Study On The Fracture Mode Of The Material In The Main Shear Zone Of Dry Hard Cutting

During the metal cutting process,the formation of chips is due to shear unstable fracture in the main shear zone(ie,the first deformation zone).According to different materials and changes in cutting conditions,four types of chips are usually produced in the interface of the knife-chip interface,namely,band-shaped chips,saw-toothed chips,unit chips and crushed chips.In the dry hard cutting process,the periodic saw-toothed chips are the most common cutting type in the machining of high hard materials.The forming process of periodic saw-toothed chip brings the high frequency of cutting force,which directly affects the surface quality and tool life of the workpiece.Therefore,the research on the unstable fracture model in the main shear zone has been a hot spot in the field of cutting.The study of the unstable fracture model has long been carried out by exploring the microscopic evolution of the chip.In the process of dry hard machining,in order to find the critical condition of the unstable fracture,it is necessary to find out the formation mechanism of the saw-toothed chip,and then reveal the deep reason of the shear failure in the main shear zone,and analyze the internal relationships between the formation of saw-toothed chip and cutting force,cutting heat and cutting parameters and other factors.So as to provide a scientific basis for the study of unstable fracture model and the reasonable selection of cutting parameters in the actual production and processing.During the dry hard cutting process,the shear instability will form serrated chips in different hardened state.In order to solve this problem,equipments used to collect and process data are CNC machine tools,three-way dynamometer,infrared thermal imager,scanning electron microscope,micro hardness tester and others.The effects of different cutting parameters on the microstructure of chips were studied and the cutting model was verified.The main working contents and research results are as follows:(1)The cutting model is established by finite element simulation method,and the force-heat effect in the process is analyzed.The accuracy of the model is verified by comparing the test values.(2)The single factor method,scanning electron microscopy(SEM),X-ray energy dispersive spectrometer(EDS)and microhardness tester was used to analyze,observe and measure the chip samples.(3)Based on the analysis of the cutting force and cutting heat data,the change trend of chip micro-topography,and the existing theoretical results in the literature,the critical conditions of chip shear deformation and deformation rate,cutting heat prediction and shear instability are studied in this paper.According to the study of material unstable fracture model in the cutting process,it can be found that the saw-toothed shear failure of high hard material chips is the result of force-heat interaction.The force-thermal effect causes the strain hardening effect and the temperature softening effect of the material.Among the many influencing factors,the material properties of the material are more obvious,followed by the cutting speed of this cutting parameters.The critical conditions for shear failure in the main shear zone are determined by shear stress,shear strain rate and deformation temperature.In the critical state,the higher the deformation temperature,the higher the critical strain rate.