BACKGROUND OF THE INVENTION 1. Field of the Invention

[0001] The invention relates to a method of casting a light alloy produced by adding an additional element to a light metal, and to a casting manufactured by the method.

2. Description of the Related Art

[0002] As the related art, in the case where a casting product is manufactured through casting using a light alloy such as an aluminum alloy or the like as a coarse material, it is known that an aging treatment is performed after subjecting the casting product to a hardening treatment, with a view to improving the mechanical strength (tensile strength, elongation or the like) of the casting product. The light alloy is produced mainly from a light metal as a base material and an additional element added to the light metal. Then, after a molten metal is poured into a casting mold (a pouring process), the coarse material is coagulated while gradually cooling the entire casting mold (a casting mold cooling process). The coarse material is then released from the casting mold (a releasing process). Subsequently, after the released coarse material is further cooled to a temperature equal to or lower than 100°C (a coarse material cooling process), the coarse material is heated up again to a predetermined temperature and held at this temperature for a predetermined time. The coarse material is thereby subjected to an aging treatment (an aging process). As a result, the mechanical strength of the casting product is improved.

[0003] In order to ensure uniform mechanical strength for the casting product manufactured through casting by this casting method, the coarse material made of the supersaturated light alloy needs to be subjected to the aging treatment after the hardening treatment is completed while minimizing to the best possible extent the dispersion of the amount of solid solution of the additional element in the coarse material. However, the supersaturated light alloy has the properties of allowing the solid solution element to separate out when being held at a predetermined temperature for a predetermined time. Further, the time needed for the separation of the supersaturated solid solution element from the light alloy (hereinafter referred to as a "separation time") slightly differs depending on the types or the like of the light metal and the additional element, but has the properties of changing in accordance with the temperature of the light alloy. Consequently, in the course of cooling the coarse material from the casting mold cooling process to the coarse material cooling process, a large amount of the solid solution element non-uniformly separates out from the light alloy when the coarse material is cooled for a certain time or when the temperature of the coarse material is distributed according to a certain pattern.

[0004] For this reason, in order to complete the hardening treatment for the coarse material made of the supersaturated light alloy while minimizing to the best possible extent the dispersion of the amount of solid solution in the coarse material, it is considered appropriate to simply make the time for cooling the coarse material as short as possible. As the related art, the function of cooling the casting mold is reinforced to shorten the cooling time. However, even when the function of cooling the casting mold is reinforced to shorten the cooling time, uniform mechanical strength cannot be actually ensured for the casting product. On the contrary, the mechanical strength decreases in some cases.

[0005] The following reason is conceivable for this decrease in the mechanical strength of the casting product. That is, the coarse material has a temperature range in which the solid solution element in the light alloy rapidly separates out, namely, a temperature range in which the "separation time" is relatively short (hereinafter referred to as a "separation temperature range"). Then, when the function of cooling the casting mold is reinforced, a vicinity of that region (a surface portion) of the coarse material which is in contact with the casting mold is cooled to a temperature equal to or lower than the "separation temperature range" in a short time, and a large amount of the solid solution element is held in a supersaturated state. On the other hand, that region (a center portion) of the coarse material which is spaced apart from that region (the surface portion) of the coarse material which is in contact with the casting mold starts to be cooled later than the vicinity of the surface portion of the coarse material. Then, the center portion of the coarse material is gradually cooled while discharging heat locally (non-uniformly) toward the vicinity of the surface portion of the coarse material, which has already been cooled to the temperature equal to or lower than the "separation temperature range". In the vicinity of the surface portion of the coarse material, a location toward which heat is discharged from the center portion of the coarse material is heated up again, but the temperature of this location reaches a temperature within the "separation temperature range". Then, when the aforementioned "separation time" elapses, a large amount of the solid solution element in the light alloy in a supersaturated manner separates out. For this reason, in the vicinity of the surface portion of the coarse material, there arises a location where the amount of the solid solution element in the light alloy is remarkably small, and the amount of the solid solution element in the coarse material disperses. Then, when the region where the solid solution element separates out is further subjected to the aging treatment, the mechanical strength decreases due to an overaging phenomenon. Therefore, when the amount of the solid solution element in the coarse material disperses, the subjection of the coarse material to any aging treatment can no longer ensure sufficient mechanical strength for the aforementioned region.

[0006] On the other hand, each of Japanese Patent Application Publication No. 8-225903 (JP-A-8-225903), Japanese Patent Application Publication No. 2005-169498 (JP-A-2005-169498), and Japanese Patent Application Publication No. 2008-13791 (JP-A-2008-13791) discloses an art as means for shortening a time for cooling a coarse material with a view to completing a hardening treatment for the coarse material in a supersaturated state while allowing a larger amount of a solid solution element to remain in a light alloy. In Japanese Patent Application Publication No. 8-225903 (JP-A-8-225903), there is disclosed an art about a method of manufacturing a high-pressure-cast aluminum alloy casting. In this method, after the coagulation of an aluminum alloy (a coarse material) poured into a mold (a casting mold) is completed, the coagulated aluminum alloy (the coarse material) is removed from the mold (the casting mold), immediately soaked in water, and then subjected to hardening. An aging treatment is performed after the completion of this hardening.

[0007] Further, in Japanese Patent Application Publication No. 2005-169498

(JP-A-2005-169498), there is disclosed an art about a method of manufacturing a light alloy casting. This method is characterized in the following respects. After the coagulation of an aluminum alloy (a coarse material) poured into a casting mold, whose cavity is constituted by a sand mold forming a dead head portion and a mold arranged apart from the dead head portion to form part of the cavity, is completed, only the mold is separated with the sand mold left. This coagulated aluminum alloy (the coarse material) and this sand mold are soaked as a whole in water stored in a tank and subjected to a hardening treatment. Subsequently, the aluminum alloy (the coarse material) is subjected to an aging treatment with the aid of the heat retained by the dead head portion in a heat retention container.

[0008] Further, in Japanese Patent Application Publication No. 2008-13791 (JP-A-2008-13791), there is disclosed an art about an aluminum alloy (a coarse material) manufactured in the following manner. After solution heating is carried out to hold a cast metal at a treatment temperature between 450°C and 510°C for a time equal to or longer than 30 minutes, the cast metal is subjected to water hardening. After that, this cast metal is subjected to an aging treatment to be held at a treatment temperature between 170°C and 230°C for 1 to 24 hours.

[0009] In each of the arts disclosed in Japanese Patent Application Publication No. 8-225903 (JP-A-8-225903) and Japanese Patent Application Publication No. 2005-169498 (JP-A-2005-169498), the coarse material is directly soaked in water. Therefore, the coarse material is uniformly cooled to a temperature equal to or lower than the "separation temperature range". Besides, since the coarse material has already been released from the casting mold, the temperature of the vicinity of the surface portion of the coarse material does not rise again to be held in the "separation temperature range". Further, according to the art disclosed in Japanese Patent Application Publication No. 2008-13791 (JP-A-2008-13791), the coarse material can be reliably held in a supersaturated state by carrying out solution heating and water hardening prior to the aging treatment. Thus, according to each of the arts disclosed in Japanese Patent Application Publication No. 8-225903 (JP-A-8-225903), Japanese Patent Application Publication No. 2005-169498 (JP-A-2005-169498), and Japanese Patent Application Publication No. 2008-13791 (JP-A-2008-13791), the hardening treatment can be completed while minimizing to the best possible extent the dispersion of the amount of solid solution of the additional element in the light alloy. As a result, it seems possible to ensure uniform mechanical strength for the casting product.

[0010] However, as for the art disclosed in Japanese Patent Application Publication No. 8-225903 (JP-A-8-225903), the management of the temperature of the casting mold before releasing the coarse material from the casting mold is not considered to be an indispensable factor. That is, the coarse material may already have been cooled to a temperature equal to or lower than the "separation temperature range" before being released from the casting mold. In such a case, even when the coarse material is directly soaked in water immediately after being released from the casting mold and then subjected to the aging treatment, uniform mechanical strength cannot be ensured for the casting product as in the aforementioned case where the function of cooling the casting mold is reinforced. On the contrary, the mechanical strength decreases in some cases.

[0011] Further, as for the art disclosed in Japanese Patent Application Publication No. 2005-169498 (JP-A-2005-169498), the coarse material is soaked in water from a temperature between 480°C and 580°C, at which the temperature of the coarse material is higher than the "separation temperature range" of the additional element. Therefore, there is no need to bother to provide means for managing the temperature of the casting mold. In this art, however, the temperature of the casting mold significantly falls every time a single casting product is manufactured through casting. This art is therefore not suited to the continuous manufacture of a large quantity of such casting products through casting. Further, in this art, the coagulated coarse material and the sand mold are soaked in water as a whole. Therefore, the sand mold absorbs water, becomes hard, and significantly deteriorates in collapsibility, thereby causing a deterioration in operability. As a result, this art is not suited to the continuous manufacture of a large quantity of such casting products through casting.

[0012] Furthermore, as for the art disclosed in Japanese Patent Application Publication No. 2008-13791 (JP-A-2008-13791), the processes concerning solution heating and water hardening are added to the case where the casting product is manufactured through casting through the heat treatment composed solely of the aging treatment. Thus, this casting method for the casting product leads to high costs as a whole and hence is unrealistic.

SUMMARY OF THE INVENTION

[0013] The invention provides a method of casting a light alloy which allows a casting product to be manufactured through casting using a light alloy as a coarse material and ensures mechanical strength for the casting product through an aging treatment. In this method, uniform mechanical strength can be ensured for the casting product by performing the aging treatment after a hardening treatment is completed while minimizing to the best possible extent the dispersion of an amount of solid solution element in the light alloy. The invention also provides a casting manufactured by the method.

[0014] A first aspect of the invention relates to a method of casting a light alloy to manufacture a casting product through casting. This casting method includes heating and melting a light alloy composed of a light metal as a base material and an additional element, and coagulating the molten light alloy through cooling such that a solid solution element remains within a separation temperature range, namely, a temperature range corresponding to a shortest elapsed time needed for separation of the solid solution element, which is contained in the light alloy, from the light metal when the light alloy has the solid solution element therein before being heated and when the light alloy is held at a predetermined temperature. [0015] In the casting method according to this aspect of the invention, the coagulated light alloy may be removed from a casting mold into which the molten light alloy may be poured to be coagulated. The casting mold may have cooling means. The cooling means may cool the casting mold so that the light alloy is cooled to a temperature higher than the separation temperature range. The light alloy may be removed from the casting mold before the temperature of the light alloy reaches the separation temperature range.

[0016] In the casting method according to this aspect of the invention, the light metal may be an aluminum-silicon-copper-magnesium alloy. The additional element may be magnesium. The separation temperature range may be a range between 300°C and 400C°.

[0017] A second aspect of the invention relates to a casting manufactured by the casting method according to the foregoing first aspect of the invention.

[0018] The following effect is achieved by the invention. That is, according to the method of casting the light alloy in the invention, uniform mechanical strength can be ensured for the casting product by performing the aging treatment after the hardening treatment is completed while minimizing to the best possible extent the dispersion of the amount of the solid solution element in the light metal, without causing an increase in the cost of equipment.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The foregoing and further objects, features and advantages of the invention will become apparent from the following description of an exemplary embodiment of the invention with reference to the accompanying drawings, wherein like numerals are used to represent like elements and wherein:

FIG. 1 is a diagrammatic view showing a relationship between a temperature of a coarse material (unit (°C)) and an elapsed time at a time when a supersaturated solid solution element begins to separate out from a light alloy;

FIG. 2 is a schematic view showing the overall configuration of a casting device that realizes "a method of casting a light alloy" according to the embodiment of the invention;

FIG. 3 is a schematic view showing the overall configuration of a coarse material cooling device that realizes the "method of casting the light alloy" according to the embodiment of the invention;

FIG 4 is a process chart showing the overall flow of a method of casting a light alloy according to one example of the invention;

FIG 5 is a diagrammatic view showing how the temperature of the coarse material changes with the elapsed time in respective processes;

FIG 6 is a diagrammatic view showing a contrast between a case where a mold is subjected to temperature control and a case where the mold is not subjected to temperature control, in respect of changes with time in the temperature of the coarse material from a pouring process to a coarse material cooling process; and

FIG. 7 is a graph showing a contrast between the case where the mold is subjected to temperature control and the case where the mold is not subjected to temperature control, in respect of the solid solubility and hardness of the coarse material after an aging treatment.

DETAILED DESCRIPTION OF EMBODIMENT

[0020] Next, the embodiment of the invention will be described.

[0021] [Outline] First of all, the outline of this example will be described. The casting method in this example is a method of casting a coarse material made of a light alloy, and aims at ensuring uniform mechanical strength (tensile strength, elongation or the like) for a casting product manufactured through casting. That is, the light alloy (hereinafter referred to as the coarse material) is mainly produced from a light metal as a base material and a solid solution element in the light metal. It should be noted herein that the light metal may not necessarily be a pure metal, but may be an alloy obtained by mixing two or more elements such as light metals with one another in advance. In this example, for instance, an aluminum-silicon alloy is used as the base material, and an Al-Si-Cu-Mg-type aluminum alloy, which is produced by solidly dissolving magnesium as an additional element into the aluminum-silicon alloy and contains magnesium as the additional element, is used as the coarse material. The coarse material in a molten state is then coagulated while regulating the temperature for cooling a later-described casting mold 2 (see FIG. 2). The coagulated coarse material is then cooled in a short time, subjected to a hardening process to be tumed into a supersaturated solid solution, and then subjected to an aging treatment. As a result, the mechanical strength of the casting product is improved.

[0022] It should be noted herein that the aging treatment refers to a treatment for developing a curing phenomenon by holding a coarse material made of a supersaturated solid solution at a predetermined temperature.

[0023] More specifically, when the coarse material is rapidly cooled through a hardening treatment, the solid solution element in the light alloy in a supersaturated manner is mostly prevented from separating out. Thus, the coarse material is cooled while remaining in the supersaturated state.

[0024] After that, the coarse material is heated up again to a temperature (e.g.,

100 to 160°C) slightly higher than a room temperature, and is held at this temperature for a predetermined time. The solid solution element in the supersaturated manner thereby forms a Guinier-Preston (G-P) zone, and a strain develops in a crystal lattice of the light metal. However, since the developed strain serves as a resistance against the deformation of the coarse material, the curing phenomenon of the coarse material emerges.

[0025] For this reason, in order to ensure uniform mechanical strength for the casting product through the aging treatment, the hardening treatment needs to be completed while minimizing to the best possible extent the dispersion of the amount of solid solution element in the coarse material.

[0026] Thus, means for reinforcing the function of cooling the casting mold 2 to shorten the time for cooling the coarse material has been employed as means for completing the hardening treatment while minimizing to the best possible extent the dispersion of the amount of solid solution element in the light metal according to the related art. In reality, however, it is difficult to ensure uniform mechanical strength for the casting product manufactured through casting by simply reinforcing the function of cooling the casting mold 2.

[0027] On the other hand, a surplus of the solid solution element in the saturated manner has the properties of separating out to the outside. The elapsed time needed for the separation of the surplus of the solid solution element from the light metal changes in accordance with the temperature of the coarse material.

[0028] That is, as shown in FIG. 1, an axis of ordinate represents the temperature (unit (°C)) of the coarse material, and an axis of abscissa represents an elapsed time (unit (sec)) in a state where the coarse material is held at the temperature. In FIG. 1, a relationship between the temperature of the coarse material and the elapsed time is expressed by a curve at a time point when the supersaturated solid solution element begins to separate out from the light alloy. This curve protrudes leftward. That is, the "separation time" temporarily decreases with falls in the temperature of the coarse material from a high-temperature side, but increases with falls in the temperature of the coarse material in a temperature range lower than a predetermined temperature (hereinafter referred to as a "peak temperature (T)").

[0029] Although the "peak temperature (T)" slightly differs depending on the types or the like of the light metal and the additional element, it is known that the "peak temperature (T)" of magnesium is approximately equal to 350°C in the Al-Si-Mg-type aluminum alloy used as the coarse material in this example.

[0030] Thus, as a result of repeated and strenuous studies, the inventors have focused attention on the "peak temperature (T)" and hence established the method of casting the coarse material which makes it possible to ensure uniform mechanical strength for the casting product manufactured through casting.

[0031] That is, the inventors have decided to prescribe "a separation temperature range" as a temperature range in which the solid solution element begins to separate out from within the light metal by "a separation time" relatively shorter than in other temperature ranges in the course of cooling the molten coarse material, and to set this "separation temperature range" to a range of about 50°C around the "peak temperature (T)", namely, between 300°C and 400°C. This "separation temperature range" can be said to be a temperature range conesponding to the shortest elapsed time needed for the separation of the solid solution element, which is contained in the light alloy, from the light metal when the coarse material is held at a predetermined temperature.

[0032] Then, by managing the temperature of the casting mold 2 in the "separation temperature range" (i.e., by managing the temperature to shorten the time during which the coarse material remains in the "separation temperature range" in the course of cooling the molten coarse material as will be described later), the inventors have made it possible to complete the hardening treatment while minimizing to the best possible extent the dispersion of the amount of solid solution in the light metal, thereby establishing the method of casting the coarse material in this example.

[0033] [Casting Device 1] Next, the overall construction of a casting device 1 that realizes the "method of casting the light alloy" according to the embodiment of the invention will be described using FIG. 2. It should be noted for the sake of convenience that the following description will be given on the assumption that the vertical direction in the drawing coincides with the vertical direction of the casting device 1.

[0034] The casting device 1 is a device for manufacturing a casting product with a desired shape through casting by gradually cooling and coagulating a coarse material in a molten state. The casting device 1 is mainly composed of the casting mold 2, first cooling means 3, first temperature detecting means 4, a first control device 5, and the like.

[0035] The casting mold 2 is composed of a plurality of regions. In this example, for instance, the casting mold 2 is composed of a stationary mold 2A, and a movable mold 2B provided above the stationary mold 2A. The stationary mold 2A is removably secured to an upper face portion of a stationary frame 11 installed in the casting device 1. The movable mold 2B is removably secured to a lower face portion of a movable frame 12 provided above the stationary frame 11.

[0036] Then, when the movable frame 12 is lowered (moved downward) and raised (moved upward) by drive means (not shown), the movable mold 2B is thereby moved toward and away from the stationary mold 2A. As a result, the casting mold 2 is "closed" and "opened".

[0037] It should be noted that the drive means for the movable frame 12 is electrically connected to the first control device 5, which will be described later. The first control device 5 controls the "closing" and the "opening" of the casting mold 2.

[0038] Recess portions 2a and 2b are formed in an upper face portion of the stationary mold 2A and a lower face portion of the movable mold 2B respectively. Then, when the casting mold 2 is "closed", these recess portions 2a and 2b are thereby fitted to each other to form a cavity 21.

[0039] A runner channel portion 2c is formed inside the stationary mold 2A. The runner channel portion 2c is a through-hole through which the outside of the casting mold 2 communicates with the cavity 21. The coarse material in a molten state is caused to flow into (poured into) the cavity 21 via the runner channel portion 2c.

[0040] By using the casting mold 2 thus constructed, the coarse material in the molten state is coagulated into the desired shape, and the casting product is manufactured through casting. That is, the cavity 21 is formed in a shape corresponding to that of the casting product. After the coarse material in the molten state is poured into the cavity 21, the casting mold 2 is cooled by the first cooling means 3, which will be described later. The coarse material is thereby coagulated, and the casting product is manufactured through casting.

[0041] It should be noted that when, for example, a cylinder head of an automobile engine or the like is manufactured through casting as a casting product by the casting device 1 in this example, a core (not shown) is provided, for example, inside the casting mold 2.

[0042] The first cooling means 3 is means for managing the temperature of the coarse material poured into the cavity 21 by cooling the casting mold 2. The first cooling means 3 is composed of a supply source 31 that supplies a cooling medium such as coolant, circulating oil or the like, a plurality of communication channels 32 disposed inside the casting mold 2 (more specifically, the stationary mold 2A and the movable mold 2B), a pipeline member 33 that joins this supply source 31 and these communication channels 32 together, and the like. Besides, a first electromagnetically controlled valve 34 that controls the flow rate of the cooling medium is disposed in a midway portion of the pipeline member 33 and in the vicinity of the supply source 31.

[0043] The first electromagnetically controlled valve 34 is electrically connected to the first control device 5, which will be described later. Then, when the cooling medium supplied from the supply source 31 is introduced into the communication channels 32 through the pipeline member 33, the flow rate of the cooling medium is changed to an arbitrary amount by operating the first electromagnetically controlled valve 34 on the basis of an electric signal transmitted from the first control device 5.

[0044] As described above, when the flow rate of the cooling medium introduced into the communication channels 32 is controlled by the first control device 5, the casting mold 2 is thereby cooled to a predetermined temperature. Then, when the casting mold 2 is cooled to the predetermined temperature, the coarse material poured into the cavity 21 is thereby cooled. As a result, the temperature management of the coarse material is carried out.

[0045] The first temperature detecting means 4 is means for detecting the temperature of the coarse material, which is in contact with the cavity 21 from inside, by detecting the temperature of the casting mold 2. The first temperature detecting means 4 is designed as a known contact-type temperature sensor, and is secured to each of the stationary mold 2A and the movable mold 2B, which constitute the casting mold 2.

[0046] The first temperature detecting means 4 is electrically connected to the first control device 5, which will be described later. A measurement value (a temperature of the casting mold 2) measured by the first temperature detecting means 4 is then converted into an electric signal and transmitted to the first control device 5. That is, the temperature of the casting mold 2 is detected as the temperature of the coarse material, which is in contact with the cavity 21 from inside, by the first temperature detecting means 4, and is transmitted to the first control device 5.

[0047] It should be noted that the first temperature detecting means 4 can also be constituted by a known non-contact-type temperature sensor. This non-contact-type temperature sensor may be provided through each of the stationary mold 2A and the movable mold 2B to directly detect the temperature of the coarse material, which is in contact with the cavity 21 from inside.

[0048] The first control device 5 is a device equipped with a storage portion and a calculation portion to control the operation of the entire casting device 1. The first control device 5 is designed to have input thereto a detection signal from the first temperature detecting means 4, and to control the operations of the drive means for the movable frame 12, the first electromagnetically controlled valve 34, and the like, thereby controlling the operation of the entire casting device 1.

[0049] [Coarse Material Cooling Device 50] Next, the overall construction of a coarse material cooling device 50 that realizes the "method of casting the light alloy" according to the embodiment of the invention will be described using FIG. 3. It should be noted for the sake of convenience that the following description will be given on the assumption that the vertical direction in the drawing coincides with the vertical direction of the coarse material cooling device 50.

[0050] The coarse material cooling device 50 is a device for cooling a coarse material 100 released from the casting mold 2 to subject the coarse material 100 to a hardening treatment. The coarse material cooling device 50 has a heat retention chamber 51 into which the coarse material 100 to be cooled is thrown. The heat retention chamber 51 is equipped with second cooling means 52, second temperature detecting means 53, a second control device 54, and the like.

[0051] The second cooling means 52 is means for cooling the coarse material

100 thrown into the heat retention chamber 51 to subject the coarse material 100 to a hardening treatment. The second cooling means 52 has a supply source 55 that supplies the cooling medium, a plurality of nozzles for injecting the cooling medium toward the coarse material 100, a pipeline member 57 that joins this supply source 55 and these nozzles 56 together, and the like. Besides, a second electromagnetically controlled valve 58 that controls the flow rate of the cooling medium is disposed in a midway portion of the pipeline member 57 and in the vicinity of the supply source 55.

[0052] The second electromagnetically controlled valve 58 is electrically connected to the second control device 54, which will be described later. Then, when the cooling medium supplied from the supply source 55 is introduced into the nozzles 56 through the pipeline member 57, the flow rate of the cooling medium is changed to an arbitrary value by operating the second electromagnetically controlled valve 58 on the basis of an electric signal transmitted from the second control device 54.

[0053] As described above, the flow rate of the cooling medium introduced into the nozzles 56 is controlled by the second control device 54. The temperature of the coarse material 100 is thereby cooled to a predetermined temperature. As a result, the coarse material 100 is subjected to the hardening treatment.

[0054] It should be noted that the cooling medium is not limited to any particular type, but may be any substance, for example, a liquid such as cooling water, lubricating oil or the like, or a gas such as cooling air or the like.

[0055] The second temperature detecting means 53 is means for detecting the temperature of the coarse material 100 thrown into the heat retention chamber 51. The second temperature detecting means 53 is designed as a known non-contact-type temperature sensor, and is so disposed inside the heat retention chamber 51 as to protrude toward the coarse material 100.

[0056] The second temperature detecting means 53 is electrically connected to the second control device 54, which will be described later. A measurement value measured by the second temperature detecting means 53 (a temperature of the coarse material 100) is then converted into an electric signal and transmitted to the second control device 54. It should be noted that the second temperature detecting means 53 can also be constituted by, for example, a known contact-type temperature sensor.

[0057] The second control device 54 is a device that is equipped with a storage portion and a calculation portion to control the operation of the entire coarse material cooling device 50. The second control device 54 is designed to have input thereto a detection signal from the second temperature detecting means 53, and to control the operations of the second electromagnetically controlled valve 58 and the like, thereby controlling the operation of the entire coarse material cooling device 50.

[00S8] It should be noted that although the separate control devices, namely, the first control device 5 and the second control device 54 are provided for the casting device 1 and the coarse material cooling device 50 respectively in this embodiment of the invention, the invention is not limited to this configuration. A single control device common to the casting device 1 and the coarse material cooling device 50 may be provided to control the operations of this casting device 1 and this coarse material cooling device 50 in a concentrated manner.

[0059] [Method of Casting Coarse Material] Next, a method of casting a coarse material, which realizes the "method of casting the light alloy" according to the embodiment of the invention, will be described using FIGS. 4 and 5.

[0060] As shown in FIG. 4, the method of casting the coarse material in this embodiment of the invention mainly has a pouring process (step S 101), a casting mold cooling process (step S102), a releasing process (step S103), a coarse material cooling process (step S104), a post-treatment process (step S105), and an aging treatment process (step S106).

[0061] The pouring process (step S101) is a process of pouring a coarse material in a molten state, which is obtained by heating and melting a light alloy in a preceding process, into the cavity 21 of the casting mold 2 (see FIG. 2). That is, in the casting device 1, the casting mold 2 is "closed", and the coarse material in the molten state is poured into the cavity 21 through the runner channel portion 2c. The cavity 21 is then filled with the coarse material in the molten state. The pouring process (step S101) thus ends.

[0062] It should be noted that the temperature of the coarse material with which the cavity 21 is filled falls through the transfer of heat to the casting mold 2, and the coarse material begins to coagulate in the pouring process (step S101) as shown in FIG. 5. [0063] When the pouring process (step S101) ends, the casting mold cooling process (step S102) starts. The casting mold cooling process (step S102) is a process of cooling the temperature of the casting mold 2 to a predetermined "casting mold cooling temperature (Tl)" to coagulate the coarse material in the molten state.

[0064] It should be noted herein that the "casting mold cooling temperature

(Tl)" is set to a temperature higher than the aforementioned "separation temperature range" and lower than an eutectic temperature of the coarse material. That is, as shown in FIG 5, in this embodiment of the invention, it is known that the eutectic temperature of the Al-Si-type alloy used as the coarse material is about 575°C, which is a temperature higher than a temperature range prescribed as the "separation temperature range", namely, a temperature range between 300°C and 400C°.

[0065] Thus, the "casting mold cooling temperature (Tl)" is set to an arbitrary temperature in a range between 400°C and 575°C, and the information on the "casting mold cooling temperature (Tl)" is stored in advance in the storage portion of the first control device 5 (see FIG. 2). Then, when the cavity 21 of the casting mold 2 is filled with the coarse material in the molten state, the first control device 5 controls the first electromagnetically controlled valve 34 on the basis of the measurement value measured by the first temperature detecting means 4 to regulate the flow rate of the cooling medium supplied to the communication channels 32 by the first electromagnetically controlled valve 34. Thus, the casting mold 2 is gradually cooled before reaching the "casting mold cooling temperature (Tl)".

[0066] In this manner, the coarse material in the molten state reaches the "casting mold cooling temperature (Tl)", which is higher than the "separation temperature range", and coagulates.

[0067] It should be noted that when the temperature of the casting mold 2 measured by the first temperature detecting means 4 becomes approximately equal to the "casting mold cooling temperature (Tl)", the first control device 5 determines that the temperature of the coarse material has also become approximately equal to the "casting mold cooling temperature (Tl)", and controls the first electromagnetically controlled valve 34 to stop the supply of the cooling medium to the communication channels 32. The casting mold cooling process (step S102) thus ends.

[0068] When the casting mold cooling process (step S102) ends, a releasing process (step S103) starts. The releasing process (step S103) is a process of removing the coagulated coarse material from the casting mold 2 to release the coarse material from the casting mold 2.

[0069] That is, when the casting mold cooling process (step S102) ends, the first control device 5 controls the drive means for the movable frame 12 (see FIG. 2) to raise the movable frame 12, thereby "opening" the casting mold 2. The coarse material is then removed from the "opened" casting mold 2. The releasing process (step S103) thus ends.

[0070] It should be noted that the temperature of the released coarse material, which is slightly cooled through contact with outside air, reaches a temperature (T2) within the "separation temperature range" in the releasing process (step S103) as shown in FIG. 5.

[0071] When the releasing process (step S103) ends, a coarse material cooling process (step S104) starts. The coarse material cooling process (step S104) is a process of rapidly cooling the released coarse material to a predetermined "hardening temperature (T3)" to subject the coarse material to the hardening treatment.

[0072] It should be noted herein that the "hardening temperature (T3)" is usually set to a temperature equal to or lower than 100°C. That is, as shown in FIG. 5, the "hardening temperature (T3)" is set to a temperature much lower than the aforementioned "separation temperature range". This "hardening temperature (T3)" is then set in the second control device 54 (see FIG. 3), and is stored in advance in the storage portion of the second control device 54.

[0073] When the coarse material cooling process (step S104) is started, the released coarse material is conveyed to the coarse material cooling device 50 and thrown into the heat retention chamber 51. When the coarse material is thrown into the heat retention chamber 51, the second control device 54 controls the second electromagnetically controlled valve 58 on the basis of the measurement value measured by the second temperature detecting means 53 to regulate the flow rate of the cooling medium supplied to the nozzles 56 by the second electromagnetically controlled valve 58. The coarse material is then rapidly cooled before reaching the "quenching temperature (T3)".

[0074] In this manner, the released coarse material is rapidly cooled to the "quenching temperature (T3)", which is much lower than the "separation temperature range". Thus, while remaining in a supersaturated state, the coarse material is subjected to the hardening treatment.

[0075] It should be noted that when the temperature of the coarse material measured by the second temperature detecting means 53 becomes approximately equal to the "quenching temperature (T3)", the first control device 5 determines that the quenching treatment is completed, and controls the second electromagnetically controlled valve 58 to stop the supply of the cooling medium to the nozzles 56. The coarse material cooling process (step SI 04) thus ends.

[0076] When the coarse material cooling process (step S104) ends, a post-treatment process (step S105) starts. The post-treatment process (step S105) is a process of removing, for example, the core or the like from the coarse material, which is subjected to the quenching treatment through the coarse material cooling process (step S104), cleaning a surface portion of the coarse material, and the like so as to prepare for a subsequent aging treatment process (step S106).

[0077] It should be noted that although the temperature of the coarse material is more or less lowered (or raised) through contact with outside air in the post-treatment process (step S105), the coarse material is held substantially at a predetermined temperature due to a small difference between the "quenching temperature (T3)" and the temperature of outside air, as shown in FIG. 5.

[0078] When the post-treatment process (step S105) ends, the aging treatment process (step S106) starts. The aging treatment process (step S106) is a process of subjecting the coarse material, which is subjected to the quenching treatment through the coarse material cooling process (step S104), to an aging treatment to ensure uniform mechanical strength for a casting product manufactured through casting.

[0079] That is, when the post-treatment process (step S105) ends, the coarse material is thrown into a heat treatment furnace (an aging furnace) (not shown). As shown in FIG. 5, the coarse material thrown into the heat treatment furnace (the aging furnace) is rapidly heated up to a predetermined "aging temperature (T4)". After that, the coarse material is held at the "aging temperature (T4)" for a "predetermined time". Then, after the lapse of the "predetermined time", the coarse material is rapidly cooled again to the "quenching temperature (T3)" (i.e., the temperature of the coarse material before the aging treatment), and is thereby subjected to the aging treatment. The aging treatment process (step S106) thus ends.

[0080] When the aging treatment process (step S106) ends, the coarse material is removed from the heat treatment furnace (the aging furnace). The casting of the coarse material in this example thus ends.

[0081] [Substantiated Data] Next, substantiated data obtained by the inventors in the method of casting the coarse material in this example will be described using FIGS. 6 and 7. First of all, FIG. 6 is a graph in which changes with time in the temperature of the coarse material are recorded from the pouring process (step S101) to the coarse material cooling process (step S104). In FIG. 6, an axis of ordinate represents the temperature (unit (°C)) of the coarse material, and an axis of abscissa represents an elapsed time (unit (°Q). In this graph, a relationship between the temperature of the coarse material and the elapsed time is expressed by a curve as to each of the casting method in this example (indicated by a solid line) and the casting method according to the related art (indicated by broken lines).

[0082] It should be noted that the casting method in this example is designed, as described above, to set the "casting mold cooling temperature (Tl)", which is higher than the "separation temperature range", in the casting mold cooling process (step S102) and control the function of cooling the casting mold 2 (more specifically, the first cooling means 3) such that the coarse material is gradually cooled toward the "casting mold cooling temperature (Tl)". On the other hand, the casting method according to the related art is designed to reinforce the function of cooling the casting mold 2 and perform control such that the temperature of the coarse material becomes low (i.e., a temperature t shown in FIG. 6) in the shortest possible time in the casting mold cooling process (step S102).

[0083] As shown in FIG. 6, while time elapses from the start of the pouring process (step S lOl) until the temperature of the coarse material reaches a temperature close to the eutectic temperature in the casting mold cooling process (step S102), there is no difference observed in changes with time in the temperature of the coarse material between the casting method in this example and the casting method according to the related art.

[0084] In the casting method of this example, after the temperature of the coarse material changes past the eutectic temperature, the first control device 5 (see FIG. 2) controls the operation of the first cooling means 3, so that the temperature of the coarse material gently falls. Then, when the casting mold cooling process (step S102) ends, the temperature of the coarse material reaches the "casting mold cooling temperature (Tl)", which is higher than the "separation temperature range".

[0085] On the other hand, in the casting method according to the related art, even after the temperature of the coarse material changes past the eutectic temperature, the casting mold continues to be cooled rapidly. Then, the temperature of the coarse material already reaches the "separation temperature range" while the casting mold cooling process (step S102) is carried out. As a result, when the casting mold cooling process (step S102) ends, the temperature of the coarse material reaches the temperature t within the "separation temperature range".

[0086] In the casting method of this example, after the casting mold cooling process (step S102) ends, the coarse material comes into contact with outside air through the releasing process (step S103), so that the temperature of the coarse material gently falls to reach the "separation temperature range". In other words, while the coarse material is cooled, the releasing process (step S103) is carried out before the temperature of the coarse material reaches the "separation temperature range". While the releasing process (step S103) is carried out, the temperature of the coarse material reaches the "separation temperature range". Consequently, in the casting method of this example, it takes longer for the temperature of the coarse material to reach the "separation temperature range" than in the casting method according to the related art.

[0087] Then, after the releasing process (step S103) ends, the coarse material cooling process (step S104) is swiftly started. The temperature of the coarse material then changes past the "separation temperature range" in a short time, and the coarse material is further cooled rapidly.

[0088] On the other hand, in the casting method according to the related art, after the casting mold cooling process (step S102) ends, the temperature of the coarse material is already approximately equal to the temperature of the coarse material (the temperature within the "separation temperature range") immediately after the start of the coarse material cooling process (step S104) in the casting method of this example. In consequence, the temperature of the coarse material does not change very much in the releasing process (step S103). After the releasing process (step S 103) ends, the coarse material cooling process (step S104) is swiftly started. The temperature of the coarse material changes past the "separation temperature range" in a short time, and the coarse material is further cooled rapidly.

[0089] As described above, in the casting method of this example, the temperature of the coarse material remains within the "separation temperature range" from a certain moment during the performance of the releasing process (step S 103) to an initial stage of the coarse material cooling process (step S104) (a range indicated by an arrow XI shown in FIG. 6). On the other hand, in the casting method according to the related art, the temperature of the coarse material remains within the "separation temperature range" from a certain moment during the performance of the casting mold cooling process (step S 102) to the initial stage of the coarse material cooling process (step S104) (a range indicated by an arrow X2 shown in FIG. 6). Consequently, as shown in FIG. 6, according to the casting method of this example, it is apparent that the time during which the temperature of the coarse material remains within the "separation temperature range" can be reliably shortened in comparison with the casting method according to the related art, without taking any measure, for example, to change (shorten) the time required for each of the processes.

[0090] Next, a comparison between the mechanical strength of a casting product manufactured through casting by the casting method of this example and the mechanical strength of a casting product manufactured through casting by the casting method according to the related art will be described using FIG 7. FIG. 7 is a view showing measurement values obtained by measuring two selected items, namely, a solid solubility (an amount of the solid solution element in the light metal) and a Vickers' hardness of the casting product manufactured through casting by the casting method of this example and the casting product manufactured through casting by the casting method according to the related art, with a view to quantitatively grasping the mechanical strength of each of the casting products. That is, an axis of ordinate on the left side on the sheet of FIG. 7 represents the solid solubility (unit (mass Mg)), and an axis of ordinate on the right side on the sheet of FIG. 7 represents the Vickers' hardness (unit (Hv)). The measurement values concerning each of the items are shown for the casting method of this example (indicated as "THIS EXAMPLE" in FIG. 7) and the casting method according to the related art (indicated as "COMPARATIVE EXAMPLE'' in FIG. 7) distinctly. The solid solubility is expressed in the form of a bar chart, and the Vickers' hardness is expressed by black dots.

[0091] It should be noted that the casting method in this example is designed, as described above, to set the "casting mold cooling temperature (Tl)", which is higher than the "separation temperature range" and control the function of cooling the casting mold 2 (more specifically, the first cooling means 3) such that the coarse material is gradually cooled toward the "casting mold cooling temperature (Tl)" in the casting mold cooling process (step S102). On the other hand, the casting method according to the related art is designed to reinforce the function of cooling the casting mold 2 and perform control such that the temperature of the coarse material becomes low (e.g., the temperature t shown in FIG 6) in the shortest possible time in the casting mold cooling process (step S102).

[0092] As shown in FIG. 7, while the solid solubility of the casting product manufactured through casting by the casting method according to the related art is al (mass g), the solid solubility of the casting product manufactured through casting by the casting method of this example is a2 (mass%Mg) (al < a2). It is apparent that the amount of the solid solution element into the light metal is reliably increased by manufacturing the casting product through casting by the casting method of this example.

[0093] Further, while the Vickers' hardness of the casting product manufactured through casting by the casting method according to the related art is bl (Hv), the Vickers' hardness of the casting product manufactured through casting by the casting method of this example is b2 (Hv) (bl < b2). A larger amount of the additional element can be solidly dissolved into the light metal by manufacturing the casting product through casting by the casting method of this example. It is therefore apparent that the Vickers' hardness is enhanced as well.

[0094] As described above, the method of casting the coarse material in this example is a method of casting a coarse material (a light alloy) to manufacture a casting product through casting by heating and melting the coarse material (the light alloy), which is composed of a light metal as a base material and an additional element, and coagulating the molten coarse material (the light alloy) through cooling. In cooling the solid solution element, when the coagulated coarse material (the light alloy) is held at the predetermined temperature, the coarse material (the light alloy) is cooled such that the time during which the coarse material remains within the separation temperature range, namely, the temperature range corresponding to the shortest elapsed time needed for the separation of the solid solution element, which is contained in the coarse material (the light alloy) from the coarse material (the light alloy) becomes short.

[0095] Owing to the use of this method of casting the coarse material, uniform mechanical strength can be ensured for the casting product by perform ing the aging treatment after the completion of the hardening treatment while minimizing to the best possible extent the dispersion of the amount of solid solution element in the light metal without causing an increase in the cost of equipment.

[0096] That is, as shown in FIG 6, in the method of casting the coarse material in this example, the time during which the temperature of the coarse material remains within the "separation temperature range" is shortened in comparison with the casting method according to the related art in the course of cooling the coarse material from the casting mold cooling process (step S102) to the coarse material cooling process (step S 104) without separately adding a process leading to an increase in the cost of equipment, for example, a process of liquefaction heating or the like. Thus, the amount of the additional element separating out from the light metal decreases, and the quenching treatment can be completed while minimizing to the best possible extent the dispersion of the amount of solid solution element in the light metal. Then, by subjecting to the aging treatment the coarse material with the dispersion of the amount of solid solution element minimized to the best possible extent, the casting product having uniform mechanical strength is manufactured through casting.

[0097] Further, in the method of casting the coarse material in this example, the casting method is equipped with the casting mold cooling process (step S102) of coagulating the coarse material (the light alloy) in the molten state, which is poured into the casting mold 2, and the releasing process (step S103) of removing from the casting mold 2 the coarse material (the light alloy) coagulated through the casting mold cooling process (step S102). The casting mold 2 has the first cooling means 3. In the casting mold cooling process (step S 102), the casting mold 2 is cooled by the first cooling means 3, so that the coarse material (the light alloy) is cooled to the casting mold cooling temperature (Tl), which is higher than the "separation temperature range". The releasing process (step S103) is carried out before the temperature of the coarse material (the light alloy) reaches the "separation temperature range".

[0098] As described above, in the method of casting the coarse material in this example, the releasing process (step S103) is carried out before the temperature of the coarse material reaches the "separation temperature range", and the temperature of the coarse material reaches the "separation temperature range" in the course of the releasing process (step S103). Thus, according to the method of casting the coarse material in this example, in the process of cooling the coarse material from the casting mold cooling process (step S102) to the coarse material cooling process (step S104), the time taken until the temperature of the coarse material reaches the "separation temperature range" is longer than in the casting method according to the related art, and the time during which the temperature of the coarse material remains within the "separation temperature range" can be shortened in comparison with the casting method according to the related art. Accordingly, as described above, the amount of the additional element separating out from the solid solution element decreases, and the hardening treatment can be completed while minimizing to the best possible extent the dispersion of the amount of solid solution of the additional element in the light metal. The coarse material with the dispersion of the amount of solid solution element thus minimized to the best possible extent is then subjected to the aging treatment. The casting product having uniform mechanical strength is thereby manufactured through casting.

[0099] Further, in the method of casting the coarse material in this example, the coarse material (the light alloy) is an alloy containing aluminum-silicon alloy as the light metal and magnesium as the additional element. The separation temperature range is prescribed as the range between 300°C and 400°C.

[0100] That is, as described above, the coarse material made of the light alloy in the supersaturated state has the properties of allowing the solid solution element to separate out when being held at the predetermined temperature for the predetermined time. Further, the time (the separation time) needed for the separation of the solid solution element slightly differs depending on the types or the like of the light metal and the additional element, but has the properties of changing in accordance with the temperature of the coarse material. That is, in the temperature range of the coarse material, while the temperature (the peak temperature (T)) corresponding to the shortest "separation time" exists, it is known that the "peak temperature (T)" concerning the alloy containing the aluminum-silicon alloy as the light metal and magnesium as the additional element is approximately equal to 350°C. Thus, the range of about 50°C around the "peak temperature (T)", namely, the range between 300°C and 400°C is set as the "separation temperature range", and the time during which the temperature of the coarse material remains within the "separation temperature range" is shortened. The dispersion of the amount of solid solution element in the coarse material made of the aluminum alloy can thereby be minimized to the best possible extent.

[0101] While the embodiment of the invention has been illustrated above, it is to be understood that the invention is not limited to the details of the illustrated embodiment thereof, but may be embodied with various changes, modifications or improvements, which may occur to those skilled in the art, without departing from the scope of the invention.