Claims
- 1. A stabilized metal foam body, comprising:
- a metal matrix having dispersed therethrough a plurality of completely closed cells substantially filled with gas;
- and finely divided solid stabilizer particles dispersed within said matrix, wherein the stabilizer particles contained in the matrix are concentrated adjacent the interfaces between the matrix metal and the closed cells.
- 2. A foam body according to claim 1 wherein the stabilizer particles are present in the metal matrix composite in an amount of less than 25% by volume.
- 3. A foam body according to claim 1 wherein the stabilizer particles have sizes in the range of about 0.1 to 100 .mu.m.
- 4. A foam body according to claim 3 wherein the stabilizer particles have sizes in the range of about 0.5 to 25 .mu.m and are present in the composite in an amount of 5 to 15% by volume.
- 5. A foam body according to claim 3 wherein the stabilizer particles are ceramic or intermetallic particles.
- 6. A foam body according to claim 3 wherein the stabilizer particles are metal oxides, carbides, nitrides or borides.
- 7. A foam body according to claim 3 wherein the stabilizer particles are selected from the group consisting of alumina, titanium diboride, zirconia, silicon carbide and silicon nitride.
- 8. A foam body according to claim 3 wherein the closed cells have average sizes range from 250 .mu.m and 50 mm.
- 9. A foam body according to claim 3 wherein the matrix metal is aluminum or an alloy thereof.
SUMMARY OF THE INVENTION
This is a continuation-in-part of application Ser. No. 403,588, filed Sept. 6, 1989, now U.S. Pat. No. 4,973,358.
This invention relates to lightweight foamed metal, particularly a particle stabilized foamed aluminum, and its production. This is a continuation-in-part of now U.S. Pat. No. 4,973,358.
Lightweight foamed metals have high strength-to-weight ratios and are extremely useful as load-bearing materials and as thermal insulators. Metallic foams are characterized by high impact energy absorption capacity, low thermal conductivity, good electrical conductivity and high absorptive acoustic properties.
Foamed metals have been described previously, e.g. in U.S. Pat. Nos. 2,895,819, 3,300,296 and U.S. Pat. No. 3,297,431. In general such foams are produced by adding a gas-evolving compound to a molten metal. The gas evolves to expand and foam the molten metal. After foaming, the resulting body is cooled to solidify the foamed mass thereby forming a foamed metal solid. The gas-forming compound can be metal hydride, such as titanium hydride, zirconium hydride, lithium hydride, etc. as described in U.S. Pat. No. 2,983,597.
Previously known metal foaming methods have required a restricted foaming temperature range and processing time. It is an object of the present invention to provide a new and improved metal foaming method in which it is not necessary to add a gas-evolving compound nor to conduct the foaming in the restricted melt temperature range and restricted processing time.
According to the process of this invention, a composite of a metal matrix and finely divided solid stabilizer particles is heated above the liquidus temperature of the metal matrix. Gas is introduced into the the molten metal composite below the surface of the composite to form bubbles therein. These bubbles float to the top surface of the composite to produce on the surface a closed cell foam. This foamed melt is then cooled below the solidus temperature of the melt to form a foamed metal product having a plurality of closed cells and the stabilizer particles dispersed within the metal matrix.
The foam which forms on the surface of the molten metal composite is a stabilized liquid foam. Because of the excellent stability of this liquid foam, it is easily drawn off to solidify. Thus, it can be drawn off in a continuous manner to thereby continuously cast a solid foam slab of desired cross-section. Alternatively, it can
The success of this foaming method is highly dependent upon the nature and amount of the finely divided solid stabilizer particles. A variety of such refractory materials may be used which are particulate and which are capable of being incorporated in and distributed through the metal matrix and which at least substantially maintain their integrity as incorporated rather than losing their form or identity by dissolution in or chemical combination with the metal.
Examples of suitable solid stabilizer materials include alumina, titanium diboride, zirconia, silicon particles in the foam is typically less than 25% and is preferably in the range of about 5 to 15%. The particle sizes can range quite widely, e.g. from about 0.1 to 100 .mu.m, but generally particle sizes will be in the range of about 0.5 to 25 .mu.m, with a particle size range of about 1 to 20 .mu.m being preferred.
The particles are preferably on average substantially equiaxial. They normally have an average aspect ratio (ratio of maximum length to maximum cross-sectional dimension) of no more than about 2:1. There is also a relationship between particle sizes and the volume fraction that can be used, with the preferred volume fraction increasing with increasing particle sizes. If the particle sizes are too small, mixing becomes very difficult, while if the particles are too large, particle settling becomes a significant problem. If the volume fraction of particles is too low, the foam stability is then too weak and if the particle volume fraction is too high, the viscosity becomes too high.
The metal matrix may consist of any metal which is aluminum, steel, zinc, lead, nickel, magnesium, copper and alloys thereof.
The foam-forming gas may be selected from the group consisting of air, carbon dioxide, oxygen, water, inert gases, etc. Because of its ready availability, air is usually preferred. The gas can be injected into the molten metal composite by a variety of means which provide sufficient gas discharge pressure, flow and distribution to cause the formation of a foam on the surface of the molten composite. It has been found that the cell size of the foam can be controlled by adjusting the gas flow rate, the impeller design and the speed of rotation of the impeller, where used.
It is also possible to operate an impeller such that a vortex is formed in the molten metal composite and the bubble-forming gas is then introduced into the molten metal composite via the vortex to form the gas bubbles within the molten composite. With this batch method, the gas is slowly drawn into the melt, e.g. over a period of 10 minutes, and produces a foam in which the cells are very small, spherical-shaped and quite evenly distributed. Typically the cell sizes are less than 1 mm, compared to cell sizes of 5-30 mm when the gas is injected below the surface of the melt.
According to another method of the invention, gas is introduced into the melt by both above techniques. Thus, the gas is both injected directly beneath the surface of the melt and induced via a vortex. This makes it possible to tailor both the structure and properties of the foam.
In forming the foam according to this invention, the majority of the stabilizer particles adhere to the gas-liquid interface of the foam. This occurs because the total surface energy of this state is lower than the surface energy of the separate liquid-vapour and liquid-solid state. The presence of the particles on the bubbles tends to stabilize the froth formed on the liquid surface. It is believed that this may happen because the froth is restricted by the layer of solids at the liquid-vapour interfaces. The result is a liquid metal foam which is not only stable, but also one having uniform , pore sizes throughout the foam body since the bubbles tend not to collapse or coalesce.
The stabilized metal foam of the present invention can form a wide variety of products. For example, it may be in the form of acoustic absorbing panels, thermal insulation panels, fire retardant panels, energy absorbing panels, electro-magnetic shields, buoyancy panels, packaging protective material, etc.
US Referenced Citations (4)
Foreign Referenced Citations (3)
| Number |
Date |
Country |
| 0210803 |
Feb 1987 |
EPX |
| 1259163 |
Mar 1961 |
FRX |
| 2282479 |
Mar 1976 |
FRX |
Continuation in Parts (1)
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Number |
Date |
Country |
| Parent |
403588 |
Sep 1989 |
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Patent Grant
5112697
