Patent Grant
7500511
Investment casting, also known as lost wax, lost pattern or precision casting, is a process employed in a number of industries to make metal, glass, and ceramic articles that meet relatively close dimensional tolerances. Typically, an investment casting is made by first making a facsimile or pattern from a meltable substrate of the object to be made by investment casting. Suitable meltable substrates may include, for example, wax, polystyrene, or plastic.
Next, a ceramic mold, known as an investment casting shell, is formed around the pattern. This process may include dipping the pattern into a slurry containing a mixture of liquid refractory binders and a refractory powder and then sieving dry refractory grains onto the freshly dipped pattern. The most commonly used dry refractory grains include quartz, fused silica, zircon, alumina and aluminosilicate. The steps of dipping the pattern into a refractory slurry and then sieving dry refractory grains onto the freshly dipped pattern may be repeated until the mold has sufficient thickness and strength for further processing. However, it is preferable if each coat of slurry and refractory grains is air-dried before subsequent coats are applied. The investment slurry is then given time to set and dry. Drying can be accelerated by forced air and other techniques.
After drying, the shell is heated to at least the melting point of the meltable substrate. The heat melts the substrate away, leaving only the shell and possibly some residual substrate. The shell may be heated to a temperature high enough to vaporize any residual meltable substrate from the shell. Generally, the shell is filled with molten metal before the shell has cooled from the high temperature heating. Various methods have been used to introduce molten metal into shells including gravity, pressure, vacuum and centrifugal methods. When the molten metal in the mold has solidified and cooled sufficiently, the casting may be removed from the shell.
Although investment casting has been known and used for many years, the investment casting market continues to grow as the demand for more intricate and complicated parts increase. Because of the great demand for high quality, precision castings, there continuously remains a need to develop new ways to make investment casting shells more quickly, efficiently, cheaply and of higher quality. For instance, if the strength of investment casting shells could be increased, less material would be required. If an investment casting shell could be made with fewer coatings, it could be made more quickly, resulting in time and cost savings.
The present invention is directed to slurry composition for a mold and method of use thereof. The composition includes about 45-80% by weight alumina, about 10-30% by weight silicon carbide, and about 10-50% by weight colloidal silica. In one aspect, the alumina component comprises a material selected from the group consisting of brown fused alumina, white fused alumina, tabular alumina, calcined alumina, and mixtures thereof. In another aspect, the composition includes fumed silica at 2-5% by weight. In another aspect, the composition includes a setting agent at 0.05-2% by weight.
The casting method includes the steps of providing a meltable patterned substrate, coating the substrate with a slurry composition, allowing the slurry composition to set, and removing the substrate from the mold. The slurry composition includes about 45-80% by weight alumina, about 10-30% by weight silicon carbide, and about 10-50% by weight colloidal silica. In one aspect, the method includes coating the substrate with the slurry composition by dipping the substrate into the slurry composition. In another aspect, the method includes coating the substrate with the slurry composition by spraying the slurry composition onto the substrate. In another aspect, the method includes coating the substrate with the slurry composition by brushing the slurry composition onto the substrate.
The composition of the present invention has several advantages. One is that the slurry can be applied in as few as one or two coatings, instead of the multiple coatings of conventional compositions for investment casting molds. Another advantage is that the set time can be controlled by varying the amount of setting agent. A further advantage is reducing the necessary finishing work because the shell sticks less to the cast piece. Other advantages include less material consumption, quicker turn around time, excellent thermal shock resistance, and very good detail in the final product.
The foregoing paragraphs have been provided by way of general introduction, and are not intended to limit the scope of the following claims. The presently preferred embodiments, together with further advantages, will be best understood by reference to the following detailed, description taken in conjunction with the accompanying drawings.
The present invention is directed to a slurry composition for casting and a method of use thereof. The investment casting process begins with the substrate 12 shown in
Alternatively, the substrate may be dipped into the slurry composition, as shown in
Following a standard investment casting process, which is well known to those skilled in the art, the mold is then heated to melt out the substrate. Before forming the casting, the mold may either be allowed to cool, or used while still hot. The mold may then be filled with liquid metal to form castings of the desired design.
The composition of the invention may include about 45-80% by weight of an alumina component, preferably about 50-65% by weight of the alumina component. The alumina component preferably has an average particle diameter of about 100 micrometers to about 3 mm, and is preferably selected from brown fused alumina, white fused alumina, tabular alumina, and mixtures thereof. In one embodiment, the alumina component has the following size distribution: particles of screen size 6×14 (1.1 to 3.0 mm) are present at about 0-10% by weight of the composition, particles of screen size 14×70 (0.2 to 1.1 mm) are present at about 40-60% by weight, and alumina particles of screen size −70 (0.150 mm) are present at about 2-10% by weight.
The composition of the invention may also include about 2-12% by weight calcined alumina having an average particle diameter of about 0.2-70 microns. The calcined alumina is believed to react with the colloidal silica binder to form a sediment phase which causes further improved binding characteristics, especially at higher temperatures.
The composition of the invention includes about 10-30% by weight silicon carbide. Preferably, the slurry composition includes about 15-25% by weight silicon carbide. The silicon carbide should have an average particle diameter between about 30 micrometers and about 3.5 millimeters, in order to promote flow of the composition during application and improve the strength of the resulting shell. A wide distribution of particle sizes facilitates flow and workability of the composition, as well as enhancing the strength of the shell.
The composition of the invention also includes about 10-50% by weight of an aqueous colloidal silica binder, and preferably includes about 15-30% by weight of this binder. The binder should include about 10-70% by weight colloidal silica in water, preferably about 30-50% by weight colloidal silica in water. The colloidal silica should have an average silica particle diameter of about 4-100 nanometers, preferably about 8-20 nanometers.
The colloidal silica binder serves two important purposes. During application of the composition, the aqueous colloidal silica imparts excellent flow and suspension properties. After the slurry composition has been formed and dried, the colloidal silica acts as an excellent binder, thereby contributing strength and erosion resistance to the shell.
In one embodiment, the composition of the invention includes about 1 -5% by weight of fumed silica. Fumed silica improves the flow and mixing properties of the composition and also helps to prevent caking.
The composition of the invention preferably includes about 0.05-2.0% by weight of a setting agent. Examples of suitable setting agents include calcium aluminate cement, magnesium oxide, and mixtures thereof. By adding an appropriate amount of setting agent, the set time of the composition can be adjusted from under 15 minutes to over 10 hours.
In one embodiment, the composition includes about 0.05-0.5% by weight of a fiber, preferably polypropylene fiber. Examples of suitable polypropylene fiber include Herculon™, available from Hercules Inc. In a preferred embodiment, the fibers are around 2-10 mm in length and 1-5 denier in diameter.
In one embodiment, the composition of the invention includes about 0.01-1.0% by weight of a welan gum. Welan gum is a fermentation polysaccharide with excellent thermal stability and retention of viscosity at elevated temperatures. It improves the workability of the composition by improving the suspension characteristics so that the components of the composition will not separate during application. It helps to provide a uniform and stable distribution of the components of the mixture and reduces the need for multiple coats to produce a shell. Welan gum is available from Kelco-Crete®.
The slurry composition of the invention may also include about 0-10% by weight free carbon, preferably about 2-6% free carbon, having an average particle diameter of about 40 microns to about 0.5 mm. Typically, the carbon is in the form of pitch, which is a mixture of carbon and volatile organic compounds. The slurry composition of the invention preferably includes about 0.05-5% by weight of petroleum pitch.
An investment casting composition was prepared by mixing the following components together in the stated quantities.
The resulting casting composition had excellent flow properties and was applied to substrates by both the brushing and dipping methods. The composition had a set time between three and four hours. The slurry composition yielded molds with excellent density, porosity and strength, and acceptable casts were made from the molds.
The embodiments described above and shown herein are illustrative and not restrictive. The scope of the invention is indicated by the claims rather than by the foregoing description and attached drawings. The invention may be embodied in other specific forms without departing from the spirit of the invention. Accordingly, these and any other changes which come within the scope of the claims are intended to be embraced therein.
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| Number | Date | Country | |
|---|---|---|---|
| 20050061471 A1 | Mar 2005 | US |
