Patent Application
20140238632
The present invention generally relates to casting processes and materials. More particularly, this invention relates to cores and processes for repairing cores formed with defects such as cracks.
Metal alloy materials can be formed into components by various casting techniques, a notable example being investment casting (lost wax) processes. Investment casting typically entails dipping a wax or plastic model or pattern of the desired component into a slurry comprising a binder and a refractory particulate material to form a slurry layer on the pattern. A common material for the binder is a silica-containing material, for example, colloidal silica. A stucco coating of a refractory particulate material is typically applied to the surface of the slurry layer, after which the slurry/stucco coating is dried. The preceding steps may be repeated any number of times to form a shell mold of suitable thickness around the wax pattern. The wax pattern can then be eliminated from the shell mold, such as by heating, after which the mold is fired to sinter the refractory particulate material and achieve a suitable strength.
To produce hollow components, such as turbine blades and vanes having intricate air-cooling channels, one or more cores must be positioned within the shell mold to define the cooling channels and any other required internal features. Cores are typically made using a plasticized ceramic mixture that is injection molded or transfer molded in a die or mold, and then hardened by firing or baking Typical ceramic compositions contain silica and/or alumina. For example, U.S. Pat. Nos. 7,287,573 and 7,732,526 to McNutty et al. disclose ceramic cores formed from a slurry comprising a ceramic powder comprising materials such as alumina, fused alumina, fused silica, magnesia, zirconia, spinels, mullite, glass fits, tungsten carbide, silicon carbide, boron nitride, silicon nitride, and mixtures thereof suspended in a silicone fluid comprising silicone monomers and/or oligomers having alkenyl and hydride functionalities. A metal catalyst is added to the suspension to cross-link the silicone monomers and/or oligomers yielding a rigid core of ceramic particles in a silicone based polymeric matrix. McNutty discloses firing the core to substantially decompose the matrix to produce a silica char.
One or more fired cores are then positioned within a pattern die cavity into which a wax, plastic or other suitably low-melting material is introduced to form the wax pattern. The pattern with its internal core(s) can then be used to form a shell mold as described above. Once the shell mold is completed and the pattern selectively removed to leave the shell mold and core(s), the shell mold can be filled with a molten metal, which is then allowed to solidify to form the desired component. The mold and core are then removed to leave the cast component with one or more internal passages where the core(s) formerly resided.
When forming cores as described above, defects such as cracks and voids can occur within the core material. Cores with defects generally cannot be used in the component casting process as the cores may break during casting, resulting in casting defects. Commonly, defective cores are disposed of rather than repaired due to the difficulty in restoring the structural integrity of the cores, especially in regards to hollow cores. An example of a prior attempt to repair the cores is reported in U.S. Pat. No. 4,804,562 to Ferguson et al. wherein the cores were repaired by (a) softening a thermoplastic binder in the core; (b) while the binder is soft, applying loose ceramic particles to the defect, the particles having a composition similar to the overall composition of the core; (c) allowing the binder to reharden; and (d) heating the core to volatilize the binder and sinter the ceramic particles to each other.
In view of the above, it can be appreciated that it would be desirable if a method were available for repairing defective cores that resulted in the cores having a structural integrity suitable for use in a casting process.
The present invention provides methods suitable for repairing ceramic cores having defects, such as one or more cracks and/or voids, to enable a core to exhibit sufficient structural integrity suitable for use in a casting process.
According to a first aspect of the invention, a method is provided for repairing defects in a ceramic core adapted for use in a casting process. The ceramic core is formed from a slurry comprising at least one refractory powder material suspended in a liquid vehicle. The method includes forming a repair mixture comprising a liquid diluent and the at least one refractory powder material suspended in the liquid vehicle of the slurry. The repair mixture is applied to a region of the ceramic core that includes at least one defect and then the ceramic core is fired to burn off the liquid vehicle in the repair mixture and form a ceramic composition that closes the defect.
According to a second aspect of the invention, a method is provided for repairing defects in a silica-containing ceramic core adapted for use in a casting process. The silica-containing ceramic core is formed from a slurry comprising at least one refractory powder and a siloxane binder. The method includes forming a repair mixture comprising a siloxane diluent and the at least one refractory powder material and the siloxane binder of the slurry. The repair mixture is applied to a region of the silica-containing ceramic core that includes at least one defect and then the silica-containing ceramic core is fired to burn off the the siloxane binder and the siloxane diluent in the repair mixture and form a silica-containing ceramic composition that fills the defect.
A technical effect of the invention is the ability to repair defective ceramic cores that might otherwise be unsuitable for using in the casting process. In particular, it is believed that, by repairing a defective ceramic core with a repair mixture that is similar in composition to the slurry used to form the ceramic core, the repaired core will have sufficient structural integrity to survive the casting process. In addition, it is believed that diluting the repair mixture with a binder/diluent can promote adhesion of the repair mixture to a ceramic core.
Other aspects and advantages of this invention will be better appreciated from the following detailed description.
The present invention is generally applicable to processes for repairing cores formed with defects such as cracks. Although the processes hereinafter will be discussed in reference to cores comprising silica-containing materials, it is foreseeable and therefore within the scope of the invention that the processes could be performed on cores comprising other materials.
As represented in
According to a preferred aspect of the invention, the openings 16 and cracks in the core 10 can be repaired by applying to the core 10 a repair mixture 20, represented in
In order to promote the structural integrity of the repairs, the repair mixture 20 should be diluted to have not more than about 70 vol. % of solids (including the refractory powder materials) to promote the ability of the repair mixture 20 to fully infiltrate the crack 18, with the balance being the binder, diluent, and any other liquid vehicle constituent. Preferably, the repair mixture 20 comprises, by volume, about 30 to about 50 vol. % refractory powder materials, about 50 to about 70 vol. % binder and diluent combined, and optionally up to about 100 ppm catalyst. As noted above, the binder and diluent may be the same, for example, a siloxane, in which case the stated amounts for the binder and diluent are simply combined to reflect the total amount of siloxane (or other binder/diluent) used in the repair mixture 20.
The openings 16 and cracks 18 (and/or any other defects) can be filled through one or more applications of the repair mixture 20. In certain embodiments of the invention, a more diluted formulation of the repair mixture 20 can be used for one or more initial applications to the core 10 to promote the ability of the repair mixture 20 to fully infiltrate into the cracks 18 and other defects. It is believed that the additional amounts of diluent with lower the viscosity of the repair mixture 20 thereby promoting infiltration of hairline cracks. Such a more diluted repair mixture 20 may contain, by volume, about 20 to about 40 vol. % refractory powder materials, about 60 to about 80 vol. % binder and diluent, and optionally up to about 100 ppm catalyst.
The repair mixture 20 may be applied to any region of the core 10, including the surface and the cavity 14, to be repaired by any means known in the art such as, but not limited to, brushing or injection with a manual or power actuated syringe. The surfaces of the region may first be wetted with the diluent prior to applying the repair mixture 20 to promote wetting of the core surfaces by the repair mixture 20. If the core 10 is relatively small in size, the repair mixture 20 may be used that does not contain the catalyst. It is believed that the catalyst may change the viscosity of the repair mixture 20 during application of the repair mixture 20, and eventually the repair mixture may become a solid prior to completion of the repair. In addition, repair mixtures 20 containing the catalyst may have shorter life spans and therefore can be difficult to handle and store. It is believed that if core 10 is small, the repair mixture 20 can fill and remain within a small defect without the need for the catalyst. If the core 10 is relatively larger in size, the repair mixture 20 preferably contains the catalyst. It is believed that the catalyst will promote cross-linking of the repair mixture 20 thereby improving the durability of the repair. The repair mixture 20 may be applied as many times as necessary to fill the crack 18 and any other defect in the surface of the core 10. In addition, the repair mixture 20 (without catalyst if the core 10 is small) or the original slurry can be injected into the cavity 14 within the hollow core 10 to close all of the openings 16 of the core 10 prior to the core 10 being used in a casting process. Preferably, the repair mixture 20 is applied in a manner to ensure that the thickness of wall 12 is balanced (i.e. relatively uniform) prior to firing. After applying the repair mixture 20, the repaired region is allowed to dry and the surface is smoothed. If the repair mixture 20 comprises the catalyst, the core 10 may be cured to cross-link the binder and diluent once the repair mixture 20 has be applied to the core 10 as desired. After the binder has cross-linked, the core 10 is fired to burn off the binder and diluent and sinter the refractory solids of the repair mixture 20. The core 10 is preferably fired at a temperature of more than about 1000° C. to ensure that the solids content of the repair mixture 20 is fully sintered to bond the refractory powder particles to each other and to the surfaces of the core 10.
While the invention has been described in terms of specific embodiments, it is apparent that other forms could be adopted by one skilled in the art. For example, the order and methods by which the repair mixture 20 is applied to the core 10 could differ, and materials and processes other than those noted could be used. Therefore, the scope of the invention is to be limited only by the following claims.
