Investigation on Conductive Layer, Delamination, and Recast Layer Characteristics of Electro-discharge Machined Holes in
- PDF / 5,340,898 Bytes
- 10 Pages / 593.972 x 792 pts Page_size
- 19 Downloads / 150 Views
JMEPEG DOI: 10.1007/s11665-017-2654-2
Investigation on Conductive Layer, Delamination, and Recast Layer Characteristics of Electro-discharge Machined Holes in TBCs Li Wang, Yongfeng Guo, and Guowei Zhang (Submitted September 12, 2016; in revised form March 8, 2017) In this study, electrical discharge machining (EDM) is used to directly drill normal effusion cooling holes in thermal-barrier-coated nickel-based superalloys (TBCs) via the assisting electrode method. The formation of the conductive layer was studied using scanning electron microscopy and energy-dispersive spectroscopy. The effects of the EDM process parameters including peak current, pulse duration, and duty cycle on delamination and recast layer characteristics were investigated. The analysis results indicate that the conductive layer possesses a feature of bilayer structure for the EDM of TBCs. The bottom layer is generated first due to the deposition of carbon-based products and molten brass debris, and its composition primarily contains C, Cu, Zr, and Zn; the surface layer is the result of the overlying of subsequently molten superalloy debris and carbon-based products, and its composition primarily consists of Ni, C, Cr, Nb, Co, Al, Fe, Cu, and Zr. The microcracks of the superalloy substrate only reside in the recast layer during the EDM of TBCs. The thickness of recast layer sharply increases with increasing peak current, pulse duration, and duty cycle, respectively. The delamination occurs at the ceramic coating/bond coating interface for the EDM of drilling normal holes in TBCs, and it can be eliminated by the selection of low discharge energy and appropriate duty cycle. Additionally, the length of delamination increases with increasing peak current, pulse duration, and duty cycle, respectively. The spalling of ceramic coating appears at the entrance of the hole due to the thermal-shock brittle fracture if excessive peak current, pulse duration, or duty cycle is selected. Keywords
assisting electrode, conductive layer, delamination, electrical discharge machining, normal effusion cooling holes, recast layer, thermal-barrier-coated nickelbased superalloys
1. Introduction The increasing demand for higher efficiency of aircraft engine has been facilitating the development of thermal barrier coating (TBC) and the effusion cooling technique (Ref 1, 2). In the aircraft engine manufacturing process, the aero-engine hot end components such as turbine blades, after burners, and combustion rings are covered with insulating TBC to protect them from direct exposure to corrosive high temperatures (Ref 3). The effusion cooling approach is then introduced to further improve the working temperature of the aircraft engine. With the combination of the two methods, the aero-engine hot end components can approach a higher working temperature of 1150 °C (Ref 4). Therefore, this process route has been extensively used by aircraft engine manufacturers. However, the hot end components are often made of tough nickel-based superalloys, and approximately 100,000 small ho
Data Loading...
