Study On Cutting Force And Surface Integrity Of TC18 Titanium Alloy By Longitudinal-torsional Ultrasonic Vibration-assisted Milling

Near-β titanium alloys are considered a promising development trend of highstrength titanium alloys.As a typical near-β titanium alloy,TC18 titanium alloy features excellent comprehensive properties,including low density,high fracture toughness,and high hardenability.It is the titanium alloy with the highest strength under annealing conditions.At present,TC18 titanium alloy has been widely applied in structural parts(e.g.,landing gear)and load-bearing parts,receiving increasing attention in recent years as an important aeronautical material.However,it has the same cutting characteristics as other series of titanium alloys,namely severe tool wear,large cutting force,and uncontrollable processing quality,which restricts its further application and development.Ultrasonic vibration cutting,especially multi-dimensional ultrasonic vibration cutting,can improve the shortcomings of titanium alloys during cutting due to its pulse enhancement effect and ability of intermittent cutting.Thus,it is a good choice for titanium alloys to adopt ultrasonic vibration cutting.However,existing research on the ultrasonic vibration cutting of titanium alloys mainly focuses on atypical Ti-6Al-4V alloys.Regarding process research,turning,grinding,and drilling are hotspots,with few studies of milling reported.Therefore,this paper put forward to apply longitudinal-torsional ultrasonic vibration-assisted milling to the processing of high-strength TC18 titanium alloy.Its cutting force,surface morphology,surface roughness,surface residual stress,and subsurface microstructure were all investigated.On this basis,the paper made clear the influence laws of ultrasonic amplitude and milling parameters on cutting force and machined surface integrity during the longitudinal-torsional ultrasonic vibration-assisted milling of TC18 titanium alloy.The main contents include the followings:(1)This paper analyzed the conversion mechanism underlying the longitudinaltorsional ultrasonic vibration of the horn of a helical groove type and the kinematic trajectory of the tool tip,separation speed characteristics,and impact ironing characteristics of longitudinal-torsional ultrasonic vibration-assisted milling.Considering the ultrasonic softening effect,a mathematical model of ultrasonic vibration-assisted milling force was established with the help of a comprehensive analysis regarding the generation of vibration impact force,chip formation force,and friction force in light of the oblique cutting theory in end milling.In addition,analysis was also made on the generation mechanism and influencing factors of surface morphology and surface residual stress in the case of conventional milling and ultrasonic milling respectively.This part lays a theoretical foundation for subsequent experimental research.(2)Depending on the theoretical analysis of cutting force,single-factor experiments were conducted to explore the influence of ultrasonic amplitude,feed per tooth,and cutting speed on cutting force,with the results compared with those of conventional milling.The experiments proved that the longitudinal-torsional ultrasonic vibrationassisted milling can reduce cutting force.More specifically,proper selection of ultrasonic amplitude,feed per tooth,and cutting speed can effectively reduce cutting force during longitudinal-torsional ultrasonic vibration-assisted milling.(3)Single-factor experiments were also employed to investigate the influence of ultrasonic amplitude,feed per tooth,and cutting speed on the micromorphology,roughness,and residual stress of milled surfaces.This aims to give full play to the advantages of the longitudinal-torsional ultrasonic vibration-assisted milling in improving the surface integrity of parts,obtain controllable micromorphology of machined surfaces,and improve surface quality after machining.The research shows that proper settings of ultrasonic amplitude,feed per tooth,and cutting speed are conducive to ideal surface micromorphology and larger surface residual compressive stress.(4)The depth of the subsurface deformation layer,the grain boundary of the material structure,and the grain structure state of specimens were studied at different ultrasonic amplitudes,feeds per tooth,and cutting speeds.The influence of longitudinal-torsional ultrasonic vibration-assisted milling on the structural behaviors of subsurface structure and matrix structure are clarified and compared with that of conventional milling.The results indicate that ultrasonic vibration-assisted milling is capable of refining surface grains and enables a deeper and severer plastic deformation of subsurface,thereby strengthening the machined surface.This research provides an experimental research basis for the surface strengthening and processing of parts made of TC18 titanium alloy by ultrasonic milling.(5)With ultrasonic amplitude,cutting speed,feed per tooth and depth of cut as experimental factors and cutting force during face milling,surface roughness,and surface residual stress as evaluation indexes,an orthogonal experiment was designed to study the significance levels at which the experimental factors influenced the evaluation indexes.The factor–level combinations were evaluated,and the optimal combination was finally determined.This dissertation lays an important foundation for realizing high-quality cutting of TC18 titanium alloy and broadening the application scope of ultrasonic vibration cutting.In addition,it has application prospects in surface processing of TC18 titanium alloy by longitudinal-torsional ultrasonic vibration-assisted milling and also the quality control of machined surfaces.