Ingot Casting

Abstract

This specification discloses an open mould conveyer casting apparatus for forming a metal ingot including: a conveyer for conveying one or more ingot moulds from at least a first location where an ingot mould receives a molten metal having an exposed surface, to a second location where the molten metal has partially or completely solidified into a metal ingot; one or more magnetic field applicators configured to apply a varying magnetic field to the molten metal in the ingot mould between the first location and the second location, the magnetic field being of a magnetic field strength to induce stirring within the molten metal. This specification also discloses an open mould conveyer casting method for forming a metal ingot including: filling an ingot mould with a molten metal, the molten metal in the mould having an exposed surface; solidifying the molten metal to form the metal ingot; and applying a varying magnetic field to the molten metal of a magnetic field strength and frequency to induce stirring within the molten metal during the step of solidifying the molten metal.

Claims

1. An open mould casting method for forming a metal ingot including: filling an ingot mould with a molten metal, the molten metal in the mould having an exposed surface; solidifying the molten metal to form the metal ingot; and applying a varying magnetic field to the molten metal of a magnetic field strength and frequency to induce stirring within the molten metal during the step of solidifying the molten metal.

2. The method of claim 1, wherein the varying magnetic field is an oscillating magnetic field having a frequency of from about 1 to about 100 Hz.

3. The method of claim 2, wherein the oscillating magnetic field has a frequency of from about 20 to about 30 Hz.

4. The method of claim 1, wherein the magnetic field is applied by one or more permanent magnets and/or one or more electromagnets at an offset distance of about 5 mm to about 30 mm above the exposed surface.

5. The method of claim 4, wherein the offset distance is about 10 to about 20 mm.

6. The method of claim 1, wherein the magnetic field strength is at least about 0.3 T.

7. The method of claim 1, wherein the step of applying the magnetic field includes applying the magnetic field for a period of up to 20% of the total solidification time.

8. The method of claim 1, wherein the step of solidifying the molten metal to form the metal ingot further includes blowing air or a water mist on to the exposed surface to cool the exposed surface.

9. The method of claim 1, wherein the method is selected from a group consisting of: an open mould stationary casting method; an open mould direct chill continuous casting method; and an open mould conveyor casting method.

10. The method of claim 9, wherein the method is an open conveyor casting method and after the step of filling the ingot mould, the method further includes: conveying the ingot mould from at least a first location where the ingot mould received the molten metal, to a second location where the molten metal has partially or completely solidified into a metal ingot; and wherein the step of conveying the ingot mould includes transporting the ingot mould through one or more magnetic field zones to apply the varying magnetic field.

11-13. (canceled)

14. An open mould casting apparatus for forming a metal ingot including: an ingot mould for receiving a molten metal having an exposed surface; and one or more magnetic field applicators configured to apply a varying magnetic field to the molten metal in the ingot mould via the exposed surface, the magnetic field being of a magnetic field strength to induce stirring within the molten metal.

15. An open mould conveyer casting apparatus for forming a metal ingot including: a conveyer for conveying one or more ingot moulds from at least a first location where an ingot mould receives a molten metal having an exposed surface, to a second location where the molten metal has partially or completely solidified into a metal ingot; one or more magnetic field applicators configured to apply a varying magnetic field to the molten metal in the ingot mould via the exposed surface between the first location and the second location, the magnetic field being of a magnetic field strength to induce stirring within the molten metal.

16. The apparatus of claim 14, wherein the one or more magnetic field applicators are configured to apply an oscillating magnetic field having a frequency of from about 1 to about 100 Hz.

17. (canceled)

18. The apparatus of 14, wherein the magnetic field applicator includes one or more permanent magnets and/or one or more electromagnets located at an offset distance above the exposed surface, the apparatus further including a sensor configured to determine the offset distance between the exposed surface and the magnetic field applicator, the apparatus being configured to adjust the offset distance between the exposed surface and the magnetic field applicator to maintain the offset distance within a range of about 5 mm to about 30 mm.

19. The apparatus of claim 18, wherein the offset distance is about 10 mm to about 20 mm.

20. (canceled)

21. The apparatus of claim 14, wherein the magnetic field strength of about 0.1 T to about 1 T.

22. The apparatus of claim 16, wherein the one or more magnetic field applicators include one or more permanent magnets that are rotated over the exposed surface, the rotation generating the oscillating magnetic field.

23. The apparatus of claim 15, wherein the one or more magnetic field applicators is located at a fixed position between the first location and the second location, and the conveyer conveys the one or more ingots under the one or more magnetic field applicators.

24. The apparatus of claim 23, wherein the one or more magnetic field applicators is free to rotate about the fixed position, or the one or more permanent magnets are free to rotate about the fixed position.

25. The apparatus of claim 15, wherein the one or more magnetic field applicators are configured to move between at least the first position and the second position to apply the magnetic field to the molten metal in at least one of the one or more ingot moulds.

26. The apparatus of claim 14, wherein the apparatus further includes an air blower or water mist spray configured to cool the exposed surface, the air blower being located near to a first of the one or more magnetic field applicators to cool the exposed surface as the first of the one or more magnetic field applicators applies the magnetic field.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0049] FIG. 1: Illustration of a typical ingot mould.

[0050] FIG. 2: Illustration of an embodiment of a magnet disc array.

[0051] FIG. 3: Illustration of an embodiment showing an arrangement of a magnet array with respect to the ingot mould.

[0052] FIG. 4: Illustration of an embodiment showing an arrangement where two or more magnets are arranged in a truncated cone with alternating poles.

[0053] FIG. 5: Illustration of an embodiment showing an arrangement where one or more horizontally aligned magnets are rotated about a horizontal axis through their centre.

[0054] FIG. 6: Illustration of an embodiment showing an arrangement where two or more magnets are arranged in the periphery of a barrel with alternating poles so that as the barrel is rotated about an horizontal axis through its centre an oscillating magnetic field is created.

[0055] FIG. 7: Illustration of an embodiment showing an arrangement where a magnetic field is applied via an AC current carrying coil.

[0056] FIG. 8: Photograph showing cross section of an air cooled 99.85% aluminium ingot with no magnetic treatment.

[0057] FIG. 9: Photograph showing top surface of an air cooled 99.85% aluminium ingot with no magnetic treatment.

[0058] FIG. 10: Photograph showing cross section of an air cooled 99.85% aluminium ingot with magnetic treatment applied during solidification.

[0059] FIG. 11: Photograph showing top surface of an air cooled 99.85% aluminium ingot with magnetic treatment applied during solidification.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0060] The invention generally relates to the use of non-contact stirring by oscillating magnetic fields during solidification of a metal melt to control the shrinkage cavities forming in open mould conveyor cast ingots, such as sow casting carousels and continuous lines. The method can be applied to all metals and conducting materials cast in open moulds including aluminium, ferro alloys, silicon, and electrical conducting slags or salts which are sometimes cast into sow moulds such as NaCl-KCl salts used in the recycling of aluminium.

[0061] The embodiments and examples generally describe the application of a magnetic field using rotating magnets. However, it is to be understood that the oscillating magnetic fields can be created either by AC current and coils or by rotating magnets to achieve the same effects. One advantage of electromagnetic fields created by AC current over moving magnets is that multiple frequency components can be applied at the same time. For example, a low frequency of 10 Hz could be applied at the same time as a higher frequency of 40 Hz. Thereby, different parts of the ingot can be stirred at the same time. Thus, in certain forms of the invention, the magnetic field applicators are AC electromagnets, and the step of applying a magnetic field can include simultaneously applying magnetic fields of different frequency to the molten metal. Despite this, permanent magnets are preferred over AC current as these are more economical due to lower energy consumption and operating costs.

[0062] To induce mixing/stirring within a metal melt, or a solidifying metal melt, an oscillating magnetic field is applied through the use of permanent magnets or electromagnets. In a preferred form of the invention, the oscillating magnetic field is provided by permanent magnets that are rotated above the molten metal or solidifying ingot. In any event, the application of the oscillating magnetic field induces liquid movement within the solidifying ingot. This liquid movement disperses solid fragments (which may be referred to as crystallites or seed crystals) throughout the bulk of the molten metal. This advantageously: [0063] (i). grows and settles onto the solidification front in the base of the ingot encouraging solidification to proceed from the base of the mould and less so from the sides; [0064] (ii). creates a semi-solid region where shrinkage is dispersed over a larger volume and not only in the central region; and [0065] (iii). reduces shrinkage cavities and disperses porosity.

[0066] The timing and duration of the oscillating magnetic field to the solidifying ingot is important. Generally, it is preferred that the oscillating magnetic field is applied at least during the initial phases of solidification during which solid crystallites have formed adjacent the walls and base of the ingot mould so that these crystallites may be dispersed throughout the bulk molten metal of the solidifying ingot. In this regard, it is preferably to include enhanced air cooling of the top surface of the ingot during this initial phase to assist in the formation of seed crystals which can then be dispersed within the melt via the mixing/stirring force created by the oscillating magnetic field. This enhanced air cooling may be provided by standard means known in the art, such as using a fan or blower. Water mist cooling may also be used to similar effect.

[0067] It is further preferred that the oscillating magnetic field is again applied near the end of solidification in order to homogenise the temperature distribution in the melt. The timing of the duration of the application of the magnetic field is to be adjusted according to the solidification time of the ingot which will depend on the size, shape and dimensions of the ingot.

[0068] While the ingot solidification time is dependent on the size, shape and dimensions of the ingot; the mould cooling method (air or water); and the melt temperature, generally the solidification process includes a number of distinct phases. Generally, for the costing temperature of the liquid entering the mould is up to 100 C. above the melting point. A temperature 50 C. above is reached after 10 to 20 seconds for 23 kg ingots. The melting point is usually reached after 40 seconds for 23 kg ingots. For a standard 23 kg pure aluminium remelt ingot cooled with water cooling of the mould, the typical solidification time is around 300 seconds. For large aluminium sows of around 700 kg the solidification time may be around 4800 seconds.

[0069] Initially, grains nucleate on the mould and form a solid shell defined by the inner dimensions of the mould and the solidification front. The solidification front from the walls and the base of the mould migrates, over time, toward the centre of the ingot where final solidification occurs. The time taken for solidification after filling the mould, is the time it takes for the solidification front to progress from adjacent the mould walls and into the centre of the ingot so that no molten metal liquid remains.

[0070] The solidification time can be determined by freezing in thermocouples into the ingot, and mathematically modelling the solidification to make solidification time predictions. Observations of the top liquid surface during solidification also give some indication of the solidification time. In this manner the solidification time is known for a given alloy and ingot size and shape and the oscillating magnetic field treatment stages and times can be adjusted accordingly.

[0071] In the case of a standard 23 kg pure aluminium remelt ingot cooled with water cooling of the mould, the oscillating field is ideally applied for a short duration part way through the solidification e.g. at around 60 seconds after filling the ingot mould for a duration of 10 to 20 seconds. The oscillating field helps to disperse seed crystals which will lead to a semisolid mush at the end of solidification. Toward the end of solidification the field is applied again in order to homogenise the temperature distribution in the melt.

[0072] Generally, the magnetic field is applied from above the ingot. This is because the walls and base of the ingot mould act as a magnetic shield that prevents the magnetic field from adequately penetrating the solidifying melt to induce mixing/stirring. As above, the use of permanent magnets is desired due to more favourable economics. Neodymium magnets are particularly preferred. However, magnets formed of other materials may be used if they have suitable field strength and pulling force.

[0073] In order to provide an oscillating magnetic field, the permanent magnets are rotated over the surface of the solidifying melt. The frequency of field oscillation depends on the RPM of the device and the number of magnets. To achieve a suitable penetration depth into the ingot of the forces acting on the metal, a low frequency in the range 1 to 100 Hz, preferably 5 to 50 Hz, and most preferably 20-30 Hz is used. To get sufficient stirring force requires a small offset distance between the magnets and the surface of the melt of 5-20 mm preferably around 10 mm.

[0074] Generally, one or more magnets will be held in a magnetic field applicator or jig above the solidifying melt. The jig used to hold and rotate the magnets also should be made of a non-magnetic material, mechanically sound and sufficiently heat resistant. Preferably, the jig is formed from a material selected from the group consisting of aluminium, PTFE, wood, or copper based alloys. Preferably the jig includes milled slots for containing the magnets in set positions and a cover plate to hold them in the milled slots.

[0075] The magnets may be held in a number of different arrangements within the jig. The magnets can be rotated around a vertical or horizontal axis above the moulds. The axis of rotation may be held in a fixed position as the moulds on the conveyor pass underneath or may travel a defined path in relation to the moulds. For example the axis of rotation may travel up and down the length of the ingot within a defined time frame as it moves along the conveyor.

[0076] The jig can be fixed in position above the conveyor or move along with the moulds. A number of different arrangements of the jig and the magnets held therein are contemplated.

[0077] In one example, permanent magnets are held within the jig (or magnetic field applicator) such that the permanent magnets are rotated about a vertical axis above the exposed surface of the molten metal (such as in a typical ingot mould 100 filled with a metal ingot 102 as shown in FIG. 1). In this case, the term vertical axis denotes an orientation that is substantially perpendicular to the exposed molten metal surface. This is best illustrated in FIGS. 2 and 3. FIG. 2 illustrates a disc shaped jig or magnetic field applicator 200 holding four permanent magnets (each 502012.5 mm) 202, 204, 206, 208 in an alternating pole arrangement retained held in a planar manner (in a plane parallel with the exposed surface of the molten metal). The jig 200 is rotated about a vertical axis in the direction shown. The rotation may be either clockwise or anticlockwise. Still further, the direction of rotation may be alternated between clockwise and anti-clockwise in order to perturb the thermal field at the solidification front and encourage crystal detachment from the solidification front. FIG. 3 shows a side view of the jig 200 of FIG. 2 held 5 to 10 mm above an ingot mould 100 filled with a molten material 102. The jig 200 is rotated about a vertical axis 210 in the direction shown.

[0078] Alternatively, as shown in FIG. 4, two magnets 400 and 402 with alternating poles are or mounted into a jig or magnetic field applicator 404 having an inverted frustoconical shape which is rotated about a vertical axis 406 above the exposed surface of the molten metal 408 held in a ingot mould 410, such that each of the magnets are angled relative to the surface if the molten metal (i.e. not arranged parallel to the molten metal surface). In this case, the magnets are angled away from the exposed surface with distance from the vertical axis.

[0079] The position of the vertical axis can vary in relation to the mould/ingot for example moving back and forward from one end of the ingots to the other. That is, in one or more embodiments, the vertical axis can translate over the exposed surface. The jig or magnetic field applicator may also be raised and lowered over the exposed surface. The vertical axis may also be in a fixed position relative to the ingot, this position may be in the centre of the ingot above the exposed surface or in an off centre position, for example at one quarter of the ingot width.

[0080] An array of an even number of more than four magnets radially spaced at equal angles could also be used. Alternate magnet arrays and stirring patterns are also possible. For example, in an alternate configuration, a row of long magnets is provided with poles on their sides rotating about horizontal axes through their centre. This arrangement is illustrated in FIGS. 5 and 6 in respect of a single rotating magnet that is rotated about a horizontal axis (that is, the axis is in the same plane as the exposed molten metal surface). It will be appreciated that a plurality of magnets in this arrangement could be employed.

[0081] FIG. 5 shows an ingot mould filled with a molten metal have an exposed molten metal surface 502. Held above the exposed molten metal surface is a jig or magnetic field applicator 504 having four faces with alternating polarity. The faces are rotated about a horizontal axis 506 in the illustrated direction.

[0082] FIG. 6 shows a further arrangement of a jig or magnetic field applicator 600 having a cylindrical shape and including therein four magnets 602, 604, 606, and 608 for rotation about a horizontal axis in a similar manner to that illustrated in FIG. 5.

[0083] In each case, the invention encompasses embodiments where the jig or magnetic field applicators are held in position, or are able to move. Typically, for a conveyor type system, the jig or magnetic field applicators are held in a static position relative to the process, with a magnetic field generating portion that is free to rotate (in the case of permanent magnets) to provide the varying or oscillating magnetic field. Alternatively, the jig or magnetic field applicators may be able to move with the ingot moulds along the conveyor, such that each ingot is associated with its own jig or magnetic field applicator. In still another alternative arrangement, the jig or magnetic field applicators may be free to move within a confined portion of the process, such that the jig or magnetic field applicator can move with an ingot from an initial point in the process where application of the oscillating magnetic field is commenced to a final point where application of the oscillating magnetic field is ceased. Once the jig has reached this final point, the jig returns to the initial point to be associated with a new ingot, while the ingot continues to be conveyed for further downstream processing (such as further cooling etc.).

[0084] In a preferred arrangement, the magnets are held within a star arrangement of four magnets, for example, in one embodiment four 502012.5 mm Neodymium magnets of 0.6 Tesla strength were used. The magnets are positioned in a frame of the jig with adjacent magnets having alternating poles. The magnets are rotated about a vertical axis in a horizontal plane above the surface of the melt.

[0085] In yet another embodiment, as illustrated in FIG. 7, the magnetic field is applied by a series of AC current carrying coils 700 and 702. In this embodiment the coils 700 and 702 are held in a fixed position and the mould 704 is carried by a conveyor underneath the series of AC current carrying coils.

[0086] Irrespective of the specific arrangement that is chosen, during normal operation there is some variation in ingot pour weight and consequently the height of the liquid in the mould. To maintain a constant offset distance between the magnets (such as, in certain embodiments, the plane of rotation of the magnets) and the liquid surface of the solidifying ingot with every ingot cast, a laser sensor is used to measure the level of metal in the mould and the vertical position of the magnets (which in certain embodiments is in the form of a rotating disk containing the magnets) is adjusted accordingly.

[0087] The variation in ingot weight from a nominal weight causes the solidification time of the ingots to vary and thus the last stage of solidification along the conveyor also varies. Multiple magnetic field treatment heads or stations may be used to ensure all ingots received treatment at the correct time in the solidification process. For example for nominal 23 kg ingots, there may be a variation in the weight of the ingots of plus or minus 1 kg from the nominal weight. As a result, the solidification time may vary by up to 30 seconds which, depending on the conveyor speed, can result in a 2 metre variation in the point of final solidification along the conveyor. In this case, the use of two treatment stations 2 meters apart can ensure all ingots are treated before final solidification for a sufficient period.

EXAMPLE

[0088] Tests were conducted on a square steel mould 12.5 mm thick with a cross section typical of 23 kg remelt ingots (see FIG. 1). Ingots produced were around 4.5-5 kg. When natural air cooling was used the solidification time was about 5 minutes.

[0089] FIG. 2 shows a disc array of four magnets seated in a rotating disc above the surface of the melt, with alternating north and south poles. The disc array was constructed of wood with an aluminium cover plate to hold the magnets in place. Four 50 mm long 2012.5 mm 0.64 Tesla magnets were used with alternating poles. Magnets were positioned 31 mm from the central axis within the disk on the underside of the disk (FIG. 2). A shaft was connected to the disk allowing the disk to be rotated with an electric motor. The array was rotated at 400 RPM, giving a field oscillation frequency of 27 Hz. The array was held 5-10 mm above the mould and 20-25 mm from the melt surface in the mould after filling as shown in FIG. 3.

[0090] When casting with natural air cooling, the resulting shrinkage cavities were large and located in the centre of the ingot (FIGS. 8 and 9). When the oscillating magnetic field was applied to induce mixing/stirring during solidification the central cavity was eliminated (FIGS. 10 and 11).

[0091] It will be understood that the invention disclosed and defined in this specification extends to all alternative combinations of two or more of the individual features mentioned or evident from the text or drawings. All of these different combinations constitute various alternative aspects of the invention.