Research On Flow Field And Discharge State Of EDM Small Hole Machining For Thermal Barrier Coated Superalloy

Thermal barrier coated(TBC)material has a good heat insulation effect,can effectively improve working temperature of superalloy and is widely used in domestic and foreign aero-engine turbine blade.However,TBC superalloy is a multi-layer structural material.Because the top layer is coated with hard and brittle insulating ceramic coating,there is a problem of quality in electrical discharge machining(EDM)hole forming process on TBC superalloy.Aiming at the above problems,the effects of flow field and electric field on the EDM performance of TBC superalloy,the detection of gap discharge state in EDM and the EDM experiment of TBC superalloy were studied in this thesis,this research provided an effective means for high quality EDM of TBC superalloy and has important research significance for expanding the application scope of EDM.The geometric model of EDM flow field simulation was established and simulated by Fluent,aiming at the problems of rapid discharge process,narrow discharge gap and difficult to directly observe the distribution of the debris in the gap flow field.The results show that with the increase of internal rush hydraulic pressure,the debris in machining gap thrown acceleratedly,which is beneficial to the improvement of machining efficiency and quality.During EDM,the size of the debris will cause the change of the electric field strength between the electrode and workpiece,the simulation analysis of the effect of the particle on the electric field between the discharging gaps showed that the larger the particle diameter,the more serious the electric field distortion between the electrodes,and the greater the maximum value of the electric field strength between electrodes under the same processing conditions,the more likely the breakdown discharge is to occur.Due to the difference of melting point and conductivity between ceramic coatings and superalloy matrix materials in TBC superalloy,and the characteristics of high resistance and low resistance spark discharge of TBC ceramics,a detection circuit for gap discharge state of EDM TBC superalloy was designed,which included voltage divider circuit,threshold comparison circuit,photoelectric isolation circuit and high frequency component detection circuit.Assisting electrode method with rotational electrode and inner jetted dielectric,gap discharge detection circuit and multi-mode pulsed power supply were used to machine TBC superalloy,the experimental results showed that with the increase of internal rush hydraulic pressure,machining time and relative electrode wear ratio had a trend of increasing firstly and then decreasing,and the average thickness of the resolidification layer of the hole wall increased with the increase of internal rush hydraulic pressure.It is because excessive internal rush hydraulic pressure will cause mechanical system vibration intensification,leading to concentrated discharge and side discharge phenomenon increased.When machining the tilted hole,with the increase of the tilted angle,the machining speed decreased,relative electrode wear ratio and the average thickness of the resolidification layer of the hole wall became larger.By analyzing the output waveform of the circuit,it is proved that the circut can recognize the discharge state effectively.