| Titanium alloy is a new alloy material developed in 1950 s,it is widely used in biomedicine,aerospace,artificial intelligence and other fields due to its special material properties such as high specific strength,low density,good corrosion resistance,good fatigue resistance and excellent biocompatibility.However,titanium alloy is a typical difficult-to-machine material,which is prone to cause problems such as large cutting force and cutting heat,rapid tool wear,and low machining efficiency in micro-milling.Ultrasonic assisted machining can effectively solve the defects of conventional micro-milling technology due to its intermittent cutting characteristics,which has become a research hotspot in the field of titanium alloy micro parts processing.In this paper,taking TC4 titanium alloy as the research object,the surface formation mechanism and the evaluation of surface functional characteristics of longitudinal-torsional ultrasonic micro-milling were studied,combining longitudinal-torsional ultrasonic vibration with ordinary micro-milling technology.The main research contents and conclusions are as follows:Based on the wave equation,the structure design and simulation analysis of the single excitation longitudinal-torsional ultrasonic horn were carried out.The longitudinal-torsional ultrasonic micro-milling system of TC4 titanium alloy was built and its vibration performance was tested.The results show that the system has good electro-acoustic conversion efficiency and longitudinal-torsional conversion ratio,which meets the test requirements.The kinematics of the machining process of longitudinal-torsional ultrasonic micro-milling was analyzed,and the instantaneous undeformed chip thickness model and longitudinal-torsional ultrasonic micro-milling force model were established.It is found that the introduction of longitudinal-torsional ultrasonic vibration can reduce the instantaneous undeformed chip thickness and micro-milling force,which is helpful to improve the surface quality of longitudinal-torsional ultrasonic micro-milling of TC4 titanium alloy.Considering the influence of radial runout,flexible deformation and elastic recovery of micro-milling cutter,the cutting edge trajectory equation of longitudinal-torsional ultrasonic micro-milling of TC4 titanium alloy was established,which is suitable for the actual machining.On this basis,the probability model of the residual material height of TC4 titanium alloy longitudinal-torsional ultrasonic micro-milling surface was established.Surface topography of TC4 titanium alloy in longitudinal-torsional ultrasonic micro-milling was simulated by MATLAB software.The simulation results show that the distribution of convex peaks and concave valleys on the surface of TC4 titanium alloy longitudinal-torsional ultrasonic micro-milling is more uniform and dense compared with ordinary micro-milling.The effects of different process parameters and ultrasonic amplitude on the three-dimensional surface morphology of TC4 titanium alloy in longitudinal-torsional ultrasonic micro milling were investigated by single factor experiment.The experimental results show that the longitudinal-torsional ultrasonic micro-milling surface of TC4 titanium alloy exhibits fish-scale microstructure.With the increase of spindle speed,feed per tooth and milling depth,the fish-scale microstructure on the workpiece surface gradually disappears and the surface quality gradually deteriorates.In the range of 0-3μm,the fish scale microstructure on the workpiece surface becomes more and more dense and uniform with the increase of ultrasonic amplitude,and the flatness of the workpiece surface is gradually improved.Based on the evaluation criteria of three-dimensional roughness parameters and the theoretical model of three-dimensional roughness parameters,the prediction model of three-dimensional roughness parameters of ultra-precision machining surface was established.Several three-dimensional roughness parameters of different classifications are selected to constitute the evaluation parameter group,and the evaluation indexes of three surface functional characteristics of ultra-precision machining surface were established,including fatigue resistance,wear resistance and lubrication performance.Through single factor experiment and orthogonal experiment,the influence of process parameters and ultrasonic amplitude on the evaluation parameters of surface functional characteristics of longitudinal-torsional ultrasonic micro-milling of TC4 titanium alloy was explored,and the optimal processing parameters of each functional characteristic within the range of experimental parameters were obtained.The test results show that the fatigue resistance,wear resistance and lubrication performance are reduced to varying degrees with the increase of spindle speed,feed per tooth and milling depth.The increase of ultrasonic amplitude can effectively enhance the fatigue resistance,wear resistance and lubrication performance of TC4 titanium alloy longitudinal torsional ultrasonic micro milling surface.Ultrasonic amplitude and spindle speed have significant effects on the evaluation parameters of surface functional properties.Under the condition of high spindle speed and ultrasonic amplitude,low feed per tooth and milling depth,longitudinal-torsional ultrasonic micro-milling of TC4 titanium alloy can obtain better surface functional characteristics. |
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