Specialized metallurgical processes – compositions for use therei – Processes – Free metal or alloy reductant contains magnesium
Reexamination Certificate
2000-03-22
2003-04-15
Cooley, Charles E. (Department: 1723)
Specialized metallurgical processes, compositions for use therei
Processes
Free metal or alloy reductant contains magnesium
C366S169100, C366S305000, C366S306000, C266S235000
Reexamination Certificate
active
06547850
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to particle mixing technology and, more particularly, to an apparatus and method for mixing particles into a liquid or semi-liquid medium. In certain preferred embodiments, the invention relates to the mixing of nonmetallic reinforcing particles into molten metals or metal alloys for the production of stir-cast metal matrix composite (MMC) materials.
BACKGROUND OF THE INVENTION
Metal matrix composites (MMCs), particularly those based upon aluminum alloys, have gained increasing popularity and recognition as alternative structural materials, especially for applications requiring increased stiffness, wear resistance, and strength. MMCs are usually produced by mixing nonmetallic reinforcing particles such as grit, powder, fibers or the like into a metallic matrix. For example, aluminum-based MMCs are composed typically of aluminum alloys (e.g., 6061, 2024, 7075, or A356) reinforced with ceramic particles such as silicon carbide or aluminum oxide (alumina) powder. The reinforcement provided by these particles contributes strength, stiffness, hardness, and wear resistance, in addition to other desirable properties, to the composite.
Despite their growing market, the high cost of manufacturing MMCs has hampered their ability to be priced competitively with unreinforced metallic materials. Traditionally, the fabrication of metal matrix composites has employed non-liquid methods such as the compaction of blends of ceramic particles or fibers and aluminum powders, or the metal spraying of continuous fibers in a lay-up process. Unfortunately, the high cost of metallic powders and the explosion and pyrophoric hazards associated with large quantities of powders have prevented a significant reduction in the cost of MMCs produced by this approach.
In addition, the use of liquid metals in MMC fabrication has largely been limited to the infiltration of ceramic preforms. The mixing of ceramic particles into molten aluminum using stir-cast methods has not been advantageous due to problems with the incomplete wetting of fine particles having a large surface area, as well as the rapid oxidation of a chemically reactive molten metal (e.g., aluminum) during agitation. On the other hand, the simplicity of this approach and its potential for producing low cost MMCs has led to numerous studies on the fabrication of aluminum-based MMCs through stir-casting. Numerous researchers have reported experiments involving the mixing of various ceramic powders and fibers into molten aluminum-based matrices. The equipment and methods utilized in many of these experiments were extremely simple. The equipment usually consisted of a heated crucible containing molten aluminum alloy and a motor to rotate a paddle-style impeller made of graphite or coated steel in the molten aluminum while ceramic particles were added to the surface of the molten metal (i.e., the melt). The vortex formed by the rotating impeller drew the ceramic particles into the melt and the shear developed between the impeller and the walls of the crucible helped wet the particles. The temperature was usually maintained below the liquidus temperature (in the two-phase region) to keep the aluminum alloy in a semi-solid condition, since the higher viscosity of the partially solid melt further increased the shear force created by the simple impeller. This process has been called compocasting.
The aluminum-based MMCs made by compocasting suffered from various problems. In particular, since the process was carried out under atmospheric pressure, the vortex formed by the impeller rotation drew considerable amounts of gas into the melt. Also, because the composite is sensitive to turbulence and the particles act as sites for the entrapment of gas bubbles, the solidified composites produced by compocasting were often porous. In addition, it was common for these compocast MMCs to contain numerous oxide skins due to the passing of the particles through the surface oxide into the body of the melt. Another problem with the compocasting process is the low level of shear developed by the rotating impeller in the semi-liquid matrix. Since shear is needed for wetting, the particles are generally incompletely wetted by the molten metal alloys. In sum, the quality of the composites produced by these stir-cast approaches was poor and not considered commercially viable.
The aforementioned compocasting process and other prior processing techniques used in the manufacture of metal matrix composite materials are described in detail in U.S. Pat. No. 5,531,425 to Skibo et al., the disclosure of which is incorporated herein by reference.
Today, Duralcan, a division of Alcan Aluminum Corporation, is a leader in the manufacture and sale of stir-cast aluminum-based MMCs. The technological development which led to the Duralcan process is based on an improvement in mixing efficiency combined with a reduction in gas entrapment. In this process, a low vacuum of approximately 1-5 torr is drawn over molten aluminum heated above the liquidus temperature (in the fully liquid region). The reinforcing particles are added to the surface of the melt and an impeller capable of creating a moderately high level of shear in a low viscosity melt is inserted into the molten metal and stirred at high rotational speed, as measured in revolutions per minute (rpm). The vacuum removes the air which tends to act as a buffer, cushioning the particles and preventing intimate contact with the metal. With the particles in contact with the metal from the start of the process, wetting can begin immediately. The high shear impeller physically shears the particles into the aluminum alloy, spreading the aluminum over the high surface area of the fine particles, thereby rapidly wetting them. The quality of the resulting MMC is much improved over that produced by the other techniques described above. The particles are essentially 100% wetted and there is little or no porosity in the Duralcan MMC. However, while the end product of the Duralcan process is of high quality, the high cost of manufacture, due in large part to the inefficiency of particle mixing and wetting, prevents Duralcan from fully exploiting the potential MMC market.
The Duralcan process is a batch process that can be divided into three general stages. The first stage is the incorporation of the particles into the molten aluminum, i.e., bringing the particles into intimate contact with the aluminum so that wetting can begin. This stage relies on the formation of a vortex to draw the particles into the body of the melt and a vacuum for eliminating the cushioning effect of gas at atmospheric pressure. In the second stage, the particles must be sheared into the melt through the use of a rotating impeller which produces high shear force. In general, the impeller must have sharp teeth and rotate at sufficient rotational speed in order to break up agglomerates of particles such that each particle may individually come into contact with the aluminum melt. The rotational speed requirement seems to be related to a minimum level of shear generated at a specific surface velocity of the impeller in the melt. Typically, if the rotational speed of the impeller, as measured in rpm, is too low and/or the edges of the teeth are dull, low porosity MMC material comprising well-wetted particles cannot be produced. To further enhance the level of shear, a stationary bar or baffle is positioned proximate to the perimeter of the rotating impeller. A small region of increased shear is created between the outer periphery of the impeller and the baffle. The third stage involves the slow general motion of the composite in the mixing vessel so that substantially all of the composite eventually passes through the region of high shear several times. This motion also ensures uniformity of particle distribution throughout the batch.
However, the Duralcan process, and other similar stir-cast processes practiced presently, have certain shortcomings and disadvantages. In particular, the wetting of the particles, which is the
Schuster David M.
Skibo Michael D.
Cooley Charles E.
MC21 Incorporated
Sorkin David
Wagner , Murabito & Hao LLP
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