Abstract:
A method and apparatus for stirring a molten thixotropic aluminum alloy comprising a first solid particulate phase suspended in a second liquid phase so as to maintain its thixotropic character by degenerating forming dendritic particles into spheroidal particles while simultaneously equilibrating the melt temperature by quickly transferring heat between the melt and its surroundings. The melt is stirred by a magnetomotive force field generated by a stacked stator assembly. The stacked stator assembly includes a stator ring adapted to generate a linear/longitudinal magnetic field positioned between two stator rings adapted to generate a rotational magnetic field. The stacked stator rings generate a substantially spiral magnetomotive mixing force and define a substantially cylindrical mixing region therein.

Description:
TECHNICAL FIELD OF THE INVENTION  
       [0001]     The present invention relates generally to metallurgy, and, more particularly, to a method and apparatus for controlling the microstructural properties of a molded metal piece by efficiently controlling the temperature and viscosity of a thixotropic precursor metal melt through precisely controlled magnetomotive agitation.  
       BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates in general to an apparatus which is constructed and arranged for producing an “on-demand” semi-solid material for use in a casting process. Included as part of the overall apparatus are various stations which have the requisite components and structural arrangements which are to be used as part of the process. The method of producing the on-demand semi-solid material, using the disclosed apparatus, is included as part of the present invention.  
         [0003]     More specifically, the present invention incorporates electromagnetic stirring techniques and apparata to facilitate the production of the semi-solid material within a comparatively short cycle time. As used herein, the concept of “on-demand” means that the semi-solid material goes directly to the casting step from the vessel where the material is produced. The semi-solid material is typically referred to as a “slurry” and the slug which is produced as a “single shot” is also referred to as a billet.  
         [0004]     It is well known that semi-solid metal slurry can be used to produce products with high strength, leak tight and near net shape. However, the viscosity of semi-solid metal is very sensitive to the slurry&#39;s temperature or the corresponding solid fraction. In order to obtain good fluidity at high solid fraction, the primary solid phase of the semi-solid metal should be nearly spherical.  
         [0005]     In general, semi-solid processing can be divided into two categories; thixocasting and rheocasting. In thixocasting, the microstructure of the solidifying alloy is modified from dendritic to discrete degenerated dendrite before the alloy is cast into solid feedstock, which will then be re-melted to a semi-solid state and cast into a mold to make the desired part. In rheocasting, liquid metal is cooled to a semi-solid state while its microstructure is modified. The slurry is then formed or cast into a mold to produce the desired part or parts.  
         [0006]     The major barrier in rheocasting is the difficulty to generate sufficient slurry within preferred temperature range in a short cycle time. Although the cost of thixocasting is higher due to the additional casting and remelting steps, the implementation of thixocasting in industrial production has far exceeded rheocasting because semi-solid feedstock can be cast in large quantities in separate operations which can be remote in time and space from the reheating and forming steps.  
         [0007]     In a semi-solid casting process, generally, a slurry is formed during solidification consisting of dendritic solid particles whose form is preserved. Initially, dendritic particles nucleate and grow as equiaxed dendrites within the molten alloy in the early stages of slurry or semi-solid formation. With the appropriate cooling rate and stirring, the dendritic particle branches grow larger and the dendrite arms have time to coarsen so that the primary and secondary dendrite arm spacing increases. During this growth stage in the presence of stirring, the dendrite arms come into contact and become fragmented to form degenerate dendritic particles. At the holding temperature, the particles continue to coarsen and become more rounded and approach an ideal spherical shape. The extent of rounding is controlled by the holding time selected for the process. With stirring, the point of “coherency” (the dendrites become a tangled structure) is not reached. The semi-solid material comprised of fragmented, degenerate dendrite particles continues to deform at low shear forces.  
         [0008]     When the desired fraction solid and particle size and shape have been attained, the semi-solid material is ready to be formed by injecting into a die- mold or some other forming process. Solid phase particle size is controlled in the process by limiting the slurry creation process to temperatures above the point at which the solid phase begins to form and particle coarsening begins.  
         [0009]     It is known that the dendritic structure of the primary solid of a semi-solid alloy can be modified to become nearly spherical by introducing the following perturbation in the liquid alloy near liquidus temperature or semi-solid alloy: 
        1) Stirring: mechanical stirring or electromagnetic stirring;     2) Agitation: low frequency vibration, high-frequency wave, electric shock, or electromagnetic wave;     3) Equiaxed Nucleation: rapid under-cooling, grain refiner;     4) Oswald Ripening and Coarsening: holding alloy in semi-solid temperature for a long time.        
 
         [0014]     While the methods in (2)-(4) have been proven effective in modifying the microstructure of semi-solid alloy, they have the common limitation of not being efficient in the processing of a high volume of alloy with a short preparation time due to the following characteristics or requirements of semi-solid metals: 
        High dampening effect in vibration.     Small penetration depth for electromagnetic waves.     High latent heat against rapid under-cooling.     Additional cost and recycling problem to add grain refiners.     Natural ripening takes a long time, precluding a short cycle time.        
 
         [0020]     While most of the prior art developments have been focused on the microstructure and rheology of semi-solid alloy, temperature control has been found by the present inventors to be one of the most critical parameters for reliable and efficient semi-solid processing with a comparatively short cycle time. As the apparent viscosity of semi-solid metal increases exponentially with the solid fraction, a small temperature difference in the alloy with 40% or higher solid fraction results in significant changes in its fluidity. In fact, the greatest barrier in using methods (2)-(4), as listed above, to produce semi-solid metal is the lack of stirring. Without stirring, it is very difficult to make alloy slurry with the required uniform temperature and microstructure, especially when the there is a requirement for a high volume of the alloy. Without stirring, the only way to heat/cool semi-solid metal without creating a large temperature difference is to use a slow heating/cooling process. Such a process often requires that multiple billets of feedstock be processed simultaneously under a pre-programmed furnace and conveyor system, which is expensive, hard to maintain, and difficult to control.  
         [0021]     While using high-speed mechanical stirring within an annular thin gap can generate high shear rate sufficient to break up the dendrites in a semi-solid metal mixture, the thin gap becomes a limit to the process&#39;s volumetric throughput. The combination of high temperature, high corrosion (e.g. of molten aluminum alloy) and high wearing of semi-solid slurry also makes it very difficult to design, to select the proper materials and to maintain the stirring mechanism.  
         [0022]     Prior references disclose the process of forming a semi-solid slurry by reheating a solid billet formed by thixocasting or directly from the melt using mechanical or electromagnetic stirring. The known methods for producing semi-solid alloy slurries include mechanical stirring and inductive electromagnetic stirring. The processes for forming a slurry with the desired structure are controlled, in part, by the interactive influences of the shear and solidification rates.  
         [0023]     In the early 1980&#39;s, an electromagnetic stirring process was developed to cast semi-solid feedstock with discrete degenerate dendrites. The feedstock is cut to proper size and then remelt to semi-solid state before being injected into mold cavity. Although this magneto hydrodynamic (MHD) casting process is capable of generating high volume of semi-solid feedstock with adequate discrete degenerate dendrites, the material handling cost to cast a billet and to remelt it back to a semi-solid composition reduces the competitiveness of this semi-solid process compared to other casting processes, e.g. gravity casting, low-pressure die-casting or high-pressure die-casting. Most of all, the complexity of billet heating equipment, the slow billet heating process and the difficulties in billet temperature control have been the major technical barriers in semi-solid forming of this type.  
         [0024]     The billet reheating process provides a slurry or semi-solid material for the production of semi-solid formed (SSF) products. While this process has been used extensively, there is a limited range of castable alloys. Further, a high fraction of solids (0.7 to 0.8) is required to provide for the mechanical strength required in processing with this form of feedstock. Cost has been another major limitation of this approach due to the required processes of billet casting, handling, and reheating as compared to the direct application of a molten metal feedstock in the competitive die and squeeze casting processes.  
         [0025]     In the mechanical stirring process to form a slurry or semi-solid material, the attack on the rotor by reactive metals results in corrosion products that contaminate the solidifying metal. Furthermore, the annulus formed between the outer edge of the rotor blades and the inner vessel wall within the mixing vessel results in a low shear zone while shear band formation may occur in the transition zone between the high and low shear rate zones. There have been a number of electromagnetic stirring methods described and used in preparing slurry for thixocasting billets for the SSF process, but little mention has been made of an application for rheocasting.  
         [0026]     The rheocasting, i.e., the production by stirring of a liquid metal to form semi-solid slurry that would immediately be shaped, has not been industrialized so far. It is clear that rheocasting should overcome most of limitations of thixocasting. However, in order to become an industrial production technology, i.e., producing stable, deliverable semi-solid slurry on-line (i.e., on-demand) rheocasting must overcome the following practical challenges: cooling rate control, microstructure control, uniformity of temperature and microstructure, the large volume and size of slurry, short cycle time control and the handling of different types of alloys, as well as the means and method of transferring the slurry to a vessel and directly from the vessel to the casting shot sleeve.  
         [0027]     While propeller-type mechanical stirring has been used in the context of making a semi-solid slurry, there are certain problems and limitations. For example, the high temperature and the corrosive and high wearing characteristics of semi-solid slurry make it very difficult to design a reliable slurry apparatus with mechanical stirring. However, the most critical limitation of using mechanical stirring in rheocasting is that its small throughput cannot meet the requirements of production capacity. It is also known that semi-solid metal with discrete degenerated dendrite can also be made by introducing low frequency mechanical vibration, high-frequency ultra-sonic waves, or electric-magnetic agitation with a solenoid coil. While these processes may work for smaller samples at slower cycle time, they are not effective in making larger billet because of the limitation in penetration depth. Another type of process is solenoidal induction agitation, because of its limited magnetic field penetration depth and unnecessary heat generation, it has many technological problems to implement for productivity. Vigorous electromagnetic stirring is the most widely used industrial process permits the production of a large volume of slurry. Importantly, this is applicable to any high-temperature alloys.  
         [0028]     Two main variants of vigorous electromagnetic stirring exist, one is rotational stator stirring, and the other is linear stator stirring. With rotational stator stirring, the molten metal is moving in a quasi-isothermal plane, therefore, the degeneration of dendrites is achieved by dominant mechanical shear. U.S. Pat. No. 4,434,837, issued Mar. 6, 1984 to Winter et al., describes an electromagnetic stirring apparatus for the continuous making of thixotropic metal slurries in which a stator having a single two pole arrangement generates a non-zero rotating magnetic field which moves transversely of a longitudinal axis. The moving magnetic field provides a magnetic stirring force directed tangentially to the metal container, which produces a shear rate of at least 50 sec −1  to break down the dendrites. With linear stator stirring, the slurries within the mesh zone are re-circulated to the higher temperature zone and remelted, therefore, the thermal processes play a more important role in breaking down the dendrites. U.S. Pat. No. 5,219,018, issued Jun. 15, 1993 to Meyer, describes a method of producing thixotropic metallic products by continuous casting with polyphase current electromagnetic agitation. This method achieves the conversion of the dendrites into nodules by causing a refusion of the surface of these dendrites by a continuous transfer of the cold zone where they form towards a hotter zone.  
         [0029]     It is known in the art that thixotropic metal melts may be stirred by the application of a sufficiently strong magnetomotive force. Known techniques for generating such a magnetomotive force include using one or more static magnetic fields, a combination of static and variable magnetic fields, moving magnetic fields, or rotating magnetic fields to stir the metal melt. However, all of these techniques suffer from the same disadvantage of inducing three-dimensional circulation primarily at the container walls, resulting in inhomogeneous mixing of the metal melt. While the above-mentioned known magnetomotive mixing techniques all produce a shear force on the thixotropic melt by inducing rotational movement thereof, three-dimensional circulation is only achieved to the extent that centripetal forces acting on the rotating melt force a top layer of molten metal against the container wall where it travels down the wall and back into the melt at a lower level. Although sufficient to maintain the thixotropic character of the melt, this process is inefficient for uniformly equilibrating the temperature or composition of the entire melt. Obviously, it would be desirable to stir the melt so as to maintain its thixotropic character while simultaneously quickly and efficiently transferring heat between the melt and its surroundings. The present invention is directed toward achieving this goal.  
       SUMMARY OF THE INVENTION  
       [0030]     The present invention relates to a method and apparatus for magnetomotively stirring a metallic melt so as to maintain its thixotropic character (prevent bulk crystallization) by simultaneously quickly and efficiently degenerating dendritic particles formed therein and transferring heat between the melt and its surroundings. One form of the present invention is a stacked stator assembly including a stator ring adapted to generate a linear/longitudinal magnetic field positioned between two stator rings adapted to generate a rotational magnetic field. The stacked stator rings define a generally cylindrical magnetomotive mixing region therein.  
         [0031]     One object of the present invention is to provide an improved magnetomotive metal melt stirring system. Related objects and advantages of the present invention will be apparent from the following description.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0032]     FIG. A is a schematic illustration of a 2-pole multiphase stator.  
         [0033]     FIG. B is a schematic illustration of a multipole stator.  
         [0034]     FIG. C is a graphic illustration of the electric current as a finction of time for each pair of coils of the stator if FIG. A.  
         [0035]     FIG. D is a schematic illustration of a multiphase stator having pairs of coils positioned longitudinally relative a cylindrical mixing volume.  
         [0036]      FIG. 1A  is a schematic front elevational view of a magnetomotive stirring volume defined by a stacked stator assembly having three individual stators according to a first embodiment of the present invention.  
         [0037]      FIG. 1B  is a schematic front elevational view of a magnetomotive stirring volume defined by a stacked stator assembly having two individual stators according to a second embodiment of the present invention.  
         [0038]      FIG. 1C  is a schematic front elevational view of a magnetomotive stirring volume defined by a stacked stator assembly having four individual stators according to a third embodiment of the present invention.  
         [0039]      FIG. 1D  is a schematic front elevational view of a magnetomotive stirring volume defined by a stacked stator assembly having five individual stators according to a fourth embodiment of the present invention.  
         [0040]      FIG. 2A  is a schematic front elevational view of the magnetomotive stirring volume of  FIG. 1A  illustrating the simplified magnetic field interactions produced by each individual stator of a first stator assembly.  
         [0041]      FIG. 2B  is a schematic front elevational view of the combination of magnetomotive forces from each stator of the stator assembly of  FIG. 2A  to generate a substantially spiral resultant magnetic field.  
         [0042]      FIG. 2C  is a schematic front elevational view of the magnetomotive stirring volume of  FIG. 1A  illustrating the simplified magnetic field interactions produced by each individual stator of a second stator assembly.  
         [0043]      FIG. 2D  is a schematic front elevational view of the combination of magnetomotive forces from each stator of the stator assembly of  FIG. 2C  to generate a substantially spiral resultant magnetic field.  
         [0044]      FIG. 3A  is a schematic diagram illustrating the simplified shape of a magnetic field produced by a rotating field stator of  FIG. 1A .  
         [0045]      FIG. 3B  is a schematic diagram illustrating the simplified shape of a magnetic field produced by a linear field stator of  FIG. 1A .  
         [0046]      FIG. 3C  is a schematic diagram illustrating the simplified substantially spiral magnetic field produced by combining the rotating field and linear field stators of  FIG. 1A .  
         [0047]      FIG. 3D  is a perspective schematic view of the cylindrical spiral magnetomotive mixing volume of  FIG. 1A  separated to illustrate an inner cylindrical core portion and an outer cylindrical shell portion.  
         [0048]      FIG. 3E  is a perspective schematic view of the outer portion of  FIG. 3D .  
         [0049]      FIG. 3F  is a perspective schematic view of the inner portion of  FIG. 3D .  
         [0050]      FIG. 4  is a schematic view of a sixth embodiment of the present invention, a magnetomotive stirring apparatus having an electronic controller connected to a stator assembly and receiving voltage feedback.  
         [0051]      FIG. 5  is a schematic view of a seventh embodiment of the present invention, a magnetomotive stirring apparatus having an electronic controller connected to a stator assembly and receiving temperature feedback from temperature sensors.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0052]     For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiment illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, and alterations and modifications in the illustrated device, and further applications of the principles of the invention as illustrated therein are herein contemplated as would normally occur to one skilled in the art to which the invention relates.  
         [0053]     One of the ways to overcome the above challenges, according to the present invention, is to apply modified electromagnetic stirring of substantially the entire liquid metal volume as it solidifies into and through the semi-solid range. Such modified electromagnetic stirring enhances the heat transfer between the liquid metal and its container to control the metal temperature and cooling rate, and generates a sufficiently high shear inside of the liquid metal to modify the microstructure to form discrete degenerate dendrites. Modified electromagnetic stirring increases the uniformity of metal temperature and microstructure by means of increased control of the molten metal mixture. With a careful design of the stirring mechanism and method, the stirring drives and controls a large volume and size of semi-solid slurry, depending on the application requirements. Modified electromagnetic stirring allows the cycle time to be shortened through increased control of the cooling rate. Modified magnetic stirring may be adapted for use with a wide variety of alloys, i.e., casting alloys, wrought alloys, MMC, etc. It should be noted that the mixing requirement to produce and maintain a semi-solid metallic slurry is quite different from that to produce a metal billet through the MHD process, since a billet formed according to the MHD process will have a completely solidified surface layer, while a billet formed from a semi-solid slurry will not.  
         [0054]     In the past, MHD stirring has been achieved by utilizing a 2-pole multiphase stator system to generate a magnetomotive stirring force on a liquid metal. While multipole stator systems are well known, they have not been in the MHD process because, for a given line frequency, multiphase stator systems generate rotating magnetic fields having only one half the rotational speed of fields produced by 2-pole stator systems. FIG. A schematically illustrates a 2-pole multiphase stator system  1  and its resulting magnetic field  2 , while FIG. B schematically illustrates a multipole stator system  1 ′ and its respective magnetic field  2 ′. In general, each stator system  1 ,  1 ′ includes a plurality of pairs of electromagnetic coils or windings  3 ,  3 ′ oriented around a central volume  4 ,  4 ′ respectively. The windings  3 ,  3 ′ are sequentially energized by flowing electric current therethrough.  
         [0055]     FIG. A illustrates a 3-phase 2-pole multiphase stator system  1  having three pairs of windings  3  positioned such that there is a 120 degree phase difference between each pair. The multiphase stator system  1  generates a rotating magnetic field  2  in the central volume  4  when the respective pairs of windings  3  are sequentially energized with electric current. In the instant case, there are three pairs of windings  3  oriented circumferentially around a cylindrical mixing volume  4 , although other designs may employ other numbers of windings  3  having other orientations.  
         [0056]     Typically, the windings or coils  3  are electrically connected so as to form a phase spread over the stirring volume  4 . FIG. C illustrates the relationship of electric current through the windings  3  as a function of time for the windings  3 .  
         [0057]     In use, the magnetic field  2  varies with the change in current flowing through each pair of windings  3 . As the magnetic field  2  varies, a current is induced in a liquid electrical conductor occupying the stirring volume  4 . This induced electric current generates a magnetic field of its own. The interaction of the magnetic fields generates a stirring force acting on the liquid electrical conductor urging it to flow. As the magnetic field rotates, the circumferential magnetomotive force drives the liquid metal conductor to circulate. It should be noted that the magnetic field  2  produced by a multipole system (here, by a 2-pole system) has an instantaneous cross-section bisected by a line of substantially zero magnetic force.  
         [0058]     FIG. D illustrates a set of windings  3  positioned longitudinally relative a cylindrical mixing volume  4 . In this configuration, the changing magnetic field  2  induces circulation of the liquid electrical conductor in a direction parallel to the axis of the cylindrical volume  4 .  
         [0059]     In FIG. B, a multipole stator system  1 ′ is illustrated having four poles, although the system  1 ′ may have any even integral number P of poles. Assuming sinusoidal distribution, the magnetic field B is expressed as 
 
 B=B   m cos  P/ 2θ s , 
 
 where B m  is the magnetic density at a given reference angle θ sin . The value P/2 is often referred to as the electrical angle. It should be noted that the magnetic field  4 ′ produced by the multipole multiphase stator system  1 ′ produces a resultant magnetic field  2 ′ having two-dimensional cross-section with a central area of substantially zero magnetic field. 
 
         [0060]     Typically, known MHD systems for stirring molten metals use a single 2-pole multiphase stator to rapidly stir a metal melt. One disadvantage of using such a system is the requirement of excessive stirring forces applied to the outer radius of the melt in order to assure the application of sufficient stirring forces at the center of the melt. Additionally, while a single multiphase multistator system is usually sufficient to thoroughly stir a molten metal volume, it may be insufficient to provide uniformly controlled mixing throughout the melt. Controlled and uniform mixing is important insofar as it is necessary for maintaining a uniform temperature and viscosity throughout the melt, as well as for optimizing heat transfer from the melt for its rapid precision cooling. In contrast to the steady-state temperature and heat transfer characteristics of the MHD process, the production of a semi-solid thixotropic slurry requires rapid and controlled temperature changes to occur uniformly throughout the slurry in a short period of time. Moreover, in the thixotropic range, as the temperature decreases the solid fraction, and accordingly the viscosity, rapidly increases. In this temperature and viscosity range, it is desirable to maintain steady, uniform stirring throughout the entire volume of material. This is especially true as the volume of molten metal increases.  
         [0061]     To this end, the present invention utilizes a combination of stator types to combine circumferential magnetic stirring fields with longitudinal magnetic stirring fields to achieve a resultant three-dimensional magnetic stirring field that urges uniform mixing of the metal melt. One or more multiphase stators are included in the system, to allow greater control of the three-dimensional penetration of the resulting magnetomotive stirring field. In other words, while the MHD process requires a stator having only two poles and producing a non-zero electromotive field across the entire cross-section of the metal melt or billet, the system of the present invention utilizes a combination of stator types to achieve greater control of the resulting magnetomotive mixing field. Otherwise, as the outer layer of the volume of molten metal solidifies, the shear force on the remaining liquid metal in the interior of the volume would be insufficient to maintain dendritic degeneration, resulting in a metal billet having an inhomogeneous microstructure. In order to produce a thixotropic slurry billet, a stator assembly having four poles may be used to stir the slurry billet with greater force and at a faster effective rate to mix the cooling metal more thoroughly (and uniformly throughout the slurry billet volume) to produce a slurry billet that is more homogeneous, both in temperature and in solid particle size, shape, concentration and distribution. The four pole stator produces faster stirring since, although the magnetic field rotates more slowly than that of a two pole stator, the field is more efficiently directed into the stirred material and therefore stirs the melt faster and more effectively.  
         [0062]     FIGS.  1 A,  2 A- 2 B, and  3 A- 3 F illustrate a first embodiment of the present invention, a magnetomotive agitation system  10  for stirring volumes of molten metals (such as melts or slurry billets)  11 . As used herein, the term “magnetomotive” refers to the electromagnetic forces generated to act on an electrically conducting medium to urge it into motion. The magnetomotive agitation system  10  includes a stator set  12  positioned around a magnetic mixing chamber  14  and adapted to provide a complex magnetic field therein. Preferably, the mixing chamber  14  includes an inert gas atmosphere  15  maintained over the slurry billet  11  to prevent oxidation at elevated temperatures.  
         [0063]     The stator set  12  preferably includes a first stator ring  20  and a second stator ring  22  respectively positioned above and below a third stator ring  24 , although the stator set may include any number of stators (ring shaped or otherwise) of any type (linear field, rotational field, or the like) stacked in any convenient sequence to produce a desired net field magnetomotive shape and intensity (see, for example,  FIGS. 1B-1D ). As used herein, a ‘rotating’ or ‘rotational’ magnetic field is one that directly induces circulation of a ferromagnetic or paramagnetic liquid in a plane substantially parallel to a central axis of rotation  16  extending through the stator set  12  and the magnetic mixing volume  14 . Likewise, as used herein, a ‘linear’ or ‘longitudinal’ magnetic field is one that directly induces circulation of a ferromagnetic or paramagnetic material in a plane substantially parallel the central axis of rotation  16 . Preferably, the stator ring set  12  is stacked to define a right circular cylindrical magnetic mixing volume  14  therein, although the stator set  12  may be stacked to produce a mixing volume having any desired size and shape.  
         [0064]     A physical mixing vessel or container  26  is positionable within the stator set  12  substantially coincident with the mixing volume  14 . Preferably, the mixing vessel  26  defines an internal mixing volume  14  shape identical to that of the magnetomotive field generated by the stator ring set  12 . For example, if a substantially right oval cylindrical magnetomotive force field were to be produced, the mixing vessel  26  would likewise preferably have an interior mixing volume  14  having a right oval cylindrical shape. Likewise, the stator set  12  may be stacked high to accommodate a relatively tall mixing vessel  26  or short to accommodate a small mixing vessel  26 .  
         [0065]     The first and second stators  20 , 22  are preferably multiple phase stators capable of producing rotating magnetic fields  30 ,  32 , while the third stator  24  is capable of producing a linear/longitudinal (axial) magnetic field  34 . When all three stators  20 ,  22 ,  24  are actuated, the magnetic fields  30 ,  32 ,  34  so produced interact to form a complex substantially spiral or pseudo-spiral magnetomotive field  40 . The substantially spiral magnetomotive field  40  produces an electromotive force on any electrical conductors in the magnetic mixing chamber  14 , such that they are circulated throughout the melt  11 , both axially and radially. Electrical conductors acted on by the spiral magnetomotive field  40  are therefore thoroughly randomized.  
         [0066]     FIGS.  1 A,  2 C- 2 D, and  3 A- 3 F illustrate an alternate embodiment of the present invention, a magnetomotive agitation system  10 ′ as described above, but having a stator ring set  12 ′ including a first and second stator  20 ′,  22 ′, each adapted to produce a linear magnetic field  30 ′,  32 ′, and a third stator  24 ′ adapted to produce a rotational magnetic field  34 ′. As above, when all three stators  20 ′,  22 ′,  24 ′ are actuated, the magnetic fields  30 ′,  32 ′,  34 ′ so produced interact to form a complex substantially spiral or pseudo-spiral magnetomotive field  40 . The substantially spiral magnetomotive field  40  produces an electromotive force on any electrical conductors in the magnetic mixing chamber  14 , such that they are circulated throughout the melt  11 , both axially and radially. Electrical conductors acted on by the spiral magnetomotive field  40  are therefore thoroughly dispersed. This stator set  12 ′ design offers the advantage of directly inducing longitudinal circulation in both ends of the mixing volume  14  to ensure complete circulation of the slurry billet  11  at the ends of the mixing volume  14 .  
         [0067]      FIGS. 3A-3F  illustrate the stirring forces resulting from the interaction of the magnetic forces generated by the present invention in greater detail.  FIGS. 3A-3C  are a set of simplified schematic illustrations of the combination of a rotational or circumferential magnetic field  30  with a longitudinal or axial magnetic field to produce a resultant substantially spiral magnetic field  40 . By itself, the rotational magnetic field produces some circulation  42  due to the centripetal forces urging stirred material against and down the vessel walls, but this is insufficient to produce even and complete circulation. This is due primarily to frictional forces producing drag at the interior surfaces of the mixing vessel  26 . The circumferential flow generated by the rotational magnetic field  30  (shown here as a clockwise force, but may also be opted to be a counterclockwise force) is coupled with the axial flow generated by the longitudinal magnetic field  34  (shown here as a downwardly directed force, but may also be chosen to be an upwardly directed force) to produce a downwardly directed substantially spiral magnetic field  40 . As the molten metal  11  flowing downward near the interior surface of mixing vessel  26  nears the bottom of the mixing volume  14 , it is forced to circulate back towards the top of the mixing volume  14  through the core portion  48  (see FIGS.  3 D-F) of the mixing vessel  26 , since the magnetomotive forces urging downward flow are stronger nearest the mixing vessel walls  26 . Likewise, the direction of the longitudinal magnetic field  34  may be reversed to produce an upwardly directed flow of liquid metal having a downwardly directed axial portion. It should be noted that the stator set  12  may be controlled to produce net magnetic fields having shapes other than spirals, and in fact may be controlled to produce magnetic fields having virtually any desired shape. Likewise, it should also be noted that the spiral (or any other) shape of the magnetic filed may be achieved by any stator set having at least one stator adapted to produce a rotational field and at least one stator adapted to produce a linear field through the careful control of the field strengths produced by each stator and their interactions.  
         [0068]      FIGS. 3D-3F  schematically illustrate the preferred flow patterns occurring in a metal melt  11  magnetomotively stirred in the substantially cylindrical magnetic mixing chamber or volume  14 . For ease of illustration, the magnetic mixing volume  14  is depicted as a right circular cylinder, but one of ordinary skill in the art would realize that this is merely a convenient approximation of the shape of the magnetomotive force field and that the intensity of the field is not a constant throughout its volume. The magnetic mixing volume  14  may be thought of as comprising a cylindrical outer shell  46  surrounding a cylindrical inner axial volume  48 . The downwardly directed spiral portion  54  of the flowing liquid metal  11  is constrained primarily in the cylindrical outer shell  46  while the upwardly directed axial portion  56  of the flowing liquid metal  11  is constrained primarily in the cylindrical inner axial volume  48 .  
         [0069]     In general, it is preferred that a thixotropic metal melt  11  be stirred rapidly to thoroughly mix substantially the entire volume of the melt  11  and to generate high shear forces therein to prevent dendritic particle formation in the melt  11  through the application of high shear forces to degenerate forming dendritic particles into spheroidal particles. Stirring will also increase the fluidity of the semi-solid metal melt  11  and thereby enhance the efficiency of heat transfer between the forming semi-solid slurry billet  11  and the mixing vessel  26 . Rapid stirring of the low viscosity melt also tends to speed temperature equilibration and reduce thermal gradients in the forming semi- solid slurry billet  11 , again enjoying the benefits of more thoroughly and efficiently mixing the semi-solid slurry billet  11 .  
         [0070]     It is further preferred that the stirring rate be decreased as the viscosity of the cooling melt/forming semi-solid slurry billet  11  increases, since as the solid fraction (and thereby the viscosity) of the slurry billet  11  increases the required shear forces to maintain a high stirring rate likewise increase and it is desirable to mix the high viscosity slurry billet  11  with high-torque low-speed stirring (since low speed magnetic stirring is produced by using more penetrating low frequency oscillations.) The stirring rate may be conveniently controlled as a function of the viscosity of the melt (or as a function of a parameter coupled to the viscosity, such as the temperature of the melt or the power required to stir the melt), wherein as the viscosity of the cooling melt  11  increases, the stirring rate decreases according to a predetermined relationship or function.  
         [0071]     In operation, a volume of molten metal (i.e., a slurry billet)  11  is poured into the mixing vessel  26  positioned within the mixing volume  14 . The stator set  12  is activated to produce a magnetomotive field  40  within the magnetic mixing chamber  14 . The magnetomotive field  40  is preferably substantially spiral, but may be made in any desired shape and/or direction. The stator set  12  is sufficiently powered and configured such that the magnetomotive field produced thereby is sufficiently powerful to substantially penetrate the entire slurry billet  11  and to induce rapid circulation throughout the entire slurry billet  11 . As the slurry billet  11  is stirred, its temperature is substantially equilibrated throughout its volume such that temperature gradients throughout the slurry billet  11  are minimized. Homogenization of the temperature throughout the slurry billet  11  likewise homogenizes the billet viscosity and the size and distribution of forming solid phase particles therein.  
         [0072]     The slurry billet  11  is cooled by heat transfer through contact with the mixing vessel  26 . Maintenance of a rapid and uniform stirring rate is preferred to facilitate uniform and substantially homogenous cooling of the slurry billet  11 . As the slurry billet  11  cools, the size and number of solid phase particles therein increases, as does the billet viscosity and the amount of shear force required to stir the slurry billet  11 . As the slurry billet  11  cools and its viscosity increases, the magnetomotive force field  14  is adjusted according to a predetermined relationship between slurry billet (or melt) viscosity and desired stirring rate.  
         [0073]      FIG. 4  schematically illustrates a still another embodiment of the present invention, a magnetomotive agitation system  10 A for stirring thixotropic molten metallic melts including an electronic controller  58  electrically connected to a first stator  20 , a second stator  22  and a third stator  24 . A first power supply  60 , a second power supply  62  and a third power supply  64  are electrically connected to the respective first, second and third stators  20 ,  22 ,  24  as well as to the electronic controller  58 . A first voltmeter  70 , a second voltmeter  72  and a third voltmeter  74  are also electrically connected to the respective power supplies  60 ,  62 ,  64  and to the electronic controller  58 .  
         [0074]     In operation, the power supplies  60 ,  62 ,  64  provide power to the respective stators  20 ,  22 ,  24  to generate the resultant substantially spiral magnetic field  40 . The electronic controller  58  is programmed to provide control signals to the respective stators  20 ,  22 ,  24  (through the respective power supplies  60 ,  62 ,  64 ) and to receive signals from the respective voltmeters  70 ,  72 ,  74  regarding the voltages provided by the respective power supplies  60 ,  62 ,  64 . The electronic controller  58  is further programmed to correlate the signals received from the voltmeters  70 ,  72 ,  74  with the shear forces in the melt/slurry billet  11 , to calculate the viscosity of the forming semi solid slurry billet  11 , and to control the stators  20 ,  22 ,  24  to decrease the intensity of the substantially spiral magnetic field  40  to slow the stirring rate as the slurry billet  11  viscosity increases. Alternately, a feedback signal relating to the temperature or viscosity of the molten metal  11  may be used to provide a control signal to the electronic controller  58  for controlling the stator set  12 .  
         [0075]      FIG. 5  illustrates yet another embodiment of the present invention, a magnetomotive agitation system  10 B for stirring a thixotropic metallic melt  11  contained in a mixing vessel  26  and including an electronic controller  58  electrically connected to a first stator  20 , a second stator  22  and a third stator  24 . The electronic controller  58  is also electrically connected to one or more temperature sensors  80 ,  82  such as an optical pyrometer  80  positioned to optically sample the metallic melt  11  or a set of thermocouples  82  positioned to detect the temperature of the metallic melt  11  at different points within the mixing vessel  26 .  
         [0076]     In operation, the electronic controller  58  is programmed to provide control signals to the respective stators  20 ,  22 ,  24  (through one or more power supplies, not shown) and to receive signals from the temperature sensor(s)  80 ,  82  regarding the temperature of the cooling molten metal/forming semi-solid slurry billet  11 . The electronic controller  58  is further programmed to correlate the temperature of the metal melt/slurry billet  11  with a predetermined desired stirring speed (based on a known relationship between slurry viscosity and temperature for a given metallic composition) and to control the stators  20 ,  22 ,  24  to change the intensity of the substantially spiral magnetic field  40  to control the stirring rate as a function of temperature of the slurry billet  11 . In other words, as the temperature of the slurry billet  11  decreases, the electronic controller  58  is adapted to control the stators  20 ,  22 ,  24  to adjust the stirring rate of the slurry billet  11 .  
         [0077]     Other embodiments are contemplated wherein the stator assembly comprises a single stator capable of producing a complex spiral magnetomotive force field. Still other contemplated embodiments include a single power supply adapted to power the stator assembly.  
         [0078]     While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only the preferred embodiment has been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected.