Process Characteristics And Joint Corrosion Behavior Of SUS301L/Q450NQR1 Dissimilar Steel Laser-MAG Hybrid Welding

In recent years,stainless steel materials have been widely used in the field of urban rail transit due to their excellent corrosion resistance,wear resistance and good formability.Q450NQR1 weathering steel has a series of advantages,such as high strength,low price and excellent comprehensive mechanical properties,which can meet the requirements of the current railway freight car operation.The SUS301L/Q450NQR1composite structure can better combine the advantages of the two,which has a good application prospect in the fields of urban rail transit and high-speed trains.At present,the commonly used welding wire for SUS301L/Q450NQR1 dissimilar joints is stainless steel welding wire,but due to its high price,it increases the vehicle manufacturing cost.The commercial CHW-55CNH weathering steel welding wire is expected to replace stainless steel welding wire for SUS301L/Q450NQR1 dissimilar joint welding and realize low-cost manufacturing due to its high strength and atmospheric corrosion resistance.However,conventional arc welding tends to coarsen the grains in the heat-affected zone,resulting in a decrease in the mechanical properties of the joint,and at the same time causing large deformation of the stainless steel car body.The use of laser MAG hybrid welding can largely avoid the above-mentioned problems.From the perspective of process method,it is expected to solve the problem of insufficient mechanical properties and corrosion resistance of joints caused by CHW-55CNH replacing stainless steel welding wire.In this paper,SUS301L stainless steel material is weld with low-alloy steel Q450NQR1 by means of laser-MAG hybrid welding.Through the single factor test,the forming rules of SUS301L/Q450NQR1 dissimilar joints under various process parameters were analyzed,and the influence of high-speed welding on the microstructure and mechanical properties of the joint was systematically studied.The research shows that high-speed welding is not only beneficial to improve the grain coarsening phenomenon,but also improve the mechanical properties of the joint to a certain extent.In addition,the welding speed has a significant effect on the weld microstructure of dissimilar joints.During high-speed welding,the microstructure of the weld is mainly fine ferrite,while the low-speed welding weld is mainly composed of coarseδ-ferrite and austenite.By using ER309LSi and CHW-55CNH welding wires,as well as MAG welding and laser-MAG hybrid welding,the formation law of SUS301L/Q450NQR1 dissimilar steel welds and the microstructure and properties of the joints were systematically studied.The results show that the type of welding wire has a significant effect on the microstructure of the joint weld.When the ER309LSi welding wire is used,the microstructure of the weld is mainly composed of austenite+a small amount of ferrite;while when the CHW-55CNH welding wire is used,the weld is mainly composed of proeutectoid ferrite+acicular ferrite.Using ER309LSi welding wire,the grain size of each area of the laser-MAG hybrid welding joint is smaller than that of MAG welding;in terms of mechanical properties,the tensile properties of the joint obtained by ER309LSi welding wire is better;the mechanical properties of the hybrid welding joint are generally better than those of MAG welding.Finally,the corrosion resistance of SUS301L/Q450NQR1 dissimilar joints in3.5%Na Cl neutral solution was studied,and the corrosion laws of dissimilar steel butt joints were revealed through electrochemical and salt spray corrosion tests.Combined with the analysis results of polarization curve and impedance map,it is found that the corrosion resistance of the weld obtained by ER309LSi welding wire is better than that obtained by CHW-55CNH welding wire,and there are two time constants in the weld impedance spectrum obtained by the two welding wires,and the equivalent circuit diagram is R（Q（R（QR）））;in addition,the passivation film R_cp and charge transfer resistance R_ct of the weld obtained by ER309LSi welding wire are larger,which shows that its corrosion resistance is better;combined with the SEM results and the salt spray corrosion test,it can be seen that the corrosion resistance of the weld obtained by the ER309LSi welding wire is relatively good;compared with MAG welding,SUS301L/Q450NQR1 dissimilar steel laser-MAG hybrid welding seam has better corrosion resistance.Through the composite welding process,the disadvantage of relatively poor corrosion resistance of CHW-55CNH wire can be effectively improved.