Study On Material Selection And Technology Of Spot Welding Electrode Cap Of Unequal Thickness Coating Plate For Railway Vehicle

Carbon steel car body skeleton of railway vehicles and wallboard used coating anticorrosive coating of high strength weathering steel manufacturing,in the resistance spot welding,surface coating makes the welding between the current density is reduced,the distribution of the current field is not stable,lead to plate weldability dropped significantly,process window is narrow,greatly reduce the service life of the electrode cap,need frequent sharpening or replacement,affect the production.Because of the high strength and thickness of the weathering steel used for railway vehicle,the spot welding experience of ordinary low carbon steel coating plate is difficult to be simply transplanted and used.Therefore,it is of great significance to study the spot welding technology and the failure mechanism of spot welding electrode cap and determine the appropriate material of electrode cap for the manufacturing of carbon steel rail vehicle body.This paper through the numerical simulation analysis of the characteristics of the welding temperature field under different process parameters,choose the optimal process parameters,after using the parameters for the actual welding,and the process of spot welding joint performance study,finally using five kinds of material of electrode cap for life test and combining with the analysis of failure mechanism of the electrode cap,the best electrode cap material.The results show that for spot welding of 3mm+2.5mm thick high strength coating plate of railway vehicle,it is necessary to adopt prepressing,preheating and"hard specification"and combine with higher electrode pressure to avoid spatter and other problems.The welding current and welding time have a great influence on the temperature field distribution of the spot welding joint.Based on the comprehensive analysis of the temperature field distribution and the performance of the electrode cap,the optimal technological parameters are welding time 1200ms,welding current 11k A and electrode pressure 10k N.Under the optimum process parameters,the nuclide zone of the joint is the mixed structure of martensite and ferrite,and the hardness distribution is between 250 and 400HV.The microstructure of the heat-affected zone is a small amount of ferrite+pearlite,and the hardness distribution is between 220 and250HV.There are no shrinkage holes and cracks in the joint.Comparative analysis found that after the service life of the electrode cap made of different materials,the main form of electrode cap failure for softening and plastic deformation caused by the transverse diameter grew up,at the same time accompanied by viscous loss,alloying and recrystallization phenomenon,its basic reason is that the cooling caused by bad electrode cap position in contact with the workpiece surface temperature rise too high,these factors influence each other,eventually led to the interaction between electrode cap failure.Considering the thermal-physical properties,welding process and service life of the five materials,Cu Cr Zr alloy is the most suitable electrode cap material for spot welding conditions of this project compared with Cu Ni Be,Al₂O₃ dispersion reinforced copper and W75Cu alloy materials.The service life of electrode cap made of Cu Cr Zr alloy is greatly improved after cryopreservation treatment.