Research On Surface Residual Stresses Generated By Machining Ti6Al4V Titanium Alloy

Becaused of its great intensity,good resistance to high temperatures and chemical corrosion,and some other good performances,Ti6Al4 V alloy is widely used in aerospace engineering.Meanwhile,Ti6Al4 V alloy is a hard-to-machine material,the machining efficiency is very low,the surface residual stresses generated by machining will significantly affect the static strength,fatigue life,corrosion resistance and some other performances,as well as the service life of the machined workpiece,furthermore,and the stability of the geometry size of the machinined workpiece will be affected.Therefore,it is necessary to study and control the machining induced surface residual stresses to improve the properties and service life of the machined component.In this paper,the surface residual stresses generated by machining Ti6Al4 V alloy were studied in details based on theoretical analysis,experiments and finite element simulations.The major works can be summarized as follows:(1)Measurement of the surface residual stresses generated by machining thin-walled Ti6Al4 V alloy tube part.The surface residual stresses were measured based on strain changes and FEM correction method,furthermore,the effects of the cutting parameters on the surface residual stresses were analyzed based on the experimental results.In addition,the machinined part was annealled to analyzed the the effect of the corresponding heat treatment to the residual stresses.(2)Measurement of the surface residual stresses generated by milling Ti6Al4 V alloy.The method of measuring the surface residual stresses in pre-equilibrium state based on bending deflection changes and strain changes were proposed.Based on the finite element analysis results,it can be found that the distributions of the surface residual stresses in equilibrium state are so different from that in pre-equilibrium state,and the final state of the surface residual stresses in equilibrium state highly depends on the rigidity of the workpiece.In order to evaluate the surface residual stresses objectively without the effect of the workpiece rigidity,a measuring method based on layers removal and inverse calculation was proposed.(3)Prediction of the workpiece deformations based on the measurement of the equivalent residual stresses.As too many procedures and too much work will be needed to measure the distribution of the surface residual stresses,for the purpose of deformation prediction,the method for measuring the equivalent residual stresses and the depth of its affected layer was proposed.According to the corresponding experimental results,it can be found that the proceures and work needed will be largely cut down;furthermore,the accuracy of the deformation prediction is very high.(4)Finite element simulations of the surface residual stresses generated by machining Ti6Al4 V alloy.The effects of the cutting regime parameters and tool geometries on the surface residual stresses were studied based on the orthogonal cutting model.Because of the limitations of the orthogonal model and the low efficiency of three-dimensional model,the effects of feed rate per revolution and tool corner radius were studied based on the orthogonal indentation model.Based on the comparison of the results from experiments and simulations,it can be found that the predicted results based on the combination of the two orthogonal models can be accurate enough to evaluate the effects of the cutting regime parameters and tool geometries.(5)Research on the creep behavior of Ti6Al4 V alloy.The creep stress exponent of Ti6Al4 V alloy at room temperature was measured based on nano-indentation experiment.The Displacement-Load curves obtained under different loading rate were almost the same,it can be concluded that the displacement of the indenter only depends on the maximum load.The stress exponents obtained under different maximum loads loaded under different loading rates are almost the same,which shows the loading rate cannot affect the experimental result significantly.Based on the relationship of the changing rate of displacement and the material hardness,the creep stress exponent of Ti6Al4 V alloy at room temperature was obtained at last.Furthermore,the creep deformations of the workpiece caused by the machining induced surface residual stresses under different temperatures were studied to evaluate the effects of temperatures on the deformations of the workpiece.