| In the high-power traction motor for rail vehicles,the joints of the winding outgoing lines and outgoing lines with the conducting ring are joined by resistance brazing.During the trains operating,because of the electrical load and high heat load,high speed operation,frequent starting and braking,the welding positions of the motor easily fracture by changing distorting force,electromagnetic force,centrifugal force and thermal stress etc.The cause of the fracture is connected with brazing thermal cycle making the material performance decline.Studying the characteristics of resistance brazing welding temperature field and the influence of welding thermal cycle on the joint microstructure and performance have important reference value for production and the safe operation of the motor.In this paper,the resistance brazed joints of traction motor stator lead wire for rail vehicle were taken as the research subject.Firstly,the welding thermal cycle curves at the special locations of the joints were measured.Then the thermal-electric coupling models for lead wire during resistance brazing process were established in finite element software MSC.Marc.Through the comparison with the actual welding thermal cycle curves measured and numerical simulation welding thermal cycle curves at the same location to modify the simulation models,to complete numerical simulations of some typical joints resistor brazing processes of the stator,and to study the characteristics of its welding temperature fields and welding thermal cycles,and analyzed the influence of changes in the welding process parameters on the temperature field,and provided the theoretical basis for optimizing the welding process parameters.The results show that during the heating process,the highest temperature in the transverse center of the lead occurs in the center of the brazing joint.The maximum temperature of the lead and loop,the lead and the neutral ring occurs at the contact between the lead and the graphite electrode.The time of heat affected zones reaching the maximum temperature is later than the central area of the brazing seam.The welding temperature reaches a maximum at the time of stopping heating,and then enters cooling stage.The cooling rate is slower than the heating rate compared to the heating stage.The temperature distributions of the three joints along the lateral direction of the leads are slightly different.The temperature field of the lead resistance brazing is symmetrical,and the temperature field of the lead and loop and the lead and the neutral ring resistance brazing are similar,showing an asymmetrical form.The longitudinal highest temperature of the three joints of the lead is not in the center of the brazing joint.The brazing joint front end,brazing seam center,brazing joint end and heat affected zone are successively reduced.The temperature in the highest temperature zone is higher than the center of the brazing joint by about 10?30 0C.The temperature of the edge of the lead joint brazing seam is 85?120?lower than the center of the brazing joint.The temperature of the position of the lead and the loop is 30?60? lower than the center of the brazing joint.The temperature of the lead and the neutral ring is 50?77? lower than the center of the brazing joint.The temperature gradient of the lead joint is steeper than that of the other two joints,and the distance transmitted by the heat to the extended end of the lead is short,and the effect of the welding thermal cycle on the lead heat-affected zone is lower than that of the other two joint types.Considering the reliable connection of brazing joints and reducing the effect of welding heat cycle on the performance of the base metal,the reasonable welding parameters for the leads welding is 1.9kA of welding current,5s of welding time,and the reasonable welding parameter of lead wire and loop is 1.98kA of welding current,7.7s of welding time,and reasonable welding parameters of lead and neutral ring is 2.14kA of welding current,6.7s of welding time. |
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