Metal founding – Process – Shaping liquid metal against a forming surface
Reexamination Certificate
1998-08-07
2001-07-10
Nguyen, Nam (Department: 1722)
Metal founding
Process
Shaping liquid metal against a forming surface
C164S098000, C164S061000, C164S063000, C164S065000
Reexamination Certificate
active
06257312
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to the manufacture of metal matrix composites having high particulate loadings and, more particularly, to the concentration of composite materials with lower particulate loadings to achieve higher particulate loadings.
Metal matrix composites, for example those consisting of refractory particles such as silicon carbide or alumina distributed in aluminum alloy matrices, are most economically manufactured on a large scale by mixing techniques. One such technique, described in U.S. Pat. No. 4,759,995, disperses and wets refractory particles with molten metal using a shearing technique while avoiding the retention and incorporation of gases. Such mixing techniques are frequently restricted to relatively low particulate loadings (e.g. less than about 25 volume percent particulate) because of fluidity problems encountered during mixing.
On the other hand, metal matrix composites used in large-scale shape casting processes and similar applications most frequently use the same or lower solids loading because of similar fluidity constraints. As a result, scrap material from such casting processes having relatively low particulate loading is frequently available as a byproduct from such operations.
Higher solids-loading material is typically prepared by pressure-assisted or other infiltration techniques. These higher solids-loading materials are useful in certain structural and electronic applications, but because of the difficulty in producing and processing such materials, they have not found wide application.
There is a need for a less costly method for manufacturing higher solids loading metal matrix composites, ideally one which uses the proven cost advantages of the various processes for manufacturing low solids loading materials or which uses available scrap materials. There is further a need for methods for manufacturing high solids loading materials that can be readily cast, forged or otherwise manufactured by methods suitable for high volume production. The present invention fulfills this need, and further provides related advantages.
SUMMARY OF THE INVENTION
This invention relates to the preparation and use of metal-matrix composite materials having a high particulate loading. A metal-matrix composite material having lower particulate loading is prepared and thereafter concentrated to a higher particulate loading. The approach produces a composite material with a high particulate loading wherein the particles are not agglomerated, and is economical. The concentrated, high-particulate-loading composite material may be used in thixoforming operations or diluted with the addition of other matrix alloys to produce a lower-particulate-loading material with a different matrix than that first prepared.
In accordance with the invention, a method for preparing a metal matrix composite has as a starting material an initial composite material having a plurality of particles with an average particle size, and a molten metal matrix. The method uses a container having an exit channel and a porous element within the exit channel. The porous element has an average pore size opening such that, under a pressure differential, molten matrix metal but no substantial amount of particles pass through the porous material. The method is practiced by placing the initial composite material into the container in contact with the porous element, and creating a pressure differential in the initial composite material across the porous element sufficient to cause the matrix metal to flow through the porous element and leave a final composite material in the container. The step of creating a pressure differential is continued for a time sufficient that the final composite material is a self-supporting mass.
The initial composite material preferably is prepared by a technique such as that described in U.S. Pat. No. 4,759,995, which results in a structure having wetted particles in a metallic matrix. Such a process is most readily applied to produce composites having at least about 5 volume percent particulate, preferably from about 5 volume percent to about 25 volume percent of particulate, a relatively low particulate loading. The porous element has an average pore size larger than the average particle size, and typically at least about 2 times the average particle size. Larger pore sizes may also be used, as where the average pore size is at least about 5 times the average particle size, but less than about 25 times the average particle size.
The initial composite material, with the matrix molten, is contacted to one side of the porous element. A pressure differential, typically about 0.2-2 atmospheres, is applied across the porous element, either by application of a vacuum on the downstream side of the porous element or a pressure to the initial composite material on the upstream side of the porous element. Matrix metal from the composite material separates from the composite material and flows through the porous element, causing the particle volume fraction of the composite material remaining on the upstream side of the porous element to gradually increase.
The concentrating of the particulate by removal of the metal matrix is continued for a period of seconds to a few minutes until, when the particle volume fraction reaches the range of about 37 to about 45 volume percent, the composite material becomes self-supporting. This final composite material separates from the sidewall of the container, and may be removed from the container as a freestanding mass.
The self-supporting final composite material may be used in any appropriate fashion. For example, it may be formed directly to a useful shape having the particulate loading produced in the concentrating operation, in a manner similar to thixoforging. It may instead be modified by the addition of the same matrix metal to produce a composite material having an intermediate, lower volume fraction of particulate, such as in the range of from about 25 to about 37 volume percent of particulate. Composite materials with the particulate loading in this range may be difficult to produce directly by mixing techniques. When such composites of intermediate particulate loading are produced directly by conventional infiltration and subsequent dilution, there is an undesirable tendency for the particulate to agglomerate into clusters of particles that are only partially wetted by the matrix metal, leading to difficulties during dilution and reduced mechanical properties of the final product. By contrast, any agglomerated particles produced by the present approach are wetted by the matrix metal, resulting in acceptable properties of the final composite material. In yet another use of the concentrated composite material, a second, different molten metallic material is mixed with the concentrated composite material, simultaneously reducing the particulate loading and also altering the composition of the matrix alloy. By this approach, either intermediate or lower particle loading composite materials may be produced with matrix alloys that cannot be otherwise used.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.
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patent:
Doutre Don
Hay Gary
Jin Iljoon
Lloyd David J.
Wales Peter
Alcan International Limited
Garmong Gregory
Lin I.-H.
Nguyen Nam
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