Finite Element Simulation And Experimental Study Of Chip Formation Process In Cutting Titanium Alloy Ti6Al4V

Titanium alloy is rapidly becoming one of the most important engineering materials with excellent physical properties,and its poor cutting performance has become a typical difficult to machine materials that the serrated chip is the most common feature of chip.In this dissertation,the titanium alloy is employed as research objects.Their serrated-chip's forming process,evolution mechanism and influence of cutting parameter on geometric characteristics are studied by cutting test and finite element simulation.First of all,the cutting experiments of titanium alloy Ti6A4 V was carried out by orthogonal method.The geometrical characterization of chips and cutting forces in experiments were analyzed.The results show that the cutting force with the increase of the feed rate is also gradually increased,and is proportional to the increase with the increase of cutting depth,but is slowly decreased with the cutting speed.On the other hand,the cutting force increases gradually with the rake angle decreases and the feed force influence is more significant.Additionally,the degree of segmentation increases with the increase of cutting speed and feed rate,and performs opposite trends with the rake angle.The segmentation frequency improves with the cutting speed increases,but decreases gradually with increasing the rake angle and with decreasing the feed rate.Second,microstructure characteristics in adiabatic shear bands were observed and analyzed in different cutting conditions.With the increase of serrated chip deformation,the microstructure characteristics change from deformed band to transformed band.Because of the non-uniform strain distribution in the adiabatic shear band,the width decreases from tool tip to top surface of cutting layer.The crack at the top of the serrated chip are initiated at the juncture of adiabatic shear band and subsequent trapezoidal block,and propagate along the adiabatic shear band toward the tool tip direction.Then the influence of rake angle on crack is significant,the crack is obvious when the rake angle is 10°.Finally,a finite element model for orthogonal cutting about titanium alloy Ti6A4 V was constructed using the software ABAQUS,and the accuracy of the finite element model was verified through the contrast analysis of cutting force and chip shape.Based on the analysis of the finite element simulation results,the sawtooth chip formation process consisting of rudiment of sawtooth,sawtooth segmented and serrated chip is studied and a three stage formation model is developed.This paper discusses the evolution mechanisms at different stages.Firstly,the shear-slipping deformation begins to appear in much lower cutting layer near the tool-tip region due to temperature weakening and gradually forms a region,and if the rate of material thermal diffusivity is far lower than heat generation rate in the cutting process,this region extends in the direction from tool tip to top surface of cutting layer,which shows as "spikes".When the material near the tool-tip region reaches the state of critical thermoplastic instability,the shear-slipping region instantly extends to the top surface of cutting layer.Then the entire shear-slipping region is at the state of critical thermoplastic instability and the rudiment of sawtooth is finished which is made of the shear-slipping region and micro-deformable block.Secondly,the shear-slipping region generates local shear-slipping deformation and destructive shear surface with cutting layer material when the tool-tip moves ahead from the center area of the shear-slipping region,and then sawtooth segment is shaped.Finally,the metal is still in the state of thermoplastic instability in the local shear-slipping region after the sawtooth segment enters the second deformation zone.Under the pushing of the rake face and the continuously growing block of subsequent rudiment of sawtooth,the sawtooth segment continues to generate local shear-slipping deformation,the destructive shear surface becomes bigger and the boundary of subsequent rudiment of sawtooth is generated until it leaves the second deformation zone,and the serrated chip will finally be formed.