Description

HOT CHAMBER DIE CASTING APPARATUS FOR SEMISOLID MAGNESIUM ALLOY AND THE MANUFACTURING

METHOD USING THE SAME

Technical Field

[1] The present disclosure relates to a hot chamber die casting apparatus for magnesium alloy and a manufacturing method of magnesium alloy casting products using the same, more particularly to a hot chamber die casting apparatus where molten magnesium alloy is pressurized and formed without being exposed to the atmosphere, the apparatus including an electromagnetic stirrer adapted to stir magnesium alloy and a nozzle of which an inner diameter is adjustable and having protrusions therein, and a manufacturing method of magnesium alloy casting products using the hot chamber die casting apparatus. Background Art

[2] In general, die casting apparatuses can be divided into cold chamber die casting apparatuses and hot chamber die casting apparatuses depending on how molten metal produced in a melting furnace is transported to a mold.

[3] Particularly, a hot chamber die casting method refers to a method of directly pressurizing molten metal above atmospheric pressure, and injecting the molten metal into a mold fixed to a die. In detail, the hot chamber die casting method is performed in such a manner that molten alloy is very rapidly injected into a precision mold under high pressure at high speed to thereby form a casting product.

[4] In the hot chamber die casting method, the molten metal produced in a melting furnace is pressurized by a pressure unit without being exposed to the atmosphere, and then pressed into a mold via a nozzle. As for a transportation passageway of the molten metal, the molten metal is pressurized in a pressure chamber disposed inside the furnace, and the pressurized molten metal is transported to the nozzle via a goose neck, and then pressed into a mold cavity. Because the molten metal is pressed into the mold while not being exposed to the atmosphere, the hot chamber die casting method provides such an advantageous merit that it has less porosity in a finished casting product compared to the cold chamber die casting method. Further, since the molten metal can be airtightly solidified without being exposed to the atmosphere, the hot chamber die casting method is especially available for metals such as magnesium, which are susceptible to be oxidized rapidly under the atmosphere.

[5] Although a hot chamber die casting apparatus may be generally illustrated with reference to FIG. 1, a typical hot chamber die casting apparatus does not include a coil

for electromagnetic stirring, i.e., an electromagnetic stirrer 87.

[6] If the hot chamber die casting apparatus does not include the electromagnetic stirrer

87, metal alloy maintains its liquid state while it passes through a furnace 44, a pressure chamber 43, a goose neck 45, and a nozzle 41. Prior arts relating to the hot chamber die casting apparatus having such a configuration have been announced in Japanese Patent Laid-Open Publication No. Sho 55-136554 (October 24, 1980) and U.S. Patent No. 5,960,854 (October 5, 1999), which disclose hot chamber die casting techniques where an electric heating device or an insulating device are provided so as to prevent heat dissipation in transportation passageway from a furnace to a mold. However, molten metal in liquid state is used in the hot chamber die casting apparatus, which leads to problems in that a microstructure of a final die casting product is not densified and its strength is relatively lower than that of a die casting product obtained by using stirred semi-molten metal.

[7] If, however, a mother material is used in semi-molten state instead of liquid state, it is possible to manufacture casting products with superior mechanical properties. This is called a semi-molten or semisolid forming method (hereinafter, referred to as semisolid forming method for simplicity), in which molten metal cools down to a temperature region where solid and liquid phases coexist, and semisolid metal slurry is then casted and forged to manufacture billets or other final molded articles.

[8] This semisolid forming method has several advantages in comparison with a typical forming method using molten metal such as a casting and a liquid metal forging. For example, since slurry used in the semisolid forming method has fluidity at a relatively low temperature compared to the molten metal, the temperature of a die exposed to the slurry can be lowered than the case of using the molten metal, thus increasing a lifetime of the die. Further, when the slurry flows along a cylinder, gas entrapment can be reduced during a casting process owing to less turbulence in the semisolid metal, which reduces porosities in a final casting product. In addition, a finer, uniformly distributed, and spheroidized microstructure can be realized, which improves mechanical properties and corrosion resistance of a product.

[9] In particular, the slurry may be stirred in the semisolid forming method. FIG. 1 illustrates an injection in liquid state in the case where the electromagnetic stirrer 87 is not included, but an injection in semi-molten state in the case where the electromagnetic stirrer 87 is included.

[10] When the molten metal cools down, the slurry is stirred at a temperature between the liquidus and solidus temperatures, which breaks up already formed dendrites and prevents an initially solidified layer from growing into a dendrite, thereby obtaining spheroidal particles. To this end, a mechanical stirring and an electromagnetic stirring are commonly used for the stirring. Examples of a technology employing a stirring

process in preparing semisolid slurry are fully disclosed in Japanese Patent Laid-Open Publication No. Hei. 11-33692 (February 9, 1999) and 10-128516 (May 19, 1998).

[11] A technology of a hot chamber die casting employing an external stirrer, such as an electromagnetic stirrer and a mechanical stirrer, has been announced in US Patent Application Pub No. US 2001-0037868 (Nov. 8, 2001), in which an electromagnetic coil 87 surrounds a furnace 44 to apply electromagnetic field to semisolid metal bath, i.e., the furnace 44. Also, in the disclosure as listed above, a screw type mechanical stirrer has been introduced besides the electromagnetic coil. Such a method, however, has disadvantages that there is a limitation in reducing the perimeter of the furnace due to a pressure unit 42 provided in the furnace, and semi-molten metal 46 already supplied into a pressure chamber is substantially not affected by the electromagnetic field any longer. Moreover, the furnace is too big to apply effective electromagnetic field to semisolid metal so that the electric field does not sufficiently have an effect on the semisolid metal even if the electric field is applied. Further, there is a limitation in increasing the electromagnetic field infinitely in terms of current manufacturing process, which also brings about very high power consumption. The mechanical stirring also has a problem in that the insertion of the screw could break the vacuum state of the furnace, and thus external impurities may be entrapped in the molten metal. In addition, there is a limitation in breaking primary crystal and preventing the growth of a dendrite by the mechanical stirring. Furthermore, there is another problem in that even though it is possible to control conditions for fine and densified microstructure using the electromagnetic or mechanical stirring for the time being, the conditions are no longer maintained as soon as the semisolid metal flows into the nozzle 41 via the goose neck 45. Disclosure of Invention Technical Problem

[12] Accordingly, to substantially obviate one or more above described problems, the present disclosure is directed to provide a hot chamber die casting apparatus for semisolid magnesium alloy and a manufacturing method of magnesium alloy casting products using the same, which are capable of manufacturing die casting products with finer microstructures and thus achieving high-strength, slimness and light-weight properties of the products. Technical Solution

[13] Embodiments provide a hot chamber die casting apparatus where molten magnesium alloy is pressurized and molded without being exposed to the atmosphere, the apparatus including an electromagnetic stirrer adapted to stir molten magnesium alloy and a nozzle of which an inner diameter is adjustable and having protrusions

therein, and a manufacturing method of magnesium alloy casting products using the hot chamber die casting apparatus.

Advantageous Effects

[14] In accordance with hot chamber die casting apparatuses of exemplary embodiments, it is possible to manufacture casting products with fine microstructure to exhibit high strength. Further, the microstructure can be densified, which secures the casting product with uniform thickness. Particularly, it is possible to decrease defect rate and increase productivity of a die casting product. Brief Description of the Drawings

[15] Exemplary embodiments can be understood in more detail from the following description taken in conjunction with the accompanying drawings, in which:

[16] FIG. 1 is a schematic view illustrating a typical hot chamber die casting apparatus;

[17] FIG. 2 is a schematic view illustrating a hot chamber die casting apparatus in accordance with an exemplary embodiment;

[18] FIG. 3 is an enlarged sectional view of a portion denoted as reference numeral 100 of FIG. 2;

[19] FIG. 4 is a schematic view illustrating a hot chamber die casting apparatus in accordance with another exemplary embodiment;

[20] FIGS. 5 through 8 are sectional views illustrating various alternative embodiments depending upon a relative size between inner diameters of a nozzle and a pressure chamber, and upon whether or not a protrusion is formed;

[21] FIG. 9 is a sectional view illustrating various inner shapes of the nozzle;

[22] FIGS. 10 and 11 are micrographs illustrating microstructures of a magnesium alloy manufactured in accordance with the exemplary embodiments; and

[23] FIG. 12 is a micrograph illustrating a microstructure of a magnesium alloy prepared by a comparative example according to the related art. Best Mode for Carrying Out the Invention

[24] In accordance with an exemplary embodiment, a hot chamber die casting apparatus of pressurizing molten magnesium alloy at an atmospheric pressure or higher without being exposed to the atmosphere and forming a magnesium alloy casting product in the shape of a mold, the hot chamber die casting apparatus including a melting furnace having a heater, a pressure unit configured to pressurize the molten magnesium alloy, and a mold configured to shape a product, the hot chamber die casting apparatus being characterized in that the hot chamber die casting apparatus includes an electromagnetic stirrer disposed around a nozzle between a pressure chamber and the mold, and configured to stir semisolid magnesium alloy, the pressure chamber being connected to the melting furnace and supplying magnesium alloy in liquid state. Herein, an inner

diameter of the nozzle is smaller than that of the pressure chamber so as to crush solid phases existing in the semisolid magnesium alloy in the pressure chamber or the nozzle.

[25] That is, since the inner diameter is smaller in the nozzle than the pressure chamber and the protrusions are provided inside the nozzle, higher pressure may be exerted on the semisolid metal while the semisolid metal flowing through the nozzle. This causes slurry in the semisolid metal to collide with each other or against an inner wall of the nozzle, thus crushing solid phases and making a microstructure become finer. Due to the electromagnetic stirrer disposed around the nozzle, the crushing effect can be more increased.

[26] In accordance with another exemplary embodiment, a hot chamber die casting apparatus includes at least one protrusion narrowing the inner diameter of the nozzle, which is provided inside the nozzle, thus increasing a pressure exerted on semisolid magnesium alloy passing therethrough. Accordingly, slurry in the semisolid metal collides against the protrusion to increase the crushing effect of solid phases.

[27] In accordance with still another exemplary embodiment, an inner diameter of the nozzle is equal to or greater than that of the pressure chamber and at least one protrusion is provided inside the nozzle.

[28] In the exemplary embodiments, the nozzle may have various sectional shapes as well as a circular shape such that flow resistance at an inner surface of the nozzle can be increased on a transportation passageway of the semisolid metal to a mold. Further, the protrusion may have a variety of shapes that can impede the passage of the semisolid magnesium alloy.

[29] The stirrer may include an electromagnetic stirrer generating electromagnetic field or an ultrasonic generator generating ultrasonic wave. The stirrer may be disposed in a region on a passageway of molten metal between the furnace and the mold.

[30] An outlet of the pressure chamber may extend up to an outer wall of the furnace.

Alternatively, the outlet of the pressure chamber may be shorter or longer than the outer wall of the furnace. The nozzle may be positioned around the outlet end of the pressure chamber, and may be connected to the mold. The pressure chamber may protrude from the inside of the furnace and may be directly connected to the mold. In this case, the stirrer may be positioned on a portion of the stirrer which extends from the furnace to the outside. In the case where the pressure chamber extends to the outer wall of the furnace, the stirrer may surround the pressure chamber. Further, in the case where the pressure chamber is provided without a nozzle, the stirrer may surround the pressure chamber extending to the outside of the furnace. Mode for the Invention

[31] Hereinafter, specific embodiments will be described in detail with reference to the accompanying drawings.

[32] FIG. 2 is a schematic view illustrating a hot chamber die casting apparatus in accordance with an exemplary embodiment, and FIG. 3 is an enlarged sectional view of a portion denoted as reference numeral 100 of FIG. 2.

[33] In accordance with the exemplary embodiment, solid metal to be melted or liquid metal already melted is contained in a furnace 230. A heater of the furnace 230 may also be used for maintaining the furnace 230 at a predetermined temperature. When the temperature of the molten metal is maintained above a predetermined temperature, a piston or a plunger in a pressure chamber 220 operates to pressurize and supply molten metal to a mold 130 and 140. Here, the molten metal 240 can be pressurized through a pneumatic cylinder or a hydraulic cylinder. The molten metal 240 in the furnace flows into the pressure chamber 220 through an intake port 210 of the pressure chamber 220 in the furnace 230. Then, the molten metal 240 flows toward a goose neck 270 or an outlet end of the pressure chamber.

[34] The goose neck 270 is connected to a nozzle 105 so that the mold 130 and 140 is connected to the pressure chamber 220. The temperature of the molten metal 240 cools down while flowing through the outlet end of the pressure chamber 220. Accordingly, the metal flows toward the nozzle 105 in liquid-solid coexisting state.

[35] In accordance with the exemplary embodiment, an electromagnetic stirrer 101 may surround the nozzle 105. Considering that the molten metal 240 contained in the furnace 230 is rarely affected by external electromagnetic field because the molten metal is in liquid state, the electromagnetic stirrer 101 may be disposed in a region between the pressure chamber 220 and the mold 130 and 140 where the molten metal maintains its liquid-solid coexisting state and has an appropriate solid fraction. Therefore, the metal in semisolid state, i.e., of which a state is transformed from a liquid state into a liquid-solid coexisting state, is supplied into the mold 130 and 140 while or after being affected by electromagnetic field caused by the stirrer 101 while passing through the nozzle 105. The semisolid metal prevents nucleation of a primary dendrite or breaks already nucleated primary dendrite to enable fine and uniform nuclei to be formed, and also prevents the growth of nucleated primary dendrite. Further, even during a stand-by time between a time when the semisolid metal is primarily injected into the mold 130 and 140 and a time for a next injection, the semisolid metal may be affected by the external electromagnetic field. The mold 130 and 140 may use typical molds, and may be configured with a fixed mold part 140 and a movable mold part 130.

[36] An inner diameter DO of the nozzle 105 may be smaller than an inner diameter Dl of an outlet end of the goose neck 270 or an outlet end of the pressure chamber 220

(that is, DO<D1 in FIG. 3). Desirably, the inner diameter DO of the nozzle 105 may be half or smaller than the inner diameter Dl of the outlet end of the goose neck 270 or the outlet end of the pressure chamber 220. Accordingly, since higher pressure is exerted on the semisolid metal passing through the nozzle 105, slurry in the semisolid metals collide with each other or the semisolid metal collides against an inner wall of the nozzle, thus crushing solid phases finely.

[37] FIG. 4 is a schematic view illustrating a hot chamber die casting apparatus in accordance with another exemplary embodiment.

[38] In this exemplary embodiment, the semisolid metal is supplied into the mold 130 and 140 while or after being affected by electromagnetic field caused by the stirrer 101 disposed outside the nozzle 105 as described above. Protrusions 106 are formed on an inner surface of the nozzle 105, which increases the resistance against the flow of the semisolid metal passing through the nozzle 105. Hence, the pressure exerted on the semisolid metal is increased greatly to more improve the crushing effect of the slurry therein, whereby the microstructure can become finer. Since the pressure can be increased greatly by virtue of the protrusions 106, it is not necessary to satisfy the above condition that the inner diameter DO of the nozzle 105 should be smaller than the inner diameter Dl of the outlet end of the goose neck 270 or the outlet end of the pressure chamber 220. However, in the case where the protrusions 106 are provided on the inner surface of the nozzle 106 and also the inner diameter DO of the nozzle 105 is made to be smaller than the inner diameter Dl of the outlet end of the goose neck 270 or the outlet end of the pressure chamber 220, the crushing effect of the slurry inside the semisolid metal can be increased much more.

[39] FIGS. 5 through 8 are sectional views illustrating various alternative embodiments depending upon a relative size between inner diameters of the nozzle 105 and the outlet end of the goose neck 270 or the outlet of the pressure chamber 220 and upon whether the protrusions are formed or not.

[40] In an alternative embodiment, the inner diameter DO of the nozzle 105 may be smaller than the inner diameter Dl of the outlet end of the goose neck 270 or the outlet end of the pressure chamber 220, and the protrusions may not be formed on the inner surface of the nozzle 105, as illustrated in FIG. 5. Due to the pressure caused by the inner diameter DO of the nozzle 105 smaller than the inner diameter Dl of the outlet end of the goose neck 270 or the outlet end of the pressure chamber 220, the slurry in the semisolid metal may be crushed by collision of the slurry itself and the slurry may be simultaneously crushed by the surface resistance of an inner wall of the nozzle 105, even though there is no protrusion in the nozzle 105.

[41] In another alternative embodiment, the inner diameter DO of the nozzle 105 is smaller than the inner diameter Dl of the outlet end of the goose neck 270 or the outlet

end of the pressure chamber 220, and the protrusions are formed inside the nozzle 105, as illustrated in FIG. 6. As a result, the pressure exerted on the semisolid metal is increased more than that of the embodiment of FIG. 5, and thus the crushing effect of the slurry is also increased due to the increase in the surface resistance of the inner wall of the nozzle 105.

[42] In still another alternative embodiment, the inner diameter DO of the nozzle 105 is equal to the inner diameter Dl of the outlet end of the goose neck 270 or the outlet end of the pressure chamber 220, and the protrusions 106 are formed on the inner surface of the nozzle 105, as illustrated in FIG. 7. Alternatively, the inner diameter DO of the nozzle 105 is greater than the inner diameter Dl of the outlet end of the goose neck 270 or the outlet end of the pressure chamber 220, and the protrusions 106 are formed inside the nozzle 105, as illustrated in FIG. 8. In both the embodiments, the slurry in the semisolid metal is crushed due to the existence of the protrusions 106.

[43] FIG. 9 is a sectional view illustrating various shapes of the protrusion 106 formed inside the nozzle 105 in the exemplary embodiments.

[44] In the exemplary embodiments, the protrusion 106 may be branched into two passageways or more, and thus the pressure exerted on the semisolid metal passing through the narrow passageways and the surface resistance of the inner surface of the nozzle are more greatly increased, which makes it possible to maximize the crushing effect of the slurry in the semisolid metal.

[45] In the exemplary embodiments, the molten metal 240 is substantially in liquid state when the molten metal flows through the pressure chamber 220 in the furnace 230. However, the molten metal 240 cools down while the molten metal exits from the furnace 230 and flows through the outlet end of the pressure chamber 220 or the goose neck 270. Accordingly, the molten metal gradually is transformed from a liquid state into a liquid-solid coexisting state.

[46] The temperature around the nozzle 105 may be set to a temperature corresponding to the temperature of the liquid-solid coexisting state. In accordance with the exemplary embodiment, a heating device or an insulating unit may be provided around the nozzle 105 so as to prevent a rapid cooling of the nozzle 105. As described above, the temperature should be appropriately controlled such that the molten metal is not affected by the electromagnetic field due to the liquid state of the molten metal. Consequently, the rapid cooling can be prevented and the molten metal in liquid state can maintain its solid fraction, thus making it possible to form semisolid metal having a fine microstructure through the stirring. Moreover, both a variation in a relative size between inner diameters of the nozzle 105 and the outlet of the pressure chamber and the protrusions 106 provided inside the nozzle 105 increase the resistance of flow so that primary crystals or solids in the slurry become finer, and thus the semisolid metal

with finer microstructures can be supplied into the mold 130 and 140.

[47] In accordance with the exemplary embodiments, the electromagnetic stirrer 101 may include an electromagnetic field generator or an ultrasonic wave generator.

[48] If the stirrer 101 is an ultrasonic wave generator, the intensity of the ultrasonic wave may be approximately 5 D or more, and desirably may be in a range of approximately 13 D to approximately 10 D. The present inventor cannot obtain a sufficient stirring effect below 5 D. The ultrasonic wave may have the intensity as high as possible.

[49] If the stirrer 101 is an electromagnetic field generator, the intensity of the electromagnetic field may be at least approximately 50 Gauss, and desirably may be in the range of approximately 200 to approximately 10, 000 Gauss. The present inventor cannot obtain a sufficient electromagnetic effect below 50 Gauss. Since the stirring effect increases as the intensity of electromagnetic field increases, it is more effective to apply the electromagnetic field with intensity as high as possible theoretically. The maximum intensity of the electromagnetic field to be applied, however, may be determined taking into account an available space for the stirrer and supplied power. In particular, current of the electromagnetic field generator may be approximately 0.5 A or more, and desirably may be in the range of approximately 1 to approximately 100 A. The present inventor cannot obtain a sufficient electromagnetic effect below 0.5 A. Although the stirring effect is proportional to the increase in current, the applied current should be determined in consideration of the limit of the apparatus and economical views.

[50] A time taken for stirring does not exceed a typical process time if the current and the electromagnetic field have the intensities within the above-listed ranges, respectively. That is, a stirring is sufficiently performed while a mold 150 receives molten metal, solidifies the molten metal, discharges the solidified metal, and then prepare to receive another molten metal.

[51] The hot chamber die casting apparatus in accordance with the exemplary embodiments are suitable for low-melting-point metals such as zinc, tin, lead, aluminum and magnesium, but not limited to them. Since the molten metal is not exposed to the atmosphere in the hot chamber die casting, it is adapted to manufacture die casting products with good quality because it prevents the metal from being oxidized at a high temperature. Hence, the hot chamber die casting apparatus is very suitable for die casting products of magnesium alloy which are recently used as frames of electronic products such as notebooks and cell phones.

[52] A variety of magnesium alloys can be used in the hot chamber die casting in accordance with the exemplary embodiments. The magnesium alloys may include AZ91A, AZ91B, ZA91D, AM60A, AM60B, and AS41A, according to ASTM standard although not limited to them. Compositions of the magnesium alloys for use

in the die casting are listed in Table 1 below.

[53] Table 1

[54] Considering that the complete melting temperature of the general magnesium alloys are in the range of approximately 590 ⁰ C to approximately 630 ⁰ C, the temperature of the furnace 230 may be set to a temperature slight higher than the above range, i.e., in the range of approximately 620 ⁰ C to approximately 750 ⁰ C.

[55] The molten magnesium alloy cools down to a solid-liquid coexisting temperature while flowing toward the nozzle 105 or the outlet end of the pressure chamber 220 via the pressure chamber 220 in the furnace 230. When the temperature of the nozzle 105 is controlled to be too low, the content ratio of solid phases to liquid phases is so high that the semisolid metal cannot be easily supplied to the mold 130 and 140, which may increase the probability of casting defects in die casting products. On the other hand, when the temperature of the nozzle 105 is controlled to be too high, the content ratio of solid phases to liquid phases is so low that the semisolid metal cannot be affected sufficiently by an electromagnetic wave or an ultrasonic wave. Since the temperature of the nozzle is to be controlled to be substantially lower than an ideal temperature of the semisolid magnesium alloy, the temperature range of the nozzle 105 should be substantially higher than the ideal temperature, for example, may be in a range of approximately 480 ⁰ C to approximately 620 ⁰ C.

[56] Hereinafter, specific embodiments will be more fully described with reference to the accompanying drawings. It is noticed that following specific embodiments are merely provided for general understandings of the present invention, and thus the present invention is not limited to the specific embodiments.

[57] Embodiment 1 [58] In this embodiment, a hot chamber die casting was performed under conditions that

a magnesium alloy of AZ91D was used, a temperature of the furnace was 65O ⁰ C, a pressure of the plunger was 10 MPa, a temperature of nozzle was 600 ⁰ C, an electromagnetic field was 700 Gauss, an electric current was 7 A, and an injection rate was 2.5m/sec. Further, in the hot chamber die casting apparatus in accordance with this embodiment, the nozzle was designed to have a smaller inner diameter than the pressure chamber, and protrusions were provided inside the nozzle. The resultant mi- crostructure is illustrated in FIG. 10. Grain size of the microstructure is approximately 9 D.

[59] Embodiment 2

[60] In this embodiment, a hot chamber die casting was performed under conditions that a magnesium alloy of AM60B was used, a temperature of the furnace was 68O ⁰ C, a pressure of the plunger was 7 MPa, a temperature of nozzle was 58O ⁰ C, an electromagnetic field was 900 Gauss, an electric current was 10 A, and an injection rate was 1.5m/sec. In the hot chamber die casting apparatus in accordance with this embodiment, the nozzle was also designed to have a smaller inner diameter than the pressure chamber, but protrusions were not provided inside the nozzle. The resultant microstructure is illustrated in FIG. 11. Grain size of the microstructure is approximately 5.5 D.

[61] Comparative example

[62] In this comparative example, a hot chamber die casting was performed under conditions that a magnesium alloy of AZ91D was used, a temperature of the furnace was 65O ⁰ C, a pressure of the plunger was 10 MPa, a temperature of nozzle was 600 ⁰ C, an electromagnetic field was 1,000 Gauss, an electric current was 15 A, and an injection rate was 2.5m/sec. A related art hot chamber die casting apparatus was used in the comparative example. Further, the nozzle size, i.e., ratio of the inner diameter of the nozzle to that of the pressure chamber, was not controlled, and protrusions were not formed on the inner surface of the nozzle either. The resultant microstructure is illustrated in FIG. 12. Grain size of the microstructure is approximately 40 D.

[63] Referring to the embodiments and FIGS. 10 through 12, it can be observed that the grain size of the magnesium alloy manufactured by the hot chamber die casting apparatus of the embodiments ranges from approximately 2 D to approximately 10 D, approximately 5 D on the average. From this result, it can be appreciated that the magnesium alloy of the embodiments become finer and more densified than that of the comparative example manufactured by the related art (grain size of approximately 40 D).

[64] As described above, in the hot chamber die casting apparatus of the exemplary embodiments, molten metal is gradually transformed from liquid state into semisolid state, i.e., solid- liquid coexisting state, as the molten metal cools down while flowing

to the nozzle 105 from the furnace 230. The semisolid metal alloy in the solid- liquid coexisting state is affected by the electromagnetic stirrer 101 surrounding the periphery around the nozzle 105 so that primary crystals and dendrites growing from the primary crystals existing in slurry of the semisolid metal alloy becomes finer and more densified. Due to the resistance caused by the shape of the nozzle 105 having a smaller inner diameter DO than the outlet end of the goose neck or the outlet end of the pressure chamber and/or the resistance caused by the protrusions 106 provided inside the nozzle 105, the solid phases in the slurry are crushed while the semisolid metal alloy flowing through the nozzle 105. Accordingly, the metal alloy with much finer microstructure can be supplied into the mold 130 and 140, thus making it possible to obtain a die casting product with very densified and high-strength microstructure.

[65] Although the hot chamber die casting apparatus and the manufacturing method using the same have been described with reference to the specific exemplary embodiments, they are not limited thereto. Therefore, it will be understood that various modifications, changes and supplements can be made by those skilled in the art. Accordingly, following claims should be construed that they include all the modifications, changes and supplements without departing from the spirit and scope of the present invention. Industrial Applicability

[66] In accordance with the exemplary embodiments, it is possible to manufacture a casting product with high strength in virtue of fine microstructure, and further manufacture a casting product with uniform thickness due to a densified microstructure. This contributes to the decrease in defect rate and the increase in productivity of die casting products.

[67]