Removal of recast layer in the laser drilled film cooling holes by a two-step inner streaming electrochemical technique

High-power millisecond laser is an efficient tool to manufacture micro holes, but the laser induced thermal defects, such as recast layer (RL), micro cracks and heat affected zone, are difficult to avoid. However, some important components, like turbine blades, are very sensitive to these defects, therefore, they must be eliminated in the manufacturing process of film cooling holes. We propose a combined processing method which includes high-power laser pre-drilling and electrochemical post-processing. One novelty of this study is that we applied a special back-bonded material (silicate mixture) under the Inconel 718 substrate, which functions as barricade to protect the opposite inner surface of the hollow blade in laser drilling, while in the electrochemical process, it functions as bottom support to form electrolyte back flow. A novel two-step inner streaming electrochemical post-processing technique was designed according to the characteristics of RL distribution in the laser pre-drilled holes. The processing strategy was optimized by a series of experiments. Finally, recast-layer free holes with clear and straight hole walls could be drilled in less than 30 s each, which is much faster than ultrafast laser drilling. (paper)