Research On Macro-mesoscopic Phase Transition And Multi-field Coupling Residual Stress Of Grinding Surface Layer

In machining process,as a final machining method of components,grinding determines the final surface integrity of components,so grinding surface integrity plays an important role in guaranteeing the service performance of components.Great grinding force and heat are produced in grinding process because of the high speed of grinding wheel and the high strain rate of material.In addition to plastic deformation,the surface layer of components may have phase transition.Phase transition not only directly changes the properties of surface materials,but also causes additional deformation and stress and changes the residual stress.Residual stress not only affects the dynamic and static contact performance,corrosion resistance and service life of components,but also affects geometric precision of components by causing warpage.Therefore,the accurate prediction and control of phase transition and residual stress of grinding surface layer are of great significance to ensure the surface integrity and improve performance of component.However,researches on phase transition and residual stress in grinding process are not enough.The simulation of phase transition often neglects the effect of stress.The simulation of residual stress also neglects the effect of phase transition.These lead to the prediction accuracy of phase transition and residual stress decreases.Moreover,the effect of stress on phase transition and effects of phase transition on residual stress and etc.are still unrevealed.Based on the analysis above and the project of the National Natural Science Foundation of China"Research on mechanism and surface integrity in pre-stressed hardening grinding process"(No.51375083),this paper mainly studies the phase transition and residual stress of grinding surface layer.The main research contents are listed as following:(1)Modeling of grinding force-heat distribution based on the analysis of grain interaction and simulation of dynamic material removal.The grinding wheel topography is reconstructed according to the wheel parameters.The instantaneous characteristics of all active grains,such as the cut-in depth and interaction stage,are obtained by motion analysis and simulation of material removal.Based on the instantaneous characteristics,a grain-interaction force model including the magnitude of force and the location of grain is established by distinguishing the grain-interaction mechanisms.Furthermore,the dynamic grinding force model is established,the variation rule of grinding force within the full time domain and the fluctuation of grinding force are obtained and analyzed,and the force-heat distribution within the grinding zone is determined.(2)Research of macro-mesoscopic phase transition of grinding surface layer.1)Based on the TTT and TTA kinetics curves,the macro phase transition model of surface layer considering the effect of stress is established by using JMAK equation,superposition rule,K-M equation,Clausius-Clapeyron equation and Inoue equation.The calculation of macro phase transition is realized through the secondary development of ABAQUS subroutines.The evolution of phase transition is studied,and the effects of stress on phase transition and characteristics of transition layer in grinding are revealed.2)Based on the temperature history obtained by the macro phase transition model,the cellular automata algorithm and the austenite transition theory are used to establish the mesoscopic austenite transition model.The nucleation-growth process of austenite and solute diffusion process are studied,and the effects of grinding depth on the austenite transition and carbon distribution of surface layer are studied.(3)Research of phase transition effects and thermal-phase transition-mechanical coupling residual stress.The residual stress is the result of multi-field coupling when the phase transition occurs,the multi-field coupling relationship and phase transition effects are analyzed and modeled.Based on elastic-plastic mechanics and finite element theory,a thermal-phase transitionmechanical direct coupling finite element model is established by secondary development of ABAQUS subroutines with considering three phase transition effects,latent heat of phase transition,yield strength change caused by phase transition and volume change caused by phase transition.The influences of various phase transition effects on temperature,stress and strain evolutions,residual deformation and residual stress of surface layer in grinding are revealed by comparative analysis.(4)Analytical calculation of residual stress of grinding surface layer incorporating the effect of phase transition.The temperature field of workpiece is analytically calculated.An analytical formula for calculating the microstructure field is constructed based on the kinetic models of phase transition,then the microstructure field of workpiece is obtained.The stress of workpiece induced by grinding forces is calculated based on the Boussinesq stress solution.Considering the volume change caused by phase transition,the stresses of workpiece induced by inhomogeneous temperature field and microstructure field are calculated based on the thermoelastic assumption and Duhamel similarity law.Based on the stress of the workpiece under the combined actions of grinding forces,inhomogeneous temperature field and micro structure field and the incremental plastic theory,the residual stress of surface layer incorporating the effect of phase transition is obtained through stress loading and unloading and stress release by using McDowell’s hybrid algorithm with considering yield strength change caused by phase transition.(5)Grinding and surface integrity experiments.The dry grinding experiments are carried out under various grinding parameters.The grinding force,surface micro-topography,the metallographic structure and surface residual stress are measured or observed.The grinding force model is validated by the grinding force measurement results and the simulation of dynamic material removal is verified by the measurement results of workpiece surface micro-topography.The macro-mesoscopic phase transition model is validated by the observation results of the metallographic structure of the grinding surface layer and the established residual stress model is validated by the measured surface residual stress.