Study On Dynamic Deformation Viscosity Behavior And Chip Formation Mechanism Of TC4

TC4 titanium alloy is widely used in various fields such as national defense and medical treatment due to its excellent mechanical and biological characteristics.However,the current research on TC4 is generally below 104s-1 strain rate,which is inconsistent with the conditions of ultra-high strain rate in actual high-speed cutting.Whether TC4 has the same plastic deformation control mechanism as copper,aluminum and other materials near 103s-1 strain rate remains to be further studied.Therefore,this paper will explore and verify the dynamic deformation viscous behavior of TC4 by SHPB experiment,orthogonal cutting experiment and two-dimensional simulation verification,and attempt to explain the viscous behavior and chip formation mechanism of material during actual deformation from a micro perspective by theoretical analysis methods such as dislocation increment,Orowan formula and chip evolution theory.In the first part,three TC4 cylinder specimens(Φ3×3mm,Φ2×2mm,Φ2×1mm)were subjected to single variable dynamic impact tests at 20℃ and high temperature with SHPB device to study the effects of strain rate(1×103s-1～1.7×104s-1),work hardening,multiple loading and temperature(20℃～550℃)on their viscous behavior.The results show that the material is strengthened with the increase of the number and density of internal dislocations during dynamic impact,resulting in a positive correlation between the rheological stress and strain rate of the material.However,the hardening rate of the material decreases slightly with the increase of strain rate and the proportion of dislocation increment decreases.The control mechanism of plastic deformation of materials changes from thermal activation to dislocation damping mechanism between 5000s-1 and 6000s-1,which shows the sharp increase of rheological stress and the phase change of yield stress-logarithmic strain rate curve before and after the transition point.Multiple loading increases the internal stress of the material as in forging and strengthens to a certain extent,but dislocation entanglement makes it difficult to deform the material and the material will be destroyed when the accumulated internal stress reaches its limit.High temperature has significant effect on thermal softening of TC4.In addition,JC constitutive parameters of TC4 are obtained by fitting the data collected in this chapter.In the second part,the viscous behavior of TC4 at higher strain rates was studied by orthogonal cutting experiments.The chip under realized conditions(cutting speed 50.24 m/min～703.36m/min)is measured in microstructure and hardness in different directions.The results show that the sticker of TC4 is serious at high cutting speed and the macro surface roughness of chip increases with the increase of cutting speed.After the cutting speed reaches 100.48m/min,the chip shape changes from band to serrated,and the degree and frequency of serration increase until the brittleness of the material increases and the chip changes to knuckle-shaped chip.With the increase of cutting speed,the shear band changes from deformation band to deformation band+transition band and then to transition band completely,and the white layer phenomenon becomes more and more serious.During this process,the grains in the material are elongated and aggregated to form an adiabatic shear band until the distortion fracture forms ultrafine grains which are not visible under a high power microscope.The microhardness near the shear band and the cutting first deformation zone increases significantly.The third part uses JC parameters fitted by SHPB experiment to simulate the cutting experiment with Abaqus simulation software,and then compares the results of simulation and experiment to verify its reliability.The cutting experiments are refined and expanded in terms of tool rake angle,cutting depth and workpiece temperature from the angles of strain,cutting force and temperature.The simulation results show that,as expected,the chip will turn into nodular chip after 2700 m/min cutting speed.TC4 is controlled by dislocation damping mechanism during cutting at ultra-high strain rate.During material removal,the internal stresses are mainly concentrated in the first deformed area of the tool tip and near the front of the chip root.In addition,both the rake angle of the tool and the cutting depth have a significant influence on the size,fluctuation and frequency of the cutting force.Due to thermal softening,a higher initial temperature of the workpiece results in a smoother chip surface and a smaller chip crimp radius.