Study On The Deformation And Extrusion Load Of Equal Channel Angular Pressing

Ultra-fine grained(UFG)matertals with the outstanding mechanical properies and special physical property has attracted much attention by material science researcher in recenet years.The equal channel angular pressing or extrusion technique,ECAP/ECAE,is a severe plastic deformation process employed to produce bulk ultra-fine grained materials.During ECAP processing,a well-lubricated billet is forced to pass through two intersected channel with constant cross-sectional area.The workpiece undergoes a large amount of plastic strain by simple shear within the deformation zone located at the channels die intersection.The mechanical properties of the sample are impoved greatly by SPD,and grain size refined,micro structure improved.In this paper,the deformation process of Equal channel angular pressing was analyzed and formulas for simple shear strain and normal strain were proposed according to the principle.The formulas were verified by using finite element methods.The validity of numerical simulation results were further verified by the experimental results.The reasons of inhomogeneity were found by finite element methods,inhomogeneous structure was determined by strain status.The extrusion load formula was proposed according to slip line solution,upper bound solution and finite element methods.And experiments were taken to validate the formula.By studying the principle of simple shear deformation,the formula of shear strain was proposed.According to the results of formulas,the shear strain was linearly dependent on the difference value between the ratio of fillet radius to channal width((Rint-Rext)/D)and the intersection angle ? determined the slop of strain curves.New formulas were verified by finite element simulation.The simulation results indicated that the velocities of the points from different zone were different in the sample and the motion trajectories of different points didn't follow geometrical laws,which cause inhomogeneity of shear strain.Considering the influences of the average velocity and the motion trajectory,a more comprehensive formula of shear strain during ECAP process was proposed.The microhardness value and martensitic phase transformation results of the 304L austenitic stainless steel and the Ti49.2Ni50.8 alloys were analyzed.The results showed that the deformation was larger in the central zone of the specimen,and the simulation results were verified.By studying the principle of normal deformation,the normal strain formulas of shear strain was proposed-According to formula,the normal strain was linearly dependent on the difference value between the ratio of fillet radius to channal width((Rint-Rext)/D)and the intersection angle ? determined the slop of strain curves,also.If Rext-Rint=D or the internal angle was 180°;the normal strain was zero,namely,there was no nomal strain.New formulas were verified by finite element simulation.A comprehensive formula of normal strain was proposed by using actual fillet radius to replace fillet radius of dies.According to metallograph of 304L specimen,there was compression strain in the external zone and tensile stain in the internal zone.The influence of die parameters,material parameters and routes on the inhomogeneity of the ECAPed sample was studied by simulation.The results showed that the sample was more homogeneity prepared by ECAP dies with R0-1R0-0.5 according to residual sum of squares results.Considering the strain status,dies with R0-0.3R0-0.2 were better.The homogeneity were better with increasing the internal angle ? and work hardening exponent n.With the increaseing of friction coefficient,the homogenetiy increased first and then decreased.The the samples prepared by route C after 2 passes and route BC after both 4 and 8 passes were more homogenous,due to rotating between consecutive passes in different routes and the less strain in external zone.The simulation results were verified by metallograph and microhardness test.Compared with the annealed material,the yield strength and tensile strength of the ECAPed sample increase while elongation decreases.The equivalent strain was a direct proportion to strength and microhardness,and the distribution of equivalent strain was expressed by microhardness results.The microstructure was determined by strain status,and the material performance was determined by microstructure.So the strain status in different zones of ECAPed sample should be taken into consideration when the inhomogeneity was studied.The resistance included deformation resistance and frictional resistance during ECAP processing.According to slip line solution and upper bound solution,the same formula of deformation resistance was proposed.The deformation resistance formula was verified by finite element simulation.The influence of extrusion process and material parameters on deformation resistance were investigated by FEM.The curves of actual load have two sections.The difference value between peak load of different length sample was constant in the first stage.The value was proportional to the difference of the length.The slope of the curve was the same in the second stage.The slope was proportional to the difference between the stress of true stress and true strain compression curve.The frictional resistance was proportional to the deformation resistance,and the proportional coefficient was twice as friction coefficient.The strength parameters of the formula in different zone were confirmed by the curves of actual load.The extrusion load formula was verified by actual load-stroke curve.