Influence of Electrical Discharge Machining on Thermal Barrier Coating in a Two-Step Drilling of Nickel-Based Superalloy
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RESEARCH ARTICLE-MECHANICAL ENGINEERING
Influence of Electrical Discharge Machining on Thermal Barrier Coating in a Two-Step Drilling of Nickel-Based Superalloy Changshui Gao1 · Zhuang Liu1
· Tianhai Xie1 · Chao Guo1
Received: 27 May 2020 / Accepted: 23 September 2020 © King Fahd University of Petroleum & Minerals 2020
Abstract The influence of electrical discharge machining (EDM) on thermal barrier coating (TBC) in a two-step drilling of nickelbased superalloy has been studied. The TBC was coated on the superalloy using electron beam physical vapor deposition (EB-PVD). Simulation result shows that the temperature at TBC/metal interface increases with pulse-on time and peak current. The temperature will exceed the melting point of the coating material when using relatively large pulse energy. Experimental result reveals that large pulse-on time or high peak current caused defect such as cracking at coating sidewall and delamination at the TBC/metal interface. For EDM drilling holes with inclined angle, the leading edge had worse condition of debris evacuation, and consequently caused secondary discharge which resulted in melt and spalling of the coating material. In addition, relatively larger pulse-on time and greater peak current might result in greater thermal stress gradient at the interface and subsequently lead to cracking or spalling at the coating layer. Therefore, process conditions of short pulse-on time, longer pulse-off time, and small peak current are recommended for EDM drilling of the EB-PVD coated nickel-based superalloy. Keywords EB-PVD thermal barrier coating · Electrical discharge machining · Cooling hole · Abrasive slurry jet machining · Micro-drilling
1 Introduction The hot sections of aero-engines, e.g., turbine blade and vanes, operate at temperatures beyond the melting point of the material to achieve more engine efficiency. Therefore, the thermal barrier coating (TBC) and film cooling technologies are applied concurrently on these hot sections of modern aero-engines to provide thermal insulation against high-temperature gas streams [1, 2]. There are two widely used deposition methods for TBC, i.e. air plasma spray (APS) and electron beam physical vapor deposition (EB-PVD) [3]. EB-PVD deposited TBC features a columnar microstructure which gives the ceramic top coat improved strain and thermo-shock tolerance compared to APS technology. The EB-PVD coated turbine blades, mostly made up of nickelChangshui Gao and Zhuang Liu have contributed equally to this paper.
B 1
Zhuang Liu [email protected] College of Mechanical and Electrical Engineering, Nanjing University of Aeronautics and Astronautics, 29 Yudao Street, Nanjing 210016, China
based superalloys, are normally designed to have tens to hundreds micro-sized cooling holes needed to be drilled. These cooling holes can be drilled before deposition of the TBC (drilling pre-coating technology) or after deposition of the TBC (drilling post-coating technology) [4]. For the drilling pre-coating technology, mature machining processes are e
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