Research On Mechanism Of Segmentation Chip Formation In High Speed Cutting

The Mechanism of high speed cutting is one of the striking problems that high speed machining technologies is confronted with, and seriously hinders the evolution of high speed machining manufactures. Consequently it has great value in academic and engineering area to realize the prediction and manipulation of chip shape. The research of high speed cutting on chip morphology involves many disciplines, such as mechanical manufacture, continuum medium mechanics and material science and etc. In view of the complexity in nature, a method combined theoretical modeling with numerical computation and experimental verification is proposed in this dissertation. Subsequently initiation with the metal cutting principle, the analytical models of plastic instability and deformation localization are established due to dominant thermal softening in high speed cutting. Furthermore, the analytical models are used to investigate the mechanism of segmentation chip morphology, time and space characteristics of adiabatic shear bands, and the influence factor on segmentation chip formation.1. The adiabatic shear model and the mechanical plastic instability induced by thermal softening model are established for the rate-dependent and strain gradient-dependent material in high speeding. The general criterion of plastic instability is derived physically in nature.(1) In view of the effect of intensive localization deformation on the flow stress, the constitutive relation is modified by integrating high strain gradient and applicable to multi-axial stress state. The present constitutive relation extends the plastic deformation gradient theory on the adiabatic shear and adapts to represent the adiabatic shear deformation in the multi-axial stress state.(2) It is released that the adiabatic shear instability is due to the constitutive relation instability in nature. Whether one-dimensional shearing deformation or multi-axial stress state, it is demonstrated in the plastic instability criterion that the material becomes instable when the thermal softening dominates over the strain hardening. (3) The present instability condition of the adiabatic shear deformation includes rate independent case as well as rate dependent one.The strain gradient is integrated to the present criterion, it prohibits the locallized degormation and delays the initiation of plastic instability.2. The continuum medium model of the primary zone in the high speed cutting is presented. On basis of the absolute perturbation analysis and the relative perturbation analysis, it is discussed that the single adiabatic shear band initiates and evolutes in the high speed cutting.(1) The modified relative perturbation analysis is used to determine the threshold wavelength for the rate dependent and strain gradient dependent material. The present analysis of the shear band width is constituent to the result for the neither strain hardening or strain gradient materials, but it isn't confined to the case and is propone to include the effect of the strain hardening and gradient.(2) In accordance with the comparison of the absolute perturbation analysis with the relative perturbation analysis, the classical perturbation analysis conforms to the relative perturbation analysis for strain rate-independent materials while the latter makes more precious approximation than the former for strain rate-dependent materials.3. On the basis of the relative perturbation analysis, the interaction among the repeated multiple adiabatic shear bands is investigated.(1) The rate and strain gradient dependent constitutive relation is used to predict the adiabatic shear band spacing on the basis of the modified relative perturbation analysis. Compared to the emerging theoretical models, the present prediction result is consistent to the experimental one verifying the present prediction model.(2) The adiabatic shear band spacing decreases with the increase of the strain rate and the thermal softening, and it is indicated that the interaction with the adiabatic shear bands is weak and the shear bands are prone to grow to maturity. The adiabatic shear band spacing increases with the strain hardening, the strain rate hardening, the thermal conduction as well as the strain gradient, and it is indicated that the interaction with the adiabatic shear bands is strong and the shear bands have no chance to grow to maturity. Moreover, the strain gradient plays dominant the restriction to the growth of shear bands over the thermal conduction.4. Based on the combined Oxley cutting model with the relative perturbation method, the formation and evolution of the segmentation chip is investigated in orthogonal cutting.(1) Compared to the emerging theoretical models, the modified Oxley's model is proposed to predict the adiabatic shear in high speed cutting considering the normal velocity effect in the primary zone. Moreover, the present predition model is general puepose for predicting the adiabatic shear of various meatal materials in high speed cutting.(2) The width of transformed adiabatic shear bands decreases with the cutting velocity as V ? 1. The separation distance decreases approximately as V 3/4. However the dependence of the separation frequency f upon the cutting velocities, f∝V7/4. It is indicated that the above analysis is unanimous with the recent document verifying the theoretical modeling of the segmentation chip adiabatic in high speed cutting.(3) The shear localization criterion derived from the localization analysis indicates that it is the interaction of the thermal energy induced by the intensive shear deformation with the thermal dispersion induced by the thermal conduction and the strain gradient brings about the shear localization chip. It is demonstrated that the strain gradient more lessens the localization deformation than the thermal conduction, especially under high strain rate.Finally, the main topics and conclusions of this thesis are summarized, and the future research direction has been proposed.