GB2275634A

GB2275634A - Metal casting employing electromagnetic levitation

Info

Publication number: GB2275634A
Application number: GB9402610A
Authority: GB
Inventors: M J Broomfield; Eric Andrew Feest; Richard Stuart Nelson
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1993-03-03
Filing date: 1994-02-08
Publication date: 1994-09-07
Also published as: GB9402610D0; GB9304340D0

Abstract

In a method for continuously casting a metal, a jet 18 of molten metal emerging from a crucible 12 is controlled by magnetohydrodynamic forces producing electromagnetic levitation provided by electrical coils 24. The jet 18 falls vertically and is levitated by a tubular linear induction motor 20 so as to fall at constant speed, while being cooled by a coolant sleeve 28 and by forced gas flow 36, 32, to emerge as a solid. The jet may be of circular cross-section, to form a rod 18, or of annular cross-section to form a tube 60. The forces maintain the desired cross-sectional shape of the jet and control the rate of flow of the jet. <IMAGE>

Description

Metal Casting This invention relates to methods for casting metals in which a molten metal is continuously poured out of a container to form a continuous length of a desired shape.

UK Patent No. 1 481 301 describes a method of casting a molten alloy or pure metal from a container in which the level of molten metal is maintained substantially constant.

The molten metal flows through an outlet spout into a mould which may be defined by the walls of a groove in the peripheral surface of a wheel and an endless band that overlies the groove over a part of its length. An alternating electromagnetic field is established about the outlet spout, and the rate of flow of the metal controlled thereby; in addition the field causes the molten metal to be compressed inwardly to reduce the cross-section of the stream such that there is little for no contact between the molten metal and the internal wall of the outlet spout.

This method has the aim of reducing the rate of chemical and physical erosion of the spout, and is described as applicable to aluminium and copper.

Apparatus is also known, for example in European Patent Application 0 049 937A, for continuously casting a molten metal from a tundish through an orifice onto a rapidly moving casting surface. The casting surface, which may be the surface of a cylindrical wheel, is located within about 3mm of the orifice, and may move past the orifice at a speed of several hundred metres per second.

The orifice is slot-shaped, and the strip of molten metal emerging therefrom is rapidly quenched to form a solid strip.

The use of the casting surface in the above apparatus effectively limits the shape of the product to being a thin flat strip.

According to the present invention there is provided apparatus for continuously casting metal, the apparatus comprising a container for the molten metal with an orifice passage through which the molten metal is delivered from the container so the metal emerges as a jet having a desired cross-sectional shape, cooling means surrounding a first portion of the trajectory of the jet, and electromagnet coils around the first portion for providing a force to maintain the desired cross-sectional shape of the jet and to control the rate of flow of the jet, the first portion being of such a length that the jet has solidified by the time it leaves the first portion.

This apparatus enables jets to be cast by a continuous non-contact process, and the jet may for example be of elliptical, circular or tubular cross-section. Preferably the apparatus includes means to cause coolant gas to flow over the first portion of the jet. The electromagnet coils are wound and energised as a linear induction motor; they may utilise a single layer winding or a multiple layer winding, energised at a frequency of for example between 25 and 150 Hz, and carrying a current of between about 3 and 30 A. The relative field speed must be sufficient to control the rate of flow of the jet, but not so large as to cause excessive eddy-current heating, and might for example be about 25 m/s. Where the jet is of tubular cross-section, means may be provided to supply gas under pressure within the tube, to counterbalance the inward force due to the electromagnet coils; alternatively coil may be provided within the tube to provide a counterbalancing electromagnetic force, and these coils preferably are excited at the same frequency as the electromagnet coils. Additional electromagnetic pressure forces may be provided by means of additional windings energised at a higher frequency (up to 20 kHz for example).

The invention also provides a method for continuously casting a metal, the method comprising the steps of causing molten metal to pass through an orifice of a desired crosssectional shape so as to create a jet, allowing the jet to attain a desired speed, and supporting the jet by non-contact means along a portion of its trajectory for sufficient distance that the jet solidifies.

A further potential application of the combined linear induction motor and electromagnetic pressure forces is in the consolidation of continuous-fibre-reinforced metal matrix composites. Here arrays of tows could be arranged in the form of the required cross section, melt impregnated by passing through a container of molten metal, and then passed through an orifice, below which electromagnetic forces could control the jet containing both the tows and the molten metal such that the molten matrix would remain in the desired cross-section until solidified. This would give the economic benefit of impregnating and consolidating in one continuous process and could also provide technical benefits over current consolidation processes due to the reduction of mechanical damage to the fibres and elimination of bonding defects between infiltrated tows.

The invention will now be described by way of example only and with reference to the accompanying drawings, in which: Figure 1 shows a medial sectional view in a vertical plane of a metal casting apparatus; and Figure 2 shows a medial sectional view in a vertical plane of an alternative metal casting apparatus.

Referring to Figure 1, a metal casting apparatus 10 comprises a crucible 12 containing molten metal 14, for example an aluminium alloy, which is maintained at a constant depth. In the centre of the bottom of the crucible 12 is a circular orifice 16 from which emerges a jet 18 of molten metal of circular cross-section falling freely under gravity.

Just below the crucible 12 is a tubular cylindrical linear induction motor 20, defining a vertical bore 22 through which the jet 18 passes. The motor 20 comprises multiple layer electrical windings 24 within a cylindrical casing 26, and a water jacket 28 of tubular form inside the windings 24, spaced apart from the windings 24 by axially spaced rings 30. Several ducts 32 extend through the water jacket 28, between annular chambers 34 defined between the windings 24 and the water jacket 28, and the bore 22. A pipe 36 communicates with each annular chamber 34; and pipes 38 and 39 communicate with the water jacket 28 at its upper and lower ends respectively.

In operation of the apparatus 10 the linear induction motor windings 24 are energised at a frequency of 50 Hz with current of about 30 A so as to levitate the jet 18, which consequently falls at a steady speed through the bore 22. Water is passed upwardly through the water jacket 28; and a coolant gas is pumped in and out of adjacent annular chambers 34 through the pipes 36. Gas pumped into one chamber 34 flows into the bore 22 through the ducts 32, and either escapes from one end of the bore 22 or passes along the bore 22 and through another duct 32 into an adjacent chamber 34 from which it is extracted.

The jet 18 consequently loses energy by radiation to the water jacket 28 and by conduction to the coolant gas.

The linear induction motor 20 is of such a length that the jet 18 has solidified to form a rod by the time it emerges from the lower end 40, and below the motor 20 are arranged rollers 42 to handle the emerging circular rod 18.

Referring now to Figure 2, an alternative metal casting apparatus 50 is shown suitable for the continuous casting of metal tubing. The apparatus 50 comprises a crucible 52 containing molten metal 54, the crucible 52 having a circular spout 56 in its bottom, within which is a coaxially arranged tube 58. Molten metal consequently falls as a tubular jet 60 from the annular orifice thus defined between the tube 58 and the spout 56.

Just below the crucible 52 is a tubular cylindrical linear induction motor 20, structurally the same as that of the apparatus 10 of Figure 1, with a bore 22 through which the tubular jet 60 falls. In passing through the bore 22, the jet 60 is cooled by radiation to a water jacket 28 and by conduction to a coolant gas, in the same manner as described above, and emerges from the lower end 40 as a solid tube 60.

A cylindrical electromagnet coil 62 covered by a cooling sleeve 64 is supported coaxially within the tube 58 in the crucible 52, and extends downwardly within the tubular jet 60 to the lower end 40 of the linear induction motor 20. Water flows into and out of the cooling sleeve 64 through pipes 66 and 68.

In operation of the apparatus 50, the linear induction motor 20 is energised as described above at a frequency of 50 Hz and with a current of 3 A to levitate the tubular jet 60 so it falls at a steady speed. The windings 24 exert both an upwards, levitating force and also an inwards pressure on the tubular jet 60. The electromagnet coil 62 is energised at a frequency of 50 Hz and with a current of 10 A to produce a balancing outwards pressure on the tubular jet 60. The coil 62 may also be wound so as to act as a linear induction motor.

In an alternative apparatus (not shown) to that of Figure 2, the electromagnet coil 62 and the sleeve 64 are replaced by means providing sufficient gas pressure on the tubular jet 60 to balance the inwards pressure due to the windings 24. In each apparatus, to ensure stability of the falling liquid, additional pressure forces may be applied electromagnetically to the surface of the liquid stream.

This may be achieved by means of additional windings incorporated into the windings 24 (and into the coil 62, where present) and energised at a higher frequency, for example at between 1 and 10 kHz.

Claims

Claims:

1. An apparatus for continuously casting metal, the apparatus comprising a container for the molten metal with an orifice passage through which the molten metal is delivered from the container so the metal emerges as a jet having a desired cross-sectional shape, cooling means surrounding a first portion of the trajectory of the jet, and electromagnet coils around the first portion for providing a force to maintain the desired cross-sectional shape of the jet and to control the rate of flow of the jet, the first portion being of such a length that the jet has solidified by the time it leaves the first portion.

2. An apparatus as claimed in Claim 1 including means to cause coolant gas to flow over the first portion of the jet.

3. An apparatus as claimed in Claim 1 or Claim 2 wherein the electromagnet coils are wound and energised as a linear induction motor utilising a single layer winding carrying a current of between 10 and 50 A, at a frequency of 10 kHz and a relative field speed of about 200 m/s.

4. An apparatus as claimed in Claim 1 or Claim 2 wherein the electromagnet coils are wound and energised as a linear induction motor utilising a multiple layer winding energised at a frequency of about 50 Hz.

5. An apparatus as claimed in any one of the preceding Claims wherein the jet is of tubular cross-section, and means are provided to supply gas under pressure within the tube, to counterbalance the inward force due to the electromagnet coils.

6. An apparatus as claimed in any one of Claims 1 to wherein the jet is of tubular cross-section, and internal electromagnet coils are provided within the tube to provide a counterbalancing electromagnetic force.

7. An apparatus as claimed in Claim 6 wherein the internal electromagnet coils are excited at a frequency of about 500 Hz.

8. A method for continuously casting a metal, the method comprising the steps of causing molten metal to pass through an orifice of a desired cross-sectional shape so as to create a jet, allowing the jet to attain a desired speed, and supporting the jet by non-contact means along a portion of its trajectory for sufficient distance that the jet solidifies.

9. An apparatus for continuously casting metal substantially as hereinbefore described with reference to, and as shown in, Figure 1 or Figure 2 of the accompanying drawings.