Thermodynamic Behavior Of Ultrafine Grained Pure Cooper Material

As one of the newly developed severe plastic deformation methods,large strain extrusion machining(LSEM)has becoming the focus of the industry for its strong ability of efficiently producing ultrafine grained material with a good geometry.However,the ultrafine grained material obtained by this process has a small grain size,high dislocation density and plenty of lattice defects,which makes the deformation stored energy and the atomic activity of the material increase.During the temperature rise environment,the ultrafine-grained material may undergo a microstructure transition,leading to performance degradation.Therefore,in order to clarify the microstructure and performance evolution of the ultrafine-grained material during the heating process and determine the application range of the material,the thermodynamic behavior of ultrafine-grained pure copper materials prepared by LSEM was systematically studied in this paper.Besides,the effects of LSEM parameters,such as rake angle and chip thickness compression ratio,on deformation parameters,microstructure and thermodynamic behavior of ultrafine grained copper were investigated.The main contents of this paper are as follows:(1)The LSEM processes were simulated in this paper and the effects of LSEM parameters,such as tool rake angle and the chip thickness compressive ratio,on the equivalent strain,strain rate,stress,cutting temperature and main cutting force were analyzed.The results show that the smaller the rake angle or the greater the chip thickness compressive ratio,the greater the strain and the more severe the deformation of the material.(2)The microstructure of ultra-fine grained copper was observed by using super-depth microscope,metallographic microscope,scanning electron microscope and EBSD.The hardness value of the material was measured.And the influence of the LSEM processing parameters,such as tool rake angle and the chip thickness compressive ratio,on the microstructure and mechanical properties of ultra-fine grained copper were analyzed.The results show that the smaller the rake angle or the greater the chip thickness compressive ratio,the finer the grain of the ultrafine grained copper,and the higher the hardness of the material.(3)The ultrafine-grained pure copper material was subjected to isochronous and isothermal annealing heat treatment.The effects of annealing temperature and holding time on the microstructure,texture and hardness of the ultrafine-grained pure copper were analyzed by ESBD observation and hardness measurement.The results show that,with the annealing temperature rising,ultrafine-grained pure copper material has experienced a recovery,recrystallization and grain growth stage.Also,the hardness values are subjected to a slight decrease,a drastic drop and a slow descent in three stages.With the extension of the holding time,the internal structure of the material is more uniform,the grain remains stable after a slight growth,and the hardness value remains stable after a slow descent.(4)With the help of EBSD observation and hardness measurement,the influence of the parameters such as tool rake angle and chip thickness compression ratio on the thermodynamic behavior of ultrafine-grained pure copper was analyzed.The results show that the smaller the rake angle or the greater the chip thickness compression ratio is,the lower the initial recrystallization temperature and the lower the recrystallization temperature of the ultrafine-grained pure copper material become.