joints for both the welding methodsis higher than the S355J2H base metal’s lowest value. The fatigue strength of hybridwelding and MAG welding joints is131.5MPa and110.6MPa, respectively. The resultsabove show that it is feasible to weld the bogie beamed pipe with the laser-arc hybridwelding method.