Preparation of Ultrafine Grain TC4 Alloy by Non-isothermal ECAP Technology and Its Pulse Current Treatment

Author:Zhao Zhi Yong

Supervisor:wang guo feng hou hong liang


Degree Year:2019





Compared with the traditional materials,ultrafine grain titanium alloy has obvious advantages in strength,toughness,plasticity and wear resistance.Equal channel angular pressing(ECAP)technology is one of the most effective methods to prepare ultrafine grain metals,and has been proved to produce the ultrafine grain TC4 alloy.However,the technical difficulty and cost of ultrafine grain TC4 alloy produced by ECAP are both very high.Therefore,the ultrafine grain TC4 alloy was successfully prepared by non-isothermal ECAP process with improving the mold optimal design and operatingconditions.The mechanical properties of ECAP-produced ultrafine grain TC4 alloy were also discussed.The microstructure and mechanical properties of TC4 alloy with severe plastic deformation were both improved by the pulsed electric current treatment.Compressing and shearing are the two main plastic deformations in ECAP process.The compressing and shearing deformationbehaviors of TC4 alloy were studied based on heating with pulsed electric current,and discussed the effects of thermal and non-thermal effects induced by pulsed electric current on the microstructure and mechanical properties of TC4 alloy.Meanwhile,the non-isothermal deformation behavior of TC4 alloy was also investigated.On one hand,Joule heating effect was the main reason for the decreasing of deformation resistance,and the temperature variation of TC4 alloy samples was closely related to current density,size of sample,contact pressure,andadditives,etc.on the other hand,the dynamic recrystallization and α → β phase change were induced by non-thermal effects of pulsed electric current in compression deformation;intracrystalline deformation was also increased by pulsed electric current in the rapid shearing deformation and avoiding microcracks.The non-thermal effect of pulsed electric current plays an important role in improving the plastic deformability of TC4 alloy.The results of mechanics characteristic curves indicate that higher deformation rate is benefit for the plasticity of TC4 alloy in the non-isothermal deformation characterized by gradually decreasing of temperature,which provides reference for the non-isothermal ECAP process of TC4 alloy.The non-isothermal ECAP process of TC4 alloy was successfully operated by improving heat conduction and lubrication conditions,decreasing process time and increasing extrusion speed.The ultrafine grain TC4 alloy was produced by four-pass ECAP and the cumulative shear strain has been reached to 4.5.Lots of cellular structure and grains with hundreds of nanometers were observed in the ECAP-produced TC4 alloy,and the gradually increasing twins indicate that twinning is an important coordination mechanism during the multi-pass ECAP process.The results of mechanical properties measurements show that the microhardness and compressive strength of ECAP-produced ultrafine grainTC4 alloy at room temperature were significantly increased by 6.5% and 12%,with corresponding values of 392 HV and 1435 MPa,respectively.The thermo-compressive behavior of TC4 alloy produced by ECAP(Post-ECAP)and original TC4 alloy(Pre-ECAP)were comparativeanalyzed by Gleeble experiments.The strength of Post-ECAP is higher than Pre-ECAP at 500 ℃,but the plasticity of Post-ECAP is better than Pre-ECAP at temperature higher than 600 ℃,which characterized by a lower deformation resistance.Fastener were prepared by hotupsetting-extrudingmethod from Pre-ECAP and Post-ECAP at temperature of 650~700℃ and a high strain rate.There is a evident crack at the end of fastener produced from Pre-ECAP,which indicates that the hot deformability of Post-ECAP is better than Pre-ECAP.However,the room temperature tensile tests show that the strength of fastener produced from Post-ECAP reached 1190 MPa,and 15.5% higher than that of fastener produced from Pre-ECAP.Those results prove that the ultrafine grain TC4 alloy has advantages in both deformability and mechanical properties.The strength of ECAP-produced ultrafine grain TC4 alloy is very high at room temperature;however,its plasticity is really poor due to the high dislocation density caused by the severe plastic deformation.Annealing treatment is the traditional technology to improve the plasticity of materials with severe plastic deformation,but the mechanical properties will be inevitably decreased due to grains growth in the long time annealing process at high temperature.The ECAP-produced ultrafine grain TC4 alloy was pretreated by pulsed electric current instead of traditional annealing.The results show that high dislocation density and unstable microstructure in ECAP-produced TC4 alloy transform into low dislocation density and stable homogeneous submicron scale grains underthe action of pulsed electric current,accompanied by the phase transformation of α→β.The results of tensile tests show that elongation of ECAP-produced TC4 alloy was significantly improved by 66.5%,while its strength still retain very high.In comparison with traditional heat treatment process,pulsed electric current treatment is a high efficiency,and environmentally friendly method to improve the mechanical properties and microstructure of metal materials,particularly those produced by severe plastic deformation.