Modeling And Simulation Of Machined Surface White Layer In High Speed Dry-hard Cutting

High speed dry-hard cutting has many advantages,such as high precision and high efficiency,which has been widely used in mold,aerospace and other fields.However,in the high speed dry-hard cutting process,the change layer formes easily on the machined surface,that is named“white layer”.The white layer is a brittle-hard tissue with the thickness of only a few micrometers,which can easily cause spalling failure and fatigue cracking.It is an important factor in determining the surface quality of the workpiece.Therefore,it is of great significance to study the influencing factors and forming mechanism of the white layer machined surface.In this paper,the white layer was formed by cutting hardened GCr15 bearing steel(HRC60)with PCBN tool.The finite element model of high speed dry-hard cutting was established.The effect of heat and force on the martensitic volume fraction of white layer was analyzed.By cellular automata method,the austenite transformation and martensitic transformation evolution model about white layer were established.First,based on the ABQAUS software,the high-speed dry-hard cutting finite element model was established to simulate the cutting process dynamically.By comparing the cutting force and the cutting temperature measured in the medium and low speed cutting conditions to the simulation results,the feasibility of the model was verified from two aspects of heat and force.The simulation results of high speed cutting temperature were compared with the critical phase transition temperature of austenite,the result shows that the hardened GCr15 steel machined surface white layer is phase transition product dominated by temperature combined with analyzing phase composition of white layer.Then,the yield strength of quenched GCr15 steel under different temperatures was obtained by SHPB test equipment and the contents of main elements were corrected by EMPA.The value of martensite critical phase transition temperature under the influence of the cutting heat was deduced based on the change of Gibbs free energy.The theoretical martensite fraction of the white layer under the action of cutting heat was calculated based on the M_S.As the volume fraction calculated by the theoretical model was compared with that measured by the experiments in the white layer,the results show that the theoretical value of martensite fraction is higher than the experimental value.The martensite fraction in the white layer is affected by cutting heat and cutting force,the plastic deformation inhibits the martensitic transformation in the white layer.At the same time,the effects of different cutting speed and flank wear on the martensitic transformation in the white layer are analyzed combined the TEM experiment.In order to visualize the process of austenite transformation and martensitic transformation during the formation of the white layer,the TEM dark field images of the white layer on machined surface were obtained by TEM experiments,then the grain sizes were measured according to the images.The number of austenite nucleation was calculated by the grain size.Combining the mechanism of phase transition,a phase evolution model of cellular automata for white layer formation was established.The austenite transformation and martensitic transformation about the evolutionary process of white layer were simulated,and the martensitic nucleation density of white layer with different flank wear was obtained.The simulation result shows that reducing the flank wear can improve the strength of martensite in white layer on machined surface to improve the fatigue resistance of machined surface.