Abstract:
A system for two stage casting of a metal alloy is disclosed that dispenses multiple feedstock metals into an arc melting crucible via a pressurized inert gas or metal vapor chamber to lower the volatilization rate of metals in an arc melting crucible at a rate proportional to the composition of the final desired alloy. The melt from the melting crucible enters a second stage cold wall crucible through a passage, where the melt cools and solidifies. A casting piston is used to slowly and progressively withdraw the solidified alloy from the cold wall crucible as it cools.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims benefit of priority of U.S. Provisional Patent Application No. 62/368,113, filed Jul. 28, 2016, entitled “TWO STAGE MELTING AND CASTING SYSTEM AND METHOD”, which is incorporated herein by reference in its entirety. 
     
    
     SUMMARY OF THE DISCLOSURE 
       [0002]    A “metal alloy” as used herein is defined as an alloy based on a metal. One species is a multi-component alloy wherein the multi-component alloy realizes an entropy of mixing of at least 1.25. Species within the genus of “metal alloy” includes aluminum alloys, nickel alloys, titanium alloys, steels, cobalt alloys, and chromium alloys. 
         [0003]    As used herein, “multi-component alloy product” and the like means a product with a metal matrix, where a plurality of elements, typically four or more different elements make up the matrix, and where the multi-component product comprises 5-35 at. % of the four or more elements. In one embodiment, at least five different elements make up the matrix, and the multi-component product comprises 5-35 at. % of the at least five elements. In one embodiment, at least six different elements make up the matrix, and the multi-component product comprises 5-35 at. % of the at least six elements. In one embodiment, at least seven different elements make up the matrix, and the multi-component product comprises 5-35 at. % of the at least seven elements. In one embodiment, at least eight different elements make up the matrix, and the multi-component product comprises 5-35 at. % of the at least eight elements. As described below, additives may also be used relative to the matrix of the multi-component alloy product to achieve an alloy generated by the system. 
         [0004]    This disclosure presents a system for two stage casting of any metal alloy product such as a multi-component alloy. The first stage involves melting multiple feedstock of elements or known alloys and generating a desired composition of matter by varying the speed of the feedstock advancement into a molten form in a high pressure inert gas or metal vapor environment such that all metals introduced into the first crucible are retained in a liquid state in the first crucible. The second stage involves casting the desired composition by pulling the liquid phase crucible contents from the first stage into a second cooling crucible through a passageway using a casting piston attached to the cooling crucible and permitting the composition to cool into a solid state as the piston slowly withdraws the cooling crucible. 
         [0005]    Briefly, a metal alloy with a specific composition is selected to be casted. The elemental components for this metal alloy are prepared as feedstock for the two stage casting system and loaded via a high pressure vacuum chamber into a melting crucible that has a surface layer several inches thick of a metal salt. This metal salt typically comprises CaF 2  along with minor additives and is heated via resistive heating current supplied by an electrical circuit. The first melting crucible is electrically connected to an electrical current power supply. The primary element feedstock acts as an electrode in the electrical circuit. Electrical current through the primary electrode, through the slag to the surface of the first crucible causes the metal salt layer to heat up, generating a high temperature slag layer, which in turn causes the primary feedstock electrode and secondary feedstock elements immersed in the slag to melt and puddle in the first melting crucible. 
         [0006]    Preferably the primary element feedstock electrode and secondary feedstocks are dispensed through a vacuum pressure chamber on top of the metal salt/slag layer on the melting crucible. This chamber is pressurized with inert gas or metal vapor and maintained at a temperature and pressure suitable to stop element evaporation during the melting process, since various metals melt at different pressures and temperatures. Once all of the feedstock elements have reached liquid phase in the crucible, the melted feedstock is stirred, preferably by inductive or electromagnetic stirring, to ensure consistent uniform distribution of each element or constituent of the melt. After being stirred to a homogenous state, the mixture is withdrawn through an extraction valve, passage or port into a second stage cooling crucible beneath the first or melting crucible using negative pressure from a casting piston. In the second stage crucible, preferably a cold wall crucible, the mixture is cooled and forms a quiescent metal head on the casting piston. The casting piston is then slowly withdrawn as the melt solidifies and the cooled and solidified metal alloy can then be removed for further treatment or modification. 
         [0007]    A system for two stage casting of a metal alloy in accordance with the present disclosure preferably has in a first stage a first melting crucible, a pressurized inert gas or metal vapor chamber connected to the first crucible to adjust a volatilization rate of metals in the melting crucible such that all metals introduced into the first crucible are retained in a liquid state in the first crucible, and a feedstock control system to dispense multiple feedstock metals into the chamber and into the melting crucible. The feedstock metals are dispensed at a rate sufficient to achieve a target composition of a final metal alloy. At least one of the multiple metal feedstock metals is in the form of an electrode, part of an electrical power supply supplying electrical current to the electrode. 
         [0008]    The second stage includes a second cooling crucible connected to the first melting crucible via a passageway. The system preferably includes a layer of metal salt/slag disposed on an upper surface of the melting crucible. A distal tip of the electrode is submerged below the upper surface of the metal salt/slag layer. Electrical current through the electrode passes through the upper surface layer of the metal salt/slag and resistively heats the slag layer to a temperature above the melting point of the electrode. Secondary feedstock elements are also positioned in the high pressure vacuum chamber so as to extend into the metal salt/slag layer. Some of the secondary feedstock elements may be high density materials. Other of the secondary feedstock elements may be hollow so as to carry low density materials into the slag layer and into the first melting crucible. 
         [0009]    The slag layer preferably has an increasing temperature gradient from the upper surface of the layer to a bottom of the layer, and is preferably controlled such that the upper surface has a temperature below the melting point of the primary or secondary elements. The bottom surface of the slag layer preferably has a temperature greater than the melting temperature of the element having the highest melting temperature. Preferably the slag layer has a thickness sufficient to achieve a first temperature associated with its upper surface, and a second temperature associated with its lower surface, wherein the first temperature is lower than the melting point of the electrode and wherein the second temperature is higher than the melting point of the electrode. 
         [0010]    A two stage method of producing a metal alloy in accordance with the present disclosure comprises placing a metal salt layer in a first crucible, wherein the first crucible is connected to a second crucible via a passageway, introducing a first electrode into the metal salt layer, passing an electrical current through the first electrode to produce a slag layer in the first crucible from the metal salt layer via resistance heating, pushing the electrode into the slag layer so that a tip of the electrode begins to melt into a molten composition below the slag layer in the first crucible, introducing secondary feedstock elements into the heated slag layer to melt the secondary feedstock elements into the molten composition in the first crucible and continuing to melt the electrode and the secondary feedstock elements into the composition until a desired volume of composition is reached. Once the desired volume of molten composition is achieved, the method comprises opening the passageway to the second crucible such that the molten composition flows into the second crucible; and cooling the composition in the second crucible to a solid state. 
         [0011]    The method may further include progressively lowering a piston attached to a bottom of the second crucible as the molten composition solidifies bottom up in the second crucible. Preferably during the first stage the primary electrode is a metal having a highest melting point of any of the elements to be introduced into the first crucible. In one embodiment the electrode is a hollow tube. The electrode may be titanium or a titanium alloy. In an embodiment the resistive heating of the metal salt by the first electrode heats the slag layer to a temperature above a secondary element melting point. In one embodiment, as a bottom portion of the composition in the secondary crucible solidifies the secondary crucible is withdrawn via a piston such that the bottom portion is progressively lowered relative to a top of the secondary crucible, and this progressive lowering is preferably continued until a solid ingot of the composition can be withdrawn for removal from the secondary crucible. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a schematic cross-sectional diagram of an exemplary embodiment of a two stage multi-component alloy casting system in accordance with the present disclosure. 
           [0013]      FIG. 2  is a schematic cross-sectional diagram of the upper portion of the embodiment shown in  FIG. 1  illustrating one embodiment of an electrical power supply circuit. 
           [0014]      FIG. 3  is a schematic cross-sectional diagram of an upper portion of the embodiment shown in  FIG. 1  illustrating an exemplary electromagnetic stirring arrangement. 
           [0015]      FIG. 4  is a schematic cross-sectional diagram of the crucible passageway shown in  FIG. 1  illustrating an exemplary passageway closure. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    In the description that follows, like numerals are utilized to describe like components and subcomponents in the various views. 
         [0017]    As noted above, this disclosure presents a system/apparatus for two stage casting of a metal alloy such as a multi-component alloy. The system includes a first melting crucible  6  and a second cooling crucible  8  connected to the first crucible  6  via a selectively closable passageway  7 . The upper surface of the first crucible  6  is layered with a metal salt that when resistively heated forms a relatively thick slag layer  20  on the upper surface of the first crucible  6  and the melt  11  formed thereon. 
         [0018]    This slag layer  20  may be 4 to 6 or more inches thick. It must be thick enough to have a large temperature gradient from top to bottom such that the upper slag layer surface temperature is lower than the lowest melting point of the feedstock element. The bottom surface of the slag layer  20  preferably has a temperature higher than the melting point of any of the feedstock elements. 
         [0019]    The feedstock elements  1 ,  2 ,  3  to produce the desired alloy composition shown as melt  11  include at least one feedstock element that acts as a first electrode  1  connected to a remote electrical power supply  21  via a feedstock controller  4 . Secondary solid elements  2 ,  3  are also included, whose feed rate is also controlled by the feedstock controller  4 , that add secondary elements to achieve the desired end composition of melt  11 . These elements  1 ,  2 ,  3  may be solid, for high density materials. The distal ends of these solid elements will sit below at least the surface of the slag layer  20 . Hollow elements that act as a tube to feed high volatile/low density materials to below the surface of the slag layer  20  may also be utilized. 
         [0020]      FIG. 1  shows basic diagram of a two stage metal alloy casting system  100  in accordance with one embodiment of the present disclosure. The system  100  allows feedstocks  1 ,  2  and  3  to be fed from a feedstock controller  4  into a melting first crucible  6 . The exemplary feedstock elements  1 ,  2 , and  3  are each comprised of elemental metals or pre-alloys which can be melted together to form a desired molten multi-component alloy 11. The feedstocks  1 ,  2 , and  3  and crucible  6  are disposed within a pressurized gas chamber  5  that may be under a vacuum or pressurized with an inert gas (He, Ar, N) or metal vapor, in some embodiments, to lower the volatilization rate of the various metal feedstocks. Many metal elements utilized in alloying processes volatize or melt at different temperatures and pressures. Preferably the chamber  5  is maintained at a desired temperature and pressure to maintain all constituent elements in a liquid state during processing as described herein. Use of a pressure chamber  5  results in an as cast microstructure of the melt as well as the end product solidified alloy 9 that includes volatile ingredient elements such as Li, Mg, and Zn in mixture with Titanium that would otherwise have been vaporized if pressure chamber  5  were not utilized. 
         [0021]    The feedstock motion and power controller  4  is electrically powered via a DC power supply  21  shown in  FIG. 2 . DC power is supplied to the system  100  via the power supply  21  such that current is fed through a primary feedstock electrode element  1 . The feedstock controller  4  is given feed rate instructions based on the specific amounts of each feedstock  1 ,  2 , or  3  needed to produce the desired multi-component alloy product. The primary feedstock element electrode  1  is fed through the vacuum chamber  5  into the melting first crucible  6  which has a surface layer typically several inches thick of slag  20 . This slag layer  20  typically comprises CaF 2  along with minor additives and is heated via the arc melting electrical circuit shown in  FIG. 2 . The primary element feedstock  1  acts as an electrode in the melting electrical current circuit shown in  FIG. 2 . The melting first crucible  6  is electrically connected to the power supply  21 , as a return, thus completing the electrical circuit. The slag  20  acts as a series resistive element in this electrical circuit of the power supply  21 . The current passing through the electrode  1  resistively heats the slag  20  and melts the tip of the primary electrode  1  into the first crucible  6  initially forming a melt  11 . Electrical current fed through the feedstock controller  4  via the primary electrode  1 , and through the slag  20  to the first crucible  6  via resistive heating causes the slag  20  to heat up, which in turn causes the primary feedstock electrode  1  and then the secondary feedstock elements  2  and  3 , also immersed in the heated slag  20 , to melt and puddle as a common melt  11  in the melting first crucible  6 . 
         [0022]    The feedstock controller  4  regulates the feed rate of each of the feedstocks  1 ,  2  and  3  into the crucible  6  in proportion to the desired composition melt  11  to be generated. Furthermore, the feedstock controller  4  adjusts the position of the primary electrode  1  tip in the slag  20  so as to promote melting at a controlled rate. 
         [0023]    The composition melt  11  is preferably stirred in the first crucible  6 . Stirring of the melt  11  may be accomplished by induction or electromagnetic stirring, mechanical stirring, sonic or ultrasonic agitation, or other mechanism. One exemplary arrangement for electromagnetic stirring is illustrated in  FIG. 3 . Multi-component alloy melts  11  may contain elements which have a significant difference in density. Since the properties of a multi-component alloy depend on the uniformity of the elemental composition throughout the material, it is necessary to stir the liquid phase metal components together to ensure uniformity before they solidify. The composition  11  may be stirred electromagnetically by providing AC power to at least one induction coil  13  using a magnetic stirring control system  12 . 
         [0024]      FIG. 3  shows an electromagnetic stirring control  12 . The magnetic stirring control  12  allows the system  100  to dynamically modify the parameters which control the magnetic stirring of liquid phase metals  11  in the first crucible  6 . The magnetic stirring control  12  is a component capable of adjusting the power to a magnetic stirring mechanism, such as a series of coils  13 , in order to vary the magnetic field allowing magnetic stirring of materials with different densities. An AC power source  14  supplies the magnetic stirring controller  12 . The magnetic stirring controller  12  adjusts the power and phasing to the magnetic stirring induction coils  13 , in order to vary the magnetic field allowing magnetic stirring of materials with different densities. 
         [0025]    Once the melt  11  is adequately stirred to form the desired consistency of the multi-component alloy product, the melt  11  is transported through an extraction valve, passageway, or port  7  into a second chamber including a cold wall cooling crucible  8 . The cold wall crucible  8  is cooled so that a quiescent metal alloy composition head  9  comprising a solid metal alloy composition may form in the cold wall crucible  8  on the casting piston  10 . The casting piston  10  may then be lowered or withdrawn and the solid metal head  9  removed from the top of the piston  10  for further use or treatment as may be desired. 
         [0026]    The feedstocks  1 ,  2 ,  3  described herein include at least two separate sources of raw material for the multi-component alloy product, and may include any form of elemental metals (e.g. Li, Ti, Mn, Cr, Fe, Co, Ni, Cu, Ag, W, Mo, Nb, Al, Cd, Sn, Pb, Bi, Zn, Ge, Si, Sb, and Mg) or pre-alloys, which can be in cylindrical wire form, granulated pellets, or powdered, for example. Preferably the primary element electrode  1  is the highest melting temperature element or alloy, such as Titanium. This way, as current is fed through the electrode  1  into the slag  20 , it will be heated high enough to progressively melt the Titanium. The heated slag  20  will in turn heat and melt the secondary feedstocks  2  and  3  such that they melt through the slag  20  into the first crucible  6  to coalesce into the melt  11 . 
         [0027]    Optionally, the first crucible  6  may be constructed of a consumable metal material itself such that a portion of the first crucible  6  melts into and forms part of the melt  11  in the first stage. Also, one of the feedstock elements may be a pre-alloy such as an Aluminum and/or Titanium alloy or one or more of the feedstock elements  1 ,  2 ,  3  may be a more complex multi-component alloy such as one that comprises at least three or four or more element metals pre-alloyed together in a prior two stage process as above described. 
         [0028]    In the embodiments described herein, the feedstock elements and alloys may be in a cylindrical wire form, granulated pellets, or powdered, etc. The electrode  1  may be a solid rod or may be hollow, or a hollow tube filled with another component element or alloy to become a part of the melt  11 . Furthermore, the slag  20  may also contain one or more feedstock elements or additives within it that combine with the feedstock elements  1 ,  2 , and  3  during formation of the melt  11 . 
         [0029]      FIG. 4  shows one exemplary embodiment of the system  100  in which a cooled valve pin  30  is utilized to controllably open a conical entrance portion  29  of the passageway  7  out of the crucible  6  into the solidifying head  9  on top of the cold crucible  8 . The entrance  29  to the passageway  7  is closed during the melting and formation of the melt  11  as above described. At least the entrance  29  of the passageway  7  is closed by a hollow trapezoidal tip shaped valve disk pin  30  during those operations. The passageway  7  is shown in  FIG. 4  exaggerated in size for explanation purposes. The passageway  7  may be essentially eliminated downstream of entrance  29  such that the entrance  29  is all that exists of passageway  7  into the second cooling crucible  8 . When it is desired to transfer the melt  11  into the crucible  8 , the valve pin  30  is slowly withdrawn while a cooling liquid  31  is circulated within the valve pin  30 . Raising the pin  30  opens a gap A which is carefully controlled such that the melt  11  passing by the tip of the pin  30  and through the passageway  7  via gap A does not change to a solid state prior to dropping onto the head  9 . This may be controlled by reducing or increasing the gap A and by regulating the temperature of the cooling fluid  31  within the pin  30  during the transfer operation. The first crucible  6 , if made of a conductive metal such as copper, may also be cooled or thermally regulated such that the melt  11  formed via resistive heating of the slag layer  20  remains liquid during the first stage formation of melt  11  described above and during the transfer process through passageway  7 . 
         [0030]    While various embodiments of the new technology described herein have been described in detail, it is apparent that modifications and adaptations of those embodiments will occur to those skilled in the art. For example, the two stage process and apparatus may be utilized over and over again utilizing one or more intermediate solid multi-component alloys produced in a previous stage as a pre-alloy element  1 ,  2  or  3  in a subsequent use of the system  100 . It is to be expressly understood that such modifications and adaptations are within the spirit and scope of the presently disclosed technology.