CA1192372A - Prioritized electromagnetic casting control system - Google Patents
Prioritized electromagnetic casting control systemInfo
- Publication number
- CA1192372A CA1192372A CA000405832A CA405832A CA1192372A CA 1192372 A CA1192372 A CA 1192372A CA 000405832 A CA000405832 A CA 000405832A CA 405832 A CA405832 A CA 405832A CA 1192372 A CA1192372 A CA 1192372A
- Authority
- CA
- Canada
- Prior art keywords
- molten material
- sump
- hydrostatic pressure
- rate
- replenishing
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Classifications
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B22—CASTING; POWDER METALLURGY
- B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
- B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
- B22D11/01—Continuous casting of metals, i.e. casting in indefinite lengths without moulds, e.g. on molten surfaces
- B22D11/015—Continuous casting of metals, i.e. casting in indefinite lengths without moulds, e.g. on molten surfaces using magnetic field for conformation, i.e. the metal is not in contact with a mould
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B22—CASTING; POWDER METALLURGY
- B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
- B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
- B22D11/16—Controlling or regulating processes or operations
- B22D11/20—Controlling or regulating processes or operations for removing cast stock
- B22D11/201—Controlling or regulating processes or operations for removing cast stock responsive to molten metal level or slag level
Landscapes
- Engineering & Computer Science (AREA)
- Mechanical Engineering (AREA)
- Continuous Casting (AREA)
Abstract
ABSTRACT OF THE DISCLOSURE
An apparatus and process for casting a material into a desired shape. The apparatus and process comprise the structure and steps for electromagnetically forming the material in molten form into the desired shape. An electromagnetic forming device applied an electromagnetic field to the molten material. This magnetic field defines a containment zone for the molten material. A sump of molten material is opera-tively associated with the forming structure for delivery of molten material to the forming structure.
A device is provided for withdrawing the casting from the forming device at a desired withdrawal rate.
Another device replenishes the sump of molten material at a replenishing rate as the casting is formed. The improvement comprises a control device for maintaining the hydrostatic pressure exerted by the molten material in the containment zone at a substantially constant desired level. The control device includes a first structure for providing a substantially constant material volume balance by maintaining the ratio of the withdrawal rate to the replenishing rate substantially constant when the hydrostatic pressure in the contain-ment zone varies less than a desired percentage from the desired level. A second device senses variations in the level of the hydrostatic pressure of the molten material in the containment zone. A third device is responsive to the second device for providing a material volume imbalance by varying the ratio of the control rate to the replenishing rate when the hydro-static pressure level of molten material varies more than said desired percentage from the desired level until the hydrostatic pressure of molten material in the containment zone returns to the desired level.
An apparatus and process for casting a material into a desired shape. The apparatus and process comprise the structure and steps for electromagnetically forming the material in molten form into the desired shape. An electromagnetic forming device applied an electromagnetic field to the molten material. This magnetic field defines a containment zone for the molten material. A sump of molten material is opera-tively associated with the forming structure for delivery of molten material to the forming structure.
A device is provided for withdrawing the casting from the forming device at a desired withdrawal rate.
Another device replenishes the sump of molten material at a replenishing rate as the casting is formed. The improvement comprises a control device for maintaining the hydrostatic pressure exerted by the molten material in the containment zone at a substantially constant desired level. The control device includes a first structure for providing a substantially constant material volume balance by maintaining the ratio of the withdrawal rate to the replenishing rate substantially constant when the hydrostatic pressure in the contain-ment zone varies less than a desired percentage from the desired level. A second device senses variations in the level of the hydrostatic pressure of the molten material in the containment zone. A third device is responsive to the second device for providing a material volume imbalance by varying the ratio of the control rate to the replenishing rate when the hydro-static pressure level of molten material varies more than said desired percentage from the desired level until the hydrostatic pressure of molten material in the containment zone returns to the desired level.
Description
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PRIORITIZED ELECTROMAGNETIC CASTING CONTROL SYSTEM
This invention relates to an improved process and apparatus for producing thin strlp from materials, particularly including semi-conductive materials such as silicon. The process and apparatus include a control system that provides improved cross-sectional uniformity of the thin strip casting.
A ~ariety of processes have been developed ~or forming semi-conductive materials such as sillcon into a thin strip shape. Examples of such approaches can be found in National Technical Information Service Report PB 248,963 "Scale-Up of Program on Continuous Silicon Solar Cellst' by A. D. Mor~ison, published in September 1975, and a paper entitled "The Role of Surface ~ension in Pulling Single Crystals of Controlled Dimensions" by G. K. Gaule et al. ~rom Metal}urgy of Elemental and Compound Semiconductors, published by Interscience - Publishers, Inc., New York in 1961, pages 201-226.
Also, U.S. Patent No. 4,242,553 to Frosch et al. dis-closes an apparatus for producing ribbon-shaped crystals from a silicon melt. The Morrison publication is exemplary of the state of the art wi~h respect to the pullin~ o~ strip-type materials from a melt of silicon. The Gaule et al. publication is similarly exemplary and of particular interest insofar as it discloses the use of electromagnetic forces for applying external pressure at the growth interface.
The material produced by Morrison as exampled in Figure 20 is sub~ect to substantial thickness variation.
At page 82 of the publication, it is indicated tha~ the ribbons which are grown ~re not of'uniform thickness.
It is the intent o~ the present inventlon to provide a process and apparatus which can form the strip with improved thickness uniformity.
A considerable body o~ art has developed with respect to the use o~ electromagne~ic containment for 3~ ~
PRIORITIZED ELECTROMAGNETIC CASTING CONTROL SYSTEM
This invention relates to an improved process and apparatus for producing thin strlp from materials, particularly including semi-conductive materials such as silicon. The process and apparatus include a control system that provides improved cross-sectional uniformity of the thin strip casting.
A ~ariety of processes have been developed ~or forming semi-conductive materials such as sillcon into a thin strip shape. Examples of such approaches can be found in National Technical Information Service Report PB 248,963 "Scale-Up of Program on Continuous Silicon Solar Cellst' by A. D. Mor~ison, published in September 1975, and a paper entitled "The Role of Surface ~ension in Pulling Single Crystals of Controlled Dimensions" by G. K. Gaule et al. ~rom Metal}urgy of Elemental and Compound Semiconductors, published by Interscience - Publishers, Inc., New York in 1961, pages 201-226.
Also, U.S. Patent No. 4,242,553 to Frosch et al. dis-closes an apparatus for producing ribbon-shaped crystals from a silicon melt. The Morrison publication is exemplary of the state of the art wi~h respect to the pullin~ o~ strip-type materials from a melt of silicon. The Gaule et al. publication is similarly exemplary and of particular interest insofar as it discloses the use of electromagnetic forces for applying external pressure at the growth interface.
The material produced by Morrison as exampled in Figure 20 is sub~ect to substantial thickness variation.
At page 82 of the publication, it is indicated tha~ the ribbons which are grown ~re not of'uniform thickness.
It is the intent o~ the present inventlon to provide a process and apparatus which can form the strip with improved thickness uniformity.
A considerable body o~ art has developed with respect to the use o~ electromagne~ic containment for 3~ ~
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the purposes of casting metals. Such electromagnetic casting apparatuses comprlse a three-part mold con-sisting Qf a water cooled inductor, a non-magnetic screen, and a manifold for applying cooling water to the resultant casting, Such an apparatus is exempli-fied in U.S ~atent NoO 3,467,166 to Getselev et al.
Cont2inment of the molten metal is achieved without direct contact between the molten metal and any component of the mold. Solidification of the molten me1;al is achieved by the direct application of water ~rom a cooling manifold to the solidi~ying shell of the casting .
The present invention is particularly related to the process and apparatus for controlling the electro-ma~;netic casting system. Various approaches have beendescribed in the prior art ~or co~trolling the excitatlon of the inductor in a manner so as to provide in~;ots of uni~orm cross section. In U.S. Patent No.
4,C14,379 to Getselev~ a control system is described for controlling the magnitude of the current flowing through the inductor as a function of the deviations of the dimensions of the liquid zone Or the ingot ~rom a prescribed value~
In Russlan Patent No~ 537,750 to Getselev, an alternative control approach ls described wherein the-potential on the inductor is regulated to reduce a deviation of the phase angle from a prep~ogrammed value.
In Russian Patent No. 273,226 to Kabakov~ there i3 disclosed a control system ~or operating an ingot withdrawal mechanism to withdraw the ingot from the mold after the molten metal reaches a desired level on the mold.
In Russian Patent No. 338,297 to Irkutsk, the ele^tromagnetic casting mold is fitted with measuring
the purposes of casting metals. Such electromagnetic casting apparatuses comprlse a three-part mold con-sisting Qf a water cooled inductor, a non-magnetic screen, and a manifold for applying cooling water to the resultant casting, Such an apparatus is exempli-fied in U.S ~atent NoO 3,467,166 to Getselev et al.
Cont2inment of the molten metal is achieved without direct contact between the molten metal and any component of the mold. Solidification of the molten me1;al is achieved by the direct application of water ~rom a cooling manifold to the solidi~ying shell of the casting .
The present invention is particularly related to the process and apparatus for controlling the electro-ma~;netic casting system. Various approaches have beendescribed in the prior art ~or co~trolling the excitatlon of the inductor in a manner so as to provide in~;ots of uni~orm cross section. In U.S. Patent No.
4,C14,379 to Getselev~ a control system is described for controlling the magnitude of the current flowing through the inductor as a function of the deviations of the dimensions of the liquid zone Or the ingot ~rom a prescribed value~
In Russlan Patent No~ 537,750 to Getselev, an alternative control approach ls described wherein the-potential on the inductor is regulated to reduce a deviation of the phase angle from a prep~ogrammed value.
In Russian Patent No. 273,226 to Kabakov~ there i3 disclosed a control system ~or operating an ingot withdrawal mechanism to withdraw the ingot from the mold after the molten metal reaches a desired level on the mold.
In Russian Patent No. 338,297 to Irkutsk, the ele^tromagnetic casting mold is fitted with measuring
3~
coils to control the metal ~lowing into the mold is controlled by a value.
An lmportant function of control systems for electromagnetic castlng systems ~s to take into account changes in the molten metal head due to fluctuations such as in t;he position o~ ~he solid-llquid interface between the molten metal and the solidified casting.
These changes in the interface posit;lon occur because of instablllties in the withdrawal mechanism, insta-bilities in the coolant application system, etcO Theresult of increasing or reducin~ the height of the molten metal head whether due to a repositioning o~ the solid~liquid interface or the upper surface of the molten metal or both is to lncrease or decrease, respectively, the hydrostatic pressure exerted by the molten metal head. These changes in hydrostatic pressure may be offset b~ the control system for con-trolling the excitation of the inductor.
In U.S. Patent No. 4,1619206 to Yarwood et al., a control system for electromagnetic casting is utilized for minimizing variations in the gap between the molten metal and the inductor. ~ha~ approach includes determining a reactive electrical parameter of the inductor which varies with the magnitude of the gap and comparing it to reference values ~o generate an error signal for controlling the inductor excit;ation.
The system described in Yarwood et al. preferably provides solidi~ied castings of more uniform cross section by overcoming the instablities associated wit~
changes in hydroskatic pressure of the molten metal head. The control system for controlling the inductor e~citatlon operates most effectively over a preferred range of cont;rol. Therefore~ it is preferable to minimize any long ~erm changes in hydrostatic pressure due to variations in the height of the molten metal head. Consequently~ it is thought t,o be desirable to ~9~37~
coils to control the metal ~lowing into the mold is controlled by a value.
An lmportant function of control systems for electromagnetic castlng systems ~s to take into account changes in the molten metal head due to fluctuations such as in t;he position o~ ~he solid-llquid interface between the molten metal and the solidified casting.
These changes in the interface posit;lon occur because of instablllties in the withdrawal mechanism, insta-bilities in the coolant application system, etcO Theresult of increasing or reducin~ the height of the molten metal head whether due to a repositioning o~ the solid~liquid interface or the upper surface of the molten metal or both is to lncrease or decrease, respectively, the hydrostatic pressure exerted by the molten metal head. These changes in hydrostatic pressure may be offset b~ the control system for con-trolling the excitation of the inductor.
In U.S. Patent No. 4,1619206 to Yarwood et al., a control system for electromagnetic casting is utilized for minimizing variations in the gap between the molten metal and the inductor. ~ha~ approach includes determining a reactive electrical parameter of the inductor which varies with the magnitude of the gap and comparing it to reference values ~o generate an error signal for controlling the inductor excit;ation.
The system described in Yarwood et al. preferably provides solidi~ied castings of more uniform cross section by overcoming the instablities associated wit~
changes in hydroskatic pressure of the molten metal head. The control system for controlling the inductor e~citatlon operates most effectively over a preferred range of cont;rol. Therefore~ it is preferable to minimize any long ~erm changes in hydrostatic pressure due to variations in the height of the molten metal head. Consequently~ it is thought t,o be desirable to ~9~37~
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control the ~ariation ~n molten metal head height during a cas~ing run in addition to the inductor exc~itation.
The British Patent No. 1,516,306 to ~regory dis-closes a method and apparatus for forming a flexible elongate metallic member. Although this British patent is of interest, it does not provide a prioritized control system as in the present invention.
The present in~ention substantially overcomes the 1~ deficiencies described above to maintain accurate hyclrostatic pressure head control. The instant electromagnetic casting system relies on a material _ _ /
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volume balance casting control while the hydrostatic pressure of the molten material is substantially constant. However, when significant variation in the hydrostatic pressure occurs, the prioritized control system rapidly restores the hydrostatic pressure to a desired level.
It is a problem underlying the present invention to provide superior hydrostatic pressure control of molten material in a sump of an electromagnetic cast-ing system.
It is an advantage of the present invention toprovide an apparatus for casting a material into a desired shape ~hich obviates one or more of the limita-tions and disadvantages of the described prior arrange-rnents.
It is a further advantage of the present invention t:o provide an apparatus for casting a material into a clesired shape wherein the control system includes a c:ontrol based on the material volume balance.
It is a still further advantage of the present invention to provide an apparatus for casting a mat-erial into a desired shape wherein a prioritized control apparatus rapidly adjusts the hydrostatic pressure due to significant variations in the volume balance of the replenishing and withdrawing material.
Accordingly, there has been provided an apparatus and process for electromagnetically forming material in molten form into a casting of a desired shape. An electromagnetic forming device applies an electro-magnetic field to the molten materials. This magneticfield defines a containment zone for the molten mat-erial. A sump of molten material is operatively associated with and delivers molten material to the forming apparatus. A device is provided for 237~
withdrawing the casting frorh the forming device at a desired withdrawal rate. Another device replenishes the sump of molten material at a replenishing rate as the cas~lng is formed. The improvement comprises a control device for maintaining the hydrostatic pressure exerted by the molten material in the containment zone at a substantially constant desired level. The control device inciudes a ~irst structure for providing a sub-stantially constant material volume balance by maintaining the ratio o~ the wit~drawal rate to the replenishing rate substantially constant when the hydrostatic pressure in the containment zone varies less than a desired percentage ~rom the desired level.
A second device senses variations in the level of the hydrostatic pressure of the molten material in the containment ~one. A third device is responsive to the second device for providing a material volume imbalance by varying the ratio of the withdrawal rate to the replenishing rate when the hydrostatic pressure level of molten material varies more than a desired percent-age -~rom the desired level until the hydrostatic pressure o~ molten material in the containment zone returns to the desired level.
The invention and further developments of the invention are now elucidated by means of preferred embodirnents shown in the drawings:
Figure 1 is a schematic representation o~ an apparatus in accordance with the present invention.
Figure 2 is a schematic representation of a second embodiment o~ a~ ap~ara~us in accordance with the present invention.
Figure 3 is a schematic representation of a third embodlment o~ an apparatu3 in accordance with the present invention.
An apparatus 10 is provided for casting a materlal 12 into a cast strip 13 having a desired shape~ The ~9~3~
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apparatus 10 includes structure 14 for electromag-nekically forming the material in molten form into the desired shape. An electromagnetic forming device 15 applies an electromagnetic field to the molten material. This magnetic field defines a contalnment zone 16 for the mol~en material. A sump 17 of molten material is operatively assoclated with the forming structure 14 whereby molten materlal is delivered to the forming structure. A device 18 withdraws the casting from the forming structure at a desire~ with-drawal rate. Another de~ice 20 replenishes the sump 17 of molten material at a desired replenishing rate as the casting is formed. The improvement comprises a control device 22 for malntaininæ the hydrostatic pressure exerted by the molten material in the contain-ment zone 16 at a substantially constant desired level.
The control device 22 includes a first structure 24 for providing a substantially constant material volume balance by maintaining the ratio of the withdrawal rate to the replenishing rate substantially constant when the hydrostatic pressure in the containment zone varies less than a desired percentage from the desired level.
A second device 26 senses va~iations in the level of the hydrostatic pressure of the molten material in the containment zone. A third device 28 is responsive to the second device 26 for providing a material volume lmbalance by var~lng the ratio of the withdrawal rate to the replenlshing rate when the hydrostatic pressure level of molten material varies more than the desired percentage from the desired level until the hydrostatic pressure o~ molten material in the containment zone returns to the desired le~el.
Referring now to Figure 1, there is shown by way of example an electromagnetic casting appara~us of this invention. The electromagnetic casting apparatus ls comprised of a cooling manifold 32 for applying a coolant to the peripheral surface of the material strip 13 being cast and non-magnetic screen 34. A repleni-shing bar 36 of material, such as silicon, is continuously introduced into the sump 17 during a casting run. The forming structure 14 includes an eleckromagnetic forming device 15 for applying a ~agnetic fieId to the molten material. The device 15 may be an inductor which is excited by an alternating current from a power source 38. Although the inductor la is preferably water cooled and a single section inductor as shown, it is within the scope of the present invention for it to be divided into two sections. In a first portion 40 of the containment zone 16, sol;dification takes place and the molten material is formed into the desired cross-sectional shape of the resulting strip casting 13. A second portion 42 of the containment zone 16 upstream of the first portion, defines the molten material sump 17.
The volume of the molten material sump 17 is sufficiently great to insure that the temperature dif~erentials within the ~olten material sump are minimized and further to insure that the molten material head height which controls the hydrostatic pressure of the molten material within the solidifica-tion portion 40 of the containment zone is maintainedsubstantially constant. This reduces fluctuations in the hydrostatic pressure and provides a resultant strip product of a higher cross_sectional and thickness uniformity. Maintaining the hydrostatic pressure relatively constant ls an important aspect of the present invention as will be further e~plained. While it is preferred to use an electromagnetic force ~ield to support the sump as shown, it ~s also within the scope of the present invention to support the sump by 7i~
other means such as crucibles. In addition, the sump may be located at a remote location from the formlng inductor.
The current in the inductor 15 concentrates in the first portion 40 of the containment zone 16 because lt represents the shortest path. However, at a suitable power level, suf~icient current also ~lows ln the second portion 42 to~support the molten material sump 17. This is a highly desired characteristic of the inductor 15 as shown, since the hydrostatic pressure of the molten material is the greatest in the containment zone where the head height is also the highest. There-~ore, it is desirable that the current density or current per unit area o~ surface 46 in -the contalnment zone 16 also be the greatest. As one proceeds along the ~lared portion 48 of the inductor 15, the current density gradually decreases as the current path increases. This is also desirable because the molten material head height which is supported at each ~o succee~ing point outwardly along the ~lared portion 48 decreases correspondingly. The angle of inclination of the surface 48 is pre~erably selected so that for the material being cast, there is a general balance between the current magnitude in the inductor and the hydro-static pressure exerted by the molten material at eachpoint in the portion 42 of the containment zone. For example, the molten material head height and, there-~ore, the hydrostatic pressure exerted by the molten material at each point o~ the surface 48 o~ the inductor 15 can ~enerally be incr~ased by making the surface 48 more vertically oriented and vice versa.
The non-magnetic screen 34 is provided to ~ine tune and balance the magnetic pressure with the hydro-~tatic pressure of the molten material head near the upper sur~ace 44 o~ the molten material. The non-~agnetic screen 34 may comprise a separate element ~d~V ~ ~
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as shown or may be integrated into other structural elements o~ the apparatus as known in the prior art.
In practice, the non-magnetic screen or shield is employed to intercept a portion o~ the electromagnetic field from the inductor 15 near the top surface 44 to prevent undue rounding off of the top corners of the molten material sump 17. In practice, however, it may be possible due to the particular shape o~ the inductor 15 to eliminate the need for the shield which, there~
fore, is not believed to be an essential element in this apparatus.-Solidification of the mol~en material, which ismagnetically contained in the forming structure 14, may be achieved by direct application of a suitable coolant from the cooling manifold 32 to the solidifying casting structure. The coolant may be supplied to the manifoid from a storage location 39. In the embodiment shown in Figure l, the coolant may be applied to the casting surface directly below the inductor and in very close proximity therewith. Alternatively, if desired, the coolant may be applied to the casting surface within the inductor by providing suitable coolant eJection slots or ports in the inductor itself.
The apparatus lO as shown may be contained within an atmosphere control chamber (not shown) whereby it is possible to perform the process in a desired atmosphere which will reduce the possibility of contamination. A
suitable atmosphere could comprise argon gas. However, any desired atmosphere may be utilized.
The device 20 for replenishing the sump of molten materials at a replenishing rate as the casting is formed includes a capstan drive 50 for controlllng the rate of speed at which the solid replenishing bar 36 is fed into the sump 17. The capstan drive 50 may be driven by a motor 52 whose speed may be controlled by a conventional speed control 53 in a manner as described 9~37~
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hereinbelow. The drive 50 may comprise opposed driven and idling pinch rolls 54 and 56, respectively. In addition, a guide device (not shown) which may be comprised of rollers, slides, or brushes may be inter-posed between the capstan drive and the sump 17. It isalso within the scope of the present invention to co~trol the addition of molten material to the sump by a val~e.
The deuice 18 for withdrawing the casting from the forming structure 14 at a desired withdrawal rate includes a second capstan drive 60 which may be driven by a mo~or ~2 at a speed controlled by a conventional speed control 63 in a manner which will be described hereinbelow. The capstan drive 60 comprises opposed driven and ldllng pinch rolls 64 and 66, respectively.
The cast strip is threaded through the capstan drive rolls 60 and may be fed onto a conventional takeup reel.
The present invention is concerned with the control o~ the casting process and apparatus in order to provide cast strips or ingots which have a substan~
tially uniform cross section over the length of the strip and which are preferably formed of semi-conductor material such as silicon, The control is accomplished in accordance with the present invention by controlling the molten material hydrostatic head in the casting zone so as to maintain a substantially uniform hydro-static pressure. The molten material head corresponds to the pool of molten material arranged above the ~0 solldif~in~ strip or ribbon which exerts ~he aforenoted hydrostatic pressure in the magnetlc casting zone 40.
In the vertical casting apparatus 10 of Figure 1, the molten metal head extends-~rom the top surface 44 of the sump 17 of molten material to the solid-liquld interf'ace or solidification front 68 o~ the strip 13.
In the prior art as noted in the background of this application, various systems have been described 237~
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with the aim of providing cast ingots by the electro-ma~netic casting process which have substantially unifor~ cross sections. Certain of these approaches control the excitation of the inductor in a way so as to compensate for ~ariations in the molten material head in order to maintain uniform dimensions in the cast ingot. ~n example of this approach, as suggested in U.S. Pate~t No. 4,161,206 to Yarwood et al., may be incorporated with the present invention as described below.
With any of these latter described approaches~
some parameter of the casting process or system 10 is sensed by a control system 26 in order to generate an error signal which is applied to the power supply 38.
The power supply, in turn, excites the inductor 15 in order to provide the inductor with the amount of current required to overcome varations in the hydro-static pressure of the molten material head. Any such control system optimally operates at peak efficiency over a given range of such a sensed parameter. During a casting run~ however, there may be trends or changes which can shift the range of the sensed parameter over a period of time adversely with respect to its optimum control range.
O~her systems, as ncted above in the prior art, ~ay provide for longer term changes in the molten material head by controlling the replenishment of the molten material in the sump or the flow of molten material into the containment zone so as to-maintain the hydrostatic pressure witnln desired limits.
In contrast to the approaches mentioned abo~e, the present in~ention is directed to an integrated approach whereby the hydrostatic pressure of the molten ~aterial is maintained at a substantially constant desired le~el~ The control system 22 includes a control de~ice 24 ~or pro~iding a substantially constant material 3~;~
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~olume balance by maintaining the ratio of the ~ith-dra~al rate to the replenishing rate su~stantially constant as long as the hydrostatic pressure of the cont~inment zone ~aries less than a desired percentage fro~ the desired level. In addition~ a control circuit device 28, responsi~e to variations in the level of the hydrostatic pressure of the molten material in the containment zone~ pro~ldes a material volume imbalance ~y varying the ratio of the withdrawal rate to the replenishing rate when the hydrostatic pressure le~el of the molten ~aterial varies more than a desired percentage from the desired level. In this manner, the control system 26 for the inductor 15 is always able to operate`eff.lciently in a gi~en range of values since long term variations in the molten material head ; ~re compensated for so that the hydrostatic pressure of the ~olten material in the containment zone remains in a~desired range of values. In the terms of the present invention, a given percent variation in the hydro-static pressure of the molten material in the containment zone is substantially equal to the same percent variation in /
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the height o~ the sur~ace 44 o~ the sump 17 since the height o~ -the molten head is directly proportional to the pressure exerted by the head.
Referring again to ~igure 1, the prioritized control device 22 includes a control circuit 24 for providing a substantially constant material volume balance by maintaining the ratio o~ the withdrawal rate to the replenishing rate substantially constant when the hydrostatic pressure of the sump 17 varies less than a desired percentage from the desired level. The control system 24 may comprise a conventional swltch back arrangement wherein a plurality of selectable s~eed le~els can be selected for each motor 52 or 62 through speed controls 53 or 63. The rotary speed of the capstan drives 50 and 60 may ~e sensed mechanically during the casting process. The conventional devices are able to directly sense rotary motion and convert to electrical signals by any number o* desired means.
However, in ~iew o~ the lirnitations posed by the possible low rpm o~ the feed motors ln the present application, conventional digital optical tachometers 70 and 71 may be preferable since low rpm motor generation of currents may be inherently noisy. These tachometers can sense and convert the rotary speed of the capstan drives to corresponding output signals.
The output signals from the optical tachometers~may be directed to the structure 24 through llnes 72 and 73 so that the ratlo of the strip exit speed to the replenishment rod entry speed is accurately malntained.
This pro~ides for a control of the material volume balance or imbalance. The control system 24 may utilize a computer, microprocessor, or other desired ci:rcuitry to automatically regulate the speed controls 53 and 63 so as to drive the motors 52 and 62, ~espectively, at the desired speeds ~or achie~ing a desired ~nput to output material Yolume ratio.
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The present invention performs optimally when the cross-sectionai areas o~ the replenishing rod 36 and the exiting cast strip l3 both remain essentially constant. If the cross-sectional area of the repleni-shing rod a~d exiting strip are each uniformlyconsistent ~rom a geometrical standpolnt, the control system 24, as described above, can accurately maintain the hydrostatic head in the casting zone 40 substan-tially constant, However, small varlations in the cross~sectional area of either the rod 36 or exiting strip 13 may lead to a long term error in t~e material volume balance, In particular, this error may result in variations in the level of the upper surface 44 of the sump and thereby the hydrostatic pressure in the containment zone 40~ A change in this hydrostatic pressure causes a change in the cross-sec~ional area of the strip 13 being ~ithdrawn from the inductor.
To alleviate this problem, the present invention includes a circuit device 26 for sensing variations in the level of the hydrostatic pressure of the molten material in the containment zone 16 as well as a con-trol device 28, responsive to the device 26g for varying the ratio of the withdrawal rate to the replenishing rate as requlred. The circuit device 26 may be of any desired desi~n However, preferably it is in accordance with the teachings o~ U.S. Patent No.
4,1615206 to Yarwood et al Using that approach~
curren~ in the inductor 15 is contralled in a manner so as to maintain the inductance substantially constant. This maintains a uniform air ga~ between the molten material and the surrounding inductor as the casting run proceeds. The device ~or controlllng and exciting the lnductor 15 may comprise a separate power supply 38 and an electrical control system 26 as shown, or they could be combined in a single unit. For purposes of the present invention, the control device .~923~
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26 senses the electrical signal of the power source po~tion 38 which excites the inductor. The device 26 generates an electrical error signal in response to the electrical signal of the inductor which is sub-s~antially proportional.to changes in hydrostaticpressure of the molten material head in the containment zone~ The error signal is directed to a control circuit device 28 through line 75.
The circuit device 28 for varying the ratlo o~ the withdrawal rate to the replenishing rate includes clrcuitry for overriding the control device 24. When the h~drostatic pressure level of the molten material in the containment zone varies from a desired level by more than a given percentage, such as ~or example 1~, the error signal is transmitted to the control 28.
Then, the control device 28 provides circuitry ~or si~nalling the devlce 24 to vary the ratio of the with-drawal rate to the repleNishing rate throu~h varying the speed control 53 so that the replenishing rate of the bar 36 entering the sump 17 changes while keeping the withdrawal rate of the cast strip 13 substantially constant. It i5 within the scope of the invention to eit;her increase or decrease the replenishing rate depending on whether the hydrostatic pressure decreases or increases, respectively, or as otherwise deslred.
The new ratio provided by the control device 24 is in effect until the hydrostatic pressure o~ the molten material in the containment zone returns to the appro~-imate desired level. At that time~ the circuitry in control device 28 may signal the control device 24 to take over the control o~ the speed controls. Then, the control 24 resumes its ~unction o~ providing a constant material volume balance by maintaining the ratio o~ the withdrawal rate to the replenishing rate substantially constant. Although the control devlce 28 is pre~erably set to sverride the control device 24 when the hydrostatic pressure varies more than about 1% (plus or minus) ~rom the desired level, it is within the scope o~ the present invention to override the control 24 when the hydrostatic pressure varies more than a value in the range of about 0.5-to 5% (plus or minus) from the desired level.
Thus, the present invention provides a prioritized system 22 where the control device 24 maintains the rat;io of the withdrawal rate to the replenishing rate of the material in the apparatus 10 so that the material volume balance is maintained substantially constant. In the event that thè hydrostatic pressure of the molten materlal at the containment zone varies above a given percentage such as 1% from a desired level, a control device 28 alters the ratio of the device 24 whereby the replenishing rate changes while the withdrawal rate remains substantially constant. Once the hydrostatic pressure returns to the desired level, the control device 28 stops altering the control function of device 24 so that device 24 reverts back to its preferred functioning of providing a constant material volume balance by maintaining a constant ratio of the with-drawal to replenishing rate.
Referring to Figure 2, there is shown a second embodiment of the present invention which is substan-tially identical with the ~irst embodiment with the exception of the manner in whlch the override control circuit 28 changes the ratio o~ the speed o~ the repLenishing rate to the withdrawal rate. The numbers are identlcal for the elements of Figure 2 which are substantially identica~ to the elements of Figure l.
The circuikry o~ the control devlce 28 is connected by line 74 to the control box 24 and by line 76 to the speed control 53. In the event that the hydrosta~ic pressure variqs above about 1% ~rom the desired level~
the control device 28 overrides the normal functionin~
control 24~ Control 28 signals the control 24 to ~Z37;~
continue providing a constant speed of withdrawal of the casting 13 while stopping any control of the speed o~ t~e replenishing bar 36. In addition, control 28 signals the speed control 53 to change the speed of motor 52 to provide a dl~ferent speed of replenishment o~ bar 36 ~o rapidly bring the molten sur~ace 44 b~ck to the desired height. Once the hydrostatic pressure in the containment zone is in the desired range of not more than about 1% variation from the desired le~el, the control device 28 stops both overriding the ratio control 24 and controlling the speed control 53. Then, the ratio control box 24 resumes its function of controlling the speed controls 53 and 63 to provide a constant material volume balance. It is within the scope of the present invention to provide any desired speed, to be ad~usted by speed control 53, in accord-ance with a desired replenishing rate. The control device 28 may either directly control the speed signal provided by control 53 or signal the control 53 to provide an increased or decreased speed of motor 52.
Also, it is within the scope o~ the invention to alter the speed o~ the replenishment in any other desired manner.
Re~erring to Figure 3, a third embodiment of the present invention is disclosed wherein the changes in hydrostatic pressure in the containment zone are detected by sensing the height of the surface 44 of the molten material in the sump. The numbering o~ this embodiment is identical to Figures 1 and 2 where the elements may be identical. The system ~or sensing the height may be of any desired design including any OL
those described in the background o~ this application.
~owever, in ~he preferred embodiment of this system, khe position o~ the head a~ the molten material is carried out through the use of an infrared 37~
imaging system 90 which may ~e secured within an element or elemen~s o~ the electromagnetic castlng apparatus and thus is removed from the primary electro-magnetic casting zone. In this system, the infrared si~nals from the load are delivered by filaments to a signal processor 92 which may include a processor and memory system. The in~ormation output or electrical error signal from ~he slgnal processor may be relayed through 11ne 94 to a control device 28. The control device 28 operates as descrlbed with regards to Figures 1 and 2 above.
In the operation of the embodiment as shown in Flgure 3, the infrared sensing device 90 sense~ the variations in the level of the molten material within the sump 17. When the top surface 44 of the molten material within the sump varies less than about 1~ from a desired level, whereby the hydrostatic pressure correspondingly varies less than about 1% from the desired level, the control circuit 24 operates to provide a constant material volume balance by main-talning the ratio of the withdrawal rate of the cast strip to the replenishing rate of the bar 36 substan-tially constant. In the event that the height o~ the surface o~ the molten material varies more than about 1% from the desired level9 an error signal ~rom processor 92 is sent to con~rol 28. The control device 28 signals the ratio control structure 24 to change the replenishing ra~e wi~hout changing the withdrawal rate.
This process has been described more fully above with regards to the explanation of the embodiment o~
Figure 1~ It is also within the scope o~ the present inventlon to operate the third embodiment with the control device 28 overriding the conkrol device 24 as described with regards to the embodiment of Figure 2.
The power supply 96 of ~he embodimen~ shown in Figure 3 may be combined with an electrical control 37~
system 98. The control system 98 may control the electrical signal sent to the inductor in any desired manner including control as described in U.S. Patent No 4,161,206 to Yarwood et al. as described herein-above.
While the invention has been described generally by reference to semi-conductor materials such as silicon, it is adapted ~or use with a wide range of such semi-metals, metalloidsg semi-conductive, or compound semi-conductive materials includ~ng germanium, sapphire, galliumarsenide or the like. These materials are mentioned by way of example, and it is not intended to exclude other metalloids or semi-metal type materials. In additiong the present invention may be adapted ~or use with various metals including copper and copper alloys, steel and steel alloys, aluminum and a:Luminum alloys, nickel and nickel alloys, titanium, z~rconium, vanadium, tantalum; molybdenum, although ot;her metals and alloys are not intended to be excluded.
It is apparent that there has been provided in accordance with this invention an electromagnetic casting apparatus and method which fully satis~ies the objects, means, and advantages set ~orth hereinabove.
While the invention has been described in combination wi.th the specific embodimen~s thereof, it is evident that many alternatives, modi~icationsg and variations will be apparent to those skilled in the art in light of' ~he foregoing description Accordingly9 lt is intended to embrace all such alternatives, modi~ica-tions~ and variations as ~all within ~he spirit andbroad scope of the appended claims.
control the ~ariation ~n molten metal head height during a cas~ing run in addition to the inductor exc~itation.
The British Patent No. 1,516,306 to ~regory dis-closes a method and apparatus for forming a flexible elongate metallic member. Although this British patent is of interest, it does not provide a prioritized control system as in the present invention.
The present in~ention substantially overcomes the 1~ deficiencies described above to maintain accurate hyclrostatic pressure head control. The instant electromagnetic casting system relies on a material _ _ /
g237~
volume balance casting control while the hydrostatic pressure of the molten material is substantially constant. However, when significant variation in the hydrostatic pressure occurs, the prioritized control system rapidly restores the hydrostatic pressure to a desired level.
It is a problem underlying the present invention to provide superior hydrostatic pressure control of molten material in a sump of an electromagnetic cast-ing system.
It is an advantage of the present invention toprovide an apparatus for casting a material into a desired shape ~hich obviates one or more of the limita-tions and disadvantages of the described prior arrange-rnents.
It is a further advantage of the present invention t:o provide an apparatus for casting a material into a clesired shape wherein the control system includes a c:ontrol based on the material volume balance.
It is a still further advantage of the present invention to provide an apparatus for casting a mat-erial into a desired shape wherein a prioritized control apparatus rapidly adjusts the hydrostatic pressure due to significant variations in the volume balance of the replenishing and withdrawing material.
Accordingly, there has been provided an apparatus and process for electromagnetically forming material in molten form into a casting of a desired shape. An electromagnetic forming device applies an electro-magnetic field to the molten materials. This magneticfield defines a containment zone for the molten mat-erial. A sump of molten material is operatively associated with and delivers molten material to the forming apparatus. A device is provided for 237~
withdrawing the casting frorh the forming device at a desired withdrawal rate. Another device replenishes the sump of molten material at a replenishing rate as the cas~lng is formed. The improvement comprises a control device for maintaining the hydrostatic pressure exerted by the molten material in the containment zone at a substantially constant desired level. The control device inciudes a ~irst structure for providing a sub-stantially constant material volume balance by maintaining the ratio o~ the wit~drawal rate to the replenishing rate substantially constant when the hydrostatic pressure in the containment zone varies less than a desired percentage ~rom the desired level.
A second device senses variations in the level of the hydrostatic pressure of the molten material in the containment ~one. A third device is responsive to the second device for providing a material volume imbalance by varying the ratio of the withdrawal rate to the replenishing rate when the hydrostatic pressure level of molten material varies more than a desired percent-age -~rom the desired level until the hydrostatic pressure o~ molten material in the containment zone returns to the desired level.
The invention and further developments of the invention are now elucidated by means of preferred embodirnents shown in the drawings:
Figure 1 is a schematic representation o~ an apparatus in accordance with the present invention.
Figure 2 is a schematic representation of a second embodiment o~ a~ ap~ara~us in accordance with the present invention.
Figure 3 is a schematic representation of a third embodlment o~ an apparatu3 in accordance with the present invention.
An apparatus 10 is provided for casting a materlal 12 into a cast strip 13 having a desired shape~ The ~9~3~
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apparatus 10 includes structure 14 for electromag-nekically forming the material in molten form into the desired shape. An electromagnetic forming device 15 applies an electromagnetic field to the molten material. This magnetic field defines a contalnment zone 16 for the mol~en material. A sump 17 of molten material is operatively assoclated with the forming structure 14 whereby molten materlal is delivered to the forming structure. A device 18 withdraws the casting from the forming structure at a desire~ with-drawal rate. Another de~ice 20 replenishes the sump 17 of molten material at a desired replenishing rate as the casting is formed. The improvement comprises a control device 22 for malntaininæ the hydrostatic pressure exerted by the molten material in the contain-ment zone 16 at a substantially constant desired level.
The control device 22 includes a first structure 24 for providing a substantially constant material volume balance by maintaining the ratio of the withdrawal rate to the replenishing rate substantially constant when the hydrostatic pressure in the containment zone varies less than a desired percentage from the desired level.
A second device 26 senses va~iations in the level of the hydrostatic pressure of the molten material in the containment zone. A third device 28 is responsive to the second device 26 for providing a material volume lmbalance by var~lng the ratio of the withdrawal rate to the replenlshing rate when the hydrostatic pressure level of molten material varies more than the desired percentage from the desired level until the hydrostatic pressure o~ molten material in the containment zone returns to the desired le~el.
Referring now to Figure 1, there is shown by way of example an electromagnetic casting appara~us of this invention. The electromagnetic casting apparatus ls comprised of a cooling manifold 32 for applying a coolant to the peripheral surface of the material strip 13 being cast and non-magnetic screen 34. A repleni-shing bar 36 of material, such as silicon, is continuously introduced into the sump 17 during a casting run. The forming structure 14 includes an eleckromagnetic forming device 15 for applying a ~agnetic fieId to the molten material. The device 15 may be an inductor which is excited by an alternating current from a power source 38. Although the inductor la is preferably water cooled and a single section inductor as shown, it is within the scope of the present invention for it to be divided into two sections. In a first portion 40 of the containment zone 16, sol;dification takes place and the molten material is formed into the desired cross-sectional shape of the resulting strip casting 13. A second portion 42 of the containment zone 16 upstream of the first portion, defines the molten material sump 17.
The volume of the molten material sump 17 is sufficiently great to insure that the temperature dif~erentials within the ~olten material sump are minimized and further to insure that the molten material head height which controls the hydrostatic pressure of the molten material within the solidifica-tion portion 40 of the containment zone is maintainedsubstantially constant. This reduces fluctuations in the hydrostatic pressure and provides a resultant strip product of a higher cross_sectional and thickness uniformity. Maintaining the hydrostatic pressure relatively constant ls an important aspect of the present invention as will be further e~plained. While it is preferred to use an electromagnetic force ~ield to support the sump as shown, it ~s also within the scope of the present invention to support the sump by 7i~
other means such as crucibles. In addition, the sump may be located at a remote location from the formlng inductor.
The current in the inductor 15 concentrates in the first portion 40 of the containment zone 16 because lt represents the shortest path. However, at a suitable power level, suf~icient current also ~lows ln the second portion 42 to~support the molten material sump 17. This is a highly desired characteristic of the inductor 15 as shown, since the hydrostatic pressure of the molten material is the greatest in the containment zone where the head height is also the highest. There-~ore, it is desirable that the current density or current per unit area o~ surface 46 in -the contalnment zone 16 also be the greatest. As one proceeds along the ~lared portion 48 of the inductor 15, the current density gradually decreases as the current path increases. This is also desirable because the molten material head height which is supported at each ~o succee~ing point outwardly along the ~lared portion 48 decreases correspondingly. The angle of inclination of the surface 48 is pre~erably selected so that for the material being cast, there is a general balance between the current magnitude in the inductor and the hydro-static pressure exerted by the molten material at eachpoint in the portion 42 of the containment zone. For example, the molten material head height and, there-~ore, the hydrostatic pressure exerted by the molten material at each point o~ the surface 48 o~ the inductor 15 can ~enerally be incr~ased by making the surface 48 more vertically oriented and vice versa.
The non-magnetic screen 34 is provided to ~ine tune and balance the magnetic pressure with the hydro-~tatic pressure of the molten material head near the upper sur~ace 44 o~ the molten material. The non-~agnetic screen 34 may comprise a separate element ~d~V ~ ~
110~2-MB
as shown or may be integrated into other structural elements o~ the apparatus as known in the prior art.
In practice, the non-magnetic screen or shield is employed to intercept a portion o~ the electromagnetic field from the inductor 15 near the top surface 44 to prevent undue rounding off of the top corners of the molten material sump 17. In practice, however, it may be possible due to the particular shape o~ the inductor 15 to eliminate the need for the shield which, there~
fore, is not believed to be an essential element in this apparatus.-Solidification of the mol~en material, which ismagnetically contained in the forming structure 14, may be achieved by direct application of a suitable coolant from the cooling manifold 32 to the solidifying casting structure. The coolant may be supplied to the manifoid from a storage location 39. In the embodiment shown in Figure l, the coolant may be applied to the casting surface directly below the inductor and in very close proximity therewith. Alternatively, if desired, the coolant may be applied to the casting surface within the inductor by providing suitable coolant eJection slots or ports in the inductor itself.
The apparatus lO as shown may be contained within an atmosphere control chamber (not shown) whereby it is possible to perform the process in a desired atmosphere which will reduce the possibility of contamination. A
suitable atmosphere could comprise argon gas. However, any desired atmosphere may be utilized.
The device 20 for replenishing the sump of molten materials at a replenishing rate as the casting is formed includes a capstan drive 50 for controlllng the rate of speed at which the solid replenishing bar 36 is fed into the sump 17. The capstan drive 50 may be driven by a motor 52 whose speed may be controlled by a conventional speed control 53 in a manner as described 9~37~
~ 11032-MB
hereinbelow. The drive 50 may comprise opposed driven and idling pinch rolls 54 and 56, respectively. In addition, a guide device (not shown) which may be comprised of rollers, slides, or brushes may be inter-posed between the capstan drive and the sump 17. It isalso within the scope of the present invention to co~trol the addition of molten material to the sump by a val~e.
The deuice 18 for withdrawing the casting from the forming structure 14 at a desired withdrawal rate includes a second capstan drive 60 which may be driven by a mo~or ~2 at a speed controlled by a conventional speed control 63 in a manner which will be described hereinbelow. The capstan drive 60 comprises opposed driven and ldllng pinch rolls 64 and 66, respectively.
The cast strip is threaded through the capstan drive rolls 60 and may be fed onto a conventional takeup reel.
The present invention is concerned with the control o~ the casting process and apparatus in order to provide cast strips or ingots which have a substan~
tially uniform cross section over the length of the strip and which are preferably formed of semi-conductor material such as silicon, The control is accomplished in accordance with the present invention by controlling the molten material hydrostatic head in the casting zone so as to maintain a substantially uniform hydro-static pressure. The molten material head corresponds to the pool of molten material arranged above the ~0 solldif~in~ strip or ribbon which exerts ~he aforenoted hydrostatic pressure in the magnetlc casting zone 40.
In the vertical casting apparatus 10 of Figure 1, the molten metal head extends-~rom the top surface 44 of the sump 17 of molten material to the solid-liquld interf'ace or solidification front 68 o~ the strip 13.
In the prior art as noted in the background of this application, various systems have been described 237~
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with the aim of providing cast ingots by the electro-ma~netic casting process which have substantially unifor~ cross sections. Certain of these approaches control the excitation of the inductor in a way so as to compensate for ~ariations in the molten material head in order to maintain uniform dimensions in the cast ingot. ~n example of this approach, as suggested in U.S. Pate~t No. 4,161,206 to Yarwood et al., may be incorporated with the present invention as described below.
With any of these latter described approaches~
some parameter of the casting process or system 10 is sensed by a control system 26 in order to generate an error signal which is applied to the power supply 38.
The power supply, in turn, excites the inductor 15 in order to provide the inductor with the amount of current required to overcome varations in the hydro-static pressure of the molten material head. Any such control system optimally operates at peak efficiency over a given range of such a sensed parameter. During a casting run~ however, there may be trends or changes which can shift the range of the sensed parameter over a period of time adversely with respect to its optimum control range.
O~her systems, as ncted above in the prior art, ~ay provide for longer term changes in the molten material head by controlling the replenishment of the molten material in the sump or the flow of molten material into the containment zone so as to-maintain the hydrostatic pressure witnln desired limits.
In contrast to the approaches mentioned abo~e, the present in~ention is directed to an integrated approach whereby the hydrostatic pressure of the molten ~aterial is maintained at a substantially constant desired le~el~ The control system 22 includes a control de~ice 24 ~or pro~iding a substantially constant material 3~;~
~13- 11032-~lB
~olume balance by maintaining the ratio of the ~ith-dra~al rate to the replenishing rate su~stantially constant as long as the hydrostatic pressure of the cont~inment zone ~aries less than a desired percentage fro~ the desired level. In addition~ a control circuit device 28, responsi~e to variations in the level of the hydrostatic pressure of the molten material in the containment zone~ pro~ldes a material volume imbalance ~y varying the ratio of the withdrawal rate to the replenishing rate when the hydrostatic pressure le~el of the molten ~aterial varies more than a desired percentage from the desired level. In this manner, the control system 26 for the inductor 15 is always able to operate`eff.lciently in a gi~en range of values since long term variations in the molten material head ; ~re compensated for so that the hydrostatic pressure of the ~olten material in the containment zone remains in a~desired range of values. In the terms of the present invention, a given percent variation in the hydro-static pressure of the molten material in the containment zone is substantially equal to the same percent variation in /
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the height o~ the sur~ace 44 o~ the sump 17 since the height o~ -the molten head is directly proportional to the pressure exerted by the head.
Referring again to ~igure 1, the prioritized control device 22 includes a control circuit 24 for providing a substantially constant material volume balance by maintaining the ratio o~ the withdrawal rate to the replenishing rate substantially constant when the hydrostatic pressure of the sump 17 varies less than a desired percentage from the desired level. The control system 24 may comprise a conventional swltch back arrangement wherein a plurality of selectable s~eed le~els can be selected for each motor 52 or 62 through speed controls 53 or 63. The rotary speed of the capstan drives 50 and 60 may ~e sensed mechanically during the casting process. The conventional devices are able to directly sense rotary motion and convert to electrical signals by any number o* desired means.
However, in ~iew o~ the lirnitations posed by the possible low rpm o~ the feed motors ln the present application, conventional digital optical tachometers 70 and 71 may be preferable since low rpm motor generation of currents may be inherently noisy. These tachometers can sense and convert the rotary speed of the capstan drives to corresponding output signals.
The output signals from the optical tachometers~may be directed to the structure 24 through llnes 72 and 73 so that the ratlo of the strip exit speed to the replenishment rod entry speed is accurately malntained.
This pro~ides for a control of the material volume balance or imbalance. The control system 24 may utilize a computer, microprocessor, or other desired ci:rcuitry to automatically regulate the speed controls 53 and 63 so as to drive the motors 52 and 62, ~espectively, at the desired speeds ~or achie~ing a desired ~nput to output material Yolume ratio.
-15- 11032-~B
The present invention performs optimally when the cross-sectionai areas o~ the replenishing rod 36 and the exiting cast strip l3 both remain essentially constant. If the cross-sectional area of the repleni-shing rod a~d exiting strip are each uniformlyconsistent ~rom a geometrical standpolnt, the control system 24, as described above, can accurately maintain the hydrostatic head in the casting zone 40 substan-tially constant, However, small varlations in the cross~sectional area of either the rod 36 or exiting strip 13 may lead to a long term error in t~e material volume balance, In particular, this error may result in variations in the level of the upper surface 44 of the sump and thereby the hydrostatic pressure in the containment zone 40~ A change in this hydrostatic pressure causes a change in the cross-sec~ional area of the strip 13 being ~ithdrawn from the inductor.
To alleviate this problem, the present invention includes a circuit device 26 for sensing variations in the level of the hydrostatic pressure of the molten material in the containment zone 16 as well as a con-trol device 28, responsive to the device 26g for varying the ratio of the withdrawal rate to the replenishing rate as requlred. The circuit device 26 may be of any desired desi~n However, preferably it is in accordance with the teachings o~ U.S. Patent No.
4,1615206 to Yarwood et al Using that approach~
curren~ in the inductor 15 is contralled in a manner so as to maintain the inductance substantially constant. This maintains a uniform air ga~ between the molten material and the surrounding inductor as the casting run proceeds. The device ~or controlllng and exciting the lnductor 15 may comprise a separate power supply 38 and an electrical control system 26 as shown, or they could be combined in a single unit. For purposes of the present invention, the control device .~923~
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26 senses the electrical signal of the power source po~tion 38 which excites the inductor. The device 26 generates an electrical error signal in response to the electrical signal of the inductor which is sub-s~antially proportional.to changes in hydrostaticpressure of the molten material head in the containment zone~ The error signal is directed to a control circuit device 28 through line 75.
The circuit device 28 for varying the ratlo o~ the withdrawal rate to the replenishing rate includes clrcuitry for overriding the control device 24. When the h~drostatic pressure level of the molten material in the containment zone varies from a desired level by more than a given percentage, such as ~or example 1~, the error signal is transmitted to the control 28.
Then, the control device 28 provides circuitry ~or si~nalling the devlce 24 to vary the ratio of the with-drawal rate to the repleNishing rate throu~h varying the speed control 53 so that the replenishing rate of the bar 36 entering the sump 17 changes while keeping the withdrawal rate of the cast strip 13 substantially constant. It i5 within the scope of the invention to eit;her increase or decrease the replenishing rate depending on whether the hydrostatic pressure decreases or increases, respectively, or as otherwise deslred.
The new ratio provided by the control device 24 is in effect until the hydrostatic pressure o~ the molten material in the containment zone returns to the appro~-imate desired level. At that time~ the circuitry in control device 28 may signal the control device 24 to take over the control o~ the speed controls. Then, the control 24 resumes its ~unction o~ providing a constant material volume balance by maintaining the ratio o~ the withdrawal rate to the replenishing rate substantially constant. Although the control devlce 28 is pre~erably set to sverride the control device 24 when the hydrostatic pressure varies more than about 1% (plus or minus) ~rom the desired level, it is within the scope o~ the present invention to override the control 24 when the hydrostatic pressure varies more than a value in the range of about 0.5-to 5% (plus or minus) from the desired level.
Thus, the present invention provides a prioritized system 22 where the control device 24 maintains the rat;io of the withdrawal rate to the replenishing rate of the material in the apparatus 10 so that the material volume balance is maintained substantially constant. In the event that thè hydrostatic pressure of the molten materlal at the containment zone varies above a given percentage such as 1% from a desired level, a control device 28 alters the ratio of the device 24 whereby the replenishing rate changes while the withdrawal rate remains substantially constant. Once the hydrostatic pressure returns to the desired level, the control device 28 stops altering the control function of device 24 so that device 24 reverts back to its preferred functioning of providing a constant material volume balance by maintaining a constant ratio of the with-drawal to replenishing rate.
Referring to Figure 2, there is shown a second embodiment of the present invention which is substan-tially identical with the ~irst embodiment with the exception of the manner in whlch the override control circuit 28 changes the ratio o~ the speed o~ the repLenishing rate to the withdrawal rate. The numbers are identlcal for the elements of Figure 2 which are substantially identica~ to the elements of Figure l.
The circuikry o~ the control devlce 28 is connected by line 74 to the control box 24 and by line 76 to the speed control 53. In the event that the hydrosta~ic pressure variqs above about 1% ~rom the desired level~
the control device 28 overrides the normal functionin~
control 24~ Control 28 signals the control 24 to ~Z37;~
continue providing a constant speed of withdrawal of the casting 13 while stopping any control of the speed o~ t~e replenishing bar 36. In addition, control 28 signals the speed control 53 to change the speed of motor 52 to provide a dl~ferent speed of replenishment o~ bar 36 ~o rapidly bring the molten sur~ace 44 b~ck to the desired height. Once the hydrostatic pressure in the containment zone is in the desired range of not more than about 1% variation from the desired le~el, the control device 28 stops both overriding the ratio control 24 and controlling the speed control 53. Then, the ratio control box 24 resumes its function of controlling the speed controls 53 and 63 to provide a constant material volume balance. It is within the scope of the present invention to provide any desired speed, to be ad~usted by speed control 53, in accord-ance with a desired replenishing rate. The control device 28 may either directly control the speed signal provided by control 53 or signal the control 53 to provide an increased or decreased speed of motor 52.
Also, it is within the scope o~ the invention to alter the speed o~ the replenishment in any other desired manner.
Re~erring to Figure 3, a third embodiment of the present invention is disclosed wherein the changes in hydrostatic pressure in the containment zone are detected by sensing the height of the surface 44 of the molten material in the sump. The numbering o~ this embodiment is identical to Figures 1 and 2 where the elements may be identical. The system ~or sensing the height may be of any desired design including any OL
those described in the background o~ this application.
~owever, in ~he preferred embodiment of this system, khe position o~ the head a~ the molten material is carried out through the use of an infrared 37~
imaging system 90 which may ~e secured within an element or elemen~s o~ the electromagnetic castlng apparatus and thus is removed from the primary electro-magnetic casting zone. In this system, the infrared si~nals from the load are delivered by filaments to a signal processor 92 which may include a processor and memory system. The in~ormation output or electrical error signal from ~he slgnal processor may be relayed through 11ne 94 to a control device 28. The control device 28 operates as descrlbed with regards to Figures 1 and 2 above.
In the operation of the embodiment as shown in Flgure 3, the infrared sensing device 90 sense~ the variations in the level of the molten material within the sump 17. When the top surface 44 of the molten material within the sump varies less than about 1~ from a desired level, whereby the hydrostatic pressure correspondingly varies less than about 1% from the desired level, the control circuit 24 operates to provide a constant material volume balance by main-talning the ratio of the withdrawal rate of the cast strip to the replenishing rate of the bar 36 substan-tially constant. In the event that the height o~ the surface o~ the molten material varies more than about 1% from the desired level9 an error signal ~rom processor 92 is sent to con~rol 28. The control device 28 signals the ratio control structure 24 to change the replenishing ra~e wi~hout changing the withdrawal rate.
This process has been described more fully above with regards to the explanation of the embodiment o~
Figure 1~ It is also within the scope o~ the present inventlon to operate the third embodiment with the control device 28 overriding the conkrol device 24 as described with regards to the embodiment of Figure 2.
The power supply 96 of ~he embodimen~ shown in Figure 3 may be combined with an electrical control 37~
system 98. The control system 98 may control the electrical signal sent to the inductor in any desired manner including control as described in U.S. Patent No 4,161,206 to Yarwood et al. as described herein-above.
While the invention has been described generally by reference to semi-conductor materials such as silicon, it is adapted ~or use with a wide range of such semi-metals, metalloidsg semi-conductive, or compound semi-conductive materials includ~ng germanium, sapphire, galliumarsenide or the like. These materials are mentioned by way of example, and it is not intended to exclude other metalloids or semi-metal type materials. In additiong the present invention may be adapted ~or use with various metals including copper and copper alloys, steel and steel alloys, aluminum and a:Luminum alloys, nickel and nickel alloys, titanium, z~rconium, vanadium, tantalum; molybdenum, although ot;her metals and alloys are not intended to be excluded.
It is apparent that there has been provided in accordance with this invention an electromagnetic casting apparatus and method which fully satis~ies the objects, means, and advantages set ~orth hereinabove.
While the invention has been described in combination wi.th the specific embodimen~s thereof, it is evident that many alternatives, modi~icationsg and variations will be apparent to those skilled in the art in light of' ~he foregoing description Accordingly9 lt is intended to embrace all such alternatives, modi~ica-tions~ and variations as ~all within ~he spirit andbroad scope of the appended claims.
Claims (20)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. In an apparatus for casting a material into a desired shape, said apparatus comprising:
means for electromagnetically forming said material in molten form into said desired shape, said electromagnetic forming means including means for applying a magnetic field to said molten material, said magnetic field defining a containment zone for said molten material, sump means of said molten material operatively associated with said forming means for delivery of said molten material to said forming means, means for withdrawing the casting from said forming means at a withdrawal rate, means for replenishing said sump means of molten material at a replenishing rate as said casting is formed, the improvement comprising:
control means for maintaining the hydrostatic pressure exerted by said molten material in said containment zone at a substantially constant desired level, said control means including first means for providing a substantially constant material volume balance by maintaining the ratio of said withdrawal rate to said replenishing rate substantially constant when the hydrostatic pressure in said containment zone varies less than desired percentage from the desired level, second means for sensing variations in the level of the hydrostatic pressure of the molten material in said containment zone, and third means responsive to said second means for providing a material volume imbalance by varying the ratio of the withdrawal rate to the replenishing rate when said hydrostatic pressure level of molten material varies more than said desired percentage from the desired level until the hydrostatic pressure of molten material in said containment zone returns to approxi-mately said desired level.
means for electromagnetically forming said material in molten form into said desired shape, said electromagnetic forming means including means for applying a magnetic field to said molten material, said magnetic field defining a containment zone for said molten material, sump means of said molten material operatively associated with said forming means for delivery of said molten material to said forming means, means for withdrawing the casting from said forming means at a withdrawal rate, means for replenishing said sump means of molten material at a replenishing rate as said casting is formed, the improvement comprising:
control means for maintaining the hydrostatic pressure exerted by said molten material in said containment zone at a substantially constant desired level, said control means including first means for providing a substantially constant material volume balance by maintaining the ratio of said withdrawal rate to said replenishing rate substantially constant when the hydrostatic pressure in said containment zone varies less than desired percentage from the desired level, second means for sensing variations in the level of the hydrostatic pressure of the molten material in said containment zone, and third means responsive to said second means for providing a material volume imbalance by varying the ratio of the withdrawal rate to the replenishing rate when said hydrostatic pressure level of molten material varies more than said desired percentage from the desired level until the hydrostatic pressure of molten material in said containment zone returns to approxi-mately said desired level.
2. An apparatus as in claim 1 wherein said third means includes a first circuit means for overriding said first means to prevent said first means from con-trolling said replenishing rate, said first circuit means further providing means for changing said re-plenishing rate without altering said withdrawal rate.
3. The apparatus of claim 1 wherein said third means includes a second circuit means for changing the ratio of said first means so that said replenishing rate changes without changing said withdrawal rate.
4. The apparatus of claim 2 wherein said second means includes means for generating a first electrical error signal corresponding to sensed desired varia-tions in said hydrostatic pressure to which said third means is responsive.
5. The apparatus of claim 2 wherein said second means includes means for sensing the height of the molten material in said sump means, said means for sensing the height includes means for generating a second electrical error signal corresponding to sensed desired variations in the height of the material in said sump means, and wherein said third means is responsive to said second electrical error signal.
6. The apparatus of claim 4 or 5 wherein said means for withdrawing includes means for conveying the casting from said forming means at a withdrawal rate of a first desired speed, said means for replenishing including means for conveying a rod of material into said sump means at a replenishing rate of a second desired speed, and said first means for providing a substantially constant material volume balance including means for regulating the first and second speeds so that the volume of the rod of replenishing material conveyed into the sump means is substantially equal to the volume of the casting being withdrawn from said forming means.
7. The apparatus of claim 1 wherein said first means provides a substantially constant material volume balance when the hydrostatic pressure of said sump varies less than about 5% from the desired level and said third means is responsive when the hydrostatic pressure level varies more than about 5% from the desired level.
8. In a process for casting a material into a desired shape, the process comprising the steps of:
providing an electromagnetic field defining a containment zone for forming said material in molten form into said desired shape, delivering said molten material to said electro-magnetic field from a sump of said molten material, withdrawing a cast strip from said electromagnetic field at a withdrawal rate, replenishing said sump of molten material at a replenishing rate as said cast strip is formed, the improvement comprising the steps of:
maintaining the hydrostatic pressure exerted by the molten material in said containment zone at a substantially constant desired level, maintaining a substantially constant material volume balance by keeping the ratio of said withdrawal rate to said replenishing rate substantially constant when the hydrostatic pressure of the molten material in the containment zone varies less than a desired percentage from the desired level, providing a material volume imbalance by varying the ratio of the withdrawal rate to the replenishing rate when the hydrostatic pressure level of molten material varies more than said desired percentage from the desired level until the hydrostatic pressure of molten material in said containment zone returns to the desired level.
providing an electromagnetic field defining a containment zone for forming said material in molten form into said desired shape, delivering said molten material to said electro-magnetic field from a sump of said molten material, withdrawing a cast strip from said electromagnetic field at a withdrawal rate, replenishing said sump of molten material at a replenishing rate as said cast strip is formed, the improvement comprising the steps of:
maintaining the hydrostatic pressure exerted by the molten material in said containment zone at a substantially constant desired level, maintaining a substantially constant material volume balance by keeping the ratio of said withdrawal rate to said replenishing rate substantially constant when the hydrostatic pressure of the molten material in the containment zone varies less than a desired percentage from the desired level, providing a material volume imbalance by varying the ratio of the withdrawal rate to the replenishing rate when the hydrostatic pressure level of molten material varies more than said desired percentage from the desired level until the hydrostatic pressure of molten material in said containment zone returns to the desired level.
9. The process of claim 8 further comprising the step of overriding the step of providing a sub-stantially constant material volume balance and varying the replenishing rate without altering the withdrawal rate.
10. The process of claim 8 further including the step of varying the ratio of the withdrawal rate to the replenishing rate so that the replenishing rate changes without changing the withdrawal rate.
11. The process as set forth in claim 9 includ-ing the step of generating an electrical error signal corresponding to sensed desired variations in the hydrostatic pressure of more than said desired per-centage from the desired level.
12. The process of claim 11 including the steps of sensing the height of the molten material in the sump, and generating a second electrical error signal corresponding to sensed variations in the height of the material in the sump whereby the step of varying the ratio is in response to said second electrical error signal.
13. The process of claim 12 wherein said step of withdrawing a cast strip includes conveying a cast strip of material from the electromagnetic field at a withdrawal rate of a first desired speed.
14. The process of claim 13 wherein said step of replenishing said sump includes the step of conveying a rod of material into said sump at a replenishing rate of a second desired speed.
15. The process of claim 8 wherein the step of maintaining a substantially constant material volume balance is provided when the hydrostatic pressure of the sump varies less than about 5% from the desired level while the step of varying the ratio occurs when the hydrostatic pressure level varies more than about 5% from the desired level.
16. The apparatus of claim 3 wherein said second means includes means for generating a first electrical error signal corresponding to sensed desired varia-tions in said hydrostatic pressure to which said third means is responsive.
17. The apparatus of claim 3 wherein said second means includes means for sensing the height of the molten material in said sump means, said means for sensing the height includes means for generating a second electrical error signal corresponding to sensed desired variations in the height of the material in said sump means, and wherein said third means is res-ponsive to said second electrical error signal.
18. The apparatus of claim 16 or 17 wherein said means for withdrawing includes means for conveying the casting from said forming means at a withdrawal rate of a first desired speed, said means for replenishing including means for conveying a rod of material into said sump means at a replenishing rate of a second de-sired speed, and said first means for providing a substantially constant material volume balance includ-ing means for regulating the first and second speeds so that the volume of the rod of replenishing material conveyed into the sump means is substantially equal to the volume of the casting being withdrawn from said forming means.
19. The process as set forth in claim 10 including the step of generating an electrical error signal corres-ponding to sensed desired variations in the hydrostatic pressure of more than said desired percentage from the desired level.
20. The process of claim 19 including the steps of sensing the height of the molten material in the sump, and generating a second electrical error signal corresponding to sensed variations in the height of the material in the sump whereby the step of varying the ratio is in response to said second electrical error signal.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US27777181A | 1981-06-26 | 1981-06-26 | |
| US277,771 | 1981-06-26 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1192372A true CA1192372A (en) | 1985-08-27 |
Family
ID=23062278
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000405832A Expired CA1192372A (en) | 1981-06-26 | 1982-06-23 | Prioritized electromagnetic casting control system |
Country Status (4)
| Country | Link |
|---|---|
| EP (1) | EP0068826B1 (en) |
| JP (1) | JPS589748A (en) |
| CA (1) | CA1192372A (en) |
| DE (1) | DE3269961D1 (en) |
Families Citing this family (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN102527967B (en) * | 2012-02-16 | 2013-06-19 | 云南通变电磁线有限公司 | Oxygen-free copper rod production line |
Family Cites Families (8)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| SU537750A1 (en) * | 1972-04-24 | 1976-12-05 | Предприятие П/Я В-2996 | Method of controlling continuous and semi-continuous casting of metals |
| US4157373A (en) * | 1972-04-26 | 1979-06-05 | Rca Corporation | Apparatus for the production of ribbon shaped crystals |
| GB1516306A (en) * | 1975-08-29 | 1978-07-05 | Bicc Ltd | Method of and apparatus for continuously forming a flexible elongate metallic member |
| US4161206A (en) * | 1978-05-15 | 1979-07-17 | Olin Corporation | Electromagnetic casting apparatus and process |
| US4158379A (en) * | 1978-07-03 | 1979-06-19 | Olin Corporation | Electromagnetic casting method and apparatus |
| US4242553A (en) * | 1978-08-31 | 1980-12-30 | Samuel Berkman | Apparatus for use in the production of ribbon-shaped crystals from a silicon melt |
| CA1161107A (en) * | 1979-07-11 | 1984-01-24 | Olin Corporation | Controlled water application for electromagnetic casting shape control |
| JPS5946705B2 (en) * | 1979-10-04 | 1984-11-14 | 川崎製鉄株式会社 | Method for controlling molten metal level in continuous casting mold |
-
1982
- 1982-06-23 CA CA000405832A patent/CA1192372A/en not_active Expired
- 1982-06-24 DE DE8282303302T patent/DE3269961D1/en not_active Expired
- 1982-06-24 EP EP19820303302 patent/EP0068826B1/en not_active Expired
- 1982-06-25 JP JP10968282A patent/JPS589748A/en active Pending
Also Published As
| Publication number | Publication date |
|---|---|
| EP0068826B1 (en) | 1986-03-19 |
| EP0068826A1 (en) | 1983-01-05 |
| JPS589748A (en) | 1983-01-20 |
| DE3269961D1 (en) | 1986-04-24 |
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