Patent Publication Number: US-2004055734-A1

Title: Metallic materials for rheocasting or thixoforming and method for manufacturing the same

Description:
[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 2003-13498 filed on Mar. 4, 2003, in the Korean Intellectual Property Office, the disclosures of which are incorporated herein in their entireties by reference.  
       BACKGROUND OF THE INVENTION  
       [0002] 1. Field of the Invention  
       [0003] The present invention relates to metallic materials for rheocasting or thixoforming, and a method for manufacturing the same.  
       [0004] 2. Description of the Related Art  
       [0005] Semi-solid or semi-molten metal processing combines casting and forging processes and can be further divided into two categories—rheocasting and thioxforming. In the rheocasting process, a slurry prepared in a semi-solid state is directly cast into final products. In the thixoforming process, billets which has been formed from its semi-solid state is reheated to a semi-molten state and then cast into final products through forging or die casting.  
       [0006] Metal slurry for rheocasting or thixoforming refers to a metallic material consisting of solid particles suspended in a liquid phase in an appropriate ratio at temperature ranges for semi-solid state, changing its form easily even by a small force due to its thixotropic properties, and being cast like a liquid due to its high fluidity. Billet can easily be processed back to a metal slurry in a semi-molten state by reheating and, therefore, is very useful metallic material for rheocasting or thioxforming.  
       [0007] Rheocasting or thixoforming, which uses metallic slurries or billets, is more advantageous than processes which use liquid metal alloys of the same composition. For example, metallic slurries have fluidity at a temperature lower than the temperature at which liquid metal alloys of the same composition completely melt, so that the die casting temperature can be lowered, thereby ensuring an extended lifespan of the die. In addition, when a metallic slurry is extruded, turbulence does not occur and less air is incorporated during a casting process, thereby preventing formation of air pockets in final products. Therefore, the final product can be subjected to a subsequent thermal process for improving mechanical properties thereof. Besides, the use of metallic slurries or billets leads to reduced shrinkage during solidification, improved working efficiency and anti-corrosion, and lightweight products. Therefore, such metal slurries can be used as new materials in the productions of automobiles, airplanes, and information communications equipment.  
       [0008] In conventional semi-solid alloy manufacturing methods, dendritic particles are broken up into spherical particles suitable for rheocasting, mainly by stirring molten metal at a temperature lower than its liquidus temperature. Stirring methods include mechanical stirring, electromagnetic stirring, gas bubbling, electric shock agitation, and low-frequency, high-frequency, or electromagnetic wave vibration and the like.  
       [0009] As an example, U.S. Pat. No. 3,948,650 discloses a method for manufacturing a liquid-solid mixture. In this method, alloys are heated to a temperature at which most alloys reach a liquid phase, and the resulting molten metal is cooled while being vigorously stirred. Specifically, by stirring and cooling the molten metal until the percentage of solids in the molten metal reaches 40-65%, the formation of dendritic particles is prevented or dendritic particles on primary solid particles are eliminated or reduced.  
       [0010] U.S. Pat. No. 4,465,118 discloses a method for manufacturing a semi-solid alloy slurry. In this method, a molten metal in a vessel is mixed electromagnetically by a moving, non-zero magnetic field provided over substantially all of a solidification zone within the vessel. The magnetic field causes the shearing of dendrites formed in the solidification zone at a desired shearing rate.  
       [0011] U.S. Pat. No. 4,694,881 discloses a method for manufacturing a thixotropic material. In this method, an alloy is heated to a temperature above its liquidus 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.  
       [0012] Japanese Patent Laid-open Application N0 11-33692 discloses a method for producing a metallic slurry for rheocasting. In this method, a molten metal is poured into a slurry manufacturing container 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 the metallic slurry having spherical particles for rheocasting.  
       [0013] In particular, dendritic particle structures, which are considered to be grown from discrete nuclei at the initial stage of solidification, are broken into separate particles by applying an appropriate force near its liquidus temperature and then slowly cooled to form a spherical shape of particles without an interaction between the nuclei. This method also uses a physical force, such as ultrasonic vibration, to break up the dendritic particle structures grown at the early stage of solidification. In this method, if the pouring temperature is greatly higher 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 pouring a molten metal into a slurry manufacturing container and vibrating the molten metal using a vibrating bar dipped in the molten metal to directly transfer its vibrating force to the molten metal. In particular, an alloy of a liquid phase having crystal nuclei at temperatures above its liquidus temperature or a semi-solid thixotropic alloy containing crystal nuclei in a temperature range between its liquidus temperature and forming temperature is formed first. Next, the molten metal in the container is cooled down to a temperature at which it has a predetermined liquid fraction and held from 30 seconds to 60 minutes to allow micronuclei in the alloy to grow larger, thereby resulting in a semi-molten metal. This method provides relatively large particles of about 100 μm and requires 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 a molten metal to form the 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 techniques use shear force to break the previously formed dendritic structures into spherical structures during a cooling cycle. 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 caused by the formation of initial solidification layers is generated. As a result, there are many disadvantages such as reduced cooling rate and increased manufacturing time. In addition, there is a need to precisely control the temperature during loading the molten metal into the vessel. Otherwise, dendritic structures are inevitably formed at the early stage of solidification near the inner vessel wall due to a temperature difference between the inner wall and the center of the vessel. Therefore, the prior art necessitates the precise control of the loading temperature and the cooling processes.  
       SUMMARY OF THE INVENTION  
       [0017] The present invention provides metallic materials for rheocasting or thixoforming and a method for manufacturing the same, with the advantages of finer spherical particles, improved energy efficiency, reduced manufacturing costs, improved mechanical properties, convenient casting process, and reduced manufacturing time, compared to conventional methods.  
       [0018] According to an aspect of the present invention, there is provided a method for manufacturing metallic materials for rheocasting or thixoforming, comprising: applying an electromagnetic field to a vessel and loading a molten metal into the vessel; and cooling the molten metal to form a metallic material for rheocasting or thixoforming.  
       [0019] According to another aspect of the present invention, there is provided a metallic material for rheocasting or thixoforming in the form of slurries or billets manufactured according to the above method, the metallic material having spherical particles grown from uniform crystal nuclei. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0020] 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:  
     [0021]FIG. 1A is a graph illustrating a process for manufacturing a metallic material for rheocasting or thixoforming according to an embodiment of the present invention, and FIG. 1B is a photograph showing the microstructure of the metallic material manufactured according to the process shown in FIG. 1A;  
     [0022]FIGS. 2 through 5 are photographs showing the microstructures of metallic materials for rheocasting or thixoforming manufactured at various pouring temperatures of a molten metal using the method according to the present invention;  
     [0023]FIGS. 6 through 9 are photographs showing the microstructures of metallic materials for rheocasting or thixoforming manufactured at various cooling rates of a molten metal after terminating the application of an electromagnetic field, using the method according to the present invention;  
     [0024]FIGS. 10 through 12 are photographs showing the microstructures of metallic materials for rheocasting or thixoforming manufactured at various termination point of the application of the electromagnetic field, using the method according to the present invention;  
     [0025]FIGS. 13 through 16 are photographs showing the microstructures of metallic materials for rheocasting or thixoforming manufactured at various cooling end temperatures of the molten metal, using the method according to the present invention;  
     [0026]FIG. 17 is a photograph showing the microstructure of the metallic material for rheocasting or thixoforming manufactured by pouring molten metal and applying an electromagnetic field at the same time according to the present invention;  
     [0027]FIG. 18 is a photograph showing the microstructure of the metallic material for rheocasting or thixoforming manufactured by applying an electromagnetic field in the middle of pouring a molten metal according to the present invention;  
     [0028]FIGS. 19A and 19B are photographs of the surface and core regions, respectively, of a metallic material manufactured according to another embodiment of the present invention;  
     [0029]FIGS. 20A and 20B are photographs of the surface and core regions, respectively, of a metallic material manufactured according to yet another embodiment of the present invention;  
     [0030]FIGS. 21A and 21B are photographs of the surface and core regions, respectively, of a metallic material manufactured according to a conventional method; and  
     [0031]FIGS. 22A and 22B are photographs of the surface and core regions, respectively, of a metallic material manufactured according to another conventional method. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0032] In a method for manufacturing metallic materials for rheocasting or thixoforming according to the present invention, a molten metal in a vessel has a uniform temperature. In particular, since the temperature of the entire vessel containing the molten metal is uniform throughout; at the center, inner wall, and upper and lower regions, latent heat caused by a solidification in a particular region is not generated at the early stage of cooling, thereby enabling the molten metal to be cooled rapidly within a short time. As a result, the density of crystal nuclei in the molten metal markedly increases, leading to the formation of micro, spherical particles.  
     [0033] Hereinafter, the present invention will be described in greater detail.  
     [0034] According to the present invention, an electromagnetic field is applied to a vessel before the completion of loading a molten metal into the vessel, i.e., before, simultaneously, or in the middle of loading of the molten metal into the vessel. Ultrasonic waves instead of the electromagnetic field may be used. Suitable metals which can be used in the method according to the present invention include any metals available for rheocasting or thixoforming, in which preferable metals are selected from the group consisting of aluminum, magnesium, copper, zinc, iron, and alloys of the forgoing metals. Such alloys may contain various kinds of optional metals depending on the physical properties required for final molded products.  
     [0035] It is preferable that the temperature 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 100° C. above its liquidus temperature, without the need to cool the temperature of the molten metal to near its liquidus temperature.  
     [0036] On the other hand, in conventional methods, an electromagnetic field is applied to a vessel after the completion of loading a molten metal into the 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 at the inner wall of the vessel at the early stage of cooling. Because the latent heat 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.  
     [0037] However, according to the present invention, since an electromagnetic field is applied to a vessel before the completion of loading a molten metal into the vessel, the entire vessel containing the molten metal has a uniform temperature throughout, i.e., at the inner wall, center region, and upper and lower regions of the vessels. As a result, the molten metal does not solidify near the inner wall of the vessel, which occurs in conventional methods, and the entire molten metal in the vessel can be cooled down rapidly below its liquidus temperature, thereby enabling simultaneous formation of numerous crystal nuclei. In the present invention, such a uniform temperature throughout the vessel is directly related with the electromagnetic field applied to the vessel before the completion of loading the molten metal into the vessel. The electromagnetic field applied to the vessel before the completion of loading the molten metal into the vessel induces the entire molten metal to be vigorously stirred in the space between the inner wall and the center of the vessel and facilitates heat transfer throughout the molten metal in the vessel, thereby suppressing the formation of solidification layers of the molten metal near the inner vessel wall at the early stage of cooling. In addition, while the molten metal is being thoroughly stirred, conductive heat transfer from the molten metal to the comparatively low-temperature inner vessel wall is facilitated, so that the temperature of the entire molten metal is rapidly lowered. In the present invention, as the molten metal is loaded into the vessel and simultaneously stirred by the electromagnetic field, solid particles in the molten metal scatter as crystal nuclei throughout the vessel. As a result, a temperature disparity in the molten metal at the various regions of the vessel does not occur. However, in conventional methods, as a molten metal is loaded into a low-temperature vessel, conductive heat transfer from the molten metal to the vessel abruptly occurs, thereby resulting in the formation of dendritic particles at the early stage of solidification.  
     [0038] The principles of the present invention will become more apparent when described in connection with latent heat of solidification. In a method for manufacturing metallic materials for rheocasting or thixoforming 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 is generated from solidification. 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 1/400 of the latent heat. Therefore, the temperature of the molten metal can be lowered within a short time, uniformly throughout the vessel, without the formation of dendritic particles at the early stage of solidification. It takes merely about 1-10 seconds to lower the temperature to a desired temperature from the point of time at which the molten metal is loaded. As a result, numerous crystal nuclei are created and dispersed uniformly throughout the entire molten metal in the vessel, and the increased density of crystal nuclei shortens the distance between the crystal nuclei, thereby resulting in the growth of spherical particles instead of dendritic particles.  
     [0039] The application of the electromagnetic field is stopped when the temperature of the molten metal in the vessel reaches near its liquidus temperature. However, the application of the electromagnetic field may be stopped at any point between the completion of nucleation of the molten metal and the cooling process. The application of the electromagnetic field is stopped when the solid fraction of the molten metal reaches, preferably, 0.001-0.7, more preferably, 0.001 to 0.4, and most preferably, 0.01-0.1 for energy efficiency.  
     [0040] After the application of the electromagnetic field to the vessel is stopped, the molten metal is cooled until the solid fraction of the molten metal reaches, preferably, 0.1-0.7.  
     [0041] In the cooling process, the molten metal is cooled, preferably, at a rate of 0.2-5.0° C./sec, and more preferably, 0.2-2.0° C./sec for more uniform distribution of nuclei and smaller particle formation.  
     [0042] According to the present invention, after the loading of molten metal into a vessel, a metallic material as slurry with a solid fraction of 0.1-0.7 can be manufactured shortly in 30-60 seconds. This metal slurry can be processed into billets by rapid cooling.  
     [0043] Metallic materials in the form of slurries or billets according to the present invention may be subjected to secondary molding, such as die casting, squeeze casting, forging, and press, etc. Alternatively, metallic materials in the form of billets according to the present invention may be cut to proper length to form slugs. This slug is melted back to semi-solid state by reheating for secondary forming.  
     [0044] A metallic material for rheocasting or thixoforming manufactured using the method according to the present invention contains metal particles that are spherical and have an average diameter of 10-60 μm and uniform distribution.  
     [0045] Hereinafter, the present invention will be described in greater detail with reference to the following examples. The following examples are for illustrative purposes and are not intended to limit the scope of the invention.  
     EXAMPLE 1  
     [0046] An aluminum alloy, A356, was used for a molten metal. 500 g of A356 alloy was melted using a graphite crucible in an electrical furnace (10 kW) by heating at about 750° C. for 1 hour. The temperature of the resulting molten metal was measured at a K-type thermal conduction sheath equipped with a digital thermometer to maintain temperature of 100° C. above the liquidus temperature (about 615° C. for A356 alloy) of the molten metal.  
     [0047]FIG. 1A is a graph illustrating a working process for manufacturing a metallic material according to the present invention. An electromagnetic field was applied to a vessel using an electromagnetic stirrer (EMS), which was manufactured by the inventors, at a voltage of 250V, a frequency of 60 Hz, and an intensity of 500 Gauss. Before pouring the molten metal into the vessel, power was supplied to the EMS to operate and generate an electromagnetic field. When the temperature of the molten metal reached a pouring temperature (Tp) of 650° C. (see FIG. 1A), the molten metal was poured into the vessel.  
     [0048] After pouring the molten metal into the vessel with the electromagnetic field to induce stirring of the molten metal, EMS was shut off when the temperature of the molten metal reached near its liquidus temperature (point “a” in FIG. 1A). The EMS was operated only for the time interval “p” of FIG. 1A. Next, the molten metal was cooled at a rate of 1° C./sec to a temperature at which the molten metal had a solid fraction of 0.6 (point “b” of FIG. 1A, corresponding to about 586° C.) to obtain a metal slurry. It took about 40 seconds from the pouring of the molten metal into the vessel until the solid fraction of the metal slurry became 0.6.  
     [0049] After point “b” of FIG. 1A, the metal slurry was subjected to secondary forming process, such as die casting, squeeze casting, forging, press, etc.  
     [0050] To observe the microstructure of the metallic material manufactured according to the method of Example 1, sliced samples were prepared as follows. The metal slurry was rapidly cooled, and sliced using a bandsaw, polished, and etched in Keller solution (20 mL of H 2 O, 20 mL of HCL, 20 mL of HNO 3 , and 5 mL of HF), and used as sliced samples for image analysis. The structure of the sliced sample was observed using an image analyzer (LEICA DMR). The result is shown in FIG. 1B. As is apparent from the image of FIG. 1B, the metallic material manufactured using the method according to the present invention has a structure of micro, spherical particles whose size is uniform, from the core to surface regions of the cross-section.  
     EXAMPLES 2 THROUGH 5  
     [0051] Metallic materials were manufactured in the same manner as in Example 1, except that the pouring temperature Tp of the molten metal was varied to 720° C. (Example 2), 700° C. (Example 3), 650° C. (Example 4), and 620° C. (Example 5), the operation of the EMS was stopped when the solid fraction of the molten metal became 0.05 (slightly above the liquidus temperature), and the molten metal was cooled to obtain a metal slurry having a solid fraction of 0.6. The metal slurries were rapidly cooled, sliced samples were prepared according to the same method as used in Example 1, and the microstructures thereof were observed. The total time spent for manufacturing metallic materials was less than 1 minute. FIGS. 2 through 5 show images obtained from the image analysis for the samples of Examples 2 through 5, respectively. As shown in FIGS. 2 through 5, metal alloys of micro, uniform particles that are spherical and have an average diameter of 10-60 μm can be manufactured with the range of pouring temperatures of the molten metal from 720-620° C., within a short time of less than 1 minute. The high density of crystal nuclei results in narrow distances between the particles formed at the early stage of stirring and is believed to enable the formation of semi-solid materials having particles of uniform size and shape at a higher cooling rate than conventional methods.  
     EXAMPLES 6 THROUGH 9  
     [0052] Metallic materials were manufactured in the same manner as in Example 1, except that the cooling rate of the molten metal was varied to 0.2° C./sec (Example 6), 0.4° C./sec (Example 7), 0.6° C./sec (Example 8), and 2.0° C./sec (Example 9) to obtain metallic slurries. The resulting metal slurries were rapidly cooled, sliced samples were prepared according to the same method as used in Example 1, and the microstructures thereof were observed. The results are shown in FIGS. 6 through 9.  
     [0053] As shown in FIGS. 6 through 9, metallic materials of spherical particles can be manufactured at the various cooling rates of the molten metal. The spherical particles are fine with an average particle diameter of 10-60 μm and have uniform distribution.  
     EXAMPLES 10 THROUGH 12  
     [0054] Metallic materials were manufactured in the same manner as in Example 1, except that the applications of the electromagnetic field were terminated when the solid fractions of the molten metal were 0.2 (Example 10), 0.6 (Example 11), and 0.7 (Example 12). The resulting metal slurries were rapidly cooled, sliced samples were prepared according to the same method as used in Example 1, and the microstructures thereof were observed. The results are shown in FIGS. 10 through 12.  
     [0055] As is apparent from the images of FIGS. 10 through 12, although the termination point of the application of the electromagnetic field is varied, metal alloys of micro, spherical particles can be manufactured with uniform distribution.  
     EXAMPLES 13 THROUGH 16  
     [0056] Metallic materials were manufactured in the same manner as in Example 1, except that the cooling end temperature of the molten metal was varied to 610° C. (Example 13, equivalent to a solid fraction of about 0.2), 600° C. (Example 14), 590° C. (Example 15), and 586° C. (Example 16, equivalent to a solid fraction of about 0.6) to obtain metallic slurries. The resulting metal slurries were rapidly cooled, sliced samples were prepared according to the same method as used in Example 1, and the microstructures thereof were observed. The results are shown in FIGS. 13 through 16.  
     [0057] As is apparent from the images of FIGS. 13 through 16, although the cooling end temperature of the molten metal is varied, metal alloys of micro, spherical particles can be manufactured with uniform distribution. In other words, when the electromagnetic field is applied to the vessel prior to the loading of the molten metal and the electromagnetic stirring is continued until the temperature of the molten metal reaches its liquidus temperature, according to the method of the present invention, metal alloys of uniform, micro, spherical particles can be manufactured regardless of the changes of the cooling end temperature.  
     EXAMPLE 17  
     [0058] A metallic material was manufactured in the same manner as in Example 1, except that the pouring temperature was 630° C., and the pouring of the molten metal and the application of the electromagnetic field were performed simultaneously. The resulting metal slurries were rapidly cooled, sliced samples were prepared according to the same method as used in Example 1, and the microstructures thereof were observed. The results are shown in FIG. 17.  
     [0059] As is apparent from the images of FIG. 17, although the pouring of the molten metal and the application of the electromagnetic field were performed simultaneously, a metal alloy of micro, spherical particles can be manufactured with uniform distribution. In other words, the microstructure of the metallic material prepared by applying the electromagnetic field simultaneously with the pouring of the molten metal was substantially the same as the one prepared by applying the electromagnetic field prior to the pouring of the molten metal.  
     EXAMPLE 18  
     [0060] A metallic material was manufactured in the same manner as in Example 1, except that the pouring temperature was 630° C., and the application of the electromagnetic field was performed in the middle of (50% of the pouring process completed) pouring the molten metal. The resulting metal slurries were rapidly cooled, sliced samples were prepared according to the same method as used in Example 1, and the microstructures thereof were observed. The results are shown in FIG. 18.  
     [0061] As is apparent from the images of FIG. 18, although the electromagnetic field was applied in the middle of the pouring of the molten metal, a metal alloy of micro, spherical particles can be manufactured with uniform distribution. In other words, the microstructure of the metallic material prepared by applying the electromagnetic field in the middle of the pouring process was not much different from the ones prepared by the above-described examples, even though the effect of applying the electromagnetic field can be varied or reduced depending on the point of time at which the electromagnetic field is applied.  
     EXAMPLE 19  
     [0062] A metallic material was manufactured in the same manner as in Example 1, except that, the pouring temperature of the molten metal was set to 650° C., and the molten metal after being stirred by the electromagnetic field was cooled at a rate of 1.5° C./sec until the solid fraction reached 0.6. It took 35 seconds from the loading of the molten metal to the point of time at which the metal slurry had a solid fraction of 0.6. Sliced samples were prepared using the same method as in Example 1 for microstructure observation, and the surface and core regions on their cross-section were observed. The results are shown in FIGS. 19A and 19B.  
     EXAMPLE 20  
     [0063] A metallic material was manufactured in the same manner as in Example 1, except that the pouring temperature of the molten metal was set to 700° C., and the molten metal after being stirred by the electromagnetic field was cooled at a rate of 1.5° C./sec until the solid fraction reached 0.6. It took 40 seconds from the loading of the molten metal to the point of time at which the metal slurry had a solid fraction of 0.6. Sliced samples were prepared using the same method as in Example 1 for microstructure observation, and the surface and core regions on their cross-section were observed. The results are shown in FIGS. 20A and 20B.  
     COMPARATIVE EXAMPLE 1  
     [0064] For comparison, a metallic material was manufactured in the same manner as in Example 19, except that, after the molten metal was loaded into the vessel, an EMS was operated at a temperature slightly lower than the liquidous temperature of the molten metal for 10 seconds, and the molten metal was cooled at a rate of 0.8° C./sec until the solid fraction reached about 0.6. It took 75 seconds from the loading of the molten metal to the point of time at which the metal slurry had a solid fraction of 0.6. Sliced samples were prepared using the same method as in Example 1 for microstructure observation, and the surface and core regions on their cross-section were observed. The results are shown in FIGS. 21A and 21B.  
     COMPARATIVE EXAMPLE 2  
     [0065] For comparison, a metallic material was manufactured in the same manner as in Example 20, except that, after the molten metal was loaded into the vessel, an EMS was operated at a temperature slightly lower than the liquidous temperature of the molten metal for 10 seconds, and the molten metal was cooled at a rate of 1.0° C./sec until the solid fraction reached about 0.6. It took 85 seconds from the loading of the molten metal to the point of time at which the metal slurry had a solid fraction of 0.6. Sliced samples were prepared using the same method as in Example 1 for microstructure observation, and the surface and core regions on their cross-section were observed. The results are shown in FIGS. 22A and 22B.  
     [0066] Comparing the results of Examples 19 and 20 and Comparative Examples 1 and 2, the metallic materials manufactured in Examples 19 and 20 contain spherical particles that are fine and uniform in average diameter at the core and surface regions of the cross-section. However, for the metallic materials manufactured using conventional methods in Comparative Examples 1 and 2, where, after the molten metal was loaded into the vessel and the temperature of the molten metal dropped below its liquidus temperature, an electromagnetic field was applied to stir the molten metal, there was a difference in the microstructure of the core and surface regions of the cross-section, wherein spherical particles appear at the core region and dendritic particles appear at the surface region. Also, by using the method according to the present invention, the manufacturing time of metallic materials for rheocasting or thixoforming was greatly reduced. This is because the initial density of crystal nuclei created from the molten metal increases so that a predetermined solid fraction can be reached through the growth of the crystal nuclei for a short time.  
     [0067] As is apparent from the above-described examples and comparative examples, in a method for manufacturing metallic materials for rheocasting or thixoforming according to the present invention, it is possible to load the molten metal into a vessel at a temperature about 100° C. above its liquidus temperature, and metallic materials for rheocasting or thixoforming having micro, spherical particles can be manufactured in the form of slurries or billets from alloys through electromagnetic stirring for a short time.  
     [0068] Although manufacture of metallic materials for rheocasting or thixoforming from commercially available A356 alloy has been described in the above examples according to the present invention, the present invention is not limited to this alloy, and other various metals and alloys, for example, aluminum, magnesium, zinc, copper, iron, and alloys of the forgoing metals can be used according to the present invention.  
     [0069] As described above, in a method for manufacturing metallic materials for rheocasting or thixoforming according to the present invention, the entire volume of molten metal in the vessel can be rapidly cooled below the liquidus temperature of the molten metal uniformly throughout the center, peripheral, upper and lower regions of the vessel, without generating latent heat caused by the formation of solidification layers at the early stage of cooling. As a result, the density of crystal nuclei is markedly increased, so that alloys of uniform, micro, spherical particles of even distribution can be manufactured with improved mechanical properties.  
     [0070] A method for manufacturing metallic materials for rheocasting or thixoforming according to the present invention is simple and easy to control the overall procedure and can save the time and energy for electromagnetic stirring. Therefore, the total time and cost for manufacturing final products can be saved.  
     [0071] 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.