Thinning Process of Recast Layer in Hole Drilling and Trimming by EDM

In order to reduce the recast layer thickness of film cooling holes on turbine blade, a novel thinning process of electrical discharge machining (EDM) is investigated. To avoid the contradiction between holes drilling speed and recast layer of nickel-based superalloy, the EDM holes process consists of two steps, the drilling process for high machining speed and the trimming process for thin recast layer. To measure and evaluate the uneven and inconsistent recast layer thickness of film cooling holes, a mesh partition and multi-point averaged measurement by optical microscope is discussed. Experimental investigation is conducted to determine the EDM drilling and trimming parameters in machining Inconel 738 with 6 mm thickness. By changing the discharge polarity in the trimming process, the recast layer thickness is obviously reduced. Over the range of parameters tested, the recast layer thickness can be reduced from 10.472 μm to 2.416 μm, and the entire process including drilling and trimming could be completed within 20 s. The experimental results show that the hybrid process containing drilling and trimming by EDM is feasible for film cooling holes with thin recast layer thickness.