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Thermal barrier coatings (TBCs) are used to provide thermal insulation for superalloy components from high-temperature gas streams. Due to the insulativity of the top coat and the different physical and chemical properties of each layer, it is difficult to machine small holes in TBC-coated components. In this paper, electrical discharge machining (EDM) was used to machine small holes in high-velocity oxy-fuel-sprayed TBC-coated nickel superalloys using the assisting electrode method. The signal-to-noise (S/N) ratio of the experimental results was used to study the influence of the processing parameters on the response characteristics. Based on the S/N ratio results, the machining parameters of multiple responses were optimized using the gray relational analysis method. Experiments were conducted using an orthogonal array, and the factors selected in this study were peak current, pulse duration, duty factor, and flushing pressure. The responses (i.e., machining time (MT), electrode wear (EW), maximum falling distance of 8 %YO-stabilized ZrO (MFD-YSZ), and surface roughness (Ra)) were chosen to evaluate the effects of machining. The results showed that peak current influences MT, EW, Ra, and MFD-YSZ more than the other factors and that pulse duration is the most significant factor for multiple responses. A confirmation experiment showed that the optimal process parameters improved the gray relational grade index for the multiple responses from 0.7393 to 0.7785. [ABSTRACT FROM AUTHOR]

Electrical discharge machining (EDM) is capable of fabricating thin wall structures due to the absence of physical contact between the tool and workpiece. However, the presence of impulsive force does exist during the sparking process, which could give rise to limitations in fabricating small features. The L9 Taguchi method together with the analysis of variance was used to determine the significance of various machining parameters on the specific impulsive force. Through the statistical analysis, it was observed that discharge energy was the most significant factor that influenced the specific impulsive force. Hence, it is important to understand the effects of specific impulsive force in EDM and to investigate its influence that limits the fabrication of thin wall structures. The limit in which the machined part is able to maintain its geometrical integrity is described as the critical wall thickness. The effects of specific impulsive force on thin wall structure were studied through determining the critical wall thickness for various discharge energies. The experimental work showed that both electrothermal erosion and brink erosion, which is attributed by the specific impulsive force was responsible for the loss of geometrical integrity of the wall structure. As the wall thickness falls below a critical value in a given discharge energy setting, the brink erosion was observed. The limit that established the critical wall thickness was elucidated by conflating the thermal erosion and brink erosion. [ABSTRACT FROM AUTHOR]

In most of the commercially available EDM systems, the control parameters such as the electrode jump height and the discharge machining time between two consecutive electrode jump actions are set manually to constant values which can ensure stable machining even under the worst machining conditions. It is beneficial to keep the machining process stable, but much time is consumed in the electrode jump process. In order to improve the EDM efficiency, a new adaptive control strategy which directly and automatically regulates the electrode jump height and discharge machining time has been developed in this paper. Experimental results show that the adaptive control system can obviously improve the machining efficiency regardless of the surface area, shape, and depth of the machined dies. Meanwhile, the accuracy and surface roughness of the fabricated dies can be well retained. [ABSTRACT FROM AUTHOR]