Figure 1: Managing heat in the engine's hot zone. Composites Horizons (Covina, CA, USA) manufactures the OX-to-OX CMC mixer (far right), centrosome, and fairing parts for the Passport 20 jet engine developed by General Electric Aviation (Cincinnati, Ohio, USA).
Figure 2: Focus on high-temperature composites. Since its inception in 1974, Composite Horizons has been a pioneer in high-temperature materials, dedicating its entire 3,252 square meter building to CMC component manufacturing.
Figure 3: Double Challenge: Heat and Shape. The GE PASSPORT 20 engine shows off its metal outboard tube (A) and Ox-Ox CMC components: core fairing (B), mixer (C) and centrosome (D).
Step 1: After the metal mold is specially demolded, the cutting pattern is hand-laid on the mold according to the specified layup plan. The diagram shows the placement of a mixer in a multi-piece mold.
Step 2: The finished laminated layer is vacuum-bagged, and the parts are transferred to an autoclave to be cured under high temperature and pressure. The picture shows the autoclave opened during the installation of the vacuum line.
Step 3: After curing, remove the vacuum bag and caulk, demold the part, and transfer it to the sintering furnace. The autoclave-cured mixer parts (top two shelves) and core fairing sections (bottom shelves) are shown as loaded on the movable floor of the furnace. After the loaded floor slides into the furnace, the door (as shown on the left) closes, bringing the furnace to high temperatures.
Step 4: After the parts are removed from the sintering furnace and cooled, they are machined on a fooke five-axis CNC milling machine. This ** shows a central body being machined.
Step 5: Quality control inspection includes surface measurements using a ROMER CMM. **Shows that the blender is taking measurements.
Step 6: At this point, the part will undergo non-destructive testing. The picture shows the mixer in NDI. In this photo**, the mixer appears black because each CMC part is treated with a temporary non-reflective coating prior to laser IR inspection to improve the quality of the thermal imaging images.
Step 7: After curing, machining, and inspection, the metal and ceramic parts are joined and the finished parts are packaged in the assembly area inside the CMC's dedicated building for shipment to GE.
Aircraft engine manufacturers have been researching exotic materials for years in response to passenger, regulatory and cost pressures that have the potential to reduce engine noise, emissions and specific fuel consumption (a measure of the efficiency of an engine design in terms of thrust output, primarily related to mass or weight) in commercial airliners. To do this, they took a closer look at ultra-heat-resistant materials that show the potential for reliable performance under aircraft engine life expectancy and extreme temperatures (1093°C) produced under various conditions.
Among the candidates, including nickel superalloys, ceramic matrix composites (CMC-ceramic-matrix composites) appear to be a winner, and their different forms of composites have been used in GE **iation, Cincinnati, Ohio, US, Rolls-Royce (Manchester (UK), Pratt & Whitney (Pratt & Whitney, Hartford, CT, US) and other major aircraft engine manufacturers have been engaged in research and development for many years.
High temperature pioneer
Covina, Calif., USA, Covina, USA, Coposites Horizons LLC (CHI), a high-temperature materials pioneer, won a competitive contract from GE Aviation in 2010 to manufacture CMC mixers, center bodies, and engine core fairing assemblies for GE's new FAA-certified PASSPORT 20 engine. The engine will be used in Bombardier's long-range business jets, the Global 7000 and Global 8000, as well as similar models from other OEMs. By the end of 2017, CHI had produced more than 300 ceramic components under General Electric's contract.
Acquired in 2016 by Precision Castparts (Portland, Oregon, USA), a Berkshire Hathaway company, CHI has focused on high-temperature composites since its founding in 1974 in Covina, California. It starts with the high-temperature organic polymers that are available. "We were one of the early innovators of polyimide composites in the late '70s and early '80s and worked with engine and aircraft manufacturers to develop all key polyimide materials," said Jeff Hynes, President and CEO of CHI. ”。But these also have their limitations. "Our drive to research and pursue higher temperature materials in inorganic ceramics outside of the organic world is a continuing interest of our engine customers in the higher temperature areas that organic composites can achieve. So our whole business is really focused on high-temperature materials. ”
With its high-temperature experience, and a clear and growing demand for the highly prized properties of hot-zone materials in the aerospace and industrial markets, CHI made the strategic decision to fully invest in CMC technology. However, "you can't simply put these materials on a PMC-polymer-matrix composite cutter or put them in a PMC ply room," Haynes explains, "and you have contamination problems." ”
With this in mind, the company built a 3,252-square-meter building in 2014 dedicated to CMC component manufacturing. The facility includes an impressive range of CMC resources, including CNC tools (Gerber Technology, Tolland, CT, US) with blades designed to cut abrasive ceramic materials; Laser projection systems (Virtek Vision International, Inc. *** Waterloo, Ontario, Canada); 5-axis CNC milling machine (Boken Fooke GmbH, Germany); Autoclave (ASC Process Systems, Valencia, CA, USA); an 8-foot by 8-foot by 8-foot (512 cubic feet Celsius) dedicated sintering furnace; and Digital Laser NDT Infrared Ash Thermography Equipment, designed by Thermal W**e Imaging Co., Ltd. (TWI, FERNDALE, MI, US) and operated under a contract with Chi by X-Ray Industries (XRI, Troy, MI).
In addition, Bill Roberts, who worked on the Boeing (Chicago, Illinois) space shuttle program for 32 years, was appointed vice president of ceramics operations.
Advantages of CMC in engines compared to metal
As mentioned earlier, an important feature of the CMC is its ability to withstand very hot engines and very hot exhaust gases. This is a significant advantage in this application, as jet engines typically emit less carbon dioxide (CO2) and nitrogen oxides (NOx-nitrogen oxide) when operating at high temperatures, propelling the aviation industry toward an often elusive but important goal.
Another advantage of the CMC is that, like PMCs, which are less resistant to high temperatures, it is flexible and can form very small radii – for example, small radii and complex shapes for engine mixers. Hynes notes that this is in stark contrast to the difficulties experienced when forming a less flexible metal for this or similarly complex parts.
Mixers are used in small to medium-sized aircraft engines to improve the mixing of hot core engine exhaust with cooler bypass air (air driven by an engine fan that bypasses the core engine). CMC mixers can achieve a smooth, compact shape that helps to mix bypass air and exhaust air more efficiently. The more efficiently these airstreams are mixed, the better the specific fuel consumption. This increases engine efficiency and reduces engine noise.
"For the same reason, CMC is also better resistant to acoustic fatigue (vibration) than welded titanium," says Haynes. "Especially in designs like mixers. Hynes explains that metals that bend or form with very small radii tend to experience fatigue failure, but CMC is much more forgiving and durable.
Roberts adds that CMC "allows you to build multiple shapes with exotic geometries without the risk of fatigue of welded joints." ”
In addition, the CMC helps to reduce the weight of the engine structure. "Oxide ceramics are about one-third as dense as nickel alloys and have similar or even higher temperature properties," Hynes said. ”。As a result, the use of CMC can significantly reduce the overall mass of the engine structure. ”
GE part and material design
GE Aviation has a separate plant in Huntsville, Alabama, USA, and through a joint venture with Safran Aero Engines (Courcouronnes, France), produces silicon carbide (SiC) CMC materials for the manufacture of turbo guards, nozzles and other "super-hot zone" parts for LEAP and other engines. However, in order to control costs, GE opted for a different CMC for the Passport 20. Tim Shumate, vice president of sales and marketing at CHI, explains, "The SiC SiC is a more expensive material, and the temperature requirements of the Passport 20 exhaust components do not mean that the SiC SiC is required to provide higher temperature capabilities. ”。
For CMC parts of the mixer, center body, and core fairing, GE designed a CMC prepreg billet material, called OX OX-CMC, using alumina fiber and alumina matrix, which subsequently made the prepreg blank suitable for Passport 20 as well as many military applications. Prepregs are manufactured by Axiom Materials, Inc. *** Santa Ana, CA, USA) in accordance with General Electric's specifications. Axiom uses 3M Nextel 720 continuous filament ceramic oxide fibers manufactured by 3M Advanced Materials (St. Paul, Minnesota, USA) for GE woven fabrics. The fabric is then woven with an alumina matrix pre-impregnated.
Ceramic matrices are slurries of alumina and other fillers and materials. When the matrix is heated in an autoclave and then sintered, the fibers and substrate are fused together. The process differs from standard PMCS, in which two very different organic materials, such as carbon fiber and epoxy thermoset resins, are combined and thermoformed into new molecular structures. Hynes explains that the fibers and matrix in the OX-OX-CMC material have very similar chemical structures to powders and various other mixtures. When these materials are sintered and hardened, their chemical composition remains the same.
The design loads for these components come from the air loads of the engine's core airflow and the external airflow, primarily pressure loads, not tensile or compressive loads, Haines said. Like carbon or glass fibers, oxide fibers can be oriented in stacks to achieve a quasi-isotropic structure.
Manufacture of Ox Ox CMC parts
For the Passport 20 engine, CHI's Ox Ox CMC components include a mixer, exhaust center body, and engine core fairing. Both the mixer and the center body are part of the engine nozzle. For this purpose, the Axiom multi-axis prepreg fabric is cut into a pre-designed pattern on a Gerber automatic flatbed CNC cutting machine. Ox-Ox CMC prepregs use solvents with volatile gray spots, so they must be refrigerated before use. As a result, they have a limited cumulative time out at room temperature and require strict control procedures.
The precision-cut patterns are placed by hand on custom metal molds and are previously demolded. The stacking is aided by a laser projection system supplied by Virtek Vision International, a subsidiary of Gerber. The molds were supplied by GE or AIP Aerospace, the former owner of CHI (the components are produced in Santa Ana, California, USA. ）
The mixer is the most complex component, with a diameter of 965 mm and a length of 610 mm. The mixer is formed in a multi-segment metal tool that is assembled for lamination and then disassembled after the autoclave has cured to allow the tool to be safely removed from the part. After lamination, the part was vacuum-bagged using a nylon bag from Airtech International Co., Ltd. ***Huntington Beach, CA, USA) and cured in an autoclave at high temperature and pressure (details not disclosed) before demolding. Because the autoclave cycle leaves the ceramics in a green state – that is, compacted but not fully cured – Haynes explains, "We had to be very careful to demould this complex part before it was fully hardened in the sintering furnace. ”
The engine exhaust center body is a conical structure with a diameter of about 460 mm (front end) and a height of 610 mm. It is mounted inside the mixer and protrudes to the rear of the trailing edge of the mixer. For this part, the cut pattern is placed by hand into a female die and covered with a backing plate made of an undisclosed material to apply and equalize the pressure. It is then also vacuum-bagged, autoclave-cured and demolded before entering the sintering cycle.
The core fairing consists of four sections, each about 152 meters, bent to form a cylinder that surrounds the core of the engine. The parts are also placed in single-sided dies, covered with partitions and vacuum bags. They are also autoclave cured, demolded and finished in a sintering furnace.
In each case, the cured part is transferred to the sintering furnace after demolding. The custom-designed furnace features a computer-controlled, movable floor that protects workers from entering the furnace. Once the parts are loaded, the floor kicks in, automatically slides under the furnace, and is lifted into place. After the loading floor is set inside the furnace, the insulation door is closed and the furnace reaches operating temperature. When the furnace temperature reaches 1093°C, the ceramic matrix melts and the residual organic material burns out.
Once the parts are removed from the sintering furnace and cooled, they are transferred to a 5-axis CNC milling machine at Fooke GmbH (Boken, Germany) where the counterbore is milled, ground and drilled using specially designed cutting tools.
The next step is quality control: the outer surface is first measured and the surface size and shape are confirmed using a Romer CMM (Coordinate Measuring Machine) from Hexagon Metrology Co., Ltd. *** Cobham, Surrey, UK. Next, they undergo non-destructive testing (NDI) on site, using infrared gray thermal imaging equipment. The automated inspection system includes a robot and a variety of other articulated devices that enable a FLIR (FLIR) forward-looking infrared camera to maintain a direct line of sight on complex parts and scan 100% of the surface of each part to detect porosity and delamination. In grey imaging, a heat source, such as a brief pulse of light, is used to heat the sample surface while an infrared camera records changes in surface temperature. As a sample cools, its surface temperature can be affected by internal defects, including degumming, porosity, or inclusions that prevent heat from flowing into the sample.
Finally, the metal parts and ceramic parts are assembled using mechanical fasteners, and then the finished products are packaged and shipped to GE for final assembly.
Hynes noted that fastener technology for engine parts is intellectual property and is often closely protected by OEMs because of the different coefficients of thermal expansion (CTE-thermal expansion) of materials in engine components. Although oxide CMC is resistant to high temperatures in operation, its CTE is similar to that of aluminum. Therefore, the thermal mismatch between the oxide components attached to the nickel alloy is an important design consideration.
Markets and applications
In 2015, with the successful signing of the GE contract, CHI also began to develop its own OX-OX CMC matrix materials, named AXC-610 and AXC-720. Mr Haines said CHI is currently using the materials for two other applications: one for aerospace and the other for industrial applications.
Hynes further explained, "In search of a low-cost alternative to the industry-standard ceramic fabric form, CHI began working with 3M Advanced Materials, a nextel fiber provider, and Axiom Materials, a prepreg manufacturer, to test several oxide fabrics using an AXC oxide matrix. ”
Most OX-OX-CMC work is done using very fine (and expensive) 1500 denier yarn, but 3M can supply higher denier yarn and 10000 deniers), increasing the number of filaments in the tow while maintaining the same individual filament diameter. This creates a significant cost advantage in fiber production, which can be passed on to the first chain and part manufacturer. Roberts clarified: "It's the same fiber, but the tow is larger, which reduces the cost of manufacturing. ”
However, it is highly recommended to carefully consider the part profile and layup characteristics when selecting the lowest cost CMC fabric for a particular application. For example, a tight profile has been achieved with prepreg blanks up to 3,000 deniers, but 4,500 denier yarns are not recommended. But milder geometries are possible and higher deniers.
The results of these surveys are reported in two articles**. 3M, Axiom and CHI presented their first report at the American Ceramic Society conference in Toronto, Ontario, Canada, in June 2015, titled "Oxide-Oxide Ceramic Matrix Composites – Making Industry Widely Adoptable." Another report, presented at the American Association of Fine Ceramics conference in Delicious Beach, Florida, USA, in January 2017 discussed "Designing with Oxide CMC: Understanding the Value for Money Relationship in New Fabric Designs." ”
Using a lower-cost fabric form, Hynes said, the project successfully demonstrated mechanical properties comparable to the more expensive industry-standard ceramic fabric form, with a maximum temperature of 1,177°C. In addition to woven fabrics, ox ox cmc tows, tapes or chopped fibers can also be made. Markets for this lower-cost, higher-denier ox-ox-cmc material include high-performance aerospace applications, as well as industrial, energy, and possibly high-end automotive applications.
The use of new materials yields a lot
Ceramic matrix composites have contributed greatly to what some consider to be a disruptive advance in aircraft engines, and it is the game-changer for the transition to integrated propulsion systems that integrate advanced materials and technologies, including GE's innovative carbon fiber composite fan housing. The result is an 8% reduction in specific fuel consumption, reduced vibration and noise, significantly reduced emissions, and long-term performance.
-END --Note: For the original text, see Ceramic Matrix Composites: Hot Engine Solution 201711.4.
Chaofan Yang 20239.7