Research On The Material Microstructure Scale Effects In Micro Electrical Discharge Machining

Micro electrical discharge machining(micro EDM)is a new micro machining method,which is developed to meet the market for the increasingly high demand of product miniaturization and precision.Due to its contactless,electro-thermal material removal mechanism,micro EDM is capable of machining all kinds of electrically conductive materials regardless of their mechanical properties.Therefore,micro EDM is suitable for machining difficult-to-cut material,low stiffness and thin wall workpieces,micro holes,micro shafts and complex three-dimensional micro cavities.However,micro EDM performances vary from those of traditional macro EDM,because of the micro-scaled tool electrodes,very low electrical discharge energy and very high discharge frequency.Many influential factors that can be neglected in macro EDM must be closely considered in micro EDM,due to scale effects.As a result,revealing the formation mechanism of scale effect in micro EDM is helpful to improve the machining speed,machining precision and surface quality by optimizing the machining parameters in micro EDM.The formation and expansion process of discharge plasma channel are analyzed by taking discharge duration as starting point,and a model of discharge plasma channel is established.The relationship between the expansion acceleration and the internal pressure of discharge channel is established considering the effect of surface tension,viscous force and magnetic restraint force.The theoretical results show that the internal pressure,temperature,electron density and extended acceleration of discharge channel decrease with the increase of discharge duration,while the radius and expansion speed of discharge channel increase with the increase of the discharge duration,until the discharge channel reaches equilibrium.Single pulse discharge experiment was carried out to study the discharge waveform and discharge crater morphology at microscale.The changes of Shandong University Doctoral Dissertation inter-electrode dielectric state,electrode micro-surface irregularities and other factors cause instability of the discharge state.There may be multiple discharges during one single pulse,and the discharge does not always last until the pulse width ends,which is an important cause of the abnormal electrical discharge characteristics under micro-scale conditions.The influence of discharge duration and open-circuit voltage on the discharge sustaining voltage and discharge current during micro-scale EDM process is insignificant.There is a positive correlation between discharge energy,discharge crater volume and discharge duration.The resistance of discharge channel can be estimated by the ratio of the discharge sustaining voltage to the discharge current,the value fluctuates in the range of 38?45?.The evolution of discharge crater radius and heat affected zone radius with discharge duration is analyzed by observing the discharge crater morphology.It is found that the discharge energy transferred to the workpiece(positive electrode)mainly acts on the melting and vaporization of electrode material,at the beginning of discharge.The shorter the discharge duration,the higher the energy utilization rate of spark discharge.An empirical formula of discharge crater radius and volume with discharge duration is obtained by regression analysis,which provides the theoretical basis for the parameter selection of the micro-scale EDM power supply.Based on the results of single pulse experiment,the oscillation characteristics of discharge channel are theoretical studied.The influence of the transverse vibration and longitudinal vibration of discharge channel on material erosion process is analyzed,and the oscillation frequency of discharge channel is estimated by substituting the relevant experimental parameters,its value is about 132.8MHz.Based on the theory of discharge channel movement,the expression of total removal amount of a single pulse discharge is deduced.It is found that,in addition to the discharge duration,the current density and the oscillation frequency of the discharge channel,the microstructure characteristics of the electrode material also have influence on the volume of the discharge crater.Polarity effect of discharge channel is analyzed by calculating the motion characteristics of charged particles in discharge channel.The results show that ABSTRACT the movement time of the charged particles in discharge channel is much less than the discharge duration.The acceleration and velocity of electrons are very high.Therefore,even if the positive ions mass is much larger than the electron mass,the kinetic energy of positive ions is still smaller than that of electrons when bombarding the electrode surface.What's more,in the same time,the number of electron bombardment of positive electrode surface is 600 times more than that of positive ion bombardment of negative electrode surface.Therefore,the removal amount of positive electrode material is significantly higher than that of negative electrode.The effects of material microstructure scale on micro EDM performances are researched.The effects of grain size,grain boundary,crystal orientation and crystal defects on material micro-physical properties and micro EDM performances are analyzed.Micro EDM experiments are carried out by adopting 304 stainless steels with different grain sizes,monocrystalline silicon and porous steel with different specifications as workpiece,respectively.The results show that the grain boundaries of polycrystalline materials contain defects and impurities which can decrease their melting point,thermal conductivity and electrical conductivity,the decrease of grain size is accompanied by the increase of grain boundary volume fraction which causes decrease of energy loss during micro EDM process,leading to the increase of energy utilization rate and material removal rate(MRR);The influences of electrostatic force,thermal stress and Joule heat on micro EDM of brittle materials are significant at the beginning of discharge.Not only heat erosion,there are stress erosion during micro EDM of single crystal silicon,surface roughness and MRR change with the change of crystal orientation of monocrystalline silicon,due to anisotropy;The increase of porosity and pore size improves the abilities of debris removal and machining status,leads to the increase of the effective thermal conductivity of porous material,resulting in the increase of MRR and decrease of relative tool wear ratio(RTWR).The effects of electrode scale on micro EDM are studied by means of theoretical analysis and micro EDM experiment.The results show that,in the specific tool diameter range(100?500?m),the increase of the tool diameter causes the increase of actual discharge energy and discharge frequency due to area effect,leading to the increase of MRR;The effect of skin effect and area effect is enhanced with the increased tool diameter,causing more drastic tool wear and more side discharges,leads to the increase of RTWR and taper ratio(TR);The surface layer model of micro EDM is established.It is found that with the decrease of workpiece scale,the effects of surface layer increases.The higher surface free energy causes better surface wettability of workpiece,which improves electrode cooling,inter-electrode ionization and discharge state,so that higher MRR can be obtained;The lower surface free energy of workpiece without surface treatment causes deterioration of discharge status and increase of side discharge,resulting in larger TR of machined micro holes;The lower the discharge energy,the more significant the effects of surface layer of workpiece.Similarity theory is introduced and single factor experiment of open circuit voltage and capacitance is carried out.The quantitative study on the scale effects in micro EDM is carried out based the experimental results by discussing corresponding similarity difference and similarity precision.The results show that,the smaller the scale of capacitor or open circuit voltage used in micro EDM,the more drastic the change of similarity precision,which indicates that scale effects become significant as discharge parameter decreases;When the capacitor or open circuit voltage is scaled down,the variation of the similarity difference is relatively gentle while the similarity precision increases rapidly,therefore,similarity precision is more effective in reflecting scale effects and their fluctuation than similarity difference.Similarity theory is suitable for the quantitative evaluation of scale effects in micro EDM,The study of the scale effect can help to optimize the process parameters of micro EDM so as to promote the machining performance.