IMPROVING AEROSPACE ENGINES WITH ADVANCED MATERIALS

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

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: THE 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 less cooling air used equates to 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 important to making engine improvements, and Morgan Technical Ceramics (Fairfield, NJ) 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 deg 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 [1].

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 [2].

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 (gold, silver, platinum, palladium, etc.) 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, Nioro is 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 deg C (2350 deg 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.