Improving Aerospace Engines with Advanced Materials

13/07/2010

Ceramics and Metal Alloys Survive High Temperatures

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.

Brazing and investment casting, two ancient arts that have been adapted for use in repairing and manufacturing aerospace engines, make use of ceramic’s extreme heat resistance, in addition to its other unique wear and corrosion resistance, light weight, and electrical and heat insulation. New high performance metal brazing alloys are being used for high temperature braze repairs and for sealing ceramic-to-metal pressure sensor and temperature sensor components.

Hot and Hotter – Ability to Withstand Increasing Engine Heat is Key

To achieve greater engine fuel efficiencies, engines are running at higher and higher temperatures and must be cooled with more intricate cooling schemes, requiring the casting of complex cooling passages. Stronger and stronger metal alloys are being used in the casting process, and a core material must be able to withstand the extremely high temperatures used to pour these alloys.

Gas turbine engine efficiency is largely determined by turbine temperature, since the less cooling air used the more air available for propulsion. Increasing temperature capability of the turbine is therefore key to improving engines. Since engines run hotter as processing temperature is increased, there is a need for demanding materials to put together the engines.

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 and Morgan Technical Ceramics plays an important role in the following areas:

Maintenance repair and overhaul (MRO) of engines 

  • Design and manufacture of ceramic-to-metal assemblies used in engine monitoring applications 
  • Manufacturing critical aerospace engine components, particularly engine turbine blades and vanes
  • The Ancient and Modern Art of Brazing: Using ceramics and high-temperature metal alloys for assembling components, engine maintenance and repair, and sealing monitoring sensors

Brazing alloys are used for metal-to-metal bonding in engine MRO, assembly of aerospace components, and repair of micro-cracks. They are also used for ceramic-to-metal assemblies requiring joining by metalizing ceramic surface and brazing of components, including pressure and temperature sensors, thermocouple housings, and fire detection feedthrus.

Brazing is a term used for high temperature joining at temperatures above 600 degrees Celsius, and it has a long and storied history. It’s actually an ancient art, used more than 5,000 years ago to make jewelry and statuary. Before 1000 BC, iron was forge-welded for tools, weapons, and armor; however, the high temperatures required for modern welding processes became possible only with the development of electric power in the 19th century.

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.

As shown in Table 1, a number of new brazing alloys are available for use in 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 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 1300°C (2350°F) 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 inches (0.3 millimeters) to 0.200 inches (5 millimeters). Figure 1 shows examples of PSP plate forms and shapes available. In addition to plates, Morgan Technical Ceramics-Wesgo Metals business supplies PSPs in pastes, ideal for filling oxidation corrosion fatigue cracks and paints, best suited for deep, narrow micro-cracks. As new superalloys are developed and used by original equipment manufacturers’ assemblies, MTC-Wesgo Metals follows along with a new non-precious PSP for use in MRO of that component.

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 feedthrus 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 an extremely reliable seal.

Investment Casting – Another Ancient and Modern Art: Advanced ceramics used in the manufacture of aeroengine turbine blades

Investment casting is a key process used in the production of aerospace engine blades, and 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°C to 1100°C, and along with that change has been an evolution in materials that meet the demand for surviving higher temperatures.

Investment casting, also known as lost wax (or cire-perdue) casting, is yet another age old process, this one reliably traced back to at least 4000 BC. Its earliest use was for idols, ornaments and jewelry, using natural beeswax for patterns, clay for the molds and manually operated bellows for stoking furnaces . Originally, investment casting meant 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.

The ancient investment process has received a significant modern makeover, which began during World War II, when the process was seriously adopted for engineering aircraft components. 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 permeated 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.

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. 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.

MTC-Certech developed a proprietary injection molding process used to create the ceramic cores faster, allowing high volumes to be manufactured in less time. Also, the 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.

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).

Bright Future for Ceramics in Aerospace Engines

Driven by the aerospace industry’s demand for higher performance and lower costs, material scientists and ceramics component manufacturers will continue to develop new materials and processes that take advantage of advanced ceramic materials’ properties, particularly those that let engines run hotter and more efficiently.