Milling Induced Surface Integrity And Its Effects On Fatigue Life Of The Titanium Alloy Ti6Al4V

In the fields of aerospace and defense industries,the use of titanium alloys can not only meet the requirements of lightweight design,but also ensure the high reliability and long service life of machined components.However,the material properties of titanium alloy with low elastic modulus and low thermal conductivity,and the microstructure characteristics with the mixed forms of ca and β phases make it a typical difficult to machine material.Optimization and control of surface integrity are the key technology to guarantee the function and service life of machined titanium alloy components.Previous studies of the machined surface integrity mainly concentrated on the analysis of data from many experiments,while the generation and control mechanism of the machined surface integrity was rarely involved.It still lacks deep insights into the physical and mechanical properties of the machined surface integrity.Using a systematic approach with combination of machining process simulation,cutting testing,surface integrity prediction,and fatigue life analysis of machined components,the thesis aims to elucidate the generation mechanism of machined surface integrity and reveal the relationship between the machined surface integrity and fatigue life.Firstly,machined surface topography characteristics in peripheral milling of Ti6A14V are analyzed.The effects of cutting parameters on surface defects and surface topography parameters are discussed.The physical properties of the machined surface topography generation mechanism are then revealed.Autocorrelation analysis of machined surface profiles is carried out to realize the quantitative evaluation of surface defect,and the response pattern between machined surface topography parameters and cutting parameters is obtained.In addition,the surface topography prediction model,which takes into account of the effects of cutting tool geometrical parameters,cutting parameters and error parameters,is established.The results show that the surface defect quantity has a change of descends firstly then ascends with the increase of cutting speed.Increasing the feed per tooth will lead to an increase of surface defect quantity,while the effect of the optimized cutting depth on the surface defect quantity is not significant.Surface topography parameters are most sensitive to feed per tooth,followed by cutting speed and then radial depth of cut.The larger milling cutter radius tends to generate relatively small machined surface profile height and cycle values,and the greater the difference of the numerical values,the more rough the machined surface.With the increase of the eccentricity of milling cutter,the distance between adjacent peaks and the peak-valley height difference increases.The eccentric angle causes the translation of the machined surface profile.but has less effect on the periodic and height characteristics.Secondly,the theoretical analytical model,the finite element simulation model and the hybrid prediction model are proposed to clarify the generation mechanism of the machined surface residual stress.The peripheral milling tests are carried out to verify the effectiveness of the proposed models.The results show that the models are reasonable and effective in judging and analyzing the form and distribution of the machined residual stress field.With the cutter rake angle changes from-5° to 5°,the peak value of compressive stress in the cutting direction becomes larger,and the reverse transition depth decreases.However,the depth of residual stress has no obvious variation.At the cutting width direction,the depth of the reverse transition and the residual stress decrease,while the change of the peak value for compressive stress is not obvious.With the cutting edge radius of cutting tool changes from 0.01 mm to 0.03 mm,the value of surface residual stress increases,and the peak value of compressive stress and the depth of reverse transition decrease gradually,while the change of the residual stress depths is not obvious both in the cutting and cutting width directions.When the value of cutting speed or feed per tooth is small,the machined surface residual stress is compressive.With the values of both cutting speed and feed per tooth increase,the residual stress changes from compressive to tensile stress,and the peak value of compressive stress decreases,while the depth of reverse transition and residual stress increase.Thirdly,the plastic strain of machined surface is investigated.The prediction model of plastic deformation depth is established to reveal the machined surface plastic deformation mechanism.Meanwhile,a quantitative description method based on the image recognition for the metallographic volume fraction of dual phase titanium alloy is presented.By means of metallographic identification technology and micro hardness tests,the effect of cutting parameters on grain refinement,metallographic change and micro hardness of the machined surface are discussed.The results show that the plastic strain of machined surface is mainly related to the yield strength,elastic modulus,linear thermal expansion coefficient of the machined material and temperature change,while the depth of plastic deformation mainly depends on the value of cutting force.The rate of grain refinement and phase volume fraction of the machined surface is inversely proportional to the cutting speed and the radial depth of cut,and the hardening rate increases with the increase of cutting speed and radial depth of cut.Feed per tooth has no significant effect on the grain refinement,the variation of the β phase volume fraction and the hardening rate of the machined surface.Finally,the high and low cycle fatigue behaviors of Ti6Al4V samples are investigated,and the macroscopic and microscopic properties of fracture are revealed.The effects of surface integrity induced by various cutting conditions on the fatigue properties under low cycle fatigue and high cycle fatigue are analyzed,respectively.The optimization intervals of cutting parameters are presented.The results show that the fatigue source region of the high cycle fatigue fracture shows a brittle fracture,while a ductile fracture exhibited under the low cycle condition.The fatigue stress concentration factor is the most important one for the low cycle fatigue life when the residual stress meets the relaxation condition.When the residual stress does not relax,the influence of residual stress on the fatigue life is smaller than that of the surface micro hardness,which is higher than that of the fatigue stress concentration factor.Optimization intervals of cutting parameters are those at cutting speed 20～50 m/min,feed per tooth 0.02～0.03 mm/z and radial depth of cut 0.5～1.0 mm.