Abstract:
In a casting furnace employing a liquid metal bath to allow directional solidification of an article from a melt, and a method of operating such a furnace, a system is provided for lowering the mold from within a heating chamber into a liquid metal bath situated immediately beneath the heating chamber. An automated system is provided for automatically maintaining the level of the liquid metal bath at a relatively constant position immediately beneath the heating chamber, wherein simultaneously upon the level of the bath having risen due to immersion of the mold within the bath, the bath is lowered to thereby permit the level of the liquid metal bath to be maintained in a substantially fixed position.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to casting of metals, and more specifically, relates to a metal bath furnace used in the directed solidification of superalloys, and a method for carrying out the casting of superalloys using directional solidification. 
     BACKGROUND OF THE INVENTION 
     Certain components, such as for example turbine blades and stator vanes for gas turbine engines, because of their relatively complex shapes and harsh operating environment to which these components are composed, are typically cast of nickel-based and cobalt-based alloys which are conventionally known as superalloys, which have high strength and typically very high melting temperatures. 
     The strength of such components is enhanced by forming the turbine vanes (stators), and particularly the turbine blades, using directional solidification casting for obtaining substantially single crystal components. Such a process is conventionally known. 
     Various processes and apparatus for directional solidification casting, such as directional solidification casting apparatus and method disclosed in U.S. Pat. Nos. 4,108,236 and 4,175,609, are known in the industry and vary in effectiveness. In these processes, a suitable ceramic mold is specifically configured for the particular component being cast, such as a gas turbine engine blade or vane. The mold is lowered into a heating chamber where it is preheated, and subsequently then filled with a desired superalloy in a superheated liquid melt condition. Thereafter, the bottom of the mold is then subjected to preferential cooling to commence the unidirectional solidification process necessary for single crystal formation, which travels upwardly through the mold. 
     Cooling of the mold may be accomplished in different manners. In one conventional process, a suitable liquid metal coolant, such as molten tin or aluminum, is contained in a bath below the mold, with the mold then being immersed into the cooling bath for effecting a substantially large temperature gradient in the melt for enhancing directional solidification. 
     In a typical directional solidification casting furnace, solid superalloy known as a charge is initially placed inside a melting crucible surrounded by a suitable heater, such as an induction heater, which melts the charge to form the liquid melt with suitable superheat. The mold is initially positioned inside a heating chamber within a furnace, which preheats the mold to a suitable elevated temperature. And, the furnace and mold are disposed above a liquid metal cooling bath. These components are typically disposed within a common pressure vessel or housing which makes up the furnace, which is typically evacuated, or filled with a suitable inert gas. 
     During the process, the melt is poured from the melting crucible into the preheated mold. The mold is then lowered, bottom end first, into the bath for immersion cooling thereof to directionally solidify the melt upwardly inside the mold. Upon completion of melt solidification inside the mold, the mold is removed upwardly from the bath, furnace, and housing. A new charge and mold are placed inside the housing and the process is repeated to cast additional parts. 
     It is known that in order to obtain unidirectional crystal growth vertically upward, there need be uniform high thermal gradient in the axial (vertical) direction, so that there is a horizontal liquid-solid interface within the mold, with such interface moving vertically upwards as the metal cools. Accordingly, the cooling must occur unidirectionally in the vertical (axial) direction. Heat loss or a thermal gradient in the radial direction (ie. radially outwards of the mold) is undesirable and has a detrimental effect on unidirectional crystal formation with exterior portions of the mold tend to cool prior to interior portions of the mold resulting in a non-planar liquid-solid interface which has a detrimental effect on unidirectional cooling and thus unidirectional crystal growth formation. Any space between the heating chamber for the mold and the level of the liquid metal bath means that a portion of the mold, particularly its exterior, upon being lowered out of the heating chamber for immersion into the bath, will lose heat through radiation quicker than the interior of the so-exposed mold, resulting in an undesirable radial thermal gradient. 
     Accordingly, to achieve a purely axial (vertical) thermal gradient for unidirectional cooling, it is known to make the liquid metal bath variably positionable so as to be able to position the liquid metal container, and in particular the level of the liquid metal bath, immediately beneath the heating chamber. In such manner there is no space between the heating chamber and the liquid metal bath, and the mold when lowered out of the heating chamber for immersion into the coolant will be immediately thrust into such coolant. Such a furnace, having a liquid metal bath which may be positionable directly under the heating member, is taught in published EPO application 0631832A1 filed Feb, 2, 1993 and assigned to one of the applicants herein, namely ALD Vacuum Technologies Gmbh. In circumstances where the level of the liquid metal coolant is positioned immediately beneath the heating chamber, upon immersion of the mold within the container containing the liquid metal coolant, the level of the liquid metal coolant is caused to rise due to liquid metal within such container being displaced by the immersed mold. Any rise in the level of such molten liquid coolant is highly undesirable, since it will, being in such proximity to the heating chamber, immediately enter the heating chamber, and likely cause extensive damage to such heating chamber and other problems, such as vaporization of liquid metal coolant. Such problems are very acute where the heating chamber employs induction or resistance heating coils, and the liquid metal contacts such heating coils. In such circumstances damage to the heating coils typically results, due to their temperature generally being significantly higher than that of the liquid metal coolant. 
     To overcome such problems, the prior art practice has been generally to fill the bath container right to the top with liquid metal coolant, so that any excess will not rise into the heating chamber but will instead spill over the upper lip of the bath container and fall onto the floor of the furnace. This has the disadvantage not only of mess and build-up of solidified coolant on the floor of the furnace, but also has the definite disadvantage that liquid metal coolant must always be added to the bath container after each casting in order to conduct the next casting. Otherwise if no additional liquid metal is added to make up for the quantity of liquid metal lost through displacement, if in the next casting operation the bath container is moved so that the level of the remaining liquid metal is immediately beneath the heating chamber, upon immersion the level of the liquid metal coolant, the liquid metal now having room to rise in the bath container since some metal coolant had previously been lost, will then rise and enter the heating chamber, with undesirable and detrimental results. 
     A known means to overcome the problem of spillage of the liquid metal to the floor of the furnace is to provide a bath container having a spillway which catches and contains liquid metal coolant overflow, as disclosed and depicted in aforementioned published EPO application 0631832A1, namely in FIG. 4 thereof. Unfortunately, although the liquid metal “spillover” is retained by such spillway, it need subsequently be re-added to the bath container prior to the next casting operation. Frequently, such liquid metal has become solidified, which means it needs to be re-heated, and such wastes time and heat energy. More importantly, however, re-adding the “spillover” for each casting operation makes casting of numerous parts in succession time-consuming and inefficient, increasing the cost of each individually cast article. Alternatively, if in order to compensate for spillover, the bath container is made deep enough so that no “spill-over” over the top lip thereof occurs upon immersion of the mold therein, the operator of the furnace is left with two alternatives, neither of which is desirable. The operator may choose to position the level of the molten liquid metal in the bath container immediately below the heating chamber, in which case upon the immersion of the mold therein the level will rise and enter the heating chamber, with the detrimental results mentioned previously. Alternatively, the operator may choose to position the level of liquid metal slightly below the heating chamber, so that only after full immersion of the mold will the level of the liquid metal rise to the lowermost extremities of the heating chamber. Unfortunately, this means that a “space” will initially exist between the coolant bath and the heating chamber, which will cause the non-directional cooling problems discussed earlier. 
     Accordingly, a need exists for a furnace apparatus for unidirection solidification of superalloys which overcomes the above problems of the prior art. 
     SUMMARY OF THE INVENTION 
     In order to overcome the problems of the prior art and provide for an efficient operation of a furnace for unidirectional solidification of superalloys, the present invention, in one of its broad aspects, provides for a furnace apparatus for directional solidification of an article from a melt, comprising 
     a housing; 
     a heating chamber within said housing, adapted when activated to pre-heat a mold member to allow said mold member to receive said melt and to maintain said melt in liquid state in said mold member; 
     a crucible member situated beneath said heating chamber, for containing a liquid metal bath; 
     means for lowering said mold member from within said heating chamber into said crucible member; 
     means permitting vertical movement of said crucible member; and automated means, independent from human intervention, for automatically lowering said crucible member upon the level of said liquid metal bath having risen upon said mold member and melt being lowered into said liquid metal bath, to thereby permit said level of liquid metal bath to be maintained in a substantially fixed position immediately beneath said heating chamber during lowering of said mold member into said liquid metal bath. 
     Advantageously, a furnace of the above configuration allows the level of liquid metal coolant to be continuously maintained immediately beneath the heating chamber, effectively creating a seal at the lowermost end of the heating chamber and eliminating radiation loss therefrom. No “space” need be left between the level of liquid metal coolant and the lowermost extremities of the heating chamber, which would otherwise cause radial thermal cooling gradients, as explained earlier, resulting in loss of unidirectional solidification. Moreover, by eliminating spillover, not only is any mess avoided, but likewise the need to re-add spillover to the bath and the time consuming steps of doing so and having to wait until such “spillover” becomes melted again (if in the interim it has solidified) are overcome. 
     In addition, the aforementioned feature permits successive castings using the same liquid metal in the bath, without having to stop the operation after each casting operation to make up for spillover. As a result, numerous successive castings can be more quickly carried out than was heretofore the case. 
     In one embodiment of the furnace apparatus of the present invention, the automated means for automatically lowering the crucible member comprises a detection means for indicating if the level of the liquid metal bath, when the mold member is immersed therein, has risen above a predefined point relative to the heating chamber, and subsequently lowering the crucible member upon the detection means indicating the level of the bath has risen above such point. 
     As more particularly described herein, the detection means may comprise an electrically-conductive sensor, and more particularly a sensor of the type wherein an electrical circuit is closed upon electrodes from the sensor contacting the rising, electrically-conductive liquid metal coolant. 
     In an alternative embodiment, instead of employing a sensor, the automated means comprises making use of a known displacement relationship between the distance the mold member is lowered into the liquid metal bath, and the distance which the liquid metal bath correspondingly rises due to displacement of liquid metal within the liquid metal bath, and means for automatically lowering the crucible member into such bath in accordance with said known relationship so as to maintain the level of such liquid metal bath relatively constant. 
     The present invention further provides a method for the directed solidification of molten metal superalloy utilizing the liquid metal bath furnace apparatus described above, such method comprising: 
     preheating a mold in a heating chamber directly above a crucible containing a bath of liquid metal coolant; 
     pouring the molten metal superalloy into the mold; 
     positioning the liquid metal bath, in particular, the level of the liquid metal in the liquid metal bath, immediately beneath the heating chamber; 
     lowering the mold into the liquid metal bath; and lowering the crucible containing the liquid metal bath to prevent substantial rise of the level of the liquid metal bath relative to the heating chamber upon the mold being lowered into the bath. In using directional solidification, not infrequently the mold, being typically of a refractive ceramic material and being brittle and relatively fragile, will during the initial melt-pouring process into the preheated mold, break or crack, or alternatively, the melt when poured into the mold will spill over, with the result that all such materials, including any broken mold, would typically fall through the heating chamber downwardly into the cooling bath. Undesirable contamination of the bath results which must be remedied prior to the next molding operation. Accordingly, to protect the cooling bath and mold furnace (which may, due to splashing from mold debris, become damaged) from contamination in the event of mold failure, the apparatus of the present invention is particularly suited for further interposing a catch basin or receptacle between the heating chamber and the crucible member for catching mold debris. The means permitting vertical movement of the crucible member is adapted to move the crucible member and liquid metal bath from a lowered position beneath the basin when the basin member is in an interposed position between the heating chamber and the crucible member, to a raised position immediately beneath the heating chamber when the basin is removed from its interposed position, Likewise, the method of the present invention, providing as it does for a moveable bath, is likewise easily adaptable to including such further features. In partiular, the method of the present invention may be further adapted to include locating a basin between the heating chamber and liquid metal bath prior to pouring the melt into the mold to allow capture of mold debris in the event of breakage or leakage of the mold, and after pouring the melt into the preheated mold member, removing the basin member and raising the liquid bath so that the level of the liquid metal bath is immediately beneath the heating chamber. 
     Finally, due to the advantages attained in unidirectional solidification which is accomplished by being able to retain the liquid metal bath in close proximity to the heating chamber during immersion of the mold thereon, the invention further comprises an article formed by the method of directional solidification set out above. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The following drawings show particular embodiments of the present invention in which: 
     FIG. 1A is a side cross-sectional view of a directional solidification furnace of the prior art, having both means for lowering the mold into a liquid metal bath, and means for positioning molten metal in a bath container immediately below a heating chamber. Such figure shows the mold about to be immersed in the bath; 
     FIG. 1B is a side cross-sectional view of the directional solidification furnace of the prior art shown in FIG. 1A, showing the mold in the process of being immersed in the liquid metal bath; 
     FIG. 2 is a side cross-sectional view of a directional solidification furnace of the present invention, showing the liquid metal bath raised so as to position it immediately beneath the heating chamber; 
     FIG. 3 is a side cross-sectional view of the directional solidification furnace of FIG. 2, showing the mold partially immersed in the liquid metal bath, and the liquid metal bath correspondingly lowered in accordance with the method of the present invention so as to maintain the liquid metal bath level in a fixed position immediately beneath the heating chamber; 
     FIG. 4 is a side cross-sectional view of the same directional solidification furnace of the present invention, shown during the melt-pour stage, showing the further utilization of a receptacle to receive mold debris, wherein such receptacle is in the extended position; 
     FIG. 5 shows the directional solidification furnace of FIG. 4, with the receptacle to receive mold debris in the retracted position; and 
     FIG. 6 shows a directional solidification furnace of FIG. 4, showing the mold partially immersed in the liquid metal bath, and the liquid metal bath level correspondingly lowered in accordance with the method of the present invention to compensate for the increase in level of the liquid metal bath, further showing the receptacle in the retracted position. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1A &amp; B show two views of a directional solidification furnace  14  of the prior art, during two separate stages of the directional solidification casting process. Such furnace  14  typically comprises a housing  15  for conducting the casting process under a vacuum, in the example shown comprising three sections  15   a,    15   b  and  15   c  respectively. 
     A heating chamber  6  is provided, having induction heating coils  7  adapted to pre-heat a mold  5  placed therein. Mold  5 , in which the molten superalloy is placed for subsequent directional solidification casting, is adapted to receive a pour of molten superalloy through aperture  20 . An elevator chamber  1 , which may be moved up and down by piston  2 , may support a cooling plate  4  on which mold  5  sits. 
     A liquid metal container  8 , containing a liquid metal quenching bath  10 , typically comprised of molten tin or aluminum, adapted to have the mold  5  immersed therein to quench such mold, is mounted on a boom arm  9 . By means of threaded rod  12 , the liquid metal bath  10  is verticallly positionable so as to allow location of such bath  10 , and in particular, the liquid metal  10  therein, beneath heating chamber  6 , as shown in FIGS. 1A &amp; 1B. The boom arm  9  is permitted to move upward by virtue of aperture  11  in elevator chamber  1  until quenching bath  10  is situated under heating chamber  6 . The bath  10  may have a thermal insulative layer  13  which is penetrable by the mold  5  when lowered into bath  10 . 
     Once container  8  and liquid metal bath  10  is positioned under heating chamber  6  as shown in FIG. 1A, in order to conduct directional solidification of the superalloy present within mold  5 , elevator chamber  1  is lowered by slidable piston  2 , as shown in FIG. 1B, causing mold  5  and cooling plate  4  to be immersed in cooling bath  10 , thereby commencing the directional solidification of the mold  5 , with progressive cooling of the mold from bottom to top occurring as mold  5  is gradually immersed in bath  10 . 
     To avoid problems of radial thermal gradients within the mold  5  during the cooling process, the bath container  8  is typically filled with molten metal  10  right up to its uppermost edges, and subsequently the bath container  8  and molten metal bath  10  positioned immediately beneath heating chamber  8  to effectively seal the chamber  8 , thereby reducing radiation heat loss via the bottom of the heating chamber  8  (ref. FIG.  1 A). 
     Disadvantageously, however, when piston  2  is lowered so as to immerse the mold  5  in cooling bath  10 , as shown in FIG. 1B, due to displacement of molten liquid metal  10  when mold  5  is immersed therein, a quantity of liquid metal coolant  10   a  will spillover onto the floor of the furnace, as shown in FIG.  1 B. In order to conduct a subsequent casting operation, the bath container  8  will have to be refilled with the spillover, or with a quantity of metal equal to the spillover, in order to “top up” the bath container  8 . This is highly unsatisfactory, for reasons given under Background. 
     Accordingly, to overcome these problems, the present invention shown in FIGS. 2-6 provides a directional solidification casting furnace  30 , comprising a cylindrical housing  32  composed of three separable sections, namely an upper section  32   a,  a middle section  32   b,  and a lower section  32   c.  The cylindrical closed housing  32 , typically of steel, creates an atmospheric seal which permits the heating, pouring, casting and directional solidification of a superalloy melt  40  to be carried out under near total vacuum conditions, or alternatively, in presence of an inert gas, to thereby avoid oxidation of some of the trace metals present in the superalloy melt  40  which would otherwise occur if the processes of the present invention were to be carried out in air under normal atmospheric conditions. Accordingly, for the purpose of creating a vacuum within casting furnace  30  a vacuum pump  42  is provided. Sighting view ports  20   a  and  20   b  are provided in the locations shown in FIGS. 2-6, should viewing of the process be desired at any time. 
     Suitably disposed inside the housing  32  is a heating chamber  44  which acts as a mold furnace to preheat a mold  46 , prior to receiving the liquid melt  40  and also for times thereafter. Such heating chamber may take any conventional form. In the exemplary embodiment illustrated, the heating chamber  44  is a multi-zone furnace having three vertically aligned zones each having a respective, independently powered zone heater, which may comprise resistance heating elements  48   a, b,  and  c.  Surrounding the zone heaters  48   a, b  and  c  is an insulating chamber or box  49  of suitable heat insulating material for containing the heat from zone heaters  48   a, b  and  c  inside the heating chamber  44 . The heating chamber  44  is open at its top and bottom. 
     The mold  46  is initially disposed inside the heating chamber  44  at the beginning of the casting process, and is formed of a suitable refractory or ceramic material for receiving the superalloy melt  40 . 
     A first or upper elevator  50  is preferably disposed above the upper housing  32   a,  and extends in part into the housing  32  for selectively lowering and raising the mold  46  into and out of the heating chamber  44  and housing  32  via port  92 . The upper elevator  50  may take any conventional form for transferring in turn individual molds  46  into position in the heating chamber  44  and for subsequent vertical travel during the casting process as hereinafter described. 
     The melt  40  is initially provided in the form of a solid charge which may be suitably delivered into housing  32  through an access port  52  in the top housing member  32   a,  and initially deposited in a crucible  60 . The crucible  60  may take any conventional form, and is typically formed of a suitable refractory material surrounded by an induction heater  62 . The induction heater  62  is independently powered for melting the solid superalloy charge inside crucible  60  to create the liquid superalloy melt  40 . 
     The melt material  40  may be a nickel-based or cobalt-based superalloy for casting high-temperature strength gas turbine engine rotor blades and stator vanes, for example. Correspondingly, the mold  46  may take any conventional form for casting suitable components such as the blades and vanes, and is specifically configured for promoting directional solidification of the melt  40  into a single crystal component upon solidification of the melt  46  therein. The mold  46 , therefore, typically has a complex shape and varying outer configuration or profile and may be arranged simply, or in multiple units or gangs within a suitably sized heating chamber  44 . The mold  46  may further have at its base a cooling plate  90 , typically of a highly thermally conductive material, which assists in ensuring cooling of the mold occurs at the lowest point, and which therefore assists in the directional solidification process. 
     The crucible  60  is initially disposed inside the housing  32  adjacent the heating chamber  44 , and is attached to a suitable carriage  65  for transporting the crucible  60  to the top of the heating chamber  44  and tilting the crucible  60  to pour the melt  40  into the top of the mold  46  inside the heating chamber  44 . The carriage  65  may take any conventional form for transporting the crucible  60  back and forth to the heating chamber  44  for allowing repeated recharging of the crucible  60  with a new melt charge  40  and filing successive molds  46  in sequence operation. 
     An open crucible or bath container  70  of a suitable refractory material, is disposed vertically below the heating chamber  44 . Such crucible  70  allows for immersion cooling of the mold  46  as hereinafter more fully described to directionally solidify the melt  40  within mold  46  when such mold  46  is lowered into bath  70 . The bath  70  contains a suitable liquid metal coolant  72 , which may be molten tin or aluminum for example, having a lower melting point than the superalloy melt  40 , which is used to cool the mold  46 . A suitable bath heater comprised of resistive heater elements  74  may surround the bath  70  for melting and maintaining the coolant  72  at a suitable temperature useful for cooling the mold  46  containing the superheated melt  40 . 
     In the exemplary embodiment illustrated in FIG. 2, the bath  70  is moved vertically by a second (lower) elevator  76  which is disposed vertically below the housing  32   c  and extends in part through the bottom of the housing  32   c  and upwardly to support the bath  70 . The second elevator  76  may take any conventional form for selectively raising and lowering the bath  70  into and out of position below the heating chamber  44 , but in the preferred embodiment may be activated electrically and comprise an electrically-driven hydraulic pump  94  which supplies hydraulic oil under pressure to a hydraulic ram/piston  96  which is used to raise/lower the bath  75 . 
     Importantly, a sensor  80  is provided. Such sensor  80  may be fixedly attached to the heating chamber  44  or to anywhere on the furnace housing  32 , and operates in conjunction with the lower elevator  76  in the manner hereinafter described, to permit the level of liquid metal  72  to be maintained in a substantially fixed position immediately beneath the heating chamber  44  during lowering of the mold  46  into the coolant  72 . Such sensor may be anyone of the commercially available sensors for detecting the level of a liquid, but in one embodiment is of the type having two electrodes  81   a,    81   b,  which create a closed electrical circuit when contacted by the liquid metal coolant, where the coolant is highly electrically conductive, such as aluminum. The electrodes  81   a,    81   b  of the sensor  80  are positioned to contact the coolant  72  at a position at or just slightly above the maximum desired level of the coolant. Upon the electrodes  81   a,    81   b  being contacted by the coolant, an electrical signal is provided to lower elevator  76  to cause it to lower the melt bath  70 , thereby lowering the level of the coolant  72 . As soon as the electrical circuit is opened by loss of electrical connection between the electrodes  81   a,    81   b  due to the lowering of the coolant  72 , the electric signal to the elevator  76  is interrupted, with the result that the bath  70  and coolant  72  ceases to continue to be lowered. The process will be iteratively repeated as the mold  46  is gradually lowered into coolant  72 , with each lowering of the mold causing a resultant increase in the level of the coolant, which will be detected by the electrodes  81   a,    81   b,  when the electrical circuit is closed by the rising coolant  72 , resulting in an electrical signal being sent to the lower elevator  76  to cause it to lower the bath  70  and cool at  72  therein. 
     The method of the present invention for directional solidification casting of an article in one of its broadest aspects will now be described. 
     An empty mold  46  is lowered by first elevator  50  into position inside the heating chamber  44  and is preheated to ready the mold  46  to receive the melt  40 . Such preheating of mold  46  avoids thermal shock to the mold  46  when it receives melt  40 . A solid billet of superalloy is lowered through port  52  into crucible  60 , and heated so as to create a liquefied melt  40 . 
     The melt  40  is then moved via carriage  65  over heating chamber  44 , and poured into preheated mold  46 , where it may optionally continue to have heat supplied to it by heating chamber  44 , if necessary, to maintain the melt  40  in a liquid state prior to or during the directional solidification process. 
     Bath  70  which contains liquid metal coolant  72  is then raised by lower elevator  76 , so as to position the level of the liquid metal coolant  72  in bath  70  immediately below heating chamber  44 . By positioning the liquid metal coolant  72  at such position it is possible to substantially reduce or eliminate any space between the bottom of the heating chamber  44  and the level of the liquid metal bath, which would otherwise allow heat from the mold  46  to be radially emitted prior to the mold  46  being immersed in the coolant  72 . Indeed, the bath  70  may be raised and sensor  80  positioned so that the level of liquid metal coolant  72  therein is maintained so as to actually contact the lowest portion of the heating chamber and/or the cooling plate  90  on the base of mold  46 , thereby eliminating or reducing as much as possible any thermal heat loss in a radial direction from within the heating chamber  44  and from the mold  46 . 
     Upon bath  70  and coolant  72  being raised to so that coolant  72  is at the  10  desired level, mold  46  is gradually lowered by upper elevator means  50  downwardly from within the heating chamber  44  into coolant  72  to start the directional solidification process of the melt  40  within the mold  46 . During this process the directional solidification starts at the bottom of the mold  46  and propagates vertically upwardly therein as the mold  46  is gradually lowered into coolant  72 . This obtains a substantially single crystal solid which grows in a desired unidirectional direction vertically upward. 
     Importantly, as the mold  46  is gradually immersed in coolant  72 , the level of coolant  72  rises, such rise in level being detected by the sensor  80 . After a signal from sensor  80 , lower elevator  76  is then immediately activated to cause hydraulic ram  88  to be lowered, thereby lowering bath  70  so as to maintain the level of coolant  72  at the desired height beneath the heating chamber  44 . If bath  70  was not lowered, the level of coolant  72  would rise and either spill over from bath container  70 , or rise into heating chamber  44 , thereby damaging lower heating resistance elements  48   c,  and possibly  48   b.  As may be clearly seen from FIGS. 3 and 6, due to the immersion of the mold  46  in coolant  72  in accordance with the method of the present invention, hydraulic ram  96  operated by lower elevator  76  has lowered bath  70  a distance ‘a’ to thereby maintain the level of the coolant  72  at the same level as that prior to immersion, namely immediately below heating chamber  44  as shown in FIGS. 2 and 5. 
     Upon entire immersion of the mold  46  in coolant  72 , the upper elevator  50  is then reversed for removing upwardly the mold  46  from bath  70  and heating chamber  44 , and outwardly through the upper housing  32   a  via exit port  92 , for replacement with the next empty mold  46  for repetition of the casting process. 
     Of note, since the bath  70  and coolant  72  are disposed directly below heating chamber  44  during the melt-pouring process (see FIG.  4 ), failure or breakage of the mold  46  at this step would contaminate coolant  72  in bath  70  with the resulting mold debris and poured melt  40 . Furthermore, the fallen mold debris and poured melt may cause the coolant  70  to splash upwardly into one or more of the core heaters  48   a, b,  or  c  causing undesirable damage thereto. 
     Accordingly, as an added feature to the apparatus and method of the present invention, an empty receptacle or basin  100 , as shown in FIGS. 4-6, is provided, removably disposed between heating chamber  44  and bath  70 . A suitable actuator  110  is coupled to basin  100  for selectively deploying the basin  100  into position under heating chamber  44  during the melt-pour step as shown in FIG. 4, and for withdrawing and storing the basin  100  away from the heating chamber  44  at a suitable location within the housing  32  after completion of the melt pour step, as shown in FIGS. 5 &amp; 6. 
     The basin  100  is preferably comprises an open container sized to capture substantially all mold debris and melt  40  in the event of breakage of mold  46 , to prevent any contamination of coolant  72  in bath  70 . It is further of a suitable refractory material that will not be damaged by falling melt. 
     The method of the present invention wherein a catch basin is further utilized will now be described, with reference to FIGS. 4-6. Commencing with the mold heating and pouring step, the mold  46  situate in heating chamber  44  is preheated to ready it to receive the liquid melt  40 , which is heated in crucible  60 . Bath  70  is lowered by second elevator  76 , and catch basin  100  is extended by actuator  110  to position it immediately beneath mold  46  within heating chamber  44 . Liquid melt  40  is moved by carriage  65  above mold aperture  20  and poured into mold  46 , as shown in FIG.  4 . Catch basin  100  is then removed by actuator  110  from its extended position beneath mold  46  to a stowed position away from heating chamber  44 , and bath  70  is then raised to a position which the sensor  80  no longer permits upward travel of hydraulic piston  96 , namely to a position wherein the level of the liquid metal coolant  72  in bath  70  is immediately below heating chamber  44 , as shown in FIG.  5 . 
     Thereafter the mold  46  is lowered into coolant  72  by upper elevator  50 . As the level of the coolant  72  rises due to immersion of mold  46  therein, such rise in level is detected by sensor  80 , and the lower elevator  76  is activated so as to cause the bath  70  to be lowered, thereby maintaining the level of coolant  72  at a consistent desired level. 
     It is contemplated that the method of the present invention, namely while lowering the mold  46  into the coolant  72  lowering the bath  70  so as to maintain the coolant level at a fixed desired level, may be practiced without the use of a sensor  80 . In particular, there is a direct proportionate relationship between the distance the mold  46  is lowered into the liquid coolant  72 , and the rise in the level of the coolant  72  that results, such relationship depending on the surface area of the mold as a function of its height and the total volume of the bath container as a function of its height. Accordingly, such relationship may be calculated mathematically, or established empirically through experimental trial runs wherein the mold is immersed in the bath and the relationship established between the amount of lowering of the upper elevator and the resultant increase in the level of the coolant level. Upon establishing such relation, the lower elevator  76  may be operated to lower the bath  70  commensurate with the lowering of the upper elevator  50  at the respective proportionate rate which ensures the level of the coolant  72  relative to the heating chamber  44  remains at a fixed desired location. 
     While there have been described herein what are considered to be preferred and exemplary embodiments of the present invention, other modifications of the invention shall be apparent to those skilled in the art from the teachings herein, and it is, therefore, desired to be secured in the appended claims all such modifications as fall within the true spirit and scope of the invention.