Metal founding – Means to shape metallic material – United particle type shaping surface
Reexamination Certificate
1999-07-14
2002-08-13
Elve, M. Alexandra (Department: 1725)
Metal founding
Means to shape metallic material
United particle type shaping surface
C164S411000, C164S516000
Reexamination Certificate
active
06431255
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to metal casting. More specifically, it relates to shell molds used in the casting of metal components, e.g., components made from superalloys.
The casting of metals is carried out by various techniques, such as investment casting. Ceramic shell molds are used during investment-casting, to contain and shape the metal in its molten state. The strength and integrity of the mold are very important factors in ensuring that the metal part has the proper dimensions. These shell mold characteristics are especially critical for manufacturing high performance components, such as superalloy parts used in the aerospace industry.
Investment casting techniques often require very high temperatures, e.g., in the range of about 1450° C. to 1750° C. Many conventional shell molds do not exhibit sufficient strength at those temperatures. The molds become susceptible to bulging and cracking when they are filled with the molten metal. (Bulging can also occur when very large parts are being cast—even at lower temperatures). Bulging can alter the dimensions of the mold, thereby causing undesirable variation in the component being cast. Cracking could result in failure of the mold as the molten material runs out of it.
Clearly, greater strength and dimensional stability are required for shell molds used at very high casting temperatures, or for those used to cast very large parts. The problem is addressed by J. Lane et al in U.S. Pat. No. 4,998,581. In that disclosure, shell molds are strengthened by wrapping a fibrous reinforcing material around the shell mold as it is being made. In preferred embodiments, the reinforcing material is said to be an alumina-based or mullite-based ceramic composition having a specific, minimum tensile strength. The reinforcing material is apparently wrapped in spiral fashion around the shell mold with a tension sufficient to keep it in place as ceramic layers are applied to the mold to build it up to its desired thickness.
The Lane patent appears to provide answers to some of the problems described above. However, there appear to be some considerable disadvantages in practicing the invention disclosed in that patent. For example, mullite-based materials are difficult to produce without second phase inclusions of either silica- or alumina-containing compounds. These inclusions can degrade the physical properties of the mold. In addition, many of the reinforcing materials employed in U.S. Pat. No. 4,998,581 have thermal expansions much less than the mold. These large thermal expansion differences will make fabrication of a crack-free mold more difficult.
It should thus be apparent that further improvements in the properties of shell molds used under the conditions described above would be welcome in the art. The shell molds should have the strength to withstand high metal-casting temperatures, and should be suitable for casting large parts. The molds should also be dimensionally stable at elevated temperatures, and throughout various heating/cooling cycles. Moreover, if the molds are to be improved by the use of reinforcing materials, such materials should be flexible enough, before being fired, to satisfy the shape requirements for the mold, especially when intricate metal components are being cast. Finally, the preparation of improved shell molds should be economically feasible, e.g., not requiring the use of a significant amount of additional equipment. The use of the new molds should not result in undesirable increases in the cost for manufacturing metal parts in the investment casting process.
SUMMARY OF THE INVENTION
The desired improvements discussed above have been obtained by way of the discoveries upon which the present invention is based. In one aspect, the invention is a ceramic casting shell mold having a pre-selected shape, and comprising repeating layers of a ceramic material which define the thickness and shape of the mold, and a ceramic-based mat disposed in the layers of ceramic material. The mat substantially conforms to the shape of the mold, providing the mold with structural reinforcement. In many embodiments, the casting shell comprises:
(a) alternate, repeating layers of a ceramic coating material and a ceramic stucco, defining a total thickness of the shell mold; and
(b) a ceramic-based mat of reinforcing material disposed in the alternate, repeating layers of coating material and stucco at an intermediate thickness.
The reinforcing material for the mat is usually a silicon carbide-based material, or an alumina- or aluminate-based material. Mixtures of any of these materials can also be used. In preferred embodiments, the reinforcement mat comprises fibers having a bi-directional orientation. Moreover, the mat is preferably placed within about 10% to about 40% of the thickness from the inner wall of the mold, or within about 10% to about 25% of the thickness from the outer wall of the mold.
Furthermore, openings within the surface of the mat are large enough to allow the passage of ceramic particles when the mat is prepared from the coating material and the stucco. Moreover, in preferred embodiments, the coefficient of thermal expansion (CTE) of the mat is within about 50% of the CTE of the shell mold layers in which it will be inserted.
A method for making a ceramic casting shell mold is also described, comprising the steps of:
(I) applying a ceramic-based reinforcement mat to a ceramic layer-surface of a partial shell mold, e.g., one being made by an investment casting process;
(II) completing the shell mold by applying additional ceramic layers over the reinforcement mat; and then
(III) firing the shell mold at an elevated temperature.
Shell molds prepared by the method of the present invention exhibit substantial improvements in strength and dimensional stability at high temperatures, as compared to many of the shell molds of the prior art. Many metals or metal alloys can efficiently be cast in such shell molds, such as nickel-based superalloys.
DETAILED DESCRIPTION OF THE INVENTION
The ceramic shell molds which are reinforced according to the present invention are known in the art. Moreover, information related to shell molds for investment casting is widely available. Exemplary sources of useful information are as follows:
Kirk
-
Othmer Encyclopedia of Chemical Technology,
3rd Edition, Vol. 7, p. 798 et seq.;
Modern Metalworking
, by J. R. Walker, The Goodheart-Willcox Co., Inc., 1965;
Shell Molding and Shell Mold Castings
, by T. C. Du Mond, Reinhold Publishing Corp., 1954; and
Casting and Forming Processes in Manufacturing
, by J. S. Campbell, Jr., McGraw-Hill Book Company, Inc., 1950.
Shell molds are usually composed of refractory particles (e.g., refractory oxide particles) bonded together by a silica or phosphate gel. Examples of the typical refractory particles are alumina-based materials, aluminate-based materials (such as yttrium aluminate), or mixtures of these materials. Various patents describe many aspects of conventional shell-molding processes. The following are exemplary, and are all incorporated herein by reference: U.S. Pat. No. 4,998,581 (Lane et al); U.S. Pat. No. 4,097,292 (Huseby et al); U.S. Pat. No. 4,086,311 Huseby et al); U.S. Pat. No. 4,031,945 (Gigliotti, Jr. et al); U.S. Pat. No. 4,026,344 (Greskovich); U.S. Pat. No. 3,972,367 (Gigliotti, Jr. et al); and U.S. Pat. No. 3,955,616 (Gigliotti, Jr. et al).
One investment casting technique which is especially suitable for the present invention is the “lost wax” process. In one version of this technique, a wax pattern (i.e., a replica of the part being cast) is immersed repeatedly in a liquid slurry of refractory oxide particles in a silica- or phosphate-bearing binder. Usually, the slurry is highly loaded with the ceramic solids, e.g., at least about 40 volume percent, with the remainder being deionized water, an organic solvent, or a mixture thereof. Sufficient time is provided between immersions to allow the slurry coat to partially or completely dry on the wax. After a sufficient thickness of ceramic
Ghosh Asish
Giddings Robert Arthur
Klug Frederic Joseph
Monaghan Philip Harold
Svec Paul Steven
Elve M. Alexandra
General Electric Company
Johnson Noreen C.
Santandrea Robert P.
Tran Len
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