Machining Mechanism Of Vibration Asissted Cutting And Machinability Investigations Of Typical Difficult-To-Machine Materials

Vibration-assisted machining technology is a cutting process in which a certain frequency of micron amplitude is applied to the cutting tool in order to realize tool-workpiece separation periodically;thus,some aspects of performance better than conventional machining can be obtained.It has shown remarkable superiority in the processing of difficult-to-machine materials and has attracted more and more scholars' attention.Due to the variety of materials and different characteristics of difficult-to-machine materials,the progress of vibration-assisted cutting experiments is still insufficient,and the cutting mechanism is still under study.In this paper,the widely used titanium alloy is represented as the representative,and the mechanism of vibration-assisted cutting is analyzed and validated by experiments.Besides,experimental investigations on machinability of three typical difficult-to-machine materials are presented in order to provide experimental basis for the processing mechanism.An improved analytical model for orthogonal cutting of chip formation is presented,which can predict chip formation and cutting force of continuous and sawtooth chip.Based on the non-equidistant shear zone model and the Calamaz modified Johnson-Cook material model,the strain field,strain rate field and temperature field in the primary shear zone are predicted.A simplified tool-chip interface model is established based on the characteristic of material flow in tool-chip interface.An improved,accurate,efficient prediction model of orthogonal cutting process is proposed based on the coupling relationship between the primary shear zone and the second deformation zone.The relationship between serrated chip geometry and variables in shear zone is established.Prediction models of chip formation and cutting force for one-dimensional and elliptical vibration cutting are proposed.The principle of one-dimensional vibration-assisted cutting kinematics,as well as the length of transient tool-chip contact length and transient shear angle in a single cycle are analyzed.The variation of the shear angle of ordinary cutting and one-dimensional vibration-assisted cutting with machining parameters is verified by experiments.The transient depth-of-cut and transient shear angle of elliptical vibration cutting are analyzed.The principle of cutting force fluctuation of main cutting force and thrust force in each stage of elliptical vibration cutting is described.The microstructural evolution model of surface and chip in vibration-assisted cutting was established,and the conclusion that high-frequency vibration assisted cutting can achieve low damage processing was found.Based on the analysis and comparison of the three material constitutive equations,the finite element model of vibration-assisted cutting was established.By introducing the material dynamic recrystallization model,the dynamic recrystallization of the machined surface and chip after vibration assisted cutting and conventional cutting was compared.The recrystallization grain size and average grain size were used to verify the effects of the two processing methods on the surface and chip.Experiments show that the average grain size of the processed surface of high-frequency vibration-assisted cutting(ultrasonically assisted cutting)is larger than the average grain size of conventional cutting,which is closer to the grain size of the material matrix.The average grain size of machined surface of high-frequency vibration-ssisted cutting distributed more evenly along the depth.The machinability of nickel base superalloy in high-frequency vibration-assisted turning is investigated,and the conclusion that vibration-assisted turning can reduce the net value of the tensile residual stress produced by conventional turning was found.The cutting force,surface topography and surface roughness,residual stress of conventional turning and high-frequency vibration-assisted turning are performed respectively with Inconel 718 and 625 alloys.When the cutting speed is lower than the critical speed,the feed and cutting depth are lower and the vibration amplitude is higher,the high-frequency vibration-assisted turning can reduce the cutting force significantly.The surface profile shows that the surface of the vibration assisted turning is smoother than that of conventional turning,and the high-frequency vibration-assisted turning can improve the surface topography under different processing parameters.The improvements of surface roughness by high-frequency vibration-assisted turning decrease with the increase of feed and increase with the increase of vibration amplitude.The residual stress on the machined surface of nickel-based superalloy by high-frequency vibration-assisted turning is analyzed experimentally for the first time.The results show that high-frequency vibration-assisted turning generates more compressive residual stresses.The machinability of particle reinforced metal matrix composite by high-frequency vibration-assisted turning is investigated,the surface quality obtained by high-frequency vibration-assisted turning with WC tools is approximately comparable to that obtained by PCD tools.Conventional turning and vibration-assisted turning experiments with SiC particle reinforced aluminum matrix composite are performed,cutting forces and cutting temperatures are compared under different cutting tools and lubrication conditions,the mechanism of different chip formation is analyzed,and the roughness and morphology of machined surface are compared.High-frequency vibration-assisted turning achieves a 68% reduction of the main cutting force under the dry cutting conditions of WC tool.Vibration assisted turning converts discontinuous C-type chips into continuous and semi-continuous chips.Vibration-assisted turning reduces the generation of BUE and improves the surface morphology of machined surface.Abrasive and adhesive wear appears in conventional and vibration-assisted turning when using WC tools.Cutting mechanism and experiments of cortical bone in conventional cutting,one-dimensional high-frequency and two-dimensional low-frequency vibration-assisted cutting are investigated,vibration-assisted cutting can change crack propagation of cortical bone and reduce cutting forces.The chip formation and in conventional cutting in different cutting depth and direction are analyzed.The chips gradually change from continuous chips to serrated chips to fractured chips with the increase of DOC(depth of cut).Crack propagation in different cutting direction is different.One-dimensional high-frequency and two-dimensional low-frequency vibration-assisted cutting device are developed.The one-dimensional high-frequency vibration-assisted cutting of cortical bone can transform the large fracture chip generated by conventional cutting into discontinuous or continuous-small-triangular chip,the direction of crack propagation is also changed from irregular direction to the primary shear direction,and the surface cracks and defects of processed bone materials are reduced.Two-dimensional low-frequency vibration-assisted cutting changed the chip formation of c cortical bone in conventional cutting,resulting in smaller chip curvature and chip fracture.Besides,two-dimensional low-frequency vibration-assisted cutting reduces cutting force in different cutting directions.