Research On Surface Microstructure And Characteristics Using The Ultrasonic Vibration Assisted End Milling

With the rapid development of high-end equipment such as aviation,defense,bioengineering and precision machinery,the performance,reliability,and service life of the components should be greatly improved.However,the conventional machining can not match the increasing precision and surface quality requirements.Titanium alloys is widely used in advanced manufacturing such as aerospace,military and artificial intelligence,due to their high hardness,high specific strength,high temperature resistance and corrosion resistance.This leads to its cutting performance poor.In addition,defects such as surface grooves and micro-cracks in the machined parts cut down the fatigue cycle of materials,which is particularly presented in high-strength materials.Generally,the surface properties of high-performance parts often depend on the surface microstructure characteristics,and thus the surface microstructure must be effectively controlled during processing.The designation and manufacture of surface microstructures have a promising application in drag reduction,vibration resistance,viscosity reduction,desorption,wear resistance and noise reduction.To address the above problems,the morphological characteristics,creation mechanism and surface characteristics of ultrasonic assisted end milling microstructures under different vibration directions are studied.The performance of the key components can be improved by controlling the micro-structural features of the surface through the control of the vibration direction and processing parameters.The general design theory and method of inhomogeneous media ultrasonic amplitude variation system are studied on the base of acoustic theory.The longitudinal-torsional composite ultrasonic milling system is realized,and the ultrasonic vibration milling platform is built on the CNC machining center.The influence mechanism of the motion trajectory on the microstructure of the machining surface is studied after analyzing the trajectory of the tool tip by ultrasonic vibration milling kinematics.The two-dimensional orthogonal vibration ultrasonic milling tool-workpiece separation conditions is obtained:when the number of teeth of the tool is N=4,only when conditions that the feed amplitude ratioδ<2 and the oscillating ratio is not 4 integral multiples are satisfied,the adjacent two cusp trajectories will intersect to form a closed region.When“?”is an odd multiple of 2,the distance in the X direction of the closed area is the largest,and the distance in the Y direction increases as the“?”decreases.In the longitudinal-torsional composite vibration ultrasonic milling,the concept of"torsional vibration ratio"is proposed.Whenηis constant,that is,the ratio of the vibration frequency to the spindle rotation speed keeps constant the form of the knife tip trajectory is consistent.When the rotational speed is increased,the high-speed ultrasonic composite machining can be realized by appropriately increasing the vibration frequency.According to the spatial transformation theory,the geometric model of ultrasonic assisted end milling surface microstructure is established,and a"reduction approximation algorithm"is proposed to solve the three-dimensional surface of the workpiece.A surface micro-structural simulation tool for ultrasonic vibration end milling is developed and operated well using the software of Matlab.Based on orthogonal experiment and single factor experiment,the effects of longitudinal and torsional composite end milling parameters and acoustic parameters on surface roughness are studied.It is concluded that the primary influence processing parameters on the surface roughness value in order is the feed per tooth,the ultrasonic amplitude,the axial cut depth and the milling speed.The variation trend of the roughness curve of different torsion longitudinal amplitudes are consistent,where the curve of A_t/A_l=0.3 is relatively flat.The surface roughness value of the machined surface obtained by ultrasonic end milling in different vibration directions conforms to the following rules:the surface roughness value of the two-dimensional orthogonal vibration milling is the smallest,followed by the R_a value obtained by the longitudinal-torsional composite vibration milling when the torsion longitudinal amplitude ratio is A_t/A_l=0.3 and the longitudinal vibration along feeding direction.The surface roughness value obtained in the other vibration directions is large.Aiming at the typical vibration micro-units formed on the surface of ultrasonic vibration end milling,the microstructure characteristics of milling surfaces in different vibration directions are studied.The longitudinal-torsional composite vibration milling surface changes with the increase of longitudinal amplitude ratio from a pit-like microscopic surface to a sheet-like texture or strip-like texture.A regular sinusoidal scale microstructure is produced by the longitudinal vibration along feeding direction.Quadrilateral microstructure appears on the surface of two-dimensional orthogonal vibration milling.The surface microstructure unit becomes sparse as the milling speed increases and the structure size is large.On the contrary,when the milling speed is low,the vibration texture is dense and fine.In order to obtain the tribological properties of the processed surface texture,the friction and wear properties of dry friction surfaces are investigated.It is indicated that the friction coefficient of the ultrasonic vibration milling surface is significantly lower than that of ordinary milling,and the running-in phase is shortened with the amplitude A.The surface of the longitudinal torsional composite vibration milling has the lowest friction coefficient of the machined surface when A_t/A_l=0.21.During the ultrasonic vibration in the feed direction,the friction coefficient is the smallest when the ultrasonic amplitude is A=3μm,of which there is no obvious running-in phase.Compared with the conventional milling,the friction coefficient is decreased by 60%.For the two-dimensional orthogonal vibration milling,the friction coefficient is minimum when the ultrasonic amplitude A=2μm.The fractal characteristics of ultrasonic machining are analyzed based.It is concluded that the fractal dimension of the longitudinally torsional composite vibration ultrasonic milling is relatively large,and the fluctuation with the processing parameters is relatively stable.The axial vibration and the fractal dimension vibrating in the feed direction are relatively small,and fluctuate greatly with the processing parameters.The fractal dimensions of axial vibration and vibration in the feed direction are relatively small,and it fluctuates greatly with the processing parameters.Fractal dimension of ordinary milling is the smallest and the fluctuation is the largest.When the fractal dimension is large and the surface roughness value is small,the machined surface with a small friction coefficient can be obtained.