Simulation Study On Upsetting Process Of Rail Flash Butt Welding Based On Finite Element Method

Seamless track is the main component of modern railway,and rail welding is a key link in the construction of seamless lines.Flash butt welding(FBW)has become one of the most important methods of rail welding due to its advantages of high degree of automation,good joint quality,fast welding speed and stable welding quality.The quality of the rail FBW joint mainly depends on the reasonable temperature field distribution,the state of the end surfaces before the upsetting process and the upsetting process matching the temperature field.Among them,the temperature field distribution and the state of the end surface before upsetting can be adjusted by changing the flashing process.Due to the special nature of rail FBW,there is no shielding gas during the welding process,so it mainly relies on the metal vapor generated at high temperature during the flashing process and the self-oxidation of the high-temperature blasting liquid metal particles to prevent oxidation of the welded joint.Oxide inclusions are inevitably formed on the end surface before upsetting,if the oxide inclusions remain in the joint after welding,gray spots will be formed and crack sources will be generated,hazardous substances must be extruded through a suitable upsetting process.Therefore,it is very necessary to study the plastic deformation of upsetting process and the effect of upsetting deformation on impurity extrusion behavior.Owing to the complex working conditions of rail FBW,it is difficult to compare the effects of different processes on the extrusion of harmful impurities in the upsetting process by debugging the welding parameters.In addition,the internal metal flow cannot be observed and the experimental cost is very high.Therefore,this paper establishes a finite element model of the rail FBW upsetting process by numerical simulation,and explores the plastic deformation law of the upsetting process as well as the influence of different thermal field distributions and different upsetting processes on the impurity extrusion behavior.In this paper,the thermal-mechanical coupling model of FBW upsetting process of U75 V rail was established by ABAQUS finite element software.The Johson-Cook constitutive equation was used to describe the plastic deformation behavior of the welded joint during upsetting process,and the plastic deformation law of the high-temperature metal at the end surface of the joint during upsetting was obtained;The effect of different temperature fields and upsetting parameters on the extrusion behavior of impurities was studied by embedding solid gray impurities on the end faces of the joints;The influence of the feeding position on the current distribution and the influence of the charged upsetting process on the joint temperature field are analyzed by establishing the electro-thermal coupling model of the preheating stage of preheating FBW and the charged upsetting stage.Simultaneously,the Infrared Thermal Camera was used to monitor the dynamic changes of the temperature field of the joint during the upsetting process to verify the reliability of the model.The analysis results of the upsetting process show that the areas where the displacement of the plastic metal is large are mainly concentrated in the rail top,the rail bottom,the joint of rail web and rail bottom,and the joint of the rail head and rail web.When the lateral deformation of the joint is not considered,selecting the proper upsetting length and increasing the width of the high-temperature plastic zone are beneficial to the extrusion of harmful impurities.Besides,using variable speed upsetting or increasing the speed of the early upsetting also facilitates the extrusion of impurities.However,the impurities in the deeper area of the rail are difficult to move,and they are easy to form gray spot defects once the oxide inclusions remain.The simulation results in the preheating stage show that the electrode feeding method on the rail web of the mobile FBW electrode will cause the current to concentrate on the rail web,which is consistent with the "blue" phenomenon of the rail web during the high current welding in the actual welding process.The fixed FBW will make the current mainly through the rail head and rail bottom,"Blue" phenomenon occurs on the rail top and rail bottom.The electric field distribution obtained by the hybrid electrode clamping method is the most uniform.The results of electric field simulation analysis and the heat affected zone morphology both show that the electrode feeding method of the fixed FBW is more conducive to the uniform heating of the joint area.The simulation and temperature measurement experiments in the charged upsetting stage show that the charged upsetting will directly change the temperature field formed by flashing process,the temperature gradient and the high-temperature plastic zone width can be controlled by adjusting the charged upsetting time and voltage.The clamping method of the mobile FBW electrode will make the isothermal surface of the rail web wider during the charged upsetting stage,which is consistent with the conclusion obtained by the thermal imaging temperature measurement experiment.In the fixed FBW electrode clamping method,the whole joint heating is relatively uniform during the charged upsetting stage.