Research On EDM-ECM Combined Machining Process For Small-hole Without Recast Layer

High speed small hole EDM has the superiorities of high efficiency and stabilization, which make itapplicable to machining film cooling holes on turbine blades. However, EDM machining producesrecast layer on the machined surface, which contains imperfections such as micro cracks. Thus a postprocess (like abrasive flow machining (AFM) and chemical polishing machining) must be employedto remove the recast layer. But sometimes the recast layer can not be fully removed by AFM andworkpiece materials may corroded by the solution used in chemical polishing machining. To solvingthese problems, a combined electrical machining process for small hole without recast layer isproposed in this paper.This process fully utilizes the high efficiency of high-speed small hole EDM and the advantage ofno metamorphic layer of ECM. The same tool electrode and working fluid are used during the wholeprocess. First, the small hole is machined by high-speed EDM on directionally solidified Ni-basesuperalloy, then ECM is employed to remove the recast layer formed during the EDM process in situ.In this paper, the following tasks have been done:(1) A combined machining experimental platform was built. Scheme of the combined electricalmachining process was made and the experimental devices were developed. Electrolyte flushing flowfield simulation was made and analyzed to help designing a special electrolyte flushing device, whichcan ensure the ECM processing smoothly;(2) The effects of the EDM parameters (including discharge pulse width and peak current) on thesmall hole were discussed in small hole EDM experiment. The dimension of the small hole and thethickness of recast layer that were convenient for subsequent ECM process were selected. Small holeswith the average dimension of568.6μm were obtained base on the optimal EDM parameters (peakcurrent6A, pulse width10μs) and the average thickness of recast layer is20μm;(3) The ECM experiment was carried out to remove the recast layer formed during the EDMprocess. A revolving electrode and a fixed electrode were used respectively during ECM process, andthe processing effects were discussed and compared. The processing efficiency and stability of twodifferent power output modes were discussed. The processing results detected by metallographic testand SEM show that the recast layer was removed and ideal surface quality was obtained after ECMprocess.In the EDM-ECM combined machining experiment, small holes without recast layer were obtainedwithin about2min on workpiece with the thickness of1.5mm. The processing results show the feasibility and high efficiency of the combined machining process proposed in this paper.