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Insights into drilling film cooling holes on ceramic‑coated nickel‑based superalloys

Vasudevan Balaji, Natarajan Yuvaraj, Sivalingam Vinothkumar, Krolczyk Grzegorz, Tandon Puneet

Archives of Civil and Mechanical Engineering

2022

Vol. 22, no. 3

art. no. e141

This article aims at providing insights into various drilling operations relating to the production of holes of small diameter for film cooling, particularly inclined holes, in aero-engine turbine blades, made of Yttria-stabilized Zirconia (YSZ) coated superalloy. Drilling inclined holes on YSZ coated superalloys that find use in aerospace industries is difficult due to the presence of two sections of materials, each section possessing different material properties. This article indicates five machining processes, namely conventional drilling, electrical discharge drilling, laser drilling, electro chemical discharge machining (ECDM), and abrasive water jet machining (AWJM). Machinability and the challenges involved in the fabrication of micro straight and inclined holes on ceramic-coated nickel-based superalloys have been discussed. Previous researchers have attempted at drilling tiny holes (≤ 2 mm) on ceramic-coated and uncoated nickel-based superalloys at different hole angles varying from 10° to 90°. Material removal was seen as difficult to control in the YSZ coated section when compared to the superalloys. Despite the occurrence of a controlled manner of material removal in different machining techniques, the holes produced had poor geometry and surface finish with cavities on the ceramic coat and burr formation at the hole exit. There was also the problem of thermal spalling, delamination, flakes, spatters, recast layer and cracks seen over the ceramic coat due to the employment of thermal-based machining techniques including laser drilling with advanced pulse settings. Keeping this in view, the AWJM technique is found to be a good alternative technique for drilling inclined holes with size constraints as the cold process maintains material integrity, followed by the ECDM and the conventional drilling processes. However, the hybrid process, namely ECDM, has still been a complex phenomenon in the material removal mechanism of the YSZ coated section consuming a lot of time. Based on the key results from the different setups, the outcome of this work provides a platform for scientists and engineers working in aerospace industries for finding a suitable machining process for drilling precision holes on the YSZ coated nickel-based superalloys.