Thermal Contact Resistance For TiAlN Coated Tool And Its Influencing On Cutting Temperature

The increase in cutting temperature of the cutting tool changes the tool cutting performance.Thermal deformation of the tool is caused by the elevated cutting temperature.Wear of the cutting tool is accelerated by the elevated cutting temperature.The accuracy of the machined workpiece is deteriorated and the processing cost is increased.The thermal contact resistance between cutting tool rake face and chip affects the cutting heat conduction between cutting tool and chip.The thermal contact resistance plays an important role in the distribution of the cutting heat in cutting tool and chip.Coatings such as TiAIN,CrAlN,TiAlCrN,CrAlSiN change the tool-chip contact characteristics,which in turn affects the cutting temperature of cutting tool.This paper takes the thermal contact resistance formed at tool-chip contact interface for TiAlN coated tool during metal cutting process as the research object.By analyzing the incomplete tool-chip contact interface and the Al2O3 oxide layer formed on the surface of the TiAlN coating,the formation mechanism of thermal contact resisatance generated at TiAlN coated tool rake face and chip contact interface is revealed.Analytical model for the incomplete contact-oxide layer thermal contact resistance is established to calculate the thermal contact resistance for TiAlN coated tool.The cutting heat conduction process between chip and tool rake face is simulated by the heat conduction experiment.According to the experiment,the thermal contact resistance is calculated by analyzing the temperature gradient between contact interfaces and heat flux to verify the correctness of the established analytical model.By comparing the thermal contact resistance formed for TiAlN coated tool,TiN coated tool and uncoated carbide tool,the influencing of thermal contact resistance generated at tool-chip contact interface on cutting temperature is analyzed.The effect of Al2O3 oxide layer formed on the coating surface on tool wear is explained.How to choose cutting speed to extend the tool life through analyzing the thermal contact resistance generated at tool-chip contact interface is discussed.Firstly,TiAlN coated tool is applied to orthogonally machining H13 hardened steel.By observing the surface morphology of the uncut tool rake face and the used tool rake face by scanning electron microscope,the formation mechanism of thermal contact resistance caused by incomplete tool-chip contact is revealed during the initial stage and the stable stage of cutting process.Analytical model for the thermal contact resistance caused by tool-chip incomplete contact is established by analyzing the advantages and disadvantages of the analytical models established by previous researchers.According to the cutting force,tool-chip contact area and the surface morphology characteristics of tool rake face and chip bottom surface,the thermnal contact resistance generated at tool-chip contact interface is calculated with the established analytical model.Influencing of the parameters of analytical model on the thermal contact resistance caused by tool-chip incomplete contact is investigated.By comparing thermal contact resistance caused by tool-chip incomplete contact for different cutting speeds and different workpiece material,the influence of analytical model parameters is analyzed.Secondly,conditions for the oxidation reaction and oxidation products of TiAIN coating material are studied according to the high-temperature oxidation experiment.Influence of the oxide layer on thermal conductivity of the TiAlN coating is analyzed.By using scanning electron microscope and X-ray energy spectrometer,the morphology and element distribution on cutting tool rake face is observed.Morphology and element distribution on fracture cross-section of TiAIN coating of the tool-chip contact area are analyzed,too.The formation mechanism of thermal contact resistance generated at tool-chip contact interface caused by Al2O3 oxide layer is revealed.Analytical model for the thermal contact resistance is extablished based on analysis of the influencing factors.According to thickness and thermal conductivity of the Al2O3 oxide layer obtained by measurement and analysis,the thermal contact resistance caused by Al2O3 oxide layer is calculated.By comparing the thermal contact resistance for different workpiece material and TiAlN coated tools with different Al element content,the influence of workpiece material and tool coating is analyzed.Thirdly,relationship between the thermal contact resistance casued by tool-chip incomplete contact and the thermal contact resistance caused by Al2O3 oxide layer is analysized.It can be concluded from the equivalent circuit that the analytical model of the thermal contact resistance generated at tool-chip contact interface is the sum of the two analytical models.The process of cutting heat conduction from chip into cutting tool is simulated by the heat conduction experiment.By measuring and analyzing the temperature gradient between the contact interfaces and the heat flux,the thermal contact resistance is calculated.The analytical model of incomplete contact-oxide layer thermal contact resistance is verified by the heat conduction experiment.According to the established analytical model,the cutting temperature is predicted with finite element method.The thermal contact resistance values calculated by the established analytical model and the Umbrello analytical model are compared.The values obtained by the two analytical models are applied for finite element simulation,respectively.Cutting temperature,shear angle,tool-chip contact length and chip thickness obtained by the finite element simulations are compared with the cutting experimental results to verify the validity of the established analytical model.Finally,by analyzing the formation mechanism and numerical calculation to compare differences in thermal contact resistance formed at tool-chip contact interface between TiAlN coated tool,TiN coated tool and uncoated cemented carbide tool when H13 hardened steel was machined.It is found that the value of thermal contact resistance formed at tool-chip contact interface for TiAIN coated tool is much greater than that for TiN coated tool and uncoated cemented carbide tool.Applying the two kinds cutting tool to machine H13 hardened steel under same cutting conditions.Influencing of thermal contat resistance generated at tool-chip contact interface on cutting temperature is analyzed by comparing chip cutting temperature,tool substrate temperature and other parameters.The influence of Al2O3 oxide layer formed at TiAlN coating surface on the TiAlN coated tool wear is analyzed.By comparing the tool life of TiAlN coated tools at different cutting speeds with considering the thermal contact resistance generated at tool-chip contact interface,how to control the cutting speed to change the contact thermal resistance and extend the tool life is clarifed.