High-speed Cutting Mechanism Of High Manganese Steel And Its Influencing Factors

High-speed cutting（HSC）theory is the foundation of the development and application of high-speed machining technology,which can provide important support for the development of high-speed machine tools,tools and processes.Therefore,it is of great theoretical significance and engineering value to study HSC and reveal its essential law for effective control of HST process as well as research and development of high-speed equipment.In this paper,the method of theoretical analysis and modeling,experiment and numerical simulation is used to test and analyze the HSC of high manganese steel and medium carbon steel with cermet tool.By using metallography of chip,tribology,heat transfer theory and known cutting theory,the special phenomena in HSC are compared and analyzed,and their cutting mechanism and influencing factors are studied.Through test and analysis of the cutting deformation characteristics,it offers the geometry model during the evolution of chip and the related parameter equation when HSC is established,and gives the relationship of mean stress with strain,strain rate and the cutting temperature in zigzag section chip and between the segments of chip.The hardening and softening effects and the coupling mechanism of multiple factors in the process of HST are studied.The high-speed cutting temperature field,strain field,strain rate,cutting force and cutting deformation were analyzed qualitatively and calculated quantitatively,and the cutting force model was established.The experimental and simulation results verify the conclusion and model obtained from the analysis of HST characteristics.On this basis,the influences of cutting parameters,material properties and other factors on cutting deformation,cutting force,cutting temperature field and tool wear are studied.The main research contents and results of this paper include the following aspects:（1）Based on the analysis of chip formation and shape characteristics in HSC,the geometric model of HSC layer deformation and the relationship models of deformation coefficient,cutting ratio,shear angle and cutting parameters are established.By comparative analysis,it shows that the high speed and large deformation of cutting layer cause high strain rate and hardening phenomena.At the same time,the deformation work is instantly transformed into cutting heat,and the"adiabatic shear"softening effect is formed in the deformation region of the processed material with low thermal conductivity.（2）The geometric model of sawtooth chip with the characteristics of adiabatic shear band is established,and the calculation model of the average shear strain and shear strain rate of sawtooth chips and between the sawtooth segments is deduced.According to the full differential equation of shear flow stress and strain,strain rate and temperature coupling in high speed cutting,the factors and mechanism of sawtooth chip formation are analyzed.It is found that the cause of sawtooth chip formation is thermoplastic shear instability,and the critical condition of sawtooth chip formation is the critical cutting speed.Banded chips form below the critical speed and zigzag chips form above the critical speed.Thermoplastic shear instability is caused by the coupling of strain,strain rate and temperature.With the increase of cutting speed,chips transform from uniform strip to non-uniform zigzag shape and are marked by shear band.The increase of strain and strain rate has a strong hardening effect,while the increase of temperature has a significant softening effect.Sawtooth chips are formed during the alternation of deformation hardening and temperature softening.The critical shear strain and shear strain rate of thermoplastic shear instability increase with increasing cutting temperature.（3）By using the elastic-plastic shear theory and considering the high-frequency impact in high speed cutting,the mechanical model of single edge right angle cutting is established,and the calculation method of main cutting force and cutting power in high speed cutting is given.The relationship curve between the average value of dynamic cutting force and cutting speed was determined by means of mean value method,and the trend of cutting force changing with cutting amount parameters in HSC was analyzed,and the range of cutting amount with the minimum cutting force was determined,which could be used to optimize machining technology.Both of the single-factor and multi-factor models of cutting force and cutting quantity obtained can be used to calculate the cutting force and predict the machining accuracy under certain conditions.（4）The heat balance equation was established by analyzing the cutting heat source and heat efferent path.The formulas of average temperature of the shear deformation region and the efferent ratio of cutting heat from deformation region into the chip,tool and workpiece were derived,and the curves of heat transfer rate R₁ and R₃ with the change of cutting speed were obtained by numerical algorithm.It is found that the heat transfer rate R₁ from the first deformation zone is 96.24%,while the heat transfer rate R₃ from the third deformation zone is less than 3‰.The results show that most of the heat is taken away by the chips,and the cutting heat has little influence on the tool wear and workpiece deformation,so the tool life and machining accuracy are higher in high speed cutting.（5）It is found that among the factors affecting the high-speed cutting temperature and tool wear,the cutting speed has the greatest influence,followed by the feed quantity,and the depth of cut has the least influence.When the hardness of the tool material is high,the thermal conductivity is large,and the friction coefficient is small,the cutting temperature can be lower by reducing the front angle and the tool cutting edge angle.The cutting temperature increases when the tool wear increases.The analysis of tool wear morphology and tool surface chemical composition shows that the tool wear is mainly mechanical wear and thermal wear,and in which the wear of the rear tool surface is mainly abrasive wear.There are mechanical wear,bonding wear,diffusion wear and oxidation wear on the front cutter surface.The maximum temperature point is a distance away from the cutter edge,and the deviation distance increases with the increase of the plasticity of the workpiece material.The temperature gradient in the normal direction of the shear plane is the largest,and the maximum thermal stress caused by this leads to shear instability in the deformation zone and the formation of sawtooth chips.The chip shape changes with the cutting speed.The morphology of chip,temperature field,stress field and cutting force of high manganese steel in cutting simulation test are consistent with the experimental results.