Formation Mechanism And Characterization Of Serrated Chip In High Speed Machining

In order to adapt to the development of manufacturing industry, improvement production quality and machining efficiency and reducing processing costs, high speed machining is widely used in aerospace, automotive, power generation equipment, et al. Compared to the traditional cutting conditions, the chip morphology in high speed machining will produce great change. Most of the plastic-metal materials will produce continuous chips under traditional cutting conditions, however, those under high speed machining will produce serrated chips. This phenomenon leads to high frequency fluctuations of cutting force, to accelerate the wear rate of cutting tool, to reduce the machined surface quality and the machining accuracy. Therefore, it is necessary to study the mechanism of serrated chip formation in high speed machining. Study on the characterization of serrated chip is conducive to a correct understanding of serrated chip formation mechanism in order to develop a reasonable high speed machining process, to improve the efficiency of cutting, to ensure that parts machined surface quality and to reduce tool wear or breakage and to improve tool life.Based on the above reasons, in this paper through machining three plastic metal materials, using the method of combining theoretical analysis and experimental research, mechanism and characterization of serrated chip formation in high speed machining are studied. The main contents are as follows.Firstly, through theoretical prediction and experimental verification, the critical cutting condition of chip serration in high speed machining of plastic metals is studied. Through analyzing the characteristics of plastic deformation in the primary deformation zone under the critical cutting condition, the formula of the shear strain, the shear strain rate and the cutting temperature are obtained. The effect of cutting parameters and material physical and mechanical parameters on the critical cutting condition of chip serration is studied. The results show that the increase of the yield strength and the modulus of strain hardening will promote the chip serration. However, the increase of the strain rate hardening coefficient, the strain hardening coefficient, the coefficient of thermal softening thermal conductivity, the uncut chip thickness and the tool rake angle increase will hamper the chip serration. Secondly, the adiabatic shear band formation in high speed orthogonal cutting plastic metal materials is studied. The momentum conservation equation, the energy conservation equation and the coordination equation in the primary deformation zone are established. Based on the linear perturbation analysis, regarded the cutting speed and the uncut chip thickness as the basic perturbation, the theoretical prediction model of adiabatic shear band spacing is proposed. Through experiments verification, the effect of cutting parameters on the adiabatic shear band spacing is analyzed. The results show that the increase of the cutting speed will promote the formation of adiabatic shear band and the increase of the uncut chip thickness will delay the formation of adiabatic shear band. With the increase of the cutting speed, the adiabatic shear band spacing decreases and with that of the uncut chip thickness, the adiabatic shear band spacing increases.Thirdly, through analyzing the mechanism of serrated chip formation in high speed machining plastic metals, the deformation of the serrated chip is regarded as the characterization about mechanics of materials. The deformation of the serrated chip is classified into two types the segment shear deformation which contains the segment shear strain and the segment shear strain rate and the adiabatic shear deformation which contains the adiabatic shear strain and the adiabatic shear strain rate. The characterized model of mechanics of materials is established. Through high speed machining experiment, the effect of the cutting speed on the serrated chip deformation and the internal parameters is analyzed. The results show that the adiabatic shear deformation is much larger than the segment shear deformation and the adiabatic shear band width can be approximated as1/10of adiabatic shear spacing. With the increase of cutting speed, the segment shear deformation and the adiabatic shear deformation are increased.Finally, the characterization methods of the geometric pattern, the mechanical properties and the metallkunde pattern are proposed. The serration frequency, the segmented degree, the basic angle and the vertical angle are used to characterize the geometric characterization. The micro-hardness in the serrated chip is used to characterize the mechanical properties characterization. The change of important chemical element in the serrated chip is used to characterize the metallkunde characterization. Through high speed orthogonal cutting experiment, the relationship between the cutting speed and the geometric characterization, mechanical properties characterization and metallkunde characterization in the serrated chip is studied. The serration frequency and the segmented degree increases with the increase of the cutting speed. The value of the basic angle and the vertical angle decreases with the increase of the cutting speed. The micro-hardness increases with the increase of the cutting speed. The degree of oxidation in adiabatic shear band and the bottom surface of the serrated chip increases with the cutting speed, and there is no apparent diffusion phenomenon of the carbon in the cutting process.