Study On The Influence Mechanism Of System's Chatter On Phase Transformation Of Grinding Hardening Layer

Grinding process is one of the finishing processes commonly used in machining field to obtain high precision working surface.Factors that affect the surface quality of parts besides equipment and process parameters,the vibration is one of the main factors.There are different forms of vibration during grinding process,which are forced vibration and self-excited vibration respectively.Self-excited vibration that occurs between the grinding wheel and the workpiece is called grinding chatter.The chatter causes the dynamic characteristics of the system to change.The dynamic characteristics will affect the temperature field distribution in the contact zone of grinding in the form of grinding force and cutting depth.The distribution of the temperature field is also the key factor of the transformation of the microstructure.Grinding hardening is the initial part of the workpiece with the contact zone temperature changes,the microstructure of the first austenitizing and then martensitic process.The whole phase change process in the process of grinding hardening directly affects the quality of the parts.The study on the mechanism of the influence of grinding chatter on the microstructure transformation helps to guide the actual grinding process by adjusting the system dynamics parameters and optimizing product quality and improve product performance.In this paper,the research on how chatter affects the transformation process of the microstructure of the grinding hardening layer has been carried out.The main research work of this thesis is as follows:Firstly,this thesis studied on the difference of chatter between grinding and other process and proposed that the contact stiffness is critical to evaluate the intensity of grinding chatter.Established the dynamic model of grinding system,solved the time-delay differential dynamic equation by Runge-Kutta method.Obtained the influence mechanism of grinding chatter on the property parameters of grinding system by analysing the simulation results of dynamic model.Secondly,The temperature field in the grinding contact zone is analyzed and numerically solved.According to the principle of heat transfer,a two-dimensional heat conduction partial differential equation of the workpiece is established.Finite difference method was used to solve the steady-state and unsteady temperature fields in the contact zone of grinding and compared with the steady-state temperature field solved by Analytical method to verify the feasibility of the finite difference method.Coupled the dynamic results of grinding system into the temperature field analysis,obtained the dynamic temperature field distribution of grinding hardening process.Thirdly,elaborated the principle of cellular automata method and the general principle of austenization process simulation by cellular automata method.Based on dynamic temperature field distribution,established the mathematics model of austenization process,mainly divided into pearlite dissolved,ferrite-austensite transformation and grain coarsening of austensite.Simulation by MATLAB and analysed the influence of grinding chatter on austenization process.Forthly,based on the results of the austenitization of coupled dynamic characteristics and the distribution of the dynamic temperature field,martensitic analysis is carried out.based on thermodynamics of phase transformation of martensite,analysed the mechanism of martensite transformation and predicted the start and finish of martensite transformation by calculation of thermodynamics equation.Solved driving force and resistance force of martensite transformation and the critical temperature of martensite transformation.Calculated the volume fraction of martensite formation.Simulation by MATLAB and analysed the the influence of grinding chatter on martensite transformation.The results show that The chatter vibration intensity of the grinding wheel is greater than the chatter intensity of the workpiece,and the chatter vibration of the grinding wheel can be approximated as the chattering of the system;The higher the contact stiffness results in the more violent chatter of grinding system,therefore,it is verified that the magnitude of the contact stiffness can determine the intensity of the grinding chatter;The more violent the grinding chatter,the greater the maximum temperature and the heating rate of the workpiece temperature field;Grinding chatter will make the grain size of the austenite microstructure becomes larger;The more violent the grinding chatter,the lower the amount of martensite in the final microstructure,thereby reducing the hardness and mechanical strength of the workpiece surface.