Abstract:
An apparatus for producing a thixotropic metallic melt by simultaneously controlledly cooling and stirring the melt to form solid particles of a first phase suspended in a residual liquid second phase. Vigorous stirring of the metallic melt results in the formation of degenerate dendritic particles having substantially spheroidal shapes. The metallic melt is stirred to rapidly and efficiently circulate the forming semi-solid slurry. Circulation of the forming semi-solid slurry results in a substantially uniform temperature throughout. Through precision stirring and cooling, a semi-solid slurry is formed having a first solid phase of about 70-80 wt. % suspended in a second liquid phase.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention relates generally to metallurgy, and, more particularly, to a method and apparatus for producing a thixotropic metallic melt through precisely controlled heat transfer and magnetomotive agitation. 
     BACKGROUND OF THE INVENTION 
     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. 
     More specifically, the present invention incorporates electromagnetic stirring and various temperature control and cooling control techniques and apparata to facilitate the production of the semi-solid material within a comparatively short cycle time. Also included are structural arrangements and techniques to discharge the semi-solid material directly into a casting machine shot sleeve. 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. 
     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. 
     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. 
     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. 
     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 force. 
     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. 
     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. 
     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. 
     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. 
     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. 
     Prior references disclose the process of forming a semi-solid slurry by reheating a solid billet forming by thixocasting or by 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. 
     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. 
     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. 
     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. 
     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. 
     While propeller-type mechanical stirring has been used in the context of making a semi-solid slurry, there are certain problems or 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. 
     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 quasiisothermal 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. 
     It is known in the art that thixotropic metal melts may be produced by agitating a cooling metal melt. As the metal melt approaches its liquidus temperature, a particulate sold phase begins to precipitate out. As the melt cools, the amount of solid phase increases relative to the remaining liquid phase. Also, the composition at the liquid phase may vary as a function of its the ratio of the amount of remaining liquid phase to the total amount of solid and liquid phases. The viscosity of the cooling melt is sensitive to its temperature, its solid-to-liquid ratio, the composition of the remaining liquid phase, and the relative size, number, and shape of the solid particles. In particular, if the forming solid particles are irregular, the viscosity of the forming semi-solid slurry tends to be substantially greater than if the particles are spherical or spheroid. The viscosity of the semi-solid slurry is even greater if the forming metallic particles are dendritic. 
     It is well known that a semi-solid metallic slurry may be produced having substantially regularly shaped particles by agitating the cooling melt to degenerate the forming dendrites. Known agitation techniques include mechanical stirring, vibration, induction agitation, undercooling, and high-voltage electric pulse injection. However, these techniques do not address the issue of maintaining the slurry at a uniform, equilibrated temperature. If temperature differentials exist within the melt, the distribution and growth of the solid particulate phase will be irregular and the viscosity of the slurry will likewise be non-uniform. Moreover, temperature differentials in the slurry increase the likelihood of the onset of cascade crystallization of all or part of the slurry. This is especially true with regard to the formation of a solid metallic skin around the slurry, since heat extraction from the slurry occurs primarily at the container-slurry interface. 
     Another disadvantage with the known techniques and apparata for producing semi-solid slurries is that they are ill suited for continuous or large-scale processing. In addition to the above-described disadvantages, the prior art techniques take on the order of 6-8 minutes to process a molten metal charge into a thixotropic slurry ready for molding. Moreover, the known techniques necessitate a step for transferring molten metal from a melting furnace into a separate stirring vessel, exposing the molten metal to ambient gasses and increasing the possibility of reaction contaminants forming in the liquid metal. 
     There is therefore a need for a system capable of both quickly and efficiently producing molten metal charge and of mixing the melt to produce a thermally equilibrated thixotropic metal slurry ready for molding from the molten metal charge under a controlled atmosphere. The present invention addresses this need in a novel and unobvious manner. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a method and apparatus for producing a thixotropic metallic melt by simultaneously controlledly cooling and stirring the melt such that solid particles of a first phase begin to precipitate in a residual liquid second phase. Dendritic growth of the solid particles is curtailed by vigorously stirring the metallic melt, resulting in degenerate dendritic particles having a substantially spheroidal character. The metallic melt is stirred such that the metal is rapidly and efficiently circulated, so as to quickly reach a substantially uniform temperature throughout. Through precision stirring and cooling, the metallic melt is maintained with about 70-80% of the melt being solid spheroidal particles of a first phase suspended in a liquid medium of a second phase. 
     One form of the present invention is an apparatus for forming a molten metal mass from solid metal processors under an inert gas atmosphere, automatically transferring a portion of the molten metal mass into a mixing chamber, and rapidly cooling and stirring the transferred portion of molten metal to form a thixotropic semi-solid metallic slurry suitable for molding. 
     One object of the present invention is to provide an improved system for the production of a thixotropic metallic melt comprising a first phase of degenerate dendritic solid particles suspended in a second liquid phase, wherein the first phase comprises about 70-80 percent of the melt. Related objects and advantages of the present invention will be apparent from the following description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a schematic illustration of a first embodiment of the present invention detailing an automatic system for producing a thixotropic semi-solid metal slurry from a molten metal precursor. 
     FIG. 1B is a schematic illustration of the embodiment of FIG. 1A, wherein a temperature gradient is maintained along the length of the mixing vessel. 
     FIG. 1C is a schematic illustration of a second embodiment of the present invention, an automatic system for producing a thixotropic semi-solid metal slurry from a molten metal precursor. 
     FIG. 2A is a schematic illustration of a third embodiment of the present invention detailing an automatic system for producing a thixotropic semi-solid metal slurry from a molten metal precursor. 
     FIG. 2B is a schematic illustration of a fourth embodiment of the present invention detailing an automatic system for producing a thixotropic semi-solid metal slurry from a molten metal precursor. 
     FIG. 2C is a schematic illustration of a fifth embodiment of the present invention detailing an automatic system for producing a thixotropic semi-solid metal slurry from a molten metal precursor. 
     FIG. 3 is a schematic illustration of the FIG. 2A embodiment wherein the mixing vessel is horizontally displaced from the melting furnace. 
     FIG. 4 is a schematic illustration of the FIG. 2A embodiment wherein the mixing vessel is adapted to discharge the billet onto a shot sleeve. 
     FIG. 5 is a schematic illustration of a sixth embodiment of the present invention, an automatic system for producing a thixotropic semi-solid slurry from a molten metal precursor. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     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. 
     One of the ways to overcome the above challenges, according to the present invention, is to apply modified magnetomotive stirring of substantially the entire liquid metal volume as it solidifies into and through the semi-solid range. Such modified magnetomotive 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 magnetomotive 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 magnetomotive stirring allows the cycle time to be shortened through increased control of the cooling rate. Modified magnetomotive 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 one important advantage of the present invention is that the exposure of the molten metal to uncontrolled atmospheres (i.e., oxygen) is minimized, since the melting furnace is connected to the mixing vessel such that a controlled, inert atmosphere can be maintained over the metal at all times subsequent to its entry into the furnace. This reduces the risk of contamination due to the formation of oxide impurities or the like in the highly reactive molten metal charge. Another advantage of the present invention is the elimination of a ladle or other mechanical containment means from the furnace to mixing vessel transfer process. In addition to further reducing the risk of oxidation contamination, the elimination of the ladle eliminates a source of flash inclusion contamination, since residual metal adhering to the ladle may act as a contaminant. This is especially important as the residual metal adhering to the ladle is readily oxidized, thereby rendering the ladle a substantial source of oxide contamination. Moreover, the elimination of the ladle from the system serves to reduce the transfer time of molten metal from the furnace to the mixing vessel, thereby reducing overall system cycle time and increasing efficiency. 
     Yet another advantage of the present invention arises from the presence of a thermal cooling jacket around the mixing vessel, allowing for a predetermined temperature profile over the length of the mixing vessel. The thermal cooling jacket may be adapted to yield a constant heat transfer profile over its length, or it may be adapted to yield a variable heat transfer profile over its length as a function of any convenient parameter, such as time, melt temperature or melt viscosity. An independently programmable thermal cooling jacket allows for an increased resident time of the metal melt in the mixing vessel, since only part of the vessel content is discharged at once. Increased resident time means more time for better mixing without sacrificing cycle time or efficiency. Control of the heat transfer and/or temperature profiles provides for increased stability and consistency of heat transfer from the mixing vessel and enables better stirring and mixing to maximize product consistency. A part formed according to this invention will typically have equivalent or superior mechanical properties, particularly elongation, as compared to castings formed by a fully liquid-to-solid transformation within the mold, the latter castings having a dendritic structure characteristic of other casting processes. 
     FIGS. 1A and 1B illustrate a first embodiment of the present invention, a system  10  for producing a semi-solid thixotropic metallic slurry from solid metal precursors. The slurry making system  10  includes a metal-melting furnace  12  fluidly connected to a slurry mixing vessel  14 . The metal melting furnace is typically capable of holding and melting about 5000-20000 pounds of metal. The operating temperatures of the melting furnace  12  and the mixing vessel  14  are similar, with the mixing vessel  14  maintained at a slightly lower temperature than the melting furnace  12 . For example, for processing an aluminum alloy, such as Al357, the melting furnace is preferably maintained at about 630-700° C. and the mixing vessel  14  is maintained at about 580-605° C. In general, the operating temperatures of the system  10  are functions of such variables as the metal composition, the heat generation techniques applied to the furnace  12  and mixing vessel  14 , the size of the mixing vessel  14  and melting furnace  12 , and the desired throughput speed. 
     The metal melting furnace  12  includes an inlet port  20  for loading solid metal precursors (ingots)  22  into the furnace interior  24 . Preferably, the precursor ingots  22  have the same alloy composition as desired for the end products, however the precursor ingots  22  may be of different compositions in ratios predetermined to form the desired end product alloy composition. Alternately, the inlet port  20  may be used to load premelted liquid metal precursors into the furnace interior  24 . One or more heat sources  26  are coupled in thermal communication to the furnace  12  for providing heat sufficient to melt the solid metal precursors  22 . A pressurized inert gas supply  28  is connected in fluid communication to a gas inlet  30  formed through the furnace  12 , with a gas valve  32  governing the pressure and flow of gas into the furnace  12 . Preferably, the pressurized gas is an inert gas, such as nitrogen (N 2 ), although any convenient inert gas (such as argon, helium or the like) may be chosen. The pressurized gas supply  28  may therefore provide a positive pressure inert gas atmosphere  33  above the metal melt  34  formed in the furnace  12  as the solid metal precursors  22  are melted. A mixing vessel inlet  36  formed between the mixing vessel  14  and the melting furnace  12  provides a connection through which fluid communication may occur therebetween. 
     The mixing vessel  14  defines an interior mixing volume  38 . The mixing vessel  14  is substantially surrounded by a thermal jacket  40 . The thermal jacket  40  may be unitary, or may be formed of linked sections. The thermal jacket  40  is typically formed from a material having a relatively high melting point and good thermal conductivity (such as bronze, graphite or stainless steel) and includes conduits formed therethrough through which a coolant fluid (such as air, oil, or water) may be flowed. The thermal jacket  40  may also include separate heating means (such as conduits for flowing hot fluids or electric heating rods) to provide precision temperature control. The thermal jacket  40  is connected to the mixing vessel  14  in thermal communication therewith to facilitate rapid heat transfer therebetween. The thermal jacket  40  is preferably used to provide a predetermined temperature profile along the mixing vessel  14 , wherein the temperature of the mixing volume  38  is greatest at the mixing vessel inlet  36  and decreases along the length of the mixing vessel  14  according to the temperature curve  41  (see FIG.  1 B). However, the mixing volume  38  may be maintained at a substantially constant temperature if so desired. The thermal jacket  40  and mixing vessel  14  are preferably formed from non-magnetic materials to facilitate electromagnetic flux penetration with minimal interference or distortion. A detailed thermal jacket design is provided in the related U.S. patent application Ser. No. 09/584,859 and attorney docket number 9105-5, filed on Jun. 1, 2000, by inventors Lombard and Wang, and is incorporated herein by reference. 
     FIG. 1C illustrates an alternate embodiment of the present invention, a system  10 ′ for producing a semi-solid metallic slurry with a solid particulate phase characterized as having degenerated dendrites from solid metal precursors as described above, with the exception that this system  10 ′ does not require a thermal jacket for temperature control. Instead, the mixing vessel  14  is cooled through other means, such as air jets directed at the exterior of the mixing vessel  14 . 
     A stator assembly  42  is also positioned around the mixing vessel  14  such that a magnetomotive force field generated by the stator assembly  42  can substantially permeate the mixing volume  38 . As used herein, “magnetomotive” refers to the electromagnetic forces generated to act on an electrically conducting medium to urge it into motion. The stator assembly  42  in each embodiment typically includes a number of individual stators  44  stacked together around the mixing vessel  14 . The stator assembly  42  preferably provides a field of varying magnetomotive force, to provide more rapid stirring while the solid fraction of the slurry billet  46  is low and to provide greater stirring force as the solid fraction of the slurry billet  46  increases. However, the stator assembly  42  may, if desired, provide a substantially constant magnetomotive force over the length of the mixing vessel  14 . A detailed discussion of magnetomotive mixing is provided in the related U.S. patent application Ser. No. 09/585,060 and attorney docket number 9105-6, filed on Jun. 1, 2000, by inventors Lu, Wang and Norville, and is incorporated herein by reference. 
     During use, a thixotropic semi-solid metallic slurry billet  46  may be formed in the mixing vessel  14 . The upstream portion of the slurry billet  46  in the mixing vessel  14  is not yet in a condition ready for discharge from the mixing vessel  14 , due to the temperature profile maintained along the length of the mixing vessel  14 . Preferably, the thixotropic billet  46  is formed at one end of the mixing vessel  14  (in the case of a mixing vessel  14  having a thermal gradient, at the cool end), but may be formed throughout the mixing vessel  14  (in the case of an isothermal mixing vessel  14 .) The slurry billet  46  is formed from a portion of liquid metal transferred into the mixing vessel  14  from the melting furnace  12 . The mixing vessel  14  includes a slurry outlet  48  formed therethrough for directly transferring the processed, thixotropic semi-solid billet  46  portion nearest the slurry outlet  48  into a shot sleeve  56  (either directly or by means of an intermediate mechanism). The slurry billet  46  is then immediately transferred from the shot sleeve  56  into a mold  58  via injection molding or the like. Preferably, the slurry billet  46  moving through the mixing vessel  14  is stirred and cooled such that a portion of the slurry billet  46  at and near the slurry outlet  48  is maintained having the desired thixotropic properties to molding; when desired, the slurry outlet is opened, a measured portion of the thixotropic billet  46  is discharged onto the shot sleeve  56 , and the slurry outlet  48  is closed. 
     In operation, the slurry making system  10  typically receives a predetermined amount solid metal ingots  22  through an inlet port  20 . The solid metal ingots  22  are preferably of the same composition as desired for the final billet  46 , but they may alternately have different compositions preselected to form the desired slurry composition upon melting. The furnace is heated to a predetermined temperature T f  to melt the solid metal precursors  22  into a pool of low viscosity molten metal  34 , having a desired composition and temperature T f . An inert gas is introduced into the furnace during the melting process to minimize contamination of the metal melt  34  from oxidation and other chemical reactions. 
     Once the metal melt  34  has reached the desired temperature T f  (and, accordingly, a desired relatively low viscosity) a predetermined portion of the molten metal  34  (e.g., the slurry billet  46 ) is transferred into the mixing vessel  14 . It is preferable that for each slurry billet charged into the mixing vessel  14 , an equal mass of precursor metal ingots  22  is added to the melting furnace  12 . Alternately, new metal ingots  22  may be added at regularly scheduled intervals or metal ingots  22  may be added to the melting furnace  12  continuously. In this embodiment, the mixing vessel inlet  36  comprises a valve that may be opened to allow liquid metal to flow from the melting furnace  12  into the mixing vessel  14 . However, the mixing vessel inlet  36  may also be provided as a gate, as an aperture positioned such that liquid metal may flow therethrough only after the level of the melt  34  reaches a certain depth, as a small aperture positioned between the furnace  12  and the mixing vessel  14  such that the surface tension of the molten metal or gas pressure differential between the furnace  12  and the mixing vessel  14  prevents flow through the mixing vessel inlet  36  unless positive gas pressure  33  is applied thereto, or by any other transfer means convenient to the design choice. 
     Once the molten metal charge  34  has been measuredly transferred into the mixing vessel  14 , the stator assembly  42  is activated to generate a magnetomotive force field sufficient to stir the entire forming slurry billet  46 . This process may be either incremental or continuous. The magnetomotive force field is preferably non-uniform in strength, such that the portion of the slurry billet  46  nearest the mixing vessel inlet  36  (i.e., the lower solid fraction portion) is stirred rapidly to achieve mixing and cooling, while the portion of the slurry billet  46  further away from the inlet  36  (i.e., the higher solid fraction portion) is stirred more slowly due to the higher shear magnetomotive stirring force necessary to keep the slurry in motion. However, the magnetomotive force field may be maintained having a constant (albeit variable) strength, such that the entire billet is stirred at a uniform rate. As the slurry billet  46  is stirred, its temperature is controlledly decreased from T f  by the thermal jacket  40 . Preferably, the billet temperature is maintained according to the temperature curve  41 , wherein the substantially flat portion of the curve  41  represents the portion of the slurry billet  46  ready for molding. The thermal jacket  40  quickly removes heat from the slurry billet  46  such that the billet temperature rapidly decreases to a point T m  a few degrees above its liquidus point T l . Preferably, the slurry billet  46  is cooled at a rate of between about 0.1° C. per second to about 10° C. per second, and more preferably at a rate from about 0.1° C. per second to about 3° C. per second. As the slurry billet  46  is cooled, it is continuously stirred by the magnetomotive force field generated by the stator set  42  to maintain the slurry billet  46  at a substantially uniform temperature/stirring profile at any point in the mixing volume  14 . In other words, a cross-section of the slurry billet  46  is maintained at a substantially homogeneous temperature as it moves through the mixing vessel  14 , indicated by the corresponding point on temperature curve  41 . However, as the billet temperature decreases, the volume percent of solid phase of the slurry billet  46  increases, as does its viscosity. Although for a given magnetomotive force field an increase in billet viscosity will likewise be accompanied by a decrease in stirring rate, it is desirable to control the strength of the magnetomotive force field to more precisely control the stirring rate of the slurry billet  46  as it cools close to its liquidus temperature. 
     Once the slurry billet  46  has been stirred and cooled to a desired temperature T m , viscosity, and volume fraction of solid phase particles, the portion of the slurry billet  46  that now behaves as a semi-solid thixotropic metallic slurry is transferred upon demand from the mixing vessel  14  by means of the slurry outlet  48  into a waiting shot sleeve  56 . The slurry outlet  48  preferably includes a slurry valve  50  sufficient to control the portions of the slurry billet  46  discharged and to maintain an inert gas atmosphere within the slurry maker system  10 . Once transferred to the shot sleeve  56 , the slurry billet  46  is immediately transferred into a mold  58 , wherein it is cast into a desired final form. The casting process is performed rapidly, and is completed before the slurry billet  46  cools below its liquidus temperature T l  to some temperature T c  at which it no longer behaves thixotropically. A typical slurry billet  46  may be processed as described above in about 5 to 100 seconds. 
     FIG. 2A illustrates a second embodiment of the present invention, a system  10 A for producing a semi-solid thixotropic metallic slurry from metal precursors  22 A (preferably ingots). The slurry making system  10 A includes a metal-melting furnace  12 A fluidically connected to a slurry mixing vessel  14 A. The metal melting furnace  12 A includes a metal inlet port  20 A for loading solid metal ingots  22 A or the like into the furnace interior  24 A. One or more heat sources  26 A are coupled in thermal communication to the furnace  12 A for providing heat sufficient to melt the solid metal precursors  22 A. An inert gas supply  28 A is connected in fluid communication to a gas inlet formed through the furnace  22 A, with a gas valve  32 A governing the flow of gas into the furnace  22 A. The inert gas supply  28 A preferably provides a positive pressure inert gas atmosphere  33 A above the metal melt  34   a  formed in the furnace  22 A as the solid metal precursors  22 A are melted. A mixing vessel inlet  36 A formed between the mixing vessel  14 A and the melting furnace  12 A provides a connection through which fluid communication may occur therebetween. A sprue or pipe  37 A extends upwardly from the melting furnace  12 A into the mixing vessel  14 A. Liquid metal may be controlledly forced from the melting furnace  12 A up the sprue  37 A and into the mixing vessel  14 A by increasing the inert gas pressure  33 A upon the metal melt  34 A. Preferably, the mixing vessel inlet  36 A comprises a valve operable to allow liquid metal to fill the mixing vessel  14   a  and further operable to contain the liquid metal within the mixing vessel  14 A in isolation from the melting furnace  12 A. 
     The mixing vessel  14 A defines an interior mixing volume  38 A positioned above the melting furnace  12 A. The mixing vessel may be positioned directly above the melting furnace (see FIGS. 2A-2B) or the mixing vessel may be horizontally displaced from the melting furnace  12 A (see FIG.  3 ). 
     The mixing vessel  14 A is substantially surrounded by a thermal jacket  40 A. The thermal jacket  40 A may be unitary, or may be formed of linked sections. The thermal jacket  40 A is typically formed from a material having a relatively high melting point and good thermal conductivity (such as bronze or stainless steel) and includes conduits formed therethrough through which a coolant fluid (such as air, oil, or water) may be flowed. The thermal jacket  40 A may also include separate heating means (such as conduits for flowing hot fluids or electric heating rods) to provide precision temperature control. The thermal jacket  40 A is connected to the mixing vessel  14 A in thermal communication therewith to facilitate rapid heat transfer therebetween. 
     Alternately, as shown in FIG. 2B, the system  10 A′ may be cooled without the use of a thermal jacket for temperature control. Instead, the mixing vessel  14 A′ is cooled through other means, such as air jets directed at the exterior of the mixing vessel  14 A′. 
     A stator assembly  42 A is also positioned around the mixing vessel  14 A such that a magnetomotive force field generated by the stator assembly  42 A can substantially permeate the mixing volume  38 A. The stator assembly  42 A typically includes a number of individual stators  44 A stacked together around the mixing vessel  14 A. 
     During use, a semi-solid metallic slurry billet  46 A having a suspended solid particulate phase characterized by degenerated dendrites may be formed in the mixing vessel  14 A. The slurry billet  46 A is formed from a portion of liquid metal transferred into the mixing vessel  14 A from the melting furnace  12 A. The mixing vessel includes a slurry outlet  48 A formed therethrough for transferring the processed, thixotropic semi-solid billet  46 A into a shot sleeve  56 A, from where the slurry billet  46 A is immediately transferred into a mold  58 A. The slurry outlet  48 A may comprise an aperture formed atop the mixing vessel  14 A through which the slurry billet  46 A may be discharged (when the mixing vessel is tilted see FIG. 2C) or the slurry outlet  48 A may comprise an aperture formed in the side or bottom of the mixing vessel  14 A through which the slurry billet  46 A may be discharged (see FIG.  4 ). Alternately, the mixing vessel  14 A may be detachable, such that a robot arm can be used to grab the mixing vessel  14 A, to move the mixing vessel  14 A to a desired location, and to tilt the mixing vessel  14 A to facilitate discharge of the slurry billet  46 A. 
     As illustrated in FIG. 2C, a robot arm assembly  50 A is used to move the mixing vessel  14 A from its mixing position (i.e., connected to the sprue  37 A and in liquid communication with the melting furnace  12 A) to a discharge position, wherein the mixing vessel  14 A is aligned with a piston  52 A adapted to engage the bottom portion  54 A of the mixing vessel  14 A and move the bottom portion  54 A therethrough to discharge the slurry billet  46 A onto a waiting shot sleeve  56 A. In this embodiment, the bottom portion  54 A is adapted to be pushed through the mixing vessel  14 A. Alternately, the slurry billet  46 A may be discharged by tilting the mixing vessel  14 A (with or without the assistance of the robot arm  50 A) to utilize gravity to force the slurry billet  46 A onto a shot sleeve  56 A or the like. 
     In operation, the slurry making system  10 A receives a predetermined amount solid metal precursors  22 A through an inlet port  20 A. The solid metal precursors  22 A may be of the same composition as desired for the final billet  46 A, or they may have different compositions selected to form the desired slurry composition upon melting. The furnace is heated to a predetermined temperature to melt the solid metal precursors  22 A into a pool of molten metal  34 A, having a desired composition and temperature. An inert gas is introduced into the furnace during the melting process to minimize contamination of the metal melt  34 A from oxidation and other chemical reactions. 
     Once the metal melt  34 A has reached a desired temperature (and, accordingly, a desired relatively low viscosity) a predetermined portion of the metal melt  34 A (e.g., the slurry billet  46 A) is transferred into the mixing vessel  14 A. In this embodiment, the mixing vessel inlet  36 A includes a sprue  37 A positioned to connect the lower melting furnace  12 A to the raised mixing vessel  14 A in fluidic communication. Positive gas pressure  33 A is applied above the melt  34 A, forcing liquid metal up the sprue  37 A and into the mixing vessel  14 A. Precise control of the inert gas pressure  33 A allows precise measurement of the amount of liquid metal flowing into the mixing vessel to form a billet  46 A. 
     Once the slurry billet  46 A has been measuredly transferred into the mixing vessel  14 A, the stator assembly  42 A is activated to generate a magnetomotive force field sufficient to rapidly stir the entire billet  46 A. As the slurry billet  46 A is stirred, its temperature is controlledly decreased by the thermal jacket  40 A. The thermal jacket  40 A quickly removes heat from the slurry billet  46 A such that the billet temperature rapidly decreases to a point a few degrees above its liquidus point, and then the temperature is further decreased as a solid phase forms in the liquid matrix. As the slurry billet  46 A is cooled, it is continuously stirred by the magnetomotive force field generated by the stator set  42 A to maintain the slurry billet  46 A at a substantially uniform temperature. However, as the billet temperature decreases, the volume percent of solid phase of the slurry billet  46 A increases, as does its viscosity. Although for a given magnetomotive force field an increase in billet viscosity will likewise be accompanied by a decrease in stirring rate, it is desirable to control the strength of the magnetomotive force field to more precisely control the stirring rate of the slurry billet  46 A as it cools close to its liquidus temperature. 
     Once the slurry billet  46 A has been stirred and cooled to a desired temperature, viscosity, and volume fraction of solid phase particles, the slurry billet  46 A (now a semi-solid thixotropic metallic slurry) is transferred from the mixing vessel  14 A by means of the slurry outlet  48 A into a waiting shot sleeve  56 A. The slurry outlet  48 A preferably includes a slurry valve  50 A sufficient to maintain an inert gas atmosphere within the slurry maker system  10 A. Once transferred to the shot sleeve  56 A, the slurry billet  46 A is immediately transferred into a mold  58 A, wherein it is cast into a desired final form. 
     FIG. 5 illustrates a third embodiment of the present invention, a system  10 B for producing a semi-solid thixotropic metallic slurry from metal precursors  22 B (again, preferably ingots). As in the case of the previous embodiments, the slurry making system  10 B includes a metal-melting furnace  12 B fluidically connected to a slurry mixing vessel  14 B. The metal melting furnace  12 B includes a metal inlet port  20 B for loading solid metal ingots  22 B or the like into the furnace interior  24 B. One or more heat sources  26 B are coupled in thermal communication to the furnace  12 B for providing heat sufficient to melt the solid metal precursors  22 B. The heat sources may be gas-fed flame jets, electrical resistance or inductance coils, or any convenient heating apparati. An inert gas supply  28 B is connected in fluidic communication to a gas inlet formed through the furnace  22 B, with a gas valve  32 B governing the flow of gas into the furnace  22 B. The inert gas supply  28 B preferably provides a positive pressure inert gas atmosphere  33 B above the metal melt  34 B formed in the furnace  22 B as the solid metal precursors  22 B are melted. A mixing vessel inlet  36 B formed between the mixing vessel  14 B and the melting furnace  12 B provides a connection through which fluid communication may occur therebetween. A sprue or pipe  37 B extends from the melting furnace  12 B into the mixing vessel  14 B. Liquid metal may be controlledly forced from the melting furnace  12 B through the sprue  37 B and into the mixing vessel  14 B by sufficiently increasing the inert gas pressure  33 B upon the metal melt  34 B. In this embodiment, the sprue  37 B is curved, such that liquid flowing out of either the mixing vessel  14 B or the melting furnace  12 B must first flow against the pull of gravity. In other words, the curve and positioning of the sprue relative the mixing and melting vessels  14 B,  12 B provides an added safety benefit, reducing the likelihood of accidental transfer of molten metal therebetween. Preferably, the mixing vessel inlet  36 B comprises a valve operable to allow liquid metal to fill the mixing vessel  14 B and further operable to contain the liquid metal within the mixing vessel  14 B in isolation from the melting furnace  12 B. 
     The mixing vessel  14 B defines an interior mixing volume  38 B positioned near, and preferably elevated at least slightly above, the melting furnace  12 B. The mixing vessel  14 B may be substantially surrounded by a thermal jacket  40 B. The thermal jacket  40 B may be unitary, or may be formed of linked sections. The thermal jacket  40 B is typically formed from a material having a relatively high melting point and good thermal conductivity (such as bronze or stainless steel) and includes conduits formed therethrough through which a coolant fluid (such as air, oil, or water) may be flowed. The thermal jacket  40 B may also include separate heating means (such as conduits for flowing hot fluids or electric heating rods) to provide precision temperature control. The thermal jacket  40 B is connected to the mixing vessel  14 B in thermal communication therewith to facilitate rapid heat transfer therebetween. In the absence of a thermal jacket  40 B, the mixing vessel  14 B may be cooled through other means, such as air jets directed at the exterior of the mixing vessel  14 B. 
     A stator assembly  42 B is also positioned around the mixing vessel  14 B such that a magnetomotive force field generated by the stator assembly  42 B can substantially permeate the mixing volume  38 B. The stator assembly  42 B typically includes a number of individual stators  44 B stacked together around the mixing vessel  14 B. 
     During use, a semi-solid metallic slurry billet  46 B having a suspended solid particulate phase characterized by degenerated dendrites may be formed in the mixing vessel  14 B. The slurry billet  46 B is formed from a portion of liquid metal transferred into the mixing vessel  14 B from the melting furnace  12 B. The mixing vessel includes a slurry outlet  48 B formed therethrough for transferring the processed, thixotropic semi-solid billet  46 B into a shot sleeve  56 B, from where the slurry billet  46 B may be easily and immediately transferred into a mold. The slurry outlet  48 B preferably comprises an aperture formed atop the mixing vessel  14 B through which the slurry billet  46 B may be discharged, although the slurry outlet  48 B may comprise an aperture formed in the side or bottom of the mixing vessel  14 B. Alternately, the mixing vessel  14 B may be detachable, such that a robot arm can be used to grab the mixing vessel  14 B, to move the mixing vessel  14 B to a desired location, and to tilt the mixing vessel  14 B to facilitate discharge of the slurry billet  46 B. 
     Preferably, a piston  52 B is positioned in contact with the bottom portion  54 B of the mixing vessel  14 B, which is adapted to either move through the mixing vessel  14 B or yield to the piston  52 B. Preferably, the piston  52 B engages the bottom portion  54 B of the mixing vessel  14 B, pushing the bottom portion  54 B and the slurry billet  46 B through the mixing vessel  14 B until the slurry billet  46 B emerges onto the shot sleeve  56 B. Alternately, the slurry billet  46 B may be discharged by tilting the mixing vessel  14 B to utilize gravity to force the slurry billet  46 B onto a shot sleeve  56 B or the like. 
     In operation, the slurry making system  10 B receives a predetermined amount solid metal precursors  22 B through an inlet port  20 B. The solid metal precursors  22 B may be of the same composition as desired for the final billet  46 B, or they may have different compositions selected to form the desired slurry composition upon melting. The furnace is heated to a predetermined temperature to melt the solid metal precursors  22 B into a pool of molten metal  34 B, having a desired composition and temperature. An inert gas is introduced into the furnace during the melting process to minimize contamination of the metal melt  34 B from oxidation and other chemical reactions. 
     Once the metal melt  34 B has reached a desired temperature (and, accordingly, a desired relatively low viscosity) a predetermined portion of the metal melt  34 B (e.g., the slurry billet  46 B) is transferred into the mixing vessel  14 B. In this embodiment, the mixing vessel inlet  36 B includes a sprue  37 B positioned to connect the melting furnace  12 B to the spaced mixing vessel  14 B in fluidic communication. Positive gas pressure  33 B is applied above the melt  34 B, forcing liquid metal through the sprue  37 B and into the mixing vessel  14 B. Precise control of the inert gas pressure  33 B allows precise measurement of the amount of liquid metal flowing into the mixing vessel to form a billet  46 B. 
     Once the slurry billet  46 B has been measuredly transferred into the mixing vessel  14 B, the stator assembly  42 B is activated to generate a magnetomotive force field sufficient to rapidly stir the entire billet  46 B. As the slurry billet  46 B is stirred, its temperature is controlledly decreased by the thermal jacket  40 B. The thermal jacket  40 B quickly removes heat from the slurry billet  46 B such that the billet temperature rapidly decreases to a point a few degrees above its liquidus point, and then the temperature is further decreased as a solid phase forms in the liquid matrix. As the slurry billet  46 B is cooled, it is continuously stirred by the magnetomotive force field generated by the stator set  42 B to maintain the slurry billet  46 B at a substantially uniform temperature. However, as the billet temperature decreases, the volume percent of solid phase of the slurry billet  46 B increases, as does its viscosity. Although for a given magnetomotive force field an increase in billet viscosity will likewise be accompanied by a decrease in stirring rate, it is desirable to control the strength of the magnetomotive force field to more precisely control the stirring rate of the slurry billet  46 B as it cools close to its liquidus temperature. 
     Once the slurry billet  46 B has been stirred and cooled to a desired temperature, viscosity, and volume fraction of solid phase particles, the slurry billet  46 B (now a semi-solid thixotropic metallic slurry) is transferred from the mixing vessel  14 B by means of the slurry outlet  48 B into a waiting shot sleeve  56 B. The slurry outlet  48 B preferably includes a slurry valve  50 B sufficient to maintain an inert gas atmosphere within the slurry maker system  10 B. Once transferred to the shot sleeve  56 B, the slurry billet  46 B is immediately transferred into a mold  58 B, wherein it is cast into a desired final form. 
     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.