Study On Improving Surface Integrity Of Micro-hole ECDM Machining

Film-cooling holes structure is widely used in advanced aero engine turbine blades for cooling.The machining accuracy and quality of film-cooling holes greatly affect the performance of aero engines.Electrochemical discharge(ECDM)drilling combines the mechanism of EDM and ECM in one step by using working fluid of specific concentration.Electrolytic reaction will remove the recast layer remained by EDM,improving the quality of machined small holes.But the electrolytic reaction during ECDM is hard to control,and excessive electrochemical will cause corrosive damage to the surface of the small holes,especially around the entrance.This paper analyses the machining parameter which will influence electrolytic reaction during ECDM.Several methods are tried to improve to localization of electrolytic reaction,in order to reduce corrosive damage around the entrance of the small holes and improve the surface integrity of small holes.The main research contents are as follows:(1)The electric distribution on the surface of workpiece during ECDM drilling is analyzed and the process parameters during experiments are optimized according to the results of simulation and experiments.The optimized parameters are used to conduct ECDM drilling experiments on Nickle-based single crystal alloy(pulse width 12?s,pulse interval 36?s,peak current 12A).The surface topography of the entrance of machined micro can be divided into three zones: the edge zone,the transition zone and the workpiece surface zone,which all have different improving demand and difficulty.Due to the distribution difference of alloy elements,selective corrosion occurs on the surface of the workpiece during ECDM drilling,and carbon content on the surface of the workpiece rises.The difficult-to-corrode phase of the alloy remains on the surface of the workpiece in the form of particles,and peels off with the continuous progress of the electrochemical reaction,leaving pits on the surface of the workpiece,which damage the surface integrity of the workpiece.Besides,the residual tensile stress zone on the surface of the workpiece caused by electric discharge machining can be removed by electrochemical machining,which has a positive significance for improving the fatigue life of machined workpiece.(2)According to the formation mechanism of stray corrosion on the surface of the workpiece during ECDM drilling,a method is proposed to shield the workpiece from the working fluid by using insulating medium to improve the surface integrity of the workpiece.Two insulating medium,insulating lacquer and airflow,are tried.Protecting the surface of the workpiece by insulating lacquer can ensure that the surface of workpiece will not be corroded by stray current during drilling without affecting the removal of recast layer on the side wall.This method is easy to operate,but can effectively protect the surface of the workpiece from being corroded by stray current and is highly practicable.The airflow shielding method uses air flow to impact the surface of the workpiece,accelerating the working fluid away from the surface of the workpiece,so that the workpiece need no pre-insulation but also can be protected from stray corrosion,improving its surface quality.However,airflow will disturb the stability of machining state,and carbide will remain on the edge of machined micro-hole.This method needs future improvement.(3)To solve the appearance of increased corrosion caused by sudden change of current density at the edge of the shield area,an auxiliary anode of positive potential difference is used to optimize the distribution of the electric field during ECDM drilling,and the parameters of the auxiliary anode are optimized according to the simulation analysis and experimental verification.The inner diameter of the auxiliary anode is 0.7mm,the gap between the auxiliary anode and the workpiece is 0.05 mm and the potential difference is +15V.Affected by the application of this auxiliary anode,the surface integrity and dimensional accuracy of the micro-holes machined by ECDM drilling have been improved.This method has stable processing condition and significant effect,but it requires external circuits and is relatively complicated to operate.This method has poor practicability and is only recommended for high-quality processing of a small number of micro-holes.