Research On The Discharge And Material Removal Mechanisms Of PRMMCs In EDM

Particle reinforced metal matrix composites(PRMMCs)possess both the properties of matrix metal materials and non-metallic particles.They have good strength and toughness and special advantages such as heat resistance and wear resistance.Compared with pure metal materials,the introduction of high strength and high hardness refractory ceramic particles makes the machining process of PRMMCs more difficult,especially there are serious problems of temperature rise and tool wear in the machining process,resulting in low machining efficiency and difficulty in meeting the machining accuracy requirements.Electrical discharge machining(EDM)has the characteristics of non-contact processing and is not limited by material strength and hardness.It is very suitable for precision processing of difficult-to-machine materials such as PRMMCs.However,due to the large difference in physical properties between the reinforced particles and the matrix,the micro discharge and the material removal mechanisms compared with the traditional homogeneous material have obvious particularity,the mechanism of EDM considering the heterogeneous characteristics of PRMMCs is still not very clear.In this dissertation,the mechanisms of discharge and material removal were studied,the microscopic processes of discharge breakdown and discharge channel formation were simulated,and the melting,ejecting and solidification model of PRMMCs in EDM were established.A full cycle simulation of the material removal process was carried out,and the temperature distribution of the material surface and the molten pool under different discharge parameters were analyzed.The microscopic process of the high-speed ejecting of the molten matrix material and the solid particle reinforcement,and the formation process of the crater were revealed.The rule of residual stress distribution during solidification process and its effect on micro-cracks were analyzed.This dissertation mainly conducted research work from the following aspects:(1)The physical change of the formation,oscillation and vanishing in the plasma channel during discharge breakdown were analyzed.Based on the theory of plasma motion,the mathematical model of charged particle oscillation was derived,and the magnetic fluid motion equation of charged particle in magnetic field in discharge channel was obtained.With the help of Fluent fluid dynamics software,a magnetohydrodynamic coupling module was introduced to establish a micro model of plasma motion in the discharge region,and the breakdown process of discharge channel and its pinch effect were simulated.The dynamic characteristics of the transient velocity field and pressure field in the discharge region were investigated,and the influence of the discharge channel oscillation on the crater morphology was revealed by simulation and experiment.(2)Study on the melting process of PRMMCs.According to the physical properties of PRMMCs,the energy conversion process after the discharge breakdown was analyzed.Considering the existence of interfacial thermal resistance,the thermal conductivity coefficient of PRMMCs was calculated,and the constitutive heat conduction equations of metal matrix and particle reinforcement were derived based on the Fourier heat transfer equation.With red copper as the electrode and aluminum-based silicon carbide composite as the workpiece,the heat flux expression was compiled into a program using the secondary development interface and loaded onto the surface of the electrodes as the heat source.The surface heating and melting processes of the two electrodes were simulated,and the temperature distribution along the radius and depth was obtained.(3)Study on the ejecting process of PRMMCs.Based on the theories of magnetohydrodynamics and fluid mechanics,the mathematical model of composite material ejection was deduced.The fluid-solid coupling kinetic model between molten liquid metal matrix and particle reinforcement was established,and the mathematical model of material ejection was compiled by using the secondary development interface as the discharge ejection velocity condition.The influence of magneto-fluid force on the movement of molten metal and particle reinforcement in molten pool under high pressure was studied,and the high-speed ejecting process of particle reinforcement with molten metal was investigated,and the removal volumes with time of the tool and workpiece were obtained.(4)Study on the solidification process of PRMMCs.Based on the theory of phase transition of materials and considering the physical properties and the particularity of phase transition process of composite materials,the evolution of material properties and solidification process of each phase were analyzed.A one-way simulation analysis of fluid-structure coupling was realized,and the phase transition process of molten material with time was obtained.The stress redistribution state of the crack tip in the recast layer and the microscopic surface morphology of the discharge crater after the solidification were obtained.Scanning electron microscope was used to observe the surface morphology of the crater,and the distribution of surface cracks was recorded,which verified the correctness of the simulation results.(5)Full cycle analysis of PRMMCs in EDM.A multi-field coupling model based on thermal-structure-magnetic-fluid coupling field was constructed and the material removal process in EDM was completely analyzed.With red copper as the electrode,a complete simulation model of the discharge breakdown process,the melting process,the ejecting process and the solidification process of the PRMMCs were established.Considering the effects of different discharge parameters,the effects of machining parameters on the material removal volume,tool relative wear rate,the evolution of the properties of multiphase materials and the microcosmic surface morphology in the cooling process of residual materials were studied.The results showed that the tool relative wear rate increased with the increase of discharge energy,and the experimental results verified the correctness of the simulation results.