Investigation Of Heat Transfer And Cutting Temperature In Machining With Coated Tools

With the rapid development of advanced tools,coated tools with high performance have become a symbol of modern cutting tools.Coatings covered on the carbide cutting tools can improve the tool wear resistance and heat resistance,and reduce the friction between cutting tool and workpiece.The coatings of cutting tools have great influences on cutting heat generation and cutting temperature field in the tool during machining.The advanced tool coatings can work as the thermal barrier through the heat generated in the cutting deformation zone flowing into cutting tools.Heat transfer of coated tools is still not fully understood because of thermo-mechanical coupling in the metal cutting process.Thus,investigation of heat transfer mechanism for coated tools can not only help instruct the optimization of cutting temperature in order to reduce the tool wear and prolong the coated tools life,but also lay a theoretical foundation for design of coating materials and coated tools.In this thesis,three kinds of nomo-layer coated tools such as TiN,TiAIN and Al2O3 and multi-layer coated tools are selected as the research objects to investigate cutting heat transfer in the coated tools.The research methods including theoretical calculation,finite element simulation and real cutting experiments are adapted to study the cutting heat partition and mechanism of heat transfer.The research is focused on rake face temperature field under the effects of shear deformation heat source and friction heat source and cutting temperature field of tool coating.It is expected to reveal the heat transfer mechanism of coated tools and influences mechanism of tool coating properties and coating thickness.The tool coating microstructure is observed and the properties of coated tools are tested.Through the microstructure observation of the coated tool cross section,the microcosmic appearance and grain morphology are observed.The chemical elements of coating are tested.The thickness of coating layer is measured.The friction coefficient at ambient temperature and high temperature between tool coating and H13 steel are measured.The interfacial adhesion to the carbide substrates of coating layer is obtained through the large load scratch tests and the coating film hardness is tested by the nanoindentation.The thermal conductivity of coating film material has carried on the theoretical derivation,size effect on coating film thermal conductivity will be revealed.According to the characteristics of the coated tools,the cutting heat partition and heat transfer for coated tools is investigated.Base on the cutting heat partition model,the quantitative research of heat partition coefficient for different coated tools under different cutting is developed.The parameters used for calculated heat partition are obtained by the finite element simulation.The variation mechanism of the cutting heat partition will be revealed.The influences of cutting speed,tool coating materials and coating thickness on heat partition coefficients are investigated.The rake face temperature model of coated tools is developed,and the effects of shear deformation heat source and tool-chip friction heat source on tool rake face temperature are analyzed.Rake face temperature fields with different coatings are researched under different cutting speeds.The influences mechanism of tool coatings on rake face temperature is revealed.The results showed that thermo-physical properties of tool coating materials have important influence on heat partition coefficient and rake face temperature of the coated tool.Rake face heat partition coefficient relate to and the thermophysical properties of workpiece and the thermophysical properties of coating for cutting tools.High thermal conductivity coefficient makes the rake face heat partition coefficient small.The higher thermal conductivity coefficient,the more friction heat that transfer into the chip.The rake face temperature of coated tool also showed an increasing tendency from the tool tip along the rake face.The rake face temperature is mainly depended on the friction heat between tool coating and workpiece and the frictional coefficient difference between tool coating materials and workpiece.Base on the investigation for the heat distribution of coated tool,the heat conduction and the cutting temperature is investigated.And the influence mechanism of coated cutting tool cutting temperature is investigated in cutting heat transfer of coated tools.The influence factors including thermo-physical properties of tool coatings and thickness of coating layer in heat transfer process of monolayer coated tools are discussed and illustrated.The steady and transient heat transfer models for coated tool with the first boundary condition are established base on Fourier and non-Fourier law respectively.The steady and transient temperature distributions in the coated tools are obtained.The variations of cutting temperature due to heat transfer under different coated tools are explored.The influences of cutting speed,coating material and coating thickness on the cutting heat transfer are analyzed.The application scopes of the Fourier and Non-Fourier heat transfer model are analyzed in the cutting heat transfer of coated tools.When machining for a long time,the cutting heat transfer reaches the steady-state phase,Fourier heat transfer model is suitable for solving steady-state heat transfer problems.When the heat transfer presents a strong transient or transient stats,Non-Fourier heat transfer model is more applicable to be used to describe thermal shock and thermal transfer delay in the transient heat transfer process.The results showed that thermo-physical properties of coating material and the thickness of coating layer have important influence on cutting temperature.Low thermal diffusivity coefficient of coating and high thickness of coating layer makes low transient cutting temperature incoated tools.Low thermal conductivity coefficient of coating and high thickness of coating layer makes low steady cutting temperature in coated tools.Coating temperature decreased obviously with the increasing of the coating thickness,the roles of coating thickness in the cutting heat transfer appeared obviously.Based on research for heat transfer of the nomo-layer coated tool,the heat transfer of the multi-layer coated tool is analyzed.The equivalent mono-layer composite coating is developed to represent the multi-layer coated tool cutting tool.The equivalent thermal conductivity coefficient,the equivalent heat capacity,and the equivalent thermal diffusion coefficient of the coating are obtained.The equivalent heat transfer model of multi-layer coated tools is deduced according to the heat transfer model of mono-layer.The rake face temperature flied and cutting temperature flied of coating are calculated as the equivalent mono-layer composite coated tool.The finite element simulation and cutting experiment of the cutting temperature for multi-layer coated tool are carried out to research the influences of coatings on the heat transfer.The results showed that the equivalent model of multi-layer coated tools was suitable for the investigation of cutting heat transfer and cutting temperature,compared with the cutting experiment,the error was within 10%.This research is supported by the National Natural Science Fund(U1201245)and National High Technology Research and Development Program of China(2015ZX04005008).