Patent Publication Number: US-2004055735-A1

Title: Method and apparatus for manufacturing semi-solid metallic slurry

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001] This application claims priority from Korean Patent Application Nos. 2002-58163 filed on Sep. 25, 2002; 2002-63162 filed on Oct. 16, 2002; 2003-3250 filed on Jan. 17, 2003; and 2003-13517 filed on Mar. 4, 2003, the disclosures of which are incorporated herein in their entirety by reference.  
       BACKGROUND OF THE INVENTION  
       [0002] 1. Field of the Invention  
       [0003] The present invention relates to a method and apparatus for manufacturing a semi-solid metallic slurry, and more particularly, to a method and apparatus for manufacturing a semi-solid metallic slurry, in a solid and liquid combined state, containing fine, uniform spherical particles.  
       [0004] 2. Description of the Related Art  
       [0005] Semi-solid metallic slurries refer to metallic materials in a solid and liquid combined phase for use in a rheocasting or thioxocasting process. Semi-solid metallic slurries consist of spherical solid particles suspended in a liquid phase in an appropriate ratio at temperature ranges for semi-solid state, and thus, they change form easily by a small force due to their thioxotropic properties and can be cast easily like a liquid due to their high fluidity. Rheocasting refers to a process of manufacturing billets or mold products from metallic slurries having a predetermined viscosity through casting or forging. Thixocasting refers to a process involving reheating billets manufactured through rheocasting back into a metal slurry and casting or forging the metal slurry to manufacture final products.  
       [0006] Such rheocasting orthixocasting is more advantageous than general forming processes, such as casting or forging, using molten metal. For example, semi-solid or semi-molten slurries used in rheocasting or thixocasting have fluidity at a lower temperature than molten metal, so that the die casting temperature can be lowered in rheocasting or thixocasting, thereby ensuring an extended lifespan of the die. In addition, when a semi-solid or semi-molten metallic slurry is extruded through a cylinder, turbulence is less likely to occur, and less air is incorporated during casting, thereby preventing formation of air pockets in final products. Besides, the use of semi-solid or semi-molten metallic slurries leads to reduced shrinkage during solidification, improved working efficiency, mechanical properties, and anti-corrosion, and lightweight products. Therefore, such semi-solid or semi-molten metallic slurries can be used as new materials in the fields of automobiles, airplanes, and electrical, electronic information communications equipment.  
       [0007] As described above, semi-solid slurries solidified from molten metal by a predetermined method are used in rheocasting, and semi-molten slurries obtained by reheating solid billets are used in thixocasting. Throughout the specification of the present invention, the term “semi-solid metallic slurry” means a metallic slurry in a solid and liquid combined stage at a temperature range, between the liquidus temperature and the solidus temperature of the metal, which can be manufactured by rheocasting through solidification of molten metal.  
       [0008] In conventional rheocasting, molten metal is stirred at a temperature of lower than the liquidus temperature while cooling, to break up dendritic structures into spherical particles suitable for rheocasting, for example, by mechanical stirring, electromagnetic stirring, gas bubbling, low-frequency, high-frequency, or electromagnetic wave vibration, electrical shock agitation, etc.  
       [0009] As an example, U.S. Pat. No. 3,948,650 discloses a method and apparatus for manufacturing a liquid-solid mixture. In this method, molten metal is vigorously stirred while cooled to be solidified. A semi-solid metallic slurry manufacturing apparatus disclosed in this patent uses a stirrer to induce flow of the solid-liquid mixture having a predetermined viscosity to break up dendritic crystalline structures or disperse broken dendritic crystalline structures in the liquid-solid mixture. In this method, dendritic crystalline structures formed during cooling are broken up and used as nuclei for spherical particles. However, due to generation of latent heat of solidification at the early stage of cooling, the method causes problems of low cooling rate, manufacturing time increase, uneven temperature distribution in a mixing vessel, and non-uniform crystalline structure. Mechanical stirring applied in the semi-solid metallic slurry manufacturing apparatus inherently leads to non-uniform temperature distribution in the mixing vessel. In addition, the apparatus is operated in a chamber, thereby making it difficult to continuously perform a subsequent process.  
       [0010] U.S. Pat. No. 4,465,118 discloses a method and apparatus for manufacturing a semi-solid alloy slurry. This apparatus includes a coiled electromagnetic field application unit, a cooling manifold, and a vessel, which are sequentially formed inward, wherein molten metal is continuously loaded down into the vessel, and cooling water is flowed through the cooling manifold to cool the outer wall of the vessel. In manufacturing a semi-solid alloy slurry, molten metal is injected through a top opening of the vessel and cooled by the cooling manifold, thereby resulting in a solidification zone in the vessel. Cooling is sustained while a magnetic field is applied by the electromagnetic field application unit to break up dendritic crystalline structures formed in the solidification zone and to pull an ingot from the slurry through the lower end of the apparatus. The basic technical idea of this method and apparatus is to break up dendritic crystalline structures after solidification by applying vibration. However, many problems, such as complicated processing and non-uniform particle structure, arise with this method. In the manufacturing apparatus, since molten metal is continuously supplied to grow an ingot, it is difficult to control the state of the metal ingot and the overall process. Moreover, the vessel is cooled using water prior to applying an electromagnetic field, so that there is a great temperature difference between the peripheral and core regions of the vessel.  
       [0011] Other types of rheocasting and thixocasting described later are available. However, all of the methods are based on the technical idea of breaking up dendritic crystalline structures after formation, to generate nuclei of spherical particles, and arise such problems described in conjunction with the above patents.  
       [0012] U.S. Pat. No. 4,694,881 discloses a method for manufacturing thixotropic materials. In this method, an alloy is heated to a temperature at which all metallic components of the alloy are present in a liquid phase, and the resulting molten metal is cooled to a temperature between its liquidus and solidus temperatures. Then, the molten metal is subjected to a sufficient shearing force to break dendritic structures formed during the cooling of the molten metal, so that thixotropic materials are manufactured.  
       [0013] Japanese Patent Laid-open Application No. 11-33692 discloses a method for producing a metallic slurry for rheocasting. In this method, a molten metal is supplied into a vessel at a temperature near its liquidus temperature or 50° C. above its liquidus temperature. Next, when at least a portion of the molten metal reaches a temperature lower than the liquidus temperature, i.e., the molten metal is cooled below a liquidus temperature range, the molten metal is subjected to a force, for example, ultrasonic vibration. Finally, the molten metal is slowly cooled into a metallic slurry for rheocasting containing spherical particles. This method also uses a physical force, such as ultrasonic vibration, to break up the dendrites grown at the early stage of solidification. In this method, if the casting temperature is greater than the liquidus temperature, it is difficult to form spherical particle structures and to rapidly cool the molten metal. Furthermore, this method leads to a non-uniformity of surface and core structures.  
       [0014] Japanese Patent Laid-open Application No. 10-128516 discloses a casting method of thixotropic metal. This method involves loading a molten metal into a vessel and vibrating the molten metal using a vibrating bar dipped in the molten metal to directly transfer its vibrating force to the molten metal. A molten alloy containing nuclei, which is a semi-solid and semi-liquid state, at temperatures lower than its liquidus temperature is formed and cooled to a temperature at which it has a predetermined liquid fraction and held from 30 seconds to 60 minutes to allow nuclei in the molten alloy to grow larger, thereby resulting in thixotropic metal. This method provides relatively large particles of about 100 μm and takes a considerably long processing time, and cannot be performed in a larger vessel than a predetermined size.  
       [0015] U.S. Pat. No. 6,432,160 B1 discloses a method for making a thixotropic metal slurry. This method involves simultaneously controlling the cooling and the stirring of molten metal to form a thixotropic metal slurry. In particular, after loading a molten metal into a mixing vessel, a stator assembly positioned around the mixing vessel is operated to generate a magnetomotive force sufficient to stir the molten metal in the vessel rapidly. Next, the temperature of the molten metal is rapidly dropped by means of a thermal jacket equipped around the mixing vessel for precise control of the temperature of the mixing vessel and the molten metal. The molten metal is continuously stirred during cooling cycle in a controlled manner. When the solid fraction of the molten metal is low, high stirring rate is provided. As the solid fraction increases, a greater magnetomotive force is applied.  
       [0016] Most of the above-described conventional methods and apparatuses for manufacturing semi-solid metal slurries use shear force to break dendritic structures into spherical structures during a cooling process. Since a force such as vibration is applied after the temperature of at least a portion of the molten metal drops below its liquidus temperature, latent heat is generated due to the formation of initial solidification layers. As a result, there are many disadvantages such as reduced cooling rate and increased manufacturing time. In addition, due to a non-uniform temperature between the inner wall and the center of the vessel, it is difficult to form fine, uniform spherical metal particles. This structural non-uniformity of metal particles will be greater if the temperature of the molten metal loaded into the vessel is not controlled.  
       SUMMARY OF THE INVENTION  
       [0017] The present invention provides a method and apparatus for manufacturing a semi-solid metallic slurry containing fine, uniform spherical particles, with improvements in energy efficiency and mechanical properties, cost reduction, convenience of casting, and shorter manufacturing time.  
       [0018] The present invention also provides a method and apparatus for manufacturing a high-quality semi-solid metallic slurry within a short time, which can be readily and conveniently applied to a subsequent process.  
       [0019] In accordance with an aspect of the present invention, there is provided a method for manufacturing a semi-solid metallic slurry, the method comprising applying an electromagnetic field to a space containing a slurry vessel; loading a molten metal into the slurry vessel in a state where the electromagnetic field is applied to the space; and drawing the slurry vessel out from the space.  
       [0020] According to a specific embodiment of the semi-solid metallic slurry manufacturing method, applying the electromagnetic field to the space is performed prior to loading the molten metal into the slurry vessel. In this case, the slurry vessel is placed in the space after applying the electromagnetic field to the space.  
       [0021] Further, applying the electromagnetic field to the space may be performed at the start of loading the molten metal into the slurry vessel. Alternatively, applying the electromagnetic field to the space may be performed in the middle of loading the molten metal into the slurry vessel.  
       [0022] Further, applying the electromagnetic field to the space may be sustained until a slurry in the slurry vessel has a solid fraction of 0.001-0.7, preferably, 0.001-0.4, more preferably, 0.001-0.1.  
       [0023] The method for manufacturing a semi-solid metallic slurry may further comprising cooling the slurry vessel containing the molten metal after loading the molten metal into the slurry vessel. In this case, cooling the slurry vessel containing the molten metal is sustained until a slurry in the slurry vessel has a solid fraction of approximately 0.1-0.7. Cooling the slurry vessel containing the molten metal may be performed at a rate of 0.2-5.0° C./sec, preferably, 0.2-2.0° C./sec.  
       [0024] In accordance with another aspect of the present invention, there is provided an apparatus for manufacturing a semi-solid metallic slurry, the apparatus comprises at least one slurry vessel; at least one stirring unit having a space for the at least one slurry vessel and applying an electromagnetic field to the space; a driving unit moving the slurry vessel at least up and down to place the slurry vessel in the space; and a loading unit loading a molten metal in liquid state into the slurry vessel.  
       [0025] According to specific embodiments of the semi-solid metallic slurry manufacturing apparatus, the at least one stirring unit applies the electromagnetic field to the space prior to loading the molten metal into the at least one slurry vessel. The at least one stirring unit applies the electromagnetic field to the space at the start of loading the molten metal into the at least one slurry vessel. The at least one stirring unit applies the electromagnetic field to the space in the middle of loading the molten metal into the at least one slurry vessel.  
       [0026] Further, the driving unit may move the slurry vessel up after a predetermined time from loading the molten metal into the at least one slurry vessel to draw the at least one slurry vessel out from the space.  
       [0027] Further, the driving unit may laterally shift the at least one slurry vessel. In this case, the driving unit may comprise a rotary plate supporting the at least one slurry vessel at an edge. The driving unit with the rotary plate moves the at least one slurry vessel down after a predetermined time from loading the molten metal into the at least one slurry vessel, and rotates the rotary plate to draw the at least one slurry vessel out from the space.  
       [0028] Further, the driving unit may be constructed to be laterally moveable along a rail, so that it moves the at least one slurry vessel down after a predetermined from loading the molten metal into the at least one slurry vessel, and is moved along the rail to draw the at least one slurry vessel out from the space.  
       [0029] According to a specific embodiment of the semi-solid metallic slurry manufacturing apparatus, the at least one stirring unit applies the electromagnetic field to the space until a slurry in the at least one slurry vessel has a solid fraction of approximately 0.001-0.7, preferably, 0.001-0.4, more preferably, 0.001-0.1.  
       [0030] In specific embodiments, the at least one slurry vessel used in the semi-solid metallic slurry manufacturing apparatus may include a temperature control element. This temperature control element may comprise at least one of a cooler installed in the at least one slurry vessel and an external electric heater. The temperature control element cools a slurry in the at least one slurry vessel to reach a solid fraction of approximately 0.1-0.7. The temperature control element cools a slurry in the at least one slurry vessel at a rate of approximately 0.2-5.0° C./sec, preferably, 0.2-2.0° C./sec. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0031] The above and other features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which:  
     [0032]FIG. 1 is a graph of temperature profile applied in a method for manufacturing a semi-solid metallic slurry according to the present invention;  
     [0033]FIGS. 2 and 3 illustrate a structure of an apparatus for manufacturing a semi-solid metallic slurry according to an exemplary embodiment of the present invention;  
     [0034]FIG. 4 is a sectional view of an example of a slurry vessel used in a semi-solid metallic slurry manufacturing apparatus according to the present invention;  
     [0035]FIG. 5 illustrates a structure of a semi-solid metallic slurry manufacturing apparatus according to another exemplary embodiment of the present invention;  
     [0036]FIG. 6 illustrates a structure of a semi-solid metallic slurry manufacturing apparatus according to still another exemplary embodiment of the present invention;  
     [0037] FIGS.  7 ( a )-( e ) illustrate the operation of the semi-solid metallic slurry manufacturing apparatus of FIG. 6; and  
     [0038]FIG. 8 illustrates a structure of a semi-solid metallic slurry manufacturing apparatus according to yet still another exemplary embodiment of the present invention.  
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0039] The present invention will be described more fully in the following exemplary embodiments of the invention with reference to the accompanying drawings.  
     [0040] Unlike the above-described conventional techniques, a method of manufacturing a semi-solid metallic slurry according to the present invention includes stirring molten metal by applying an electromagnetic field prior to the completion of loading the molten metal into a vessel. In other words, electromagnetic stirring is performed prior to or at the start or in the middle of loading the molten metal into the vessel, to prevent formation of dendritric structures. Ultrasonic waves instead of the electromagnetic field can be applied for stirring.  
     [0041] In particular, an empty vessel is positioned in a space of a semi-solid metallic slurry manufacturing apparatus. An electromagnetic field is applied to the space, and molten metal is loaded into the vessel. The intensity of the applied electromagnetic field is strong enough to stir the molten metal.  
     [0042]FIG. 1 is a graph of temperature profile applied in a method for manufacturing a semi-solid metallic slurry according to the present invention. As shown in FIG. 1, molten metal is loaded into the vessel at a temperature Tp. As described above, an electromagnetic field may be applied to the vessel prior to loading molten metal into the vessel. Alternatively, the vessel may be positioned in the space after the application of an electromagnetic field into the space. However, the present invention is not limited to this, and electromagnetic stirring may be performed at the start or in the middle of loading the molten metal into the vessel.  
     [0043] Due to the electromagnetic stirring performed prior to the completion of loading molten metal into the vessel, the molten metal does not form dendritic structures near the inner wall of the vessel at the early stage of solidification, and numerous micronuclei are concurrently generated throughout the vessel because the temperature of the entire molten metal is rapidly dropped to a temperature lower than its liquidus temperature.  
     [0044] Applying an electromagnetic field to the vessel prior or simultaneously to loading molten metal into the vessel leads to active stirring of the molten metal in the center and the inner wall regions of the vessel and rapid heat transfer throughout the vessel, thereby suppressing the formation of solidification layers near the inner wall of the vessel at the early stage of cooling. In addition, such active stirring of the molten metal induces smooth convection heat transfer between the higher temperature molten metal and the lower temperature inner vessel wall, so that the molten metal can be cooled rapidly. Due to the electromagnetic stirring, particles contained in the molten metal scatter upon loading into the vessel and are dispersed throughout the vessel as nuclei, so that a temperature difference in the vessel hardly occurs during cooling. However, in conventional techniques where molten metal is stirred after the completion of loading into a vessel, the temperature of the molten metal suddenly drops as soon as it contacts the low temperature inner vessel wall, so that dendritic crystals grow from solidification layers formed near the inner wall of the vessel at the early stage of cooling.  
     [0045] The principles of the present invention will become more apparent when described in connection with latent heat of solidification. In a method for manufacturing a semi-solid metallic slurry according to the present invention, molten metal does not solidify near the inner vessel wall at the early stage of cooling, and no latent heat of solidification is generated. Accordingly, the amount of heat to be dissipated from the molten metal for cooling is equivalent only to the specific heat of the molten metal that corresponds to about {fraction (1/400)} of the latent heat of solidification. Therefore, dendrites, which are generated frequently when using conventional methods near the inner vessel wall at the early stage of cooling, are not formed, and the entire molten metal in the vessel can be uniformly cooled. It takes merely about 1-10 seconds from the loading of the molten metal. As a result, numerous nuclei are created and dispersed uniformly throughout the entire molten metal in the vessel. The increased density of nuclei shortens the distance between the nuclei, and spherical particles instead of dendritic particles are grown.  
     [0046] The same effects can be achieved even when an electromagnetic field is applied in the middle of loading the molten metal into the vessel. In other words, solidification layers are hardly formed near the inner vessel wall even when electromagnetic stirring begins in the middle of loading the molten metal into the vessel.  
     [0047] It is preferable that the temperature, Tp, of the molten metal be maintained in a range from its liquidus temperature to 100° C. above the liquidus temperature (melt superheat=0˜100° C.) at the time of being loaded into the vessel. According to the present invention, since the entire vessel containing the molten metal is cooled uniformly, it allows for the loading of the molten metal into the vessel at a temperature of 100° C. above its liquidus temperature, without the need to cool the temperature of the molten metal to near its liquidus temperature.  
     [0048] On the other hand, in conventional methods, an electromagnetic field is applied to a vessel after the completion of loading molten metal into a vessel and a portion of the molten metal reaches below its liquidus temperature. Accordingly, latent heat is generated due to the formation of solidification layers near the inner wall of the vessel at the early stage of cooling. Because the latent heat of solidification is about 400 times greater than the specific heat of the molten metal, it takes much time to drop the temperature of the entire molten metal below its liquidus temperature. Therefore, in these conventional methods, the molten metal is loaded into the vessel after the molten metal has cooled to a temperature near its liquidus temperature or to a temperature of 50° C. above its liquidus temperature.  
     [0049] According to the present invention, the electromagnetic stirring may be stopped at any point after at least a portion of the molten metal in the vessel reaches a temperature lower than its liquidus temperature T 1 , i.e., after nuclei are created in the molten metal at a solid fraction of about 0.001, as illustrated in FIG. 1. For example, an electromagnetic field may be applied to the molten metal in the vessel throughout all the cooling process of the molten metal, but prior to a subsequent forming process such as die casting or hot forging. This is because, once nuclei are distributed uniformly throughout the vessel, the electromagnetic stirring does not affect the growth of crystalline particles from the nuclei in the metallic slurry. Therefore, the electromagnetic stirring can be sustained until the solid fraction of the molten metal reaches at least 0.001-0.7. However, the electromagnetic stirring is sustained until the solid fraction of the molten metal reaches the range of, preferably, 0.001-0.4, and more preferably, 0.001-0.1, for energy efficiency.  
     [0050] After the electromagnetic stirring is completed, the vessel containing the metallic slurry is drawn out from the electromagnetic field application space for a continuous subsequent process, for example, die casting, hot forging, billet formation. The vessel containing the metallic slurry may be drawn out from the electromagnetic field application space irrespective of the termination of the electromagnetic stirring, i.e., after or during the application of the electromagnetic field.  
     [0051] According to the present invention, the application of an electromagnetic field performed prior to the completion of loading the molten metal into the vessel to form and uniformly distribute nuclei in the molten metal is followed by cooling to facilitate the growth of the nuclei. This cooling process may be performed simultaneously to loading the molten metal into the vessel.  
     [0052] As described above, the application of the electromagnetic field may be sustained throughout all the cooling process. In other worlds, cooling may be performed when the vessel containing the metallic slurry stays in the electromagnetic field application space, i.e., prior to the vessel being drawn out from the electromagnetic field application space. As a result, a semi-solid metallic slurry is manufactured in the electromagnetic field application space and readily subjected to a following forming process.  
     [0053] The cooling process may be sustained just prior to a subsequent forming process, preferably, until the solid fraction of the molten metal reaches approximately 0.1-0.7, i.e., up to time t 2  of FIG. 1. The molten metal may be cooled at a rate of approximately 0.2-5.0° C./sec, preferably, 0.2-2.0° C./sec depending on a desired distribution of nuclei and a desired size of particles.  
     [0054] A semi-solid metallic slurry containing a predetermined amount of solid is manufactured through the above-described processes and readily subjected to billet formation, by rapid cooling, for thixocasting or die casting, forging, or pressing to form final products.  
     [0055] According to the present invention described above, a semi-solid metallic slurry can be manufactured within a short time, merely in 30-60 seconds from loading the molten metal into the vessel for a metallic slurry with a solid fraction of approximately 0.1-0.7. In addition, formed products having a uniform, dense spherical crystalline structure can be manufactured from the semi-solid metallic slurry formed by the method according to the present invention.  
     [0056] The above-described method for manufacturing a semi-solid metallic slurry according to the present invention can be performed using an apparatus according to an embodiment of the present invention illustrated in FIGS. 2 and 3.  
     [0057] Referring to FIG. 2, a semi-solid metallic slurry manufacturing apparatus according to an embodiment of the present invention includes a space  13  to which an electromagnetic field is applied, a stirring unit  1  equipped with a coil  11  for applying an electromagnetic field to surround the space  13 , at least one slurry vessel  2  that can be accommodated in the space  13 , a driving unit  3  for moving the slurry vessel  2  at least up and down, a loading unit  4  via which molten metal is loaded into the slurry vessel  2 , and a controller  5 .  
     [0058] The stirring unit  1  is mounted on the top of a base plate  14  having a hollow member  14   a . The base plate  14  is installed at a predetermined height from the ground while supported by a support member  15 . The coil  11  for applying an electromagnetic field is mounted on the base plate  14  around the hollow member  14   a , while supported by a frame  12  having an inner space for the space  13 . The coil  11  is electrically connected to the controller  5  and applies a predetermined intensity of electromagnetic field to the space  13  to electromagnetically stir the molten metal contained in the slurry vessel  2  placed in the space  13 . Although not illustrated in FIG. 2, the stirring unit  1  may be implemented as an ultrasonic stirrer.  
     [0059] The slurry vessel  2  may be formed of a metallic material or an insulating material and may have any size and any shape that can be accommodated in the space  13  surrounded by the stirring unit  1 . However, it is preferable that the slurry vessel  2  is formed of a material having a higher melting point than the molten metal to be loaded thereinto. The slurry vessel  2  may have a lower stepped portion  21  that fits a vessel receiver  33 , to be described later, to lock the slurry vessel  2  into the space  13 . Although not illustrated in FIG. 2, a thermocouple may be installed in the slurry vessel  2  and connected to the controller  5  to provide temperature information on the slurry vessel  2  to the controller  5 .  
     [0060] The slurry vessel  2  may be formed with a simple structure for containing molten metal, as illustrated in FIGS. 2 and 3. However, the slurry vessel  2  may further comprise a temperature control element  20 , as illustrated in FIG. 4. The temperature control element  20  is comprised of a cooler and/or a heater. In the embodiment of FIG. 4, a cooling water pipe  23  is embedded in a vessel body  22 . Although not illustrated, alternatively, an electric heater with a heating coil may be further installed outside the slurry vessel  2 . In addition, the cooler may be implemented as a water jacket additionally attached outside the slurry vessel  2 , instead of the cooling water pipe  23 . The cooler, the heater, or a combination of the two may be installed in the slurry vessel  2  to cool the molten metal contained in the slurry vessel  2  at an appropriate rate. It will be obvious that such a slurry vessel  2  can be applied to all of the following embodiments of a semi-solid metallic slurry manufacturing apparatus according to the present invention.  
     [0061] The driving unit  3  moves the slurry vessel  2  to place it in the space  13  and to draw it out from the space  13 . The driving unit  3  is implemented with a driving motor and a gear or a hydraulic cylinder, etc. For example, the driving unit  3  may comprise a power system  31  electrically connected to the controller  5 , a piston  32  connected to and actuated by the power system  31  to move up and down in the space  13 , and a vessel receiver  33  attached to an end of the piston  32  near the space  13  to support the slurry vessel  2  therein. The slurry vessel  2  is placed in the space  13  to fit the vessel receiver  33 .  
     [0062] In a state where the driving unit  3  is operated to raise the piston  32  to place the slurry vessel  2  in the space  13 , the loading unit  4  supplies a molten metal in liquid state into the slurry vessel  2 . The loading unit  4  may be implemented with a general ladle, which is electrically connected to the controller  5 . Any device for loading molten metal into the slurry vessel  2  can be used for the loading unit  4 .  
     [0063] In the embodiment of a semi-solid metallic slurry manufacturing apparatus according to the present invention illustrated in FIG. 2, after the driving unit  3  is operated to place the slurry vessel  2  in the space  13 , an electromagnetic field having a predetermined frequency is applied to the space  13  at a predetermined intensity by the coil  11  of the stirring unit  1 . Alternatively, the slurry vessel  2  may be placed in the space  13  after the application of the electromagnetic field to the space  13 . Next, a metal molten in a separate electrical furnace is loaded via the loading unit  4  into the slurry vessel  2  under the electromagnetic field. Applying an electromagnetic field to the space  13  may be performed at the start or in the middle of loading molten metal into the vessel  2 , in addition to prior to the loading, as described above.  
     [0064] After a predetermined period of time from the loading of the molten metal into the slurry vessel  2 , the driving unit  3  is operated to raise the slurry vessel  2 , as illustrated in FIG. 3, to draw the slurry vessel  2  out from the space  13  and to load another empty vessel into the space  13 , for example, using a transfer unit such as a robot. Next, the slurry vessel  2  drawn out from the space  13  is subjected to cooling at a predetermined rate until the solid fraction of a resulting semi-solid metallic slurry reaches the range of 0.1-0.7. The molten metal in the slurry vessel 2 may be cooled at a rate of approximately 0.2-5.0° C./sec, preferably, 0.2-2.0° C./sec. Alternatively, the molten metal in the slurry vessel 2 may be cooled prior to being drawn out from the space 13 by the driving unit 3. In this case, after the completion of cooling the molten metal in the slurry vessel 2, the slurry vessel 2 is drawn out from the space 13 and replaced with another empty vessel.  
     [0065] The application of an electromagnetic field to the space 13 may be sustained throughout all the cooling process, i.e., prior to drawing the slurry vessel 2 out from the space 13 by the operation of the driving unit 3, until the solid fraction of the resulting semi-solid metallic slurry reaches the range of approximately 0.001-0.7. However, the application of an electromagnetic field to the space 13 by the coil 11 is sustained after loading the molten metal into the slurry vessel 2 until the solid fraction reaches, preferably, at least 0.001-0.4, more preferably, 0.001-0.1, for energy efficiency, as described above. The time required for the solid fraction to reach such a level can be experimentally measured. It is obvious that cooling can be performed while the electromagnetic field is applied to the space 13, as described above.  
     [0066] As illustrated in FIG. 5, a semi-solid metallic slurry manufacturing apparatus according to another embodiment of the present invention includes at least two slurry vessels  2   a  and  2   b  for simultaneous slurry formations. The basic structure of the semi-solid metallic slurry manufacturing apparatus in this embodiment is the same as in the previous embodiment, and thus, a detailed description thereon will be omitted here. In the embodiment of FIG. 5, two vessel receivers  33   a  and  33   b  for the at least two slurry vessels  2   a  and  2   b  are mounted on a receiver plate  34 . It is preferable that the height of the vessel receivers  33   a  and  33   b  is substantially equal to the height of the spaces  13   a  and  13   b  such that the slurry vessels  2   a  and  2   b  fitted into the vessel receivers  33   a  and  33   b  can be raised up to the top of the spaces  13   a  and  13   b  to be drawn out therefrom, respectively.  
     [0067]FIG. 6 illustrates a semi-solid metallic slurry manufacturing apparatus according to still another embodiment of the present invention, which differs from the previous embodiments in that the driving unit  3  is constructed to be able to laterally shift the slurry vessel  2 . The following description will be focused on this difference from the previous embodiments.  
     [0068] Referring to FIG. 6, a semi-solid metallic slurry manufacturing apparatus according to still another embodiment of the present invention includes a rotary plate  35  attached to an end of the piston  32  of the driving unit  3 . The piston  32  is attached to nearly the center of the rotary plate  35 . At least two vessel receivers  33   a  and  33   b  are mounted at the edge of the rotary plate  35 , and the slurry vessels  2   a  and  2   b  are fitted into the vessel receivers  33   a  and  33   b , respectively. The power system  31  is constructed to be able to move up and down and rotate the piston  32 . As the rotary plate  35  is rotated by the power system  31 , the slurry vessel  2   a  is laterally shifted away from the space  13 , as shown in FIG. 6. The operation of the semi-solid metallic slurry manufacturing apparatus of FIG. 6 will be described detail with reference to FIG. 7.  
     [0069]FIG. 7 sequentially illustrates the operation of the semi-solid metallic slurry manufacturing apparatus of FIG. 6. As illustrated in FIG. 7( a ), the piston  32  is raised to place the first slurry vessel  2   a  in the space  13 , and an electromagnetic field is applied to the space  13  by the coil  11  of the stirring unit  1 . Alternatively, the first slurry vessel  2   a  may be placed in the space  13  after the application of an electromagnetic field.  
     [0070] Next, as illustrated in FIG. 7( b ), molten metal is loaded into the first slurry vessel  2   a  from the loading unit  4  and left in the electromagnetic field for a predetermined time. As described above, the electromagnetic field may be applied at the start or in the middle of loading the molten metal into the first slurry vessel  2   a.    
     [0071] Next, as illustrated in FIG. 7( c ), the piston  32  is moved down to draw the first slurry vessel  2   a  out from the space  13 . Next, as illustrated in FIG. 7( d ), the piston  32  is rotated to switch the first slurry vessel  2   a  and the second slurry vessel  2   b , which is empty. The slurry in the first slurry vessel  2   a  is cooled at an appropriate rate to form a semi-solid metallic slurry containing a predetermined amount of solid. The piston  32  is raised again, as illustrated in FIG. 7( e ), to repeat the above processes on the second slurry vessel  2   b . The first slurry vessel  2   a  containing the semi-solid metal is transferred by a transfer unit, such as a robot  6 , for a subsequent forming process.  
     [0072] A large amount of semi-solid metallic slurry can be manufactured continuously using an apparatus according to the present invention described above with more convenience when applied to a subsequent process and enhanced overall processing efficiency.  
     [0073] Laterally shifting a slurry vessel can be achieved in various other ways, in addition to the method described in the above embodiment. For example, the driving unit  3  may be constructed to be laterally movable along a rail  36 , as shown in FIG. 8.  
     [0074] The methods and apparatuses for manufacturing a semi-solid metallic slurry according to the present invention are compatible with various kinds of metals and alloys, for example, aluminum, magnesium, zinc, copper, iron, and alloys of the forgoing metals, for rheocasting.  
     [0075] Semi-solid metallic slurries manufactured according to the present invention contain micro spherical particles of even distribution with an average diameter of 10-60 μm and provide improved mechanical properties, even for alloys. According to the present invention, such uniform spherical particles can be formed within a short time through electromagnetic stirring initiated at a temperature above the liquidus temperature of a source metal to generate more nuclei throughout the slurry vessel.  
     [0076] When using a semi-solid metallic slurry manufacturing apparatus according to the present invention, the overall slurry manufacturing process can be simplified, and the duration of electromagnetic stirring and forming (casting) time can be greatly shortened, thereby saving energy for the stirring and costs. The semi-solid metallic slurry manufacturing apparatus according to the present invention makes it convenient to perform a subsequent process and increases the yield of formed products.  
     [0077] While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.