Allison Engine Company

Indianapolis, Indiana, USA

Cannon-Muskegon Corporation
[SPS Technologies]

Muskegon, Michigan, USA

ABSTRACT

Increased operating temperatures and higher rotational speeds resulting in increased component stresses, are primary goals in the continuing development of the gas turbine to provide improved fuel efficiency and power-to-weight performance. Cost reduction, from improvements in turbine component producibility and process yield, and through gains in airfoil component durability, is an additional objective Turbine engine operational requirements such as maximum airfoil temperature capability, reduced use of cooling air to increase engine efficiency and airfoil component life are also critical considerations.

A team approach involving a turbine engine company [Allison] and its single crystal casting facilities and a superalloy developer and ingot manufacturer [Cannon-Muskegon], utilizing the concepts of simultaneous engineering, has been used to successfully develop CMSX-4 alloy for turbine blade and vane applications. CMSX-4 alloy is a second generation nickel-base single crystal superalloy containing 3% (wt) rhenium (Re) and 70% volume fraction of the coherent g_ precipitate strengthening phase. Its finely balanced composition offers an attractive range of properties for turbine airfoil applications. In particular the alloy's combination of high strength in relation to creep-rupture, mechanical and thermal fatigue, good phase stability following extensive high temperature, stressed exposure, and oxidation, hot corrosion and coating performance, are attractive for turbine engine applications where engine performance and turbine airfoil durability are of prime importance.

The paper details the results from the single crystal casting process and heat treatment manufacturing development, including hot-isostatic-pressing (HIP), for turbine blades and vane segments in CMSX-4 alloy. Competitive single crystal casting yields with low levels of grain defects are demonstrated with over 1000 molds cast to date (March 1994) in the Allison production facilities (SCO) with twelve different CMSX-4 airfoil configurations. Significant numbers of Lamilloy® complex cooled single crystal blades and vanes have been cast in CMSX-4 alloy. Vacuum solution heat treatment and HIP experience with close to one hundred batches of airfoils confirm the alloy to have a practical solution heat treat/homogenization "window".

The creep-rupture data base on CMSX-4 alloy now includes 350 data points from eighteen heats including fifteen 3630 kg (8000 lb) production size heats. An appreciable portion of this data was machined-from blade (MFB) properties which indicate turbine blade component capabilities based on single crystal casting process, component configuration and heat treatment. Longer term creep-rupture testing out to 9359 hrs life at 114 MPa/982°C (16.5 ksi/1800°F) and 5613 hrs at 100 MPa/1038°C (14.5 ksi/1900°F) show no fall-off in the log-stress to log-life linear property relationships due to undesirable microstructural changes, such as the significant occurrence of topologically-close-packed (TCP) phases. The use of HIP has been shown to eliminate single crystal casting micropores which along with the essential absence of g/g_ eutectic phase, carbides, stable oxide, nitride or sulphide inclusions results in remarkably high mechanical fatigue properties,with smooth and particularly notched specimens. The Re addition has been shown to benefit not only creep and mechanical fatigue strength (with and without HIP), but also bare oxidation, hot corrosion (sulfidation) and coating performance.

The high level of balanced properties determined by extensive laboratory evaluation has been confirmed during engine testing a variety of Allison engines, utilizing both conventional single crystal and advanced CastCool® Lamilloy airfoils.