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Title:
TITANIUM REINFORCED WITH ALUMINUM MATRIX COMPOSITE
Document Type and Number:
WIPO Patent Application WO/1997/033009
Kind Code:
A1
Abstract:
A metal or metallic alloy part reinforced with a metal matrix composite material is disclosed. The materials are selected such that the metal or metallic alloy which forms the part is of a different metal or metallic alloy than that used as the matrix of the metal matrix composite. A method for forming such parts is described as well.

Inventors:
Waxon, Ronald F.
Anderson, Tracy L.
Kieschke, Robert A.
Sorensen, James P.
Werner, Paul S.
Pribyl, Joseph G.
Application Number:
PCT/US1997/003171
Publication Date:
September 12, 1997
Filing Date:
February 28, 1997
Export Citation:
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Assignee:
MINNESOTA MINING AND MANUFACTURING COMPANY.
International Classes:
C22C47/08; (IPC1-7): C22C1/09
Foreign References:
GB1309519A
US4053011A
US4570316A
EP0369931A1
Download PDF:
Claims:
CLAIMS
1. A metal matrix composite reinforced part which comprises: a) a monolithic segment which comprises a first metal or metallic alloy, and b) a reinforcing segment substantially encapsulated by the monolithic segment, wherein the reinforcing segment comprises a metal matrix composite material, the metal matrix composite material comprising at least one ceramic fiber contained within a matrix comprising a second metal or metallic alloy; wherein the first metal or metallic alloy differs from the second metal or metallic alloy.
2. A metal matrix composite reinforced part as in Claim 1 wherein the first metal or metallic alloy comprises titanium or a titanium alloy.
3. A metal matrix composite reinforced part as in Claim 2 wherein the titanium alloy comprises Ti6A14V.
4. A metal matrix composite reinforced part as in Claim 2 wherein the composite material comprises at least one fiber of polycrystalline CIAI2O3 contained within a matrix of elemental aluminum.
5. A metal matrix composite reinforced part as in Claim 4 wherein the polycrystalline αAl2O3 has a tensile strength of at least about 2.8 GPa.
6. A metal matrix composite reinforced part as in Claim 4 wherein the matrix of elemental aluminum is substantially free of material phases or domains capable of enhancing brittleness in the fiber or matrix.
7. A metal matrix composite reinforced part as in Claim 4 wherein said at least one fiber is substantially continuous.
8. A metal matrix composite reinforced part as in Claim 4 wherein the composite material comprises between about 3070% by weight polycrystalline α Al2O3 fibers.
9. A metal matrix composite reinforced part as in Claim 4 wherein the composite material comprises between about 4060% by weight polycrystalline α AI2O3 fibers.
10. A metal matrix composite reinforced part as in Claim 4 wherein said elemental aluminum matrix contains less than about 0.03% by weight iron.
11. A metal matrix composite reinforced part as in Claim 4 wherein said elemental aluminum matrix contains less than about 0.01% by weight iron.
12. A metal matrix composite reinforced part as in Claim 4 wherein said elemental aluminum matrix has a yield strength of less than about 20 MPa.
13. A metal matrix composite reinforced part as in Claim 4 wherein said elemental aluminum matrix contains up to about 2% by weight copper.
14. A metal matrix composite reinforced part as in Claim 1 wherein the monolithic segment contacts the metal matrix composite material at an interface.
15. A metal matrix composite reinforced part as in Claim 1 wherein the monolithic segment comprises a jacket encapsulating a cylindrical core of the metal matrix composite material.
16. A metal matrix composite reinforced part as in Claim 1 which comprises a tube, wherein the metal matrix composite material is encapsulated between a cylindrical jacket of monolithic material and a cylindrical sleeve of monolithic material.
17. A metal matrix composite reinforced part as in Claim 1 wherein the monolithic material is in the form of a turbine engine blade which encapsulates a segment of reinforcing metal matrix composite material positioned within at least a portion of said blade.
18. A metal matrix composite reinforced part as in Claim 1 which comprises a turbine engine blade having a blade body and a root wherein the monolithic material comprises the root and the metal matrix composite material comprises the blade body, and further wherein a portion of the root substantially encapsulates a portion of the blade body.
19. A method for making a metal matrix composite reinforced tube which comprises the steps of: a) providing a cylindrical sleeve of a first monolithic material, the sleeve having an outer diameter; b) providing at least one preform comprising at least one tow of a ceramic fiber; c) wrapping said at least one preform around the cylindrical sleeve; d) providing a cylindrical jacket of a second monolithic material, the jacket having an inner diameter that is greater than the outer diameter of the sleeve; e) inserting the sleeve wrapped with the preform into the jacket; f) infiltrating said at least one preform with a matrix of a metal or metallic alloy, the metal or metallic alloy being different than each of the first and second monolithic materials.
20. The method of Claim 19 wherein the first and second monolithic materials comprise the same material.
21. The method of Claim 19 wherein the first and second monolithic materials comprise titanium or a titanium alloy.
22. The method of Claim 19 wherein the ceramic fiber comprises polycrystalline α AI2O3 fiber.
23. The method of Claim 22 wherein the preform is infiltrated with elemental aluminum.
24. The method of Claim 23 wherein the elemental aluminum contains less than about 0.03% by weight iron.
25. The method of Claim 23 wherein the elemental aluminum contains up to about 2% by weight copper.
26. A method for making a metal matrix composite reinforced part which comprises the steps of: a) providing a shaped part of a monolithic material, the part having an aperture for receiving a reinforcing segment; b) providing at least one preform comprising at least one tow of a ceramic fiber; c) inserting said at least one preform into the aperture; d) infiltrating said at least one preform with a matrix of a metal or metallic alloy, the metal or metallic alloy being different than the monolithic material.
27. The method of Claim 26 wherein the monolithic material comprises titanium or a titanium alloy.
28. The method of Claim 26 wherein the ceramic fiber comprises polycrystalline OCAI2O3 fiber.
29. The method of Claim 28 wherein the preform is infiltrated with elemental aluminum.
30. The method of Claim 29 wherein the elemental aluminum contains less than about 0.03% by weight iron.
31. The method of Claim 29 wherein the elemental aluminum contains up to about 2% by weight copper.
32. A metal matrix composite reinforced part which comprises: a) a monolithic segment which comprises a first metal or metallic alloy, and b) a matrix segment which comprises a metal matrix composite material, the metal matrix composite material comprising at least one ceramic fiber contained within a matrix comprising a second metal or metallic alloy; wherein at least one of the monolithic segment and the matrix segment includes at least a portion that is substantially encapsulated by the other segment, and further wherein the first metal or metallic alloy differs from the second metal or metallic alloy.
33. A metal matrix composite reinforced part as in Claim 32 which comprises a turbine engine blade having a blade body and a root wherein the monolithic material comprises the root and the metal matrix composite material comprises the blade body.
34. A metal matrix composite reinforced part as in Claim 32 wherein the first metal or metallic alloy comprises titanium or a titanium alloy.
35. A metal matrix composite reinforced part as in Claim 34 wherein the composite material comprises at least one fiber of polycrystalline αAl2O3 contained within a matrix of elemental aluminum.
36. A metal matrix composite reinforced part as in Claim 35 wherein the composite material comprises between about 3070% by weight polycrystalline α Al2O3 fibers.
37. A metal matrix composite reinforced part as in Claim 35 wherein said elemental aluminum matrix contains less than about 0.03% by weight iron.
Description:
TITANIUM REINFORCED WITH ALUMINUM MATRIX COMPOSITE

Government License Rights

The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of contract No. MDA 972-90-C-0018 awarded by the Defense Advanced Research Projects Agency (DARPA).

Field of the Invention

The present invention relates to hybrid parts incorporating titanium and aluminum matrix composites. More particularly, the invention relates to parts having separate regions of titanium and an aluminum matrix composite wherein the separate regions share an interface.

Background of the Invention

The need for very strong, yet lightweight, materials exists in many industries. As one example, the aerospace industry has a continuing need for such materials in order to develop advanced aircraft and spacecraft technology. One approach for enhancing the mechanical properties of monolithic materials has been to develop specialty alloys of various metals. Thus, for example, many modern aircraft and spacecraft parts are fabricated of exotic metallic alloys that offer improvements in the strength-to-weight ratio over metals and alloys that have been used in the past. Among these alloys, titanium alloys have been very popular.

Another approach for enhancing the mechanical properties of monolithic materials has been to develop composite materials in which two dissimilar materials having different mechanical and physical properties are combined to form a product having mechanical and physical properties which are superior to those of either material alone. Such composite materials are well-known in the art. Examples include polymeric composites, in which fibers such as carbon fibers are encapsulated

within a polymeric resin, and metallic composites, in which fibers such as silicon carbide fibers are encapsulated in a metal matrix such as titanium.

One problem associated with many composite materials, particularly the metallic composites, is that chemical interactions between the fibers and the encapsulating matrix can cause the properties of the resulting composite to degrade, especially at elevated temperatures. As a result, fabricating parts from these materials is difficult because most relevant manufacturing processes use high temperatures. Alternatively, if interaction between the fiber and matrix occurs, the performance of the composite is degraded. Problems of chemical incompatibility in composite materials can be overcome, or substantially reduced, by employing fibers and matrix materials which are compatible with each other. For example, composite materials formed from continuous fibers of polycrystalline α-Al 2 03 contained within a matrix of elemental aluminum or an alloy of elemental aluminum containing up to about 2% copper appear to offer excellent performance characteristics with little of the degradation seen in other composite systems. These composite materials are described in detail in PCT application having International Publication No. WO 97/00976, published January 9, 1997.

Attempts have been made in the past to combine the advantages of composite materials with those of monolithic metals. In most of these attempts, however, the composite matrix metal has been the same metal as that of the monolith. While such structures may offer certain advantages, their performance is limited by the difficulty in selecting monolithic materials that are compatible with the fiber reinforcements and that offer desirable performance. Furthermore, although structures have been made in which a monolithic segment is affixed to a metal matrix composite material having a matrix that differs from the monolithic segment, none of these structures has been configured so as to impart the properties of the metal matrix composite material to the monolith.

Despite the advantages offered by the various approaches described above, a need still exists for yet stronger and lighter materials in many industries.

Summary of the Invention

The present invention relates to structures employing materials which combine the advantages of monolithic metals and metallic alloys with those of composites. In particular, the present invention relates to metallic alloy components having at least a portion reinforced by a metal matrix composite (MMC) material in which the matrix metal of the MMC is a different metal than that of the metallic monolith forming the remainder of the part. The invention further relates to materials having an interface between a monolithic metal or metallic alloy and an MMC, as well as to methods for fabricating parts of materials having such an interface.

In its broadest aspect, the invention comprises a monolithic segment which comprises a first metal or metallic alloy, and a reinforcing segment which comprises a metal matrix composite material. The monolithic segment substantially encapsulates the metal matrix composite material. The metal matrix composite material comprises at least one ceramic fiber contained within a matrix which comprises a second metal or metallic alloy. The metal or metallic alloy which forms the monolithic segment differs from the metal or metallic alloy which. forms the matrix of the composite material. As used herein, the term "substantially encapsulates" is intended to mean that the metal matrix composite material has a substantial portion, although not necessarily all, of its external surface surrounded by the monolithic segment.

In one embodiment of the invention, the fabricated part is a tube having a monolithic metal interior and exterior with a metal matrix composite positioned between the interior and exterior walls. Such tubes can be made by the process comprising the steps of: a) providing a cylindrical sleeve of a first monolithic material, the sleeve having an outer diameter; b) providing at least one preform comprising at least one tow of a ceramic fiber; c) wrapping said at least one preform around the cylindrical sleeve;

d) providing a cylindrical jacket of a second monolithic material, the jacket having an inner diameter that is greater than the outer diameter of the sleeve; e) inserting the sleeve wrapped with the preform into the jacket; f) infiltrating said at least one preform with a matrix of a metal or metallic alloy, the metal or metallic alloy being different that each of the first and second monolithic materials.

In another embodiment of the invention, a shaped part having a reinforcing segment of a metal matrix composite is produce. Such parts can be made by the process comprising the steps of: A method for making a metal matrix composite reinforced part which comprises the steps of: a) providing a shaped part of a monolithic material, the part having an aperture for receiving a reinforcing segment; b) providing at least one preform comprising at least one tow of a ceramic fiber; c) inserting said at least one preform into the aperture; d) infiltrating said at least one preform with a matrix of a metal or metallic alloy, the metal or metallic alloy being different than the monolithic material.

Although such materials may be used in a wide variety of industries, they offer particular advantages in the aerospace industry. Among the parts in which the inventive materials are believed to be particularly applicable are turbine engine blades (i.e., fan blades, compressor blades), fan frames, rods and tubing.

The materials may be configured in a manner in which the metal matrix composite is completely encapsulated within a metal or metallic alloy, or alternatively, the part can be configured in a manner such that it has an MMC component in one section and a metal or metallic alloy in another section. For example, in the case of a turbine engine blade, in the first instance, the MMC may be a rod-like insert within a cylindrical aperture formed in the leading edge of the blade, or, in the second instance, the blade may be formed of MMC with a root of a metal such as titanium alloy.

In a preferred embodiment, the MMC comprises continuous fibers of polycrystalline 0.-AI 2 O3 contained within a matrix of elemental aluminum or an alloy of elemental aluminum containing up to about 2% by weight copper, and the monolithic metal comprises titanium alloy. A novel process for the fabrication of such parts has been discovered as well. In this process, the titanium part is fabricated, and then used as a mold for the MMC insert. As such, the process for forming the MMC is substantially simplified because it can be made in situ. No additional tooling is required, and the insert does not have to be machined or otherwise fabricated. The resulting part combines the toughness, machineability, repairability, and existing knowledge base of conventional monolithic parts with the high strength, stiffness, and low density of MMCs. Additionally, since the MMC is used only where it is specifically needed, the cost of the resulting part is substantially less than if the entire part was fabricated of MMC.

Brief Description of the Drawings

FIG. 1 depicts a rod having an MMC insert.

FIG. 2 is a schematic representation of a monolithic fan blade having an MMC insert. FIG. 3 is a schematic representation of one embodiment of an MMC fan blade having a monolithic metal root.

FIG. 4 is a schematic representation of one cross section of the blade of FIG. 3.

FIG. 5 is a schematic representation of another cross section of the blade of FIG. 3.

FIGS. 6a and 6b are schematic representations of a tube incorporating an MMC core.

Detailed Description of the Invention Materials

The present invention relates to parts having a monolithic metal or metallic alloy in combination with a metal matrix composite (MMC) material. The materials are selected such that the metal or metal alloy which forms the monolithic segment of the part is of a different metal or metallic alloy than that used as the matrix of the MMC materials. Due to the use of different metals or metallic alloys, an interface exists between the monolithic segment of the part and the reinforcing MMC segment. In a preferred embodiment, the part is formed of a titanium alloy monolith and an aluminum matrix composite. The titanium alloy Ti-6A1-4V has been found to be particularly useful. Of course, the invention is not intended to be limited as such, since numerous other titanium alloys could be used as well.

The preferred fiber reinforced aluminum matrix composites contain continuous fibers of polycrystalline 01-AI2O3 encapsulated within either a matrix of substantially pure elemental aluminum or an alloy of pure aluminum with up to about 2% copper by weight. The preferred fibers have equiaxed grains, with a grain size of less than about 100 nm, and a fiber diameter in the range of about 1-50 micrometers. A fiber diameter in the range of about 5-25 micrometers is preferred with a range of about 5-15 micrometers being most preferred. Preferred composite materials have a fiber density of between about 3.90-3.95 grams per cubic centimeter. Among the preferred fibers are those described in U.S. Patent No. 4,954,462 (Wood et al.).

Alumina fibers are available commercially under the designation "NEXTEL" 610 CERAMIC FIBERS" from the 3M Company, St. Paul, MN. The encapsulating matrix is selected to be such that it does not react chemically with the fiber material, thereby eliminating the need to provide a protective coating on the fiber exterior. The percentage of fibers contained in the fiber may be selected to suit the strength and/or weight requirements of the part ultimately being fabricated. In one embodiment, the fibers comprise about 30-70 % by weight of the composite

material. In a preferred embodiment, the fibers comprise about 40-60 % by weight of the composite material.

As used herein, the term "polycrystalline" means a material having predominantly a plurality of crystalline grains in which the grain size is less than the diameter of the fiber in which the grains are present. The term "continuous" is intended to mean a fiber having a length which is relatively infinite when compared to the fiber diameter. In practical terms, such fibers have a length on the order of about 15 cm to at least several meters, and may even have lengths on the order of kilometers or more. In the preferred embodiments, the use of a matrix comprising either substantially pure elemental aluminum, or an alloy of elemental aluminum with up to about 2% by weight copper has been shown to produce successful composites. As used herein the terms "substantially pure elemental aluminum", "pure aluminum" and "elemental aluminum" are interchangeable and are intended to mean aluminum containing less than about 0.05% impurities by weight. Such impurities typically comprise first row transition metals (titanium, vanadium, chromium, manganese, iron, cobalt, nickel, and zinc) as well as second and third row metals and elements in the lanthanide series. In one preferred embodiment, the terms are intended to mean aluminum having less than about 0.03% iron by weight, with less than about 0.01% iron by weight being most preferred. Minimizing the iron content is desirable because iron is a common contaminant of aluminum, and further, because iron and aluminum combine to form brittle intermetallic compounds (e.g., AljFe, AfeFe, etc.). It is also particularly desirable to avoid contamination by silicon (such as from SiO., which can be reduced to free silicon in the presence of molten aluminum) because silicon, like iron, forms a brittle phase, and because silicon can react with the aluminum (and any iron which may be present) to form brittle Al-Fe-Si intermetallic compounds. The presence of brittle phases in the composite is undesirable, as such phases tend to promote fracture in the composite when subjected to stress. In particular, such brittle phases may cause the matrix to fracture even before the reinforcing ceramic fibers fracture, resulting in composite failure.

Generally, it is desirable to avoid substantial amounts of any transition metal, (i.e., Groups IB through VlilB of the periodic table), that form brittle intermetallic compounds. Iron and silicon have been particularly specified herein as a result of their commonality as impurities in metallurgical processes. Each of the first row transition metals described above is relatively soluble in molten aluminum and, as noted, can react with the aluminum to form brittle intermetallic compounds. In contrast, metal impurities such as tin, lead, bismuth, antimony and the like do not form compounds with aluminum, and are virtually insoluble in molten aluminum. As a result, those impurities tend to segregate to the fiber/matrix interface, thereby weakening the composite strength at the interface. Although such segregation may aid longitudinal strength of the ultimate composite by contributing to a global load sharing domain (discussed below), the presence of the impurities ultimately results in a substantial reduction in the transverse strength of the composite due to decohesion at the fiber/matrix interface. Elements from Groups LA and UA of the periodic table tend to react with the fiber and drastically decrease the strength of the fiber in the composite. Magnesium and lithium are particularly undesirable elements in this regard, due, in part, to the length of time the fibers and the metal must be maintained at high temperatures during processing or in use. It should be understood that references to "substantially pure elemental aluminum", "pure aluminum", and "elemental aluminum" as used herein, are intended to apply to the matrix material rather than to the reinforcing fibers, since the fibers will likely include domains of iron (and possibly other) compounds within their grain structure. Such domains typically are remnants of the fiber manufacturing process and have, at most, negligible effect on the overall characteristics of the resulting composite material, since they tend to be relatively small and fully encapsulated within the grains of the fiber. As such, they do not interact with the composite matrix, and thereby avoid the drawbacks associated with matrix contamination.

Formed Parts

A rod formed of a titanium alloy reinforced with an aluminum matrix composite is shown schematically in FIG. 1. In FIG. 1, the rod 10 has a core 12 formed from an aluminum matrix composite surrounded by a jacket 14 of monolithic titanium alloy. An interface 16 exists between the core 12 and the jacket 14. The rod was produced using a tube of Ti-6A1-4V alloy. A cylindrical preform of alumina fibers ("NEXTEL 610 CERAMIC FIBERS") was made with an outer diameter corresponding to the inner diameter of the tube. The cylindrical preform was inserted into the titanium tube. The tube and preform were then inserted into a pressure casting unit and the preform was infiltrated with aluminum to form the structure shown in FIG. 1. The details of the process are given below in Example 1.

A monolithic fan blade having an MMC insert is shown in FIG. 2. In FIG. 2, the fan blade 20 includes a monolithic body 22, a monolithic root 24, and a reinforcing segment 26. The monolithic body 22 and root 24 are formed of, for example, a titanium alloy, whereas the reinforcing segment 26 is formed of an MMC which comprises, for example, continuous fibers of polycrystalline α-Al 2 0 3 contained within a matrix of elemental aluminum or an alloy of elemental aluminum containing up to about 2% copper. By selecting a combination in which the monolithic material differs from the matrix material of the MMC, it is possible to form a part having the advantages of both the alloy and the MMC. Thus, the monolithic fan blade of FIG. 2 is provided with the advantageous temperature and chemical properties of monolithic titanium blades, and also with the high strength, high stiffness and low weight of MMC materials. Additionally, since in this instance, the MMC is provided only in an area in which reinforcement is desired, it is possible to employ lower cost monolithic metals or alloys for the majority of the blade structure.

Of course, it is not intended that the present invention be limited only to structures employing MMCs as reinforcements. Rather, the invention is intended to apply generally to systems in which an MMC and a monolithic metal or metallic alloy are joined together across an interface. The interface is that between the metal

which forms the matrix of the MMC and a differing metal which comprises the monolithic metal or metallic alloy. Once such configuration is shown in FIGS. 3-5 which are schematic representations of a fan blade 30 having an MMC blade body 32 and a monolithic metal or metallic alloy root 34. The blade body 32 and the root 34 meet at an interface 36 (shown in phantom) which joins the two dissimilar metals together. The attachment between the blade body 32 and the root 34 across the interface 36 may be strengthened by increasing the surface area of the interface 36. Thus, the root 34 may have an extension segment 38 which extends into a recess in the blade body 32. Alternatively, rather than having the blade body encapsulate an extension segment projecting from the root, the blade may be configured such that an extension segment projects from the blade body and is encapsulated by a recess in the root. One approach for the fabrication of such parts is to machine the titanium root in two halves which match at a parting plane. Pockets or recesses would be machined into the mating surfaces in order to allow the alumina fiber preform to be inserted. A preform of alumina fibers in the shape of the blade (including the extension into the root) can be made separately and then placed between the machined titanium root pieces. This assembly can then be placed into a pressure infiltration mold and cast using ordinary processing. The titanium may require titanium-nitride or other surface coatings to minimize reaction between the titanium and molten aluminum.

Another approach is to machine a one-piece titanium root and wrap the alumina fiber preform around it. This assembly can then be placed into the pressure infiltration mold and processed as described in the Examples. Again, it is preferred that the titanium root be tapered in thickness (i.e., full width at the root end and tapering to narrow at the outboard end) to allow gradual load transition.

FIGS. 6a and 6b are schematic representations of a tube 40 incorporating an MMC core 42 between an inner sleeve 44 and an outer jacket 46. The inner sleeve and the outer jacket are formed of a metal or metallic alloy which is different from that which comprises the matrix material of the MMC. Such tubular constructions are well-suited for numerous industrial applications including landing gear support

tubes and the like for the aerospace industry. A detailed discussion of such tubes and their fabrication is given below in Example 2.

Examples

Example 1 : Fabrication of a Cylinder

This example illustrates the preparation of a titanium tube reinforced with an aluminum matrix composite. A commercially available Ti-6A1-4V, titanium alloy rod was machined into a tube 10 cm long, with an outer diameter of 2.4 cm (0.9 in) and an inner diameter of 1.4 cm (0.57 in).

A preform of alumina fiber tows ("NEXTEL 610 CERAMIC FIBERS") was fashioned into a cylinder, 1.4 cm (0.57 in) and 10 cm (4 in) long. The preform was made by first level winding a layer of individual tow of fiber onto a four-sided drum (10.5 in (27 cm) per side) to form a mat. Distilled water was added to the tow as it was wound to provide some integrity to the mat. The mat was then slit parallel to the axis of the drum and removed. They are then rolled into a round cylinder of the required dimensions. Once rolled, the fiber preform was forced into the titanium alloy tube. This fiber preform was sufficient to result in a final composite core with approximately 50% fibers by volume.

The tube and the preform were placed into a pressure infiltration casting apparatus. In this apparatus, the tube was placed into an airtight vessel and positioned at the bottom of an evacuable chamber. Pieces of aluminum metal were loaded into the chamber on a support plate above the mold. Small holes (approx. 2.54 mm in diameter) were present in the support plate to permit passage of molten aluminum to the tube below. The chamber was closed and the chamber pressure was reduced to 3 milliTorr (0.4 Pa) to evacuate the air from the mold and the chamber. The aluminum metal was heated to 720 °C. and the tube, with the fiber preform in it, were heated to at least 670 °C. The aluminum melted at this temperature but remained on the plate above the tube. In order to fill the tube, the power to the heaters was turned off, and the chamber was pressured by filling with

argon to a pressure of 8.96 MPa (1300 psi). The molten aluminum immediately flowed through the holes in the support plate and into the mold. The temperature was allowed to drop to 600 °C. before venting the chamber to the atmosphere. After the chamber had cooled to room temperature, the tube was removed from the chamber.

The resulting tube was examined by cutting off one end with a diamond abrasive wheel saw. Upon inspection, it was determined that the fiber preform was infiltrated with aluminum matrix material, and that an interface between the aluminum matrix composite and the titanium tube was clearly evident.

Example 2: Fabrication of Tubes

This example illustrates the preparation of a tube having usefulness as, for example, a landing gear support tube. A structural tube section had a length of 21 cm (8 in). The structural tube section was fabricated from titanium alloy inner and outer tubes with an aluminum matrix composite between. The inner titanium tube changed in thickness at each end to provide a transition from a hybrid (titanium outer jacket and inner sleeve containing MMC in between) in the center of the structural tube to all metal (titanium) at its ends. Such a configuration offers the ability to readily attach end fittings to the tube. The hybrid, structural tube was made by first machining an inner titanium sleeve, and an outer titanium jacket. The inner tube was 8.00 inches (20.32cm) long and had a constant inner diameter of 3.259 inches (8.28cm). The outer diameter was 3.656 inches (9.29cm) at the ends and for a distance of 0.50 inches (1.27cm) in from each end. At 0.50 inches (1.27cm) from the end, the diameter tapered at a 15 degree slope (measured from the axis of the tube) in to a diameter of 3.460 inches (8.79cm) in the center section. Thirty-six equally spaced 0.062 inch (0.16cm) diameter holes were drilled in one end (only) of the inner tube (parallel to the axis of the tube) at a 3.563 inch (9.05cm) diameter to the centers of the holes. The outer tube was machined to a constant thickness with an inner diameter of 3.660 inches (9.30 cm) and an outer diameter of 3.911 inches (9.93cm). It was 8.00 inches (20.32cm) long.

A preform of alumina fibers (commercially available from the 3M Company under the trade designation "NEXTEL 610 CERAMIC FLBERS")(420 end tow) was then created by level winding a single tow around a 4-sided drum. Each side of the drum was 8.00 inches (20.32cm) long (parallel to axis of drum) and 8.36 inches (21 23cm) wide. The corners of the drum had a radius of approximately 1-inch (2.54cm) forming a transition between each of the four sides. The tow was wound with a tension of 20 pounds (9.08 kg) and distilled water was added to the tow as it was placed on the drum to allow denser packing and to allow the tow to be frozen. The tows were wrapped at a spacing of 0.050 cm. After the drum was wrapped with ten layers of fibers, it was placed in the freezer at minus 17 °C for over two hours. The drum was then removed and additional water sprayed on the fiber surface, then it was placed back in the freezer for over 15 minutes.

The drum, with frozen fiber on it, was then removed from the freezer and the frozen preforms were cut (4 individual pieces - one from each side of drum) from the sides of the drum. Each preform was then cut at a 15 degree angle along the ends so that when laid on the inner titanium tube, it would match a 15 degree taper that had previously been machined into the ends of the inner titanium tube. Speed was essential to prevent the preform from thawing. Additionally, the preform was returned to the freezer periodically to ensure that it stayed frozen. The individual preforms were then laid onto the titanium inner tube with the fibers parallel to the axis of the tube. After the first preform was wrapped onto the drum, it was compressed with two matching half-cylinder dies to debulk the preform to a minimum volume (maximum density). The second preform was placed so that its edge matched the free edge of the first piece. The seam between the two was rolled with a hand-held rubber roller to blend the edges together. Then the second preform was wrapped around and compacted like the first. This sequence was repeated until the lay-up was the full thickness (i.e., nine layers of preforms). This resulted in a final fiber volume percent in the finished composite portion of greater than 50 percent. Next, the outer tube was slid over the inner tube to complete the preform assembly. It was critical that the inner tube/preform assembly remained frozen so

that the outer tube could slide over without shaving off or disrupting the preform. One end of the inner tube was placed in a bath of liquid nitrogen during the assembly to ensure that the preforms remained frozen. A hydraulic press was used to press the outer tube down over the inner tube to ensure that the assembly process was straight and aligned (to prevent binding).

The completed inner-tube/preform/outer-tube assembly was then placed in a drying oven at 100 °C for 2 hours followed by 1 hour at 200 °C to dry out the water. The assembly was then placed in a vacuum chamber and a vacuum of at least 10"4 Torr was applied. The assembly was then removed from the vacuum chamber, sealed in a plastic bag, and transferred to the pressure infiltration casting unit.

Prior to placement in the casting unit, the inner-tube/preform/outer-tube assembly was placed inside a low-carbon steel tube with the bottom end sealed by welding in a 0.134 inch (0.34cm) thick carbon steel bottom. This low-carbon steel tube and bottom are referred to as the can. The 0.062 inch (0.16cm) diameter holes in the inner titanium tube were at the top of this assembly (opposite the bottom of the can). The joint between the steel tube and the steel bottom was air tight. The steel tube was 18.00 inches (45.72cm) long with a maximum inner diameter of 3.986 inches (10.12cm) and an outer diameter of 4.250 inches (10.80cm). The inner surface of the low carbon steel tube and bottom were painted with a protective coating to prevent a reaction between molten aluminum and the iron in the steel. The protective coating was a water-based colloidal graphite (Commercially available from Acheson Colloids Company under the trade designation "AQUADAG"). The can, with the titanium tubes and preform inside, was then placed into a pressure infiltration casting apparatus. In this apparatus, the tube was placed into an airtight vessel and positioned at the bottom of an evacuable chamber. Pieces of aluminum metal were loaded into the top of the can, on a support plate above the mold. The support plate was a 3.911 inch (9.93cm) diameter piece of graphite, one-quarter inch (0.64cm) thick. Two holes, each 0.282 inch (0.72cm) in diameter, were drilled in the graphite at 1.25 inch (3.18cm) from the center to provide a place

to install thermocouples, and thirty-six equally spaced holes, each 0.062 inch (0.16cm) in diameter, were drilled on a 3.563 inch (9.05cm) diameter, to provide a path for the molten aluminum to flow into the tube assembly. The chamber was closed and the chamber pressure was reduced to 3 milliTorr (0.4 Pa) to evacuate the air from the mold and the chamber. The aluminum metal was heated to 720 °C and the tube, and fiber preform in it, were heated to at least 670 °C. The aluminum melted at this temperature but remained on the plate above the tube. In order to fill the tube, the power to the heaters was turned off, and the chamber was pressured by filling with argon to a pressure of 8.96 MPa (1300 psi). The molten aluminum immediately flowed through the holes in the support plate and into the mold. The temperature was allowed to drop to 600 °C before venting the chamber to the atmosphere. After the chamber had cooled to room temperature, the assembly was removed from the chamber and the tube was removed from the can. The tube was examined by ultrasonic scanning radially at all points and was found to be intact (i.e., no disbonds were apparent between the titanium tubes and infiltrated composite).

Two tubes were fabricated by this process.

Example 2: Fabrication of a Tube with Titanium Nitride Coating Two more tubes were made as described in Example 2, except that the inner surfaces of the outer tubes and the outer surfaces of the inner tubes were coated with titanium nitride by reactive cathodic arc coating in order to reduce the reaction between the molten aluminum and the titanium alloy during processing. Satisfactory results were obtained using a coating thickness having a minimum on the order of approximately 1 micron. The titanium nitride coatings were applied by Vergason Technology Inc. (Van Ethen, NY).

Equivalents

Various modifications and alterations to this invention will become apparent to those skilled in the art without departing from the scope and spirit of this invention. It should be understood that this invention is not intended to be unduly

limited by the illustrative embodiments and examples set forth herein and that such examples and embodiments are presented by way of example only with the scope of the invention intended to be limited only by the claims set forth herein as follows.