Drilling of film cooling holes by a EDM/ECM in situ combined process using internal and side flushing of tubular electrode

摘要

To allow very high turbine inlet gas temperatures, film cooling holes are widely applied in gas turbine engines. Owing to the requirements of high machining efficiency and surface quality, the fabrication of film cooling holes in difficult-to-machine materials is extremely difficult. A tubular electrode electro-discharge high-speed drilling/electrochemical machining (ECM) in situ combined process has been proposed for this task. In this approach, electro-discharge machining (EDM) is efficiently employed to form the holes, following which the recast layer thereby generated is removed from the machined surface by electrochemical dissolution. In this study, a tubular electrode and a specially designed flushing device are used, with internal and side flushing being performed at the same time so that EDM and ECM can be combined in situ. The performance of the technique is investigated on a nickel-based superalloy. To achieve the best performance, the ideal values of EDM parameters (peak current, pulse duration, and pulse interval) are determined. The ECM is employed to remove the rough surface produced by the EDM. Compared with EDM alone, it is found that the EDM/ECM in situ combined process improves the surface quality of the holes and removes the recast layer. It is confirmed that the combined process is suitable for producing film cooling holes without a recast layer.