Metal founding – Process – Shaping liquid metal against a forming surface
Reexamination Certificate
1998-08-07
2001-06-26
Lin, Kuang Y. (Department: 1722)
Metal founding
Process
Shaping liquid metal against a forming surface
C164S080000, C164S493000
Reexamination Certificate
active
06250363
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to metal-matrix composite materials, and, more particularly, to the fabrication of articles from such materials by melting and casting.
In one form of a metal-matrix composite material, a reinforcement phase is embedded in a metal matrix. The reinforcement is typically equiaxed or elongated particles of a ceramic phase such as aluminum oxide or silicon carbide, and the matrix is a pure metal or alloy such as aluminum. The particle phase and the matrix metal phase each retains its separate physical and chemical identity in the composite material, and each phase contributes to the properties of the final composite material.
Several techniques are available to make useful articles of such materials. In one approach, the metallic matrix material is melted and wet to the particles, either by mixing or infiltration. The wetted mixture, in the form of a slurry of the wetted ceramic particles in a molten matrix, is then cast directly into molds in the case of the mixing approach, or diluted and then cast into molds in the case of the infiltration approach.
For some applications, the metal-matrix composite material is cast into foundry ingots at one location and shipped to the facility of a foundry user. The foundry user remelts the matrix portion of the foundry ingots, forming a remelted slurry, by heating the ingots to a temperature above the melting point of the matrix material, and then casts the remelted slurry into molds that define the shape of the final article. During the remelting operation, the remelted composite material sometimes is held at elevated temperature for several hours before casting, due to the logistics of the casting operation.
For some foundry casting operations, the remelted metal-matrix composite material must be reheated in a furnace to temperatures well above the melting point of the metal matrix. If this temperature is sufficiently high that the ceramic reinforcement chemically reacts with the matrix material to a significant degree, the resulting reaction product generally increases the viscosity of the slurry. The slurry of increased viscosity is more difficult to cast than it is prior to the chemical reaction, impairing the ability to cast many articles. Additionally, the reaction product cast into the final product may adversely affect its properties.
Several solutions to this problem are known. In one, the surfaces of the particles are coated or treated in-situ to reduce their reactivity. In another, specific matrix alloys having reduced reactivity are selected. In yet another, the remelt temperature is limited so as to reduce the extent of chemical reaction. These various approaches are workable in some circumstances, but not in others due to technical or cost issues.
There is a need for an improved approach to the remelt processing of castable metal-matrix composite materials. The present invention fulfills this need, and further provides related advantages.
SUMMARY OF THE INVENTION
The present invention provides a technique for remelting metal-matrix composite materials in a rapid fashion so as to limit the extent of the chemical reaction that occurs between the matrix and the particles at the elevated temperature. (The terms “melt” and “remelt”, as used herein in reference to a composite material or to granules, means that the metal matrix phase is melted or remelted, but that the reinforcement particles remain solid. The result of melting or remelting is a slurry of the solid reinforcement particles in the molten matrix material.) A remelt charge may be quickly heated to a high temperature and then immediately cast, so that there is little opportunity for chemical degradation to occur. The remelting approach is efficient and economical in that it uses less power than required for conventional remelting techniques, and it allows the remelting to be accomplished in air rather than requiring a vacuum or a protective atmosphere. The remelting approach may be used for large or small volumes of remelted material, making it much more suitable than prior approaches for use by small foundry operations. The remelting operation avoids the entrapment of gas within the remelted mass.
In accordance with the invention, a method for preparing a composite material comprises the steps of furnishing a plurality of granules, each granule comprising a composite material of ceramic particles in a metal matrix, furnishing an induction heater having an induction coil, placing the plurality of the granules into the induction coil, and powering the induction heater to melt the metal matrix portion of the granules to form a molten mixture.
The present approach utilizes granules of the composite material, rather than ingots or powders, in the remelting operation. These granules are initially formed by any operable processing, such as melting and subsequent granule formation, or infiltration, dilution, and subsequent granule formation. The granules may be made of any operable material, such as, for example, aluminum oxide or silicon carbide particles in an aluminum metal matrix. The granules have a particle size with a smallest dimension of from about 1 to about 10 millimeters, and desirably are of smoothly spherical, ovoid, or flattened spherical shape.
The granules are placed into an induction heating coil, induction heated to remelt the metal matrix phase, and immediately cast into molds or otherwise used. High power levels may be introduced into the granules, so that heating is rapid. Individual charges of material may be prepared for each casting event, with the result that there is not a long holding period at elevated temperature. Consequently, the remelted granules may be heated to casting temperatures greater than ordinarily possible with conventional furnace-melting procedures, without producing unacceptably large quantities of chemical reaction products within the composite material. The greater remelt temperatures permit casting from higher temperatures than possible with the conventional approach. The rapid heating and short exposure time at elevated temperature also limits the extent of oxide formation at the surface of the composite material, allowing remelting in air rather than a controlled atmosphere or vacuum.
Induction melting may be scaled over a wide range from relatively small to relatively large volumes of material, permitting this approach to be used by a wide range of users without the need for investing in expensive melting furnaces, special atmospheric control equipment, and hot-metal-handling equipment, other than casting molds. The remelted composite material may be prepared in an on-demand manner, to meet logistical requirements of the casting operation.
It is usually desirable to avoid the introduction of gas and gas bubbles into molten composite materials. Any gas in dissolved form tends to prevent wetting of the molten matrix to the particles, and any gas in bubble form may be retained when the metal solidifies and may lead to internal weakness. Even though there is a substantial volume of open space between granules prior to induction melting, the melted material is remarkably free of internal gas and porosity. The final cast product is therefore quite sound, and the particles are well wetted by the metal matrix material. As known to those skilled in the art of metal-matrix composite materials, gas entrapment is a significant problem when pieces of metal-matrix composite material are melted by other techniques, such as a resistance heating furnace. That gas entrapment is avoided when melting granules is accomplished by induction heating is a surprising and unexpected advantage of the present invention.
The present induction melting approach allows the introduction and mixing of granules of different types, to achieving a controlled alloying during the melting operation. That is, the granules melted may be all of the same type of composite material, different types of composite materials, or some of composite material and some of non-composite material. This alloying capability provides i
Bruski Richard S.
Doutre Don
Hay Gary
Wales Peter
Alcan International Ltd.
Garmong Gregory
Lin I.-H.
Lin Kuang Y.
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