Basic Research on Electrochemical Machining for Superalloy Casing
Author:Ge Yong Cheng
Supervisor:zhu zeng wei
Casing parts are one of the key components of aero engines,and their shape features are mostly cylindrical or conical revolving structures.There are many kinds of casing parts in the engine,and their structural forms are quite different depending on the role they play in the engine.Some casing parts have a large number of concave-convex structures,such as combustion casings,turbine casings,etc.Some casing parts are relatively simple,and there are no complicated concave-convex structures on the revolving surface,such as the two stage low pressure casing and combustor outer casing.Nickel-based superalloys combine excellent high-temperature strength,good oxidation resistance,thermal corrosion resistance and other comprehensive performance,becoming the important materials for aircraft engine parts.To reduce the fuselage weight and pursue higher thermal efficiency,the casing parts are usually thin-walled and their partial wall thickness is even smaller than 1 mm.The characteristics of large removal margin,difficult-to-cut,and easy deformation bring great challenges to the traditional processing methods.Therefore,according to the different structural characteristics of the casing parts,it is of great significance to find suitable processing methods.With the continuous improvement of the demand and performance of aero-engine for the defense industry,it is very meaningful to find suitable and efficient advanced machining technology according to the processing requirements of different casing structures.In this paper,the superalloy casing parts in the areo-engines are taken as the research object and focusing on the processing demand of the concave-convex structures,and the revolving surface without concave-convex structures,the site directed power interruption counter-rotating electrochemical machining technology(SDPI-CRECM),annular deep-cut electrochemical machining technology(ADCECM)and the hybrid machining technology of electrochemical machining and mechanical grinding are proposed respectively.Series of theoretical analysis and experimental research are carried out.The main studies and constributions in this paper are as followings:(1)Based on the processing requirements of concave-convex structure of the wrought superalloy casing,the SDPI-CRECM method is proposed.A mathematical model is established,and numerical simulation has been conducted to analysis the materials removal process.The electric field distribution and the simulated convex profiles show that the stray current density can be reduced effectively by using the proposed method.The correction value has an important influence on the machining accuracy.Subsequently,the experimental studies on the SDPI are carried out on the forging superalloy GH4169.The results reveal that the developed simulation algorithm is reasonable and feasible,and the SDPI method plays an important role in the protection of the non-processing area of the anode workpiece.(2)Aiming at the processing requirements of wrought superalloy casing without the concave-convex structures,the ADCECM technology is proposed and the flow mode and the cathode structure are optimized respectively.The theory of anode material removal of ADCECM technology is studied,and the electric field simulation is also carried out.The results show that the proposed method can significantly improve the current of the anode surface.The influence of cut-depth of the cathode tool on the anode machining process is also studied.The results show that the current of the anodic workpiece increases significantly with the increase of the cathode cut-depth,which theoretically proves the advantage of the proposed method on the machining of revolving parts.Besides,the flow pattern and cathode structure are also optimized to help smooth out the electrolysis products during processing.The results show that the uniformity of the flow field in the machining gap can be significantly improved by using the optimized flow pattern and cathode structure.Several typical rotary parts are successfully machined to prove the application potential of the proposed method.Finally,the annular deep-cut electrochemical machining method is carried out on the forge superalloy GH416,with cut-high of 30 mm and radial removal allowance of 12 mm.The material removal efficiency is about 90 mm~3/(min?cm).(3)For ECM of cast superalloy casing,the microstructure and the electrochemical dissolution characteristics of the typical cast superalloy K423A are studied,and the electrochemical dissolution model of cast superally K423A is established which can provided theoretical guidance for ECM process.The microstructure study shows that the microstructure of cast superalloy is more complex than wrought superalloy,a large number of carbide,oxide and impurity particles are involved in the matrix material.The study of electrochemical dissolution characteristics shows that the cast superalloy can obtain higher current efficiency than the wrought superalloy,which indicates that the ECM of cast superalloy can obtain higher processing efficiency.However,the surface quality of machined cast superalloy specimens is relatively poor,which also promotes further research on the hybrid machining of ECM and mechanical grinding.(4)For the processing requirements of casting superalloys casing without concave-convex structures and the characteristics of the insoluble particles in the cast superalloy matrix that result in the rough surface,the hybrid machining of ECM and mechanical grinding is proposed,and the removal mechanism of anode material is explored experimentally.Through the study and analysis of the surface characteristics of the machined specimens under different feed-rates and different electrolyte pressure,the removal mechanism of anode material in the proposed process is seriously discussed,and the role of the mechanical grinding during the removal of anode material is revealed.In addition,the influence of the rotating speed of cathode tool and the grain size on the processing current and the processing efficiency are discussed experimentally,which can improve an guidance for the large allowance removal experiment of the cast superalloy.(5)According to the machining characteristics and technological requirements of the hybrid machining of deep-cut ECM and grinding,the rotary joint device of the electrolyte is developed,and the auxiliary inflatable device with multi degree of freedom control is designed.In order to realize the high speed rotation of the inte-jet cathode tool and the stable supply of electrolyte for the machining gap,a precise rotary joint device is designed carefully.The auxiliary inflatable device with multi degree of freedom control is designed to achieve gas-controlled freely at any angle for different sizes,different shapes and different depth of the anode workpiece to achieve the protective effect on the machined surface of the rotary parts.(6)The hybrid machining of deep-cut ECM and grinding is performed to remove the cast allowance and cast riser for a certain type of cast case.Besides,the gas-insulated protection method is proposed in the machining of cast casing.According to the target processing object,the inter-jet cathode tool is designed specially.The protection of machined surfaces is achieved by auxiliary inflating on the side of machined area to force the electrolyte away from the machined surface.The simulation results show that the electrolyte’s covering area on the machined surface can be compressed by the auxiliary inflating,and the electrolyte conductivity of the transition region is significantly reduced.The experimental results confirm the protective effect of auxiliary inflating on the machined surface.Finally,the hybrid of deep-cut ECM and grinding is performed to remove the cast allowance and cast riser for a certain type of cast case.The machining efficiency and the surface quality are both improved effectively,and the material removal rate of the anode casing parts as high as 191 mm~3/(min?cm).