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.

Mechanical property balance, malleability, and weldability of Alloy 718 have driven widespread utilization across the aerospace and non-aerospace industries for nearly 50 years. However, the metastability of the primary strengthening gamma double prime phase is typically unacceptable for applications above about 650°C. As a result, other more costly and difficult to process alloys, like Waspaloy, are used in such applications. The latter alloys, strengthened primarily by gamma prime, are also more sensitive to weld-related cracking than Alloy 718. As part of the Metals Affordability Initiative CORE Program, several alternate alloys were identified and evaluated for aircraft engine static structural component applications for use temperatures of at least 700°C. The application-integrated project team consisting of engine manufacturers, General Electric, Honeywell, and Pratt & Whitney; forgers Firth-Rixson and Ladish Co., Inc.; primary metal producers, Allvac and Carpenter Technology; and the Air Force Research Laboratory, selected the Allvac-developed 718Plus® alloy composition for scale-up and validation. Subscale and full-scale experiments confirmed that processability and weldability of this alloy were significantly improved relative to Waspaloy, approaching that of Alloy 718. Complex rolled rings varying in size from less than 25 to nearly 250 kg have been processed validating the advantages of this alloy. Assessment suggests capability similar to Waspaloy to 704°C has been achieved along with an acceptable balance of other properties. This paper will summarize the processing, weldability, and mechanical property evaluations successfully performed in this project, as well as progress toward industrial implementation of this alloy.

Achievement of critical dimensions after wholesale removal of stock from large, ring rolled Waspaloy forgings and fabrications has been complicated by residual stresses induced via rapid quench from the solution heat treat cycle. Part movement during machining can be reduced significantly by substituting air cool for the oil quench commonly specified to achieve high mechanical strengths. However, excessive time in the sub-)/ solvus temperature range during slow cooling results in undesirable precipitate coarsening with an attendant degradation in strengths. Therefore, cooling rates must be achieved which strike a balance between manufacturing ease and Propew controlling microstructural features. This report provides microstructural and mechanical property data for flash welded, shaped Waspaloy bar stock subjected to various cooling rates from a 1018'C/4 hour solution cycle followed by a stabilize and age heat treat. Average cooling rates evaluated in this investigation ranged from 5.5 to 145"C/minute through the f precipitation temperature range of 982 through 760°C. Oil quench data from production hardware are also provided for reference. A significant improvement iu 538 and 760°C yield strengths, 760°C tensile strengths, and 704°C creep resistance were significantly improved by increasing cooling rates. Ductility simultaneously decreased at 760°C but did not exhibit dependence at 538'C. Minimum cooling rates of approximately 4OYYminute were necessary to approach the properties and microstructural features typical of oil-quenhed Waspaloy.

During the early 1990s, GEAE rotor manufacturing experienced a high frequency of pitting corrosion on as-manufactured surfaces of Alloy 7 18 components. Characterization of pits indicated aqueous chloride corrosion on a general front with delta platelets in relief. Attempts to reproduce this condition within the hard chrome electroplating shops were not successful. Pitting frequency was minimized with improved masking techniques but was not completely eliminated until the requirement for Cr-plating was deleted.Laboratory efforts to reproduce this condition yielded an electrolytic method to reliably generate pits with similar delta-rich features. Severity of pitting, frequency and size, varied with time in the electrolytic cell. Fatigue specimens of two severity levels were tested at 177OC showed a 3-5X debit on fatigue lives versus baseline Alloy 718 behavior. Although pitting severity was a statistically significant factor, the primary debit was the presence of a pit to initiate fatigue.

An investigation was performed to evaluate the effects of nickel and cobalt substitutions for all or part of the iron on the precipitation hardening behavior of Alloy 718. Isothermal exposures of solution heat treated wrought material were conducted to provide material for hardness testing and metallographic evaluation. A time-temperature-hardness diagram was prepared for each iron, nickel, and cobalt combination. Scanning electron microscopy was performed to document microstructural features while phase identification was performed using X-ray diffraction and energy dispersive spectroscopy of extracted residues.