CASTING SYSTEMS AND PROCESSES WITH AN OFF-CENTER INCOMING SOURCE OF LIQUID METAL

Abstract: A casting system comprises an off-center incoming source (300) of refined liquid metal that is provided to a casting (145) being formed by the casting system. The casting comprising a liquidus portion (148) that receives the refined liquid metal and a solidified portion, the casting further comprising a fine-grain, homogeneous microstructure that is essentially oxide- and sulfide-free and segregation defect free. The casting system comprises a source of refined liquid metal, the refined liquid metal having oxides and sulfides refined out of the metal; and a nucleated casting system for forming the casting. The off-center incoming source of refined liquid metal is provided to the nucleated casting system off-set from a centerline (350) of the casting to disperse heat contained in the refined liquid metal throughout the liquidus portion thereby reducing a depth of the liquidus portion of the casting.

Description: CASTING SYSTEMS AND PROCESSES WITH AN OFF-CENTER INCOMING SOURCE OF LIQUID METAL This application claims priority of a provisional application entitled "Clean Metal Nucleated Casting Systems and Methods" by Benz, et al, US Serial No. 60/121,187, filed February 23, 1999. BACKGROUND OF THE INVENTION The invention relates to a casting systems and processes. In particular, the invention relates to casting systems and processes with an off-center incoming source of liquid metal for clean melt nucleated casting systems and processes. Metals, such as iron- (Fe), nickel- (Ni), titanium- (Ti), and cobalt- (Co) based alloys, are often used in turbine component applications, in which fine-grained microstructures, homogeneity, and essentially defect-free compositions are desired. Problems in superalloy castings and ingots are undesirable as the costs associated with superalloy formation are high, and results of these problems, especially in ingots formed into turbine components are undesirable. Conventional systems for producing castings have attempted to reduce the amount of impurities, contaminants, and other constituents, which may produce undesirable consequences in a casting made from the casting. However, the processing and refining of relatively large bodies of metal, such as superalloys, is often accompanied by problems in achieving homogeneous, defect- free structure. These problems are believed to be due, at least in part, to the bulky volume of the metal body. Known casting systems include electroslag refining (ESR) and electroslag refining with a cold induction guide (CIG) structure. Electroslag refining systems are known in industry, such as disclosed in US Patent Nos. 5,160,532; 5,310,165; 5,325,906; 5,332,197; 5,348,566; 5,366,206; 5,472,177; 5,480,097; 5,769,151; 5,809,057; and 5,810,066, all of which are assigned to the Assignee of the instant invention. While these electroslag refining systems are very effective for forming castings, they do not include anything to control the depth of the liquidus portion of the casting. One such problem that often arises in superalloys comprises controlling the depth of the liquidus portion of a casting (often known in the art as a "melt pool"), regardless of the casting system used to form the casting. For example, the grain size and microstructure of the casting may be dependent on the depth of the liquidus portion of the casting during solidification. A deep liquidus portion may cause ingredient macrosegregation that could lead to a less desirable microstructure, such as a microstructure that is not a fine-grained microstructure, or segregation of the elemental species so as to form an inhomogeneous structure. A subsequent processing operation has been proposed in combination with the electroslag refining process to overcome this deep melt pool problem. This subsequent processing may comprise vacuum arc remelting (VAR). Vacuum arc remelting is initiated when an ingot is processed by vacuum arc steps to produce a relatively shallow melt pool, whereby an improved microstructure, which may also possess a lower hydrogen content, is produced. Following the vacuum arc refining process, the resulting ingot is then mechanically worked to yield a metal stock having a desirable fine-grained microstructure. Such mechanical working may involve a combination of steps of forging and drawing. This thermo-mechanical processing requires large, expensive equipment, as well as costly amounts of energy input. An attempt to provide a desirable casting microstructure has been proposed in US Patent No. 5,381,847. US Patent No. 5,381,847 discloses a vertical casting process that attempts to control grain microstructure by controlling dendritic growth. The process may be able to provide a useable microstructure for some applications, however, the vertical casting process of US Patent No. 5,381,847 does not control the depth of the liquidus portion. Therefore, a need exists to provide metal casting process that controls the depth of the liquidus portion of the casting. Controlling the depth of the liquidus portion of the casting should produce a casting that comprises a relatively homogeneous, fine-grained microstructure. Further, a need exists to provide a metal casting system that that controls the depth of the liquidus portion of the casting to produce a casting with a relatively homogeneous, fine-grained microstructure. Further, a need exists to provide a metal casting process and system that that controls the depth of the liquidus portion of the casting to produce a casting that is essentially free of oxides, for turbine component applications. SUMMARY OF THE INVENTION An aspect of the invention sets forth a casting system. The casting system comprises an off-center incoming source of refined liquid metal that is provided to a casting being formed by the casting system. The casting comprising a liquidus portion that receives the refined liquid metal and a solidified portion, the casting further comprising a fine-grain, homogeneous microstructure that is essentially oxide- and sulfide-free and segregation defect free. The casting system comprises a source of refined liquid metal, the refined liquid metal having oxides and sulfides refined out of the metal; and a nucleated casting system for forming the casting. The off-center incoming source of refined liquid metal is provided to the nucleated casting system off-set from a centerline of the casting to disperse heat contained in the refined liquid metal throughout the liquidus portion thereby reducing a depth of the liquidus portion of the casting. In another aspect of the invention, a casting process is provided with an off-center incoming source of refined liquid metal, which is provided to a casting formed by the casting process. The casting comprising a liquidus portion that receives the refined liquid metal and a solidified portion, the casting further comprising a fine- grain, homogeneous microstructure that is essentially oxide- and sulfide-free and segregation defect free. The casting process comprises providing a source of refined liquid metal, the refined liquid metal having oxides and sulfides refined out of the metal; and forming a casting by nucleated casting. The step of providing a source of refined liquid metal comprises providing an off-center incoming source of refined liquid metal off-set from a centerline of the casting so as to disperse heat contained in the refined liquid metal throughout the liquidus portion thereby reducing a depth of the liquidus portion of the casting. These and other aspects, advantages and salient features of the invention will become apparent from the following detailed description, which, when taken in conjunction with the annexed drawings, where like parts are designated by like reference characters throughout the drawings, disclose embodiments of the invention. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic generalized illustration of a casting system with an off-center incoming source of liquid metal that is provided to the casting, as embodied by the invention; Figure 2 is a schematic illustration of a clean metal nucleated casting system with an electroslag refining system and nucleated casting system for producing a casting; Figure 3 is a partial schematic, vertical sectional illustration of the clean metal nucleated casting system, as illustrated in Fig. 1, that illustrates details of the electroslag refining system; Figure 4 is a partial schematic, vertical section illustration in detail of the electroslag refining system of the clean metal nucleated casting system for producing a casting; Figure 5 is a partial schematic, part sectional illustration of the electroslag refining system of the clean metal nucleated casting system for producing a casting; and Figure 6 is a casting system comprising an off-center incoming source of liquid metal that is provided to the casting, in which the casting is formed by a nucleated casting system. DESCRIPTION OF THE INVENTION A casting system and process for producing castings articles controls the depth of the liquidus portion of the casting by providing an off-center incoming source of liquid metal to the casting. The off-center incoming source of liquid metal to the casting provides less heat to the center of the liquidus portion of the casting. The less heat at the center of the p liquidus portion occurs since the source liquid metal will generally be at a higher temperature than the remainder of the casting, in which the remainder of the casting comprises both the liquidus portion in the casting and the solidified portion of the casting. The lower temperatures at the remaining portions of the casting is resultant of heat loss from the source liquid when it is introduced into the mold for solidification. Controlling of the casting's liquidus portion depth should produce a casting that comprises a relatively homogeneous, finegrained microstructure. Further, the controlling of the casting's liquidus portion depth should provide the casting with a relatively homogeneous, fine-grained microstructure that is essentially free of oxides, for turbine component applications. The term "essentially free" means that any constituents in the material do not adversely influence the material, for example its strength and related characteristics. The casting system with an off-center incoming source of liquid metal can comprise any appropriate casting system. The casting system may comprise an electroslag refining system (ESR); and electroslag refining system with a cold induction guide (CIG) system, an ESR/CIG system that provides refined liquid metal to a nucleated (vertical) casting system; any source of refined metal that is supplied to a nucleated casting system, and any other appropriate type of casting system. The following description will discuss exemplary casting systems with an off-center incoming source of liquid metal that is provided to the casting, as embodied by the invention, however, they are not meant to limit the invention in any manner. For example, a non-limiting casting system with an off-center incoming source of liquid metal can comprise an electroslag refining system in cooperation with a cold-induction guide (CIG), for example as set forth in the above- mentioned patents to the Assignee of the instant invention. An off-center incoming source of liquid metal that is provided to the casting, as embodied by the invention, can originate from any appropriate refined metal source, as discussed above. The refined metal source for the off-center incoming source of liquid metal may comprise a vacuum arc remelting (VAR) system, a vacuum induction melting (VIR) system, an electroslag refining (ESR) system (as discussed above) with or without a cold induction guide (CIG) system, and other systems that pertain to the purification of crude or impure metals. Another non-limiting nucleated casting system with an off-center incoming source of liquid metal that is provided to the casting can also comprise a system that permits a plurality of molten metal droplets to be formed and pass through a cooling zone, which is formed with a length sufficient to allow up to about 30 volume percent of each of the droplets to solidify on average. The droplets are then received by a mold and solidification of the metal droplets is completed in the mold. The droplets retain liquid characteristics and readily flow within the mold, when less than about 30 volume percent of the droplets is solid. The above sources are merely exemplary, and are not intended to limit the invention in any manner. The casting system with an off-center incoming source of liquid metal forms a homogeneous, fine-grained microstructure in a casting, in which the casting may be formed from many metals and alloys, including but not limited to nickel- (Ni) and cobalt- (Co) based superalloys, iron- (Fe), titanium- (Ti), alloys, which are often used in turbine component applications. The casting can be converted into a final article, a billet, or directly forged with reduced processing and heat treatment steps, due to their homogeneous, fine-grained microstructure. Accordingly, the casting can be used in many applications, such as but not limited to rotating equipment applications, such as, but not limited to, disks, rotors, blades, vanes, wheel, buckets, rings, shafts, wheels, and other such elements, and other turbine component applications. The following description will refer to turbine components formed from castings, however, this is merely exemplary of the applications within the scope of the invention. Figure 1 illustrates a non-limiting semi-schematic, part-sectional, elevational view of an exemplary casting system 3, for example a clean metal nucleated casting system 3, which comprises an electroslag refining system 1 and a nucleated casting system 2. The off-center incoming source of liquid metal that is provided to the casting in the casting system 3 will first be described in general terms with reference to Fig. 1, followed by a description of an exemplary electroslag refining system 1 and an exemplary nucleated casting system 2 to facilitate the understanding of the invention. Figure 1 schematically illustrates a casting system with a generalized off-center incoming source of liquid metal that is provided to a casting, as embodied by the invention. In Fig. 1, an off-center incoming source 300 of liquid metal, which will be provided to a liquidus portion 148 of a casting 145. The off-center incoming source 300 is provided in the form of a spray 138, such as, but not limited to, a spray formed by an atomizer assembly, as known in the art. The source of liquid metal is provided off-center from a centerline 350 of the casting 145. The off-center positioning of the source of liquid metal will dispose the source metal away from the middle of the liquidus portion of the casting, where the depth is normally greatest. Therefore, the heat that is contained in the incoming source liquid metal will be dispersed, will not be directly disposed at the center of the liquidus portion 148, and thus, less heat will be presented to the liquidus portion 148 at its area of greatest depth. The heat from the incoming source of liquid metal will be spread out throughout the liquidus portion 148 rather than at its center. The casting 145 may be formed in a rotating mold 146, for example a mold that rotates in either direction indicated by double arrow 600. The rotating mold 146 will present major portions of the entire liquidus portion 148 to the off-center source of liquid metal. Thus, the heat present in the incoming liquid metal heat will be further distributed throughout the liquidus portion 148 of the casting 145. This distribution will further enhance the solidification of the liquidus portion 148 of the casting into a solid. A further description of features of the casting system with an off-center incoming source liquid metal, as embodied by the invention, will be described hereinafter with regard to exemplary embodiments of the invention. Figure 2 illustrates an exemplary casting system 3, as embodied by the invention. In Fig. 2, the clean, refined, liquid metal for the clean metal nucleated casting system 3 and its associated clean metal nucleated casting processes can be provided by an electroslag refining system 1. The refined liquid metal can be fed to a nucleated casting system 2. The electroslag refining system 1 and nucleated casting system 2 cooperate to form a clean metal nucleated casting system 3, which in turn forms a casting. Figures 2-4 illustrate details of features in Fig. 1. The following description of the invention will discuss a clean metal nucleated casting system, however, this description is merely exemplary and is not intended to limit the invention in any manner. The partially molten/partially solidified metal droplets (referred to hereinafter as "semisolid droplets") collect in a mold 146. The mold may comprise a unitary and one-piece mold having side and bottom walls, as illustrated in the broken lines of Fig. 2. Alternatively, the mold may comprises a withdrawal mold, which includes a retractable base 246 that can be withdrawn from sidewalls of the mold 146. The following description of the invention will discuss a withdrawal mold as an exemplary, non-limiting mold, and is not intended to limit the invention in any manner. The retractable base 246 can be connected to a shaft 241 to move base away from the sidewalls in the direction of arrow 242. Further, the shaft 241 may rotate the retractable base 246 in the direction of arrow 243 to provide most portions of the mold to a cooling system, which is described hereinafter. The semisolid droplets behave like a liquid if the solid volume fraction portion is less than a viscosity inflection point, and the semisolid droplets exhibit sufficient fluidity to conform to the shape of the mold. Generally, an upper solid volume fraction portion limit that defines a viscosity inflection point is less than about 40% by volume. An exemplary solid volume fraction portion is in a range from about 5% to about 40%, and a solid volume fraction portion in a range from about 15% to about 30% by volume does not adversely influence the viscosity inflection point. The electroslag refining system 1 introduces a consumable electrode 24 of metal to be refined directly into an electroslag refining system 1, and refines the consumable electrode 24 to produce a clean, refined metal melt 46 (hereafter "clean metal"). The source of metal for the electroslag refining system 1 as a consumable electrode 24 is merely exemplary, and the scope of the invention comprises, but is not limited to, the source metal comprising an ingot, melt of metal, powder metal, and combinations thereof. The description of the invention will refer to a consumable electrode, however this is merely exemplary and is not intended to limit the invention in any manner. The clean metal 46 is received and retained within a cold hearth structure 40 that is mounted below the electroslag refining apparatus 1. The clean metal 46 is dispensed from the cold hearth structure 40 through a cold finger orifice structure 80 that is mounted and disposed below the cold hearth structure 40. The electroslag refining system 1 with an off-center incoming source of liquid metal that is provided to the casting can provide essentially steady state operation in supplying clean metal 46 if the rate of electroslag refining of metal and rate of delivery of refined metal to a cold hearth structure 40 approximates the rate at which molten metal 46 is drained from the cold hearth structure 40 through an orifice 81 of the cold finger orifice structure 80. Thus, the clean metal nucleated casting process can operate continuously for an extended period of time and, accordingly, can process a large bulk of metal. Alternatively, the clean metal nucleated casting process can be operated intermittently by intermittent operation of one or more of the features of the clean metal nucleated casting system 3. Once the clean metal 46 exits the electroslag refining system 1 through the cold finger orifice structure 80, it can enter the nucleated casting system 2. Then, the clean metal 46 can be further processed to produce a relatively large casting of refined metal. Alternatively, the clean metal 46 may be processed through to produce smaller castings, ingots, articles, or formed into continuous cast articles. The clean metal nucleated casting effectively eliminates many further refining and processing operations, such as those described above that, until now, have been necessary in order to produce a metal casting having a desired set of material characteristics and properties. In Fig. 2, a vertical motion control apparatus 10 is schematically illustrated. The vertical motion control apparatus 10 comprises a box 12 mounted to a vertical support 14 that includes a motive device (not illustrated), such as but not limited to a motor or other mechanism. The motive device is adapted to impart rotary motion to a screw member 16. An ingot support structure 20 comprises a member, such as but not limited to a member 22, that is threadedly engaged at one end to the screw member 16. The member 22 supports the consumable electrode 24 at its other end by an appropriate connection, such as, but not limited to, a bolt 26. An electroslag refining structure 30 comprises a reservoir 32 that can be cooled by an appropriate coolant, such as, but not limited to, water. The reservoir 32 comprises a molten slag 34, in which an excess of the slag 34 is illustrated as the solid slag granules 36. The slag composition used in the clean metal nucleated casting process will vary with the metal being processed. A slag skull 75 may be formed along inside surfaces of an inner wall 82 of reservoir 32, due to the cooling influence of the coolant flowing against the outside of inner wall 82, as described hereinafter. A cold hearth structure 40 (Figs. 2-4) is mounted below the electroslag refining structure 30. The cold hearth structure 40 comprises a hearth 42, which is cooled by an appropriate coolant, such as water. The hearth 42 contains a skull 44 of solidified refined metal and a body 46 of refined liquid metal. The reservoir 32 may be formed integrally with the hearth 42. Alternatively, the reservoir 32 and hearth 42 may be formed as separate units, which are connected to form the electroslag refining system 1. A bottom orifice 81 of the electroslag refining system 1 is provided in the cold finger orifice structure 80, which is described with reference to Figs. 3 and 4. A clean metal 46, which is refined by the electroslag refining system 1 so as to be essentially free of oxides, sulfides, and other impurities, can traverse the electroslag refining system 1 and flow out of the orifice 81 of the cold finger orifice structure 80. A power supply structure 70 can supply electric refining current to the electroslag refining system 1. The power supply structure 70 can comprise an electric power supply and control mechanism 74. An electrical conductor 76 that is able to carry current to the member 22 and, in turn, carry current to the consumable electrode 24 connects the power supply structure 70 to the member 22. A conductor 78 is connected to the reservoir 32 to complete a circuit for the power supply structure 70 of the electroslag refining system 1. Figure 2 illustrates a part-sectional view of the electroslag refining structure 30 and the cold hearth structure 40 in which the electroslag refining structure 30 defines an upper portion of the reservoir 32 and the cold hearth structure 40 defines a lower portion 42 of the reservoir 32. The reservoir 32 generally comprises a double- walled reservoir, which includes an inner wall 82 and outer wall 84. A coolant 86, such as but not limited to water, is provided between the inner wall 82 and outer wall 84. The coolant 86 can flow to and through a flow channel, which is defined between the inner wall 82 and outer wall 84 from a supply 98 (Fig. 4) and through conventional inlets and outlets (not illustrated in the figures). The cooling water 86 that cools the wall 82 of the cold hearth structure 40 provides cooling to the electroslag refining structure 30 and the cold hearth structure 40 to cause the skull 44 to form on the inner surface of the cold hearth structure 40. The coolant 86 is not essential for operation of the electroslag refining system 1, clean metal nucleated casting system 3, or electroslag refining structure 30. Cooling may insure that the liquid metal 46 does not contact and attack the inner wall 82, which may cause some dissolution from the wall 82 and contaminate the liquid metal 46. In Fig. 3, the cold hearth structure 40 also comprises an outer wall 88, which " may include flanged tubular sections, 90 and 92. Two flanged tubular sections 90 and 92 are illustrated in the bottom portion of Fig. 3. The outer wall 88 cooperates with the nucleated casting system 2 to form a controlled atmosphere environment 140, which is described hereinafter. The cold hearth structure 40 comprises a cold finger orifice structure 80 that is shown detail Figs. 4 and 5. The cold finger orifice structure 80 is illustrated in Fig. 4 in relation to the cold hearth structure 40 and a stream 56 of liquid melt 46 that exits the cold hearth structure 40 through the cold finger orifice structure 80. The cold finger orifice structure 80 is illustrated (Figs. 3 and 4) in structural cooperation with the solid metal skull 44 and liquid metal 46. Figure 5 illustrates the cold finger orifice structure 80 without the liquid metal or solid metal skull, so details of the cold finger orifice structure 80 are illustrated. The cold finger orifice structure 80 comprises the orifice 81 from which processed molten metal 46 is able to flow in the form of a stream 56. The cold finger orifice structure 80 is connected to the cold hearth structure 40 and the cold hearth structure 30. Therefore, the cold hearth structure 40 allows processed and generally impurity-free alloy to form the skulls 44 and 83 by contacting walls of the cold hearth structure 40. The skulls 44 and 83 thus act as a container for the molten metal 46. Additionally, the skull 83 (Fig. 4), which is formed at the cold finger orifice structure 80, is controllable in terms of its thickness, and is typically formed with a smaller thickness than the skull 44. The thicker skull 44 contacts the cold hearth structure 40 and the thinner skull 83 contacts the cold finger orifice structure 80, and the skulls 44 and 83 are in contact with each other to form an essentially continuous skull. A controlled amount of heat may be provided to the skull 83 and thermally transmitted to the liquid metal body 46. The heat is provided from induction heating coils 85 that are disposed around the cold hearth structure. An induction-heating coil 85 can comprise a cooled induction-heating coil, by flow of an appropriate coolant, such as water, into it from a supply 87. Induction heating power is supplied from a power source 89, which is schematically illustrated in Fig. 4. The construction of the cold finger orifice structure 80 permits heating by induction energy to penetrate the cold finger orifice structure 80 and heat the liquid metal 46 and skull 83, and maintain the orifice 81 open so that the stream 56 may flow out of the orifice 81. The orifice may be closed by solidification of the stream 56 of liquid metal 46 if heating power is not applied to the cold finger orifice structure 80. The heating is dependent on each of the fingers of the cold finger orifice structure 80 being insulated from the adjoining fingers, for example being insulated by an air or gas gap or by a suitable insulating material. The cold finger orifice structure 80 is illustrated in Fig. 5, with both skulls 44 and 83 and the molten metal 46 are omitted for clarity. An individual cold finger 97 is separated from each adjoining finger, such as finger 92, by a gap 94. The gap 94 may be provided and filled with an insulating material, such as, but not limited to, a ceramic material or insulating gas. Thus, the molten metal 46 (not illustrated) that is disposed within the cold finger orifice structure 80 does not leak out through the gaps, because the skull 83 creates a bridge over the cold fingers and prevents passage of liquid metal 46 therethrough. Each gap extends to the bottom of the cold finger orifice structure 80, as illustrated in Fig. 5, which illustrates a gap 99 aligned with a viewer's line-of-sight. The gaps can be provided with a width in a range from about of 20 mils to about 50 mils, which is sufficient to provide an insulated separation of respective adjacent fingers. The individual fingers may be provided with a coolant, such as water, by passing coolant into a conduit 96 from a suitable coolant source (not shown). The coolant is then passed around and through a manifold 98 to the individual cooling tubes, such as cooling tube 100. Coolant that exits the cooling tube 100 flows between an outside surface of the cooling tube 100 and an inside surface of a finger. The coolant is then collected in a manifold 102, and passed out of the cold finger orifice structure 80 through a water outlet tube 104. This individual cold finger water supply tube arrangement allows for cooling of the cold finger orifice structure 80 as a whole. The amount of heating or cooling that is provided through the cold finger orifice structure 80 to the skulls 44 and 83, as well as to the liquid metal 46, can be controlled to control the passage of liquid metal 46 through the orifice 81 as a stream 56. The controlled heating or cooling is done by controlling the amount of current and coolant that pass in the induction coils 85 to and through the cold finger orifice structure 80. The controlled heating or cooling can increase or decrease the thickness of the skulls 44 and 83, and to open or close the orifice 81, or to reduce or increase the passage of the stream 56 through the orifice 81. More or less liquid metal 46 can pass through the cold finger orifice structure 80 into the orifice 81 to define the stream 56 by increasing or decreasing the thickness of the skulls 44 and 83. The flow of the stream 56 can be maintained at a desirable balance, by controlling coolant water and heating current and power to and through the induction heating coil 85 to maintain the orifice 81 at a set passage size along with controlling the thickness of the skulls 44 and 83. The operation of the electroslag refining system 1 of the clean metal nucleated casting system 3 with an off-center incoming source liquid metal, as embodied by the invention, will now be described with reference to the figures. The electroslag refining system 1 of the clean metal nucleated casting system 3 can refine ingots that can include defects and impurities or that can be relatively refined. A consumable electrode 24 is melted by the electroslag refining system 1. The consumable electrode 24 is mounted in the electroslag refining system 1 in contact with molten slag in the electroslag refining system. Electrical power is provided to the electroslag refining system and ingot. The power causes melting of the ingot at a surface where it contacts the molten slag and the formation of molten drops of metal. The molten drops to fall through the molten slag. The drops are collected after they pass through the molten slag as a body of refined liquid metal in the cold hearth structure 40 below the electroslag refining structure 30. Oxides, sulfides, contaminants, and other impurities that originate in the consumable electrode 24 are removed as the droplets form on the surface of the ingot and pass through the molten slag. The molten drops are drained from the electroslag refining system 1 at the orifice 81 in the cold finger orifice structure 80 as a stream 56. The stream 56 that exits the electroslag refining system 1 of the clean metal nucleated casting system 3 that forms castings comprises a refined melt that is essentially free of oxides, sulfides, contaminants, and other impurities. The rate at which the metal stream 56 exits the cold finger orifice structure 80 can further be controlled by controlling a hydrostatic head of liquid metal 46 above the orifice 81. The liquid metal 46 and slag 44 and 83 that extend above the orifice 81 of the cold finger orifice structure 80 define the hydrostatic head. If a clean metal nucleated casting system 3 with an electroslag refining system 1 is operated with a given constant hydrostatic head and a constant sized orifice 81 , an essentially constant flow rate of liquid metal can be established. Typically, a steady state of power is desired so the melt rate is generally equal to the removal rate from the clean metal nucleated casting system 3, as a stream 56. However, the current applied to the clean metal nucleated casting system 3 can be adjusted to provide more or less liquid metal 46 and slag 44 and 83 above the orifice 81. The amount of liquid metal 46 and slag 44 and 83 above the orifice 81 is determined by the power that melts the ingot, and the cooling of the electroslag refining system 1, which create the skulls. By adjusting the applied current, flow through the orifice 81 can be controlled. Also, the contact of the consumable electrode 24 with an upper surface of the molten slag 34 can be maintained in order to establish a steady state of operation. A rate of consumable electrode 24 descent into the melt 46 can be adjusted to ensure that contact of the consumable electrode 24 with the upper surface of the molten slag 34 is maintained for the steady state operation. Thus, a steady-state discharge from the stream 56 can be maintained in the clean metal nucleated casting system 3. The stream 56 of metal that is formed in the electroslag refining system 1 of the clean metal nucleated casting system 3 exits electroslag refining system 1 and is fed to a nucleated casting system 2. The nucleated casting system 2 is schematically illustrated in Fig. 2 in cooperation with the electroslag refining system 1. The nucleated casting system 2 that acts to form articles comprises a disruption site 134 that is positioned to receive the stream 56 from the electroslag refining system 1 of the clean metal nucleated casting system 3. The disruption site 134 converts the stream 56 into a plurality of molten metal droplets 138. The stream 56 is fed to disruption site 134 in a controlled atmosphere environment 140 that is sufficient to prevent substantial and undesired oxidation of the droplets 138. The controlled atmosphere environment 140 may include any gas or combination of gases, which do not react with the metal of the stream 56. For example, if the stream 56 comprises aluminum or magnesium, the controlled atmosphere environment 140 presents an environment that prevents the droplets 138 from becoming a fire hazard. Typically, any noble gas or nitrogen is suitable for use in the controlled atmosphere environment 140 because these gases are generally non-reactive with most metals and alloys within the scope of the invention. For example, nitrogen, which is a low-cost gas, can be in the controlled atmosphere environment 140, except for metals and alloys that are prone to excessive nitriding. Also, if the metal comprises copper, the controlled atmosphere environment 140 may comprise nitrogen, argon, and mixtures thereof. If the metal comprises nickel or steel, the controlled atmosphere environment 140 can comprises nitrogen or argon, or mixtures thereof. The disruption site 134 can comprise any suitable device for converting the stream 56 into droplets 138, which are provided as an off-center incoming source of liquid metal that to the liquidus portion 148 of the casting 145. For example, the disruption site 134 can comprise a gas atomizer, which circumscribes the stream 56 with one or more jets 142. The flow of gas from the jets 142 that impinge on the stream can be controlled, so the size and velocity of the droplets 138 can be controlled. Another atomizing device, within the scope of the invention, includes a high pressure atomizing gas in the form of a stream of the gas, which is used to form the controlled atmosphere environment 140. The stream of controlled atmosphere environment 140 gas can impinge the metal stream 56 to convert the metal stream 56 into droplets 138. Other exemplary types of stream disruption include magneto- hydrodynamic atomization, in which the stream 56 flows through a narrow gap between two electrodes that are connected to a DC power supply with a magnet perpendicular to the electric field, and mechanical-type stream disruption devices. The droplets 138 are broadcast downward (Fig. 2) from the disruption site 134 to form a generally diverging cone shape. The droplets 138 traverse a cooling zone 144, which is defined by the distance between the disruption site 134 and the upper surface 150 of the metal casting 145 in the mold 146. The cooling zone 144 length is sufficient to solidify a volume fraction portion of a droplet by the time the droplet traverses the cooling zone 144 and impacts the upper surface 150 of the metal casting 145 off-center from the centerline 350. The portion of the droplet 138 that solidifies (hereinafter referred to as the "solid volume fraction portion") is sufficient to inhibit coarse dendritic growth in the mold 146 up to a viscosity inflection point at which liquid flow characteristics in the mold are essentially lost. The partially molten/partially solidified metal droplets (referred to hereinafter as "semisolid droplets") are disposed off-center in the liquidus portion 148 of the casting 145 in the mold 146. The semisolid droplets behave like a liquid if the solid volume fraction portion is less than a viscosity inflection point, and the semisolid droplets exhibit sufficient fluidity to conform to the shape of the mold. Generally, an upper solid volume fraction portion limit that defines a viscosity inflection point is less than about 40% by volume. An exemplary solid volume fraction portion is in a range from about 5% to about 40%, and a solid volume fraction portion in a range from about 15% to about 30% by volume does not adversely influence the viscosity inflection point. The off-center incoming source of liquid metal that is provided to the liquidus portion 148 of the casting 145 provides less heat to the center of the liquidus portion 148 of the casting 145, since the source liquid metal will generally be at a higher temperature than the remainder of the casting 145. The lower temperatures at the remaining portions of the casting 145 are resultant from a heat loss from the source liquid when it is introduced into the mold 146 for solidification. Controlling of the casting's liquidus portion 148 depth should produce a casting 145 that comprises a relatively homogeneous, fine-grained microstructure. Further, the controlling of the casting's liquidus portion depth should provide the casting 145 with a relatively homogeneous, fine-grained microstructure that is essentially free of oxides, for turbine component applications. The spray of droplets 138 creates a turbulent zone 148 at the surface of the casting in the mold 146. The turbulent zone 148 can have an approximate depth in the mold 146 in a range from about 0.005 inches to about 1.0 inches. The depth of the turbulent zone 148 is dependent on various clean metal nucleated casting system 3 factors, including, but not limited to, the atomization gas velocity, droplet velocity, the cooling zone 144 length, the stream temperature, and droplet size. An exemplary turbulent zone 148 within the scope of invention comprises a depth in a range from about 0.25 to about 0.50 inches in the mold. In general, the turbulent zone 148 in the mold 146 should not be greater that a region of the casting, where the metal exhibits predominantly liquid characteristics. Typically, a lower viscosity in turbulent zone 148 minimizes gas entrapment and resultant pores in the casting. If the solid volume fraction portion of the average droplet, which is solid in the turbulent zone 148, is less than about 50% by volume, gas entrapment in the casting is minimized. For example, if the solid volume fraction portion of the average droplet, which is solid in the turbulent zone 148, is in a range from about 5% to about 40% by volume, gas entrapment in the casting is minimized. The mold 146 extracts heat from the casting by thermal conduction through the mold 146 walls and by convection off of the top surface 150 of the casting. The turbulent zone 148 reduces a thermal gradient of the casting by the inherent turbulent nature in the turbulent zone 148. The reduction of the thermal gradients reduces hot tears and dendritic coarsening of the casting, both of which are undesirable in castings. Of course, as described above, the off-center incoming source of liquid metal that is provided to the casting 145 reduces the total heat present proximate the centerline 350 of the liquidus portion 148 of the casting 145, and thus enhances its cooling and the solidification of the liquidus portion 148. The mold 146 can be formed of any suitable material for casting applications, such as but not limited to, graphite, cast iron, and copper. Graphite is a suitable mold 146 material since it is relatively easy to machine and exhibits satisfactory thermal conductivity for heat removal purposes. Cooling coils that can be embedded in the mold to circulate a coolant may enhance the removal of heat through the mold 146. The scope of the invention comprises other means for cooling the mold, as known in the art. The mold 146 may not need as much thermal protection as in conventional molds, since the semisolid droplets are already partially solidified. Thus, some heat has already been removed from the semisolid droplets to partially solidify them and less heat needs to be removed when the semisolid droplets are in the mold, compared to conventional castings formed entirely from liquid metals. Decreased heat removal can reduce thermally induced distortion of the mold 146, and this can lead to uniform heat removal rates from the casting to enhance casting uniformity and homogeneity. As the mold 146 is filled with semisolid droplets 138, its upper surface 150 may move closer to the disruption site 134, and the cooling zone 144 is reduced. At least one of the disruption site 134 or the mold 146 may be mounted on a moveable support and separated at a fixed rate to maintain a constant cooling zone 144 dimension, while maintaining the off-center incoming source of liquid metal provided to the casting 145. Thus, a generally consistent solid volume fraction portion in the droplets 138 is formed. Baffles 152 may be provided in the nucleated casting system 2 to extend the controlled atmosphere environment 140 from the electroslag refining system 1 to the mold 146. The baffles 152 can prevent oxidation of the partially molten metal droplets 138 and conserve the controlled atmosphere environment gas 140. Heat is extracted from the casting to complete the solidification process and to form castings 145. Sufficient nuclei are formed in the casting produced by the clean metal nucleated casting process so that upon solidification, a fine equiaxed microstructure 149 can be formed in the casting and the resultant article. Porosity and hot working cracking are reduced or substantially eliminated by the clean metal nucleated casting process, which includes clean metal produced by the electroslag refining system 1 and the controlled microstructure casting formed by the nucleated casting system 2. Figure 6 illustrates another exemplary casting system 600. The casting system 600 comprises an off-center incoming source of liquid metal that is provided to the castingl45, as embodied by the invention. The casting system 600 of Figure 6 comprises a nucleated casting system 2, as described above, with any appropriate source of refined metal for the source, which is provided as an off-center incoming source of liquid metal to the liquidus portion 148 of the casting 145. For example, and in no way limiting of the invention, the source of liquid metal may be formed into a spray 138 by a disruption device 134, as described above. The spray 138 is introduced to the liquidus portion 148 of the casting 145 off-center from the centerline 350, for the reasons discussed above. Other features of the casting system 600 are as set forth in the above description of the invention. The casting system with the off-center incoming source of liquid metal that is provided to the casting inhibits undesirable dendritic growth, reduces solidification shrinkage porosity of the formed casting and article, and reduces hot tearing both during casting and during subsequent hot working of the casting and casting. Further, casting system produces a uniform, equiaxed structure in the casting, which is a result of the minimal distortion of the mold during casting, the controlled transfer of heat during solidification of the casting in the mold, and controlled nucleation. The casting system with an off-center incoming source liquid metal, as embodied by the invention, enhances ductility and fracture toughness of the article compared to conventionally castings. While various embodiments are described herein, it will be appreciated from the specification that various combinations of elements, variations or improvements therein may be made by those skilled in the art, and are within the scope of the invention.

Claims: WE CLAIM: 1. A casting system with an off-center incoming source of refined liquid metal that is provided to a casting that is being formed by the casting system, the casting comprising a liquidus portion that receives the refined liquid metal and a solidified portion, the casting further comprising a fine-grain, homogeneous microstructure that is essentially oxide- and sulfide-free and segregation defect free, the casting system comprising: a source of refined liquid metal, the refined liquid metal having oxides and sulfides refined out of the metal; and a nucleated casting system for forming the casting, the casting comprising a fine-grain, homogeneous microstructure that is essentially oxide- and sulfide-free and segregation defect free, the casting further comprising a centerline; wherein the source of refined liquid metal comprises an off-center incoming source of refined liquid metal, the off-center incoming source of refined liquid metal being provided to the nucleated casting system for forming a casting, the off-center incoming source of refined liquid metal being provided off-set from the centerline so as to disperse heat contained in the refined liquid metal throughout the liquidus portion thereby reducing a depth of the liquidus portion of the casting. 2. The casting system according to claim 1, wherein the source of refined liquid metal comprises an electroslag refining system. 3. The casting system according to claim 2, wherein the electroslag refining system comprises: an electroslag refining structure that is adapted for the electroslag refining of the source of refined liquid metal and providing molten slag; a cold hearth structure for holding a refined molten metal beneath the molten slag and providing refined molten metal in the cold hearth structure; a source of raw metal for insertion into the electroslag refining structure and into contact with the molten slag in the electroslag refining structure; an electrical power supply adapted to supply electric power to electroslag refine the source of raw metal through a circuit, the circuit comprising the power supply, the source of raw metal, the molten slag and the electroslag refining structure sufficient for resistance melting the source of raw metal where the source of raw metal contacts the molten slag and forming molten droplets of refined liquid metal; an outlet for allowing the molten droplets to fall through the molten slag; a collector for collecting the molten droplets after they pass through the molten slag as a body of refined liquid metal in the cold hearth structure directly below the electroslag refining structure; a cold finger orifice structure having an orifice at the lower portion of the cold hearth structure for draining the electroslag refined metal that collects in the cold hearth orifice structure through the orifice of the cold finger orifice structure. 4. The casting system according to claim 3, wherein the source of metal comprises an alloy selected from at least one of nickel-, cobalt-, titanium-, or iron- based metals, and the casting formed by the clean metal nucleated casting process comprises at least one of nickel-, cobalt-, titanium-, or iron-based metals. 5. The casting system according to claim 3, wherein a rate of advance of the source of metal into the refining structure corresponds to the rate at which a lower end of the ingot is melted by the resistance melting. 6. The casting system according to claim 3, wherein the orifice comprises forms a stream of molten metal. 7. The casting system according to claim 3, wherein the electroslag refining structure and the cold hearth structure comprise upper and lower portions of the same structure. 8. The casting system according to claim 3, wherein the electrical power supply comprises a circuit formed in the refined liquid metal. 9. The casting system according to claim 3, wherein the orifice establishes a drainage rate that is approximately equivalent to a rate of resistance melting. 10. The casting system according to claim 1, wherein the nucleated casting system further comprises: a disruption site for disrupting the source of refined liquid metal into molten metal droplets, wherein the molten metal droplets can be partially solidified such that, on average, from about 5% to about 40% by volume of each droplet is solid and the remainder of each droplet is molten; and a mold for collecting and solidifying the partially solidified droplets for forming the casting, in which a turbulent zone is generated by the droplets being collected at an upper surface of the mold and, the step of collecting, and, on average the droplets being solidified less than about 50% by volume. 11. The casting system according to claim 10, wherein on average about 5% to about 40% by volume of the droplet solidifies. 12. The casting system according to claim 9, wherein the disruption site comprises at least one atomizing gas jet impinging the refined liquid metal. 13. The casting system according to claim 1, wherein the casting comprises at least one of an ingot and preform. 14. The casting system according to claim 1, wherein the casting comprises at least one of nickel-, cobalt-, titanium-, or iron-based metals. 15. The casting system according to claim 1, wherein the casting is capable for use in turbine component applications. 16. The casting system according to claim 1, wherein the source of raw metal is selected from at least one of a consumable electrode, a powdered source of metal, and melt source of metal. 17. A casting process with an off-center incoming source of refined liquid metal that is provided to a casting that is being formed by the casting process, the casting comprising a liquidus portion that receives the refined liquid metal and a solidified portion, the casting further comprising a fine-grain, homogeneous microstructure that is essentially oxide- and sulfide-free and segregation defect free, the casting process comprising: providing a source of refined liquid metal, the refined liquid metal having oxides and sulfides refined out of the metal; and forming a casting by nucleated casting, wherein the step of providing a source of refined liquid metal comprises providing an off-center incoming source of refined liquid metal, the off-center incoming source of refined liquid metal being provided to the nucleated casting system for forming a casting, the off-center incoming source of refined liquid metal being provided off-set from a centerline of the casting so as to disperse heat contained in the refined liquid metal throughout the liquidus portion thereby reducing a depth of the liquidus portion of the casting. 18. The process according to claim 17, wherein the step of providing a source of refined liquid metal comprises electroslag refining, the step of electroslag refining comprises: providing a source of raw metal to be refined; providing an electroslag refining structure adapted for the electroslag refining of the source of metal and providing molten slag in the vessel; providing a cold hearth structure for holding a refined molten metal beneath the molten slag and providing refined molten metal in the cold hearth structure; mounting the source of metal for insertion into the electroslag refining structure and into contact with the molten slag in the electroslag refining structure; providing an electrical power supply adapted to supply electric power; supplying electric power to electroslag refine the source of metal through a circuit, the circuit comprising the power supply, the source of metal, the molten slag and the electroslag refining structure; resistance melting of the source of metal where the source of metal contacts the molten slag and forming molten droplets of metal; allowing the molten droplets to fall through the molten slag; collecting the molten droplets after they pass through the molten slag as a body of refined liquid metal in the cold hearth structure directly below the electroslag refining structure; providing a cold finger orifice structure having a orifice at the lower portion of the cold hearth structure; and draining the electroslag refined metal that collects in the cold hearth orifice structure through the orifice of the cold finger orifice structure. 19. The process according to claim 18, wherein the source of raw metal comprises an alloy selected from at least one of nickel-, cobalt-, titanium-, or iron- based metals, and the casting formed by the nucleated casting process comprises at least one of nickel-, cobalt-, titanium-, or iron-based metals. 20. The process according to claim 18, wherein a rate of advance of the source of raw metal into the refining structure corresponds to the rate at which of resistance melting. 21. The process according to claim 18, wherein the step of draining comprises forming a stream of molten metal. 22. The process according to claim 18, wherein the electroslag refining structure and the cold hearth structure comprise upper and lower portions of the same structure. 23. The process according to claim 18, wherein the step of supplying electric power comprises forming a circuit in the refined liquid metal. 24. The process according to claim 18, wherein the step of draining comprises establishing a drainage rate that is approximately equivalent to a rate of resistance melting. 25. The process according to claim 17, wherein the step of forming a casting further comprises: disrupting a stream of refined liquid metal from the source of refined liquid metal into molten metal droplets; partially solidifying the molten metal droplets such that, on average, from about 5% to about 40% by volume of each droplet is solid and the remainder of each droplet is molten; and collecting and solidifying the partially solidified droplets in a mold forming the casting, in which a turbulent zone is generated by the droplets at an upper surface and, the step of collecting and solidifying the partially solidified droplets collects the droplets in the turbulent zone, and, on average solidifies less than about 50% by volume of the droplet. 26. The process according to claim 25, wherein the step of partially solidifying the molten metal droplets solidifies, on the average, from about 15% to about 30% by volume of the droplet. 27. The process according to claim 25, wherein the step of collecting and solidifying the partially solidified droplets comprises collecting and solidifying about 5% to about 40% by volume of the droplet. 28. The process according to claim 25, wherein the step of disrupting comprises impinging at least one atomizing gas jet on the stream.

Inventors: BENZ MARK GILBERT, CARTER WILLIAM THOMAS JR, HUGO FRANZ WALDEMAR ERNOT, KNUDSEN BRUCE ALAN, MULLER FELIX GUENTER, ZABALA ROBERT JOHN, BENZ, MARK, GILBERT, CARTER, WILLIAM, THOMAS, JR., HUGO, FRANZ, WALDEMAR, ERNOT, KNUDSEN, BRUCE, ALAN, MULLER, FELIX, GUENTER, ZABALA, ROBERT, JOHN
Applicants: GEN ELECTRIC, GENERAL ELECTRIC COMPANY
Application year: 2000
Main classification: B22D37
IPC codes: B22D21, B22D23, B22D27, B22D37, B22F3, C23C4