Patent Publication Number: US-2003234092-A1

Title: Directional solidification method and apparatus

Description:
BACKGROUND OF THE INVENTION  
       [0001] The present invention relates to directional solidification apparatus and processes wherein heat is removed in a unidirectional manner from a metallic melt in a mold to form a columnar grain or single crystal casting.  
       BACKGROUND OF THE INVENTION  
       [0002] In the manufacture of components, such as nickel base superalloy turbine blades and vanes, for gas turbine engines, directional solidification (DS) investment casting techniques have been employed in the past to produce columnar grain and single crystal casting microstructures having improved mechanical properties at high temperatures encountered in the turbine section of the engine.  
       [0003] In the manufacture of turbine blades and vanes using the well known DS casting “withdrawal” technique where a melt-filled investment mold residing on a chill plate is withdrawn from a casting furnace, a stationary thermal baffle has been used proximate the bottom of the casting furnace to improve the unidirectional thermal gradient present in the molten metal or alloy as the investment mold is withdrawn from the casting furnace. The baffle reduces heat loss by radiation from the furnace and the melt-filled mold as the mold is withdrawn from the casting furnace.  
       [0004] Attempts to improve the thermal gradient have included introducing a cooling gas below the stationary thermal baffle of the casting furnace to extract heat from the mold and/or withdrawing the hot melt-filled investment mold from the furnace into a bath of liquid cooling metal, such as liquid tin, positioned below the furnace.  
       [0005] The low heat capacity of cooling gas creates a disadvantage in that large gas quantities are needed to effect cooling and in that the presence of the cooling gas below the baffle has a negative impact on the thermal profile of the mold heater in the casting furnace due to a chimney effect. The large quantities of cooling gas require complex and expensive vacuum pumping and recycling systems associated with the casting apparatus as well as more complex heat shielding and cooling of the casting furnace equipment.  
       [0006] Use of a liquid metal cooling bath adds significantly to complexity of the mold design and withdrawal apparatus since the mold must be lowered into a hot circulating cooling media. Complex bath circulation and level control systems are needed. In addition, the liquid metal cooling bath can be subject to contamination and reactions should a prior cast investment mold experience a leak or significant run-out of molten metal into the bath.  
       SUMMARY OF THE INVENTION  
       [0007] The present invention provides in an embodiment a method and apparatus for DS casting wherein a liquid cooling medium is sprayed directly on exterior surfaces of a melt-filled ceramic investment mold as it is withdrawn from an end of a DS casting furnace by relative movement therebetween so as to extract heat from the mold and improve the thermal gradient in the melt residing in the mold. The liquid cooling medium preferably is collected for reuse after it impinges on the exterior mold surfaces.  
       [0008] In an illustrative embodiment of the invention, a plurality of spray nozzles are disposed beneath a thermal baffle at a lower end of the DS casting furnace. The nozzles are spaced about a baffle opening through which the investment mold is withdrawn downwardly out of the casting furnace. The spray nozzles are oriented to spray a liquid metallic cooling medium in directions transverse to the path of mold withdrawal through the opening so that the sprays impinge directly on the exterior mold surfaces as the mold is withdrawn from the casting furnace by relative movement therebetween.  
       [0009] The invention provides a high heat extraction capability from the melt-filled mold without the disadvantages described above associated with use of a cooling gas and/or liquid metal cooling bath in which a mold is immersed. The invention will be described in more detail below in connection with the following drawings.  
     
    
    
     DESCRIPTION OF THE DRAWINGS  
     [0010]FIG. 1 is a schematic cross-sectional view of a DS casting apparatus in accordance with an embodiment of the invention.  
     [0011]FIG. 1A is a partial schematic cross-sectional view of the DS casting furnace of another embodiment where the nozzles are oriented at an upward angle relative to horizontal.  
     [0012]FIG. 2 is a schematic view taken along lines  2 - 2  of FIG. 1 of the bottom of the thermal baffle showing spray nozzles disposed about the baffle opening as well as liquid metal supply piping. The mold is schematically and partially shown in cross-section for convenience.  
     [0013]FIG. 3 and  4  are a schematic cross-sectional views of DS casting apparatus in accordance with other embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0014] The present invention provides a DS casting method and apparatus especially useful, although not limited, to casting of nickel, cobalt and iron base superalloys to produce a columnar grain or a single crystal cast microstructure. Referring to FIG. 1, casting apparatus in accordance with an illustrative embodiment of the invention for DS casting nickel, cobalt and iron base superalloys to produce columnar grain or a single crystal cast microstructure includes a vacuum casting chamber  10  having a casting furnace  12  disposed therein in conventional manner. Thermal insulation members  13   a ,  13   b  form a furnace enclosure. Positioned within the tubular thermal insulation member  13   a  is an inner solid graphite tubular member  15  forming a susceptor that is heated by energization of the induction coil  18 . The thermal insulation member  13   b  includes an aperture  13   c  through which molten metal or alloy (metallic melt), such as a molten superalloy, can be introduced into the mold  20  from a crucible (not shown) residing in the chamber  10  above the casting furnace  12  in conventional manner.  
     [0015] Induction coil  18  disposed about the susceptor  15  is energized by a conventional electrical power source (not shown). The induction coil  18  heats tubular graphite susceptor  15  disposed interiorly thereof. After the empty mold  20  is positioned in the furnace  12 , the mold is preheated to a suitable casting temperature for receiving the metallic melt by the heat provided by the susceptor  15 .  
     [0016] The mold  20  typically comprises a conventional ceramic investment shell gang or cluster mold formed by the well know lost wax process to include a pour cup  20   a  that receives the melt from the crucible and that communicates via sprues  20   b  to a plurality of shell molds  21  each having a mold cavity  22  in the shape of the article to be cast. Although two shell molds  21  are illustrated in FIG. 1, four or more shell molds  21  can be disposed around the center post  20   d . Each mold cavity  22  communicates to a chill plate  26  at an open bottom end of each mold cavity in conventional manner to provide unidirectional heat removal from the metallic melt residing in the mold and thus a thermal gradient in the metallic melt M in the mold extending along the longitudinal axis of the mold. In casting single crystal components, a seed or crystal selector (not shown), such as pigtail passage, will be incorporated into the mold above the open lower end thereof to select a single crystal for propagation through the metallic melt, all as is well known. The invention is not limited to use with a gang or cluster mold  20  having a plurality of shell molds  21  and can be practiced with any type of refractory shell mold having one or more mold cavities.  
     [0017] In one embodiment of the invention, the mold  20  is formed with an integral mold base  20   c  and central support post  20   d  that rest on the chill plate  26  as shown. The base  20   c  can be clamped thereto in conventional manner if desired. The chill plate resides on a ram  28  raised and lowered by a fluid actuator (not shown) to move the mold  20  into and out of the casting furnace  12 . Alternately, the invention envisions using any relative movement between mold  20  and casting furnace  12  to effect withdrawal of the melt-filled mold  20  from the end of the furnace  12 . For example, in another embodiment, the mold  20  on chill plate  26  and collection vessel  50  can be disposed in a fixed position, while the casting furnace  12  and nozzles  40  are moved together to effect withdrawal of the melt-filled mold  20  from the end of the furnace  12 .  
     [0018] In the DS casting of gas turbine engine blades or vanes, each mold cavity  22  will have a root region  22   a  corresponding to a root of the blade or vane and a relatively large platform cavity  22   b  corresponding to the platform portion of the blade or vane to be cast. Each mold cavity  22  also will have a relatively smaller or narrower airfoil cavity region  22   c  corresponding to the airfoil portion of the blade or vane to be cast.  
     [0019] A stationary thermal baffle  30  is disposed at the lower end of the casting furnace  12  and is connected in conventional manner to the walls W of the vacuum chamber  10 . The baffle  30  includes an opening  30   a  oriented perpendicular to the mold withdrawal direction (vertical direction in FIG. 1) and having a cross-sectional configuration selected to accommodate movement of the relatively large platform region or profile of the melt-filled molds  21  therepast with only a small gap (e.g. ½ inch) present between the platform region  22   b  and the inner periphery of the baffle  30 . The baffle  30  typically is made of graphite material, although other refractory materials can be used.  
     [0020] In accordance with an illustrative embodiment of the invention, a plurality of spray nozzles  40  are disposed beneath thermal baffle  30  at the lower end of the DS casting furnace  12 . The nozzles  40  are spaced about the periphery of baffle opening  30   a  through which the metallic melt-filled investment mold  20  is withdrawn downwardly out of the casting furnace. The spray nozzles  40  are oriented to spray a liquid cooling medium as a plurality of liquid cooling sprays S in generally horizontal directions transverse to the downward vertical path of mold withdrawal through the opening  30   a  so that the sprays S impinge directly on and around the exterior surfaces of shell molds  21  as the mold  20  is withdrawn from the casting furnace. In FIG. 1, the nozzles  40  are shown horizontally oriented to provide spray coverage of the exterior surfaces of the mold. Alternately, each nozzle  40  can be mounted on a bracket  41  to angle the nozzle  40  at an upward angle relative to horizontal as shown in FIG. 1A to this end. The nozzles  40  are mounted by any suitable mounting means on the structural frame F that supports the casting furnace  12  in vacuum casting chamber  10 .  
     [0021] The spray nozzles  40  preferably are of the type that produce a flat fan-shaped spray of the liquid cooling medium to provide the sharpest temperature transition and to minimize the number of nozzles needed to cool the mold surfaces. The nozzles can be made of any suitable material that can withstand prolonged contact with the cooling medium, such as for example a liquid tin cooling medium.  
     [0022] The sprays S each can comprise a relatively low melting point, relatively high heat capacity liquid metal or alloy, or other liquid material such as a molten salt or oxide, that is compatible with the shell mold material so as not to react adversely therewith. For purposes of illustration and not limitation, molten tin, molten aluminum, or any other suitable liquid cooling metal or alloy may be used. The relatively low melting point of the liquid cooling metal or alloy is with respect to the melting point of the metal or alloy to be cast and directionally solidified in the molds  21 . The relatively high heat capacity of the liquid cooling metal or alloy is with respect to the heat capacity provided by a cooling gas, such as Ar or other inert gas, used in the past to cool molds.  
     [0023] If liquid tin is used as the sprayed liquid cooing medium, the liquid tin temperature is typically in the range of 300 to 500 degrees F., whereas for purposes of illustration, the temperature of a molten nickel base superalloy residing in the molds  21  is typically in the range of 2800 to 2300 degrees F. If liquid aluminum is used as the sprayed liquid cooing medium, the liquid aluminum temperature is typically in the range of 900 to 1400 degrees F.  
     [0024] The nozzles  40  are supplied with the liquid cooling medium via a common distribution or manifold pipe  42  connected to a pump  44 , FIG. 2. The pump  44  is connected to a heated storage tank  46  containing the liquid cooling metal or alloy. The tank  46  can comprise a conventional metal (e.g. steel) or ceramic tank used to heat the liquid metal or alloy, such as molten tin or aluminum, to desired use temperature and store it for use. The pump  46  provides the liquid cooling metal or alloy under pressure to the nozzles  40  to generate sprays S. A typical pressure range of the liquid cooling metal or alloy supplied to the nozzles  40  will depend on the type of nozzle  40  selected for use and typically can be in the range of 40 to 250 psi. The pump  44  and tank  46  are disposed outside the vacuum chamber  10  and are connected to the nozzles  40  by the piping  43  that supplies the liquid cooling metal or alloy to the distribution or manifold pipe  42 .  
     [0025] After the liquid cooling medium impinges on the shell molds  21  as they are withdrawn from baffle opening  30   a , the still liquid cooling medium, such as still liquid tin or aluminum, falls by gravity from the mold  20  into a collection vessel  50  in vacuum chamber  10 . The liquid cooling medium then is returned via piping  51  and pump  52  back to tank  46  for conditioning and reuse in closed loop manner. The tank  46  can be effective to thermally condition the liquid cooling medium to an appropriate temperature as well as filter or separate out any contaminates therein that might plug the nozzles  40 .  
     [0026] In operation, an empty mold  20  is positioned in the furnace  12  by upward movement of the ram  28 . The induction coil  18  is energized to preheat via susceptor  15  the mold  20  to a suitable casting temperature, such as above 2500 degrees F. for casting nickel base superalloys. The mold is filled with molten metal or alloy to be cast from a crucible above the furnace. Then, the metallic melt-filled mold  20  is withdrawn downwardly past baffle  30  out of the furnace  12  for example by lowering of the ram  28  (or any relative movement between furnace  12  and mold  20 ) at a controlled withdrawal rate to establish a thermal gradient in the melt to achieve a solidification front that progresses upwardly through the melt residing in the shell molds  21  during withdrawal to form either a columnar grain or a single crystal microstructure, if a single crystal selector or seed is present in the mold.  
     [0027] Shortly after the mold  20  begins downward withdrawal, the nozzles  40  are supplied with the liquid cooling medium, such as a liquid metallic cooling medium, to generate cooling sprays S that impinge on the exterior surfaces of the shell molds  21  as they past the baffle  30  out of the furnace  12 . The liquid cooling metal or alloy of relatively high heat capacity and relatively low temperature impinges on and around the hot mold exterior surfaces to extract heat and improve the thermal gradient in the melt residing in the molds  21  above the solidification front progressing through the melt. After the liquid cooling metal or alloy impinges on the shell molds  21 , the still liquid cooling metal or alloy is collected in vacuum chamber  10  in vessel  50  and returned by pump  56  to tank  46  for conditioning and reuse.  
     [0028] In lieu of using a plurality of individual nozzles  40  as described above, the invention envisions using other nozzle or orifice arrangements to generate one or more sprays S of the liquid cooling medium to impinge on the exterior mold surfaces. For example, as illustrated in FIG. 3, the invention envisions using a manifold pipe  42 ′ that has a plurality of individual orifices  42   a ′ spaced about its inner periphery facing the mold  20  to generate the sprays S. Also, as shown FIG. 4, a manifold pipe  42 ″ may be provided with an annular slit or slot orifice  42   a ″ on its inner periphery facing the mold  20  to generate a spray S in the form an annulus or other suitable shape to impinge on the exterior mold surfaces.  
     [0029] Moreover, it is to be understood that the invention has been described with respect to certain specific embodiments thereof for purposes of illustration and not limitation. The present invention envisions that modifications, changes, and the like can be made therein without departing from the spirit and scope of the invention as set forth in the following claims.