Patent Application
20080257517
The present invention pertains to the art of mold assemblies and, more particularly, to a mold assembly having molybdenum chill-plate employed in a liquid metal cooled solidification furnace.
Certain components for gas turbine engines are typically cast from superalloys, for example cobalt, iron and nickel-based alloys. Superalloys have high strength and typically very high melting temperatures. Thus, superalloys well suited for gas turbine engine components such as rotor blades and stator vanes that have complex shapes and are exposed to harsh operating environments. The strength of such components is further enhanced using a directional solidification process.
In a typical directional solidification process, a superalloy charge is placed in a melting crucible surrounded by a heater. The heater melts the charge to form a molten metal. A mold is initially positioned inside a heating chamber located within a directional solidification furnace. The heating chamber preheats the mold to a suitable temperature. Once the mold is preheated, the molten metal is poured from the crucible into the mold. At this point the mold is withdrawn or immersed into a cooling bath filled with a cooling liquid. The cooling liquid is typically a liquid metal such as tin or aluminum. As the mold is withdrawn into the cooling liquid, the liquid metal directionally solidifies inside the mold.
In order to control the directional solidification and establish a desired grain structure, the mold is positioned upon a chill-plate. The chill-plate acts as a thermal interface between the mold and the heating chamber during preheat. That is, during preheat, a portion of the chill-plate extends into the liquid metal. The liquid metal establishes a thermal gradient within the chill-plate. In this manner, an upper, exposed, surface of the chill-plate remains at a temperature that is lower than the preheat temperature. With this arrangement, a bottom portion of the mold that rests upon the chill-plate also remains at a temperature that is below the preheat temperature. This temperature differential is particularly desirable during an initial stage of the directional solidification process.
Chill-plates are typically formed from copper or copper alloys and are often provided with an internal water cooling scheme. Unfortunately, copper and copper alloys react negatively to tin and aluminum. Thus, copper chill-plates have a very short service life and, due to the poor interaction with aluminum and tin, may also detrimentally affect the directional solidification process. Stainless steel is also used as a material for forming chill-plates. However, while more durable than copper, stainless steel has a relatively low thermal diffusivity and exhibits a reaction with, for example, liquid aluminum. The low thermal diffusivity allows heat from the heating chamber to rapidly transfer into the cooling liquid and raise surface temperatures of the chill-plate to approximately the preheat temperature. Therefore, stainless steel chill-plates, at best, provide only a minimal benefit for the directional solidification process.
In accordance with one aspect of the invention, a mold assembly for a liquid metal cooled directional solidification furnace having a cooling chamber provided with liquid metal and a heating chamber includes a mold member having a main body portion that defines an interior mold cavity. The mold member is adapted to be positioned in the heating chamber. The mold assembly also includes a chill-plate formed from a material having a thermal diffusivity at 600° K greater than 10 E-6 m̂2/sec, inert with at least one of molten tin and molten aluminum and adapted to be at least partially immersed in the liquid metal. The chill-plate includes a main body portion having a first surface extending to a second surface through an intermediate portion. The chill-plate is adapted to establish a thermal gradient between the mold member and the heating chamber.
In accordance with another aspect of the present invention, a liquid metal cooled directional solidification furnace is provided. The furnace includes a heating portion including a heating chamber having a first temperature and a cooling portion including a cooling chamber provided with a liquid metal having a second temperature with the second temperature being less than the first temperature. The furnace also includes a mold assembly that is positioned at least in part in each of the heating portion and cooling portion. The mold assembly includes a mold member having a main body portion that defines an interior mold cavity positioned in the heating chamber, a support yoke including at least one support member connected to the mold member, and a chill-plate. The chill-plate is formed from a material having a thermal diffusivity at 600° K greater than 10 E-6 m̂2/sec, inert with at least one of molten tin and molten aluminum, and is positioned, at least in part, in the liquid metal. The chill-plate includes a main body portion having a first surface that extends to a second surface through an intermediate portion. The chill-plate establishes a thermal gradient between the mold member and the heating chamber.
In accordance with yet another aspect of the present invention, a method of forming a cast component in a liquid metal cooled directional solidification furnace is provided. The method includes placing a mold assembly having a mold member including a bottom portion supported directly upon a chill-plate formed from a material having a thermal diffusivity at 600° K greater than 10 E-6 m´2/sec and inert with at least one of molten tin and molten aluminum into the liquid cooled directional solidification furnace. The method further requires positioning the mold member in a heating chamber having a first temperature and a portion of the chill-plate into a liquid metal bath having a second temperature with the second temperature being higher than the first temperature and raising the temperature in the heating chamber from the first temperature to a third temperature, with the third temperature being substantially higher than the second temperature. Finally, the method requires maintaining the bottom portion of the mold member at a fourth temperature, the fourth temperature being substantially less than the third temperature.
Additional objects, features and advantages of various aspects of the present invention will become more readily apparent from the following detailed description of illustrated aspects when taken in conjunction with the drawings wherein like reference numerals refer to corresponding parts in the several views.
With initial reference to
As best shown in
Chill-plate 70 is preferably formed from a material having a thermal diffusivity at 600° K greater than approximately 10 E-6 m´2/sec, more preferably, the material has a thermal diffusivity at 600° K greater than approximately 20 E-6 m´2/sec, and most preferably the material has a thermal diffusivity at 600° K greater than approximately 30 E-6 m´2/sec. In addition, the material is neutral, i.e., does not react in at least one of molten tin and molten aluminum. In accordance with one exemplary embodiment the material is molybdenum and/or alloys thereof. In accordance with another exemplary embodiment the material is tungsten and/or alloys thereof. In accordance with yet another exemplary embodiment, the material is graphite and/or alloys thereof.
As shown, chill-plate 70 includes a main body portion 74 having a first surface 80 that extends to a second surface 81 through an intermediate portion 82 which collectively define a thickness “w”. As will be discussed more fully below, second portion 48 of mold member 43 is directly supported by first surface 80 of chill-plate 70. In accordance with one aspect of the invention, thickness “w” ranges up to about 12-inches (30.48 cm). In accordance with another aspect of the invention, thickness “w” ranges between approximately 3 inches (7.62 cm) and approximately 5 inches (12.7 cm). In accordance with yet another aspect of the invention, thickness “w” is approximately 4 inches (10.16 cm).
In operation, mold cluster assembly 40 is positioned within heating chamber 8 with lower portion 48 of mold member 43 resting directly upon chill-plate 70 which, in turn, is partially immersed in liquid metal 19. (See
That is, evidence has shown, immersing chill-plate 70 two (2) inches into liquid metal 19, and raising the temperature in heating chamber 8 to approximately 2822° F. (1550° C.), results in first surface 80 remaining at approximately 824° F. (450° C.). Moreover, an overall temperature variation within chill-plate 70 is only between approximately 482° F. (250° C.) and approximately 1022° F. (550° C.). In any event, following the preheat stage, molten metal is dispensed into interior mold cavity 50 and mold cluster assembly 40 is withdrawn into cooling portion 14 so as to form a directionally solidified casting. At this point it should be understood that forming chill-plate 70 from molybdenum and/or molybdenum alloy advantageously provides for an enhanced cooling or quenching process that creates durable microstructures in resultant castings. Moreover, the use of molybdenum and/or a molybdenum alloy extends an overall service life of the mold assembly. That is, as molybdenum is compatible with tin and aluminum, no reaction will occur between the chill-plate and the liquid metal that could otherwise impact service life and possibly the resultant castings. Of course, it should be understood that while the above described mold member is configured to form a turbine bucket, the present invention can be employed to produce various components, such as turbine nozzles, blades, and vanes for turbine engines as well as other articles that would benefit from directionally solidified grain structure.
At this point it should be appreciated that the present invention provides a mold assembly for a liquid metal cooled directional solidification process having a chill-plate that possesses a high thermal diffusivity. Thus, during preheat, the bottom portion of the mold member is maintained at a temperature that is substantially lower than the temperature in the heating chamber. In this manner, the present invention achieves a desired directional solidification while providing a mold assembly having a long service period as compared to a chill-plate formed from copper. Moreover, the present invention provides a mold assembly having an extended service life without requiring the use of additional cooling circuits.
In general, this written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice 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 that occur to those skilled in the art. Such other examples are intended to be within the scope of the present invention 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 language of the claims.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 11305247 | Dec 2005 | US |
| Child | 11972124 | US |
