Fundamental Research On Electrochemical And Discharge Machining For Micro Hole

Micro-holes are widely used in the aerospace industry,and their fabrication requires high machining speed and accuracy,as well as good surface quality.In this study,attention has been paid to the fabrication of micro-holes in an advanced Nickel-based superalloy at high speeds and without recast layers.The main studies and contributions in this dissertation are as followings:(1)Tube electrode high-speed electrochemical discharge drilling(TSECDD)is proposed as a promising hybrid machining method for the fabrication of film cooling holes in difficult-to-machine superalloys.An electrochemical reaction can occur if a low-conductivity salt solution is used in the drilling.Materials can also be removed at a high speed using electrical discharge machining(EDM).Thus,This machining process achieves both a high machining speed and good surface finish.In this study,the material removal mechanism of TSECDD was studied using a low-conductivity salt solution,and comparisons with high-speed electrical discharge drilling were made.The performance of the process was investigated using salt solutions of various conductivities.Experiments confirm that the use of this process with a low-conductivity salt solution can improve the machining surface and machining efficiency achieved.The results also show that the use of a low-conductivity solution improves the material removal rate,the hole diameter,and the taper angle.(2)Super-high-pressure interior flushing,which utilises more than twice the pressure traditionally used for interior flushing,was applied to TSECDD.The resultant machining technique and its performance were examined and compared with sinking machining and traditional interior flushing.The effects of various interior flushing pressures on the TSECDD process were also investigated.In comparison to sinking machining and traditional interior flushing,the proposed super-high-pressure interior flushing was found to be more effective in removing the by-products of the machining process,and it was also found to enhance the machining efficiency and surface quality.The performance of the proposed machining technique was observed to vary with the diameter of the tube electrode.A Φ500 μm tube electrode allowed the use of a relatively high flushing pressure,which enhanced the material removal rate(MRR)and the removal of the machining products and also produced a small taper angle.However,with Φ200 μm and Φ300 μm tube electrodes,the optimal flushing pressure for obtaining a high MRR and a small average hole diameter and taper angle was found to be 10 MPa.(3)To improve the flushing condition and further enhance the machining performance,improved tube-electrode structures,obtained by varying the inner diameter and inner shape,are introduced.In this study,different inner structures are designed for the tube electrodes,and the effects of different tube-electrode inner structures on the machining performance are investigated.The results show that an increase in the tube-electrode inner diameter results in a higher material removal rate,smaller average hole diameter,and smaller taper angle.However,for the single-hole tube electrode,a larger inner hole results in the formation of a residual cylinder.Thus,the double-hole and multi-hole tube electrodes are proposed and found to be effective in removing the residual cylinder.Finally,it is verified that the tube electrodes with improved inner shapes can be used to further enhance the machining performance.The double-hole tube electrode is confirmed to have the optimal structure.(4)Film cooling holes in a turbine blade or vane are usually distributed over the blade or vane and are drilled at various angles to the surface.To satisfy the resulting complex processing requirements,the machining system used in this study was proposed with a five-axis motion table using closed-loop control for high-precision positioning.(5)TSECDD is used to machine a film-cooling hole in a Nickel-based single-crystal superalloy(DD6).The Taguchi methods of experiment are used to optimise the machining parameters.Experimental results show that TSECDD can effectively drill the film-cooling hole;the optimum parameters that give the best performance are as follows: pulse duration: 12 μs,pulse interval: 30 μs,peak current: 6 A,and salt solution conductivity: 3 mS/cm.The nonmetal is padded the back of the workpiece to completely remove the recast layer especially in the exit of the hole.Finally,a high depth-diameter hole is machined by TSECDD,and the results are compared with those obtained by electrical discharge machining.TSECDD is found to be promising for improving the machining efficiency surface quality and eliminating the recast layer.