GB2041803A - Electromagnetic casting apparatus and process - Google Patents

Abstract

A multi-turn, variable pitch inductor (20) is used to contain and shape the molten metal and which generates an electromagnetic field the flux density of which decreases in intensity with the height (h) of the molten metal column (19) forming the ingot (22), thereby balancing the reducing hydrostatic pressure within the ingot above the solidification interface (24), and giving an ingot surface free of undulations. Successive turns of the inductor are separated by insulating spacers (3) which can vary in thickness. <IMAGE>

Description

SPECIFICATION Electromagnetic casting process and apparatus This invention relates to an improved process and apparatus for electromagnetically casting metal and metal alloys, particularly high melting point metals and alloys. Electromagnetic casting processes have been known and used for many years for continuously and semi-continuously casting metals and alloys.

Known prior art electromagnetic casting apparatus comprises a three part mold consisting of a water cooled inductor, a non-magnetic screen and a manifold for appiying cooling water to the ingot. Such an apparatus is exemplified in U. S.

Patent No.3,466,166 to Getselev et al.

Containment of the molten metal is achieved without direct contact between the molten metal and any component of the mold. Solidification of the molton metal is achieved by direct application of the water from the cooling manifold to the ingot sheil.

The cooling manifold may direct the water against the ingot from above, from within, or from below the inductor, as exemplified in U. S. Patent Nos.3,735,799 to Karlson, and 3,646,988 to Getselev. In some prior art approaches the inductor is formed as part of the cooling manifold so that the cooling manifold supplies both coolant to solidify the casting and to cooi the inductor as exemplified in U. S. Patent Nos.3,773,101 to Getseiev and 4,004,631 to Goodrich et al. The water cooled inductor of Goodrich et ai.631 'is provided with a preferentiai geometry.

During electromagnetic casting of molten metal the radial component of the electromagnetic pressure against a molten column generally must be equal to the hydrostatic pressure of the molten metal being shaped. To compensate for the gradually low hydrostatic pressure of the molten metal progressing toward the upper portion thereof, it is known to provide an electromagnetic shield or screen positioned between the inductor and the top of the molten metal column to attenuate the electromagnetic field generated by the inductor and thereby gradually reduce the radial forces acting on the molten metal toward the top of the column.

Prior art non-magnetic screens have been utilized to properly shape the magnetic field for containing the molten metal as exemplified in U. S. Patent 3,605,865 to Getselev. A variety of approaches with respect to non-magnetic screens are exemplified as well in the Karlson 799' Patent and U. S. Patent No.3,985,179 to Goodrich et al.

Goodrich et al.179' describes the use of a shaped inductor to shape the field, without the use of a non-magnetic screen. Similarly, a variety of inductor designs are set forth in the aforenoted patents and in U. S. Patent No.3,741,280 to Kozheurov et al.

While the above-described patents disclose the electromagnetic casting molds for casting a single strand or ingot at a time, the process can be applied to the casting of more than one strand or ingot simultaneously, as exemplified in U. S.

Patent No.3,702,155. In addition to the aforenoted patent a further description a further description of the electromagnetic casting process can be found by reference to the following articles: "Continuous Casting with Formation of Ingot by Eiectromagnetic Field", by P. P. Mochalov and Z.

N. Getselev, Tsvetnye Met., August, 1970,43,pp.

6263; "Formation of Ingot Surface During Continuous Casting", by G. A. Balakhonpsev et al., Tsventye Met., August, 1970,43,pp. 6465; "Casting in an Electromagnetic Field", by Z. N.

Getselev, J. of Metals, October,1971, pp.

3859; "Alusuisse Experience With Electromagnetic Molds", by H. A. Meier, G.8.

Laeconte, and A. M. Odok, LightMetals, 1977,pup.

223233.

When one attempts to employ the electromagnetic casting process for casting heavier metals than aluminium, such as copper, copper alloys, steel alloys, steel, nickel, nickel alloys, etc. various problems arise in controlling the casting process. In the electromagnetic casting process the molten metal head is contained and held away from the mold walls by an electromagnetic pressure which counter balances the hydrostatic pressure of the molten metal head. The hydrostatic pressure of the molten metal head is a function of the molten metal head height and the specific gravity of the molten metal.

When casting aluminum and aluminum alloys using the electromagnetic casting method, the molten metal head has a comparatively low density with a high surface tension due to the oxide film formed on its surface. The surface tension is additive to the electromagnetic pressure and both act against the hydrostatic pressure of the molten metal head. A small fluctuation in the molten metal head therefore gives rise to a small difference in the magnetic pressure required for containment. For heavier metals and alloys, such as copper and copper alloys, comparable changes in the molten metal head cause a greater change in hydrostatic pressure and in the required offsetting magnetic pressure.

In addition copper and copper alloys display surface tensions considerably lower than seen in aluminum alloys. This effect in turn requires further compensating increases in magnetic pressure. It has been found for copper and copper alloys that the change in magnetic pressure required for containment, is approximately three times greater than for aluminum and aluminium alloys with comparable changes in molten metal head.

In orderto obtain an ingot of uniform crosssection over its full length the periphery of the ingot and molten metal head with in the inductor must remain vertical especially near the liquid solid interface of the solidifying ingot shell. The actual location of the periphery of the ingot is the plane over which the hydrostatic and magnetic pressure balance. Therefore, any variations in the absolute molten metal head height cause comparable variations in hydrostatic pressure which produce surface undulations along the length of the ingot. Those surface undulations are very undersirable and can cause reduced metal recovery during further processing.

Use of multi-turn inductors is shown in U. S.

Patent No. 3,995,678 to Zavaras et al. and U. S.

Patent No. 3,857,696 to Aldersley et al. The Patent to Zavaras 678' shows a multi-turn inductor placed about a continuously formed ingot after it emerges from a forming mold to stir the molten interior of the solidifying ingot, while the multi-turn inductor of the Patent to Aldersley et al. is utilized to prevent molten metal splashover.

In accordance with this invention an improved process and apparatus for the electromagnetic casting of metals is provided wherein a variablepitch multi-turn inductor is utilized to minimize the variations in the gap during operation of the casting apparatus. The variable-pitch multi-turn indicator comprises a coil or individual turns wherein the spacing of the individual turns or the pitch of the coil is controlled so as to provide electromagnetic pressure which accurately counter-balances the metallostatic pressure within the molten m etal being contained while maintaining substantially vertical sidewalls.

The invention will be further described with reference to the accompanying drawings in which: Figure 1 is a schematic representation of a prior art electromagnetic casting apparatus consisting in part of a single turn inductor.

Figure 2 is a schematic representation of an electromagnetic casting apparatus in accordance with the present invention, showing a variablepitch stacked inductor.

Figure 3 is a schematic representation of a variable-pitch multi-turn inductor in accordance with the present invention.

Figures 4 and 5 are different side views of the variable-pitch stacked inductor of Figure 2.

Figure 6 is a top view of the variable-pitch stacked inductor of Figure 5.

Figure 7 is a side view of a variable-pitch stacked inductor embodiment of the present invention showing unequal spacing between each turn of the inductor.

Figure 8 is a side view of a variable-pitch stacked inductor embodiment of the present invention showing a variation in the spacing between and in the height of turns of the inductor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to Figure 1 there is shown there in a prior art casting apparatus consisting in part of a single-turn inductor. The electromagnetic casting mold 10 is comprised of a single-turn inductor 11 which is water cooled; a cooling manifold 12 for applying cooling water to the peripheral surface 13 of the metal being cast; and a non-magnetic screen 14. Molten metal is continuously introduced into the mold 10 during a casting run using a through 15 and a downspout 16, with the initial surge of molten metal contacting bottom block 22 in a raised position after which bottom block 22 is withdrawn by ram 21. The indicator 11 is excited by an alternating current from a power source 17 and control system 18.The aiternating current in inductor 11 produces a magnetic field which interacts with the molten metal head 19 to produce eddy currents therein. These eddy currents in turn interact with the magnetic field and produce forces which apply a magnetic pressure to the molten metal head 19 to contain it so that it solidifies in a desired ingot cross-section. An air gap dexists during casting, between the molten metal head 19 and the inductor 11.The metal head 19 is formed or molded into the same general shape as the inductor 11 thereby providing the desired ingot cross-section. The inductor may have any desired shape including circular or rectangular as required to obtain the desired ingot cross-section.The purpose of the non-magnetic screen 14 is to fine tune and balance the magnetic pressure with the hydrostatic pressure of the molten metal head 19.

Referring to Figure 2, in accordance with this invention, single-turn prior art inductor 1 1 and non-magnetic screen 14 have been replaced by variable-pitch multi-turn indicator 20 of this invention.

The pitch, for purposes of this invention, is defined as the distance between corresponding points each located on the center-line (mid-point of the height) on two adjacent turns of the variable-pitch multi-turn indicator 20.

See a0 and a(z) in Figures 2 and 3.

The pitch or spacing between turns of variablepitch multi-turn indicator 20 can be determined according to the equation:

wherein a(z) = the pitch or spacing between inductor turns at a height z above the solidification interface a(0) = the pitch or spacing between inductor turns at the solidification interface h = the total molten metal head z = the height above the solidification interface.

It can be seen that the minimum spacing a0 between adjacent turns should be located nearest to the solidification interface of the ingot being cast so that the maximum ampere turns per the unit height counter balances the maximum metallostatic pressure p0 which occurs at that point. Above that point, the inter-turn spacing a(z) of the inductor is increased as the metallostatic pressure p within the molten metal decreases as given in the above equation.

The electromagnetic field generated by the upper portions of the electromagnetic inductor of this invention has a gradually diminishing flux density so that the radial forces on the molten metal surface are gradually reduced towards the upper portion of the molten metal head to maintain the vertical surfaces of the molten metal essentially straight and free of surface undulations.

This generation of diminishing electromagnetic field, the flux density of which diminishes in a vertical direction towards the top of the indicator, is effected by virtue of the use of multi-turn inductor pitch control.

The variable-pitch stacked inductor 20 of Figure 2 is further shown in Figures 4 through 6.

As can be seen from these figures variable-pitch stacked inductor 20 is constructed of multiple turns 2 of different cross-section which are equally spaced by insulating spacers 3. Inductor turns 2 are provided with through passages 7 for cooling fluid such as water. Hose nipples 8 are provided on each turn 2 to permit ready attachment to a cooling fluid supply source. The cross-sectional height of turns 2 increases from the bottom to the top of variable-pitch multi-turn inductor 20 to effect a decreasing magnetic field in the upward direction.

Electrical connection to a power source is provided by bus connections 6. The individual turns of multi-turn indicator 20 are shown to be connected in series but water cooling can be in series or parallel as desired. Electrical series connections 9 can consist of for example silver braze, with an optional copper spacer if necessary.

Insulator spacers 3 are constructed of insulating material, preferably of thermo-setting high temperature plastic such as fiberglass reinforced phenolic, or fiberglass reinforced silicone. Turns 2, are preferably constructed of rectangular section copper tube, but could be constructed of other materials and cross-sections.

Figures 7 and 8 show two further embodiments of the stacked inductor of this invention. In Figure 7 a decreasing magnetic field in the upper direction is provided by utilizing equal crosssection turns while varying the height of insulator spacers 3. In order to obtain reduced magnetic fields towards the top of the inductor 20 the height of spacers 3 in an upper direction. In accordance with Figure 8, both the spacers 3 and the cross-section of copper turns 2 could be varied. Again the spacers and the turn crosssections are of increased height in the upper direction so as to effect a decreasing magnetic field as one goes from the bottom to the top of multi-turn inductor?0. In both cases, the net effect is to vary, in a known way, the current per unit of coil height or amperes per inch, in order to vary the resulting magnetic field strength.

Figure 3 represents a further embodiment of this invention wherein a variable-pitch multi-turn coil 4 is utilized as an inductor to provide the desired magnetic field. Variable-pitch multi-turn coil 4 is preferably constructed of copper tube of round cross-section, but can be constructed of other materials and cross-sections, such as, for example, rectangular tube or solid coil. Coil 4 can be provided with an internal passage for cooling fluid or can be cooled externally.

The high impedance circuit of the present invention negates the need for a transformer interposed between the power source and the inductor which is necessary with prior art inductors. Moreover, the inductor of this invention provides an accurate definition of current density.

Since full current passes through each turn of the coil, the inductor of the present invention provides a means for accurately defining the current density over the full height of the inductor. In addition, the current density provided by the inductor of this invention is a function of the turn spacing or pitch of the coil only, and is not influenced by the shape of the molten metal being contained as it is in the case of massive single-turn inductors.

Another advantage of this invention lies in the fact that the use of the inductor of this invention permits a smaller air gap dthan that associated with the prior art. By virtue of the use of the pitch control of this invention rather than shape or spreading control and/or electromagnetic shields as in single-turn inductors, a reduction in air gap and consequent reduction in inductance of the circuit results in the realizing of substantial power savings.

Finally, a further advantage of the variable-pitch multi-turn inductor of this invention is that the connecting points to the power circuit are so positioned as to minimize the adverse effects upon shape control that are associated with prior art single-turn inductors.

It is apparent that there has been provided in accordance with this invention a variable-pitch inductor for use in electromagnetic casting apparatus and processes for electromagnetic casting, which fully satisfy the objects, means, and advantages as set forth hereinbefore. While the invention has been described in combination with specific embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art in light of the foregoing description. Without departing from the scope of the appended claims.

Claims (14)

1. An apparatus for electromagnetically forming molten metals or alloys into a casting of desired shape, comprising a variable-pitch multi-turn inductor defining a casting zone having a molten metal or alloy input end and a solidifying casting output end, the pitch of said inductor increasing from said output end toward said input end, whereby the electromagnetic field generated by said inductor gradually diminishes in flux density from said output end toward said input end.

2. Apparatus according to claim 1, wherein the variable-pitch multi-turn inductor comprises at least three individual turns arranged in stacked relationship, said turns being electrically connected in series and being spaced apart from each other by insulating spacers.

3. Apparatus according to claim 2, wherein the individual turns of the inductor are of different cross-section.

4. Apparatus according to claim 3, wherein the individual turns of the inductor are of increasing height toward said input end of said casting zone.

5. Apparatus according to claim 2, wherein the spacers are of different cross-section.

6. Apparatus according to claim 5, wherein the spacers are of increasing height toward said input end of said casting zone.

7. Apparatus according to claim 2, wherein both the individual turns of the inductor and said spacers are of different cross-section, said turns and spacers being of increasing height toward said input end of said casting zone.

8. Apparatus according to any one of claims 2-7, wherein the turns of said inductor have internal passages for passage of a cooling fluid.

9. Apparatus according to claim 1 , wherein the variable-pitch multi-turn inductor comprises a multi-turn coil.

1 0. Apparatus according to any one of the preceding claims, wherein the pitch between turns of said inductor has a value a(z) defined by the formula:

wherein a(z) = the pitch or spacing between inductor turns at a height z above the solidification interface a(o) = the pitch or spacing between inductor turns at the solidification interface h = the total molten metal head z = the height above the solidification interface.

11. An electromagnetic casting process which comprises pouring a molten metal or alloy into a casting zone having an input end and an output end, shaping the molten metal into an ingot in said zone by application thereto of an electromagnetic field, and withdrawing a solidified and shaped ingot from said output end, wherein the flux density of the electromagnetic field decreases in intensity towards said input end.

12. A process according to claim 11, when carried out in an apparatus as claimed in any one of claims 1-10.

1 3. Apparatus according to claim 1, substantially as hereinbefore described with reference to Figures 2-8 of the accompanying drawings.

14. A process according to claim 11, substantially as hereinbefore described with reference to Figures 2-8 of the accompanying drawings.