- THE MAGAZINE
Brazing and investment casting, two ancient arts that have been adapted for use in repairing and manufacturing aerospace engines, make use of advanced ceramics' extreme heat and corrosion resistance, unique wear, light weight, and electrical and heat insulation. New high-performance metal brazing alloys are now used for high-temperature repairs, as well as for sealing ceramic-to-metal pressure sensor and temperature sensor components.
Hot and HotterTo achieve greater engine fuel efficiencies, engines are running at increasingly higher temperatures and must be cooled with more intricate cooling schemes that require the casting of complex cooling passages. Increasingly stronger metal alloys are needed for the casting process, and the core material must be able to withstand the extremely high temperatures that are necessary to pour these alloys.
Since less cooling air means more air is available for propulsion, gas turbine engine efficiency is largely determined by turbine temperature. Increasing the temperature capability of the turbine is therefore instrumental to the improvement of overall engine performance. Because engines run hotter as the processing temperature is increased, more is demanded of the materials chosen to put the engines together. Seeking ways to lower costs and emissions while increasing fuel economy and performance, engine designers have been turning to advanced ceramics and high-temperature metal materials.
The Art of BrazingBrazing alloys are used for metal-to-metal bonding in engine maintenance, repair and overhaul (MRO); the assembly of aerospace components; and the repair of micro-cracks. They are also used for ceramic-to-metal assemblies that require joining by metalizing a ceramic surface and the brazing of components (e.g., pressure and temperature sensors, thermocouple housings, and fire detection feed-thrus).
Brazing is a term used for high-temperature joining at or above 600°C, and it has a long and storied history. This ancient art was used more than 5000 years ago to make jewelry and statuary. Before 1000 B.C., 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 or a strong structural bond (or both) between materials. The process is unique in that the metallurgical bond is formed by melting the brazing filler only; the components being joined do not melt.2
The development of advanced brazing materials for aerospace engine component repair has given rise to both precious and non-precious alloys. Precious alloys like gold, silver, platinum, and palladium are used mainly in original equipment manufacturers' assemblies for vanes, fuel and exhaust 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.
Pre-sintered preforms (PSPs) are another example of the superalloys available for high-temperature braze repair applications. PSPs, which are a customized blend of the superalloy base and a low-melting braze alloy powder, can be made in a variety of compositions and shapes (primarily preformed shapes). Other common compositions include pastes, paints, plate forms or other specific shapes (e.g., curved, tapered or cylindrical). They are used extensively for reconditioning, crack repair and dimensional restoration of aerospace engine components such as turbine blades and vanes. Thin areas and crack healing is performed with paste and paints, while plate preforms work well for dimensional restoration.
With the presence of hot corrosive gases and turbine temperatures reaching up to 1300°C (2350°F), aerospace engine components experience considerable erosion and wear. PSPs can be customized to fit the shape of the component and then tack-welded into place and brazed. Because PSPs save time and money and extend the life of engine components by up to 300%, they are a more reliable and cost-effective method than traditional welding, which requires post-braze machining or grinding. Brazing is a high-quality repair process because it allows whole components to be heated in a vacuum furnace, thereby reducing distortions and increasing consistency.
Due to advanced ceramics' ability to withstand the high temperatures, vibration and mechanical shock typically found in aircraft engines, they are ideally suited for aerospace applications that provide a physical interface between different components. Typical applications include 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 an extremely reliable seal.
Investment CastingInvestment 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 the investment casting process. Investment casting of new super engine alloy materials enables the development of more intricate designs that perform better in engines. Because operating temperatures have increased from about 400 to 1100°C, an evolution has occurred in the materials that must survive the demand for higher temperatures.
Investment casting, also known as lost wax (or cire-perdue) casting, is another age-old process that can be reliably traced back to at least 4000 B.C. Its earliest use was for idols, ornaments and jewelry. Natural beeswax and clay were used for the patterns and molds, and the furnaces were stoked by manually operated bellows.3
Originally, investment casting meant casting metal into a mold produced by surrounding (or "investing") an expendable pattern with a refractory slurry coating. The coating would then set at room temperature, after which the wax or plastic pattern would be removed through the use of heat prior to filling the mold with liquid metal.4
A significant modern makeover for this ancient process began during World War II, when investment casting was seriously adopted for engineering aircraft components. Modern investment casting techniques stem from the development of a shell process (known as the investment X process) that uses wax patterns. This method envelops a completed and dried shell in a vapor degreaser. The vapor then 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.5
Fused silica ceramic cores are used in the 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 to keep engines from overheating. Ceramic cores are capable of producing thin cross-sections and holding tight tolerances, which help to create accurate internal passageways.
In addition, 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 it is readily leached using standard foundry practices after the casting has cooled.
For example, Morgan Technical Ceramics Certech (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 crucial in 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 that is necessary in today's efficient turbine engines.
MTC-Certech also developed a proprietary injection molding process to create the ceramic cores faster, thereby allowing high volumes to be manufactured in less time. The lifespan of the cores are increased due to the fact that the cores are less abrasive on the injection molds used. Manufacturers can reduce or eliminate the use of costly platinum pins-generally needed 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).
A Bright FutureDriven by the aerospace industry's demand for higher performance and lower costs, materials scientists and ceramic component manufacturers will continue to develop new materials and processes that take advantage of advanced ceramic materials' properties, particularly those that enable engines to run hotter and more efficiently.
Visit www.morgantechnicalceramics.com for additional information.
References1. "Welding, Brazing, and Soldering," Britannica Student Encyclopedia, http://student.britannica.com/comptons/article-210127/welding-brazing-and-soldering.
2. Brazing Handbook, Fifth Edition, American Welding Society C3 Committee on Brazing and Soldering, Miami, Fla, 2007.
3. "Investment Casting," http://en.wikipedia.org/wiki/Investment_casting.
4. "Investment Casting," http://metals.about.com/library/bldef-Investment-Casting.htm.
5. "Investment Casting," http://en.wikipedia.org/wiki/Investment_casting.