Ќазвание: Aircraft engineering (ћорозова ћ. ј.)

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Metal matrix composites

 

Work with metal matrix composites (MMC) has concentrated on bo- ron/aluminum (B/A1), graphite/aluminum (GR/A1), and silicon car- bide/aluminum (SC/A1) composites but other types of matrix materials are also being studied, including titanium and magnesium. Metal matrices offer greater strength and stiffness than those provided by polymers. Fracture toughness is superior, and metal matrix composites offer less-pronounced anisotropy and greater temperature capability in oxidizing environments than do their polymeric counterparts. The greatest applications are where stiffness and light weight are needed. The following describes the metal matrix choices:

(a) Aluminum

 

Х Principal metal matrix material

 

Х Greatly improved properties when reinforced

 

Х Lightweight

 

Х Easily processed

 

(b) Titanium

 

Х Lightweight

 

Х Good resistance to high temperatures

 

Х Difficult to reinforce

 

Х Expensive

 

(c) Magnesium

 

Х Good interface with reinforcements

 

Х Poor corrosion resistance

 

Х Lightweight

 

(d)††††††† Copper

 

Х Improved shear strength over aluminum at elevated temperatures

 

Х Heavier than aluminum

 

From the standpoint of airframe design, the most interesting materials for aircraft components are SiC (Silicon Carbide) reinforced aluminum and titanium;

of main interest for space structures are graphite-reinforced aluminum or mag- nesium.

A variety of reinforcement-matrix combinations are used for metal matrix composites and some representative materials are shown in Fig. 2.3.2. Each class of material can have a broad range of properties, depending upon the spe- cific fiber, matrix, and fiber volume. Advantages of metal matrix composites vs. metals include:

Х Higher strength/density ratio

 

Х Higher stiffness/density ratio

 

Х Under highly elevated temperatures metal matrix composites still have better properties:

Чhigher strength

 

Чlower creep and creep rupture

 

Х Lower coefficient of thermal expansion

 

In comparison to organic matrices metal matrices have:

 

Х High on temperature (metal matrices have been demonstrated at tempera- tures above 2000∞F (1100∞C)

Х High transverse strength due to the fact that transverse strength is essen- tially the same as the strength of the matrix material, and, metals are much stronger than organic matrices.

Х Less moisture sensitivity but more susceptibility to corrosion

 

Х Better electrical and thermal conductivity

 

Х Less susceptibility to radiation

 

Х No outgassing contamination

 

Х Whisker and particle reinforced metals can be manufactured using exist- ing metal machinery and processes, lessening capital output required.

Currently, four methods of production are the most common for rein- forced metal matrices:

(a) Diffusion bonding Ч Diffusion bonding is most often used when the reinforcement is continuous fibers. Strands or mats of fibers are sandwiched be- tween sheets of the matrix metal. The laminate is then sealed in an evacuated can, heated and pressed to full density.

(b)Conventional casting Ч This process uses a proprietary treatment to promote wetting of the reinforcement by the molten metal.

(c) Power metallurgy (P/M) Ч Particulate reinforcements are mixed with the metal powder, and the mixture is processed in conventional P/M processes.

Some difficulties remain to be solved for metal matrix composites:

 

Х High cost

 

Х Cannot be extruded or forged

 

Х Lack of machining and joining techniques

 

Х Lack of non-destructive testing techniques

 

Х Need to improve the adhesion of the fibers to the matrix

 

Х Complex and expensive fabrication methods for metal matrix composites with continuous reinforcements

Selected metal matrix reinforced design concepts, as applied to structural components such as airframe floor beams, stiffeners, columns, and rods and tubes. To carry the primary axial loads, Boron reinforcements are selectively introduced into the beam or stiffener flanges in the form of aligned boron fila- ments threaded completely through lobes or other apertures embodied in the metal structure. This method of reinforcement makes possible a lighter structure while still permitting the retention of traditional metal joining and assembly techniques such as riveting and welding. It is claimed that boron-reinforced structural components are from 25 to 45\% lighter than metal counterparts.

ARALL (ARamid Aluminum Laminate) is made by bonding layers of thin sheets of aluminum with epoxy matrix reinforced with aramid fibers by pre- stressing the material with the aramid fibers. ARALL combines the strength and fatigue resistance of polymer composites with aluminum's machineability and formability.

 

CARBON/CARBON (C/C) MATRIX COMPOSITES

 

Carbon matrix composites, developed specifically for parts that must op- erate in extreme temperature ranges, are composed of a carbon matrix reinforced with carbon yarn fabric, 3-D (three-dimensional or three-directional) woven fa- bric, 3-D braiding, etc. depending upon application (see Section 2.5). They have

been used on aircraft brakes, rocket nozzles and nose cones, jet engine turbine wheels, high-speed spacecraft, and other planetary exploration spacecraft. The following describes a few of the current applications:

(a) Aircraft brakes Ч The rapid deceleration required for a landing air- craft generates a considerable amount of frictional heat. C/C composite brakes retain strength at high temperatures. Unliked steel brakes, C/C maintain more consistent performance over the life of the part, with no increase in stopping distance. C/C has a high heat absorption capacity, so it can act as a lightweight heat sink and can endure thousands of thermal cycles with little or no fatigue. In addition, its resistance to wear outlasts steel brakes by two fold, which means fewer overhauls and lower maintenance costs.

(b)Rocket Nozzles Ч Hot gases rush through the nozzles at extremely high velocity, stressing (up to 300 ksi (2.1 Gpa)) and eroding the nozzle walls; C/C composites can resist erosion and abrasion with very little burning away. However, rocket nozzles are not typically reused, so they are not often coated for oxidation protection and are allowed to partially burn. This burning must be taken into account in the design of the nozzle.

(c) Rocket nose cone Ч Similar to a rocket nozzle, the nose cone and lead- ing edge of the space craft must endure the searing heat and high stressing from the atmosphere during reentry, up to more than 3000∞F (1650∞C). The cone must also endure solar radiation and the attack of atomic oxygen. C/C is very good at resisting thermal shock which occurs during the rapid transistion from less than -200∞F (Ц 129∞C) in space to more than 3000∞F (1650∞C) during reen- try, and allows the nose cone to remain dimensionally stable over this wide tem- perature range under high stress.

(d)Jet engine turbine wheel† Ч Advanced materials for jet engines must withstand static and† dynamic loads caused by† high and† varying centrifugal forces, as high as more than 200,000 g's at high temperatures (at 40,000 RPM and more than 3000∞F (1650∞C)), and chemically aggressive combustion gases. The higher combustion temperature permits greater efficiency and performance, while dictating engine size, weight, and fuel consumption. To prevent burning and oxidation, a coating (e.g., ceramic), which does not bear structural loads, is

critical for C/C composites.

 

(e) Future National Aerospace Plane (NASP) program, (a) of Chapter 1.0)

 

Ч The NASP will fly at speeds up to 17,000 MPH (27350 km/h) and hypersonic velocities will produce aerodynamic heating, resulting in surface temperatures higher than most metals can endure.

(f) C/C composites meet applications ranging from aircraft to aerospace because of their ability to maintain and even increase their structural properties at extreme temperatures. The characteristics and advantages of C/C are de- scribed below:

Х Extremely high temperature resistance [up to 3500 Ц 5000∞F (1930 Ц

 

2760∞C]

 

Х Strength actually increases at higher temperatures [(up to 3500∞F (1930∞C)] which would be devastating to metal

Х High strength and stiffness

 

Х Unaffected by sharp temperature variations

 

Х Holds dimensional stability at high temperatures

 

Х Good ablative qualities

 

Х Ablates evenly thus maintaining structural properties

 

Х Good mechanical properties

 

Х Good resistance to thermal shock

 

Х Carbon fibers, when used with existing 2-D or 3-D textile technology results in reinforcing materials mat are functionally superior.

Typically, C/C composites are fabricated from both 2-D laminates 3-D pre- forms. Although most 3-D preforms are made with dry fibers, one technique is to braid phenolic prepreg. After being braided, the preform is cured. Then the freestanding structure is densified. The following briefly discusses preform con- structions:

Х 2-D preform Ч Use of woven fabric or unidirectional tape cross-plied in the X and Y planes. For additional strength, reinforcement is added in the third dimension in the form of fibers in the ЂZї plane

Х 3-D preform Ч Like cylindrical shapes, the fibers are interwoven axially, radially, and circumferentially. 3-D preforms are most important in C/C compo-

sites because the carbon matrix is inherently brittle and a 3-D preform adds toughness.

Х Multi-directional preform Ч Most multi-directional preforms used for C/C composites are represented by the orthogonal or polar constructions or by some modification of these constructions. C/C processing can be divided into six primary cycles:

Х Layup

 

Х Cure

 

Х Pyrolysis

 

Х Impregnation

 

Х Coating

 

Х Sealing

 

Manufacturing of C/C composites utilizes special forming processes and equipment and at present manufacturing C/C composites is an extremely time- consuming process, requiring slow pyrolyzing to drive off gases without crack- ing the matrix.

Two primary C/C processes, liquid impregnation and gaseous infiltration, have been developed to fabricate C/C composites.

C/C composites can be manufactured by one of the following methods:

 

(a) Liquid impregnation Ч Carbon fiber prepregs are fabricated into a de- sired shape or dry fiber is laid up into a preform and then impregnated with a liquid matrix (or resin) or pitch. Densification of fiber preforms is accomplished by impregnation and carbonization, and sometimes graphitization, of thermoset- ting resins such as phenolics or thermoplastic or petroleum based pitches. The process may be done under various pressures and temperatures depending on me precursor, desired carbon yields, density, and part shape and thickness. Examples of C/C composite manufacturing procedures used to manufacture a 3-D C/C ra- dially pierced nozzle billet. An automated weaving system for the C/C exit cone of a solid rocket, prior to densification.

(b)Chemical Vapor Infiltration (CVI) Ч CVI is used to density graphite fiber preforms starting with a hydrocarbon gas, such as methane, as a precursor. All of these involve multiple cycles to achieve final densification. High pressure

pitch impregnation processes with high final densities are favored for ablative products. The ablative capacity is a direct function of mass, so high density is a de- sirable physical property for this application. High density is not a necessary pre- requisite for high strength and stiffness. In fact, high mechanical properties and relatively low density are achieved with a phenolic matrix impregnation/CVI fol- low-up process.

Additional work has to be done after C/C densification processing: (a) Part is machined to final configuration

(b)C/C parts must be coated with silicon carbide and sealed with silicon glass to resist oxidation at high temperatures

Opportunities for broader use of C/C composites are evidenced by the re- quirements established for a variety of future aerospace vehicles and systems such as the previously mentioned NASP, (a) in Chapter 1.0) which is expected to experience surface temperatures approaching 1700∞F (930∞C) with the nose leading edge temperatures possibly twice as hot. Because of their unique prop- erties, C/C composites should become a leading candidate for high-speed space- craft and reentry vehicles and other high temperature applications.