Patent Grant
7297310
1. Field of the Invention
The present invention generally relates to the method of low cost production of aluminum metal matrix nanocomposites. More particularly, the present invention relates to the method of manufacturing nanoscale aluminum oxide particulate reinforced aluminum.
2. Description of the Prior Art
Aluminum Metal Matrix Nanocomposites reinforced by nanoscale aluminum oxide particulates (Al-MMNC) is one type of nanocomposite. A conventional particulate reinforced Aluminum Metal Matrix Composite (Al-MMC) usually has an aluminum or aluminum alloy phase and a ceramic reinforcement phase composed of micron scale particles. Most Al-MMCs material systems in use today are reinforced with aluminum oxide (Al2O3), boron carbide (B4C) and silicon carbide (SiC) particles. Ingot metallurgy (I/M) and powder metallurgy (P/M) are common methods to produce Al-MMCs. Al-MMCs have higher yield strength, stiffness and lower coefficient of thermal expansion than those of the monolithic matrix alloy.
In general, the properties of Al-MMCs are controlled by the particle size, particle distribution in the matrix and volume fraction of the reinforcement as well as the matrix alloy properties. It is found that an Al-MMC reinforced with ceramic particles possessing fine particle size have improved strength, fatigue resistance and machineability composed of Al-MMCs reinforced with coarse ceramic particles.
Optimum mechanical properties of an Al-MMC can be obtained when stable nano ceramic particulate is uniformly distributed in the aluminum matrix. This new composite is considered to be an aluminum metal matrix nanocomposite (Al-MMNC). The I/M Al-MMC manufacturing process can produce a MMC with a ceramic particle size of minimum about 20 microns.
Finer particles are different to wet and distribute uniformly in the metal matrix resulting in microporosity and poor fatigue and fracture toughness. The P/M Al-MMC process can deal with finer particles, but ceramic particles finer than 1 micron are difficult to be distributed uniformly during the mixing process resulting in particle agglomeration.
To create a uniform distribution of the nano ceramic particles, the aluminum matrix alloy powder should have a particle size close to that of the nano ceramic particle. Otherwise, the large aluminum particles are surrounded by very fine ceramic particles. Additionally, the very fine ceramic particles will also agglomerate. These agglomerations are extremely difficult to break-up during blending once they are formed and subsequent thermo-mechanical work of the resultant MMC will cause the agglomerates form “stringers” that will act as crack initiation sites in the material.
Creating a homogenous mixture of two different nano powders or one nano powder with one micron powder is extremely difficult.
High-energy ball mills as disclosed in U.S. Pat. Nos. 4,557,893 and 4,623,388 to Jatkar et al, U.S. Pat. No. 4,946,500 to Zendalis et al and U.S. Pat. No. 4,722,751 to Akechi can mix micron and submicron powders uniformly. They can also mix nano powders uniformly.
However, high-energy ball mills have very low efficiency and are very difficult to scale up for large volume production. This blending technique adds a high cost in addition to expensive nano metal powders and nano ceramic particulates.
To overcome the barrier of mixing two powders using conventional MMC processing methods, an in-situ method has been created. The reinforcing particles are synthesized in the metal matrix by chemical reactions between elements or between the element and compound during fabrication of the MMC.
A variety of in-situ processing techniques for producing a wide range of MMNCs have been developed. X. C. Tong and H. S. Fang, Metall, Mater. Trans. A, 29, 893 (1998), described using an in situ method to produce TiC particulate reinforced Al-MMNC. The TiC particles formed in-situ have a size of about 100 nanometers (nm) and are uniformly distributed in the aluminum matrix.
This processing requires a high cooling rate to reduce the particle size of reinforcement and to modify its distribution in the matrix. It is a major challenge to create uniformly distributed nano reinforcements via chemical reactions during MMNC production without difficult to maintain process control.
There have been no in-situ processes that can produce Al-MMCs reinforced with Al2O3, B4C or SiC, whether on a micronscale or nanoscale.
The present invention is a novel and unique method of manufacturing aluminum metal matrix, nanocomposites reinforced with nanoscale aluminum oxide particles.
It is an object of the present invention to create a novel, unique, and simplified process for producing aluminum metal matrix nanocomposites reinforced with nanoscale aluminum oxide particles having a controlled volume fraction and uniform distribution in the aluminum matrix.
Another object of the present invention is to provide such a technique that utilizes the Al2O3 surface layers existing on all aluminum particles as the ceramic reinforcement.
Yet another object of the present invention is to provide a method for producing Al-MMNC having unalloyed aluminum or an aluminum alloy as the metal matrix and having fine grain sizes.
Still another object of the present invention is to provide a method for producing an Al-MMNC that is heat-treatable, formable, easily machined, and weldable.
Further novel features and other objects of the present invention will become apparent from the following detailed description, discussion and the appended claims, taken in conjunction with the drawings.
Referring particularly to the drawings for the purpose of illustration only and not limitation, there is illustrated:
Although specific embodiments of the present invention will now be described with reference to the drawings, it should be understood that such embodiments are by ways of example only and merely illustrative of but a small number of the many possible specific embodiments which can represent applications of the principles of the present invention. Various changes and modifications obvious to one skilled in the art to which the present invention pertains are deemed to be within the spirit, scope and contemplation of the present invention as further defined in the appended claims.
The major sequential steps of the manufacturing method of this invention are illustrated in
Step 1. Preparing the Nano-Ceramic-Surface Aluminum Powder
In order to understand the process of producing an aluminum powder with nano surface oxide, it is necessary to understand how conventional aluminum powder is produced.
The aluminum and aluminum alloy powder can be produced by various processes including atomization, splat quenching, melt-spinning, and plasma method. Atomization, the most widely used process, can produce aluminum or aluminum alloy powder in a wide range of sizes from submicron to over 100 microns. The powder is classified into an average particle size from about 5 microns to 50 microns with normal distributions for industry applications.
The atomized aluminum particles can have shapes that go from spherical to flat particles depending on the cooling conditions. The aluminum powder for Al-MMC is usually spherical in shape. A nanoscale range of aluminum or aluminum alloy powders can be produced by the plasma method.
The aluminum or aluminum alloy powders always have an Al2O3 layer of certain thicknesses on all particles surface. All sizes of aluminum particles of the same production lot have the same thickness of Al2O3 on the particle surfaces. That is because the Al2O3 layer is formed due to the chemical reaction between aluminum and oxygen during the powder production process. The thickness of the Al2O3 layer depends on the aluminum powder manufacturing process and not the particle size. Some aluminum powder applications such as rocket engine fuel minimize the Al2O3 layer by atomizing the aluminum powder in inert gas. Aluminum powder atomized in inert gas usually has an Al2O3 surface layer of about 50 mm thickness compared to thickness of about under 100 nm for powder atomized in air.
The total percentage of Al2O3 in aluminum powder with a spherical shape is a function of the average particle size of the powder and the thickness of Al2O3 layer. The Al2O3 volume percent can be calculated by equation [1]:
Al2O3 vol %=100[1−(1−2T/D)3]% [1]
Where T is the thickness of the Al2O3 layer, D is the diameter of average aluminum powder particle size including the Al2O3 layer.
For a given thickness of the Al2O3 layer, the smaller average particle size, the more total Al2O3 is formed.
The volume percent of Al2O3 in an aluminum powder of irregular shape is also a function of the average particle size of the powder and the thicknesses of Al2O3 layer. The relationship among percentage of Al2O3, particle size and Al2O3 layer thickness for irregular shape aluminum powder has to be established by experimental test procedures.
Spherical shaped aluminum powder is preferred in this invention. A spherical shape has the largest surface to volume ratio, which provides the highest percentage of Al2O3 for the same volume particle.
Aluminum powder producers usually discharge aluminum powder that has very fine particle size because of the relationship between the particle size and the Al2O3 percentage. For example, the total Al2O3 volume percent is about 27% for aluminum powder that has a 2 micron average particle diameter and 100 nm thickness of surface Al2O3. The aluminum powder used in conventional Al-MMCs is about 25 micron average size with up to a 100 nm Al2O3 surface layer. It contains only up to about 2.4% Al2O3 according to the equation [1].
This invention uses the equation [1] to control producing the aluminum powder with a nano surface oxide. This invention also controls the volume percent of the Al2O3 in the final Al-MMNC.
The Al2O3 volume percent is controlled to between about 2% and 45% for most structure applications although aluminum powder with higher percentage Al2O3 can be produced.
There are three ways to control the Al2O3 volume percent in the aluminum powder:
The simplest way is to heat treat the aluminum powder in an air at a temperature below aluminum's melting point. A flow bed furnace is an efficient method for increasing the thickness of the Al2O3 layer. The Al2O3 thickness is controlled by the elevated temperature and the furnace time.
The Al2O3 layer needs to be thick for large average particle size. The thickness should be under 3 micron to assure that the Al2O3 shell is broken up during the powder consolidation and secondary processing steps. Aluminum powder with large average particle size should be limited to produce Al-MMNCs with low Al2O3 volume percent loadings.
Plasma is an efficient method to produce nano aluminum powder. Because of very small average particle size, nano aluminum with a very thin Al2O3 surface layer can have very high percentages of Al2O3. For example, Tetronics Limited in the United Kingdom has produced the nano aluminum powder that has a particle size in the range of 90 nm to 100 nm and contains 33 weight % (˜30 vol %) of Al2O3.
Both atomization and plasma methods are able to produce unalloyed aluminum powder and aluminum alloy powder of Aluminum Association 2000, 6000 and 7000 series as well as other special alloys.
The aluminum powder with nano surface oxide layer for producing the Al-MMNCs has a preferred average particle size from about 20 nm to 50 microns with a normal particle size distribution.
(2) Step 2. Preparing Aluminum Alloy Matrix Mixture
The aluminum matrix is the aluminum alloy that is chosen as the most appropriate for a given Al-MMC or Al-MMNC application. There are two routes to prepare the aluminum matrix alloy.
By following the procedure in Step 1, the aluminum alloy powder with predetermined percentage nano surface oxide can be produced by using a desired aluminum alloy ingot. Such aluminum alloy powder is ready for the consolidation step.
Step 2(1) is the simplest way. However, most aluminum powder producers are manufacturing unalloyed aluminum powder only. The powder metallurgy method of creating aluminum alloy is employed when the unalloyed aluminum powder with nano surface oxide is used.
The unalloyed aluminum powder and alloy elemental powders are mixed by using a blender, jet mill or ball mill. The alloy powders need to be reasonably uniformly mixed with unalloyed aluminum powder because the alloy elements defuse in the matrix to form the aluminum alloy during the hot consolidation process. The aluminum matrix is alloyed uniformly when such diffusion controlled-alloying is taking place.
The alloy elemental powders are selected from the group consisting of magnesium, copper, iron, zinc, manganese, nickel, cobalt, silicon, titanium, alloys and combination thereof. It is preferred to have average particle sizes of alloy powders in the particle size range of the unalloyed aluminum powder.
The Al2O3 volume percent should be adjusted according to the percentage of additional alloy powder being used.
Step 3. Consolidating the Powder Mixture
The powder mixture is loaded into a billet tool and is pressed at room temperature to form a compacted mixture that has between about 50% and 95% of theoretical density. The billet tool size can be very small or sized for compacting quantities approaching about 1000 kg (2,200 lb) of powder.
There are various powder metallurgy methods that can be employed to consolidate the compacted mixture. The following are typical processes:
Under vacuum, inert-gas or air, the compacted mixture is heated to a degassing temperature range and then is held at this range for more than about one-half hour for degassing purposes. The degassing temperature range depends on the matrix metal and is typically from between about 230° C. (450° F.) and less than the lowest eutectic melt temperature of the compacted mixture. The main function of degassing is to remove H2O from the metal matrix mixture.
After the degassing period, the temperature is raised to the consolidation temperature, which is the highest eutectic melt temperature of the compacted mixture. The consolidation temperature is lower than the aluminum melting point. The consolidation temperature is typically between about 230 and 615° C. (450 and 1145° F.). While the consolidation temperature and vacuum, inert-gas or air environment is maintained, the degassed, compacted mixture is pressed to full density resulting in a composite billet.
In the case of a thin Al2O3 layer, consolidation of the powder should take place under vacuum or inert gas to prevent additional oxidation.
In the Cold Isostatic Press (CIP)/Sinter process, the matrix mixture is compacted at room temperature in Step 3 to about 85% to 95% of theoretical density. Pressing the powder mixture to high density at room temperature requires pressures between about 50,000 psi and 85,000 psi. Typically, a Cold Isostatic Press is employed.
The compacted mixture is then sintered in vacuum, in inert-gas, or in air. The compacted mixture is heated to the degassing temperature range and then is held at this temperature range for more than about one-half hour to be degassed. After degassing, the degassed compacted mixture is heated to a sintering temperature that is the highest eutectic melt temperature of the compacted mixture so that sintering of the matrix takes place to form the composite billet. This sintered composite billet has a density that is still approximately that of the starting compacted mixture, between 85% and 95% of the theoretical density, but is sealed by the sintering process.
This sealing is needed to avoid internal oxidation of the billet during subsequent hot working. The composite will have 98% to 100% of the theoretical density after following metal working.
In the case of aluminum powders with a thin Al2O3 layer, sintering should take place under vacuum or inert gas to prevent additional oxidation.
In the case of aluminum powder with a thick Al2O3 layer, the matrix mixture can be consolidated without degassing since the thick Al2O3 layer prevents additional oxidization that is detrimental to the resultant composite.
The mixture is loaded into a billet tool and is pressed at room temperature to form a compacted mixture that is between about 50% and 95% of the theoretical density.
The compacted mixture is heated to the consolidation temperature in vacuum, inert-gas or in air. While the consolidation temperature and vacuum, inert-gas, or air environment continues to be maintained, the compacted mixture is hot pressed to approximately 98 to 100% of theoretical density to produce the composite billet.
Step 4. Metal Working the Composite Billet
Most of the Al2O3 surface layers are broken up during billet consolidation due to the brittle nature. Unaffected Al2O3 surface layers are broken up during cold, hot metal work, or die casting or any combination of these steps.
The cold work can be cold extrusion, cold forging, cold rolling or any combination of these steps. During hot metal working, the composite billet is plastically deformed at a temperature between about 230 and 615° C. (450 and 1145° F.), which is below the melting point of aluminum. The hot metal work can be hot extrusion, hot forging or hot rolling or any combination of these steps.
The die casting can be pressureless die casting or pressure casting. Unlike boron carbide or silicon carbide particles in conventional P/M Al/MMCs, aluminum oxide particles do not react with molten aluminum.
The metal working can be incorporated as part of fabrication processes for producing final Al-MMNC products.
After the consolidation and metal working, the Al2O surface layers of nano thickness are broken into a nano Al2O3 particulate reinforcement phase which is uniformly distributed in the aluminum matrix resulting in a homogenous aluminum metal mixture nanocomposite.
The nano Al2O3 particles also act as grain refiners that minimize the grain sizes of the aluminum alloy in the composite. As a result, the Al-MMNCs are highly ability of work hardenable to achieve higher yield strength than conventional aluminum alloys and conventional Al-MMCs.
Unlike adding reinforcing ceramic particles that contribute to impurities in the metal matrix, there are few impurities in the Al-MMNC made by the method of this invention because there is no reinforcement particle added to the aluminum matrix. Therefore, there is excellent bonding between the aluminum matrix phase and the nano Al2O3 reinforcement phase.
The nano Al2O3, excellent chemical bonding, and lower impurities in the metal matrix all contribute to a near perfect composite with significantly improved fatigue and creep resistance.
The nano aluminum oxide is less abrasive than coarse ceramic particle reinforcements. Therefore, the Al-MMNCs have better machineability than conventional Al-MMCs. The Al-MMNC is as weldable as conventional Al2O3 particulate reinforced Al-MMCs since Al2O3 does not react with the aluminum metal matrix to produce brittle components that weaken the weld zone. The Al-MMNC can be heat-treated when the matrix aluminum alloy is heat-treatable.
Of course the present invention is not intended to be restricted to any particular form or arrangement, or any specific embodiment, or any specific use, disclosed herein, since the same may be modified in various particulars or relations without departing from the spirit or scope of the claimed invention hereinabove shown and described of which the apparatus or method shown is intended only for illustration and disclosure of an operative embodiment and not to show all of the various forms or modifications in which this invention might be embodied or operated.
The present invention has been described in considerable detail in order to comply with the patent laws by providing full public disclosure of at least one of its forms. However, such detailed description is not intended in any way to limit the broad features or principles of the present invention, or the scope of the patent to be granted. Therefore, the invention is to be limited only by the scope of the appended claims.
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