IMPROVING AEROSPACE ENGINES WITH ADVANCED MATERIALS

PSP plate thicknesses range from 0.010 in (0.3 mm) to 0.200 in (5 mm). Figure 1 shows examples of PSP plate forms and shapes available. In addition to plates, PSPs are also available in pastes that are 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, new non-precious PSPs follow along 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, aerospace engine pressure and temperature monitoring sensors, thermocoupling housings, and fire detection feedthrus are 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 (see Figure 2).

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 deg C to 1100 deg 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 [3]. 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 [4].

This 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 [5].

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.

A ceramic core with proprietary P52 material has been developed which exhibits greater dimensional accuracy while maintaining tight tolerances without distortion. These 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.

A proprietary injection molding process has been developed 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). Figure 3 shows an example of ceramic cores now being used in the aerospace industry.

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.

Morgan Technical Ceramics, 26 Madison Road, Fairfield, NJ 07004, 800-433-0638, Fax: 973-227-7135, [email protected], www.morgantechnicalceramics.com.

Resources
[1]  Welding, brazing, and soldering. (2009). In Student’s Encyclopædia. Retrieved June 9, 2009, from Britannica Student Encyclopædia: http://student.britannica.com/comptons/article-210127/welding-brazing-and-soldering.
[2]  American Welding Society C3 Committee on Brazing and Soldering, (2007). Brazing Handbook, Fifth Edition. Miami, FL: American Welding Society.
[3]  http://en.wikipedia.org/wiki/Investment_casting
[4]  Investment Casting. Retrieved June 9, 2009, from About.com, a part of The New York Times Company: http://metals.about.com/library/bldef-Investment-Casting.htm
[5]  http://en.wikipedia.org/wiki/Investment_casting