Abstract:
A furnace for melting metals includes a crucible, a direct current arc source and an induction coil powered by alternating current. The furnace is particularly useful for melting charges of highly reactive metals such as titanium, zirconium and their alloys without contaminating these charges. The direct current arc source melts generally from the inside out while the water-cooled induction coil serves to cool the crucible and form a skull along the crucible sidewall which protects the crucible from interacting with the molten metal. The induction coil is thus used for cooling as well as heating and stirring the melt, and helps control the thickness of the skull.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Technical Field 
         [0002]    The present invention relates generally to a furnace and method for melting metals. More particularly, the present invention relates to the melting of metals using a DC power source and an AC power source. Specifically, the invention relates to a furnace and method which utilizes a DC direct arc electrode or plasma torch and an induction coil powered by an AC power source. 
         [0003]    2. Background Information 
         [0004]    While induction melting of metals is well known in the art, it is most restricted in the lower temperature regimes before the metal or alloy reaches its melting point. Typically, the electromagnetic coupling becomes more efficient once the charge material becomes molten. Thus, any process which aids in the initial low temperature regimes to establish a molten pool of metal helps increase the efficiency of the electromagnetic coupling of the induction field to the metal charge. 
         [0005]    Another weakness of the induction melting of metal relates to the fact that the majority of the heating is done at the crucible wall with which the molten metal is in direct contact. This is particularly true in the case of high frequency induction melting. The higher temperatures at the crucible-metal interface leads to a high potential for reactions between the molten metal and the material forming the crucible. The products of these reactions include microscopic “dirt” in the form of oxides (often called low density inclusions), oxygen, carbon, hydrogen and other reaction products. The formation of such oxides occurs because the oxides are very stable while the metals are very unstable. Thus, these compounds are often referred to as oxygen scavengers or “oxygen getters”. For this reason, significant effort and cost has gone into the development of refractory ceramics which lessen the likelihood of metal-ceramic reactions. However, it is very difficult to alleviate all likelihood of such reactions, and with regard to certain metals like titanium, zirconium and their alloys, it is virtually impossible. 
         [0006]    In addition, extremely complex and expensive systems have been designed to lessen or alleviate the problem with crucible-metal interface reactions while substituting a copper crucible for a refractory crucible and cooling the copper typically with water in an effort to prevent the melting of the copper due to the high melt temperature of molten metals such as titanium, which is substantially higher than the melting point of copper. Induction melting systems which incorporate this feature are typically referred to as I.S.M. or induction skull melters. This feature is also used with DC ingot and casting systems and is referred to by a number of names, including V.A.R. or vacuum arc remelting, or vacuum arc casting. 
         [0007]    Each of these systems has drawbacks. They are expensive to build and operate because the machines have fabricated copper components which are expensive to manufacture. They also require expensive cooling by expensive water systems. Thus, they are not practical to use except in very large size and volume applications. 
         [0008]    They can also be dangerous to operate. If the cooling system fails to work properly, the copper shell will melt, allowing the molten charge such as molten titanium to breach the system and combine with the water remaining in the crucible. This typically leads to a steam explosion at the least, and if not arrested, can quickly lead to a much larger and more devastating hydrogen explosion. Unfortunately, there have been some deadly accidents of this sort in the past. 
         [0009]    The furnace and the method of the present invention address these and other problems in the art. 
       BRIEF SUMMARY OF THE INVENTION 
       [0010]    The present invention provides a method comprising the steps of melting with a DC arc source a metal within a melting cavity of a crucible bounded by a crucible wall to form molten metal therewithin; and maintaining a skull along the crucible wall with an induction coil in electrical communication with an AC power source to protect against contact between the molten metal and crucible wall. 
         [0011]    The present invention also provides a furnace for melting a metal charge, the furnace comprising a melting crucible defining a melting cavity adapted to receive the metal charge; an electrode adjacent the melting cavity adapted to melt the metal charge; a DC power source in electrical communication with the electrode; an induction coil adjacent the crucible; and an AC power source in electrical communication with the induction coil. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0012]      FIG. 1  is a diagrammatic elevational view of the first embodiment of the furnace of the present invention showing the crucible and induction coil in section with a solid charge of metal within the crucible. 
           [0013]      FIG. 2  is similar to  FIG. 1  and shows molten material in the center of the crucible and a skull of solid material along the bottom and side walls of the crucible. 
           [0014]      FIG. 3  is similar to  FIG. 2  and show a second embodiment of the furnace of the present invention with molten material and a skull within the crucible. 
       
    
    
       [0015]    Similar numbers refer to similar parts throughout the drawings. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0016]    A first embodiment of the furnace of the present invention is indicated generally at  10  in  FIGS. 1 and 2 ; and a second embodiment of the furnace is indicated at  100  in  FIG. 3 . Furnaces  10  and  100  are configured for melting metals and are particularly useful for melting titanium, zirconium and alloys thereof. 
         [0017]    Referring to  FIG. 1 , furnace  10  includes a crucible  12 , a substantially cylindrical induction coil  14  circumscribing crucible  12 , a DC powered heat source  16 , and a DC power clamp  18  all of which are disposed within a melting chamber  20  defined by a chamber wall  22 . Furnace  10  is configured to provide an atmosphere within melting chamber  20  which typically uses an inert gas such as argon or helium and which is typically under a vacuum associated with direct arc electrodes and plasma torches. Heat source  16  includes a consumable electrode or a non-consumable electrode such as a tungsten electrode. 
         [0000]    Crucible  12  has a substantially flat bottom wall  24  and a vertical side wall  26  which is typically cylindrical and extends upwardly therefrom to define therewithin a cylindrical melting cavity  28  for receiving a metal charge  30 . Crucible  14  is preferably formed of a carbon-graphite material or a ceramic material such as zirconium oxide or yitria oxide. An electrode  32  which may be either an anode or cathode is mounted on bottom wall  24  and extends through a hole formed therein to communicate with melting cavity  28  so that it is in electrical communication with charge  30 . Electrode  32  is in electrical communication with a DC power source  34  which involves a power converter for converting AC to DC power. DC power source  34  is likewise in electrical communication with heat source  16  via clamp  18 . Heat source  16  may be a direct arc electrode or a plasma torch. Induction coil  14  is in electrical communication with an AC power source  36 . AC power source  36  has controls for regulating the amount of power (in kilowatts) and the frequency. Coil  14  is tubular and thus forms a passage so that water from water source  38  may be pumped by pump  40  through the passage to cool coil  14 . 
         [0018]    The operation of furnace  10  is now described with reference to  FIGS. 1 and 2 . Furnace  10  may be used to melt various sorts of metals and is particularly useful in the melting of highly reactive metals such as titanium, zirconium and their alloys. Furnace  10  is charged by placing metal charge  30  in melting cavity  28 . This may be achieved in a conventional manner at atmosphere or under vacuum via a vacuum lock. While under a suitable vacuum, heat source  16  is powered by DC power source  34 . While it has previously been noted that heat source  16  may be a plasma torch, it is described for the present purposes of operation as being a direct arc electrode, either consumable or non-consumable as noted above. 
         [0019]    Initially, the power to induction coil  14  from AC power source  36  is in an off condition and water is being circulated via water source  38  and pump  40  through coil  14  in order to actively cool crucible  12  primarily along side wall  26 . DC electrode  16  is then powered by DC power source  34  in order to strike an arc to charge  30 . The position of electrode  16  is controlled by a three axis positioning system (not shown) which is able to move electrode  16  along axes X, Y and Z and is typically operated via a voltage or current driven feedback loop system. The heat produced by the arc between electrode  16  and charge  30  will melt the metal to create a molten metal bath  42  which has a generally cylindrical shape. The heat produced by the arc of electrode  16  will be generally hottest at the contact with molten bath  42  with the temperatures gradually decreasing radially outward toward side wall  26 . In order to prevent molten bath  42  from contacting crucible  12 , the cooling provided by the water passing through induction coil  14  cools crucible  12  and in turn cools the metal along the walls of the crucible in order to maintain a solid or semi-solid boundary layer or skull  44  along side wall  26  and bottom wall  24 . Thus, the reactive molten metal of bath  42  either does not contact crucible  12  or only does so minimally. Thus, the problem of contamination which would otherwise occur due to the molten metal-crucible interface is eliminated or substantially avoided. 
         [0020]    During the melting process, induction coil  14  may be powered by AC power source  36  in order to provide electromagnetic heating and stirring of the molten bath  42 . A particular advantage of heating with coil  14  is the ability to maintain a uniformly thick skull  44  along crucible side wall  26 . More particularly, skull  44  includes a flat circular bottom wall portion and a cylindrical side wall portion extending vertically upwardly therefrom. In addition, coil  14  may be used to melt the material of skull  44  to allow for its easy removal in the event that the crucible is to be used for another alloy having a different chemistry. Typically, molten material  42  will be poured or otherwise transferred out of crucible  12  and used in the molding of various objects. During pouring, molten material  42  will contact crucible  12  for a relatively brief period so that contamination therebetween is minimal. Once molten material  42  is transferred out of crucible  12 , coil  14  is powered to completely melt skull  44  to form additional molten material which may be poured or otherwise transferred from crucible  12  and may be maintained separate from the original molten material  42  to prevent contamination therebetween. 
         [0021]    Furnace  100  is now described with reference to  FIG. 3 . Furnace  100  is similar to furnace  10  except that it includes a crucible  50  and an induction coil  52  each of which tapers upwardly and outwardly to define a frustoconical melting cavity  51 . More particularly, crucible  50  includes a horizontal flat circular bottom wall  54  and a substantially conical or frustoconical side wall  56  extending upwardly therefrom. Coil  52  circumscribes side wall  56  and is likewise frustoconical in shape. Side wall  56  forms an angle A relative to horizontal as represented by horizontal line  58  which is somewhere between 10° and less than 90°, in contrast to the vertical side wall  26  of furnace  10 . Coil  52  is likewise angled relative to line  58  at angle A unlike the vertical alignment of coil  14  of furnace  10 . 
         [0022]    Furnace  100  is operated in essentially the same manner as furnace  10  except that the metal charge is melted to form a molten bath  60  which is generally conical in shape and a skull  62  which forms along side wall  56  and bottom wall  54  which is frustoconical and thus has a generally V-shaped cross-section. Thus, the molten bath  60  is wider at its upper surface than at its bottom. Since melting cavity  51  has a diameter which increases from the bottom upward, it provides a greater diameter where the DC arc contacts the surface of molten bath  60  where the greatest amount of heat is produced. This configuration helps to insure that skull  62  has a substantially uniform thickness and also adds to the volume of the crucible without creating hot zones in the crucible. Skull  62  has a conical or frustoconical shape. 
         [0023]    In addition to the various advantages noted above, furnaces  10  and  100  may be used in a more standard fashion. For instance, if the metal or metal alloy to be melted is relatively non-reactive with the material of the crucible, such as a copper or stainless steel charge, it may be preferred to use the furnace as a traditional induction furnace. In addition, if it is desired to provide vigorous stirring of the molten metal in order to homogenize the bath, the use of the AC induction coil may also be preferred. 
         [0024]    On the other hand, the combined use of the DC arc source and the AC induction coil may be preferred in order to rapidly melt a given charge. For instance, if an alloy contains constituents with extremely low vapor pressure points, it may be desirable to reduce the residence time of the constituents in the molten bath in order to reduce the chances of vaporizing or oxidizing the constituents and altering the bath chemistry. Thus, furnaces  10  and  100  provide new advantages as well as versatility. 
         [0025]    In the foregoing description, certain terms have been used for brevity, clearness, and understanding. No unnecessary limitations are to be implied therefrom beyond the requirement of the prior art because such terms are used for descriptive purposes and are intended to be broadly construed. Moreover, the description and illustration of the invention is an example and the invention is not limited to the exact details shown or described.