Abstract:
A casting apparatus, comprising: a heating chamber having an open lower end defined by a periphery; a chill plate having a peripheral region for supporting one or more casting molds thereon and being in movable to move the molds from within the heating chamber to below the lower end of the heating chamber to withdraw the casting molds from the heating chamber; a cooling spool disposed at the periphery of the open lower end of the heating chamber and including a surface area for receiving heat energy radiated from at least one of the heating chamber and the casting molds; and a spool shield disposed at the cooling spool and movable to control an amount of the surface area of the cooling spool available for receiving the heat energy. In another embodiment there is an additional inner cooling spool movable inside the area surrounded by the casting molds for receiving radiant energy. A second spool shield at the inner spool controls its available surface area.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an apparatus for solidifying a casting to create a directionally solidified or single crystal casting and, more particularly, to an apparatus which is capable of introducing a cooling spool into a casting mold and withdrawing the casting mold from a stationary heating chamber. 
     2. Related Art 
     Solidifying molten materials, such as molten metal, in a mold cavity to create a directionally solidified or single crystal casting is known. FIGS. 1 a  and  1   b  illustrate a conventional apparatus  10  for producing a casting. An example of this is disclosed in U.S. Pat. No. 4,969,501. The apparatus  10  includes a heating chamber  12  defining an interior volume  16  which is heated via heating elements  14 . A plurality of casting molds  20  are disposed in an annular array on a vertically movable chill plate  22 . The molds are supported in and removable from the interior volume  16  by the movable plate  22 . The movable plate  22  is vertically displaced by column  24 . More particularly, the casting molds  20  may be removed from the interior volume  16  by displacing the plate  22  in the direction of arrow A (FIG. 1 b ) while the heating chamber  12  remains stationary. 
     Unfortunately, apparatus  10  produces directionally solidified or single crystal castings having less desirable material properties due to lower thermal gradient during casting. A thermal baffle or heat sink is not introduced into an interior region of the casting mold apparatus during the withdrawal from the heating chamber  12  to selectively absorb radiant heat being supplied from the molds  20 . Indeed, there is very little if any control of thermal gradients at the molds  20  to obtain directionally solidified castings. 
     In order to obtain a directionally solidified or single crystal casting, a casting mold must be removed from a heating chamber using special procedures. 
     FIGS. 2 a  and  2   b  show another conventional apparatus  50  to produce a directionally solidified or single crystal casting. An example of this is disclosed in U.S. Pat. No. 5,778,961. The apparatus  50  includes a heating chamber  12  defining an interior volume  16  for receiving an annular array of casting molds  20 . The casting molds  20  surround and define an interior space  21 . The molds are disposed on an annular chill plate or disk  30  which includes a central aperture  31 . A thermal baffle or heat sink  34  us shaped and sized to pass through the aperture  31  in the plate  30 , and the baffle is movable vertically upward in the direction of arrow C (FIG. 2 b ) with respect to the plate  30  by its supporting column  36 . In particular, the thermal baffle  34  may be moved into the interior space  21  by moving the column  36  upward, and vice versa. The radiation baffle  19  is disposed below the open end of the heating chamber  12 . 
     As illustrated, the casting molds  20  are maintained in a substantially fixed position and height with respect to a floor  32 . The casting molds  20  are removed from the interior volume  16  of the heating chamber  12  by raising the heating chamber  12  in the direction of arrow B (FIG. 2 b ). Thermal baffle  34  may be moved into interior space  21  while the heating chamber  12  is moved. Of course, the chamber  12  can remain stationary and the molds may be moved out downwardly. 
     The thermal baffle  34  serves as a heat sink to absorb radiant heat from the molds  20  such that the molten material within the molds  20  is solidified directionally by a thermal gradient defined from the heating chamber  12  to the thermal baffle  34 . The thermal gradient is a function of the temperature difference and relative positions of the heating chamber  12  and the thermal baffle  34 . Therefore, the higher is the temperature of the heating chamber  12  and the greater is the magnitude of heat that the thermal baffle  34  can absorb, the higher are the thermal gradients obtained. 
     Since the thermal baffle  34  may be moved relative to the molds  20 , the thermal gradient may be controlled to some extent. Unfortunately, apparatus  50  only maximizes the thermal gradient and, therefore, does not satisfactorily provide the thermal gradient control needed to produce castings of different geometries and configurations or single components having substantially complex geometries and still result in desirable directionally solidified or single crystal articles. 
     Moreover, when a component is manufactured in a fixed thermal gradient system as shown in FIG. 2 a - 2   b , the constant thermal gradient applies to the entire article and is normally not optimized over respective areas of the article. Constant, and particularly high thermal gradients may cause increases in casting scrap because hot tear prone alloys may crack as a result of thermal stresses due to the high thermal gradient. 
     Accordingly, there is a need in the art for a directionally solidified or single crystal casting apparatus which provides a high degree of control of thermal gradients when withdrawing casting molds from a heating chamber. 
     SUMMARY OF THE INVENTION 
     In order to overcome the disadvantages of the prior art, the casting apparatus of the present invention includes a heating chamber having a substantially open lower end. An outer cooling spool is disposed at the periphery of the open lower end of the heating chamber. A chill plate is movable through the lower end of the heating chamber from the lower end of the chamber to below that end by movement of at least one of the chill plate or the heating chamber. 
     A mold assembly is receivable into the lower end of the heating chamber via the movable chill plate. The assembly includes at least one, and typically includes an annular array of a plurality of mold cavities peripherally disposed around the chill plate. 
     A movable spool shield is disposed proximate to the outer cooling spool and operable to vary an amount of surface area of the outer cooling spool available for absorbing radiant heat from the heating chamber. 
     In an alternate embodiment, there is an additional inner cooling spool movable inside the area surrounded by the casting molds for receiving radiant energy. A second spool shield at the inner spool controls its available surface area. 
     Other objects, features, and advantages of the casting apparatus of the present invention will become apparent to those skilled in the art in view of the description below taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     For the purpose of illustrating the invention, there are shown in the drawing forms which are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown. 
     FIG. 1 a  is a side sectional view of a casting apparatus according to one embodiment of the prior art; 
     FIG. 1 b  is a side sectional view of the casting apparatus of FIG. 1 a  where casting molds are being withdrawn; 
     FIG. 2 a  is a side sectional view of a casting apparatus according to another aspect of the prior art; 
     FIG. 2 b  is a side sectional view of the casting apparatus of FIG. 2 a  where its heating chamber is being removed; 
     FIG. 3 a  is a side sectional view of a casting apparatus according to the present invention; 
     FIG. 3 b  is a side sectional view of the casting apparatus of FIG. 3 a  where the spool shield has been moved; and 
     FIG. 4 is a side sectional view of another embodiment of a casting apparatus according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 3 a  shows an elevational sectional view of a casting apparatus  100  according to a first embodiment of the present invention. The casting apparatus  100  includes a substantially stationary heating chamber  102  (or a susceptor when induction coils are used as the mold heater  106 ) having a pouring opening  104  through hood  105  for receiving moldable liquid (molten metal), located at its top end, and an open lower end spaced below the pouring opening  104  through hood  105 . The heating chamber  102  includes a mold heater  106 , preferably formed by electric induction coils wrapped around walls  108  of the heating chamber (susceptor)  102 . Preferably, the heating chamber  102  is in the form of a cylinder having an interior volume  110  accessible through the open lower end and heated by the mold heater  106 , typically induction coils or resistant heaters. The heating chamber  102  is divided into two heating zones  161   a  and  161   b  separated by baffle  162 . 
     An outer cooling spool  112  is disposed about and just below the periphery of the open lower end of the heating chamber  102 . The outer cooling spool  112  is preferably substantially ring-shaped so that the open lower end of the heating chamber  102  is not substantially obstructed. The outer cooling spool  112  includes a central surface area  112   a  facing radially inwardly and includes a lower surface area  112   b  facing substantially downward. The outer cooling spool  112  is capable of absorbing radiant heat. The outer cooling spool  112  is preferably formed from a fast thermal conducting material such as a copper or steel material and is internally water cooled. 
     The casting apparatus  100  also includes mold apparatus (casting mold)  114  which includes an annular array of a plurality of mold cavities  118 , as is known in art. In one preferred application, each of the mold cavities is shaped to form a turbine airfoil for an aircraft engine. The annular mold apparatus  114  defines an interior space  120 . The mold apparatus  114  includes a pouring basin  116  which receives the molten metal and communicates (connects) with the mold cavities  118 . 
     The heating chamber  102  and the mold apparatus (casting mold)  114  are sized and shaped such that the mold apparatus  114  may be received within the interior volume  110  of the heating chamber  102 . Preferably, the heating chamber  102  remains substantially stationary while the mold apparatus  114  is movable vertically into and out of the interior volume  110  through the open lower end of the heating chamber  102  by way of an elevator mechanism  200 . 
     The elevator mechanism  200  includes a chill plate  122  which is movable with respect to the substantially open, stationary lower end of the heating chamber  102 . The chill plate  122  is preferably annular to support the annular mold apparatus  114 . The chill plate  122  is sized and shaped such that it may pass coaxially through the outer cooling spool  112  and the open lower end of the heating chamber  102 . It is preferred that the chill plate is formed of a fast thermal conductor material such as copper and/or steel and is internally water cooled, the water being provided through mechanism  200 . 
     To obtain directionally solidified or single crystal castings, it is important, among other things, to control temperature gradients at the mold cavities  118  as the mold apparatus  114  is removed from the heating chamber  102  and while the castings cool. 
     The outer cooling spool  112  serves as a heat sink to absorb radiant heat from the mold apparatus  114  which has been preheated in the heating chamber  102 . In particular, the outer cooling spool  112  absorbs the radiant heat from below the heating chamber  102  such that molten material within the mold apparatus  114  is solidified directionally by a thermal gradient defined from the heating chamber  102  to the outer cooling spool  112 . The thermal gradient is a function of the temperature difference between the heating chamber  102  and the outer cooling spool  112 . Therefore, the higher the temperature of the heating chamber  102 , the greater the magnitude of heat that the outer cooling spool  112  can absorb, and thus higher thermal gradients are obtained. 
     The thermal gradient from the heating chamber  102  to the outer cooling spool  112  is also a function of the surface area of the outer cooling spool  112  exposed to radiant heat. The casting apparatus  100  of the present invention includes an outer spool shield  150  which can shield the spool or be moved off it. FIG. 3 b  shows the outer spool shield  150  spaced away from the outer cooling spool  112 . The outer spool shield  150  is preferably independently adjustable by adjustment means  151  with respect to the withdrawal of the mold apparatus  114  from the heating chamber  102 . The outer spool shield generally covers the inner surface of the cooling spool and could cover the entire inner surface. In one embodiment, the spool shield  150  can be an L-shaped cross section with a first part facing radially inward into the heating chamber  102  and a second part facing downward to cover the respective radially inwardly facing and downwardly facing surfaces  112   a ,  112   b  of the outer cooling spool  112 . 
     At least a portion of the outer surface of the outer spool shield  150  includes a reflective surface that is directed substantially towards the mold apparatus  114  and reflects radiant heat energy back toward the mold apparatus  114 . The reflective surface of the outer spool shield  150  preferably includes a monolithic refractory material, such as high purity alumina or zirconia, although other similarly functioning materials may be employed for the invention. The outer spool shield  150  may be formed in segments to obtain, for example, a 360° cylindrical shield capable of thermally isolating the outer cooling spool  112  from the heating chamber  102  and the mold apparatus  114 . 
     The outer spool shield  150  is movable axially or vertically by its adjustment means  151  to vary an amount of surface area of the outer cooling spool  112  available for receiving radiant heat energy from the heating chamber  102  and/or the mold apparatus  114 . As the outer spool shield  150  is moved away from and exposes more surface area of the outer spool shield  112 , the thermal gradient from the heating chamber  102  and/or the mold apparatus  114  to the outer cooling spool  112  is increased. According to the present invention, it is desirable to vary the thermal gradient between the heating chamber  102  and the outer cooling spool  112  to achieve desirable directionally solidified castings. 
     Since the outer spool shield  150  is capable of reflecting radiant heat energy, its position relative to the heating chamber  102  and/or the mold apparatus  114  can further reduce or increase the thermal gradient. The thermal reflectivity of the outer spool shield  150  may be increased where it faces the casting apparatus  114  by machining or coating with an appropriate material. 
     These refractory materials are capable of sustaining high temperatures, sometimes in exceed of 3,000° F., making them particularly suitable for the present invention. 
     The movement of the outer cooling shield  150  is preferably controlled by a programmable logic control, the means  151 , (such as a microprocessor under software control) or any other automation control device (not shown) to achieve a varying thermal gradient profile specific to the particular geometry or other specifications of the casting to achieve a more optimally directionally solidified article. 
     FIG. 4 illustrates a side sectional view of another embodiment of the present invention. The casting apparatus  101  of this embodiment includes an annular chill plate  222  having a central aperture  124  which communicates with the substantially open lower end of the mold apparatus  114  such that the interior space  120  of the mold apparatus  114  is accessible through the aperture  124 . 
     The apparatus  101  includes an elevator mechanism  300  having an outer annular column  126  coupled at its top end to the lower surface of the chill plate  222  and at an opposite bottom end to an actuator (not shown) capable of vertically displacing the column  126 , the chill plate  222  on the column  126 , and the mold apparatus  114  on the chill plate  222  with respect to the fixed height heating chamber  102 . The water used to internally cool chill plate  222  is provided through column  126 . 
     The elevator mechanism  300  supports an inner cooling spool  130  that is movable through the aperture  124  in the chill plate  222  and into and out of the interior space  120  of the mold apparatus  114 . The inner cooling spool  130  is substantially disk shaped and capable of absorbing radiant heat from the interior space  120  of the mold apparatus  114 . It is preferred that the inner cooling spool  130  be formed from a copper and/or steel material and be water cooled. The water used to internally cool internal cooling spool  130  is provided through column  136 . 
     An upstanding, annular, cylindrical reflective shield  132  is disposed atop the inner cooling spool  130 . The exterior of the reflective shield  132  provides a reflective surface that is directed substantially toward the mold apparatus  114  and reflects radiant heat energy back toward the mold apparatus  114 . 
     A second coaxial inner column  136  has its top end coupled to the lower surface of the inner cooling spool  130  and has its opposite bottom end coupled to another actuator (not shown). The actuator displaces the column  136 , the inner cooling spool  130 , and the reflective shield  132  together and with respect to the mold apparatus  114  and the heating chamber  102 . 
     Some further control of the temperature gradient is provided by the movable inner cooling spool  130 , the reflective shield  132 , and the movable mold apparatus  114 , as described below. 
     The elevator mechanism  300  permits variability in the temperature gradient to be obtained while the mold apparatus  114  is withdrawn from the heating chamber  102  without requiring that the heating chamber  102  be moved. Additional details on the structure and operation of the elevator mechanism  300  may be found in related U.S. patent application Ser. No. 09/304,977, filed May 4, 1999, entitled WITHDRAWAL ELEVATOR MECHANISM FOR WITHDRAWAL FURNACE WITH A CENTER COOLING SPOOL TO PRODUCE DS/SC TURBINE AIRFOILS, the entire disclosure of which is hereby incorporated by reference. 
     Outer cooling spool  112  includes an outer spool shield  150  at its lower inner corner region as in the first embodiment of FIG.  3 . Inner cooling spool  130  includes an inner spool shield  152  which generally covers the outer surface of the inner cooling spool and could cover the entire outer surface. In one embodiment the inner spool shield has an L-shaped cross section and extends around its lower end and its periphery. Depending upon the height positions of the spool shields  150 ,  152  along their respective spools, the spool shields  150 ,  152  restrict the amount of the radiant heat passing into the respective cooling spools and also reflect that heat back toward the mold apparatus  114 . The spool shields  150 ,  152  are preferably formed with refractory materials, such as alumina, zirconia or carbon-carbon composite. The spool shields  150  and  152  are each movable (e.g., vertically) with respect to their respective cooling spools  112 ,  130  to variably adjust the central, radially facing surface areas of the spools which are available to absorb radiant heat and, therefore, to control thermal gradients. Preferably, each of the spool shields  150 ,  152  is independently movable via a controller (not shown). 
     Advantageously, the introduction of one or both of spool shields  150 ,  152  provide additional control over the thermal gradient established during the withdrawal process, thereby enabling castings of even more complex configurations to be directionally solidified or single crystal. For example, the casting apparatus of the present invention is capable of changing the thermal gradients through one casting cycle. It is also capable of producing different thermal gradients through different castings during the same withdrawal process. 
     The foregoing description of the preferred embodiments of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.