Patent Publication Number: US-6214286-B1

Title: Hybrid induction skull melting

Description:
FIELD OF THE INVENTION 
     The invention relates to induction skull melting of metal and alloys. 
     BACKGROUND OF THE INVENTION 
     Induction melting processes and apparatus using a water cooled segmented, copper crucible were developed by the US Bureau of Mines, for example, as described in U.S. Pat. Nos. 3,775,091 and 4,058,668. These patents illustrate use of a CaF 2  skull in the crucible and refractory packing material/spacers between the segments. The CaF 2  skull prevents contact between the molten metal and the crucible segments. Typically, the CaF 2  is melted and solidified on the cooled crucible segments to form an insulating lining or skull between the melt and the crucible segments. 
     U.S. Pat. No. 4,738,713 illustrates an induction melting process wherein a reactive metal is melted in a water cooled segmented, copper crucible in the absence of a CaF 2  lining or skull. In this patent, a refractory packing material is required between the tubular segments of the crucible to avoid molten metal penetration therebetween and subsequent skull locking. 
     The Diehm et al. U.S. Pat. No. 4,923,508 discloses a ceramicless induction skull crucible having a plurality of upstanding, water cooled metallic fingers that collectively form an upper metallic sleeve of the melting crucible and a water cooled metallic bottom. The crucible fingers are spaced by gaps small enough to avoid penetration of molten metal between the fingers that could produce skull locking. 
     There is a need in the art for an induction skull melting apparatus and method that avoids water cooled crucible sleeve segments or fingers as well as use of CaF 2  and other refractory skulls and refractory packing materials between segments that can contaminate the melt and also provide improved service in use in melting metal or alloy charges in a production environment. 
     An object of the invention is to provide induction skull melting apparatus and method that satisfy this need. 
     SUMMARY OF THE INVENTION 
     The present invention provides induction skull melting apparatus and method wherein a charge of metal or alloy is induction melted in a crucible comprising a reusable monolithic refractory tubular sleeve disposed on a reusable water cooled metallic base by an induction coil means disposed about the sleeve. 
     In one embodiment of the invention, a charge of metal or alloy, such as a reactive superalloy or titanium alloy, is induction melted in a crucible comprising a monolithic refractory cylindrical sleeve disposed on a water cooled metallic disc-shaped base by an induction coil means disposed about the ceramic sleeve. The monolithic sleeve can comprise alumina, zirconia, and other suitable ceramic material compatible with the metal or alloy to be melted. Alternately, the cylindrical sleeve can comprise graphite. The water cooled base can comprise single or multiple copper members that define water cooling channels therebetween. 
     The induction coil is energized to inductively heat the solid charge to a molten state including forming a solidified skull of the metal or alloy on inner surfaces of the sleeve and the base to confine the molten charge. Thereafter, the molten charge can be removed, for example, by pouring from the melting chamber, leaving the solidified skull in place on the inner surfaces of the sleeve and base. 
     The invention is advantageous in that water cooled crucible sleeve segments or fingers are eliminated. In addition, the need for CaF 2  and other refractory skulls is eliminated. Morever, the need for refractory packing materials between segments that can contaminate the melt is eliminated. The induction melting apparatus of the invention provides improved service in use in melting metal or alloy charges in a production environment in that damage to or spreading of previously used metal crucible segments or fingers is eliminated as a source of crucible downtime. Moreover, practice of the invention can provide a 25% to 50% reduction in electrical power requirements for melting as compared to power requirements using a segmented melting crucible, and greater levitation of the melt in the crucible and thus less reaction with the crucible before a skull forms thereon. 
     The above objects and advantages of the invention will become more readily apparent to those skilled in the art from the following detailed description taken with the following drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of induction melting apparatus in accordance with one embodiment of the invention. 
     FIG. 2 is a plan view of the crucible base. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, an induction skull melting apparatus is illustrated for melting a solid charge of metal or alloy, such as, for example only, nickel or cobalt based superalloys, titanium and titanium alloys and other metals and alloys. 
     The apparatus includes a melting crucible  10  and induction coil  12  disposed about the crucible  10  to inductively heat the charge and melt it. The crucible  10  includes a reusable upstanding monolithic refractory tubular sleeve  14  disposed on a reusable water cooled metallic base  16 . 
     The monolithic refractory tubular sleeve  14  typically comprises a refractory right cylinder having upper annular end  14   a  and lower annular end  14   b.  The lower annular end  14   b  includes an upwardly converging upstanding slot  20  formed therein. The slot  20  is sized and shaped complementary to an upstanding upwardly converging annular rib  22  extending about the periphery of the metallic base  16  so as to sealingly receive the rib  20  therein when the sleeve  14  is assembled on the base  16  as shown. The mating of the slot  20  and rib  22  prevents molten metal from leaking out of the crucible before a solidified lining or skull is formed in the crucible. There is no need to provide any other sealant between the lower end  14   b  of the sleeve  14  and the metallic base  16 . 
     The monolithic refractory sleeve  14  can comprise alumina, zirconia, and other suitable ceramic material compatible with the metal or alloy to be melted. For example, a commercially available alumina ceramic sleeve  14  can be used in the induction melting of nickel, cobalt or iron based superalloys. A commercially available zirconia ceramic sleeve  14  can be used in the induction melting of conventional titianium and its alloys. Ceramic sleeves of these types typically comprise pressed and sintered ceramic powder tubes and are available form Howmet Corporation, Whitehall, Mich., and Thermal Ceramics, Plymouth, Mich. 
     Alternately, the monolithic refractory sleeve  14  can comprise graphite. A graphite sleeve  14  can be used in the induction melting of titanium, amorphous alloys, such as Vitreloy, and others. A graphite sleeve  14  suitable for practicing the invention is available commercially from Bay Carbon Inc., Bay City, Mich. A typical inner diameter of the refractory sleeve  14  is in the range of 3 to 15 inches with a typical wall thickness in the range of ¼ to 2 inches. The height of the ceramic sleeve  14  typically is in the range of 3 to 20 inches. 
     The water cooled base  16  comprises first and second machined disc shaped members  30 ,  32  having circular peripheries. Member  30  comprises copper while member  32  can comprise copper, steel, or aluminum. The upper base member  30  is scalloped to form a recess or cavity  33  that cooperates with the sleeve  14  to form an internal melting chamber C of the crucible. 
     The base members  30 ,  32  are connected together by a plurality of circumferentially spaced apart screws  34  received in threaded bores  36  machined in the upper base member  30  and unthreaded bores  37  machined in the lower base member  32 . 
     The lower base member  32  is machined to form a water cooling channel  38  that is closed off by the base member  30  when assembled therewith and that receives cooling water via a water inlet port  43  machined in the lower base member  32  and includes water outlet  41 . The water cooling channel  38  extends in a configuration of a spiral passage as shown in FIG.  2 . The lower base member  32  includes an annular, circumferential groove  40  in which an O-ring seal  42  is disposed to seal on the upper member  30  when the base members  30 ,  32  are connected together as shown to prevent water leakage. 
     The induction coil  12  comprises a hollow, water cooled coil energized by a conventional source of electrical power (not shown), such as a 50 Kilowatt power source, to inductively heat the charge in the crucible chamber C to a molten state. The induction coil  12  surrounds or encompasses both the sleeve  14  and the base  16  as shown in FIG.  1 . 
     A solid charge of metal or alloy, such as nickel or cobalt based superalloy, titanium or titanium alloy, is placed in the melting chamber C, and the induction coil  12  is energized at an electrical power level for a time to melt the charge to a molten state. For reactive metals and alloys such as superalloys and titanium and its alloys, the melting operation is conducted under a suitable vacuum or inert gas to prevent reaction with oxygen present in ambient atmosphere. A thin soldified lining or skull of the metal or alloy forms in-situ on the upper, inner surface of the base member  30  and on the inner surface of the monolithic sleeve  14  shortly after the charge reaches the molten state. The lining or skull typically has a thickness in the range of 0.001 to 0.25 inches. Thereafter, the molten metal or alloy is confined or contained within the solidified metal or alloy skull until the molten charge is poured or otherwise removed from the crucble  10 , for example, to a conventional mold (not shown) for vacuum or other casting with the solidified lining or skull left in place on the inner surfaces of the sleeve  14  and base  16 . The crucible comprising the sleeve  14  on the base  16  then can be reused in melting another solid charge of the metal or alloy. 
     EXAMPLE I 
     In melting a solid charge (12 pounds) of a nickel base superalloy, the crucible comprised an A 1   2 O 3  ceramic sleeve  14  with a inner and outer diameter of 5.5 inches and 6.75 inches, respectively, and height of 9 inches on a water cooled copper base  16 . The induction coil was energized at 150 Kilowatts for 10 minutes to form a melt at a temperature of 2600 degrees F. The melting operation was conducted under a vacuum of less than 1 torr. The melt then was poured into an investment mold. A thin solidified superalloy skull of approximate thickness of 0.010 inch remained in the crucible. 
     EXAMPLE II 
     In melting a solid charge (12 pounds) of a gamma titanium alloy, the crucible comprised a Zr 2 O 3  ceramic sleeve  14  with a inner and outer diameter of 5.5 inches and 6.75 inches, respectively, and height of 9 inches on a water cooled copper base  16 . The induction coil was energized at 150 Kilowatts for 15 minutes to form a melt at a temperature of 2900 degrees F. The melting operation was conducted under a vacuum of less than 1 torr. The melt then was poured into an investment mold. A thin solidified titanium alloy skull of approximate thickness of 0.010 inch remained in the crucible. 
     The invention is advantageous in that use of the monolithic sleeve  14  and base  16  eliminates the need for refractory packing materials as well as CaF 2  and other refractory foreign skulls that can contaminate the melt. Cleaner melts thus are produced using the crucible of the invention. In addition, use of the monolithic refractory sleeve  14  eliminates the water cooled crucible sleeve segments or fingers previously used. The induction melting apparatus of the invention provides improved service in melting metal or alloy charges in a production environment in that damage to or spreading of previously used metal crucible segments or fingers is eliminated as a source of crucible downtime. Moreover, the invention provides greater levitation of the melt in the crucible to reduce reaction with the crucible before the skull forms thereon. 
     Although the invention has been described hereinabove in terms of specific embodiments thereof, it is not intended to be limited thereot but rather only to the extent set forth hereafter in the appended claims.