| The titanium alloy is widely used in the field of aerospace because of its excellent characteristics of corrosion resistance,high temperature resistance,high strength and low density.It is the main material for aeroengine components and aircraft structural parts.However,the titanium alloy has poor thermal conductivity,small elastic modulus and high chemical activity,which makes the surface quality difficult to control in the machining process,and which seriously affects the service performance and service life of the parts,then it is a typical hard to machine material.With the rapid development of advanced cutting methods,the titanium alloy has made great progress in cutting,especially in ultrasonic vibration assisted cutting,which shows good machinability.However,the forming essence and control mechanism of surface microstructure of the titanium alloy in ultrasonic vibration assisted cutting are still in the exploratory stage,which lacks systematic and in-depth theoretical analysis and research.It is the high stress,high strain rate and high temperature produced in the ultrasonic vibration assisted cutting process,which leads to severe plastic deformation of the material and makes the microstructure evolution more complex.In this paper,the ultrasonic longitudinal torsional vibration side milling of titanium alloy Ti6Al4 V is taken as the research object,and the surface micro characteristics and microstructure evolution are studied to explore the essence and formation mechanism of surface microstructure of titanium alloy Ti6Al4 V processed by ultrasonic longitudinal torsional side milling.Since the force is the key factor to reveal the microstructure evolution of the material to be processed,the kinematic characteristics and the theoretical modeling of milling force for ultrasonic longitudinal torsional milling are studied in this paper:based on the kinematics theory,the cutting edge trajectory model of ultrasonic longitudinal torsional vibration milling is established,and the influence rules of different rotating speed,feed speed,ultrasonic frequency and longitudinal torsional amplitude ratio on the cutting edge trajectory are studied.The influence of ultrasonic longitudinal torsional vibration on the cutting edge motion trajectory is integrated into the common milling cutting force model,and the ultrasonic longitudinal torsional composite vibration is established The experimental and regression analysis methods were used to establish the empirical prediction model of milling force coefficient in ultrasonic longitudinal torsional vibration milling range,which verified the feasibility of the model.In order to study the relationship between the tool path,force and the micro topography of the machined surface,the surface morphology characteristics of ultrasonic longitudinal torsional vibration side milling are studied in detail: the surface morphology of ultrasonic longitudinal torsional vibration side milling is studied.Compared with the surface morphology of ultrasonic side milling and conventional side milling,the influence of different acoustic parameters on the surface morphology was analyzed.Combined with the ultrasonic longitudinal torsional cutting edge movement track and cutting force change signal in Chapter 2,the mapping relationship between the cutting edge motion track,cutting force and the machined surface topography is given.Based on the theoretical model of the machined surface residual height of the conventional side milling,the theoretical model of the residual height of the machined surface in the ultrasonic longitudinal torsional compound vibration side milling is established.Combined with the formation mechanism of the machined surface contour,the simulation model algorithm is designed,and the simulation model of the machined surface topography by the conventional side milling and the ultrasonic longitudinal torsional side milling is established,and the validity of the simulation model is verified by experiments.In order to study the surface microstructure characteristics of ultrasonic milling under high strain rate,the surface residual stress of titanium alloy machined by ultrasonic longitudinal torsional milling is studied: the surface residual stress of titanium alloy machined by ultrasonic longitudinal torsional milling is studied.Based on the equivalent plane method of oblique cutting model,the mechanical stress model of ultrasonic longitudinal torsional milling is established.Based on the workpiece temperature field theory of komanduri,the thermal stress model of ultrasonic longitudinal torsional milling is established.Based on the mechanical stress and thermal stress of ultrasonic longitudinal torsional milling,the analytical model of residual stress in ultrasonic longitudinal torsional milling is established,which is verified by numerical simulation.Through the finite element analysis,the transient loading temperature of ultrasonic longitudinal torsional milling titanium alloy Ti6Al4 V when it reaches the phase transformation temperature is extracted,and the distribution of residual stress under different processing parameters when the transformation temperature is reached is analyzed,and the experimental verification is carried out.In order to reveal the evolution law of the surface layer microstructure of the titanium alloy Ti6Al4 V by ultrasonic milling,the plastic deformation mechanism of titanium alloy machined by ultrasonic longitudinal torsional milling is simulated and experimentally studied.The mechanism of plastic deformation of the titanium alloy machined by ultrasonic longitudinal torsional milling is studied.Based on the theory of elastic-plastic loading and unloading,the prediction model of surface plastic deformation depth by ultrasonic longitudinal torsional milling titanium alloy is established.Through the establishment of finite element model,the surface plastic deformation simulation of the titanium alloy Ti6Al4 V processed by ultrasonic longitudinal torsional side milling is simulated,and the influence law of different process parameters on plastic deformation is revealed,and the single factor test of Ti6Al4 V machined by the ultrasonic longitudinal torsional side milling is designed to verify the model.With the help of back scattering electron diffraction(EBSD),the grain orientation dispersion characteristics and grain orientation difference distribution on the subsurface layer of machined surface of the titanium alloy Ti6Al4 V are analyzed,and the influence mechanism of different plastic deformation conditions on grain refinement is revealed.Finally,based on the acoustic theory,and a high-speed ultrasonic longitudinal torsional milling system test platform is built on the precision carving machine,and the micro characteristics of ultrasonic longitudinal torsional side milling titanium alloy are studied by applying vibration to the cutter.The grain refinement,phase transformation and microhardness are qualitatively and quantitatively analyzed by combining mechanism analysis and experimental test technology.The influence of microstructure evolution of ultrasonic longitudinal torsional side milling on microhardness of surface layer and sub surface layer of titanium alloy Ti6Al4 V is analyzed: with the increase of milling speed,the effect of grain refinement on the change of work hardening rate gradually increased,but it still existed at the optimal value.Therefore,it can be revealed that the essence of improving the surface microhardness and microstructure and properties is the grain refinement caused by the coupling effect of pulse force and heat in ultrasonic machining.There are 106 figures,19 tables and 162 references. |
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