Study On Precision Electrical Discharge Machining Of Si3n4 Ceramics

Due to excellent properties of high strength, high temperature resistance, silicon nitride(Si3N4) ceramics have broad application prospects. Further machining is needed to improve the precision after sintering, then it can be applied for practical use in industry. While the properties of high hardness and brittleness make it difficult to machine with traditional machining method, limiting its broader use in industry. So an exploration of a machining method to solve the problem is necessary.Electrical discharge machining(EDM), not influenced by the mechanical properties of materials, is suitable for the machining of Si3N4 ceramics. The main purpose of this paper is to enhance the machining precision of Si3N4 ceramics, investigated the discharge state, removal mechanism, formation of conductive film, process law, optimizition of precess parameters of the EDM of Si3N4 ceramics with electrical discharge machine of immersion liquid.The principle of EDM of insulating ceramics with assisting electrode method is analyzed. By comparing a large number of discharge waveform of EDM between copper and Si3N4 ceramics, we found that the sustaining voltage of the EDM of copper is basically remained unchange with the value of 17 V. While the discharge waveform of the EDM of Si3N4 ceramics is more complicated, the sustaining voltage of the EDM of Si3N4 ceramics is higher and vary in a large range. The discharge state is divided into 7 types, including 5 basic types: open circuit, spark discharge of high resistance, spark discharge of low resistance, short circuit of high resistance and short circuit of low resistance, and 2 types of combination of basic types, including: spark discharge of high resistance to spark discharge of low resistance and short circuit of high resistance to short circuit of low resistance. Surface morphology was observed by scanning electron microscope(SEM) and composition of the surface was detected by Energy Dispersive Spectrometer(EDS), silicon carbide(Si C) was found as part of the conductive film. Removal mechanism of the EDM of Si3N4 ceramics was researched and was devided into 3 types: removed by melting, gasification and thermal stress.The heat source model of single pulse discharge, thermal boundary condition and the discharge channel radius of the EDM of Si3N4 ceramics was analyzed. The temperature field of single pulse discharge machining of Si3N4 ceramics was simulated by ANSYS, the influence of pulse-on time(Ton) and peak current on temperature distribution and size of the pit was researched.Single factor experiment was conducted, law of the influence of peak current(Ip), pulse-on time(Ton), pulse-off time(Toff), servo voltage(V), height of tool retraction(H), speed of tool retraction(v), machining time of tool electrode(T) on side gap(SG), surface roughness(Ra), tool wear rate(TWR), material removal rate(MRR) are researched.Orthogonal experiment was disigned, signal-noise ratio analysis and grey relational analysis were used to optimize the process parameters, then the combination of optimal process parameters that guarantee higher machining precision, higher machining efficiency, lower Ra and lower TWR were achieved, verification experiment was conducted to validate the optimization results, the SG after optimization reached to 10.143μm.