Patent Publication Number: US-2011075697-A1

Title: Cold Crucible Induction Furnace with Eddy Current Damping

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. application Ser. No. 11/654,108, filed Jan. 17, 2007, which is a divisional of U.S. application Ser. No. 11/036,005, now U.S. Pat. No. 7,167,501, filed Jan. 14, 2005, which claims the benefit of U.S. Provisional Application No. 60/537,365, filed Jan. 17, 2004, all of which are hereby incorporated herein by reference in their entireties. 
    
    
     FIELD OF THE INVENTION 
     The present invention is in the technical field of melting electrically conductive materials, such as metals and alloys, by magnetic induction with a cold crucible induction furnace. 
     BACKGROUND OF THE INVENTION 
     A cold crucible induction furnace is used to melt and heat electrically conductive materials placed within the crucible by applying an alternating magnetic field to the materials. A common application of such furnace is the melting of a reactive metal or alloy, such as a titanium-based composition, in a controlled atmosphere or vacuum.  FIG. 1(   a ) illustrates the principle features of a conventional cold crucible furnace. Referring to the figure, cold crucible  100  includes slotted wall  112 . The interior of wall  112  is generally cylindrical. The upper portion of the wall may be somewhat conical to assist in the removal of skull as further described below. The wall is formed from a material that will not react with a hot metal charge in the crucible, when the crucible is fluid-cooled by conventional means. For a titanium-based charge, a fluid-cooled copper-based composition is suitable for wall  112 . Slots  118  have a very small width (exaggerated for clarity in the figure), typically 0.005 to 0.125-inch, and may be closed with a heat resistant electrical insulating material, such as mica. Base  114  forms the bottom of the cold crucible. The base is typically formed from the same material as wall  112  and is also fluid-cooled by conventional means. The base is supported above bottom structural element  126  by support means  122  that may also be used as the feed and return for a cooling medium. A layer of heat resistant electrical insulation  124  (thickness exaggerated in the figure) may be used to separate the base from the sidewall. Induction coil  116  is wound around the exterior of wall  112  of the crucible, and is connected to a suitable ac power supply (not shown in the figure). When the supply is energized, current flows through coil  116  and an ac magnetic field is created within and external to the coil. The magnetic flux induces currents in wall  112 , base  114  and the metal charge placed inside the cold crucible. Flux penetration into the interior of the crucible is assisted by slots  118 . Heat generated by the induced currents in the charge melts the charge. As illustrated by furnace  100  in partial detail in  FIG. 1(   b ), a portion of metal charge adjacent to the cooled wall and base freezes to form skull  190  around liquid metal  192 . The skull acts as a partial container for the molten metal, and the upper regions of the molten metal are at least partially supported by the Lorentz forces generated by the interaction of the magnetic field produced by coil  116  and the induced currents in the metal charge, to form a region of reduced contact pressure or even separation  194  between the wall and the liquid metal. Such reduced contact pressure or separation is important in reducing the thermal losses from the hot charge to the cold crucible. The Lorentz forces also cause the liquid metal to be vigorously stirred. After removal of the liquid metal product from the crucible, the skull can be left in place for a subsequent melt, or removed from the crucible, as desired. 
     As mentioned above, liquid metal in the crucible above the skull is generally kept away from the crucible&#39;s wall by Lorentz forces acting on the mass of liquid metal. Fluid motions caused by induced currents can intermittently disturb the region of separation between the wall and the mass of liquid metal. Such disturbances increase the boundary area of the melt, resulting in increased heat radiation losses from the liquid, or even increased conduction losses, if some of the liquid metal washes or splashes against the wall of the crucible. 
     It is sometimes desirable to superheat the liquid metal, for example to make it more fluid and therefore, more suitable for casting into a mold to form a casting having thin sections. However, the above apparatus and method has disadvantages when used to superheat the liquid metal. With increased superheat, there is an increased temperature difference between the liquid metal (melt) and the skull. This results in an increase in the heat transferred from the liquid metal to the skull. Consequently a portion of the formed skull melts back to liquid metal, which reduces the thickness of the skull. Decreased skull thickness increases heat losses from the liquid melt. Further the skull may be reduced in overall volume, so that parts of the liquid melt formerly contained within the skull can come into contact with the wall of the crucible, which greatly increases the heat loss from the liquid metal. In practice, the result is that for any reasonable power input to the above apparatus and process, the superheat is severely limited. 
     V. Bojarevics and K. Pericleous,  Modelling Induction Skull Melting Design Modifications, Journal of Materials Science: Special Section: Proceedings of the  2003  International Symposium on Liquid Metals , Vol. 39, no. 24 (December 2004), pp 7245-7251 (presented on 23 Sep. 2003 in Nancy, France), suggests locating a separate dc coil adjacent to the ac coil of a cold crucible arrangement (paragraph beginning at the bottom right-hand column on page 7248 and continuing on page 7249 page 4 of the Bojarevics and Pericleous paper); i.e. towards the bottom part of the crucible and below the ac coil. DC current flowing through the dc coil creates a dc magnetic field that is superimposed on the ac field. When the molten charge, driven by the Lorentz forces previously described, moves across the field lines of the dc field, additional currents are induced in the moving metal. Such currents react with the dc flux to produce a braking action that reduces the fluid velocity. Such braking action is well known and is often referred to as eddy current braking or eddy current damping. By reducing the metal flow velocity, such damping reduces the turbulence in the liquid metal near the bottom of the cold crucible, thereby reducing the heat convectively transferred from the liquid metal into the skull; thereby permitting significantly increased superheat for a given power input. Such use of a dc magnetic field for eddy current damping or braking of moving metal in an induction coil is known prior art (see e.g. U.S. Pat. No. 5,003,551). However, locating a dc coil adjacent to the ac coil as proposed in the Bojarevics and Pericleous paper, would result in the ac magnetic field inducing high losses in the large cross sectional dc conductors shown in the paper. Moreover, there is no recognition or analysis of this deleterious effect in the Bojarevics and Pericleous paper. Nor can this problem be alleviated by simply moving the dc coil away from the ac coil, or vice versa, because the magnetic field of a coil so moved would be reduced in the crucible&#39;s interior space, thus rendering the moved coil less effective. 
     Therefore, there exists the need for apparatus and a method of induction melting an electrically conductive material with a cold crucible wherein convective heat loss to the cold crucible is limited, in order to obtain more superheat. 
     BRIEF SUMMARY OF THE INVENTION 
     In one aspect, the invention is apparatus and method for induction melting of an electrically conductive material in a cold crucible induction furnace wherein a dc field is established to selectively decrease motion in the molten material. Induction melting is achieved by ac current flow in an ac coil surrounding the cold crucible. The dc field may alternatively, or in selective combinations, be established: by the flow of dc current in the ac coil; in a shielded dc coil separate from the induction coil; or by magnets selectively disposed around the exterior of the wall of the crucible. 
     In other examples of the invention the dc field is established by the flow of dc current in a dc coil disposed below the cold crucible. The coil contains a magnetic pole piece in which the magnetic field is concentrated and directed into the bottom of the cold crucible. Optionally one or more dc coils may be provided between the ac coil and the dc coil around the outside of the cold crucible, to further assist in selectively decreasing motion in the molten material. 
     These and other aspects of the invention are further set forth in this specification and the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For the purpose of illustrating the invention, there is shown in the drawings a form that is presently preferred; it being understood, however, that this invention is not limited to the precise arrangements and instrumentalities shown. 
         FIG. 1(   a ) is a partial cross sectional elevation of a conventional cold crucible induction furnace. 
         FIG. 1(   b ) is a cross sectional elevation of a formed skull and liquid metal in a conventional cold crucible induction furnace. 
         FIG. 2  is a partial cross sectional elevation of one example of the cold crucible induction furnace with eddy current damping of the present invention wherein eddy current damping is provided by the flow of dc current in the induction coil that carries ac current for inductive current heating of an electrically conductive material placed in the crucible. 
         FIG. 3  is a partial cross sectional elevation of one example of the cold crucible induction furnace with eddy current damping of the present invention wherein eddy current damping is provided by the flow of dc current in a dc field coil that is separate from the induction coil that carries ac current for inductive current heating of an electrically conductive material placed in the crucible. 
         FIG. 4  is a partial cross sectional elevation of one example of the cold crucible induction furnace with eddy current damping of the present invention wherein eddy current damping is provided by one or more magnets disposed around the exterior of the wall of the furnace. 
         FIG. 5  is a partial cross sectional elevation of another example of the cold crucible induction furnace with eddy current damping of the present invention. 
         FIG. 6  is a partial cross sectional elevation of another example of the cold crucible induction furnace with eddy current damping of the present invention. 
         FIG. 7  is a partial cross sectional elevation of another example of the cold crucible induction furnace with eddy current damping of the present invention, arranged to provide a counter gravity casting process. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As used in this specification, the term “induced currents” generally refers to currents induced by an ac coil and the term “eddy currents” generally refers to currents generated by the movement of molten electrically conductive material across dc field lines. There is shown in  FIG. 2 , one example of a cold crucible induction furnace  10 , with eddy current damping, of the present invention. For this example crucible  10  may comprise a cold crucible with wall  12  having slots  18 , and base  14 . The base may be separated from the wall by a layer of thermal and electrical insulation  24 . The base may be raised above bottom structural support element  26  by suitable support means  22 . Induction coil  16  is wound at least partially around the height of wall  12 . Induction coil  16  is suitably connected to ac power source  30 . AC current provided from the ac power source flows through coil  16  and establishes an ac field that penetrates into wall  12  and an electrically conductive material placed within the crucible. By example, and not limitation, the electrically conductive material may be a metal or alloy. The ac field couples with the metal and induces currents in the metal that heats the metal to a liquid state. The output of dc power source  32  is connected in parallel with the output of the ac power source. DC current provided from the dc power source flows through coil  16  and establishes a dc field that penetrates into wall  12 , base  14  and the liquid metal in the crucible. The dc field dampens the fluid flow induced in the melt by the ac field. Heat loss from the liquid metal to the skull takes place principally by a process of forced convection that is set up by the Lorentz-force driven molten metal flowing adjacent to the interior surfaces of the skull. This convective heat loss is reduced when the fluid velocity is reduced by the eddy current braking action of the dc field. Consequently, selectively controlling the magnitude of the dc field by controlling the magnitude of the dc current from dc power source  32  during the heating and melting process can be used to selectively reduce heat loss during the heating and melting process. 
     Suitable impedance elements can be provided at the output of the ac and dc power supplies to prevent current feedback from one supply to the other supply. In the example shown in  FIG. 2  only a single induction coil is used. In other examples of the invention two or more induction coils may be used to surround different regions along the height of the crucible, and one or more ac and dc power supplies may be selectively connected to one or more of the multiple induction coils depending upon whether a particular region requires dc field damping. In examples of the invention wherein more than one induction coil is provided, the one or more dc power supplies may be selectively applied to less than the total number of induction coils. 
     In other examples of the invention one or more dc field coils are provided separate from one or more ac current induction coils around the outer wall of the crucible. In the non-limiting example of the invention shown in  FIG. 3 , dc field coil  17  is wound around the exterior of wound induction coil  16 . AC power source  30  supplies ac current to induction coil  16  to melt and/or heat an electrically conductive material placed inside the crucible by magnetic induction of currents in the material as described above. DC power supply  32  supplies dc current to dc field coil  17  to selectively dampen fluid flow in the material. Shield  19  can be optionally provided to shield the dc field coil from the ac field produced by induction coil. The shield can be fabricated from a suitable material with high electrical conductivity. Alternatively, the one or more dc field coils may be interspaced with the one or more induction coils in substantially vertical alignment. Another non-limiting arrangement is providing one or more helically wound dc field coils below base  14  of the crucible. This concentrates the established dc field near the bottom of the melt in the crucible, where damping is most needed, to reduce forced convection heat losses to the skull. In all cases in which a separate dc coil is used, excessive induced losses in the dc coil conductors are prevented by some combination of shielding, coil location or the use of multiple, insulated small cross section conductors to carry the dc current. 
     In the above examples of the invention wherein a variable dc current is used to provide variable eddy current damping, one non-limiting method of the invention is to start with zero or low magnitude dc current early in the melting process when vigorous induced current stirring of the melt is desired to dissolve charge material (such as the skull from a prior melt) with a high melting temperature. As charge is melted the magnitude of dc current can be increased, maximum dc current being used when the charge is completely melted and the goal is to maximize superheat in preparation for transferring the liquid metal to a mold or other container. 
     In other examples of the invention one or more discrete permanent magnets may disposed around the outer perimeter of slotted wall  12  of the furnace, generally in a cylindrical region identified as region A in  FIG. 4 , and/or in a region under base  14  (not illustrated in the drawing). A plurality of discrete magnets, each with a particular magnitude of dc field strength and geometry that is dependent upon their placement around the crucible may be used. Means must be provided to prevent overheating of the magnets caused by magnetic coupling with the ac field established by ac current flow through induction coil  16 . Such means may include siting of the magnet in minimum ac field regions; magnetically shielding the magnet from the ac fields; and/or composing the magnet from electrically isolated segmented elements. Use of permanent magnets provides less flexible eddy current control than a variable dc field established by variable dc current in the above examples of the invention. Alternatively discrete electromagnets may be used to vary the dc field of the magnet, and, in turn, vary the eddy current damping. 
     In other examples of the invention, eddy current damping may be accomplished by a selective combination of two or three of the previously disclosed methods, namely: dc current flow in the induction coil; dc current flow in a dc field coil separate from the ac coil; and permanent magnets or electromagnets. 
     Other arrangements of combined ac and dc current coils, separate ac induction coils and dc field coils, and magnets are contemplated as being within the scope of the invention as long as the established dc fields are used to damp the fluid flows induced in the electrically conductive material in the crucible, in order to increase superheat, without incurring excessive induced losses in the components that are being used to generate the dc field. 
     There is shown in  FIG. 5 , another example of a cold crucible induction furnace, with eddy current damping, of the present invention. Furnace  11  has a first dc coil  52  wound around a first end section of magnetic pole piece  54 . In other examples of the invention the first dc coil can be wound around other regions of the magnetic pole piece; further more than one first dc coils may be provided. First dc coil  52  can be, but is not limited to, hollow electrical conductors wherein the interior passage is used for the flow of a cooling medium. Magnetic pole piece  54  is formed from a suitable soft magnetic material, such as high purity iron. One non-limiting shape for the magnetic pole piece is a substantially solid cylinder, although other shapes can be used to concentrate the dc magnetic field generated around the first dc coil. A magnetic pole piece flange (not shown in the figure) can be attached to the first end of the magnetic pole piece to serve as a means for holding the first dc coil in place and to control the shape of the dc magnetic field. Magnetic pole piece  54  protrudes into the base of the furnace as shown in  FIG. 5  so that the second end of the pole piece is adjacent to the crucible base plate  58 . An optional second dc coil  73  is wound around the exterior of the base of the furnace in a location between crucible base plate  58  and bottom structural support or stool plate  60 . Second dc coil  73  may be of the same or similar construction as the first dc coil. 
     Support  64  provides a means for supporting base plate  58  and the weight of the metal in the melting chamber  72 . Coolant jacket  62  provides a means for supporting and supplying coolant to segmented furnace wall  70  and base  58 . In this non-limiting example of the invention each of the segments making up the furnace wall has an interior chamber for the passage of a cooling medium, such as water. AC induction coil  68  is shown only on the left side of the furnace in  FIG. 5  since the coil insulation on the right side of the furnace in this partial cross sectional figure encloses the ac induction coil. In this non-limiting example of the invention, induction coil water inlet  80  supplies current and cooling water to hollow induction coil  68 ; water and current exit the coil through an induction coil water outlet not shown in the figure. 
     Induction coil  68  at least partially surrounds the melting chamber of the furnace and inductively heats an electrically conductive charge placed within the melting chamber when an ac current (provided by a suitable power supply not shown in the figures) flows through the induction coil. DC current flowing through first dc coil  52  from one or more suitable dc power supplies (not shown in the figures), generates a dc field that is concentrated in the magnetic pole piece  54 . The second end of the pole piece is arranged to be adjacent to crucible base plate  58  so that the dc field penetrates predominantly into the bottom and lower sides of melting chamber  72  to decrease the flow intensity and turbulence of the liquid adjacent to the base in the melting chamber that is caused by the induced ac currents in the charge. The shape and location of pole piece  54  and the location of first dc coil  52  cause the various components of the crucible assembly to shield dc pole piece  54  and first dc coil  52  from the ac fields produced by the induction coil. 
     Optional second dc coil  73  may be used to minimize the loss of dc magnetic flux from the sides of pole piece  54  and further enhance the flux density (magnetic field strength) at the top of pole piece  54  below base plate  58 . Such optional second dc coil  73  may be separately shielded from the ac field produced by induction coil  68  by coil shield  71  that is composed substantially of a material with high electrical conductivity. The currents induced in this shield by the magnetic field from ac coil  68  serve to redirect the ac field, reducing the magnitude of the currents induced in the conductors of second dc coil  73 . 
     Water inlet  84  provides cooling water to the interior passages in the segments of wall  70  and baseplate  58 . Water outlet  86  provides a return for cooling water from the interior passages in the segments of wall  70 ; water outlet  88  provides a return for cooling water from the interior passages in base  58 . 
       FIG. 6  illustrates another example of a cold crucible induction furnace, with eddy current damping, of the present invention. In this example of the invention the top of magnetic pole piece  54  is shaped to concentrate dc field penetration away from the center of crucible base plate  58  as illustrated by typical dc flux lines (shown as dashed lines  99  in the figure). The advantage of this arrangement is that the dc field is concentrated in regions in which the electromagnetically induced flow of molten metal in the melting chamber (generally represented by dotted lines  97  in the figure) has the maximum flow velocity across the dc field lines, thereby improving the eddy current braking effect of the dc field, to further reduce the convective heat loss to the skull. The shaping of the top of the pole piece in  FIG. 6  illustrates one non-limiting arrangement of achieving this advantage. In the figure magnetic pole piece  54  is of substantially solid cylindrical shape, and has a conical open volume  54   a  formed at the center of its top, which concentrates the dc field near the mid-radius of the crucible base. 
     Also shown in  FIG. 6  is optional third dc coil  75  which is disposed above and further away from wall  70  than optional second dc coil  73 . The advantage of the optional third dc coil, which can be used in any example of the invention wherein the optional second dc coil is used, is to further enhance the dc field in the region just above the crucible base. Coil shield  71   a  performs a function similar to that of coil shield  71  as previously described above. 
     In other examples of the invention the first dc coil  52  in  FIG. 6  is not used while second dc coil  73  and third dc coil dc coil  75  are used to establish a dc field that is concentrated in magnetic pole piece  54  and penetrates predominately into the bottom and lower sides of the melting chamber. All other features and options of theses examples of the invention are generally the same as those shown in  FIG. 6  and described above. 
     Once the electrically conductive material, such as a liquid metal, has been melted in the melting chamber by induction heating, various methods can be used to remove the liquid metal from the chamber. For example, the melting chamber may be mounted on a support structure providing a means for tilting of the melting chamber and pouring of the liquid metal into a suitable container such as a mold. Another non-limiting method of removing the liquid metal from the melting chamber for the cold crucible induction furnace of the present invention is by a process known as counter-gravity casting of molten metals. U.S. Pat. No. 4,791,977 generally describes the process of counter-gravity casting and is hereby incorporated herein by reference in its entirety. Referring to  FIG. 7 , in this process the lower portion of fill pipe  91  is inserted into the molten metal  93  in the melting chamber. The fill pipe is removably connected to the interior cavity  95  in mold  96 . A reduced pressure is applied to the interior cavity of the mold as further described in U.S. Pat. No. 4,791,977 to draw molten metal from the melting chamber through the fill pipe and up into the interior cavity of the mold until the mold is filled. The applied dc field in the present invention may be used to increase the superheat of the metal to enhance the filling of the cavities of the mold. 
     Alternatively in all examples of the invention any of the dc coils may comprise a suitable arrangement of a plurality of small cross sectional insulated conductors to prevent overheating of the dc coils. 
     The above examples of the invention utilize one magnetic pole piece. Two or more pole pieces suitably arranged are contemplated as being within the scope of the invention. 
     The foregoing examples do not limit the scope of the disclosed invention. The scope of the disclosed invention is further set forth in the appended claims.