To achieve greater engine
fuel efficiencies, engines are running at higher temperatures and must be
cooled with more intricate cooling schemes, requiring the casting of complex
cooling passages. Stronger metal alloys are being used in the casting process,
and the core material must be able to withstand the extremely high temperatures
used to pour these alloys.
Seeking ways to lower cost and emissions while increasing fuel economy and performance, engine designers have been turning to advanced ceramics and high-temperature metal materials. The ability of these materials to withstand heat is key to making engine improvements.
The Ancient and Modern Art of Brazing
Brazing alloys are used for metal-to-metal bonding in engine MRO (maintenance repair and overhaul), assembly of aerospace components and repair of micro-cracks. They are also used for ceramic-to-metal assemblies requiring joining by metallizing ceramic surface and brazing of components, including pressure and temperature sensors, thermocouple housings and fire-detection feed-thrus.
Brazing is a term used for high-temperature joining at temperatures above 600C. In a general sense, brazing is a joining process that relies on the wetting flow and solidification of a brazing filler material to form a metallurgical bond, a strong structural bond, or both between materials. The process is unique in that this metallurgical bond is formed by melting the brazing filler only; the components being joined do not melt.
Research into the development of advanced brazing materials for aerospace engine component repair has given rise to both precious and non-precious alloys. Precious alloys (for example, gold, silver, platinum and palladium) are used mainly in original equipment manufacturers' assemblies for vanes, nozzles, sensors and igniters. Non-precious alloys are used in MRO and are constantly evolving as better and more heat-efficient alloys are developed.
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Another example of the superalloys available for high-temperature braze repair applications are pre-sintered preforms (PSPs), a customized blend of the superalloy base and a low melting braze alloy powder in either a plate form, specific shape, paste or paint. PSPs are used extensively for reconditioning, crack repair and dimensional restoration of such aerospace engine components as turbine blades and vanes. Thin areas and crack healing is done with paste and paints, while preforms are used for dimensional restoration.
With turbine temperatures reaching up to 1,300C (2,350F) and the presence of hot corrosive gases, aerospace engine components experience considerable erosion and wear. The pre-sintered preforms are customized to fit the shape of the component and then tack-welded into place and brazed. PSPs are offered in various compositions and shapes, including curved, tapered and cylindrical, as well as paste and paint. They save time and money and extend the life of engine components by up to 300 percent, making it a more reliable and cost-effective method than traditional welding, which requires post-braze machining or grinding. Brazing allows whole components to be heated in a vacuum furnace, reducing distortions and increasing consistency, resulting in a high-quality repair process.
PSP plate thicknesses range from 0.010 inch (0.3 mm) to 0.200 inch (5 mm). In addition to plates, Morgan Technical Ceramics-Wesgo Metals supplies PSPs in pastes for filling oxidation corrosion fatigue cracks, and paints, which are best suited for deep, narrow micro-cracks.
Advanced ceramics are ideally suited for aerospace applications that provide a physical interface between different components, due to their ability to withstand the high temperatures, vibration and mechanical shock typically found in aircraft engines. For example, Morgan Technical Ceramics-Alberox business provides aerospace engine pressure and temperature monitoring sensors, thermocoupling housings, and fire detection feed-thrus constructed from a variety of metal components and high-purity alumina ceramic. Ceramic-to-metal components are sealed to metals by the high-performance brazing alloys, providing a reliable seal.
Investment casting is a key process used in the production of aerospace engine blades; high-quality ceramic cores have emerged as the material of choice for use in the investment casting process. Investment casting of new super engine alloy materials enables the development of more intricate designs that perform better in engines. Operating temperatures have increased, from about 400 to 1,100C, and along with that change has been an evolution in materials that meet the demand for surviving these higher temperatures.
Fused silica ceramic cores are used in investment airfoil casting of blades and vanes for rotating and static parts of aerospace engines. The process is used primarily with chrome-bearing steel alloys. Advanced ceramics with controlled material properties allow component designers to make special cooling channels that keep engines from overheating. These ceramic cores are capable of producing thin cross sections and holding tight tolerances, which help produce accurate internal passageways. The ceramic cores are strong enough to withstand the wax injection step in the investment casting process. While the casting is poured, the ceramic core remains stable, yet is readily leached using standard foundry practices once the casting has cooled.
For example, Morgan Technical Ceramics' Certech business (MTC-Certech) has developed a ceramic core with its proprietary P52 material, which exhibits greater dimensional accuracy while maintaining tight tolerances without distortion. The cores remain stable at high temperatures and do not prematurely deform, which is important, given the extremely high temperatures required for engine component production. The cores can be chemically dissolved after the casting has cooled, leaving the clean air passage replica needed in today's efficient turbine engines.
While dimensionally strong, the P52 core material also exhibits improved crushability during solidification. This means that it remains rigid and stable through the casting process but is crushable when it needs to be during the metal solidification process. This is particularly useful for alloys that are prone to hot-tearing (those that exhibit lower core temperature in equiax castings) and/or recrystallization (castings that are involved in directionally solidified or single-crystal castings).
Fred Kimock is vice president, technology, for Morgan Technical Ceramics. For more information on Morgan Technical Ceramics, go to http://www.morgantechnicalceramics.com/.