US20100132906A1

US20100132906A1 - Method of casting a metal article

Info

Publication number: US20100132906A1
Application number: US12/327,288
Authority: US
Inventors: Lawrence D. Graham
Original assignee: PCC Airfoils LLC
Current assignee: PCC Airfoils LLC
Priority date: 2008-12-03
Filing date: 2008-12-03
Publication date: 2010-06-03

Abstract

During the casting of the metal article, a mold is lowered from a furnace assembly into a body of an inert gas. A stream having a thermal conductivity greater than the thermal conductivity of the inert gas is directed against a first portion of the mold to initiate solidification of molten metal in the first portion of the mold. The stream is subsequently directed against portions of the mold disposed in the body of inert gas and disposed above the first portion of the mold to initiate solidification of molten metal and portions of the mold above the first portion of the mold. The stream may be formed of a molten metal. Alternatively, the stream may be formed of an inert gas in which particulate is entrained.

Description

BACKGROUND OF THE INVENTION

The present invention relates to a new and improved method of casting a metal article.
It has previously been suggested that a casting apparatus may employ a body of molten metal or a fluidized bed as a cooling bath to promote directional solidification of an article in a mold. Apparatus for doing this is disclosed in U.S. Pat. No. 6,308,767 and U.S. Pat. No. 6,695,034. When a mold is immersed in a bath of molten metal, in the manner disclosed in U.S. Pat. No. 6,308,767, there is a tendency for stress to develop in the cast metal article due to differential thermal contraction.
It has also been suggested that a flow of inert gas be directed against a mold to promote directional solidification of metal in the mold. An apparatus for doing this is disclosed in U.S. Pat. No. 7,017,646. However, an inert gas has a relatively low thermal conductivity. The low thermal conductivity of an inert gas does not promote heat transfer to initiate solidification of molten metal in the mold.

SUMMARY OF THE INVENTION

The present invention relates to a new and improved method of casting a metal article. A mold is filled with molten metal while the mold is disposed in a furnace assembly. The mold is lowered from the furnace assembly in a body of inert gas.
In accordance with one of the features of the present inventions a stream of coolant having a thermal conductivity greater than the thermal conductivity of the inert gas, is directed against a first portion of the mold to initiate solidification of molten metal in the first portion of the mold. The stream is directed against portions of the mold disposed in the body of gas and disposed above the first portion of the mold to initiate solidification of molten metal in portions of the mold above the first portion of the mold.
The stream which is directed against the mold may be formed of a molten metal. Alternatively, the stream which is directed against the mold may be formed of an inert gas in which particulate is entrained.
The present invention has a plurality of different features which are advantageously utilized together in the manner described herein. However, it is contemplated that the features may be utilized separately and/or in combination with features from the prior art.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other features of the invention will become more apparent upon a consideration of the following description taken in connection with the accompanying drawings wherein:

FIG. 1 is a schematic illustration depicting the manner in which a mold is lowered from a furnace assembly and a stream of molten metal is directed against the mold; and

FIG. 2 is a schematic illustration depicting the manner in which a mold is lowered from a furnace assembly and a stream of inert gas with particulate entrained therein is directed against the mold.

DESCRIPTION OF SPECIFIC PREFERRED EMBODIMENTS OF THE INVENTION

An improved casting apparatus 10 is illustrated schematically in FIG. 1 and is utilized in an improved method of casting metal articles in a mold structure 12. The casting apparatus 10 includes a furnace assembly 16 in which a molten metal is poured into the ceramic mold structure 12 in a known manner. Directly beneath the furnace assembly 16 is a container 20 into which the mold structure 12 is lowered from the furnace assembly 16. The furnace assembly 16 and container 20 are enclosed by a suitable housing 24 which is connected with a source of vacuum or low pressure by a conduit 26. The housing 24 is connected with a source of an inert gas, such as argon, by a conduit 27.
The housing 24 enables the body 28 of inert gas to be maintained in the container 20. The housing 24 may have any one of many known constructions, including the construction disclosed in U.S. Pat. No. 3,841,384 and/or the construction shown in U.S. Pat. No. 6,308,767. Of course, the housing 24 may have a construction which is different than the known constructions illustrated in the aforementioned patents.
The-framework 36 is provided to support the mold 12 for movement to and from the furnace assembly 16 and for movement to and from the container 20. The metal framework 36 includes a plurality of parallel support rods 38 and a mold support structure 42. The mold support structure 42 has a vertical central axis which coincident with central axes of the furnace assembly 16 and container 20. The mold support structure 42 functions as, and may be referred to as, a chill plate.
The support rods 38 are connected with an upper drive assembly 44. The upper drive assembly 44 is operable to raise and lower the framework 36 relative to the furnace assembly 16 and container 20. If desired, the support rods 38 may be disposed outside the furnace assembly 16. A lower drive assembly 48 is connected with the container 20. The lower drive assembly 48 is operable to raise and lower the container 20 relative to the furnace assembly 16. The upper and lower drive assemblies 44 and 48 may be operated simultaneously and/or sequentially to raise and/or lower the framework 36 and/or container 20.
During operation of the casting apparatus 10, the housing 24 is evacuated through the conduit 26. Inert gas is then conducted into the housing 24 through the conduit 27. In the illustrated embodiment of the invention, the housing 24 encloses both the furnace assembly 16 and the container 20. It is contemplated that an upper housing may be associated with the furnace assembly 16 and a separate lower housing may be associated with the container 20 if desired.
After the housing 24 has been filled with an inert gas, the mold structure 12 is filled with molten metal while the mold structure is in the cylindrical furnace assembly 16. The molten metal with which the mold structure is filled is a nickel-chrome super alloy which melts at a temperature which is greater than 3,000 degrees Fahrenheit. Of course, the mold structure 12 may be filled with a different molten metal which melts at a different temperature. For example, the mold structure 12 may be filled with molten titanium or a titanium alloy.
Once the mold structure 12 has been filled with the molten nickel-chrome super alloy or other metal, the upper drive assembly 44 is operated to lower the framework 36 and mold structure 12 into the cylindrical container 20. While the upper drive assembly 44 is operated to lower the mold structure 12, the lower drive assembly 48 may be operated to raise the container 20. It should be understood that the mold structure 12 may be lowered without raising the container 20. In the illustrated embodiment of the invention, the container 20 is raised to the position illustrated schematically in FIG. 1 relative to the furnace assembly 16 before the mold structure 12 is lowered into the container by operation of the upper drive assembly 44.
In accordance with one of the features of the present invention, as the mold structure 12 is lowered into the body 28 of inert gas in the container 20, a plurality of streams 54 of coolant are directed toward the mold structure from nozzles 56 disposed in a circular array around the mold structure 12. Although only two nozzles 56 are illustrated in FIG. 1, it is contemplated that a greater number of nozzles may be provided if desired. For example, ten nozzles 56 may be provided in the array of nozzles. The circular array of nozzles 56 is disposed in a coaxial relationship with the furnace 16 and container 20.
The array of nozzles 56 may have any desired configuration. The illustrated array of nozzles 56 has a circular configuration. This results in an annular array of streams 54 of coolant being directed from the nozzles 56 radially inwardly toward the mold structure 12 as the mold structure is lowered into the body 28 of inert gas. The streams 54 of coolant are effective to cool the mold structure and initiate solidification of molten metal in a portion of the mold structure against which the streams impinge.
Although a single array of nozzles 56 has been illustrated in FIG. 1 as being on one level, the nozzles 56 may be located on different levels. For example, a plurality of circular arrays of nozzles may be provided. Each array of the plurality of arrays of nozzles 56 may be located on a different level. Thus, an array of nozzles 56 may be disposed below the array of nozzles illustrated schematically in FIG. 1.
In the illustrated embodiment of the invention, coolant flows at the same rate from each of the nozzles 56. However, the nozzles 56 may be constructed so as to have different flow rates from different nozzles. For example, a nozzle 56 having a relatively small flow rate may direct a flow of coolant toward a portion of a mold structure 12 which is to be cooled at a relatively low rate. At the same time, another nozzle 56 having a greater flow rate may direct a flow of coolant toward a portion of the mold structure 12 which is to be cooled at a higher rate. If arrays of nozzles 56 are provided at different levels in the container 20 in the manner set forth above, the nozzles 56 in an array at one level may direct a flow of coolant toward a mold structure 12 at a first flow rate and nozzles in an array at another level may direct a flow of coolant toward a mold structure 12 at a second flow rate. For example, an array of nozzles 56 at a relatively high level in the container 20 may direct a flow of coolant toward a mold structure 12 at a greater flow rate than nozzles at a relatively low level in the container 20.
In accordance with one of the features of the present invention, the streams 54 of coolant from the nozzles 56 are formed of a material having a thermal conductivity which is greater than the thermal conductivity of the inert gas forming the body 28 of inert gas into which the mold structure 12 is lowered. In one embodiment of the invention, the streams 54 of coolant are formed of liquid (molten) metal.
The molten metal in the streams 54 are formed of tin and are at a temperature of approximately 500 degrees Fahrenheit. However, the streams 54 of molten metal may be formed of lead or aluminum if desired. The streams 54 of molten metal may be at any desired temperature which is sufficient to maintain the metal forming streams in a molten condition. Since the molten metal in the mold structure 12 is a nickel-chrome super alloy with a melting temperature of approximately 3,700 degrees Fahrenheit, the streams 54 of molten metal are effective to cool the mold structure 12 and the molten metal (nickel-chrome super alloy) contained within the mold structure 12.
As the upper drive assembly 44 is operated to begin lowering the framework 36 and mold structure 12 from the furnace assembly 16, the mold support structure 42 moves through an opening 60 formed in a baffle 62. The baffle 62 is mounted on the upper end portion of the container 20 and is effective to block splashing of molten metal upwardly toward the furnace assembly 16. Although the opening 60 has a circular configuration, corresponding to the generally circular configuration of the upper end portion of the container 20 and the lower end portion of the furnace assembly 16, it is contemplated that the opening in the baffle 62 may have a different configuration if desired. For example, the baffle opening 60 may have a configuration which is a function of the overall configuration of the mold structure 12 and mold support structure 42.
It is contemplated that the baffle 62 may have any one of many known constructions. If desired, the baffle 62 may have relatively movable sections in the manner disclosed in U.S. Pat. No. 6,698,493. Alternatively, the baffle 62 may have flexible projections which extend from a support portion and engage the mold structure 12 in the manner disclosed in U.S. Pat. No. 4,969,501.
The mold structure 12 and mold support framework 36 may have a construction which is the same as disclosed in co-pending U.S. patent application Ser. No. 12/145,033 filed Jun. 24, 2008 by Robert M. Garlock and entitled Method of Casting Metal Articles. The disclosure in the aforementioned U.S. patent application Ser. No. 12/145,033 is hereby incorporated herein, in its entirety by this reference thereto. Of course, the framework 36 may have a different construction if desired. For example, the mold support framework 36 and associated drive assembly 44 may have any one of the constructions disclosed in U.S. Pat. No. 6,776,213.
When the upper drive assembly 44 is operated to lower the framework 36, the mold support structure 42 passes through the opening 60 in the baffle 62. Continued lowering of the framework 36 results in a lower portion 66 of the mold structure 12 being exposed to the streams 54 of coolant. The streams 54 of coolant impinge against the lower portion 66 of the mold structure 12. As this occurs, solidification of the molten metal in the lower portion 66 of the mold structure is initiated.
After impinging, against the lower portion 66 of the mold structure 12, the coolant falls downward under the influence of gravity into a body 70 of coolant. Pumps 72 are provided to pump coolant from the body 70 of coolant upward through conduits 74 to the nozzles 56. Although only a singe nozzle 56 has been illustrated in FIG. 1 in association with each of the pumps 72, it should be understood that a plurality of nozzles may be connected with each of the pumps by a manifold so that each pump 72 may supply more than one nozzle 56 with coolant. Although only two nozzles 56 have been illustrated schematically in FIG. 1, it should be understood that a greater number of nozzles may be provided. Similarly, although only two pumps 72 have been illustrated schematically in FIG. 1, it should be understood that a greater number of pumps may be provided.
As operation of the upper drive assembly 44 continues to slowly lower the mold structure 12 into the body 28 of inert gas in the container 20, the streams 54 of coolant are directed against, portions of the mold structure disposed above the lower portion 66 of the mold structure. Heat is transferred from the portions of the mold structure 12 above the lower portion 66 of the mold structure as the streams 54 of coolant impinge against the mold structure. This results in initiation of solidification of molten metal in portions of the mold structure disposed above the lower portion 66 of the mold structure. As the mold structure 12 continues to be lowered into the container 20, the streams 54 of coolant are directed against progressively upper portions of the mold structure until the entire mold structure has been engaged by the streams 54 of coolant.
The portion of the mold structure 12 below the nozzles 56 is exposed to the body 28 of inert gas in the container 20. Since the thermal conductivity of the body 28 of inert gas is less than the thermal conductivity of the coolant in the streams 54, the rate of transfer of heat from portions of the mold structure disposed below the streams 54 of coolant is less than the rate of transfer of heat from portions of the mold structure engaged by the streams 54 of coolant. Due, to the reduced rate at which heat is transferred from the mold structure at, locations below the nozzles 56, the inducing of stresses in the portion of a casting disposed in the mold structure below the nozzles 56 is reduced. Any tendency for the occurrence of differential thermal contraction of the casting in the mold structure 12 is reduced with a resulting reduction in any tendency for the establishment of stresses in the casting.
The mold support structure 42 remains above an upper surface 76 of the body 70 of coolant. Therefore, all of the mold structure 12 which is disposed at a level below the nozzles 56 is exposed to the body 28 of inert gas and is cooled at a rate which is less than the rate at which a portion of the mold structure engaged by the streams 54 of coolant is cooled. The relatively low rate of cooling of the, portion of the mold structure 12 below the streams 54 of coolant tends to minimize any tendency for differential thermal contraction of the casting in the mold structure.
In the embodiment of the invention illustrated in FIG. 1, the streams 54 of coolant are streams of molten (liquid) metal. The molten metal in the streams 54 of coolant is at a temperature below 1,000 degrees Fahrenheit. The molten super alloy in the mold structure 12 is at a temperature above 3,000 degrees Fahrenheit. Since there is a substantial temperature differential between the molten metal in the mold structure 12 and the molten metal in the stream 54 of coolant, there is a relatively high rate of heat transfer from the mold structure 12 to the liquid metal in the streams 54 of coolant. This results in relatively rapid cooling of the area of the mold structure 12 against which the streams 54 of coolant impinge. The portion of the mold structure 12 which is at a lower level than the streams 54 of coolant is exposed to the body 28 of inert gas which has a thermal conductivity which is substantially less than the thermal conductivity of the molten metal in the streams 54. Therefore, there is a reduced rate of cooling of the mold structure after it moves below the nozzles 56.
The positions of the nozzles 56 relative to the container 20 and furnace assembly 16 remains constant as the mold structure 12 is slowly lowered by operation of the upper drive assembly 44. Therefore, solidification of the molten metal in the mold structure 12 is initiated at the level where the streams 54 of coolant impinge against the mold structure. The solidified or at least partially solidified metal in the portion of the mold structure 12 beneath the level of the nozzles 56 is exposed to the body of inert gas 28. The portion of the body 28 of inert gas which is closely adjacent to the mold structure 12 quickly heats to a relatively high temperature. However, since the thermal conductivity of the body 28 of inert gas is relatively low, compared to the thermal conductivity of the molten metal in the streams 54 of coolant, the rate of cooling the portion of the mold structure below the nozzles 56 is less than the rate of cooling of the portion of the mold structure against which the streams 54 of coolant are directed.
Heating coils 80 extend around the lower portion of the container 20 to maintain the body 70 of coolant in a molten or liquid condition. Of course, heating coils may also be provided around the upper portion of the container 20 if desired. Suitable heating coils may also be provided in association with the conduits 74 and/or nozzles 56 to maintain the flow of molten metal through the conduits and/or nozzles at a desired temperature.
In the embodiment of the invention illustrated in FIG. 1, the streams 54 of coolant are formed of liquid, that is, molten metal. In the embodiment of the invention illustrated in FIG. 2, the streams of coolant are formed of particulate entrained in a flow of inert gas. Since the embodiment of the invention illustrated in FIG. 2 is generally similar to the embodiment of the invention illustrated in FIG. 1, similar numerals will be utilized to designate similar components, the suffix letter “a” being associated with the numerals of FIG. 2 to avoid confusion.
A casting apparatus 10 a includes a furnace assembly 16 a (FIG. 2) in which a mold structure 12 a is preheated and filled with molten metal. The mold structure 12 a is disposed on a framework 36 a which includes a mold support structure 42 a. The mold support structure 42 a is connected with an upper drive assembly 44 a by support rods 38 a.
A housing 24 a encloses the furnace assembly 16 a and a container 20 a. The housing 24 a is connected with a source of low pressure (vacuum) by a conduit 26 a. The housing 24 a is connected with a source of inert gas by a conduit 27 a.
The housing 24 a is evacuated by connecting the conduit 26 a with a source of low pressure (vacuum). A valve in the conduit 26 a is then closed and a valve in a conduit 27 a opened to connect the conduit with a source of inert gas. Although any one of many different inert gases may be utilized, in the illustrated embodiment of the invention, the housing 24 a is filled with argon. The container 20 a holds a body 28 a of the inert gas (argon).
In accordance with one of the features of the present invention, a fluidized bed 70 a is provided in the lower portion of the container 20 a. The fluidized bed contains particles suspended in gas. In the embodiment of the invention illustrated in FIG. 2, the particulate is alumina particles of 325 to 90 mesh size. The alumina particles are suspended in the inert gas (argon) to form the fluidized bed 70 a.
To maintain the fluidized bed 70 a, inert gas is conducted to a cylindrical plenum chamber 90 through a conduit 92. The inert gas flows from the plenum chamber 90 through a porous layer 96 into the fluidized bed 70 a to maintain the particulate suspended in the fluidized bed. If desired, a stirrer assembly may be provided adjacent to the upper side of the porous layer 96.
A flow of particulate suspended in inert gas is conducted from the fluidized bed 70 a through conduits 74 a to nozzles 56 a. Streams 54 a of particulate entrained in inert gas are directed by the nozzles 56 a against the mold structure 12. Pumps 72 a are provided to maintain the flow of inert gas with particulate entrained therein from the fluidized bed 70 a to the nozzles 56 a.
In the embodiment of the invention illustrated in FIG. 2, the pumps 72 a are of the fluid ejector type. Of course, other types of pumps may be used if desired. The pumps 72 a are effective aspirate a flow of particulate suspended in gas from the fluidized bed to the conduits 74 a. The pumps 72 a are effective to force the flow of inert gas with particulate entrained therein to flow upward to the nozzles 56 a and radially inward through the nozzles in the streams 54 a which are directed against the mold structure 12 a.
Each of the pumps 72 a include a convergent-divergent venture nozzle or diffuser to which a flow of transport gas under pressure is directed from a conduit 100. The manner in which the fluidized bed is established and which the pumps 72 a direct streams of particulate entrained in inert gas is the same as is disclosed in U.S. Pat. No. 6,695,034. The disclosure in the aforementioned U.S. Pat. No. 6,695,034 is hereby incorporated herein in its entirety by this reference thereto.
When articles are to be cast in the mold structure 12 a, the mold structure is heated to a desired temperature in the furnace assembly 16 a. Molten metal, which may be a nickel-chrome super alloy is poured into the mold structure 12 a while the mold structure is in the furnace assembly 16 a.
After the mold structure 12 a has been filled with molten metal, the upper drive assembly 44 a is operated to lower the framework 36 a. As the framework 36 a is lowered, a lower portion 66 a of the mold structure 12 a moves into alignment with the streams 54 a of coolant. The streams 54 a of coolant are, in the embodiment of the invention illustrated in FIG. 2, particulate entrained in inert gas. Since the particulate is entrained in the inert gas, the streams 54 have a thermal conductivity which is greater than the thermal conductivity of the inert gas by itself. As the streams 54 a of particulate entrained in inert gas are directed against the lower portion 66 a of the mold structure 12 a, solidification of the molten metal in the lower portion 66 a of the mold structure 12 is initiated.
As the upper drive assembly 44 a continues to be operated to slowly lower the framework 36 a and mold structure 12 a, the streams 54 a of coolant, that is, of particulate entrained in inert gas, is directed against portions of the mold disposed above the lower portion 66 a. As this occurs, solidification of the molten in portions of the mold structure 12 above the lower portion 66 a of the mold structure is initiated.

Conclusion

In view of the foregoing description, it is apparent that the present invention provides a new and improved method of casting a metal article. A mold 12 is filled with molten metal while the mold is disposed in a furnace assembly 16. The mold 12 is lowered from the furnace assembly 16 into a body 28 of inert gas.
In accordance with one of the features of the present invention, a stream 54 of coolant having a thermal conductivity greater than the thermal conductivity of the inert gas, is directed against a first portion 66 of the mold 12 to initiate solidification of molten metal in the first portion of the mold. The stream 54 is directed against portions of the mold 12 disposed in the body 28 of gas and disposed above the first portion 66 of the mold to initiate solidification of molten metal in portions of the mold above the first portion of the mold.
The stream 54 which is directed against the mold 12 may be formed of a molten metal. Alternatively, the stream 54 which is directed against the mold 12 may be formed of an inert gas in which particulate is entrained.
The present invention has a plurality of different features which are advantageously utilized together in the manner described herein. However, it is contemplated that the features may be utilized separately and/or in combination with features from the prior art.

Claims

1. A method of casting a metal article, said method comprising the steps of filling a mold with molten metal while the mold is disposed in a furnace, lowering the mold from the furnace in a body of inert gas, directing a stream having a thermal conductivity greater than the thermal conductivity of the inert gas against a first portion of the mold disposed in the body of inert gas to initiate solidification of molten metal in the first portion of the mold, and directing the stream having a thermal conductivity greater than the thermal conductivity of the inert gas against portions of the mold disposed in the body of gas and disposed above the first portion of the mold to initiate solidification of molten metal in portions of the mold above the first portion of the mold.

2. A mold as set forth in claim 1 wherein said step of directing a stream having a thermal conductivity greater than the thermal conductivity of the inert gas against a first portion of the mold includes directing a stream of molten metal against the first portion of the mold.

3. A method as set forth in claim 1 wherein said step of directing a stream having a thermal conductivity greater than the thermal conductivity of the inert gas against a first portion of the mold includes directing a stream of the inert gas in which a particulate is entrained against the first portion of the mold.

4. A method as set forth in claim 1 further including the steps of collecting material from the stream and returning the collected material to the stream.

5. A method as set forth in claim 1 wherein said step of directing a stream having a thermal conductivity greater than the thermal conductivity of the inert gas against the first portion of the mold includes directing a stream of liquid against the first portion of the mold and allowing at least a portion of the liquid from the stream of liquid to move downwardly in the body of inert gas under the influence of gravity.

6. A method as set forth in claim 1 wherein said step of directing a stream having a thermal conductivity greater than the thermal conductivity of the inert gas against portions of the mold disposed above the first portion of the mold includes directing a stream of liquid against the portions of the mold disposed above the first portion of the mold and allowing liquid from the stream of liquid to move downwardly along portions of the mold disposed below a portion of the mold against which the stream of liquid is directed and to move downwardly in the body of inert gas under the influence of gravity.

7. A method as set forth in claim 1 wherein said step of directing a stream having a thermal conductivity greater than the thermal conductivity of the inert gas against the first portion of the mold includes directing a stream of liquid against the first portion of the mold at a first location in the body of inert gas, said step of directing a stream having a thermal conductivity greater than the thermal conductivity of the inert gas against portions of the mold disposed in the body of gas and disposed above the first portion of the mold includes directing a stream of liquid against the portions of the mold above the first portion of the mold at the first location in the body of inert gas.

8. A method as set forth in claim 7 wherein said step of directing a stream of liquid against the portions of the mold above the first portion of the mold is performed with the first portion of the mold at least partially exposed to the inert gas in the body of inert gas.

9. A method as set forth in claim 7 further including the step of collecting a body of the liquid which contains liquid which was directed against the mold in a stream of liquid, said step of collecting the body of liquid includes collecting the body of liquid at a location disposed at a level below the mold and with an upper surface of the body of liquid exposed to gas in the body of inert gas.

10. A method as set forth in claim 9 further including the step of pumping liquid from the body of liquid and at least partially forming the stream having a thermal conductivity greater than the thermal conductivity of the inert gas and which is directed against portions of the mold disposed above the first portion of the mold.