Powder metallurgy processes – Powder metallurgy processes with heating or sintering – Sintering which includes a chemical reaction
Reexamination Certificate
1999-05-05
2001-02-13
Jenkins, Daniel J. (Department: 1742)
Powder metallurgy processes
Powder metallurgy processes with heating or sintering
Sintering which includes a chemical reaction
C419S014000, C419S015000, C419S019000, C419S038000, C419S047000
Reexamination Certificate
active
06187260
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a new type of metal matrix composite (MMC) and the process for manufacturing this new MMC. MMCs are well known structures, typically comprised of a ductile metal matrix, reinforced with ceramic fibers, whiskers, particulates, or dispersions. Most frequently, a prepared reinforcing material is mixed with molten matrix metal. Occasionally, the reinforcing structure is precipitated out of the molten phase of a melt consisting of compounds dissolved in the matrix metal.
These materials often share the best characteristics of both components of the matrix. They may combine the strength, hardness, corrosion resistance, and modulus of the reinforcement phase with the ductility, thermal and electrical conductivity, and machinability of the metal matrix phase. When aluminum is used as the matrix metal, the composite may be light, strong, and hard. This is important in many applications, specifically in machine parts, automotive and transportation parts, and electronic packaging.
Mixing a prefabricated reinforcing material with molten metal has the associated problems of inter-phase bonding, anisotropic characteristics, and non-uniform dispersion of the reinforcing structures in the matrix. Much effort has gone into solving these problems. The metal matrix does not always form a strong, cohesive bond to the reinforcing material. Methods have addressed improving both the mechanical and chemical bonding aspects, resulting in elaborately prepared starting material. For example, one technique first forms a composite of silicon carbide fibers within an alumina matrix, and then combines this composite with a metal matrix. This is done to obtain an adequate bond between the metal matrix and the silicon carbide fibers, using the alumina phase as an intermediary.
Other processes use layers or woven mats of reinforcing materials infused with molten metal. These structures have strongly anisotropic characteristics. Other fabrication techniques, such as hot or cold isostatic pressing, extrusion, and arc/drum spraying can also result in isotropic characteristics, depending on the type of reinforcing material. This results in a non-uniform material, which is undesirable in many applications.
Even without using processes that result in inherently anisotropic materials, uniform dispersion of the reinforcing phase within the matrix may result in a non-uniform material. For example, dispersed reinforcing particles may settle. One method that addresses this problem pounds the reinforcing phase into a powder of the metal, and then forms the finished part by sintering, which is a solid-phase process. Many other methods pre-form the reinforcing material into a near-finished shape and infuse it with molten matrix metal. However, obtaining a uniform infusion is difficult, as is obtaining a uniform bond between the matrix and the reinforcement phase, as discussed above.
The performance of the material is known to depend on its macroscopic mechanical properties, which must be uniform to achieve the uniform benefit of the composite. Moreover, a metal matrix that is not strongly bonded to the reinforcing phase does not gain the full value of the reinforcement. Finally, exotic, difficult, or complicated fabrication processes of either the composite or its precursor materials make the use of those composites economically unattractive, if not unfeasible. What is needed is a feasible and economically viable composite and method of fabrication.
SUMMARY OF THE INVENTION
According to the invention, a reinforced metal matrix composite (MMC) is composed of an aluminum-based matrix (such as an aluminum alloy matrix) formed concurrently with the formation of sapphire whiskers, such that the sapphire whiskers are distributed randomly and uniformly within the matrix. (Sapphire is a single-crystal form of aluminum oxide). The composite is composed of a mixture of aluminum-based metal powder and a metal oxide powder that acts as an oxidizing agent to aluminum, such as tungsten or molybdenum, so that the oxygen is transferred from the metal oxide powder to the aluminum during the formation of the composite. Alternatively, the MMC is an aluminum-based matrix formed concurrently with a combination of the formation of sapphire whiskers and refractory metal intermetallics, such as aluminum molybdenide (Al—Mo) or aluminum tungstide (Al—W), the intermetallics being harder than the aluminum matrix. This formation yields a product wherein the sapphire whiskers are randomly oriented and evenly distributed throughout the matrix, and wherein the amount of the intermetallic phase is controllable.
FIG. 1
is a partial phase diagram for the aluminum-tungsten binary system. Although the present invention typically has three components, aluminum, alumina (sapphire), and the intermetallic, the binary phase diagram approximates the interaction between aluminum and the refractory metal.
FIG. 1
shows that with even small amounts (less than 2 weight %) of available tungsten, intermetallic phases form above the melting point of aluminum.
FIG. 2
is a partial phase diagram for the aluminum-molybdenum binary system. Similarly, various intermetallic phases are shown above the melting point of aluminum at low weight percents of molybdenum.
Further according to the invention is a method for producing this MMC utilizing aluminum powder mixed with refractory metal oxide powder, wherein the powders are mixed and fired at a temperature sufficient to melt the aluminum and to cause the metal oxide powder to strengthen the resulting composite by forming sapphire whiskers and intermetallic phases.
It is believed the refractory metal oxide powder contributes to the strengthening of the aluminum matrix in at least three ways. First, the metal oxide is reduced, providing oxygen to oxidize the aluminum in the matrix, forming sapphire, which grows into whiskers. Second, the oxide acts as a catalyst, allowing ambient oxygen in the firing atmosphere to combine with the aluminum, further forming sapphire whiskers. Third the reduced metal oxide provides metal atoms to combine with the aluminum to form an intermetallic phase, further strengthening and hardening the matrix.
In one embodiment, the fabrication process involves the powder mixing of WO
3
or MoO
3
with the Al metal in powder form. The mixture is then pressed and fired at a temperature of at least 660° C. and less than about 1100° C., preferably about 1000° C., in either vacuum, air or oxygen. The reinforcement phases are formed in situ from the reactions of the oxide powders with the aluminum, resulting in low cost production of the MMC. This low cost is achieved both by the simplicity of the fabrication process and also because oxide powders are used, which are inexpensive compared to the associated metal powders.
Because the sapphire whiskers are formed in situ, they are inherently uniformly distributed within the matrix (assuming a uniform temperature across the work space). Sapphire whiskers of approximately 20 &mgr;m in length and approximately 2 &mgr;m in diameter were formed. Compared to a conventional aluminum-based matrix, these whiskers within the matrix improve fracture toughness in all directions of the structure because, even though the sapphire whiskers are themselves highly anisotropic regarding modulus and strength, the orientation of the whiskers is random within the matrix.
Additional reinforcement phases also result from this process. For example, interrnetallic phases of W—Al and Al—Mo have been produced in situ. The phases are evenly distributed in the resultant Al metal matrix, thus further enhancing the hardness and the general mechanical properties of the composite. The relative amounts of these intermetallic and ceramic phases vary according to the powder compact compositions, and whether the compositions are fired in air, oxygen or vacuum.
The invention will be better understood with reference to the drawings and detailed description which follows.
REFERENCES:
patent: 3663356 (1972-05-01), Li
patent: 3890690 (1975-06-01
Ng Dickon Hang Leung
Qin Cai-Dong
Allen Kenneth R.
Jenkins Daniel J.
The Chinese University of Hong Kong
Townsend and Townsend / and Crew LLP
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