Patent Application
20090126894
The present disclosure generally relates to apparatuses for casting an article, and more specifically, to apparatuses 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 or columnar grain components. Here, a mold assembly generally includes a shell mold 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, and resulting in a non-planar liquid-solid interface.
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 from the bath into the mold assembly and egress of the casting metal from the mold assembly into the liquid metal cooling bath. Without an effective seal, the casting metal is oftentimes subject to surface attack, e.g., erosion and chemical interaction, by the liquid metal coolant. In addition, the liquid metal coolant will also get contaminated from the escaped cast metal and vice versa. Therefore, without an effective seal, the reliability of the casting process and apparatus is compromised.
Accordingly, it would be desirable to provide an effective seal between the shell mold and the chill plate.
Disclosed herein are apparatuses for directionally casting an article using liquid metal. In one embodiment, the apparatus for directionally casting an article comprises a mold assembly comprising an opening for receiving molten metal, at least one shell mold in fluid communication with the opening, and a skirt laterally extending from the at least one shell mold, wherein the skirt comprises a selected one of a channel and a boss disposed in a bottom surface that is configured to surround the at least one shell mold; and a chill plate comprising the other of the selected one of the channel and the boss having a complementary shape such that the boss is seated within the channel to define a seal about the at least one shell mold when the mold assembly is attached to the chill plate.
In another embodiment, the apparatus comprises a mold assembly comprising an opening for receiving molten metal, at least one shell mold in fluid communication with the opening, and a skirt laterally extending from the at least one shell mold, wherein the skirt comprises a channel disposed in a bottom surface that is configured to surround the at least one shell mold; a chill plate fastened to the mold assembly, the chill plate comprising a channel complementary to the channel of the mold assembly; and a seal member disposed within the channels of the chill plate and the mold assembly to define a seal between the mold assembly and the chill plate.
In yet another embodiment, an apparatus for directionally casting an article comprises a mold assembly comprising at least one shell mold in fluid communication with an opening for receiving molten metal, and a skirt laterally extending from the at least one shell mold having an opening, wherein the skirt comprises a channel disposed in a bottom surface that is configured to surround the at least one shell mold opening; a chill plate fastened to the mold assembly, the chill plate comprising a planar surface, and a support post configured to effect lowering of the mold assembly; a seal member disposed within the channel to define a sealing member between the mold assembly and the chill plate; and a liquid metal cooling bath positioned below the chill plate maintained at a temperature below a solidus temperature of the molten metal.
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 assembly.
The chill plate 12 includes a substantially planar surface 24 and a boss 26 circumscribed about a perimeter of the surface 24. The boss 26 has a shape complementary to the groove 22 such that the mold assembly 10 is seated on the boss 26 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 embodiment, the groove is formed in a top surface of the chill plate and a boss is 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. Still further, multiple grooves and bosses can be formed in an opposing relationship e.g., a labyrinth type seal.
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 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. 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 and chill plate. 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 shell mold directionally solidifies from the lower end portion of the shell 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 shell 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 another embodiment, the mold assembly and chill plate further includes a ring formed of a metal or ceramic material. 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 yet 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 metal or 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 commercially available 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 channel, boss, and/or ring, to define a seal assembly that circumscribes the shell molds, in other embodiments, the seal assembly is configured for each individual shell mold 18, i.e., each individual shell mold, such as may be beneficial in cluster mold assemblies 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 prevent 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 a 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 melt 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 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.
All cited patents, patent applications, and other references are incorporated herein by reference in their entirety. However, if a term in the present application contradicts or conflicts with a term in the incorporated reference, the term from the present application takes precedence over the conflicting term from the incorporated reference.
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.
