Reinforced ceramic investment casting shell mold and method of making such mold

Abstract: The ceramic investment casting shell mold (10) includes alternate, repeating layers of a ceramic material and a ceramic stucco defining an overall thickness of the shell mold. A fibrous reinforcing material (12) is disposed in the alternate, repeating layers at an intermediate thickness of the shell mold. The fibrous reinforcing material has high tensile strength at elevated temperature and a coefficient of thermal expansion that is less than the coefficient of thermal expansion of the ceramic material and the ceramic stucco. The fibrous reinforcing material is preferably woven into a twisted yarn and disposed in a generally spiral configuration.

Description: Field of the Invention The present invention relates to the investment casting of metals and, more particularly, to a reinforced ceramic investment casting shell mold and a method of making such mold. Background of the Invention Ceramic shell molds are used in the investment casting of metals to contain and shape the molten metal. In the casting of larger articles and in the casting of articles at higher casting temperatures, conventional ceramic shell molds are susceptible to bulging and cracking when they are filled with molten metal. When the ceramic shell mold bulges, the dimensions of the resultant casting are not accurate. Significant cracking can result in failure of the ceramic shell mold and runout of the molten metal. Accordingly, it is an object of the invention to provide an investment casting ceramic shell mold having improved strength sufficient to significantly reduce or eliminate the bulging and cracking problems experienced in conventional ceramic shell molds. It is a further object of the invention to provide a method of making an investment casting ceramic shell mold having such improved strength. Additional objects and advantages will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Documents JP-A-56 17157 (considered as the closest prior art of the present invention) and JP-A-55 64945 disclose methods of reinforcing ceramic shell molds by interposing reinforcing materials between refractory layers, but these methods have proved to give unsatisfactory results in the case of the present invention where reinforcement of ceramic shell molds at elevated temperature is sought. Summary of the Invention To achieve the foregoing objects and in accordance with the purpose of the invention, as embodied and broadly described herein, the ceramic investment casting shell mold of the present invention includes alternate, repeating layers of a ceramic material and a ceramic stucco defining an overall thickness of the shell mold, and a fibrous reinforcing material disposed in the alternate, repeating layers at an intermediate thickness of the shell mold. The reinforcing material has high tensile strength at elevated temperature and a coefficient of thermal expansion that is less than the coefficient of thermal expansion of the ceramic material and the ceramic stucco. Preferabliy, the ceramic investment casting shell molds, said shell mold comprising alternate layers of a ceramic material and a ceramic stucco defining an overall thickness of said shell mold, and a fibrous reinforcing material disposed in said alternate, repeating layers at an intermediate thickness of said shell mold is caracterized in that said fibrous reinforcing material is a substantially continuous fibrous reinforcing material, said reinforcing material being comprised of an alumina-based or mullite-based ceramic composition having tensile strength of at least 1380 MPa and having a coefficient of thermal expansion that is approximately one-half the coefficient of thermal expansion of said ceramic material and said stucco, said fibrous reinforcing material imparting a compressive load on the portion of said shell mold over which it is disposed at all temperature above the drying temperature for said shell mold. The method of making ceramic investment casting shell mold of the present invention, said method comprising the steps of providing a pattern having the shape of the desired casting, dipping said pattern into a ceramic slurry to form a coating on said pattern, applying a ceramic stucco on said coating, repeating said dipping step and said applying step to build up said shell mold to an intermediate thickness, said intermediate thickness being less than the desired overall thickness of said shell mold, disposing a fibrous reinforcing material around said shell mold, building up said shell mold to the desired overall thickness by repeating said dipping step and said applying step over said reinforcing material, and firing said shell mold, is caracterized in that said fibrous reinforcing material is a substantially continuous fibrous reinforcing material imparting a compressive load on the portion of said shell mold over which it is disposed at all temperatures above the drying temperature for said shell mold, said fibrous reinforcing material having high tensile strength at elevated temperature such that said reinforcing material remains an integral part of said shell mold after firing and said fibrous reinforcing material having a coefficient of thermal expansion that is less than the thermal expansion coefficient of the ceramic materials comprising said ceramic slurry and ceramic stucco. The step of disposing the fibrous reinforcing material around the shell mold preferably further includes wrapping the fibrous reinforcing material around the shell mold in a generally spiral configuration. More preferably, the fibrous reinforcing material is wrapped around the shell mold in a substantially continuous spiral leaving a space between successive wraps of the fibrous reinforcing material around the shell mold. The space is preferably in the range of from about 5 to about 50 mm. The accompanying drawing, which is incorporated in and constitutes a part of the specification, illustrates an embodiment of the invention and, together with the description, serves to explain the principles of the invention; Brief Description of the Drawings Fig. 1 is a side elevational view of a reinforced ceramic investment casting shell mold made in accordance with the present invention. Description of the Preferred Embodiments Reference will now be made in detail to the presently preferred embodiments of the invention, an example of which is illustrated in the accompanying drawing. A pattern having the shape of the desired casting is provided. The pattern may be made of wax, plastic, frozen mercury, or other materials suitable for use in "lost wax" casting processes. In accordance with the invention, a coating is formed on the pattern by dipping the pattern into a ceramic slurry. The initial coating formed on the pattern is generally referred to as the facecoat or facecoat layer. The ceramic slurry may be comprised of silica, alumina, zirconia, or other suitable ceramic material. After allowing excess slurry to drain from the coated pattern, ceramic stucco is applied. The ceramic stucco may be coarse alumina (120 mesh or coarser) or other suitable refractory material. The coated and stuccoed pattern is allowed to dry prior to the application of additional layers. In accordance with the invention, the dipping step and the applying step are repeated over the facecoat layer to build up the shell mold to an intermediate thickness that is less than the desired overall thickness of the shell mold. The intermediate thickness may be varied depending on the desired overall thickness of the shell mold. Preferably, the shell mold is built up to the intermediate thickness by repeating the dipping step and the applying step 6 to 9 times. At this degree of shell build up, any sharp edges and corners of the pattern are rounded. In accordance with the invention, a fibrous reinforcing material is disposed around the intermediate shell mold. The fibrous reinforcing material has high tensile strength at elevated temperature and a coefficient of thermal expansion that is less than the coefficient of thermal expansion of the ceramic materials comprising the ceramic slurry and the ceramic stucco. In connection with the description of the invention, the term "fibrous" denotes that the reinforcing material has an elongated aspect ratio. It is preferred that the fibrous reinforcing material has a length sufficient to allow it to be disposed around the intermediate shell mold in a continuous manner. Most preferably, the fibrous reinforcing material is a continuous length of material wound around the shell mold. The preferred fibrous reinforcing material is an alumina-based or mullite-based ceramic composition having a tensile strength of at least 1380 MPa and a coefficient of thermal expansion (at temperatures up to 926°C) that is approximately one-half the coefficient of thermal expansion (at temperatures up to 926° C) of the ceramic materials comprising the ceramic slurry and the ceramic stucco. Fibrous materials of this description are commercially available. NEXTEL 440 fiber manufactured by the 3M Company is the preferred reinforcing material. In a preferred embodiment, the fibrous reinforcing material is a woven twisted yarn. It has been found that a twisted yarn formed by first weaving a three roving string and then weaving four strings into the twisted yarn is particularly advantageous in terms of convenience of handling. Alternatively, the fibrous reinforcing material may be formed into a woven tape product. The preferred width for the woven tape product is about 2,5 mm to about 25 mm. The fibrous reinforcing material is disposed around the shell mold with sufficient tension so that it remains fixed during subsequent handling required to build up the shell mold to its overall thickness. If desired, ceramic adhesive or dip coat liquid may be used to locally fasten the fibrous reinforcing material to the shell mold for convenience of handling. In this case, the shell mold is dried before the application of additional layers. The step of disposing the fibrous reinforcing material around the intermediate shell mold preferably further includes wrapping the fibrous reinforcing material around the intermediate shell mold in a generally spiral configuration. More preferably, the fibrous reinforcing material is wrapped around the intermediate shell mold in a substantially continuous spiral leaving a space between successive wraps of the fibrous reinforcing material around the intermediate shell mold. The space between successive wraps of the fibrous reinforcing material is selected to allow for adequate shell build up around the reinforcing material to structurally affix the reinforcing material to the shell mold. It has been found that a space in the range of from about 5 mm to about 50 mm is sufficient for this purpose. After the fibrous reinforcing material is in place and the intermediate shell mold is dried, if necessary, the shell mold is built up to the desired overall thickness by repeating the dipping step and the applying step over the fibrous reinforcing material. The principles of the invention may be used to reinforce virtually any ceramic investment casting shell mold. By way of example, a ceramic shell mold for investment casting a large turbine airfoil reinforced in accordance with the invention is shown generally as 10 in Fig. 1. Fibrous reinforcing material 12 is wrapped around shell mold 11 at an intermediate thickness in a continuous spiral leaving space 13 between successive wraps of reinforcing material 12 around mold 11. As mentioned above, the fibrous reinforcing material has a coefficient of thermal expansion that is lower than the ceramic materials comprising the ceramic slurry and the ceramic stucco. Consequently, at all temperatures above the drying temperature for the mold, the fibrous reinforcing material imparts a compressive load on the portion of the shell mold over which it is disposed. This compressive load serves to increase the green strength, fired strength, and hot strength of the shell mold. In addition, if any cracking occurs when the shell mold is filled with molten metal, the fibrous reinforcing material holds the crack closed to prevent metal runout. The benefits of the compressive loading imparted by the fibrous reinforcing material may be enhanced by weaving twisted yarn into an open net-like member. Such an arrangement imparts compressive loading in multiple directions and can be used as a wrap in the manner described above, or as a local overlay. The principles of the present invention described broadly above will now be described with reference to specific examples. Example 1 A ceramic shell mold having a width of 256 mm and a height of 460 mm used to cast a large airfoil of the type shown in Fig. 1 was reinforced in accordance with the invention. A pattern having the shape of the airfoil was dipped into a slurry of silica and zirconia and then alumina stucco was applied. These steps were repeated 9 times to build up the shell mold to approximately one-half of its overall thickness. The shell mold was then wrapped with NEXTEL 440 mullite fiber (available from the 3M Company) that had been wound into a 12 roving yarn. Starting from the base of the mold and moving upwards, the yarn was wrapped around the mold in a continuous spiral with a space of approximately 6,4 mm between successive wraps of the yarn around the mold. The wrapping of the yarn around the mold was discontinued at the portion of the mold corresponding to the shank portion of the airfoil. The shell build up was completed by repeatedly dipping the shell mold in the slurry of silica and zirconia and applying alumina stucco. The shell mold then was subjected to conventional wax removal, firing, and casting preparation treatments. Molten metal was cast in the shell mold and it successfully held the metal. Example 2 A ceramic shell mold having a diameter of 921 mm and a height of 384 mm used to cast a large structural component was reinforced in accordance with the invention. A pattern having the shape of the structural component was dipped into a slurry of silica and zirconia and then zircon stucco was applied. These steps were repeated 6 times to build up the shell mold to approximately two-thirds of its overall thickness. The shell mold was then wrapped with the yarn described above in Example 1 in a continuous spiral from the base of the mold up to the top leaving a space of approximately 50 mm between successive wraps of the yarn around the mold. The shell build up was then completed by repeatedly dipping the shell mold in the slurry of silica and zirconia and applying the zircon stucco. The shell mold then was subjected to conventional wax removal, firing, and casting preparation treatments. The shell mold was crack-free after wax removal due to the compressive load imparted by the yarn during wax expansion. The reinforced shell mold successfully held molten metal during casting, even at high mold preheat temperatures. The present invention has been disclosed in terms of preferred embodiments. The invention is not limited thereto and is defined by the appended claims and their equivalents.

Claims: Method of making a ceramic investment casting shell mold 11, said method comprising the steps of providing a pattern having the shape of the desired casting, dipping said pattern into a ceramic slurry to form a coating on said pattern, applying a ceramic stucco on said coating, repeating said dipping step and said applying step to build up said shell mold to an intermediate thickness, said intermediate thickness being less than the desired overall thickness of said shell mold 11, disposing a fibrous reinforcing material 12 around said shell mold 11, building up said shell mold to the desired overall thickness by repeating said dipping step and said applying step over said reinforcing material, and firing said shell mold, caracterized in that said fibrous reinforcing material 12 is a substantially continuous fibrous reinforcing material imparting a compressive load on the portion of said shell mold 11 over which it is disposed at all temperatures above the drying temperature for said shell mold, said fibrous reinforcing material having high tensile strength at elevated temperature such that said reinforcing material 12 remains an integral part of said shell mold 11 after firing and said fibrous reinforcing material having a coefficient of thermal expansion that is less than the thermal expansion coefficient of the ceramic materials comprising said ceramic slurry and ceramic stucco. Method of claim 1, wherein said step of disposing a fibrous reinforcing material around said shell mold 11 is performed after said dipping step and said applying step having been alternately repeated approximately 6 to 9 times. Method of claim 1, wherein said reinforcing material 12 is an alumina-based or mullite-based ceramic composition having a tensile strength of at least 1380 MPa and a coefficient of thermal expansion that is approximately one-half the coeffcient of thermal expansion of the ceramic materials comprising said ceramic slurry and said ceramic stucco. Method of claim 1, wherein said step of disposing said reinforcing material 12 around said shell mold 11 further comprises wrapping said reinforcing materials around said shell mold in a generally spiral configuration. Method of claim 4, wherein said reinforcing material is wrapped around said shell mold 11 in a substantially continuous spiral leaving a space 13 between successive wraps of said reinforcing material around said shell mold. Method of claim 5, wherein said space 13 is in the range of about 5 mm to about 50 mm. Method of claim 1, wherein said reinforcing material 12 is a woven twisted yarn. Method of claim 1, wherein said reinforcing material (12) is a woven tape product. Method of claim 7, wherein said yarn reinforcing material (12) is an open net-like member. Ceramic investment casting shell mold (11), said shell mold comprising alternate layers of a ceramic material and a ceramic stucco defining an overall thickness of said shell mold, and a fibrous reinforcing material (12) disposed in said alternate, repeating layers at an intermediate thickness of said shell mold, caracterized in that said fibrous reinforcing material (12) is a substantially continuous fibrous reinforcing material, said reinforcing material (12) being comprised of an alumina-based or mullite-based ceramic composition having tensile strength of at least 1380 MPa and having a coeffcient of thermal expansion that is approximately one-half the coefficient of thermal expansion of said ceramic material and said stucco, said fibrous reinforcing material (12) imparting a compressive load on the portion of said shell mold over which it is disposed at all temperature above the drying temperature for said shell mold. Ceramic investment casting shell mold of claim 10, wherein said reinforcing material (12) is disposed in said alternate, repeating layers at an intermediate thickness of 6 to 9 alternate, repeating layers. Ceramic investment casting shell mold of claim 10, wherein said reinforcing material (12) is wound in a generally spiral configuration. Ceramic investment casting shell mold of claim 12, wherein said reinforcing material (12) is disposed in said shell mold (11)in a substantially continuous spiral leaving a space (13) between successive wraps of said reinforcing material. Ceramic investment casting shell mold of claim 13, wherein said space (13) is in the range of from about 5 mm to about 50 mm. Ceramic investment casting shell mold of claim 10, wherein said reinforcing material (12) is a woven twisted yarn. Ceramic investment casting shell mold of claim 10, wherein said reinforcing material (12) is a woven tape product. Ceramic investment casting shell mold of claim 15, wherein said yarn reinforcing material (12) is woven into an open net-like member.

Inventors: CORRIGAN JOHN, CROUCH PHILIP D, LANE JAN M, CORRIGAN, JOHN, CROUCH, PHILIP D., LANE, JAN M.
Applicants: HOWMET CORP, HOWMET CORPORATION
Application year: 1989
Main classification: B22C9
IPC codes: B22C9