APPARATUS AND PROCESS FOR DIRECT CHILL CASTING OF METAL INGOTS

Abstract: In direct chill casting of aluminum alloys, and particularly the casting of ingots of large cross section, macrosegregation is significantly reduced and compositional uniformity is greatly improved across the formed ingot, if the casting is carried out using a flow directing device, such as a baffle (25) or flow velocity accelerator (50), adapted to direct a major portion of the molten metal feed centrally downwardly and into substantially the bottom of a liquid zone or sump (16) formed during the casting and then outwardly and upwardly along an interface (21) between the liquid sump (16) and surrounding solidified metal (20). This ensures that the hottest metal is fed downwardly to the central core of the ingot at the bottom of the sump.

Description: Apparatus and Process for Direct Chill Casting of Metal Ingots Technical Field This invention relates to an apparatus and process for improving internal macro and microstructures and homogeneity of metal ingots and, more particularly, to reducing macrosegregation in the central region of aluminum ingots produced by direct chill casting. Background Art The primary technique used today for producing aluminum ingots is direct chill (D.C.) casting. Direct chill casting is effected in an axially vertical mould which is initially closed at its lower end by a downwardly movable platen. Molten aluminum is introduced to the upper end of the mould, which is chilled by continuous supply of coolant fluid to its external surface, and as the molten metal solidifies in the region adjacent to the periphery of the mould, the platen is moved downwardly. With effectively continuous downward movement of the platen and correspondingly continuous supply of molten metal to the mould, there is produced an ingot of the desired length. For economy of scale, the thickness of these ingots keeps increasing such that only a few years ago an 18" ingot was considered large, while today ingots with a thickness of 26" to 30" are becoming commonplace. Macrosegregation is one parameter used to measure the properties of a finished ingot so as to determine its future usefulness. Changes in macrosegregation across commercial size castings, particularly aluminum alloy ingots produced by direct chill casting, make it difficult to maintain a particular concentration of alloying elements within specification throughout the entire cross- section or thickness of a casting. The degree of macrosegregation in a casting is determined to a large extent by the casting thickness, casting speed, the alloying elements and their concentration, and by the procedure used in casting the ingots. However, the most influential parameter affecting macrosegregation is the thickness or diameter of the ingot being cast, and as the thickness or diameter increases beyond 18", macrosegre¬ gation becomes an extremely serious problem. With these large dimension ingots, the macrosegregation is a direct result of unavoidable variations in the solidification brought about by varying heat extraction rates from location to location within the ingot cross-section and by convective forces in the liquid and mushy zones of the ingot. In the direct chill casting process for aluminum alloy ingots, it is customary to have a liquid zone or sump followed by a mushy zone which in turn is followed by a solid zone, all of which are arranged in a vertical orientation. The mould is water cooled and therefore the outer surface tends to solidify prior to the central portion adjacent to the longitudinal centre line. Thus, there is created in the upper end of the forming ingot a liquid zone or sump of molten metal surrounded by solidified metal. Dendrites which are lean in eutectic elements tend to grow about the periphery of the liquid zone or sump. The presence of strong convection currents within the liquid zone are believed to cause the tips of the dendrites to detach and be carried by the convection currents to the centre of the liquid zone. The use of grain refiners is also known to result in the formation of unattached crystals. As the dendrites move toward the centre, they grow isothermally within the thermal boundary layer and are finally frozen adjacent to the longitudinal centre line. Since the dendrites are lean in alloying elements, they cause the final product to exhibit a low concentration of alloying elements in its central portion. This low concentration causes a large variation in macrosegregation of the ingot itself which is undesirable for the reasons stated above. There have been various attempts to reduce macro- segregation adjacent to the longitudinal centre line of commercial size ingots. U.S. Patent No. 4,709,747, Yu et al, describes the use of a mechanical damper positioned within the liquid zone which is intended to control the magnitude of the flow currents within the liquid zone. It was hoped that this would reduce macrosegregation in the solidified casting. U.S. Patent 3,672,431, Bryson, describes the use of a baffle structure placed beneath the metal delivery tube to a mould, this baffle being adapted to direct a major flow of molten aluminum laterally and a minor portion of the molten flow downwardly. Another attempt at solving the problem can be found in U.S. Patent 3,506,059, Burkhart et al, in which a core member or displacer is positioned within the mould cavity vertically downwards to a location within the liquid zone after casting has commenced. Again in this case the displacer is intended to direct the incoming hot liquid metal to the periphery of the mould cavity. All of the above techniques have been found to have only a marginal effect on the problem of macrosegregation. It is the object of the present invention to provide a new apparatus and process which substantially reduces macrosegregation. Disclosure of the Invention This invention relates to an apparatus and process for reducing macrosegregation and generally improving the uniformity of an ingot formed by direct chill casting of metals, such as aluminum alloys. It has been found according to the present invention that very significant improvements can be achieved if a flow directing means is provided which directs a major portion of a molten metal feed centrally downwardly and into substantially the bottom of a liquid sump within the forming ingot, then preferably outwardly and upwardly along an interface between the liquid sump and surrounding solidified metal. This system works by ensuring that the hottest feed metal is fed to the central core of the ingot, and generally contrary to existing practices which result in the hottest metal being fed to the surface region of the ingot and dispersed laterally. The system of the present invention opposes the naturally occurring buoyancy driven flows which would normally occur in the direct chill casting of large ingots, and inhibits the transport of alloy depleted dendrites to the central core of the ingot. Additionally, hot fluid is supplied to those regions of the ingot where there is the greatest thickness of mushy zone, and thus is believed to steepen the local temperature gradient and decrease the mushy zone thickness. Thus, the apparatus of the present invention in its broadest aspect includes: (a) a mould having a wall defining an axially vertical casting zone with an open lower end; (b) means for feeding a flow of molten metal downwardly into an upper region of the casting zone; and (c) means for closing the lower end of the mould, this means being positioned and adapted to support the lower extremity of an ingot being cast in the casting zone, and being movable downwardly away from the mould for effecting continuous downward advance of the ingot during casting such that the portion of the ingot emerging from the mould includes a central liquid sump or zone surrounded by solidified metal. The inventive feature comprises flow directing means for directing a major portion of the molten metal feed centrally downwardly and into substantially the bottom of the liquid sump. This can be achieved in a number of different ways, such as a baffle or vertically movable dip tube for directing the molten metal feed to substantially the bottom of the liquid sump, or means may be provided for increasing the velocity of the molten metal feed sufficiently to force a major portion of the molten metal feed centrally downwardly through the liquid sump to substantially the bottom-thereof. The molten metal feed is also preferably directed from the bottom of the sump outwardly and upwardly along an interface between the liquid sump and surrounding solidified metal. When the baffle is used, it is positioned within the upper region of the casting zone and adapted to project downwardly into the liquid sump, this baffle means having substantially closed side walls and at least one bottom opening for directing a major portion of the molten metal feed centrally downwardly through the bottom opening and into substantially the bottom of the liquid sump and then outwardly and upwardly along an interface between the liquid sump and the surrounding solidified metal. The interface between the liquid sump and solidified metal generally extends in a downward and inward direction within the forming ingot and the baffle is preferably shaped to generally conform to the shape of the liquid sump. Accordingly, the baffle is preferably downwardly and inwardly tapered. It may be in the form of a fixed baffle, or an array of baffles, the position of which may be independently adjusted during casting to impose a preferred interface profile, dynamically. The baffle may be a solid metal, e.g. alloy steel, structure, but is preferably designed as a disposable unit with walls formed of glass cloth. This glass cloth may typically be woven glass fibre fabric screen which is impermeable to liquid metal passage in the present application in the absence of a pressure differential across the screen. The same type of fabric is permeable to liquid metal passage in the presence of such a pressure drop in the known application in a flowing stream of metal in e.g. a transfer trough used for screening such metal to remove coarse inclusions. According to one preferred embodiment, the baffle can be accordion folded and then deployed downwardly stagewise as the liquid sump forms in the start up of a direct chill casting procedure. In the casting of a large commercial ingot, the liquid sump may be in the order of two to three feet deep and the baffle-preferably extends down into a bottom region of the sump. The top of the baffle typically has a width approximately 75% of the width of the ingot, with the walls tapering inwardly generally parallel to the side of the liquid sump. It may be used with any of the usual shapes of casting moulds, including rectangular, square, oblong, round, etc. The baffle device of the invention may be pushed deeper and deeper into the ingot head during casting and in some instances it may be pushed to a position below the original solidus position. By pushing the baffle into the solidifying metal during casting, the liquidus and solidus isotherms dynamically adjust themselves to fit an imposed profile. Holes or slots may be provided in the side walls of the baffle and the bottom openings and side wall openings are preferably configured to ensure that flow through them is fast enough to reduce greatly the diffusive penetration of the sub-baffle cooling into the hotter liquid maintained above the baffle. Consequently, the baffle enables metal to be supplied close to the solidification front at temperatures much hotter, e.g. more than 20 β K hotter, than would normally occur. This has the effect of increasing or steepening the local temperature gradient ahead of and within the mushy (solid and liquid) region thereby decreasing the local solidification time. It is well known that the scale of the microstructure (secondary dendrite arm spacing and size of second phase particles) depends on local solidification time and the baffle arrangement of the present invention has profound effects on the structure. The baffle functions well whether it is made of heat conductive or insulating material. In either form, it serves to thermally isolate the liquid sump from the external cooling and it is found that there is typically a 50-70"C temperature difference within the liquid sump on each side of the baffle.- Thus, the molten metal is exposed to the full effect of the external cooling only when it has passed down through the baffle to the bottom of the liquid sump. In place of the above baffle, it is also possible to utilize a vertically movable trough and down spout or manifold, preferably having a changing cross-section, for directing a major portion of the molten metal feed centrally downwardly and into substantially the bottom of the liquid sump. With the vertically movable down spout, the formation of the ingot is commenced with the bottom end of the down spout in an upper position in the upper portion of a DC mould. After a steady state casting condition has been reached with a liquid sump fully formed, the lower end of the down spout is lowered down into and immersed in the liquid sump whereby the molten metal feed is directed to substantially the bottom of the liquid sump. The flow rate of the metal and the positioning of the down spout can dynamically adjust the liquidus and solidus isotherms to fit an imposed profile. As with the baffle described above, the vertically movable down spout enables the feed metal to be supplied close to the solidification front at temperatures much hotter than would normally occur. Another way of directing the molten metal feed to substantially the bottom of the liquid sump is to increase the velocity of the molten metal feed sufficiently to force a major portion of the molten metal feed centrally downwardly through the liquid sump to substantially the bottom thereof. One convenient way of achieving this is to provide an electromagnetic device surrounding a portion of the down spout. With this arrangement, the outlet of the down spout is positioned in an upper region of the DC casting mould. The electromagnetic device is not turned on until the casting has reached a steady state with a liquid sump fully formed. At that point, the device is turned on and is adjusted such that the liquid metal feed travels to substantially- the bottom of the liquid sump. Metal hotter than the average sump temperature will then impinge on the interface between the liquid sump and the surrounding solidified metal. The velocity of the liquid metal can be adjusted to maintain the liquidus and solidus isotherms within an imposed profile. While an electromagnetic velocity increasing means is particularly desirable, the velocity can be increased by other means such as hydraulic or pneumatic systems. The invention also relates to a novel product, namely an aluminum-magnesium alloy ingot of improved homogeneity. Thus, a direct chill cast aluminum-magnesium alloy ingot is obtained having a maximum variation in magnesium content across a section of the cast ingot between +5% and -5% of the mean magnesium content of the alloy. Brief Description of the Drawings Certain preferred embodiments of this invention are illustrated by the attached drawings in which: Figure 1 is a simplified sectional elevational view of a direct chill casting apparatus embodying the present invention in a particular form; Figure 2 is a top plan view of the baffle of the invention shown in Figure 1; Figure 3 is a simplified sectional elevational view of a direct chill casting apparatus embodying a tilting baffle unit according to the invention; Figure 4 is a perspective view of one half of the baffle shown in Figure 3; Figure 5 is a simplified sectional elevational view of a direct chill casting apparatus embodying a folding baffle according to the invention with the baffle fully folded; Figure 6 is a simplified sectional view of the arrangement of Figure 5 with the baffle partly unfolded; Figure 7 is a simplified sectional elevational view showing the baffle fully unfolded; Figure 8 is a simplified sectional elevational view of a casting apparatus with a vertically movable down spout; Figure 9 is the same view as Figure 8 with the down spout lowered; Figure 10 is a simplified section view of a casting apparatus with an electromagnetic velocity accelerator; and Figure 11 is a graph showing variations in magnesium content across the thickness of an ingot. Best Mode For Carrying Out the Invention Referring to Figure 1, a direct chill ingot casting mould 10 is shown for forming an elongated rectangular ingot 17. For all practical purposes, this moulding device can be any type of continuous or semi-continuous casting mould for producing elongated ingots having circular, oblong, square or rectangular cross-section. The mould includes a water cooling chamber 11, a mould face 12 and a cooling water discharge outlet 18. Molten aluminum feed 13 is fed from a launder 14 fed into the mould 10 via dip tube 15 and controlled by stopper rod 23. At the start of the casting operation, the lower end of the casting zone between the mould faces 12 is closed by a platen 19 supported by a hydraulic ram. As the molten aluminum in the casting zone solidifies adjacent the moulding face 12, the platform 19 is drawn slowly vertically downward by operation of the hydraulic ram and the solidifying base of the ingot being cast, resting on the platform, then begins to emerge from the lower end of the casting zone. Water spray from the outlets 18 is sprayed onto the emerging solidified ingot surface immediately below the moulding faces 12. This spray of water, striking the ingot surface, acts to enhance the cooling and consequent solidification of the ingot 17 as it moves downwardly away from the mould. As will be seen from Figure 1, in the region of the forming ingot within the casting zone and immediately below the casting zone, there is a sump or pool 16 of molten metal surrounded iy solidified metal 20. The interface 21 between the solidified metal 20 and the molten sump 16 tapers downwardly and inwardly because of the cooling action of the mould faces 12 and water spray 18. Below the region of the liquid sump 16 is the fully formed ingot 17. A feature of the present invention is the baffle arrangement 25 which is designed to direct the molten metal flowing out of dip tube 15 downwardly in an axial direction and through bottom outlet 27 such that it travels to the lower end of the liquid sump 16 and then travels upwardly outside the baffle and adjacent the interface 21. This baffle has inwardly sloping sidewalls 26 which are generally parallel to the interface 21 and end walls 28. It is preferably fabricated as a disposable unit having a steel frame with sidewalls 26 and end walls 28 formed of glass cloth. At the end of each cast, the baffle is pulled out of the liquid sump, e.g. by a winch, and the glass cloth portions are removed and replaced by fresh fabric. Figures 3 and 4 show an alternate form of baffle device in which the baffle 30 comprises two half sections 30a and 30b. Each of these sections 30a and 30b is pivotally mounted by arms 34 pivotally suspended from brackets 35. Each baffle half section includes a pair of sidewalls 31 and an end wall 32. The bottom is open and the walls are preferably tapered. At the start of the casting operation, the baffle sections 30a and 30b are swung up into the position shown by the dotted lines in Figure 3 and as a depth of molten sump begins to form, the baffles can gradually be tilted downwardly such that they eventually assume the position shown by the solid lines in Figure 3. In this position, the baffle is fully functional for the remainder of the casting of the ingot. This baffle can also be made disposable by building the sections with steel frames and glass cloth walls. Another form of baffle arrangement is shown in Figures 5 to 7. In this arrangement, the baffle 40 has flexible glass cloth fabric walls 41 connected by support rods 42 such that the entire baffle can be accordion folded within a container 43 as shown in Figure 5. Sections of the baffle can then be deployed as shown in Figures 6 and 7 as the liquid sump builds up to its maximum depth as shown in Figure 7. The deployment of the baffle in this manner can be fully automated by the use of solenoids to lower the baffle sections and controlling the solenoids by a computer. Another flow directing embodiment of the invention is shown in Figures 8 and 9. In this embodiment, a down spout or manifold 15 of substantial length is used and this distribution arrangement and launder 14 are vertically movable as indicated. Figure 8 shows the beginning of a casting with a liquid sump 16 beginning to form. As the molten metal in the casting zone solidifies adjacent the moulding face 12, the platform 19 is moved slowly vertically downward with the solidifying base of the ingot being cast resting on the platform. Eventually a steady state is reached as shown in Figure 9 with a relatively deep sump or pool 16 of molten metal surrounded by solidified metal 20. The interface 21 between the solidified metal 20 and the molten sump 16 tapers downwardly and inwardly because of the cooling action of the mould faces 12 and water spray. Below the region of the liquid sump 16 is the fully formed ingot 17. When the casting operation has reached the steady state as shown in Figure 9, the down spout 15 and launder 14 are lowered as shown such that the lower end of down spout 15 is deeply immersed within the liquid sump 16. In this location, the hot feed liquid is directed down through the down spout 15 and discharges in a lower region of the liquid sump 16. An example of a casting device of the invention with a flow accelerator is shown in Figure 10. This utilizes the same basic casting system as shown in Figures 8 and 9 with a down spout 15 and launder 14. However, in the embodiment of Figure 10, the down spout 15 is surrounded by an electromagnetic device 50 adapted to increase the velocity of the flow of molten metal downwardly through down spout 15. The casting operation with the electromagnetic device is commenced in the same manner as in Figure 8 with the outlet of the down spout 15 located in an upper region within the direct chill mould 10. With the electromagnetic device turned off, the casting operation is commenced with the platform 19 being lowered. When the casting operation has reached a steady state with a sump fully formed as shown in Figure 10, the device is turned on and the metal emerging from the down spout is given a sufficient velocity to force the molten metal feed downwardly through the liquid sump 16 to substantially the bottom thereof as shown in Figure 10. A preferred embodiment of the invention is illustrated by the following non-limiting example. Example An actual plant trial was conducted using a direct chill caster and baffle as shown in Figures 1 and 2. The mould was rectangular and was dimensioned to cast an ingot 635 mm by 1350 mm. The metal which was cast was an aluminum alloy AA3004 containing a nominal 1.15% Mg and typically used for beverage can bodies. Casting procedures were carried out both with and without the baffle and the results were compared as shown in Figure 11. It will be seen that without the baffle, the magnesium variation was sufficient to exceed the Aluminum Association limits for 3004 alloy. The measured variation was about +4% to -11% about the nominal 1.15% Mg. By comparison, the ingot cast with the baffle shown in Figures 1 and 2 exhibited a magnesium variation of about +1% to -5%, falling well within the Aluminum Association limits and close to the nominal 1.15% Mg across the entire cross-section of the ingot. While this invention has been described in conjunction with certain specific embodiments, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. For instance. Figures 1 to 10 are all based upon a rectangular mould with a corresponding rectangular shaped baffle. The invention functions equally well with moulds of square, circular etc. cross-sections. For instance, when the mould is circular, the baffle has a corresponding truncated conical shape. Without being bound by any single theory to explain the complex macrosegregational effects occurring in various DC cast ingots, the inventors believe that the unique combination of fluid flow and heat flux brought about by the system of the invention controls the major macrosegregational patterns which appear in DC cast ingots. Thus, it will be seen that the invention embraces all such modifications, alternatives and variations as fall within the spirit and broad scope of the appended claims.

Claims: Claims : 1. In an apparatus for continuously casting metal ingots, including: (a) a mould having a wall defining an axially vertical casting zone with an open lower end; (b) means for feeding a flow of molten metal downwardly into an upper region of said casting zone; and (c) means for closing the lower end of said mould, said means being positioned and adapted to support the lower extremity of an ingot being cast in said zone, and being movable downwardly away from said mould for effecting continuous downward advance of the ingot during casting such that the portion of the ingot emerging from the mould includes a central liquid sump surrounded by solidified metal; the improvement which comprises flow directing means for directing a major portion of said molten metal feed centrally downwardly and into substantially the bottom of said liquid sump to thereby ensure that the hottest metal feed is fed to the central core of the ingot. 2. An apparatus as claimed in Claim 1 wherein the flow directing means is also adapted to direct the molten metal feed from the bottom of the sump in an outward and upward direction along an interface between the liquid sump and surrounding solidified metal. 3. Apparatus as claimed in Claim 1 wherein the flow directing means comprises means for increasing the downward velocity of said molten metal feed sufficient to force a major portion of said molten metal feed centrally downwardly through said liquid sump to substantially the bottom thereof. 4. Apparatus as claimed in Claim 2 wherein the flow directing means comprises baffle means positioned within the upper region of said casting zone and adapted to project downwardly into said liquid sump, said baffle means having substantially closed side walls and at least one bottom opening, for directing a major portion of said molten metal feed centrally downwardly through said bottom opening and into substantially the bottom of said liquid sump and then outwardly and upwardly along an interface between the liquid sump and surrounding solidified metal. 5. Apparatus as claimed in Claim 4 wherein the baffle has downwardly and inwardly sloping side walls which are generally parallel to the interface between the liquid sump and solidified metal. 6. Apparatus as claimed in Claim 4 wherein the walls of the baffle are formed of a glass cloth. 7. Apparatus as claimed in Claim 1 wherein the flow directing means comprises an optionally vertically movable down spout or manifold of changing cross-section adapted to direct fluid flow downwardly into the central liquid sump when the sump has formed. 8. Apparatus as claimed in Claim 3 wherein the flow velocity increasing means is selected from hydraulic, pneumatic or electromagnetic velocity increasing means. 9. Apparatus as claimed in Claim 3 wherein the flow velocity increasing means comprises a down spout at least partially surrounded by an electromagnetic accelerating device. 10. Apparatus as claimed in Claim 9 wherein said device is a pump. 11. A process for continuously casting metal ingots including the steps of: (a) feeding molten metal in continuous downward flow into an upper region of an axially vertical casting zone of a mould wherein said molten metal collects and progressively solidifies to form an ingot, by first forming a central liquid sump surrounded by solidified metal and progressively solidifying until a fully solid ingot is formed, and (b) directing the flow of molten metal feed such that a major portion of said molten metal feed travels centrally downwardly to substantially the bottom of said liquid sump to thereby ensure that the hottest metal feed is fed to the central core of the ingot. 12. A process as claimed in Claim 11 wherein the flow of molten metal is directed from the bottom of the liquid sump in an outward and upward direction along an interface between the liquid sump and surrounding solidified metal. 13. A process as claimed in Claim 11 wherein the flow is directed by increasing the velocity of the molten metal feed sufficiently to cause a major portion of the molten metal feed to travel centrally downwardly to substantially the bottom of said liquid sump to thereby ensure that the hottest metal feed is fed to the central core of the ingot. 14. A process as claimed in Claim 11 wherein the flow is directed by physical baffle means of molten metal feed such that a major portion of said molten metal feed travels centrally downwardly to substantially the bottom of said liquid sump and thence upwardly along the periphery of the liquid sump. 15. A process as claimed in Claim 11 wherein the metal is aluminum or an alloy thereof. 16. A process as claimed in Claim 15 wherein the aluminum contains an alloying amount of magnesium. 17. A direct chill cast aluminum-magnesium alloy ingot having a maximum variation in magnesium content across a section of the cast ingot between +5% and -5% of the mean magnesium content of the alloy. 18. An ingot as claimed in Claim 17 wherein the mean magnesium content is 1.15% by weight.

Inventors: BRYSON NEIL BURTON, ENRIGHT PHILIP GRAHAM, FLOOD STEPHEN C, MCCUBBIN JOHN GORDON, READ CHARLES MARK, BRYSON, NEIL, BURTON, ENRIGHT, PHILIP, GRAHAM, FLOOD, STEPHEN, C., MCCUBBIN, JOHN, GORDON, READ, CHARLES, MARK
Applicants: ALCAN INT LTD, ALCAN INTERNATIONAL LIMITED
Application year: 1991
Main classification: B22D11
IPC codes: B22D11