Study On Grain Refinement And Texture Of Machined Surface Layer Of Titanium Alloy

As a two-phase alloy(a phase and P phase),Titanium alloy has been widely applied in industry,especially in aerospace industry,due to its excellent mechanical properties,corrosion resistance and high strength-to-weight ratio,etc.But because of these mechanical advantages,titanium alloy has become a typical difficult-to-machine material,mainly due to the severe plastic defonnation during machining,resulting in severe work hardening on the machined surface,which then affects the surface quality,service life and so on.In this thesis,the plastic deformation of titanium alloy was studied from two,i.e.grain size and grain orientation.In this thesis,the orthogonal milling experiment of titanium alloy was carried out at first.The cutting forces were measured and the chips under different cutting parameters were collected.Then a two-dimensional orthogonal milling model was established using the Johnson-Cook constitutive model and shear failure model by ABAQUS/Explicit software,and the validation of the finite element model was verified by the cutting force and chip shape obtained in the experiment.The results show that the errors of cutting force and chip shape between the experiments and simulations are in an acceptable range(all less than 12%),which means the established finite element model is relatively reliable.Then the cutting force,the plastic deformation and the strain rate are analyzed under different cutting parameters of the experiment and the simulation,the results show that:(1)With the cutting speed increasing,the cutting force and the plastic deformation decreases.With the increase of feed,the cutting force and the plastic deformation increases.(2)Shear strain is the major stain in the cutting process,and the change of strain rate can be divided into three stages:Initial stage,the increasing stage of strain rate,the decreasing stage of strain rate,On the basis of the finite element simulation model,the grain refinement in the machining of titanium alloy was simulated.The subroutine of the Johnson-Mehl-Avrami-Kolmogorov(JMAK)grain size model was embedded into the established finite element model,and the grain size in the machining of titanium alloy was simulated.The results show that:(1)With the increase of cutting speed,the cutting force decreases,resulting in the decrease of plastic deformation and the grain refinement.(2)During the cutting process,the temperature has a weakening effect on the grain refinement of the titanium alloy,and the weakening of the temperature in the chip shear zone is more obvious than that in the machined layer.On the basis of strain and strain rate obtained under different parameters,the texture of the machined surface is simulated by VPSC program,and texture polar and ODF diagrams are made by using MATLAB.The results show that the typical shear textures B,P,Y,C1 and C2 fiber textures are fund on the machined surface.And the polar density decreases with the increase of cutting speed,and increases with the feed increasing.Then,the existence of texture was verified by the fact that the micro-anisotropy on the six sides of the titanium alloy close-packed hexagonal will lead to the macroscopic hardness difference of the material.The microhardness of the cross section surface(normal to the cutting speed direction)and the longitudinal section surface(parallel to the cutting speed direction)at the same depth below the machined surface are different,indicating that textures are formed in the machined titanium alloy.Finally the texture densities at different depths under the machined surface were simulated,and the microhardnesses at different depths were measured and analyzed.The results show that grain refinement and grain orientation at the meso-level are the reasons for the work hardening at the macro-level.