| Ti-6Al-4V and carbon fiber reinforced polymer(CFRP)/Ti-6Al-4V stacks are widely used in the field of aviation and aerospace equipment manufacturing because of their high specific strength and high specific stiffness.A large number of connecting holes need to be processed during the assembly of such materials components,and the requirements for hole processing quality,accuracy and reliability are more stringent.Ti-6Al-4V and CFRP are difficult-to-machine materials with poor machinability.It is difficult to ensure the machining quality of Ti-6Al-4V and CFRP/ Ti-6Al-4V stacks.In this paper,the axial and longitudinal torsional ultrasonic vibration helical milling process is proposed,the theoretical analysis and experimental research on the cutting behavior of Ti-6Al-4V and CFRP/ Ti-6Al-4V stacks by ultrasonic vibration helical milling are carried out,the motion trajectory and cutting speed prediction model of the bottom cutting edge and peripheral cutting edge relative to the machining hole surface are established,and the contact behavior between the cutting edges and the machining hole surface is studied.The micro material deformation and removal mechanism of Ti-6Al-4V and CFRP/titanium alloy stack in ultrasonic vibration helical milling are revealed,and the formation mechanism of hole surface micro texture and its action mechanism on hole diameter accuracy and hole surface integrity are analyzed.It provides theoretical support and process strategy for high quality and low damage holemaking of CFRP/Ti-6Al-4V stacks.The main work of this paper is as follows:(1)The trajectory models of the bottom cutting edge and peripheral cutting edge in the axial ultrasonic vibration helical milling of Ti-6Al-4V relative to the machined surface of the hole are established.A method is proposed to calculate the vibration frequency in the actual machining process based on the periodic cutting texture on the hole bottom surface and the kinematic relationship between the bottom cutting edge.According to the calculated cutting edge motion trajectory and the relative cutting speed between the tool and workpiece,the effect of cutting and vibration parameters on the tool chip separation state are obtained,and the mechanism and conditions of micro impact and scratching behavior of the cutting edge on the machined surface are revealed,which provides a theoretical basis for ultrasonic vibration helical milling.(2)The micro element cutting force ratio model of axial ultrasonic vibration helical milling hole and helical milling hole is established.Based on the analysis of undeformed chip thickness/width,the relationship between the micro element cutting force ratio of axial ultrasonic vibration helical milling and helical milling on the reduction ratio of macro axial force is analyzed by calculating the removal volume of discrete micro element material.It provides theoretical guidance for reducing axial cutting force in hole-making process and improving hole processing quality.(3)The calculation model of periodic texture spacing of machined surface of helical milling and axial ultrasonic vibration helical milling is established.Because the periodic cutting texture spacing of machined surface in axial ultrasonic vibration helical milling is less than that in helical milling,the hole surface roughness is better than that of helical milling.At the same time,the influence mechanism of the periodic cutting texture spacing between the machined surface of helical milling and axial ultrasonic vibration helical milling on the diameter error is analyzed.Under the action of axial ultrasonic vibration,the peripheral cutting edge has micro impact and scratch effect on the hole surface in the axial direction,the plastic deformation occurs in the range of 4-6μm depth below the hole surface layer,which changes the physical and mechanical properties of the hole surface/sub-surface.Compared with the helical milling without ultrasonic vibration,the machined surface microhardness increases by 2%-9.1%,and the residual compressive stress in the axial and circumferential directions increases by70.1%-85% and 63.5%-172.1% in axial ultrasonic vibration helical milling,respectively.(4)The trajectory model of cutting edge in longitudinal torsional ultrasonic vibration helical milling is established,and the influence of the phase difference between longitudinal and torsional vibration on the trajectory shape and direction of cutting edge is analyzed.When the phase difference satisfies 0<φ<π,the motion trajectory of the bottom cutting edge is clockwise in the xoz plane.When the vertical velocity of the bottom cutting edge is greater than the chip outflow velocity,the friction force on the chip is consistent with the chip outflow direction,which is conducive to increasing the shear angle and reducing the axial force.Compared with helical milling,the burr height at the outlet of Ti-6Al-4V is reduced by 6.3%-55%,the surface roughness is reduced by 5.63%-31%,and the hole axial and circumferential residual stresses are increased by 59.8%-75% and 55.81%-92.3% in longitudinal torsional ultrasonic vibration helical milling,respectively.Compared with the helical milling,the error value of the outlet diameter is reduced by 0.001-0.009 mm under different cutting speed and axial feed conditions in longitudinal torsional ultrasonic vibration helical milling.Combined with the relative motion trajectory between the peripheral cutting edge and the hole surface by axial and longitudinal torsional ultrasonic vibration helical milling technology,the effects of axial and radial cutting forces and micro texture of hole surface are analyzed.(5)Based on the kinematic analysis of the cutting edge in axial and longitudinal torsional ultrasonic vibration helical milling process and the study of the micro material removal mechanism,the variation laws of the instantaneous cutting temperature when the tool cutting into the thermocouple position at the CFRP/Ti-6Al-4V stacks interface and the maximum temperature generated by heat conduction at the same position in the whole hole machining process are studied.The variation law of the machined hole diameter of CFRP/ Ti-6Al-4V stacks due to the existence of laminated interface is analyzed.The influence mechanism with/without ultrasonic vibration and the the mode of ultrasonic vibration on cutting force,cutting temperature and tool wear is systematically analyzed.The variation laws of hole diameter,CFRP hole surface damage,delamination and burr damage and flank wear with the number of machined holes in the process of helcial milling on CFRP/ Ti-6Al-4V stacks components are studied.Combined with the kinematic analysis of cutting edge in ultrasonic vibration helical milling,the cutting force and cutting heat results,the cutting mechanism of ultrasonic vibration helical milling of CFRP/Ti-6Al-4V stacks is clarified,which provides a process strategy for high precision and low damage hole-making of CFRP/Ti-6Al-4V stacks. |
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