EDM Drilling and Shaping of Cooling Holes in Inconel 718 Turbine Blades

Machining capabilities of modern EDM drilling machines for drilling cooling holes and diffusers in turbine blades are described. For improving the process capability and machining efficiency of aerospace alloys, drilling of cooling holes and formation of diffusers can be combined into a single process by using the same electrode for both processes. Experimental results of material removal rate, relative tool wear and surface integrity for EDM drilling and shaping processes are shown. Particular attention is given to the process parameters analysis such as discharge current and discharge duration, their influence on the heat-affected zone and the influence of relative tool wear on the shape accuracy. Thereby the material removal rate reached 77 mm3/min, relative tool wear was reduced down to 20% and the average recast layer thickness was reduced to 8 μm. The best results were achieved separately with different sets of parameters. After diffusor shaping the roughness Sa of the internal surface is less than 1 μm. Accordingly the EDM drilling and shaping processes demonstrate the possibility to combine high processing speed with relatively low recast layer thickness and good surface quality.