To achieve greater engine
fuel efficiencies, engines are running at higher temperatures and must be
cooled with more intricate cooling schemes, requiring the casting of complex
cooling passages. Stronger metal alloys are being used in the casting process,
and the core material must be able to withstand the extremely high temperatures
used to pour these alloys.
As an example, consider the gas turbine, the efficiency of which
is largely determined by turbine temperature since the less cooling air used,
the more air is available for propulsion. Increasing temperature capability of
the turbine is therefore key to improving such 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 while increasing fuel
economy and performance, engine designers have been turning to advanced
ceramics and high-temperature metal materials. The ability of these materials
to withstand heat is key to making engine improvements.
The Ancient and Modern Art of Brazing
Brazing alloys are used
for metal-to-metal bonding in engine MRO (maintenance repair and overhaul),
assembly of aerospace components and repair of micro-cracks. They are also used
for ceramic-to-metal assemblies requiring joining by metallizing ceramic surface
and brazing of components, including pressure and temperature sensors,
thermocouple housings and fire-detection feed-thrus.
Brazing is a term used for high-temperature joining at
temperatures above 600C. 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.
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As shown in the table, 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 1,300C (2,350F) 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 inch (0.3 mm) to 0.200 inch
(5 mm). In addition to plates, Morgan Technical Ceramics-Wesgo Metals supplies
PSPs in pastes for filling oxidation corrosion fatigue cracks, and paints, which
are best suited for deep, narrow micro-cracks.
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 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 a reliable seal.
Investment Casting
Investment 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 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 to 1,100C, and along with
that change has been an evolution in materials that meet the demand for
surviving these higher temperatures.
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. These 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.
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).
Fred
Kimock is vice president, technology, for Morgan Technical Ceramics. For more
information on Morgan Technical Ceramics, go to http://www.morgantechnicalceramics.com/.
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