Microstructure Evolution Of Near Surface Layer And Mechanical Properties For Ti-Al Intermetallic Compounds In Ultrasonic Vibration Assisted Machining

Ti-Al intermetallic compounds,as a kind of rare alloy material with high strength and light weight under high temperature,are currently the most promising alternative to titanium alloy and high-temperature alloy as the next generation of high-performance components for aviation engines due to their ability to improve the engine’s thrust-to-weight ratio and self-adaptability in harsh environments.However,the material performance of Ti-Al intermetallic compounds represented by Ti₂AlNb-based alloy is still in the exploratory stage,and there are disadvantages such as low machining efficiency,high cost and poor cutting performance in the precision machining of this kind of material,which seriously affect the service performance of the parts and become one of the key challenges in development of high-performance components for large thrust to gravity ratio aviation engines.Ultrasonic assisted machining technology has been widely used in the manufacturing of high-performance parts of difficult-to-machine materials in high-end equipment,due to its ability to effectively utilize ultrasonic energy to promote material flow,change material deformation behavior,exhibit excellent cutting performance,and effectively improve the microstructure of the near surface layer of processed components.In order to promote the application of ultrasonic vibration machining technology in Ti₂AlNb-based alloys,Ti-22Al-24Nb-0.5Mo was taken as the research object in this paper to study the macro-constitutive relation,dynamic mechanical properties and macroscopic deformation mechanism of the material under high strain rate and high-temperature coupling.Micro-constitutive model,deformation behavior and fracture failure behavior of the material under ultrasonic vibration were obtained.The fundamental theory and key experimental phenomena research of longitudinal-torsional ultrasonic vibration assisted milling（L-TUVAM）were investigated to reveal the ultrasound-induced mechanism of cutting performance,machined surface and near surface layer structural evolution of Ti₂AlNb material under the coupling of multi-energy fields.The main research content and findings are as follows:（1）Based on the high-temperature synchronous separated Hopkinson pressure bar（SHPB）system and macroscopic phenomenological mechanics principles,the dynamic mechanical properties and macroscopic deformation mechanism of Ti₂AlNb material were studied in a wide strain rate and temperature range.Focusing on the rheological behavior of the material,the sensitivity of its force-thermal parameters was analyzed,and the mathematical model of its adiabatic temperature rise was established.The strain compensation and adiabatic temperature rise were introduced for model optimization,and the macroscopic Johnson-Cook improvement constitutive relation of Ti₂AlNb was constructed.The adiabatic shear band formed by thermo-viscoplastic constitutive instability of materials and its propagation law were obtained.The fracture characteristics and evolution mechanism of microstructure in the adiabatic shear band were revealed.The macroscopic deformation mechanism of materials under the coupling mechanism of force-thermal was elucidated.（2）An ultrasonic vibration assisted tension device suitable for Ti₂AlNb material was developed,and the deformation behaviors of material such as ultrasonic effect,mechanical behavior,strength-plasticity-hardness variation characteristics were studied under ultrasonic vibration.Based on the dislocation density evolution theory and crystal plasticity theory,the interactions between the stress superposition effect,acoustic thermal softening effect,acoustic softening effect and acoustic residual hardening effect were introduced to construct the microscopic Ultrasonic-K-M hybrid acoustic constitutive model.The internal mechanism of acoustic softening/hardening under ultrasonic action was analyzed,and the microstructure evolution and fracture mechanism of the material under ultrasonic vibration were revealed.The ultrasonic induction mechanism and scientific essence of the formation of Ti₂AlNb near-surface layer by ultrasonic machining are clarified.（3）A shear stress constitutive model of Ti₂AlNb materials in the shear zone under the force-thermal-acoustic multi-field coupling was established based on the kinematic separation characteristics of the cutting edge and strain rate variation characteristics of the workpiece in longitudinal-torsional ultrasonic vibration assisted machining.According to the micro-element oblique cutting theory and geometric model,the analytical model of milling force for longitudinal-torsional ultrasonic vibration assisted machining of Ti₂AlNb was developed by analyzing the shear stress of the material in the shear zone,the impact force of micro-element longitudinal and torsional vibration.Based on the diffusion model of finite-length inclined/horizontal moving line heat source,the temperature models of the shear plane heat source group and rear cutter surface-workpiece friction extrusion heat source group under ultrasonic vibration were established.The effects of shear strain and ultrasonic action on heat flux density and distribution coefficient were studied,and a analytical model of milling temperature for longitudinal-torsional ultrasonic vibration assisted machining of Ti₂AlNb was constructed.The fundamental laws of cutting performance and sensitivity for machining parameters of Ti₂AlNb in multi-dimensional ultrasonic vibration machining were obtained.（4）The longitudinal-torsional ultrasonic vibration platform was built to carry out the experimental study on Ti₂AlNb material.The influence law of milling process parameters（machining parameters:radial depth of cut,spindle speed,feed rate;acoustic parameters:ultrasonic amplitude）on cutting performance（milling force and milling temperature）,machined surface（surface microstructure and defect characteristics,surface 3D microform and roughness,surface microhardness,surface physical phase and three-phase distribution characteristics）and sub-surface layer（plastic deformation disturbance layer,work hardening phenomenon,dislocation tissue morphology and dislocation density）properties of Ti₂AlNb were obtained.The essence of precision,high efficiency and high performance in multi-dimensional ultrasonic vibration compound milling of Ti₂AlNb material was expounded.Compared with conventional milling,it was found that the maximum reduction in milling force and milling temperature of L-TUVAM were 48.55%and 37.09%,and the minimum value of surface roughness was0.276μm,and the maximum surface microhardness was 482HV,and the maximum average hardening rate within the depth of hardened layer was 115.48%,and the maximum depths of plastic deformation disturbed layer and violently disturbed layer were 25.14μm and 5.24μm.The amount of dislocation density accretion required for Ti₂AlNb material removal was effectively reduced by 1.37 times,and the entangled dislocation morphology transformed into parallel dislocation morphology distributed near the dislocation wall.This study has demonstrated that ultrasonic vibration-assisted machining process of the inherently brittle Ti₂AlNb-based alloy,is a high-quality,efficient and actively controllable precision machining method for surface conditions.It aprovides a theoretical basis and technical support for the promotion and application of high-performance components in aero-engines,and is of great significance for accelerating the development of the next-generation of aviation engines in China.Number of Figures:100,Number of Tables:16,Number of References:210.