Research On The Microstructure Refinement And Mechanical Properties Of Pure Titanium Processed By Multipass ECAP

Equal Channel Angular Pressing(ECAP),as a processing technique of severely shear deformation,was used for processing bulk materials with ultra-fine grained(UFG)which was accomplished by simply shearing when billet was pressed through the intersection of die channel.In this paper,DEFORM is a finite element simulation software which was used to analyze the variation of extrusion force,plastic flow of metal,and equivalent strain distribution of high purity titanium when it passed the mold corner.According to the results of finite element simulation.at the start,the extrusion pressure was increased,and then decreases in the process of pure titanium ECAP.In the process of metal flow,the maximum concentration of equivalent stress is located at the corner of the mold tunnel and the maximum equivalent stress at the corner also increases with the sharp trend,the elongation direction of the tissue is about 40° to the extrusion direction.The maximum stress area is concentrated at the corner of the mold,which is be 63°with the extrusion direction.In the later stage,the independently designed circular channel mold with internal angle of 120° and external fillet of 20° was successfully used to finish 4 passes of high purity titanium with 135 route and BC route,and 8 passes of BC route.Through OM,EBSD,DSC,hardness detection methods,the micro structure of ECAP samples and the trend of mechanical properties of the samples were analyzed.The deformation mechanism,microstructure refinement process and the effect of different annealing temperature on the microstructure evolution of pure titanium were studied.The main results are as follows:(1)After a single extrusion of high purity titanium,a large amount of twins appear in the tissue,and the twin effect plays a key role in the formation of fibrous texture.Compared with the first 4 passes,the X-axis grains are equiaxed and have no obvious direction,and dense fibrous structures appear on the Y-surface.The average sizes of the grains after 4 passes with BC route and 135 route 4 passes are 2.82?m and 2.36?m,and the large-angle grain boundaries in the micro structure are 22.3%and 25.9%respectively.The refining effect of the 135 path deformation specimen is better than that of the BC path.After ECAP extrusion,the transformation point of the pure titanium varies according to paths stress caused by stress cycle and the degree of organization of the fine together.(2)BC route for 8 passes are finer than BC route of 4 passes,and fibrous tissue has obvious preferential orientation.The grain size is mainly distributed in the range of 0.56?17.48?m with the average grain size of 0.9?m.The proportion of large-angle grain boundaries in 8 passes with BC route ways reaches 28.6%.(3)During annealing at 500-700?,the recrystallized grains grow along the belt of the deformed zone,and the route of 135 remains small state.However,the recrystallized grain growth phenomenon is seriously in the BC route.The thermal stability of the 135 route remains deformation structure Which is better than BC route.The hardness of the specimens with two different routes after annealing at 700? is decreased sharply.The hardness of the fourth passes of route 135 decreased from 172.7HV to 122.7HV.The hardness of fourth passes of BC route decreased from 164.3HV To 119.5HV.the sample hardness with BC route at 8 passes is dropped from 180.9HV 125.2HV.