Continuous Fiber Aluminum Metal Matrix Composites

Aluminum MMCs reinforced with continuous fibers provides high strength and stiffness. However, because of their high cost, most applications have been limited to aerospace. Boron/aluminum (B/Al) was one of the first systems evaluated.

Applications include the tubular truss members in the mid-fuselage structure of the Space Shuttle orbiter and cold plates in electronic microchip carrier boards. Boron was developed in the early 1960s and was initially used successfully in an epoxy matrix on the F-14 and F-15 aircraft. Boron is made as a single monofilament using chemical vapor deposition, as shown schematically in Fig. 9.12. A 0.5 mil tungsten wire is drawn through a long glass reactor where boron trichloride (BCl3) gas is chemically reduced to deposit boron on the tungsten core. Since this process produces a single 4.0 mil monofilament per reactor, many reactors (Fig. 9.13) are needed to produce production quantities. In addition, the process is inherently more expensive than those used for multifilament fiber forms, such as carbon fibers, in which many fibers (e.g., 12 000) are made with a single pass through a reactor. Due to the high cost of continuous fiber aluminum matrix composites and their limited temperature capabilities, the emphasis has shifted to continuous fiber titanium matrix composites that have potential payoffs in hypersonic airframes and jet engine components. However, since many of the processing procedures originally developed for aluminum MMCs have been transitioned to titanium, a brief review of these methods will be given.

Early work with boron monofilament/aluminum matrix composites in the 1970s was primarily done using diffusion bonding. Several methods, shown in Fig. 9.14, were developed for producing single layer B/Al sheets called

Tungsten Substrate Pay-out

Tungsten Substrate Pay-out

Mercury Seal/Electrode

Mercury Seal/Electrode

Variable DC Power

0.5 mil Tungsten Boride Core

0.5 mil Tungsten Boride Core

4.0 mil Boron Monofilament

4.0 mil Boron Monofilament

Gases Out

Mercury Seal/Electrode Boron Fiber Spool

Fig. 9.12. Boron Monofilament Manufacture

Fig. 9.13. Boron Monofilament Reactor Banks Source: Speciality Materials Inc.

monotapes that were provided by material suppliers. The original single ply monotapes were generally made by winding the boron fiber on a drum, applying aluminum foil over the wound fibers, and then securing the fibers with an organic binder. The material supplier then diffusion bonded each monotape layer in a vacuum hot press. These diffusion bonded monotapes were supplied to the user who layed up multilayer laminates and diffusion bonded them together. The lay-up was placed in a welded stainless steel bag using an arrangement similar to that shown in Fig. 9.15. A typical diffusion bonding cycle for B/Al is 950-1000° F at 1000-3000 psi for 60min. It was soon recognized that the boron fiber could react with aluminum during processing, forming brittle intermetallic compounds at the fiber interface that degraded the fiber strength properties. Strong interfacial bonding led to reduced longitudinal tensile strengths but higher transverse strengths, while weaker interfacial bonding resulted in higher longitudinal but lower transverse strengths.

Other methods of making monotape included placing a layer of aluminum foil on a mandrel, followed by filament winding the boron fiber over the foil in a collimated manner. An organic fugitive binder, such as an acrylic adhesive, was used to maintain the fiber spacing and alignment once the preform was cut from the mandrel. In this method often called the "green tape" method, the fibers were normally wound onto a foil-covered rotating drum, over sprayed with resin, followed by cutting the layer from the drum to provide a flat sheet

Drum Winding

Drum Winding

Diffusion Bonded Monotape

"Green" Monotape

|_P o n n n n o n o crrr\ Plasma Sprayed Monotape o o o o o o o o n~n~| Plasma Sprayed Braze Monotape ^

Fig. 9.14. MMC Monotape Product Forms of monotape. Since this product form contains a fugitive organic binder, it must be removed by outgassing before diffusion bonding. A typical process would be to place the vacuum pack between the platens of a hot press, apply a small load to hold the fibers in place, and heat under vacuum to an intermediate level and hold at that temperature until the binder is outgassed.

Plasma spraying was another method developed for B/Al montoapes. In this process, a mandrel was again covered with a thin layer of aluminum and fibers were wound onto the drum. The fibers were then plasma sprayed with the aluminum matrix material to produce a somewhat porous montoape. The spraying operation was carried out in a controlled atmosphere to minimize matrix oxidation. The advantages of this process were that no organic binders were required which could potentially lead to contamination problems, and no diffusion bond cycle was required to produce the monotape, which lessened the potential of fiber degradation.

With the advent of boron fiber coated with a thin layer of silicon carbide (Borsic) to minimize fiber degradation during processing, a variant of the plasma sprayed material was offered in which the thin aluminum foil next to the drum surface was replaced with a thin layer of 713 aluminum alloy braze foil. Although the ooooooooooo ooooooooooo ooooooooooo Consolidated Laminate

Welded Stainless Steel Vacuum Bag

■ Aluminum Pressure Equalization Plate

Boron Nitride Coated SS Slip Sheet B/Al Lay-up

Vacuum

Vacuum

Fig. 9.15. Stainless Steel Bagging B/Al for Diffusion Bonding1

-Aluminum Pressure Equalization Plate - Stainless Steel Tool ■ Boron Nitride Coated SS Slip Sheet

Fig. 9.15. Stainless Steel Bagging B/Al for Diffusion Bonding1

processing temperatures required for braze bonding were somewhat higher than for diffusion bonding, the pressures were a lot less.11 A typical consolidation cycle was 1080° F for 15 min at <300psi. A disadvantage of both plasma sprayed product forms was that the monotape was extremely stiff and had a tendency to curl due to the residual stresses introduced during plasma spraying, and therefore required vacuum annealing prior to forming structural shapes.

In the 1980s, continuous SiC monofilaments largely replaced boron because they have similar properties and are not degraded by hot aluminum during processing. The SCS-2 SiC monofilament, which was tailored specifically for aluminum matrices, has a 1 ^m thick carbon-rich coating that increases in silicon content toward its outer surface. Hot molding12 was a low pressure, hot pressing process designed to fabricate SiC/Al parts at significantly lower cost than possible with solid state diffusion bonding. Because the SCS-2 fibers can withstand molten aluminum for long periods, the molding temperature could be raised into the liquid-plus-solid region of the alloy to insure aluminum flow and consolidation at low pressure, thereby eliminating the need for high pressure die molding equipment.13 A plasma sprayed aluminum preform was laid into the mold, heated to near molten aluminum temperature, and pressure consolidated in an autoclave by a metallic vacuum bag. SiC/Al MMCs exhibit increased strength and stiffness as compared with unreinforced aluminum, with no weight penalty. In contrast to the base metal, the composite retains its tensile strength at temperatures up to 500° F.

Graphite/aluminum (Gr/Al) MMCs were developed primarily for space applications, where high modulus carbon or graphite multifilaments could be used to produce structures with high stiffness, low weight, and little or no thermal expansion over large temperature swings. Unidirectional high modulus graphite P100 Gr/6061 aluminum tubes exhibit an elastic modulus in the fiber direction significantly greater than that of steel, with a density approximately one-third that of steel.14 In addition, carbon and graphite fibers quickly became less expensive than either boron or silicon carbide monofilaments. However, Gr/Al composites are difficult to process: (1) the carbon fiber reacts with the aluminum matrix during processing forming Al4C3 which acts as crack nucleation sites leading to premature fiber failure;15 (2) molten aluminum does not effectively wet the fiber; and (3) the carbon fiber oxidizes during processing.14 In addition, Gr/Al parts can be subject to severe galvanic corrosion if used in a moist environment. Two processes have been used for making Gr/Al MMCs: liquid metal infiltration of the matrix on spread tows and hot press bonding of spread tows sandwiched between thin sheets of aluminum.

Alumina (Al2O3)/aluminum MMCs have been fabricated by a number of methods, but liquid or semi-solid state processing techniques are commonly used. The 3M Company produces a material by infiltrating Nextel 610 alumina fibers with an aluminum matrix at a fiber volume fraction of 60%. A fiber reinforced aluminum MMC is used in pushrods for high performance racing engines. Hollow pushrods of several diameters are produced, where the fibers are axially aligned along the pushrod length. Hardened steel end caps are then bonded to the ends of the MMC tubes.

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