Abstract:
A homogeneous electrode of reactive metal alloy comprises an ingot with an axially disposed core made from one induction melted heat of the reactive metal alloy and having a diameter “d” and a length “L” and a body having an outer diameter at least 2 times “d” and a length “L”. The ingot body is disposed about the core, and comprises a plurality of induction melted heats of the reactive metal alloy to provide an electrode of the required size. The electrode also includes features for receiving electrical current for arc remelting of the reactive metal alloy to produce a large homogeneous ingot. Remelting may be conducted under vacuum or controlled atmosphere conditions, as required.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to processes for producing ingots of reactive metals and alloys and more particularly to a process for producing homogeneous ingots several times larger than those which can be produced from a single induction skull melted charge. 
     Melting of reactive metal alloys containing Titanium, Zirconium, or other reactive elements is usually done in a vacuum, in an inert atmosphere, or in a partial pressure of gas which is non-reactive with the alloy constituents. Such non-reactive atmosphere melting preferably employs induction melting or arc melting in a cold crucible or mold, usually of water-cooled copper, to eliminate contamination from mold washes or from the mold itself. These melting methods, both of which are very well known in the metallurgical art, produce ingots of excellent purity; however, induction melting and vacuum arc melting each have limitations which result in ingots having less than ideal properties. 
     Induction skull melting (ISM) produces ingots having a very high degree of cleanliness and homogeneity due to melting in a skull of the alloy being processed and to the intense stirring produced in the melt by the induction field. However, the size of induction skull melted heats is limited by the presence of the water-cooled metal crucible and the size of the controlled atmosphere chamber. The crucible limits the strength of the induction field, and the chamber limits the size of the mold and the ability to manipulate the crucible for pouring the alloy into the mold. This results in small ISM ingots—typically less than 100 pounds. Consequently, for large ingots, several ISM heats must be combined. This may be accomplished by casting several small electrodes from several ISM heats, welding the electrodes together end-to-end, and using the resulting composite electrode for vacuum arc remelting (VAR) the alloy in a water-cooled copper crucible. 
     However, since hot-topping during casting is not possible in the ISM melting and casting process, the resulting ingots have at least some porosity and shrinkage pipe. These defects aggravate the costliness and difficulty of producing the VAR electrode by welding. Further, since many of the reactive alloys are brittle, they also have marginal weldability and are susceptible to cracking in the welds and in the heat affected zones adjacent to the welds. Therefore, the welded composite electrodes may contain cracks and inclusions due to contamination with oxygen and nitrogen during welding. This can lead to failure of the electrodes during VAR and can result in damage to the equipment and danger to the operators thereof. 
     Vacuum arc melting can produce very large ingots compared to those made by ISM. Electrodes of, for example, titanium alloys for vacuum arc melting are typically made by starting with titanium sponge and/or granular master alloys and/or alloying elements, which are blended together in required proportions and compacted into briquettes of about 4″ diameter and 2″ thickness. The briquettes are non-homogeneous because the alloying elements are introduced as solids and are only mechanically blended. The electrode is formed by welding the briquettes together, usually using titanium welding wire, under controlled atmosphere conditions. This is a costly and time consuming process, and, at best, produces an electrode which lacks homogeneity and may include weld defects. 
     Typically, the electrode is melted in a vacuum arc furnace using a water-cooled copper crucible having a diameter slightly larger than that of the electrode. The resulting ingot is non-homogeneous due to the non-homogeneous electrode and to the lack of stirring in the arc melting process, and it usually contains unmelted or partially melted granules of some starting materials. The ingot is used as an electrode for VAR in a water-cooled copper crucible again having a diameter slightly larger than that of the electrode to produce a second-stage ingot. This ingot is the electrode for a third stage VAR process which produces a final triple-melted ingot. Ingots of titanium alloys, made by the VAR process, may be as large as 16,000 pounds, and although they are clean, due to vacuum melting, and free of porosity and pipe, due to the hot-topping capability of arc melting, they are non-homogeneous and may still contain oxygen and nitrogen enriched inclusions due to the welding required to produce the starting electrode. Moreover, vacuum arc melting and VAR limits alloy compositions due to the difficulty of alloying some materials by mechanically mixing and the lack of stirring during arc melting. 
     The foregoing illustrates limitations known to exist in present methods for producing large ingots of reactive metal alloys. Thus it would be advantageous to provide an alternative directed to overcoming one or more of those limitations. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter. 
     SUMMARY OF THE INVENTION 
     In one aspect of the present invention, this is accomplished by making a homogeneous electrode of reactive metal alloy, comprising an axially disposed unitary core made from one induction melted heat of said reactive metal alloy and having a diameter “d” and a length “L”; and a body having an outer diameter at least 2 times “d” and length “L”, said body being disposed about said core, and comprising at least one induction melted heat of said reactive metal alloy, said homogeneous electrode being vacuum arc remelted to produce a homogeneous ingot. 
     The foregoing and other aspects of the invention will become apparent from the following detailed description, when considered in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a flow chart showing the steps for making an electrode from multiple heats of induction melted reactive metal alloy and for producing an ingot by vacuum arc remelting of said electrode according to a first embodiment of the invention; 
     FIG. 2 is an schematic sectional elevation view illustrating a mold, atop an electrical contact stub, containing a single heat of molten reactive metal alloy to form an arc melting electrode core according to the invention; 
     FIG. 3 is a schematic sectional elevation view of a mold containing a solidified electrode core, as formed in FIG. 2, and a single molten heat of induction melted reactive metal alloy to form a first portion of the electrode body at the desired finished diameter; 
     FIG. 4 is a schematic sectional elevation view of a completed electrode as formed from 5 heats of induction melted reactive metal alloy; 
     FIG. 5 is a flow chart showing the steps for making an electrode from multiple heats of induction melted reactive metal alloy for use in vacuum arc remelting to form an ingot according to an alternative embodiment of the invention; 
     FIG. 6 is a schematic sectional elevation view of a mold containing a solidified electrode core, as formed in FIG. 2, and a single molten heat of induction melted reactive metal alloy to form a first layer of the electrode body according to said alternative embodiment; 
     FIGS. 7,  8 , and  9  illustrate molds containing the electrode core, the solidified layers of the electrode body formed in the previous figures, and a single molten heat of induction melted reactive metal alloy to form the second, third, and fourth layers, respectively, of the electrode body; 
     FIG. 10 is a schematic sectional elevation view of a completed electrode formed from 5 heats of induction melted alloy according to the alternative embodiment of the invention; and 
     FIG. 11 is a schematic sectional elevation view of a homogeneous ingot made by vacuum arc remelting of an electrode produced, according to the invention, from a plurality of heats of induction skull melted reactive metal alloy. 
    
    
     DETAILED DESCRIPTION 
     Consolidation of multiple heats of induction skull melted (ISM) reactive metal alloys (for example, titanium and zirconium-based alloys) is often necessary in order to provide castings or ingots which are too large to be made from a single ISM heat. To successfully accomplish such consolidation, the preferred process uses VAR with a homogeneous electrode made by combining several ISM heats of the alloy. The flow chart shown in FIG. 1 illustrates a method for making a homogeneous electrode, from several ISM heats of a reactive metal alloy, for VAR. An electrical connecting stub  15  is placed at the bottom of a first mold  10  with a diameter “d” and a length greater than “L”, and a first induction melted heat  310  of the alloy is poured into the mold. The stub  15  has a diameter somewhat greater than the diameter “d” of the mold cavity. It also has an axially extending pin (preferably ˜2″ long) with a small (˜1″) diameter to serve as a means for, as a minimum, mechanically joining the stub to the ingot by solidification shrinkage, and the same composition as the primary constituent of the reactive metal alloy. Thus, for titanium based alloys, the stub is pure titanium. The stub  15  is partially fused by the molten alloy during casting in the mold  10 , such that, when removed from the mold, the resulting ingot  110  has a diameter “d”, a length “L”, and a stub  15  with a diameter “d∓Δd” fused and/or mechanically bonded to one axial end. The alloy ingot  110  forms a continuous core of the alloy about which the electrode is to be formed. The ingot  110  is placed in a second mold  100  with the stub  15  down, and a second heat  320  of the alloy is poured into the mold, followed by a third heat  330 , a fourth heat  340 , and a fifth heat  350  to form a body of length “L” about the core  110 . This produces an ingot  160  which is the vacuum arc electrode. The second through fifth heats bond, by fusion and/or mechanical compression from the solidification shrinkage of the molten metal, to each other and to the surface of the core  110  to provide a unitary, homogeneous, and defect free ingot  160 . The ingot is the electrode which is used in the VAR process  400  to make the final consolidated ingot  500 . Note that the description refers to a five heat combination; however, it should also be noted that fewer or more heats than five can be consolidated by the process of the invention. The number of heats required for any electrode is determined by the intended product, the alloy, the size of the induction skull melting crucible, and the range of lengths and diameters of electrodes which can be melted in the VAR equipment. 
     FIG. 2 shows a sectional view of the first mold  10  with the stub  15  at the bottom and the molten first ISM heat  310 . FIG. 3 shows the solid ingot  110  from the first mold  10  placed in a second mold  100  with the second molten ISM heat  320  poured about it. FIG. 4 shows a sectional view of the solid finished electrode  160 . The core  110 , stub  15 , and the four body portions  120 ,  130 ,  140 , and  150  are firmly bonded by limited fusion during casting or by combined fusion and mechanical compression bonding due to shrinkage during solidification. This results in an electrode with excellent strength, electrical continuity, cleanliness, and homogeneity. 
     The block diagram of FIG. 5 shows an alternative process for forming a homogeneous electrode from five ISM heats of reactive metal alloy and its use in VAR to make a final ingot  500 . An electrical connection stub  15 , with a diameter “d∓Δd” and an axially extending pin, as described above, is placed in a first mold  10 , of a diameter “d”. Note that the length of all molds of the process is greater than the desired length “L” of the finished electrode. A first ISM heat of reactive metal alloy  310  is poured in the mold and, when removed, yields an ingot  110  of length “L” and diameter “d” with the stub  15  fused and/or mechanically bonded to one axial end. This is placed in a second mold  20  of diameter “2d”, and a second ISM heat  320  is poured around the ingot  110  to form a second ingot  270  having the stub  15  at one axial end, a length “L”, and a diameter “2d”. Ingot  270  is placed in a third mold  30  of diameter “3d” and a third ISM heat  330  of reactive metal alloy is poured into the mold around the ingot. The resulting solid ingot  280  is placed in a fourth mold  40  of diameter “2d” and a fourth ISM heat  340  of alloy is poured into the mold to form a fourth ingot  290 . Ingot  290  is placed in a mold  50  of diameter “5d”, and a fifth ISM heat of alloy is poured in the mold about the ingot  290  and allowed to solidify to yield a fifth ingot  260  of diameter “5d” which, with its contact stub, forms the VAR electrode. The electrode  260  is used in the VAR process  400  to make the final ingot  500 . 
     This process is less desirable than that of FIG. 1; because it requires three more molds and significantly more labor than does the first process. In some cases it may, however, be preferred depending on the required finished length of the VAR electrode. Very long electrodes, due to the axial continuity of the body layers, would favor the alternative process, while electrodes of normal length would favor the first process with its radial continuity in the body layers. 
     The stages of the electrode fabricated by the process of FIG. 5 are illustrated in FIGS.  1  and  6 - 10 , all of which are sectional elevation views of the electrode as it is built up from the core  110  formed in the mold  10  from heat  310  in FIG. 1 to the finished multilayer electrode formed by that core and the four layers of body formed about it. Thus, FIGS. 6,  7 ,  8 , and  9  show the first ingot  110 , the second ingot  270 , the third ingot  280 , and the fourth ingot  290  in molds  20 ,  30 ,  40 , and  50 , respectively, with the molten second  320 , third  330 , fourth  340 , and fifth  350  ISM heats poured about the ingots. The resulting fifth ingot  260  is shown in FIG. 10 to comprise the core  110 , with the electrical connection stub  15  at the lower axial end, and the four body portions  220 ,  230 ,  240 , and  250  layered about the core. The ingot  260  is the electrode for the VAR process in which the final consolidated ingot  500  is produced. 
     The ingot shown in FIG. 11 results from VAR of an electrode made by either of the methods of the invention it is homogeneous and can be made several times larger than ingots which could otherwise be made from single heats of ISM metal. It, therefore, enables production of large fabrications and forgings. 
     The invention combines the ISM and VAR processes in such a manner as to virtually eliminate the limitations of each process in the unique combination process disclosed. The ingot core produced with the first ISM heat is continuous, both mechanically and electrically, and is relatively long; thereby providing a sturdy core about which the subsequently melted heats of alloy are poured to form the final ingot or electrode body with no need for welding or the potential defects associated with welding of reactive metals. This is an important advantage, since many reactive metal alloys, e.g., TiAl, have poor or no weldability. The pure stub (titanium—for titanium-based alloys; zirconium—for zirconium-based alloys; etc.) provides a means for making positive electrical contact without welding. Finally, the ingot chemistry produced by the combined ISM and VAR processes is very uniform, due to stirring action of ISM, and free of porosity and pipe, due to hot-top capability of VAR. The result is a large multi-heat ingot having the uniformity attributable to ISM and the size attributable to VAR. 
     Although the foregoing discussion relates to ISM, as the preferred induction melting process for melting reactive metal alloys; other induction melting processes may be equally well suited for some alloys in some less critical applications. Of course, such applications are contemplated in the appended claims.