Advanced materials and processes for better aerospace engines

  • 20-Nov-2009 06:24 EST
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Pre-sintered preforms are customized to fit the shape of the component and then are tack-welded into place and brazed.

Advanced ceramics and high-performance superalloys are playing an important role in improving aerospace engines, as aerospace manufacturers look for high-temperature materials that increase performance, improve fuel efficiency, and satisfy safety standards while lowering manufacturing costs.

Seeking ways to lower cost and emissions and increase fuel economy and performance, engine designers have been turning more and more to advanced ceramics and high-temperature metal materials. The ability of these materials to withstand heat is key to making engine improvements in the areas of maintenance repair and overhaul (MRO) of engines, in the design and manufacture of ceramic-to-metal assemblies used in engine-monitoring applications, and in manufacturing critical aerospace engine components, particularly engine turbine blades and vanes.

Brazing and investment casting, adapted for use in repairing and manufacturing aerospace engines, make use of ceramics’ extreme heat resistance, in addition to its wear and corrosion resistance, light weight, and electrical- and heat-insulation properties.

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. It uses ceramics and high-temperature metal alloys for assembling components, performing engine maintenance and repair, and sealing monitoring sensors.

Researchers have developed advanced brazing materials for aerospace engine component repair using precious alloys (mainly in original equipment manufacturers’ assemblies for vanes, nozzles, sensors, and igniters) and nonprecious alloys, primarily in MRO.

For example, Morgan Technical Ceramics’ Wesgo Metals business supplies Nioro, a low-erosion alloy that allows the base material to retain its properties and is a good choice for repairing fuel systems and compressors.

Another example is pre-sintered preforms (PSPs), a customized blend of the superalloy base and a low-temperature-melt braze alloy, used extensively for reconditioning, crack repair, and dimensional restoration of such aerospace engine components as turbine blades and vanes. PSP pastes are used for filling oxidation corrosion fatigue cracks, and PSP paints are best suited for deep, narrow micro cracks.

For dimensional restoration, PSPs are customized to fit the shape of the component and then are tack-welded into place and brazed. As new superalloys are developed and used by OEM assemblies, MTC-Wesgo Metals follows along with a new nonprecious PSP for use in MRO of that component.

In addition, the Morgan Technical Ceramics-Alberox business provides aerospace engine pressure- and temperature-monitoring sensors, thermocoupling housings, and fire-detection feed-throughs constructed from a variety of metal components and high-purity alumina ceramic. The materials withstand the high temperatures, vibration, and mechanical shock typically found in aircraft engines. They are suitable for aerospace applications that provide a physical interface between different components. Ceramic-to-metal components are sealed to metals by the high-performance brazing alloys, providing a reliable seal.

Investment casting is defined as casting metal into a mold produced by surrounding, or investing, an expendable pattern with a refractory slurry coating that sets at room temperature, after which the wax or plastic pattern is removed through the use of heat prior to filling the mold with liquid metal.

Investment casting of new superalloy materials enables the development of more intricate designs that perform better in engines, where operating temperatures have increased from about 400°C to 1100°C. Advanced ceramics with controlled material properties allow component designers to make special cooling channels that keep engines from overheating.

Fused silica ceramic cores have emerged as the material of choice for use in the investment airfoil casting process, where they are used primarily with chrome bearing steel alloys for casting of blades and vanes for aerospace engine rotating and static parts.

Morgan Technical Ceramics Certech business has developed a ceramic core with its proprietary P52 material, which exhibits high dimensional accuracy while maintaining tight tolerances without distortion. The cores remain stable at high temperatures, do not prematurely deform, and can be chemically dissolved after the casting has cooled. This leaves the clean air passage replica needed in today’s efficient turbine engines.

The P52 core material also exhibits improved crushability during solidification, remaining rigid and stable through the casting process but crushable during the metal solidification process. This is particularly useful for alloys that are prone to hot-tearing and/or recrystallization.

MTC-Certech has developed a proprietary injection-molding process to create the ceramic cores faster, allowing high volumes to be manufactured in less time. These cores are less abrasive on the injection molds used, increasing their lifespan. Manufacturers can reduce or eliminate the use of costly platinum pins to hold the ceramic in place and support the core during the casting process, resulting in additional cost savings.

Ann Jacobson wrote this article for Aerospace Engineering on behalf of Morgan Technical Ceramics.

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