Patent Publication Number: US-4546815-A

Title: Continuous casting using in-line replaceable orifices

Description:
DESCRIPTION 
     1. Field of the Invention 
     This invention relates to a method and apparatus for continuously forming a solid structure from a melt. It is particularly adapted for long-term continuous casting of metallic ribbon. 
     2. Description of the Prior Art 
     A common manufacturing goal is to move from a batch process to a continuous process to a long-term continuous process. The goal is particularly important in situations where high costs are associated with process startup and/or shutdown. 
     Metal casting is a process in which cost is impacted by the amount of material that can be produced continuously, without shutdown. Specifically, methods for making metal strip directly from the molten metal are known. These may involve, for example, jetting molten metal through an orifice and cooling it, either in free flight or by contact with a chill body, to obtain continuous filament. Some of these processes involve jetting of molten metal through an orifice under pressure. Typically, the molten metal is pressurized in a crucible, which has a bottom outlet in the form of an orifice in the shape of the desired cross section of the metal jet. Usually, the orifice of the casting nozzle is of small size, on the order of about 0.2 mm to about 1.0 mm in diameter, and tends to plug easily during operation. If a plug forms, casting of useable product ceases. Even in systems that incorporate replaceable nozzles (see e.g., U.S. Pat. Nos. 4,154,380; and 4,433,715), casting cannot begin again until a new nozzle is installed and the startup procedure is followed. Furthermore, plugging is only one of several nozzle failure modes. Consequently, a casting system that can operate continuously for periods longer than the useful lifetime of a casting nozzle is clearly desirable. 
     Methods for casting multiple layers of ribbon are known. For example, U.S. Pat. No. 4,229,231 issued Oct. 21, 1980, to Witt et al., discloses a melt-spinning technique that involves an array of orifices that successively deposit melt onto a moving surface to yield a multi-layered structure. 
     U.S. Pat. No. 4,428,416, issued Jan. 31, 1984, to Shimanuki et al., discloses a method of manufacturing multi-layer laminates of different metals or alloys, particularly where one of the layers is an amorphous alloy. 
     Neither Witt et al. nor Shimanuki et al. considered using a multi-orifice casting system to produce a single layer; on the contrary, it was precisely their goal to prepare multiple layers in a continuous process. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, an apparatus for continuously forming a solid structure comprises: 
     (a) a plurality of vessels, each having an opening at one end in fluid communication with an orifice at another end, with the opening in each vessel adjoining the opening in each adjacent vessel, 
     (b) means for directing a continuous stream of molten material and for moving the vessels relative to the stream, whereby the stream successively passes 
     (i) into the opening and through the orifice of a first vessel only, 
     (ii) into the openings and through the orifices of both the first vessel and a second vessel, and 
     (iii) into the opening and through the orifice of the second vessel only, and 
     (c) means for chilling the molten material that emerges from the orifices to effect quenching and solidification. 
     In an alternative embodiment, the apparatus comprises: 
     (a) a vessel that is adapted for holding molten material and that has an opening at one end and is adapted for fluid communication with a plurality of orifices at another end, 
     (b) means for controlling the flow of molten material out of the vessel and through the orifices to successively provide a flow of molten material from 
     (i) a first orifice only, 
     (ii) the first orifice and an adjacent second orifice, and 
     (iii) the second orifice only, and 
     (c) means for chilling the molten material that emerges from the orifices to effect quenching and solidification. 
     In operation, a method is provided for continuously forming a solid structure, comprising the steps of: 
     (a) providing a continuous stream of molten material, 
     (b) providing a first vessel and a second vessel, each vessel having an opening at one end and an orifice at another end and having their openings adjoining each other, 
     (c) directing the stream toward the opening in the first vessel and moving the vessels relative to the stream, whereby the stream successively passes 
     (i) into the opening and through the orifice of the first vessel only, 
     (ii) into the openings and through the orifices of both the first and second vessels, and 
     (iii) into the opening and through the orifice of the second vessel only, and 
     (d) chilling the molten material that emerges from the orifices to effect quenching and solidification of the material. 
     An alternative method comprises the steps of: 
     (a) providing molten material in a vessel that has an opening at one end and is adapted for fluid communication with a plurality of orifices at another end, 
     (b) controlling the flow of molten material out of the vessel and through the orifices to successively provide a flow of molten material from 
     (i) a first orifice only, 
     (ii) the first orifice and an adjacent second orifice, and 
     (iii) the second orifice only, and 
     (c) chilling the molten material that emerges from the orifices to effect quenching and solidification. 
     The process and apparatus of the present invention permit a casting orifice to be replaced without interrupting the casting process. Thus, they permit a casting process to continue for a period of time that exceeds the lifetime of a single casting orifice. In principle, the casting process may continue indefinitely, simply by replacing each orifice before there is a substantial likelihood of its failing (based on prior experience with orifice lifetimes). 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 depicts an embodiment of the present invention that makes use of a plurality of vessels. 
     FIG. 2 depicts a cross section through the bottom of a vessel of FIG. 1. 
     FIG. 3 depicts another embodiment involving a plurality of vessels. 
     FIG. 4 depicts an embodiment of the present invention that makes use of a single vessel and a plurality of orifices. 
     FIG. 5a and 5b depict two successive stages of casting with the apparatus of FIG. 4. 
     FIG. 6 depicts the nozzles of the apparatus of FIG. 4. 
     FIG. 7 depicts another embodiment of the invention that uses a single vessel and a plurality of orifices. 
     FIG. 8 is a cross section through an embodiment of the apparatus of FIG. 7. 
     FIG. 9 is a sectional view of another embodiment of the apparatus of this invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention provides a casting apparatus and a method that overcome the limitation that orifice lifetime has heretofore placed on the maximum duration of continuous casting. That limitation is overcome by permitting the replacement of a casting orifice without interrupting the casting process. To simplify the explanation set forth below, the invention is described primarily in terms of metallic ribbon casting. However, as is clear to one skilled in the art, the process and apparatus need be limited neither to metals nor to ribbon. Thus, it is suitable generally for use with molten materials (metal or non-metal) that can be solidified by cooling. The solid may be in continuous form, such as fiber, wire, filament, etc.; or particulate form, such as flakes, powder, splats, etc. 
     Among the metal casting processes for which the present invention is well suited is the &#34;planar flow casting&#34; (PFC) method disclosed in U.S. Pat. No. 4,142,571, issued Mar. 6, 1979, to Narasimhan. That patent discloses an apparatus comprising a movable chill body, a slotted nozzle in communication with a reservoir for holding molten metal, and means for effecting expulsion of the molten metal from the reservoir, through the nozzle orifice, and onto the moving chill surface. The slotted nozzle is located in close proximity to the chill surface. The nozzle orifice of the PFC method is susceptible, as are orifices of the earlier-disclosed casting processes, to plugging problems. For PFC, just as for the other casting processes, the present invention permits longer-duration casting, without interruption for shutdown and startup. 
     FIG. 1 displays an embodiment of the present invention, as applied to PFC. The chill body is an endless belt 1 that is placed over rolls 2 and 3, which are caused to rotate by external means (not shown). A stream of molten metal 4 is provided from a source (not shown). A plurality of vessels 5, 6, 7, and 8 periodically move under the stream of metal in the direction shown by the arrow. (Alternatively, they could also move in the opposite direction). Preferably, stream 4 has a substantially rectangular cross section, with long and short sides parallel, respectively, to the long and short sides of a rectangular opening at the top of each vessel. Because of surface tension in the molten metal, achieving a rectangular cross section generally requires that the source of molten metal have a rectangular outlet and be located close to the vessel opening. Square corners on the rectangular outlet ought to be avoided in order to minimize the likelihood of cracking from thermal shock. 
     Any conventional conveyer means (not shown) may serve to move the vessels. The vessels are in contact along the sides that extend substantially normal to the direction of motion. The stream is positioned over the vessels so that the entire stream is received first in one vessel (e.g., vessel 6), then in two vessels (e.g., at the instant shown, vessels 6 and 7), then again in one vessel (e.g., vessel 7). The stream that enters the opening at the top of a vessel is cast from an orifice at its bottom and chilled to form a solid structure. The molten metal that emerges from the orifices may be chilled into a particulate form by a gas jet (see A. Lawley, J. of Metals, January, 1981, p. 13). Preferably, the metal is quenched on a moving chill surface to form solid ribbon. Preferably, the molten metal is selected from among those alloys that can be chilled rapidly to a solid ribbon that is at least 50% amorphous (see e.g., U.S. Pat. No. 3,856,513), and the chill rate is suitably rapid. So long as the flow rate into the vessel(s) remains constant, the rate of casting remains substantially constant, although, periodically, casting is from two orifices, with material from one orifice depositing onto material that was deposited from another. After casting from a vessel (for example vessel 5) has been completed, that vessel can be removed, or several used vessels can be removed as a unit. Similarly, vessels can be added to the train, either individually or in a group, to follow vessel 8. 
     Although the vessels may move continuously, in the preferred embodiment the vessels remain stationary, and casting is through a single vessel for an extended period of time. The vessels are moved when product defects show that an orifice requires replacement, or, if no such defects appear, then after a time that is shorter than, but determined by, the mean orifice lifetime. The vessels then move until the next vessel is positioned under stream 4. Thus, the interval during which casting is from two orifices is generally a small fraction of the casting time and, if necessary, material cast during that interval may be discarded. If ribbon is wound on cores, transfer winding can provide continuous ribbon takeup on successive cores without interrupting the windup process, and a simple relay system can ensure that material to be discarded is at the end of a roll, where it can easily be removed. 
     If necessary, a vessel orifice (or the entire vessel) may be heated before molten material is to pass through it, to prevent thermal shock, which could crack the vessel and/or distort the orifice. Heating may be accomplished by any suitable method known in the art, such as radiant heating, convection heating (e.g., by a hot gas), etc. 
     FIG. 2 shows a cross section through the bottom of a vessel, showing narrow orifice 10. Orifice plugging is one of the failure mechanisms that is avoided by the use of this invention. In addition, all orifices that are elements of this invention avoid having sharp corners, where stress buildup can cause cracking. Also shown in FIG. 2 is a section of optional nozzle 11, from which a flame or hot gas can be directed at the bottom of the vessel in order to effect preheating of the orifice. 
     The chill surface depicted in FIG. 1 is an endless belt; however, the chill surface may alternatively be a wheel, such as 12 shown in FIG. 3. 
     FIG. 4 displays an exploded view of an embodiment of the present invention in which casting is from a single vessel 20. Vessel 20 has a bottom 21 that has a vessel opening 22. Mounted against bottom 21 is plate 23, which has a disk shape and has near its periphery a plurality of plate openings 24a, 24b, 24c, . . . Plate 23 is mounted so that it may be rotated about an axis that is normal to the disk. For clarity, plate 23 is shown separate from bottom 21, but, in reality, it is positioned below and in intimate contact with bottom 21. 
     As is shown in FIG. 5a, the plate openings 24a, 24b, 24c . . . are positioned so that as plate 23 rotates, each plate opening in turn is below vessel opening 22. The plate openings are smaller than the vessel opening and are closely spaced one-to-another. Thus, as plate 23 rotates, there is an interval during which parts of two adjoining plate openings are both positioned below vessel opening 22 to carry fluid from vessel 20 (see FIG. 5b). Sides s of plate openings 24 and sides S of vessel opening 22 are preferably substantially radial to provide uniform flow. Plate openings preferably have no sharp corners, where stress buildup can cause cracking. 
     Although, for clarity, they are not shown in FIGS. 4 and 5, a casting nozzle is mounted below each plate opening. Two nozzles, 25a and 25b, are shown in FIG. 6. The nozzles are mounted onto the bottom of plate 23 so as to be easily replaced and to permit close spacing. If necessary, nozzle supports, 26a and 26b for example, are provided for mechanical support. A cutaway section shows plate openings 24, against which the nozzles are mounted. The chill surface (not shown) for solidifying the material that is cast from the nozzle orifices can be of conventional form, such as the endless belt 1, shown in FIG. 1, or the chill roll 12, shown in FIG. 3. The orifices are optionally preheated using, for example, the nozzle 11 shown in FIG. 2. 
     In operation, molten material is contained in vessel 20, before being passed through vessel opening 22, a plate opening and its accompanying nozzle and nozzle orifice (e.g., 24a, 25a, and 27a) onto a chill surface. After a time that is shorter than, but determined by (from prior experience), the expected useful life of the nozzle, plate 23 is rotated so as to move plate opening 24b and accompaning nozzle 25b into place below vessel opening 22. During the rotation, there is an interval during which casting from two adjoining nozzle orifices takes place (see FIGS. 5a and 5b). Preferably, that interval is short compared with the total casting time and, if necessary, means can be provided to identify and, ultimately, discard material cast from two orifices. As plate 23 rotates, used nozzles can be replaced with new nozzles, thus permitting continuous casting for a time interval that is not limited by the lifetime of a typical nozzle. 
     Another apparatus for the practice of this invention is depicted schematically in FIG. 7, which is a top view into rotatably mounted vessel 20, showing multiple orifices 30a, 30b, . . . in nozzles attached to the bottom of the vessel. When molten material is in the vessel, casting is accomplished through nozzle orifice 30a onto endless belt 1 to form ribbon (not shown). At the instant shown in the Figure, a conventional stopper rod (40 in FIG. 8) prevents material from flowing into nozzle orifice 30b, as the orifice is being preheated by optional heater 31. At a predetermined time, or when defective product indicates premature nozzle failure, vessel 20 is rotated in a clockwise direction. The stopper rod is lifted to permit flow of molten material through nozzle orifice 30b, and nozzle 30a is cooled by a coolant supplied, for example, from optional blower nozzle 32. Alternatively, a second stopper rod may be lowered to stop flow through nozzle orifice 30a. In either case, material freezes in nozzle orifice 30a, plugging the orifice and preventing further flow through it. Until the material stops flowing from nozzle orifice 30a, material is briefly cast from both orifices 30a and 30b. As the crucible continues sequentially rotating in a clockwise fashion, the solidified material in the nozzle prevents molten material in the vessel from passing through the nozzle orifice. When the frozen nozzle again reaches the position identified in FIG. 7 as 30b, the frozen nozzle is replaced with a new nozzle, after a stopper rod is lowered into the crucible to prevent molten material flow during nozzle replacement. 
     FIG. 8 is a vertical section through the apparatus of FIG. 7 and depicts an embodiment of a stopper rod and replaceable nozzle arrangement that is suitable for the practice of this invention. Stopper rod 40 is shown lowered to the base of vessel 20, so that nozzle 41 may be replaced, while molten material 42 remains in the vessel. 
     FIG. 9 shows a cross section of an embodiment of the invention in which the vessel rotates about a horizontal axis. Vessel 50 has nozzles 51a, 51b, 51c, 51d, . . . mounted around its circumference. Shield 52 is a refractory material that prevents molten material 53 from being cast from any but the lowest nozzle orifice or, during rotation (as shown in FIG. 9), the lowest two orifices. Shield 52 fits against the vessel interior closely enough to prevent molten material from flowing into the gap between the shield and vessel, and it remains stationary as vessel 50 rotates sequentially. Nozzles are replaced at any convenient time after casting through them has finished. Additional material may be added to the vessel as casting depletes it. A conventional chill surface (not shown) serves to solidify the material that is ejected from the orifice(s) at the bottom of the crucible.