Patent Application
20090126893
The present disclosure generally relates to processes for casting an article, and more specifically, to processes for directionally casting an article.
Certain components, such as turbine blades and stator vanes for gas turbine engines, are often manufactured using a directional solidification casting. In this process, a shell mold is specifically configured for the particular component being cast, such as the turbine engine blade or vane.
Directional solidification casting can enhance the strength of these components by obtaining single crystal components. Here, a mold assembly generally, includes a shell mold (cavity) and a chill plate, wherein the chill plate is at the lowest position of the mold assembly. The entire mold assembly is then raised into a heating chamber where it is preheated, and subsequently filled with a desired superalloy in a superheated liquid melt condition. Thereafter, the bottom of the mold assembly is then subjected to preferential cooling, immersed into a liquid metal cooling bath, such as molten tin or aluminum, to create a large temperature gradient in the casting and commence the unidirectional solidification process necessary for a desired crystal formation, which travels upwardly through the mold assembly. In other words, after the mold is filled with molten metal, the mold is lowered into the liquid metal cooling bath at a controlled rate to translate the thermal gradient across the part, thus resulting in directional solidification. Upon completion of melt solidification inside the shell mold, the mold assembly is removed from the bath, furnace, and housing.
To obtain unidirectional crystal growth vertically upward, a uniform high thermal gradient in the axial (vertical) direction is established, so that there is a horizontal liquid-solid interface within the shell mold, with the interface moving vertically upwards as the metal cools. The cooling occurs unidirectionally in the vertical (axial) direction. Any heat loss or a thermal gradient in the radial direction (i.e., radially outwards of the mold assembly) can result in uncontrolled crystal growth. This happens when exterior portions of the shell mold cool prior to interior portions of the shell mold and result in a non-planar liquid-solid interface.
As noted above, a typical mold assembly has at its bottom a chill plate adapted to effect cooling of the shell mold by conducting heat from the shell mold to the liquid metal bath. However, one of the problems with current designs is the effectiveness of the seal between the chill plate and the shell mold. Current designs are prone to leakage, i.e., ingress of liquid metal into the mold assembly and egress of cast metal from the mold assembly into the cooling bath. Without an effective seal, the cast metal is oftentimes subject to surface attack, e.g., oxidation, hot corrosion, and thermal fatigue, by the liquid metal from the melt. In addition, the melt metal will also get contaminated from the escaped cast metal. Therefore, without an effective seal, the reliability of the casting process is compromised.
Disclosed herein are processes for directionally casting an article using molten metal and for sealing a shell mold in a mold assembly. In one embodiment, the process for directionally casting an article comprises compressing a seal member intermediate a mold chill plate and a mold assembly, wherein the seal member circumscribes a shell mold in the mold assembly; filling the shell mold in the mold assembly with molten metal; immersing the mold assembly into a liquid metal cooling bath at a controlled rate from a first position to a second position of the mold assembly; and transmitting heat from the mold assembly to the liquid metal cooling bath to directionally solidify the molten metal as the molten metal assembly is immersed from the first position to the second position of the mold assembly.
In another embodiment, the process for sealing a shell mold in a mold assembly comprises forming a channel in a bottom surface of the mold assembly, wherein the channel circumscribes the shell mold; placing a first ring in the channel; and securing the mold assembly to a mold chill plate and compressing the first ring against the mold chill plate and the mold assembly.
In yet another embodiment, the process for directionally casting an article comprises securing a mold assembly to a chill plate, wherein the mold assembly comprises an opening for receiving molten metal, at least one shell mold in fluid communication with the opening, and a skirt laterally extending from at least one shell mold, wherein the skirt comprises a channel disposed in a bottom surface and configured to surround the at least one shell mold, the channel further comprising a ring formed of a compressible material disposed therein, wherein the chill plate comprises a boss having a shape complementary to the channel of the mold assembly; heating the mold assembly; filling at least one shell mold in the mold assembly with molten metal; lowering the chill plate and the mold assembly into a liquid metal cooling bath at a controlled rate from a first position to a second position of the mold assembly; and transmitting heat from the mold assembly to the liquid metal cooling bath to solidify the molten metal as the mold assembly transitions from the first position to the second position.
The above described and other features are exemplified by the following figures and detailed description.
Referring now to the figures, which are exemplary embodiments, and wherein the like elements are numbered alike:
Disclosed herein is an apparatus and process for effectively sealing an interface between a shell mold and a chill plate in a casting mold assembly.
In one embodiment, the chill plate 12 includes a substantially planar surface 24 and a boss 26 radially circumscribing about a perimeter of the surface 24. The boss 26 has a shape complementary to the groove 22 such that the boss is seated within the channel prior to mechanical fastening, e.g., by a mechanical connector such as but not limited to tie rods, cords, clamps, or any other fixture that can mechanically perform the clamping function required while sustaining the high temperatures of the furnace and melt. The shape of the boss 26 or the channel 22 is not intended to be limited and is generally configured to surround the shell mold 18 or one or more shell molds in the case of a cluster mold assembly. Surface 24 also serves to enclose an opening of the shell mold that faces the chill plate, i.e., at a bottom surface of the mold assembly (shown more clearly in
Optionally, a reverse arrangement can be made to occur in this as well as the other embodiments disclosed herein. By way of example, the groove can be formed in a top surface of the chill plate and a boss formed in a bottom surface of the mold assembly, wherein the groove and boss have complementary shapes such that the boss seats within the groove when the mold assembly and the chill plate are fastened together.
Although the mold assembly may be utilized to cast many different articles, it is believed that it will be particularly advantageous to cast turbine engine blades, e.g., airfoils, or vanes formed of a nickel-based, iron-based, and/or cobalt-based superalloys. However, it should be understood that the method and apparatus is not to be limited to the casting of any particular article or metal. For example, the apparatus and method can be used during the casting of articles formed of titanium and/or other metals having any desired configuration. In cluster mold assemblies, such as the one shown, multiple parts such as blades or vanes can be simultaneously cast using multiple shell molds. The parts can be the same or different. Prior to use, the mold assembly 10 is mechanically fastened to the chill plate 12.
A furnace 30 encapsulates the mold assembly 10 and is of a conventional design. The furnace is not intended to be limited to any particular type and the illustrated furnace is exemplary. For example, the furnace can include coils 32 that are energized to provide heat within an evacuated space of the furnace in which the mold assembly is seated. Once the mold assembly 10 has been heated to a desired temperature, molten metal is poured into the mold through the funnel 14 to fill the mold cavities 18. The illustrated furnace can include an additional funnel 34 or opening that is in coaxial alignment with the funnel 14. The space around the additional funnel or opening is often evacuated to prevent contamination of the molten metal as it is poured into the mold assembly 10.
A liquid metal cooling bath 36 is disposed beneath the mold assembly 10 and chill plate 12. The liquid metal cooling bath is maintained at a temperature below the solidus temperature of the metal in the mold. As such, as the mold assembly 10 moves into the liquid metal cooling bath, the metal in the mold directionally solidifies from the lower end portion of the mold to the upper end portion of the mold. The chill plate 12 ensures the directional solidification of the casting as it cools. The directional solidification of the molten metal in the mold is particularly advantageous when it is desired to cast a metal article with a columnar grain or to cast the metal article as a single crystal. Cast material can also solidify in the runners 16. In some instances, the solidified runner castings are intended to be part of the final cast part; the rest of the time they are discarded or recycled.
In one embodiment, the mold assembly 10 and chill plate 12 further includes a ring 40, i.e., seal member, formed of a ceramic or metal material. In these embodiments, the channel 22 would have a length and height dimension effective to accommodate the boss and the ring. The ring can be configured to have a smaller diameter than the boss such that it abuts an interior surface of the boss. Alternatively, the ring can have a larger diameter than the boss such that it abuts the exterior surface of the boss. Still further, inner and outer rings relative to the boss can be utilized.
In another embodiment as shown in
As previously noted, the ring can be formed of ceramics, metals and the like. Suitable materials include, without limitation, silicon carbide, carbon, graphite, alumina, aluminum, copper, and the like. The ring can be formed of a number of filaments, which may be wound together into a single unit or left separately in a bunch. The ring can be configured to have a solid cross section or may be configured to have a hollow cross-sectional structure. By way of example, the rope can be made of ceramic-fiber filaments, for example, alumina-boria-silica fibers with high strength and low shrinkage up to 2200 degrees Fahrenheit (1204 degrees Celsius). These fibers are sold commercially as Nextel 312, Nextel 440, and the like, a trademark of 3M Ceramic Materials Department, 3M Center, St. Paul, Minn., 55144, United States. Between rope ends, the rope can be overlapped or twisted together to make a seamless connection to ensure a continuous seal.
In addition, a cloth made of ceramic fibers can be used. Some specific materials for the cloth are alumina, alumina-silica fibers, or alumina-boria-silica fibers. The cloth can be specifically layered, rolled, or twisted. A specific example of a ceramic cloth, also sold by 3M, is a cloth trademarked under the name “Nextel.”
Alternatively, the ring can be formed of ductile metals such as aluminum, copper, and the like, that can be compressed as may be desired for some applications so as to provide conformality when compressed between the mold and the chill plate. In one embodiment, the metal is selected to have a melting point higher than that exposed to during the liquid metal casting process.
Although reference has been made to a single annular channel, boss, and/or ring, to define a seal that circumscribes the molds, in other embodiments, the seal assembly is configured for each individual shell mold 18 such as may be beneficial in cluster mold assembly processes for simultaneous casting multiple parts.
When an article is to be cast in the mold assembly 10, the mold assembly, including the shell molds 18, is placed on the chill plate 12 and moved into the furnace 30. The exemplary furnace includes coils 32 that can be energized to heat the mold assembly 10. Molten metal is then poured though opening 34 into the preheated mold assembly through the funnel 14 in a known manner. The furnace maintains the molten metal at a temperature above the solidus temperature of the metal. The mold assembly is then lowered at a controlled rate into the liquid metal cooling bath 36. To lower the mold from the furnace, the support shaft 28 coupled to the chill plate 12 is moved downward. This causes the chill plate to move into the liquid metal cooling bath. As the lower end of the mold assembly is cooled, the molten metal solidifies upward from the lower end portion often mold assembly to the upper end portion of the mold assembly.
Advantageously, the seal configurations as described herein prevents molten metal from running out of the shell mold or the liquid-metal cooling agent from flowing inside of the shell mold before solidification of the cast metal. An exemplary embodiment can provide a tight seal between the shell mold and its supporting chill plate. The tight seal is necessary to prevent molten metal from leaking out of the mold before the completion of solidification, or, conversely, the cooling medium from ingression into the molds and reaction with the casting. This embodiment of this seal has several aspects including surface features in the shell and chill plate, a gasket, and a configuration of seals around mold openings.
Still further, productivity of a liquid-metal-cooled directional solidification process is beneficially increased. Better sealing decreases the ingress and egress of undesired metal into the shell mold 18 and liquid metal cooling bath resulting in less leaking and fewer corrupted castings. As such, the casting yield of a liquid metal cooled casting process will be improved and more efficient by minimizing shell mold run-out and producing castings with minimal surface attack by the cooling medium. The increased yield provided by the embodiments mentioned above will make liquid metal casting cost competitive with conventional casting processes, a critical step in the commercialization of the liquid metal casting process. Moreover, each shell mold opening will have increased protection because of the individual seal configurations and possible redundancy.
As used herein, the term “comprising” means various compositions, compounds, components, layers, steps and the like can be conjointly employed in the present invention. Accordingly, the term “comprising” encompasses the more restrictive terms “consisting essentially of” and “consisting of.”
Unless defined otherwise, technical and scientific terms used herein have the same meaning as is commonly understood by one of skill in the art to which this invention belongs. The terms “a” and “an” do not denote a limitation of quantity, but rather denote the presence of the referenced item.
Reference throughout the specification to “one embodiment”, “another embodiment”, “an embodiment”, and so forth, means that a particular element (e.g., feature, structure, and/or characteristic) described in connection with the embodiment is included in at least one embodiment described herein, and may or may not be present in other embodiments. In addition, it is to be understood that the described elements may be combined in any suitable manner in the various embodiments.
This written description uses examples to disclose the invention, including the best mode, and also to enable practice of the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.
