Application of Alloy 718 in GE Aircraft Engines: Past, Present and Next Five Years

By R. Schafrik, D. D. Ward, J. Groh. 2001

168 citations recorded by the source corpus.

GE Aircraft Engine's interest in Alloy 718 dates to the early 1960's. New jet engines were under development for the Supersonic Transport (SST) and the Air Force's C-5A, the first of the wide-body airplanes. These new engines required a stronger, more temperature-capable alloy than A286, without the fabrication limitations of Rene 41. Alloy 718 subsequently became the most widely used superalloy for aerospace applications. It is the alloy most used at GE Aircraft Engines (GEAE) with application in critical rotating parts, airfoils, supporting structures and pressure vessels. Alloy 7 18 has both business and technical characteristics that have enabled continued growth of its application base: Free license to manufacture has allowed a large material supplier and fabricator base to evolve Favorable precipitation kinetics provides substantial processing advantage, particularly weldability and castability. Relative manufacturing ease has permitted multiple manufacturing processes and product forms to develop over the years. Improvements in Alloy 718's quality and cost have fostered continued application in GEAE designs. GEAE has tailored its requirements for melting, conversion, forging, and heat treat processing for high strength Alloy 718 applications, while balancing creep and crack growth behavior. For critical rotor applications, GEAE and its specialty metals supply chain have developed high strength (HS718) and directed aged ( ~ ~ 7 1 8 ~ ) versions of Alloy 718. These variants incorporate process refinements that include cleaner input stock and finer grain processing. These improvements, along with the development of more stringent ultrasonic inspection techniques, have enabled increased Alloy 718 application in demanding GEAE rotor hardware without sacrificing durability or performance. Investment cast 718 has been extensively utilized in GEAE structural designs to reduce the cost of fabricated components. Its slow precipitation kinetics offers a weldability advantage (important for reduction in manufacturing losses and field repair of hardware), but also enables alternative process methods, such as spray forming, roll forming, and laser deposition. The attractive physical metallurgy and large usage base of Alloy 718 make it the logical first choice for the scale-up of new, low cost manufacturing methods. For the future, engine design engineers are demanding an affordable Alloy 718 derivative possessing higher temperature capability. Current applications of Alloy 718 are limited by the metastability of the y"-strengthening system. Efforts to date, with traditional approaches, to increase the temperature stability of Alloy 718 have not been successful. GEAE advocates the use of computer-based analysis tools to aid the development of a derivative alloy. Success hinges on a well-coordinated effort with government, university, and industry participants to develop the physical and data-driven models and analysis techniques necessary to realize this objective of a higher temperature Alloy 718 derivative within the decade. Early Alloy 718 Introduction to GE Aircraft Engines (GEAE) Before the advent of Alloy 7 18, turbine manufacturers primarily relied upon precipitationstrengthened stainless steels (i.e., A286) and y'-strengthened Ni-base superalloy, such as RenC 41, for elevated temperature applications. However, by the late 19501s, GEAE had bumped up against the mechanical and temperature constraints of the stainless steels. The fabrication difficulties of RenC41 were well known, such as its susceptibility to strain-age cracking during post-weld heat treatment that required costly special processing to resolve. In 1960, Huntington Alloys-INCO's Herb Eiselstein promoted Alloy 718 for turbine applications.* The prospect of an alloy with significantly improved ease of manufacture combined with mechanical properties that approach RenC 41 levels was recognized early by GEAE as a revolutionary change for GEAE designs. Interest from several GEAE engine programs supported evaluation of alloy behavior relative to the existing materials. For Alloy 718, as-quenched hardness of HRc 20 and lower, from solution could easily be achieved -a real benefit to formability in fabrication of structures as well as to hot forge of critical rotating structures. The classic slow age-overage precipitation hardening behavior of the y-y" strengthening system was credited with causing the weldability improvement and gave insight to the Ren6 41 strain-age cracking phenomenon. As precipitation strengthening is relatively slow, constraint at the weld interface is minimized, so that stresses never rise high enough to initiate cracking. Through-thickness hardening of Alloy 718 to the low HRc 40s could readily be achieved via subsequent aging using an economically viable heat treating cycle. Sensitivity to notch failure in rupture tests with the standard age was corrected via evolutionary improvements in melt processing and sulfur control. Due to its excellent balance of properties and reasonable cost, Alloy 718 quickly found adherents within GEAE. By the early 1960s it was slated for use in the X211 nuclear engine and the GE4 (Supersonic Transport) turbines; however, neither of these engines were released for production. The GE4 program did provide much-needed engine experience and a basic understanding of material behavior and manufacturing methods critical to eventual scale-up to production.3 Included in these studies was the influence of pre-exposure to 538°C-704°C [1000°F-13000F] and 10,000 hours following what has become the typical 982°C [1800°F] solution plus 71 8°C-621°C [13X°F+l 150°F] age heat treat cycles. Retention of mechanical properties was intermediate to R'41 and the less capable Astroloy. Alloy 718 did show strength due to over-aging of bct y" to form orthorhombic delta, which contributes little in the way of strengthening with the exception of grain boundary pinning during forge and heat treat processes. No evidence of embrittlement or TCP precipitation was encountered. In contrast to the two ?/-strengthened alloys, ductility of 718 actually increased with exposure. Extension of the 718 pre-exposure data from this study was supplemented with microstructures and eventually published in open ~i terature.~ Alloy 718 was first introduced into a production application for portions of the 593 (XB-70 bomber engine) turbine frame that were welded to wrought INCO 722 rings. This application provided only limited production volume. The next application led to significant production outlet; the GEl development core evolved to GE's successful, long running TF39lCF66LM2500 engine family. Selection of 718 forgings for compressor airfoils, critical rotating hardware, and static structures required a fast-paced, focused effort at GEAE and our supplier base to ensure success in the TF39 engine program, as well as in the CF6-6 and LM2500 derivatives. Figure 1 depicts alloy usage for a typical CF6 engine. Alloy 718 comprises a striking 34 percent of finished component weight. The usage is distributed over forgings and investment castings. This dominance of Alloy 718 usage by GEAE has continued to the present day. CF6 Material By Finished Weight Comwsltes 4 % powder CF6 Material By Form Cast 6% Figure 1. Alloy 718 relative input weights for a typical CF6 ~onf i~ura t ion .~ Current Alloy 718 Applications at GE Aircraft Engines All subsequent CF6 derivatives, including the CF6-50lLM5000 and CF6-80CLM6000 families, incorporated 718 in forged and cast components. Every GEAE engine family applies 718 as the material of choice to applications below approximately 650°C [1200°F]. Time at temperature and material condition is considered key to actual maximum operating temperature limits. GEAE engine families currently using 718 in production include: FlOl/FllO/CFM56, GE90, T700ICT7, TF341CF34, F404/F414/LM1600 and CFE738. Figures 2 and 3 summarize GEAE's current usage of Alloy 718 (not included are purchases by engine partners and sub-tier contractors). Figure 2 indicates usage of Alloy 718 for critical rotating components has remained strong over the last 5 years, even increasing slightly. Figure 3 summarizes the type of material used in GEAE rotating and structural forgings for 2000. These figures illustrate GEAE's strong commitment to Alloy 7 18. GEAE Rotating Parts ForglngsPurchased Welght mo\, Figure 2. Materials Used in Rotating 700% Parts' Forgings, 1995-2000 I I 60 0.0 1 i I I I GEAE Forged Metal 2000

Subfields: Aerospace Engineering, Mechanical Engineering

View Paper: https://www.semanticscholar.org/paper/6a8342645136d26bad3ddd4960ed8bbf33deb776

How researchers see this paper

The researchers whose trusted networks rank it highest, strongest advocate first. Every researcher gets a different order.

In Aerospace Engineering

  1. Stephen T. Newman
  2. Alborz Shokrani

In Mechanical Engineering

  1. I. Al-Samarrai
  2. Stephen T. Newman
  3. Alborz Shokrani
  4. A. A. Khan

Citations in the corpus