Aerospace engines must run at higher temperatures to meet the latest engine fuel efficiency requirements. But these higher temperatures require intricate schemes to cool the engines. To create the needed complex cooling passages, two ancient arts — brazing and investment casting — have re-emerged. These processes require strong metal alloys and ceramics because the core material must withstand the extremely high temperatures used to pour them.
Newer ceramics help aerospace manufacturers develop intricate cooling passages to cool engines that operate at today’s high temperatures.
Brazing alloys are used for metal-to-metal bonding in engine overhaul, aerospace component assembly, and micro-crack repair. They are also used for ceramic-to-metal assemblies including pressure and temperature sensors, thermocouple housings, and fire detection feed-through. The brazing process is unique in that the 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 application 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 maintenance and repair and are constantly evolving with better and more heat efficient alloys.
A number of new brazing alloys are available for aerospace engine repair and reassembly. For example, Morgan Technical Ceramics’ Wesgo Metals business (MTC-Wesgo Metals) 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 of the superalloys available for high temperature braze repair applications are pre-sintered preforms (PSPs), a custom 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 1300° C (2350° F) and the presence of hot corrosive gases, aerospace engine components experience considerable erosion and wear. The pre-sintered preforms are fitted to the shape of the component and then tack-welded into place and brazed. 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 in. (0.3 mm) to 0.200 in. (5 mm).
Investment casting, also known as lost wax (or cire-perdue) casting, is a key process in the production of aerospace engine blades, with high quality ceramic cores as the material of choice. Modern investment casting techniques stem from the development of a shell process using wax patterns known as the investment X process. This method envelops a completed and dried shell in a vapor degreaser. The vapor permeates the shell to dissolve and melt the wax. This process has evolved over the years into the current process of melting out the virgin wax in an autoclave or furnace. Operating temperatures have increased from about 400° C to 1100° C.
Pre-sintered preforms are a type of superalloy available for high temperature braze repair applications for reconditioning, crack repair, and dimensional restoration of such aerospace engine components as turbine blades and vanes. They are a custom blend of a superalloy base and a low melting braze alloy powder and are available in various compositions and shapes, including curved, tapered, and cylindrical, as well as paste and paint.
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 are used for special cooling channels that keep engines from overheating. Ceramic cores can produce thin cross sections and hold tight tolerances, which helps 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.
The proprietary P52 material is a good choice for a ceramic core. It exhibits good dimensional accuracy while maintaining tight tolerances without distortion. While dimensionally strong, P52 remains rigid and stable through the casting process but is crushable when it needs to be during the metal solidification process. This feature is particularly useful for alloys that are prone to hot-tearing (those that exhibit lower core temperature in equiax castings) and recrystallization (castings that are involved in directionally solidified or single crystal castings).
Morgan Technical Ceramics
Filed Under: Aerospace + defense, Automation components, Materials • advanced