EP0068826A1

EP0068826A1 - Electromagnetic casting apparatus and process

Info

Publication number: EP0068826A1
Application number: EP82303302A
Authority: EP
Inventors: Michael J. Pryor
Original assignee: Olin Corp
Current assignee: Olin Corp
Priority date: 1981-06-26
Filing date: 1982-06-24
Publication date: 1983-01-05
Also published as: CA1192372A; DE3269961D1; JPS589748A; EP0068826B1

Abstract

An electromagnetic casting apparatus and process is provided with automatic control of the hydrostatic pressure on the material in the casting zone. In the process, controller (24) maintains a constant mass volume balance by maintaining the rate of withdrawal of the casting (13) and the rate of replenishment of the sump (17) by lowering the bar (36) into the sump at a constant ratio. Sensor (26) monitors variations in the hydrostatic pressure in the sump. e.g. by monitoring an electrical parameter of the inductor (15) that is proportional to the hydrostatic pressure, and whenever the sensed hydrostatic pressure variation exceeds a given percentage of a desired value overriding controller (28) overrides or influences the ratio controller (24) to vary the feed/withdrawal ratio temporarily to create a temporary material volume imbalance sufficient to restore the hydrostatic pressure to its desired value.

Description

This invention relates to an improved process and apparatus for producing thin strip 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 variety of processes have been developed for forming semi-conductive materials such as silicon 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 Cells" by A. D. Morrison, published in September 1975, and a paper entitled "The Role of Surface Tension in Pulling Single Crystals of Controlled Dimensions" by G. K. Gaule et al. from Metallurgy 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. discloses an apparatus for producing ribbon-shaped crystals from a silicon melt. The Morrison publication is exemplary of the state of the art with respect to the pulling of 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 subject to substantial thickness variation. At page 82 of the publication, it is indicated that the ribbons which are grown are not of uniform thickness. It is the intent of the present invention to provide a process and apparatus which can form the strip with improved thickness uniformity.
A considerable body of art has developed with respect to the use of electromagnetic containment for the purposes of casting metals. Such electromagnetic casting apparatuses comprise a three-part mold consisting of a water cooled inductor, a non-magnetic screen, and a manifold for applying cooling water to the resultant casting. Such an apparatus is exemplified in U.S. Patent No. 3,467,166 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 molten metal is achieved by the direct application of water from a cooling manifold to the solidifying shell of the casting.
The present invention is particularly related to the process and apparatus for controlling the electromagnetic casting system. Various approaches have been described in the prior art for controlling the excitation of the inductor in a manner so as to provide ingots of uniform cross section. In U.S. Patent No. 4,014,379 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 of the ingot from a prescribed value.
In Russian Patent No. 537,750 to Getselev, an alternative control approach is described wherein the potential on the inductor is regulated to reduce a. deviation of the phase angle from a preprogrammed value.
In Russian Patent No. 273,226 to Kabakov, there is disclosed a control system for 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 electromagnetic casting mold is fitted with measuring coils to control the metal flowing into the mold is controlled by a value.
An important function of control systems for electromagnetic casting systems is to take into account changes in the molten metal head due to fluctuations such as in the position of the solid-liquid interface between the molten metal and the solidified casting. These changes in the interface position occur because of instabilities in the withdrawal mechanism, instabilities in the coolant application system, etc. The result of increasing or reducing the height of the molten metal head whether due to a repositioning of the solid-liquid interface or the upper surface of the molten metal or both is to increase or decrease, respectively, the hydrostatic pressure exerted by the molten metal head. These changes in hydrostatic pressure may be offset by the control system for controlling the excitation of the inductor.
In U.S. Patent No. 4,161,206 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. That approach includes determining a reactive electrical parameter of the inductor which varies with the magnitude of the gap and comparing it to reference values to generate an error signal for controlling the inductor excitation.
The system described in Yarwood et al. preferably provides solidified castings of more uniform cross section by overcoming the instablities associated with changes in hydrostatic pressure of the molten metal head. The control system for controlling the inductor excitation operates most effectively over a preferred range of control. Therefore, it is preferable to minimize any long term changes in hydrostatic pressure due to variations in the height of the molten metal head. Consequently, it is thought to be desirable to control the variation in molten metal head height during a casting run in addition to the inductor excitation.
The British Patent No. 1,516,306 to Gregory discloses 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 invention substantially overcomes the deficiencies described above to maintain accurate hydrostatic pressure head control. The instant electromagnetic casting system relies on a material 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 casting system.
It is an advantage of the present invention to provide an apparatus for casting a material into a desired shape which obviates one or more of the limitations and disadvantages of the described prior arrangements.
It is a further advantage of the present invention to provide an apparatus'for casting a material into a desired shape wherein the control system includes a control based on the material volume balance.
It'is a still further advantage of the present invention to provide an apparatus for casting a material 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 proce'ss for electromagnetically forming material in molten form into a casting of a desired shape. An electromagnetic forming device applies an electromagnetic field to the molten materials. This magnetic field defines a containment zone for the molten material. A sump of molten material is operatively associated with and delivers molten material to the forming apparatus. 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 containment 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 withdrawal rate to the replenishing rate when the hydrostatic pressure level of molten material varies more than a desired percentage from the desired level until the hydrostatic pressure of 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 embodiments shown in the drawings:

Figure 1 is a schematic representation of an apparatus in accordance with the present invention.
Figure 2 is a schematic representation of a second embodiment of an apparatus in accordance with the present invention.
Figure 3 is a schematic representation of a third embodiment of an apparatus in accordance with the present 'invention.

An apparatus 10 is provided for casting a material 12 into a cast strip 13 having a desired shape. The apparatus 10 includes structure 14 for electromagnetically 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 containment zone 16 for the molten material. A sump 17 of molten material is operatively associated with the forming structure 14 whereby molten material is delivered to the forming structure. A device 18 withdraws the casting from the forming structure at a desired withdrawal rate. Another device 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 maintaining the hydrostatic pressure exerted by the molten material in the containment 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 variations 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 ..imbalance by varying the ratio of the withdrawal rate to the replenishing rate when the hydrostatic pressure level of molten material varies more than the desired percentage from the desired level until the hydrostatic pressure of molten material in the containment zone returns to the desired level.
Referring now to Figure 1, there is shown by way of example an electromagnetic casting apparatus of this invention. The electromagnetic casting apparatus is 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 replenishing bar 36 of material, such as silicon, is continuously introduced into the sump 17 during a casting run. The forming structure 14 includes an electromagnetic forming device 15 for applying a magnetic field 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 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, solidification 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 differentials within-the molten 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 solidification portion 40 of the containment zone is maintained substantially 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 is an important aspect of the present invention as will be further explained. While it is preferred to use an electromagnetic force field to support the sump as shown, it is also within the scope of the present invention to support the sump by other means such as crucibles. In addition, the sump may be located at a remote location from the forming inductor.
The current in the inductor 15 concentrates in the first portion 40 of the containment zone 16 because it represents the shortest path. However, at a suitable power level, sufficient current also flows in 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. Therefore, it is desirable that the current density or current per unit area of surface 46 in.the containment zone 16 also be the greatest. As one proceeds along the flared 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 succeeding point outwardly along the flared portion 48 decreases correspondingly. The angle of inclination of the surface 48 is preferably selected so that for the material being cast, there is a general balance between the current magnitude in the inductor and the hydrostatic pressure exerted by the molten material at each point in the portion 42 of the containment zone. For example, the molten material head height and, therefore, the hydrostatic pressure exerted by the.molten material at each point of the surface 48 of the inductor 15 can generally be increased by making the surface 48 more vertically oriented and vice versa.
The non-magnetic screen 34 is provided to fine tune and balance the magnetic pressure with the hydrostatic pressure of the molten material head near the upper surface 44 of the molten material. The non-magnetic screen 34 may comprise a separate element as shown or may be integrated into other structural elements of the apparatus as known in the prior art. In practice, the non-magnetic screen or shield is employed to intercept a portion of 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 of the inductor 15 to eliminate the need for the shield which, therefore, is not believed to be an essential element in this apparatus.
Solidification of the molten material, which is magnetically 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 manifold from a storage location 39. In the embodiment shown in Figure 1, 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 10 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 controlling 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 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 interposed between the capstan drive and the sump 17._' It is also within the scope of the present invention to control the addition of molten material to the sump by a valve.
The device 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 motor 62 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 idling 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 of the casting process and apparatus in order to provide cast strips or ingots which have a substantially 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 hydrostatic pressure. The molten material head corresponds to the pool of molten material arranged above the solidifying strip or'ribbon which exerts the aforenoted hydrostatic pressure in the magnetic casting zone 40. In the vertical casting apparatus 10 of Figure 1, the molten metal head extends from the top surface 44 of the sump 17 of molten material to the solid-liquid interface or solidification front 68 of the strip 13.
In the prior art as noted in the background of this application, various systems have been described with the aim of providing cast ingots by the electromagnetic casting process which have substantially uniform cross sections. Certain of these approaches control the excitation of the inductor in a way so as to compensate for variations in the molten material head in order to maintain uniform dimensions in the cast ingot. An example of this approach, as suggested in U.S. Patent 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 hydrostatic 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.
Other systems, as noted above in the prior art, may 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 within desired limits.
In contrast to the approaches mentioned above, the present invention is directed to an integrated approach whereby the hydrostatic pressure of the molten material is maintained at a substantially constant desired level. The control system 22 includes a control device 24 for providing a substantially constant material volume balance by maintaining the ratio of the withdrawal rate to the replenishing rate substantially constant as long as the hydrostatic pressure of the containment zone varies less than a desired percentage from the desired level. In addition, a control circuit device 28, responsive to variations in the level of the hydrostatic pressure of the molten material in the containment zone, provides a material volume imbalance by varying the ratio of the withdrawal rate to the replenishing rate when the hydrostatic pressure level of the molten material 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 efficiently in a given range of values since long term variations in the molten material head are compensated for so that the hydrostatic pressure of the molten 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 hydrostatic pressure of the molten material in the containment zone is substantially equal to the same percent variation in the height of the surface 44 of the sump 17 since the height of the molten head is directly proportional to the pressure exerted by the head.
Referring again to Figure 1, the prioritized control device 22 includes a control circuit 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 of the sump 17 varies less than a desired percentage from the desired level. The control system 24 may comprise a conventional switch back arrangement wherein a plurality of selectable speed levels 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 be sensed mechanically during the casting process. The conventional devices are able to directly sense rotary motion and convert to electrical signals by any number of desired means. However, in view of the limitations posed by the possible low rpm of the feed motors in 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 lines 72 and 73 so that the ratio of the strip exit speed to the replenishment rod entry speed is accurately maintained. This provides for a control of the material volume balance or imbalance. The control system 24 may utilize a computer, microprocessor, or other desired circuitry to automatically regulate the speed controls 53 and 63 so as to drive the motors 52 and 62, respectively, at the desired speeds for achieving a desired input to output material volume ratio.
The present invention performs optimally when the cross-sectional areas of the replenishing rod 36 and the exiting cast strip _'13 both remain essentially constant. If the cross-sectional area of the replenishing rod and exiting strip are each uniformly consistent from a geometrical standpoint, the control system 24, as described above, can accurately maintain the hydrostatic head in the casting zone 40 substantially constant. However, small variations in the cross-sectional area of either the rod 36 or exiting strip 13 may lead to a long term error in the 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-sectional area of the strip 13 being withdrawn 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 control device 28, responsive to the device 26, for varying the ratio of the withdrawal rate to the replenishing rate as required. The circuit device 26 may be of any desired design. However, preferably it is in accordance with the teachings of U.S. Patent No. 4,161,206 to Yarwood et al. Using that approach, current in the inductor 15 is controlled in a manner so as to maintain the inductance substantially constant. This maintains a uniform air gap between the molten material and the surrounding inductor as the casting run proceeds. The device for controlling and exciting the inductor 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 ₂6 senses the,electriaal signal of the power source portion 38 which excites the inductor. The device 26 generates an electrical error signal in response to the electrical signal of the inductor which is substantially proportional.to changes in hydrostatic pressure 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 ratio of the withdrawal rate to the replenishing rate includes circuitry for overriding the control device 24. When the hydrostatic pressure level of the molten material in the containment zone varies from a desired level by more than a given percentage, such as for example 1%, the error signal is transmitted to the control 28. Then, the control device 28 provides circuitry for signalling the device 24 to vary the ratio of the withdrawal rate to the replenishing rate through 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 is within the scope of the invention to either increase or decrease the replenishing rate depending on whether the hydrostatic pressure decreases or increases, respectively, or as otherwise desired. The new ratio provided by the control device 24 is in effect until the hydrostatic pressure of the molten material in the containment zone returns to the approximate desired level. At that time, the circuitry in control device 28 may signal the control device 24 to take over the control of the speed controls. Then, the control 24 resumes its function of providing a constant material volume balance by maintaining the ratio of the withdrawal rate to the replenishing rate substantially constant. Although the control device 28 is preferably set to override the control device 24 when the hydrostatic pressure varies more than about 1% (plus or minus) from the desired level, it is within the scope of 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 ratio 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 the hydrostatic pressure of the molten material 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 withdrawal to replenishing rate.
Referring to Figure 2, there is shown a second embodiment of the present invention which is substantially identical with the first embodiment with the exception of the-manner in which the override control circuit 28 changes the ratio of the speed of the replenishing rate to the withdrawal rate. The numbers are identical for the elements of Figure 2 which are substantially identical to the elements of Figure 1. The circuitry of the control device 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 hydrostatic pressure varies above about 1% from the desired level, the control device 28 overrides the normal functioning control 24. Control'28 signals the control 24 to continue providing a constant speed of withdrawal of the casting 13 while stopping any control of the speed of the replenishing bar 36. In addition, control 28 signals the speed control 53 to change the speed of motor 52 to provide a different speed of replenishment of bar 36 to rapidly bring the molten surface 44 back 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 level, 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 adjusted by speed control 53, in accordance 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 of the invention to alter the speed of the replenishment in any other desired manner.
Referring 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 of this embodiment is identical to Figures 1 and 2 where the elements may be identical. The system for sensing the height may be of any desired design including any of those described in the background of this application. However, in the preferred embodiment of this system, the position of the head of the molten material is carried out through the use of an infrared imaging system 90 which may be secured within an element or elements of the electromagnetic casting apparatus and thus is removed from the primary electromagnetic casting zone. In this system, the infrared signals from the load are delivered by filaments to a signal processor 92 which may include a processor and memory system. The information output or electrical error signal from the signal processor may be relayed through line 94 to a control device 28. The control device 28 operates as described with regards to Figures 1 and 2 above.
In the operation of the embodiment as shown in Figure 3, the infrared sensing device 90 senses 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 maintaining the ratio of the withdrawal rate of the cast strip to the replenishing rate of the bar 36 substantially constant. In the event that the height of the surface of the molten material varies more than about 1% from the desired level, an error signal from processor 92 is sent to control 28. The control device 28 signals the ratio control structure 24 to change the replenishing rate without changing the withdrawal rate. This process has been described more fully above with regards to the explanation of the embodiment of Figure 1. It is also within the scope of the present invention to operate the third embodiment with the control device 28 overriding the control device 24 as described with regards to the embodiment of Figure 2.
The power supply 96 of the embodiment shown in Figure 3 may be combined with an electrical control 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 hereinabove.
While the invention-has been described generally by reference to semi-conductor materials such as silicon, it is adapted for use with a wide range of such semi-metals, metalloids, semi-conductive, or compound semi-conductive materials including 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 addition, the present invention may be adapted for use with various metals including copper and copper alloys, steel and steel alloys, aluminum and aluminum alloys, nickel and nickel alloys, titanium, zirconium, vanadium, tantalum, molybdenum, although other 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 satisfies the objects, means, and advantages set forth hereinabove. While the invention has been described in combination with the 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. Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the appended claims.

Claims

1. An electromagnetic casting apparatus comprising an inductor (15) defining a casting zone and energisable to produce an electromagnetic force field in said zone which shapes the cast material in said zone to the desired shape, a sump (17) for containing a supply of molten material to be fed to the casting zone, means (20) for repleneshing the molten material in said sump as the material is fed therefrom to the casting zone, means (18) for withdrawing the casting from the casting zone, and means (22) for controlling the rate of withdrawal of the casting and the rate of replenishment of the sump, characterised in that said control means comprise:

means (26) for sensing variations in the hydrostatic pressure imposed upon the material in the casting zone by the material in the sump;

means (24) responsive to said sensing means for providing a substantially constant material volume balance during the casting process by maintaining the ratio of the rate of withdrawal of the casting to the rate of replenishment of the sump substantially constant whilst the said hydrostatic pressure remains within a given percentage of a desired level;

and overriding means (28) for providing a temporary material volume imbalance by varying said ratio in the direction necessary to restore the said hydrostatic pressure to said given percentage range, whenever the said hydrostatic pressure varies from the desired level by more than said given percentage.

2. Apparatus according to claim 1, characterised in that said overriding means comprise a circuit for overriding the response of said providing means to said sensing means, whenever the sensed hydrostatic pressure varies by more than said percentage from said level, and for obtaining said material imbalance by maintaining the rate of withdrawal constant, and changing the rate of replenishment.

3. Apparatus according to claim 1, characterised in that said overriding means comprise a circuit which operates to vary the response of said providing means to said sensing means, whenever the sensed hydrostatic pressure varies by more than said percentage from said level, by increasing the rate of replenishment and maintaining the rate of withdrawal constant and thereby temporarily changing the ratio that is maintained by said providing means.

4. Apparatus according to claim 2 or 3 characterised in that said sensing means is responsive to an electrical parameter of the inductor that is proportional to the said hydrostatic pressure.

5. Apparatus according to claim 2 or 3, characterised in that said sensing means comprise means for sensing variations in the vertical height of the molgen material in said sump.

6. Apparatus according to any one of the preceding claims, characterised in that said replenishment means (20) comprise means (54,56) for supporting a solid bar of replenishment material above the sump and for gradually feeding that bar into the sump at a controlled rate.

7. An electromagnetic casting process which comprises feeding a molten material from a sump to a casting zone, electromagnetically shaping the material in said zone into a desired shape, cooling the casting, continuously withdrawing the casting from said zone, and replenishing the sump with fresh material to be cast, characterised by the steps of controlling the ratio of the rate of withdrawal and the rate of replenishment to maintain the material volume balance substantially constant throughout the casting process; sensing hydrostatic pressure variations in the head of molten material in the casting zone; and whenever the sensed hydrostatic pressure deviates by more than a given percentage from a desired level, automatically varying said ratio to provide a temporary material volume imbalance, thereby to restore the hydrostatic pressure to the desired level.

8. A process according to claim 7, characterised in that said automatic variation is effected by temporarily overriding said ratio control, and varying the rate of replenishment whilst maintaining the rate of withdrawal constant.

9. A process according to claim 7, characterised in that said automatic variation is effected by temporarily varying the controlled ratio of the rate of withdrawal to the rate of replenishment by varying the latter and keeping the former constant.

10. A process according to claim 7, 8 or 9, characterised in that said hydrostatic pressure variations are sensed by monitoring an electrical parameter of the inductor which is proportional to said hydrostatic pressure.

11. A process according to claim 7, 8 or 9, characterised in that said hydrostatic pressure variations are sensed by monitoring the vertical height of the molten material in the sump.

12. A process according to any one of claims 7-11, characterised in that the sump is replenished by gradually feeding into the sump at a controlled rate a solid bar of the material to be cast.