Research On Self-induced EDM Of Ni-Al₂O₃ Fgms And Its Control Technologies

Metal-ceramic FGMs ingeniously combine the electrical and thermal conductivity of metals with the conductive insulation and thermal insulation of ceramic materials,widely applied in aerospace,nuclear energy,biology and other fields.However,greater hardness and diverse graded composition lead to severe tool wear and high cost when materials are machined by conventional contact machining techniques.EDM as a noncontact process demonstrates great advantage on machining metal-ceramic FGMs.During the EDM process of metal-ceramic FGMs,the constantly changing material composition and conductive properties lead to special processing characteristics.Machining rules and control systems of conventional EDM machining based on the single material are not fully applied to process metal-ceramic FGMs,and limit the improvement of machining speed.Therefore,this dissertation,take Ni-Al₂O₃ FGMs as research objects,explores the EDM method,machining characteristics,discharge characteristics,and studies control technologies based on the above research.Aiming at structural characteristics of metal-ceramic FGMs,a self-induced EDM method is proposed.This method uses the conductive compositions of metal-ceramic FGMs to trigger discharges in the insulating ceramic composition.Based on this,machining characteristics of Ni-Al₂O₃ FGMs drilled by self-induced EDM have been investigated.The machining morphology,particle and surface elements have been analyzed,and three removal mechanisms are obtained.The influence of electrical parameters on machining performance has studied by single factor experiment,and basic EDM rules of Ni-Al₂O₃ FGMs are concluded.Discharge characteristics are analyzed in view of the changing conductivity during the self-induced EDM process of Ni-Al₂O₃ FGMs.Based on differences of voltage and current waveform,discharge waveforms are divided into normal spark discharge waveform,arc discharge waveform,insufficiency discharge waveform and long pulse discharge waveform,in which insufficiency discharge waveform and long pulse discharge waveform are special discharge waveforms.A metal-ceramic FGMs discharge model is established,and effects of material resistance on voltage,current and discharge energy of insufficiency and long pulse discharge have been investigated by theoretical analysis,simulation analysis and experimental study.The influence of machining parameters on insufficiency and long pulse discharge are investigated to explore controllable conditions in graded layers and ceramic layer,which can provide guidance for the development of machining state detection and control strategies.In order to realize the real-time detection of four discharge states and different layers in the self-induced EDM process of metal-ceramic FGMs,the machining state detection technology are investigated.Based on voltage features of four discharge waveforms,a new three voltage threshold monitoring method is proposed.Based on the quantitative analysis of four discharge waveforms,a graded composition identification method is proposed.Through hardware design and software design,a machining state detection module for self-induced EDM of metal-ceramic FGM has been developed which provides real-time and accurate information for power control strategy and electrode jump motion control strategy.Aiming at discharge characteristics and machining problems of different graded compositions of Ni-Al₂O₃ FGMs,a special pulse power control module and a electrode jump motion control module are developed,and corresponding control strategies are proposed.In the aspect of pulse power control,a high voltage pulse control technology is proposed to greatly increase discharge energy aiming at the insufficiency discharge feature of graded layers.An inter-pulse expansion control technique is proposed to improve machining stability aiming at the characteristics of long pulse discharge in ceramic layer.In the aspect of electrode jump motion control,a new electrode jump control strategy based on abnormal discharge rate?arc,insufficiency and long pulse discharge?is proposed,and this control strategy can adjust the electrode jump motion cycle real-timely according to the feedback information from the machining state detection module,which provides a guarantee for effective small hole machining process.To verify the machining performance of developed control system,the designed machining state detection module,pulse power control module and electrode jump control module are integrated and a series of comparative experimental studies have been conducted.The experiment results show the machining state detection module can real-timely identify four discharge states and different layers by selecting the appropriate voltage comparison threshold,statistical discharge number and discharge rate threshold.Through high voltage control strategy and inter-pulse expansion strategy,the pulse power control module can significantly increase the machining efficiency while improving the surface quality.The designed electrode jump control module can accelerate the removal of products from the discharge gap by adjusting the electrode jump motion cycle real-timely,especially the alumina particles,and increase the machining efficiency of small holes.