Assessment of surface integrity of Ni superalloy after electrical-discharge, laser and mechanical micro-drilling processes

Currently, electrical discharge machining (EDM) is the established manufacturing process for making holes through Ni superalloy turbine blades to facilitate blade cooling. Surface integrity of these sub-millimetre holes is of paramount importance since it directly influences the fatigue life of the component. In this paper, EDM is compared against laser and mechanical micro-hole drilling of Inconel 718 alloy. The motivation was to assess alternative manufacturing methods for raising the quality threshold of micro-drilled holes. Mechanical and metallurgical characterisation of surface and subsurface regions of the holes produced by the three processes was undertaken using backscatter electron (BSE) microscopy, electron backscatter diffraction (EBSD) and nano-indentation techniques to assess surface hardness, grain misorientation, plastic deformation and the heat-affected zone. The results suggest that mechanical micro-drilling offers improved mechanical, metallurgical and geometrical properties compared to both laser and EDM processes.