Sintering and Joining of Ni-Based Superalloys via FAST for Turbine Disc Applications
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THE commercial and DoD sectors aim to double the thrust-to-weight ratio of aerospace turbine engines, and one technology currently pursued to increase the thrust-to-weight ratio is a one-piece rotating turbine system composed of turbine airfoils (single crystal, directionally solidified, or equiaxed) connected to a turbine disc. The whole assembly, a blisk, is made of nickel (Ni)-based superalloy. Expected weight savings by elimination of mechanical fir-tree connectors is ~ 30 pct, leading to an increase in rotational speed and pressure ratio per stage, and thus thrust-to-weight ratio.[1] In addition, elimination of the mechanical joint can reduce vibration and associated fatigue.[2] There are two challenges that must be addressed to achieve this goal. First, the approach to joining airfoils with the turbine disc must be developed to minimize any strength penalty. Second, high-temperature properties must be maintained. Single crystal airfoils are typically operated
CHARIS I. LIN and NAMIKO YAMAMOTO are with the Department of Aerospace Engineering, The Pennsylvania State University, University Park, PA 16802. Contact e-mail: [email protected] SEBASTIAN J. NIUMAN and JOGENDER SINGH are with the Applied Research Laboratory, The Pennsylvania State University, University Park 16802. Contact e-mail: [email protected] ANIL K. KULKARNI is with the Department of Mechanical and Nuclear Engineering, The Pennsylvania State University, University Park 16802; DEREK S. KING is with the UES, Inc., Dayton, OH 45432. Manuscript submitted February 6, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
at temperatures up to 1400 C.[3] This high operating temperature will have an impact on the operating temperature of the turbine disc rim. Typically, the operating temperature of a turbine disc is about 600 C to 750 C, and the outer rim temperature may increase with a blisk configuration. Therefore, the operating temperature and other requirements of the outer rim of turbine discs must be addressed. There are two objectives in this research paper, to explore the joining of Ni-based superalloys (without any strength penalty) and to improve the operating temperature capability of the outer rim of turbine discs, which is associated with other requirements. The outer rim must have good creep resistance, which is associated with large grain size, whereas the core must have high fatigue strength and toughness, which are associated with small grain size, as shown in Figure 1.[4] This illustrates that a turbine disc should ideally have a dual microstructure. Limited research has been conducted in the development of dual microstructure turbine discs by suitable preferential heat treatment, i.e., heating the outer rim to achieve large grain size and cooling the core to retain a smaller-grained microstructure.[5–8] The shortcoming of this approach is that the chemical composition of the entire disc remains the same, and thus the outer rim of the disc will not have the needed high thermal stability. This high thermal stability requirement needs t
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