Method and apparatus for casting aluminum by casting mold

Abstract

Claims

2. The aluminum casting process using a casting mold according to claim 1, comprising the further step of heating the cavity wall with a cartridge heater embedded in the mold.

3. The aluminum casting process using a casting mold according to claim 1, comprising the further step of detecting a temperature of the cavity wall with a thermocouple embedded in the mold.

4. An aluminum casting apparatus for molding an aluminum casting in a cavity of a casting mold by supplying molten aluminum into the cavity, the apparatus comprising: an air discharging portion facing the cavity for discharging air from the cavity; an inert gas introducing portion, which faces the cavity at a position opposite to the position of the cavity where the air discharge portion meets the cavity, for introducing an inert gas into the cavity from which air has been discharged; a magnesium introducing portion having a sublimating device for sublimating magnesium to form gaseous magnesium so as to introduce gaseous magnesium into the cavity into which the inert gas has been introduced; a nitrogen gas introducing portion having a heating device for heating nitrogen gas so as to introduce heated nitrogen gas into the cavity into which gaseous magnesium has been introduced; and a control portion for controlling the air discharging, inert gas introducing, magnesium introducing, and nitrogen gas introducing portions separately to regulate the cavity to a specific pressure and also controlling the sublimating and heating devices to regulate their temperatures.

5. An aluminum casting process using a casting mold, comprising the steps of: prior to introducing gaseous magnesium, filling a cavity of a closed mold with an inert gas, while discharging air from the cavity, to establish a first pressure in the cavity that is equal to or below an atmospheric pressure; introducing gaseous magnesium into the cavity to deposit magnesium on a wall of the cavity and establish a second pressure in the cavity that is equal to or below the atmospheric pressure; introducing heated nitrogen gas into the cavity, said heated nitrogen gas reacting with said magnesium deposited on said cavity wall to form magnesium nitride on the wall of the cavity and establish a third pressure in the cavity that is equal to or below the atmospheric pressure; and supplying molten aluminum into the cavity to mold an aluminum casting in the cavity, while reducing a surface of the molten aluminum with the magnesium nitride.

6. An aluminum casting process using a casting mold and including filling a cavity of a closed mold with nitrogen gas and magnesium gas and pouring molten aluminum into the cavity, wherein the nitrogen and magnesium gases in the cavity are reacted with each other by a heat of the poured molten aluminum to form a solid magnesium-nitrogen compound, while formation of the magnesium-nitrogen compound creates a reduced pressure in the cavity, and the magnesium-nitrogen compound removes any oxide film formed on a surface of the molten aluminum.

7. The aluminum casting process using a casting mold according to claim 6, wherein the cavity is purged with an inert gas before the cavity is filled with the nitrogen and magnesium gases.

8. The aluminum casting process using a casting mold according to claim 6, wherein a pouring temperature of the molten aluminum is between about 600 to 750.degree. C.

9. The aluminum casting process using a casting mold according to claim 8, comprising the further steps of: detecting the pouring temperature of the molten aluminum with a temperature sensor; and, controlling the molten aluminum to a selected temperature based upon the pouring temperature detected by the temperature sensor.

10. The aluminum casting process using a casting mold according to claim 2, comprising the further step of detecting a temperature of the cavity wall with a thermocouple embedded in the mold.

Description

[0001] This invention relates generally to an aluminum casting process using a casting mold and to an aluminum casting apparatus and, more particularly, to an aluminum casting process using a casting mold for molding an aluminum casting in a cavity of the mold by supplying molten aluminum thereinto and to an aluminum casting apparatus.

BACKGROUND ART

[0002] When molten aluminum is supplied into the cavity of a mold for aluminum casting, it is likely that an oxide film may form on the surface of the molten aluminum and increase the surface tension of the molten aluminum and lower its fluidity. When an oxide film has formed on the molten aluminum surface, therefore, it is difficult to maintain a good distribution of the molten aluminum.

[0003] Accordingly, JP-A-2000-280063 entitled Aluminum Casting Process is, for example, proposed as a casting process making it possible to maintain a good distribution of molten aluminum for aluminum casting. This art will now be described with reference to FIG. 57 hereof.

[0004] Nitrogen gas (N.sub.2 gas) is first supplied from a nitrogen gas bottle 550 to fill the cavity 552 of a mold 551 for aluminum casting. Then, nitrogen gas is delivered to a storage tank 553 so that a powder of magnesium (Mg powder) in the storage tank 553 may be delivered into a heating oven 555 with nitrogen gas.

[0005] The magnesium powder is sublimated in the heating oven 555 and the sublimated magnesium is reacted with nitrogen gas to form a gaseous magnesium-nitrogen compound (Mg.sub.3N.sub.2).

[0006] The magnesium-nitrogen compound is introduced into the cavity 552 of the mold 551 through a pipeline 556 so that the introduced magnesium-nitrogen compound may be deposited on the wall of the cavity 552.

[0007] Then, molten aluminum 557 is supplied into the cavity 552. The supplied molten aluminum 557 is reacted with the magnesium-nitrogen compound, so that oxygen may be removed from the oxide on the surface of the molten aluminum 557.

[0008] As a result, it is possible to prevent the formation of any oxide film on the surface of the molten aluminum 557 and restrain any increase in the surface tension of the molten aluminum 557. Accordingly, it is possible to maintain a good distribution of the molten aluminum 557 in the cavity 552 and thereby produce an aluminum casting of high quality.

[0009] Description will now be made in detail of a step for the formation of the magnesium-nitrogen compound mentioned above and a step for the pouring of the molten aluminum.

[0010] Description will first be made of the step for the formation of the magnesium-nitrogen compound. The magnesium powder is sublimated in the heating oven 555 and the sublimated magnesium is reacted with nitrogen gas in the heating oven 555. As the sublimated magnesium is floating in the heating oven 555, nitrogen gas adheres to the whole surfaces of the magnesium and forms the magnesium-nitrogen compound on the whole surfaces.

[0011] Reference is now made to FIG. 58 for the description of the step for the pouring of the molten aluminum in the aluminum casting process.

[0012] FIG. 58 shows that the molten aluminum 557 has been supplied into the cavity 552 after the deposition of a layer 559 of the magnesium-nitrogen compound on the wall of the cavity 552.

[0013] When the molten aluminum 557 has been supplied into the cavity 552, its surface 557a contacts the surface 559a of the magnesium-nitrogen compound layer 559, and oxygen is removed from an oxide 557b formed on the surface 557a of the molten aluminum 557.

[0014] The contact of the surface 557a of the molten aluminum 557 with the surface 559a of the magnesium-nitrogen compound layer 559 makes it possible to remove oxygen from the oxide 557b formed on the surface 557a of the molten aluminum 557.

[0015] It, therefore, follows that it is sufficient for only the surface 559a of the magnesium-nitrogen compound layer 559 contacted by the surface 557a of the molten aluminum 557 to exist for removing oxygen from the oxide 557b formed on the surface 557a of the molten aluminum 557.

[0016] Nitrogen gas, however, adheres to the whole surfaces of magnesium, since the formation of the magnesium-nitrogen compound is carried out with magnesium floating in the heating oven 555, as explained with reference to FIG. 57. Accordingly, the magnesium-nitrogen compound is formed on the wholes surfaces of magnesium. The deposition of the magnesium-nitrogen compound on the wall of the cavity 552 forms the magnesium-nitrogen compound layer 559 having a thickness t as shown in FIG. 58.

[0017] Thus, an excessive magnesium-nitrogen compound layer 559 is deposited on the wall of the cavity 552, and the formation of the magnesium-nitrogen compound layer 559 takes a long time making it difficult to achieve high productivity.

[0018] In addition, the formation of the excessive magnesium-nitrogen compound layer 559 means the use of a large amount of nitrogen gas making it difficult to achieve a reduction of cost.

[0019] Moreover, the casting process according to the publication mentioned above is a process that includes the step of filling the cavity 552 with nitrogen gas, while air still remains in the cavity 552, before the step of forming the magnesium-nitrogen compound layer 559 on the wall of the cavity 552.

[0020] As a result, it is difficult to have air released smoothly from the cavity 552, and the creation of a nitrogen gas atmosphere in the cavity 552 take a long time making it difficult to achieve high productivity.

[0021] There is an aluminum casting having a portion of small thickness, and the known aluminum casting process shown in FIG. 57 may find it difficult to maintain a good distribution of molten aluminum in the cavity when molding an aluminum casting having a portion of small thickness.

[0022] Therefore, it is necessary to employ a somewhat prolonged pouring time for molten aluminum in order to ensure a full distribution of the molten aluminum through the whole cavity. Accordingly, the molding of an aluminum casting requires a prolonged cycle time that lowers productivity.

DISCLOSURE OF THE INVENTION

[0023] According to a first aspect of this invention, there is provided an aluminum casting process using a casting mold, comprising the step of filling the cavity of a closed mold with an inert gas, the step of introducing gaseous magnesium into the inert gas-filled cavity to have magnesium deposited on the wall of the cavity, the step of heating the mold to heat the magnesium-deposited cavity wall to a specific temperature, the step of introducing nitrogen gas into the cavity to have magnesium nitride formed on the cavity wall, and the step of supplying molten aluminum into the cavity in which the magnesium nitride has been formed, to mold an aluminum casting in the cavity, while reducing the surface of the molten aluminum with the magnesium nitride.

[0024] The formation of magnesium nitride is started by depositing magnesium on the cavity wall to form a magnesium layer thereon, and after the cavity wall is, then, heated, nitrogen gas is introduced into the cavity to form magnesium nitride on the surface of the magnesium layer.

[0025] As a result, it is possible to form magnesium nitride on only the surface of the magnesium layer and thereby shorten the time required for the formation of magnesium nitride. Accordingly, it is possible to achieve an improved productivity for an aluminum casting.

[0026] Moreover, it is possible to reduce the amount of nitrogen gas that is used, since it is sufficient to form magnesium nitride on only the surface of the magnesium layer. Accordingly, it is possible to keep down the cost of an aluminum casting.

[0027] According to this invention, the cavity wall is heated by a cartridge heater embedded in the mold. A cartridge heater is a heater which is held in a cartridge and is easy to embed in the mold.

[0028] It is usual to think of heating the whole mold as a method of heating its cavity wall. A large amount of heat energy is, however, required for heating the whole mold. Moreover, the method in which the whole mold is heated takes a long time to heat the cavity wall to a specific temperature.

[0029] According to this invention, therefore, the cartridge heater embedded in the mold is used to heat the cavity wall. The cartridge heater embedded in the mold makes it possible to heat the cavity wall by heating only a part of the mold.

[0030] Accordingly, it is possible to reduce heat energy for heating the cavity wall to a specific temperature. Moreover, it is possible to heat the cavity wall to a specific temperature within a relatively short time, since it is sufficient to heat only the necessary part of the mold. Therefore, it is possible to achieve an improved productivity for an aluminum casting.

[0031] According to this invention, moreover, the heating of the cavity wall is the heating of only its portion corresponding to a casting portion of small thickness. Generally, molten aluminum can be poured smoothly into a cavity when the cavity is a large space in a case of pouring molten aluminum into a cavity. When the cavity is a narrow space, however, molten aluminum hardly flows smoothly.

[0032] According to this invention, therefore, heating is done only of any cavity portion that is a narrow space, or that corresponds to a casting portion of small thickness. The heating of the cavity portion corresponding to a casting portion of small thickness makes it possible to form magnesium nitride in the magnesium layer on that portion. When molten aluminum has reached any cavity portion corresponding to a casting portion of small thickness, molten aluminum has its surface brought into contact with magnesium nitride. It is likely that an oxide has formed on the surface of molten aluminum, but even if such is the case, oxygen can be removed from any such oxide as a result of the reaction of the oxide with magnesium nitride. Thus, it is possible to prevent the formation of any oxide film on the surface of molten aluminum and thereby restrain any increase in surface tension of molten aluminum. Accordingly, it is possible to maintain a good distribution of molten aluminum even in any cavity portion corresponding to a casting portion of small thickness. As a result, it is possible to achieve a shortened process for molding an aluminum casting and thereby an improved productivity. Moreover, it is possible to reduce the amount of nitrogen to a still more extent, since it is only any portion corresponding to a casting portion of small thickness that is heated and have magnesium nitride formed thereon. Accordingly, it is possible to keep down the cost of any aluminum casting.

[0033] According to this invention, moreover, the temperature of the cavity wall is detected by a thermocouple embedded in the mold. A thermocouple is a device made of two different metals joined to form a closed circuit so that a temperature difference between the two junctions may develop an electromotive force. The detection of the cavity wall temperature by a thermocouple makes it possible to set the cavity wall temperature more accurately at a specific level. As a result, it is possible to have magnesium nitride formed efficiently in the magnesium layer. Accordingly, it is possible to achieve a shortened process for molding an aluminum casting and thereby an improved productivity.

[0034] According to this invention, the thermocouple is installed in a cavity portion corresponding to a casting portion of small thickness to detect the temperature of the portion. In any cavity portion corresponding to a casting portion of small thickness, the cavity has a narrow space through which molten aluminum fails to flow smoothly. According to this invention, therefore, the temperature of any cavity portion corresponding to a casting portion of small thickness is detected by the thermocouple, so that magnesium nitride may be formed efficiently on the magnesium layer in any cavity portion corresponding to a casting portion of small thickness. It is, thus, possible to remove oxygen from any oxide on the surface of molten aluminum and prevent the formation of any oxide film on the surface of molten aluminum in any cavity portion corresponding to a casting portion of small thickness by bringing the surface of molten aluminum into contact with magnesium nitride. Accordingly, it is possible to achieve a shortened process of improved productivity for molding an aluminum casting, since it is possible to maintain a good distribution of molten aluminum in any cavity portion corresponding to a casting portion of small thickness.

[0035] According to a second aspect of this invention, there is provided an aluminum casting process using a casting mold, comprising the step of filling the cavity of a closed mold with an inert gas, the step of introducing gaseous magnesium into the inert gas-filled cavity to have magnesium deposited on the wall of the cavity, the step of introducing heated nitrogen gas into the magnesium-deposited cavity to have magnesium nitride formed on the cavity wall, and the step of supplying molten aluminum into the cavity in which the magnesium nitride has been formed, to mold an aluminum casting in the cavity, while reducing the surface of the molten aluminum with the magnesium nitride.

[0036] The formation of magnesium nitride is started by depositing magnesium on the cavity wall to form a magnesium layer thereon, and nitrogen gas is introduced into the cavity to form magnesium nitride on the surface of the magnesium layer. As a result, it is possible to form magnesium nitride on only the surface of the magnesium layer and thereby shorten the time required for the formation of magnesium nitride. Accordingly, it is possible to achieve an improved productivity for an aluminum casting. Moreover, it is possible to reduce the amount of nitrogen gas that is used, since it is sufficient to form magnesium nitride on only the surface of the magnesium layer. Accordingly, it is possible to keep down the cost of an aluminum casting. Moreover, nitrogen gas is heated and heated nitrogen gas is used for forming magnesium nitride. The heated nitrogen gas makes it possible to form magnesium nitride efficiently. Accordingly, it is possible to achieve an improved productivity for any aluminum casting.

[0037] According to this invention, the temperature T (.degree. C.) of gas in the cavity and the pressure (atmosphere) in the cavity are so selected as to maintain their relationship T.gtoreq.(130.times.P+270). As the temperature T (.degree. C.) of gas in the cavity and the pressure P (atmosphere) in the cavity are relatively easy to determine based on their relationship T.gtoreq.(130.times.P+270), it is possible to perform the adjustment of equipment within a short time. It is apparent from their relationship T.gtoreq.(130.times.P+270) that when the pressure P in the cavity is, for example, 1 atmosphere, the temperature T of gas in the cavity may be set at 400.degree. C. or above for forming magnesium nitride.

[0038] According to a third aspect of this invention, there is provided an aluminum casting apparatus for molding an aluminum casting in the cavity of a casting mold by supplying molten aluminum into the cavity, the apparatus comprising an air discharging portion for discharging air from the cavity, an inert gas introducing portion for introducing an inert gas into the cavity from which air has been discharged, a magnesium introducing portion for introducing gaseous magnesium into the cavity into which an inert gas has been introduced, a nitrogen gas introducing portion for introducing heated nitrogen gas into the cavity into which gaseous magnesium has been introduced, and a control portion for controlling the air discharging, inert gas introducing, magnesium introducing and nitrogen gas introducing portions separately to regulate the cavity to a specific pressure.

[0039] The aluminum casting apparatus includes the air discharging, inert gas introducing, magnesium introducing and nitrogen gas introducing portions and the control portion controls those portions to regulate the cavity to a specific pressure. The regulation of the cavity to a specific pressure by the control portion makes it possible to deposit magnesium efficiently on the wall of the cavity and form magnesium nitride efficiently on the surface of the deposited magnesium layer. Therefore, it is possible to carry out the formation of the magnesium-nitrogen compound in a short time and thereby achieve an improved productivity. Moreover, the formation of magnesium nitride on only the surface of the magnesium layer makes it possible to avoid the formation of magnesium nitride in the inside of the magnesium layer. As a result, it is possible to reduce the amount of nitrogen gas used and thereby the relevant cost.

[0040] According to this invention, the position where the air discharging portion meets the cavity and the position where the inert gas introducing portion meets the cavity are situated in a mutually opposite relation. The mutually opposite situation of the position where the air discharging portion meets the cavity and the position where the inert gas introducing portion meets the cavity enables the inert gas supplied into the cavity to direct the air in the cavity efficiently toward the air discharging portion. It is, therefore, possible to discharge the air from the cavity efficiently through a discharging passage and thereby purge the cavity with an inert gas atmosphere within a short time and achieve an improved productivity.

[0041] According to this invention, moreover, the control portion is adapted to control the air discharging, inert gas introducing, magnesium introducing and nitrogen gas introducing portions individually. The individual control of the air discharging, inert gas introducing, magnesium introducing and nitrogen gas introducing portions by the control portion facilitates the regulation of the environment in the cavity in accordance with the conditions for the deposition of the magnesium layer and the conditions for the formation of magnesium nitride. The easy setting of the conditions for the deposition of the magnesium layer and the conditions for the formation of magnesium nitride makes it possible to carry out the deposition of the magnesium layer and the formation of magnesium nitride in a short time. Accordingly, it is possible to achieve an improved productivity for any aluminum casting.

[0042] According to this invention, moreover, the magnesium introducing portion includes a sublimating device for sublimating magnesium to form gaseous magnesium, while the nitrogen gas introducing portion includes a heating device for heating nitrogen gas, and the control portion controls the sublimating and heating devices to regulate their temperatures. The control of the sublimating and heating devices by the control portion enables the sublimating device to sublimate magnesium efficiently and the heating device to heat nitrogen gas efficiently. This makes it possible to deposit the magnesium layer efficiently and form magnesium nitride efficiently. Moreover, the deposition of the magnesium layer and the formation of magnesium nitride in a short time make it possible to achieve an improved productivity for any aluminum casting.

[0043] According to a fourth aspect of this invention, there is provided an aluminum casting process using a casting mold, comprising the step of filling the cavity of a closed mold with an inert gas, while discharging air from the cavity, to establish a first pressure in the cavity, the step of introducing gaseous magnesium into the cavity to deposit magnesium on the wall of the cavity and establish a second pressure in the cavity, the step of introducing heated nitrogen gas into the cavity to form magnesium nitride on the wall of the cavity and establish a third pressure in the cavity, and the step of supplying molten aluminum into the cavity to mold an aluminum casting in the cavity, while reducing the surface of the molten aluminum with the magnesium nitride.

[0044] Air is discharged from the cavity when the cavity is filled with an inert gas. This makes it possible to purge the cavity with an inert gas atmosphere in a short time and achieve an improved productivity.

[0045] The formation of magnesium nitride is started by depositing magnesium on the cavity wall to form a magnesium layer thereon, and nitrogen gas is introduced into the cavity to form magnesium nitride on the surface of the magnesium layer. This makes it possible to form magnesium nitride on only the surface of the magnesium layer and thereby shorten the time required for the formation of magnesium nitride and achieve an improved productivity. Moreover, it is possible to reduce the amount of nitrogen gas used and the relevant cost, since it is sufficient to form magnesium nitride on only the surface of the magnesium layer. Moreover, nitrogen gas is heated and heated nitrogen gas is used for forming magnesium nitride. The heated nitrogen gas makes it possible to form magnesium nitride efficiently and achieve an improved productivity.

[0046] The cavity is regulated to a first pressure when an inert gas atmosphere is created in it. Such regulation of the cavity pressure makes it possible to prevent efficiently any invasion of air from outside into the cavity and alter the inside of the cavity efficiently to an inert gas atmosphere.

[0047] The cavity is regulated to a second pressure when magnesium is deposited on the cavity wall. Such regulation of the cavity pressure makes it possible to establish the conditions facilitating the deposition of magnesium in the cavity and deposit magnesium efficiently.

[0048] The cavity is regulated to a third pressure when magnesium nitride is formed. Such regulation of the cavity pressure makes it possible to establish the conditions facilitating the formation of magnesium nitride in the cavity and form magnesium nitride efficiently. The regulation of the cavity to a third pressure also makes it possible to charge the cavity with molten aluminum efficiently. The regulation of the cavity pressure to the first pressure, second pressure and third pressure P for various steps of the process makes it possible to carry out aluminum casting treatment efficiently and achieve an improved productivity.

[0049] According to this invention, moreover, the first to third pressures are all so selected as not to exceed the atmospheric pressure. For the deposition of magnesium on the wall of the cavity, it is necessary to lower the temperature of the cavity wall to the specific temperature causing the deposition of magnesium. According to this invention, the second pressure in the cavity, not exceeding the atmospheric pressure, makes it easy to regulate the temperature of the cavity wall to the specific temperature. As a result, it is relatively easy to have magnesium deposited on the cavity wall. For the formation of magnesium nitride, it is necessary to select the third pressure and the temperature of gas in the cavity to specific values. According to this invention, therefore, the third pressure in the cavity is so selected as not to exceed the atmospheric pressure, so that it may be easy to regulate the temperature of gas in the cavity to the temperature at which magnesium nitride is formed. As a result, it is relatively easy to have magnesium nitride formed on the cavity wall. The third pressure not exceeding the atmospheric pressure, moreover, makes it possible to charge the cavity with molten aluminum smoothly and thereby achieve an improved productivity. The first pressure, as well as the second pressure, not exceeding the atmospheric pressure, makes it possible to reduce or eliminate any difference between the first and second pressures and thereby change from the first to the second pressure within a short time. As a result, it is possible to reduce the time lag caused by any change from the first to the second pressure and thereby achieve an improved productivity.

[0050] According to this invention, the third pressure P and the temperature T of gas in the cavity are so selected as to maintain their relationship P.ltoreq.(T-270)/130. As the third pressure P and the temperature T of gas in the cavity are relatively easy to determine based on their relationship P.ltoreq.(T-270)/130, it is possible to perform the adjustment of equipment in accordance with the aluminum casting steps within a short time and achieve an improved productivity. It is apparent from their relationship P.ltoreq.(T-270)/130 that when the temperature T of gas in the cavity is, for example, 283.degree. C., the third pressure P may be set at 0.1 atmosphere or below for forming magnesium nitride.

[0051] According to this invention, the first and second pressures may both be an atmospheric pressure, while the third pressure is a negative pressure lower than the atmospheric pressure. The first pressure set at the atmospheric level enables the pressure of the cavity to be equal to that of the open atmosphere. It is possible to prevent any invasion of air from the open atmosphere into the cavity still more reliably when an inert gas atmosphere is created in the cavity. The second pressure set at the atmospheric level makes it possible to prevent any invasion of air from the open atmosphere into the cavity still more reliably when magnesium is deposited on the cavity wall. Thus, the first and second pressures set both at the atmospheric level make it possible to have magnesium nitride formed on the cavity wall still more efficiently, since it is possible to prevent any invasion of air into the cavity still more reliably. As any invasion of air into the cavity is prevented, it is also possible to restrain the formation of any oxide on the surface of molten aluminum when the molten aluminum is supplied into the cavity. Moreover, the third pressure set at a negative pressure makes it possible to charge the cavity with molten aluminum still more smoothly. Thus, the first and second pressures set at the atmospheric pressure and the third pressure set at a negative pressure lower than the atmospheric pressure make it possible to perform aluminum casting treatment efficiently and achieve an improved productivity.

[0052] According to a fifth aspect of this invention, there is provided an aluminum casting process including filling the cavity of a closed mold with nitrogen gas and magnesium gas and pouring molten aluminum into the cavity, wherein the nitrogen and magnesium gases in the cavity are reacted with each other by the heat of the poured molten aluminum to form a solid magnesium-nitrogen compound, while the formation of the magnesium-nitrogen compound creates a reduced pressure in the cavity, and the aluminum-nitrogen compound removes any oxide film formed on the surface of the molten aluminum.

[0053] The nitrogen and magnesium gases in the cavity are reacted with each other by the heat of the molten aluminum to form a solid magnesium-nitrogen compound. The solidifying reaction of the gases in the cavity enables a reduction of the gases in the cavity. The creation of a reduced pressure in the cavity makes it possible to introduce molten aluminum efficiently into the whole area of the cavity. Moreover, the magnesium-nitrogen compound as formed serves to remove any oxide formed on the surface of the molten aluminum. It is, thus, possible to prevent the formation of any oxide film on the surface of the molten aluminum and thereby restrain any increase in surface tension of the molten aluminum. The restrained surface tension of the molten aluminum makes it possible to maintain a good distribution of the molten aluminum in the cavity. As a good distribution of molten aluminum is maintained by the removal of any oxide from its surface, and moreover as the creation of a reduced pressure in the cavity makes it easy to introduce molten aluminum into the whole area of the cavity, it is possible to achieve a still better distribution of molten aluminum. Accordingly, it is possible to achieve a shortened cycle time for the casting steps and thereby an improved productivity.

[0054] According to this invention, the cavity may be purged with an inert gas before it is filled with nitrogen and magnesium gases. If the cavity is filled with an inert gas before it is filled with nitrogen and magnesium gases, an inert gas atmosphere is created in the cavity to replace the air in the cavity with an inert gas. This makes it possible to remove oxygen from the cavity and thereby prevent the formation of any oxide or oxide film on the surface of molten aluminum when molten aluminum is poured. Accordingly, as it is possible to maintain a still better distribution of molten aluminum, it is possible to achieve a shortened cycle time for molding any aluminum casting and thereby an improved productivity.

[0055] According to this invention, moreover, the pouring temperature of molten aluminum is set at 600 to 750.degree. C. If the molten aluminum temperature is lower than 600.degree. C., the nitrogen and magnesium gases fail to react well. The molten aluminum temperature is, therefore, set at 600.degree. C. or above, so that the nitrogen and magnesium gases may react well. If the molten aluminum temperature exceeds 750.degree. C., the solidification of molten aluminum in the cavity takes a long time making it difficult to achieve high productivity. A high molten aluminum temperature is, moreover, likely to lower the durability of the mold. The molten aluminum temperature is, therefore, set at 750.degree. C. or below to obtain a shortened solidifying time. This makes it possible to achieve a shortened cycle time for molding any aluminum casting and thereby a still improved productivity. The molten aluminum temperature set at 750.degree. C. or below enables an improvement in the durability of the mold.

[0056] According to this invention, moreover, the pouring temperature of molten aluminum is detected by a temperature sensor and the molten aluminum is controlled to a selected temperature based upon information as detected by the temperature sensor. The pouring temperature of molten aluminum is detected by a temperature sensor and the molten aluminum is controlled to a selected pouring temperature based upon information as detected by the temperature sensor. This makes it possible to control the pouring temperature of molten aluminum reliably with a small amount of time and labor and thereby achieve an improved productivity.

BRIEF DESCRIPTION OF THE DRAWINGS

[0057] FIG. 1 is a perspective view of a disk rotor (brake disk) as molded by an aluminum casting process (first embodiment) using a casting mold and embodying this invention.

[0058] FIG. 2 is an overall diagram showing an aluminum casting apparatus for carrying out the aluminum casting process (first embodiment) using a casting mold and embodying this invention.

[0059] FIG. 3 is a flowchart explaining the aluminum casting process according to the first embodiment of this invention.

[0060] FIG. 4 is a diagram explaining an example in which an argon gas atmosphere is created in a cavity in the aluminum casting process according to the first embodiment of this invention.

[0061] FIG. 5 is a diagram explaining an example in which gaseous magnesium is introduced into the cavity in the aluminum casting process according to the first embodiment of this invention.

[0062] FIG. 6 is a diagram explaining an example in which the cavity wall is heated to a specific temperature after the deposition of magnesium in the aluminum casting process according to the first embodiment of this invention.

[0063] FIG. 7 is a diagram explaining an example in which nitrogen gas is introduced into the cavity in the aluminum casting process according to the first embodiment of this invention.

[0064] FIG. 8 is a diagram explaining an example in which magnesium nitride is formed on the cavity wall in the aluminum casting process according to the first embodiment of this invention.

[0065] FIGS. 9A and 9B are diagrams explaining the example in which magnesium nitride is formed in the aluminum casting process according to the first embodiment of this invention.

[0066] FIGS. 10A and 10B are diagrams explaining an example in which an aluminum casting is molded in the cavity in the aluminum casting process according to the first embodiment of this invention.

[0067] FIG. 11 is an overall diagram showing an aluminum casting apparatus for carrying out the aluminum casting process (second embodiment) using a casting mold and embodying this invention.

[0068] FIG. 12 is a diagram explaining an example in which an argon gas atmosphere is created in a cavity in the aluminum casting process according to the second embodiment of this invention.

[0069] FIG. 13 is a diagram explaining an example in which the cavity wall is heated to a specific temperature after the deposition of magnesium in the aluminum casting process according to the second embodiment of this invention.

[0070] FIG. 14 is a diagram explaining an example in which magnesium nitride is formed in the aluminum casting process according to the second embodiment of this invention.

[0071] FIGS. 15A and 15B are diagrams explaining the example in which magnesium nitride is formed in the aluminum casting process according to the second embodiment of this invention.

[0072] FIGS. 16A and 16B are diagrams explaining an example in which an aluminum casting is molded in the cavity in the aluminum casting process according to the second embodiment of this invention.

[0073] FIG. 17 is an overall diagram showing an aluminum casting apparatus for carrying out the aluminum casting process (third embodiment) using a casting mold and embodying this invention.

[0074] FIG. 18 is a flowchart explaining the aluminum casting process according to the third embodiment of this invention.

[0075] FIG. 19 is a diagram explaining an example in which a cavity is filled with an inert gas in the aluminum casting process according to the third embodiment of this invention.

[0076] FIG. 20 is a diagram explaining an example in which gaseous magnesium is introduced into the cavity in the aluminum casting process according to the third embodiment of this invention.

[0077] FIG. 21 is a diagram explaining an example in which gaseous magnesium is deposited on the cavity wall in the aluminum casting process according to the third embodiment of this invention.

[0078] FIG. 22 is a diagram explaining an example in which nitrogen gas is introduced into the cavity in the aluminum casting process according to the third embodiment of this invention.

[0079] FIG. 23 is a diagram explaining an example in which magnesium nitride is formed in the aluminum casting process according to the third embodiment of this invention.

[0080] FIGS. 24A and 24B are diagrams explaining the example in which molten aluminum is supplied into the cavity in the aluminum casting process according to the third embodiment of this invention.

[0081] FIGS. 25A and 25B are diagrams explaining an example in which an aluminum casting is molded in the aluminum casting process according to the third embodiment of this invention.

[0082] FIG. 26 is an overall diagram showing an aluminum casting apparatus for carrying out the aluminum casting process (fourth embodiment) using a casting mold and embodying this invention.

[0083] FIG. 27 is a diagram explaining an example in which an argon gas atmosphere is created in a cavity in the aluminum casting process according to the fourth embodiment of this invention.

[0084] FIG. 28 is a diagram explaining an example in which magnesium is deposited on the cavity wall in the aluminum casting process according to the fourth embodiment of this invention.

[0085] FIG. 29 is a diagram explaining an example in which magnesium nitride is formed on the cavity wall in the aluminum casting process according to the fourth embodiment of this invention.

[0086] FIGS. 30A and 30B are diagrams explaining an example in which molten aluminum is supplied into the cavity in the aluminum casting process according to the fourth embodiment of this invention.

[0087] FIGS. 31A and 31B are diagrams explaining an example in which an aluminum casting is molded in the aluminum casting process according to the fourth embodiment of this invention.

[0088] FIG. 32 is an overall diagram showing an aluminum casting apparatus (fifth embodiment) embodying this invention.

[0089] FIG. 33 is a flowchart explaining the operation of the fifth embodiment of this invention.

[0090] FIG. 34 is a diagram explaining an example in which the cavity in the apparatus according to the fifth embodiment of this invention is filled with an inert gas.

[0091] FIG. 35 is a diagram explaining an example in which air is discharged from the cavity in the apparatus according to the fifth embodiment of this invention.

[0092] FIG. 36 is a diagram explaining an example in which magnesium is introduced into the cavity in the apparatus according to the fifth embodiment of this invention.

[0093] FIG. 37 is a diagram explaining an example in which magnesium is deposited on the cavity wall in the apparatus according to the fifth embodiment of this invention.

[0094] FIG. 38 is a diagram explaining an example in which nitrogen gas is introduced into the cavity in the apparatus according to the fifth embodiment of this invention.

[0095] FIG. 39 is a diagram explaining an example in which magnesium nitride is formed in the apparatus according to the fifth embodiment of this invention.

[0096] FIGS. 40A and 40B are diagrams explaining an example in which molten aluminum is supplied into the cavity in the apparatus according to the fifth embodiment of this invention.

[0097] FIGS. 41A and 41B are diagrams explaining an example in which an aluminum casting is molded in the apparatus according to the fifth embodiment of this invention.

[0098] FIG. 42 is an overall diagram showing an aluminum casting apparatus (seventh embodiment) embodying this invention.

[0099] FIG. 43 is a diagram explaining an example in which air is discharged from the cavity in the apparatus according to the seventh embodiment of this invention.

[0100] FIG. 44 is a diagram explaining an example in which magnesium is deposited on the cavity wall in the apparatus according to the seventh embodiment of this invention.

[0101] FIG. 45 is a diagram explaining an example in which magnesium nitride is formed in the apparatus according to the seventh embodiment of this invention.

[0102] FIGS. 46A and 46B are diagrams explaining an example in which molten aluminum is supplied into the cavity in the apparatus according to the seventh embodiment of this invention.

[0103] FIGS. 47A and 47B are diagrams explaining an example in which an aluminum casting is molded in the apparatus according to the seventh embodiment of this invention.

[0104] FIG. 48 is a perspective view of a cylinder block as molded by an aluminum casting process (ninth embodiment) using a casting mold and embodying this invention.

[0105] FIG. 49 is an overall diagram showing an aluminum casting apparatus for carrying out the aluminum casting process (ninth embodiment) using a casting mold and embodying this invention.

[0106] FIG. 50 is a flowchart explaining the aluminum casting process (ninth embodiment) using a casting mold and embodying this invention.

[0107] FIG. 51 is a diagram explaining an example in which an argon gas atmosphere is created in a cavity in the aluminum casting process according to the ninth embodiment of this invention.

[0108] FIG. 52 is a diagram explaining an example in which nitrogen gas is introduced into the cavity in the aluminum casting process according to the ninth embodiment of this invention.

[0109] FIG. 53 is a diagram explaining an example in which gaseous magnesium is introduced into the cavity in the aluminum casting process according to the ninth embodiment of this invention.

[0110] FIGS. 54A and 54B are diagrams explaining an example in which molten aluminum is supplied into the cavity in the aluminum casting process according to the ninth embodiment of this invention.

[0111] FIGS. 55A and 55B are diagrams explaining an example in which the formation of any oxide or oxide film on the surface of molten aluminum is prevented in the aluminum casting process according to the ninth embodiment of this invention.

[0112] FIGS. 56A and 56B are diagrams explaining an example in which an aluminum casting is molded in the aluminum casting process according to the ninth embodiment of this invention.

[0113] FIG. 57 is a diagram explaining a known aluminum casting process.

[0114] FIG. 58 is a diagram explaining an important part of the known aluminum casting process.

BEST MODE FOR CARRYING OUT THE INVENTION

[0115] FIG. 1 is a perspective view of a disk rotor (brake disk) as molded by an aluminum casting process (first embodiment) using a casting mold and embodying this invention. The disk rotor (brake disk) 10 is a component member made of aluminum and having a cylindrical hub portion 11 and a circular disk portion 18 formed integrally with the hub portion 11.

[0116] The hub portion 11 has a lid 13 formed integrally with the outer end of its peripheral wall 12 and the lid 13 has an opening 14 formed in its center and bolt holes 15 and stud holes 16 formed around the opening 14. Bolts not shown can be inserted through the bolt holes 15 to secure the disk rotor 10 to a drive shaft (not shown). The stud holes 16 are the holes in which studs not shown are press fitted to secure a wheel to the disk rotor 10.

[0117] FIG. 2 is an overall diagram showing an aluminum casting apparatus for carrying out the aluminum casting process (first embodiment) using a casting mold and embodying this invention. The aluminum casting apparatus 20 has a casting apparatus proper 21 having a casting mold 22, an inert gas introducing portion 40 for introducing argon (Ar) gas (inert (rare) gas) into the cavity 25 defined in the casting mold 22, a magnesium introducing portion 50 for introducing gaseous magnesium (Mg) into the cavity 25 into which the inert gas has been introduced, and a nitrogen gas introducing portion 60 for introducing nitrogen (N.sub.2) gas into the cavity 25 into which the gaseous magnesium has been introduced. The casting apparatus proper 21 includes a fixed plate 31 secured to a base 30, the casting mold 22 has a stationary member 23 secured to the fixed plate 31, guide rods 32 are secured to the fixed plate 31 and support a movable plate 33, and the casting mold 22 has a movable member 24 secured to the movable plate 33. A sprue runner 34 opening to the cavity 25 is formed in the stationary member 23 of the mold and the base 30 and holds a movable plunger 35 therein. A sprue 36 is formed vertically from the sprue runner 34 and has an upper end closed by a tenon 37, while a pouring tank 38 capable of communicating with the sprue 36 is situated above it. The stationary and movable members 23 and 24 constitute the casting mold 22.

[0118] According to the aluminum casting apparatus 20, the movement of the movable plate 33 in the directions of arrows by a moving device (not shown) enables the movable member 24 of the mold to move between a mold closing position (shown) and a mold opening position. The movable member 24 held in its mold closing position enables the stationary and movable members 23 and 24 to form the cavity 25. After molten aluminum 39 is supplied into the cavity 25, it can be pressed by the plunger 35 to mold an aluminum casting in the cavity 25. Moreover, the casting apparatus proper 21 includes a heater (cartridge heater) 27 embedded in the casting mold 22 along an area 25a of the wall of the cavity 25 corresponding to the circular disk portion 18 (portion of small thickness) shown in FIG. 1, or along the outer peripheries of the stationary and movable members 23 and 24. This makes it possible to heat the area 25a corresponding to the disk portion 18 (portion of small thickness) to a specific temperature (for example, at least 400.degree. C.).

[0119] Heating the whole casting mold 22 may be thought of as a method of heating the wall area 25a of the cavity 25 to a specific temperature. Heating the whole casting mold 22, however, requires a large amount of heat energy. Moreover, it takes a lot of time to heat the area 25a to a specific temperature by heating the whole casting mold 22. On the other hand, the heater (cartridge heater) embedded in the casting mold 22 can heat the specific area 25a to a specific temperature by heating only the necessary part of the casting mold 22. Accordingly, it is possible to reduce the heat energy required for heating the specific area 25a to a specific temperature. Moreover, it is possible to heat the specific area 25a to a specific temperature within a relatively short time, since it is sufficient to heat only the necessary part of the casting mold 22.

[0120] The casting apparatus proper 21 further includes a thermocouple 28 embedded in the area 25a corresponding to the disk portion 18 (portion of small thickness) and located in the tail end of the outer periphery of the stationary member 23 of the mold. This enables the thermocouple 28 to detect the area 25a corresponding to the circular disk portion 18 (portion of small thickness) of the disk rotor 10. The detection by the thermocouple 28 of the temperature of the area 25a corresponding to the disk portion 18 (portion of small thickness) makes it possible to set the temperature of the specific area 25a more accurately to a specific temperature. This makes it possible to form magnesium nitride 58b (shown in FIG. 8) efficiently on a magnesium layer 58a. Molten aluminum fails to flow smoothly particularly along the area 25a corresponding to the disk portion 18 (portion of small thickness) as the cavity has a narrow space therein. The temperature of the area 25a corresponding to the disk portion 18 (portion of small thickness) is, therefore, detected by the thermocouple 28. This makes it possible to form magnesium nitride 58b efficiently on the magnesium layer 58a in the area 25a corresponding to the disk portion 18 (portion of small thickness). The magnesium nitride 58b reduces any oxide on molten aluminum and thereby makes it possible to maintain a good distribution of molten aluminum.

[0121] The inert gas introducing portion 40 has an argon gas bottle 42 connected to the cavity 25 by an introducing passage 41 provided with an argon valve 43 midway. The argon valve 43 is a valve for switching the introducing passage 41 between its open and closed positions. The argon valve 43 enables argon to be introduced from the argon gas bottle 42 into the cavity 25 through the introducing passage 41 when it is switched to its open position.

[0122] The magnesium introducing portion 50 has a first magnesium introducing passage 51 and a second magnesium introducing passage 52 both connected with the introducing passage 41, a sublimating device 53 connected to the first and second magnesium introducing passages 51 and 52 and a magnesium valve 57 provided in the first magnesium introducing passage 51. The sublimating device 53 has a holding case 54 connected with the outlet end 51a of the first magnesium introducing passage 51 and the inlet end 52a of the second magnesium introducing passage 52 and a sublimating heater 55 surrounding the holding case 54. The sublimating heater 55 can heat the inside of the holding case 54 to a specific temperature (for example, at least 400.degree. C.) and thereby sublimate a magnesium ingot (magnesium) 58 in the holding case 54 into a gaseous form. The magnesium valve 57 is a valve for switching the first magnesium introducing passage 51 between its open and closed positions. The magnesium valve 57 makes it possible to introduce argon gas from the argon gas bottle 42 into the holding case 54 through the first magnesium introducing passage 51 when it is switched to its open position, so that the introduced argon gas may direct gaseous magnesium into the cavity 25 through the second magnesium introducing passage 52 and the introducing passage 41.

[0123] The nitrogen gas introducing portion 60 has a nitrogen gas bottle 62 connected with the cavity 25 through a nitrogen introducing passage 61 provided with a nitrogen valve 63 midway. The nitrogen valve 63 is a valve for switching the nitrogen introducing passage 61 between its open and closed positions. The nitrogen valve 63 makes it possible to introduce nitrogen gas from the nitrogen gas bottle 62 into the cavity 25 through the nitrogen introducing passage 61 when it is switched to its open position.

[0124] Description will now be made of an example in which the casting process according to the first embodiment of this invention is carried out by the aluminum casting apparatus 20. FIG. 3 is a flowchart explaining the aluminum casting process according to the first embodiment of this invention, in which each ST-- indicates Step No.

[0125] ST10: The cavity of a closed mold is filled with an inert gas.

[0126] ST11: Gaseous magnesium is introduced into the inert gas-filled cavity to have magnesium deposited on the cavity wall.

[0127] ST12: The mold is heated to heat the magnesium-deposited cavity wall to a specific temperature.

[0128] ST13: Nitrogen gas is introduced into the heated cavity to have magnesium nitride formed on the cavity wall.

[0129] ST14: Molten aluminum is supplied into the cavity in which magnesium nitride has been formed, to mold an aluminum casting in the cavity, while the surface of molten aluminum is reduced with magnesium nitride.

[0130] Steps ST10 to ST14 of the aluminum casting process using a casting mold and embodying this invention will now be described in detail with reference to FIGS. 4 to 10. FIG. 4 is a diagram for explaining an example in which an argon gas atmosphere is created in the cavity in the aluminum casting process according to the first embodiment of this invention, and it shows ST10. The argon valve 43 is switched to its open position to introduce argon gas (shown in dots) from the argon gas bottle 42 into the cavity 25 through the introducing passage 41. The argon gas filling the cavity 25 expels air from the cavity 25 through, for example, any clearance between the stationary and movable members 23 and 24 of the mold. As a result, an argon gas atmosphere is created in the cavity 25. After an argon gas atmosphere is created in the cavity 25, the argon valve 43 is switched to its closed position.

[0131] FIG. 5 is a diagram for explaining an example in which gaseous magnesium is introduced into the cavity 25 in the aluminum casting process according to the first embodiment of this invention, and it shows ST11. The sublimating heater 55 in the sublimating device 53 is placed in operation to heat the inside of the holding case 54 to a specific temperature (for example, at least 400.degree. C.). The heating of the inside of the holding case 54 causes the sublimation of the magnesium ingot 58 into a gaseous form. The gaseous magnesium in the holding case 54 is shown in dots. The magnesium valve 57 is switched to its open position so that argon gas may be introduced from the argon gas bottle 42 into the holding case 54 through the first magnesium introducing passage 51. The introduced argon gas causes gaseous magnesium (shown in dots) to be introduced into the cavity 25 through the second magnesium introducing passage 52 and the introducing passage 41. When gaseous magnesium is introduced into the cavity 25, the second magnesium introducing passage 52 and the introducing passage 41 are preferably heated so that no magnesium may be deposited in the second magnesium introducing passage 52 or the introducing passage 41.

[0132] FIG. 6 is a diagram for explaining an example in which the cavity wall is heated to a specific temperature after the deposition of magnesium in the aluminum casting process according to the first embodiment of this invention, and it shows ST11 and the former half of ST12. The gaseous magnesium introduced into the cavity 25 as shown by arrows has its temperature lowered to 150 to 250.degree. C. by contacting the wall of the cavity 25. Its drop in temperature to 150 to 250.degree. C. causes gaseous magnesium to be deposited on the wall of the cavity 25. The deposited magnesium is called a magnesium layer 58a. After the deposition of the magnesium layer 58a on the wall of the cavity 25, the magnesium valve 57 (shown in FIG. 5) is switched to its closed position.

[0133] Description will now be made of the latter half of ST12. The heater (cartridge heater) 27 is heated after the magnesium layer 58a has been deposited on the wall of the cavity 25. It heats the area 25a (a part of the wall of the cavity 25) corresponding to the disk portion 18 (portion of small thickness) shown in FIG. 1. The temperature of the area 25a corresponding to the disk portion 18 (portion of small thickness) is detected by the thermocouple 28. When the temperature as detected by the thermocouple 28 has reached, for example, at least 400.degree. C., the heater (cartridge heater) 27 is so controlled as to maintain that temperature.

[0134] FIG. 7 is a diagram for explaining an example in which nitrogen gas is introduced into the cavity in the aluminum casting process according to the first embodiment of this invention, and it shows the former half of ST13. The nitrogen valve 63 in the nitrogen gas introducing portion 60 is switched to its open position. The nitrogen valve 63 switched to its open position allows nitrogen gas to flow from the nitrogen gas bottle 62 into the nitrogen introducing passage 61. As a result, nitrogen gas is introduced from the nitrogen gas bottle 62 into the cavity 25 through the nitrogen introducing passage 61.

[0135] FIG. 8 is a diagram for explaining an example in which magnesium nitride is formed on the cavity wall in the aluminum casting process according to the first embodiment of this invention, and it shows the latter half of ST13. The wall of the cavity 25 has been heated by the heater (cartridge heater) 27 to, for example, at least 400.degree. C. in the area 25a corresponding to the disk portion 18 (portion of small thickness) shown in FIG. 1. As a result, the magnesium layer 58a in the area 25a corresponding to the disk portion 18 (portion of small thickness) and nitrogen gas react with each other and form magnesium nitride (Mg.sub.3N.sub.2) 58b on the surface of the magnesium layer 58a in that area. When the area 25a corresponding to the disk portion 18 (portion of small thickness) is heated to, for example, at least 400.degree. C. by the heater (cartridge heater) 27 as described, the magnesium layer 58a is heated and magnesium nitride 58b can be formed easily. This enables the efficient formation of magnesium nitride 58b. After magnesium nitride 58b has been formed on the surface of the magnesium layer 58a in the area 25a, the nitrogen valve 63 is switched to its closed position.

[0136] For the formation of magnesium nitride 58b, the magnesium layer 58a is first formed by magnesium deposited on the wall of the cavity 25, then the area 25a corresponding to the disk portion 18 (portion of small thickness) is heated, and thereafter nitrogen gas is introduced into the cavity 25, as described with reference to FIGS. 6 and 8. As a result, magnesium nitride 58b is formed on the surface of the magnesium layer 58a in the heated area 25a. Accordingly, it is possible to form magnesium nitride 58b on only the surface of the magnesium layer 58a and thereby shorten the time required for forming magnesium nitride 58b. Moreover, it is possible to reduce the amount of nitrogen gas used, since it is sufficient to form magnesium nitride 58b on only the surface of the magnesium layer 58a.

[0137] FIGS. 9A and 9B are diagrams for explaining an example in which molten aluminum is supplied into the cavity in the aluminum casting process according to the first embodiment of this invention, and they show the former half of ST14. Referring to FIG. 9A, the tenon 37 in the casting apparatus proper 21 is operated to open the sprue 36, so that molten aluminum 39 may be supplied from the pouring tank 38 into the cavity 25 through the sprue 36 and the runner 34 as shown by arrows. Generally, molten aluminum 39 flows smoothly if the cavity 25 is a wide space, but it does not flow smoothly if the cavity 25 is a narrow space. Accordingly, molten aluminum 39 flows smoothly along the area 25b of the cavity forming a wide space even if any oxide 39b may be formed on the surface 39a of molten aluminum 39. On the other hand, any oxide 39b formed on the aluminum surface 39a makes it difficult for molten aluminum 39 to flow smoothly along the area 25a of the cavity forming a narrow space which makes it relatively difficult for molten aluminum 39 to flow. In the area 25a of the cavity forming a narrow space, therefore, magnesium nitride 58b is formed on the wall of the cavity 25 to reduce any oxide 39b on the molten aluminum 39. This action will be explained with reference to FIG. 9B.

[0138] Referring to FIG. 9B, the molten aluminum 39 supplied into the cavity 25 has its surface 39a contact magnesium nitride 58b upon reaching the area 25a corresponding to the disk portion (portion of small thickness) shown in FIG. 1. It is likely that any oxide 39b may have been formed on the surface 39a of molten aluminum 39, and if any oxide 39b has been formed, its reaction with magnesium nitride 58b enables the removal of oxygen from the oxide 39b. This makes it possible to prevent the formation of any oxide film on the surface 39a of molten aluminum 39 and thereby restrain any increase in surface tension of molten aluminum 39. Accordingly, it is possible to maintain a good distribution of molten aluminum 39 along the area 25a corresponding to the disk portion 18 (portion of small thickness).

[0139] FIGS. 10A and 10B are diagrams for explaining an example in which an aluminum casting is molded in the cavity in accordance with the aluminum casting process according to the first embodiment of this invention, and they show the latter half of ST14. Referring to FIG. 10A, the sprue 36 is closed by the tenon 37 after a specific amount of molten aluminum 39 has been supplied from the pouring tank 38 to the cavity 25. The plunger 35 is pushed forward toward the cavity 25 to fill the cavity 25 with molten aluminum 39. Referring to FIG. 10B, the casting mold 22 is opened for the removal of an aluminum casting 39c obtained by the solidification of molten aluminum 39 (shown in FIG. 10A). The aluminum casting 39c is a product of higher quality owing to a good distribution of molten metal as poured. The aluminum casting 39c is worked on to make the disk rotor 10 shown in FIG. 1.

Second Embodiment

[0140] Description will now be made of the second embodiment with reference to FIGS. 11 to 16. The reference numerals used for the first embodiment are used to denote like parts or materials for the second embodiment and no repeated description thereof is made.

[0141] FIG. 11 is an overall diagram showing an aluminum casting apparatus for carrying out the aluminum casting process using a casting mold and embodying this invention. The aluminum casting apparatus 80 has a casting apparatus proper 81 having a casting mold 82, an inert gas introducing portion 40 for introducing argon (Ar) gas (inert (rare) gas) into the cavity 87 defined in the casting mold 82, a magnesium introducing portion 50 for introducing gaseous magnesium (Mg) into the cavity 87 into which the inert gas has been introduced, and a nitrogen gas introducing portion 60 for introducing nitrogen (N.sub.2) gas into the cavity 87 into which the gaseous magnesium has been introduced. The casting apparatus proper 81 includes a fixed plate 91 secured to abase 90, a stationary mold member 83 is secured to the fixed plate 91, a movable plate 92 is movably mounted on the base 90, a movable mold member 84 is secured to the movable plate 92, a device 93 for moving the movable plate 92 is mounted on the base 90 and a core 85 for the casting mold 82 is supported by the base 90 so as to be capable of being raised and lowered by a raising and lowering device 94. A sprue runner 95 opening to the cavity 87 is formed in the movable mold member 84, a sprue 96 is formed vertically from the sprue runner 95, while a pouring tank 97 holding molten aluminum 39 is situated above the sprue 96, and the casting mold 82 has an opening 98 formed at its top as a vent or feeder head. The stationary and movable mold members 83 and 84 and the core 85 constitute the casting mold 82. While FIG. 11 shows the sprue 96 and the opening 98 as being large relative to the cavity 87 to provide an easier understanding of the casting apparatus proper 81, the real sprue 96 and opening 98 are sufficiently small relative to the cavity 87 to enable the cavity 87 to keep a substantially completely closed state when the casting mold 82 is closed.

[0142] According to the aluminum casting apparatus 80, the movement of the movable plate 92 in the directions of arrows by the moving device 93 enables the movable mold member 84 to move between its mold closing position (position shown in the drawing) and its mold opening position. The movement of the core 85 in the directions of arrows by the raising and lowering device 94 enables the core 85 to move between its mold closing position (position shown in the drawing) and its mold opening position. The movable mold member 84 and the core 85 held in their mold closing positions enable the stationary and movable mold members 83 and 84 and the core 85 to form the cavity 87. If molten aluminum 39 is supplied into the cavity 87, it is possible to mold an aluminum casting in the cavity 87.

[0143] The casting apparatus proper 81 differs from the casting apparatus proper 21 according to the first embodiment in that it is so constructed as to allow molten aluminum 39 to flow into the cavity 87 by its own weight at the atmospheric pressure. Moreover, the casting apparatus proper 81 has a heater (cartridge heater) 88 embedded in the casting mold 82 along the area 87a of the wall of the cavity 87 corresponding to the cylinder portion of a cylinder block (portion of small thickness), or in the left lower portion of the stationary mold member 83 and the outer periphery of the core 85. This makes it possible to heat the area 87a corresponding to the cylinder portion (portion of small thickness) to a specific temperature (for example, at least 400.degree. C.).

[0144] Heating the whole casting mold 82 may be thought of as a method of heating the wall area 87a of the cavity 87 to a specific temperature. Heating the whole casting mold 82, however, requires a large amount of heat energy. Moreover, it takes a lot of time to heat the area 87a to a specific temperature by heating the whole casting mold 82. On the other hand, the heater (cartridge heater) embedded in the casting mold 82 can heat the specific area 87a to a specific temperature by heating only the necessary part of the casting mold 82. Accordingly, it is possible to reduce the heat energy required for heating the specific area 87a to a specific temperature. Moreover, it is possible to heat the specific area 87a to a specific temperature within a relatively short time, since it is sufficient to heat only the necessary part of the casting mold 82.

[0145] The casting apparatus proper 81 further includes a thermocouple 89 embedded in the area 87a corresponding to the cylinder portion (portion of small thickness) and located in the left lower portion of the stationary mold member 83. This enables the thermocouple 89 to detect the area 87a corresponding to the cylinder portion (portion of small thickness) of a cylinder block. The detection by the thermocouple 89 of the temperature of the area 87a corresponding to the cylinder portion (portion of small thickness) makes it possible to set the temperature of the specific area 87a more accurately to a specific temperature. This makes it possible to form magnesium nitride 103 (shown in FIG. 14) efficiently on a magnesium layer 102. Molten aluminum fails to flow smoothly particularly along the area 87a corresponding to the cylinder portion (portion of small thickness) as the cavity has a narrow space therein. The temperature of the area 87a corresponding to the cylinder portion (portion of small thickness) is, therefore, detected by the thermocouple 89. This makes it possible to form magnesium nitride 103 efficiently on the magnesium layer 102 in the area 87a corresponding to the cylinder portion (portion of small thickness). The magnesium nitride 103 reduces any oxide on molten aluminum and thereby makes it possible to maintain a good distribution of molten aluminum.

[0146] An example in which the casting process according to the second embodiment of this invention is carried out by the aluminum casting apparatus 80 will now be described with reference to FIGS. 3 and 11 to 16. The step ST10 of FIG. 3 will first be explained. The argon valve 43 shown in FIG. 11 is switched to its open position to introduce argon gas from an argon gas bottle 42 into the cavity 87 through an introducing passage 41. FIG. 12 is a diagram for explaining an example in which an argon gas atmosphere is created in the cavity in accordance with the aluminum casting process according to the second embodiment of this invention. The argon gas filling the cavity 87 expels air from the cavity 87 through, for example, the sprue 96 or the vent or feeder head opening 98. As a result, an argon gas atmosphere is created in the cavity 87. After an argon gas atmosphere is created in the cavity 87, the argon valve 43 (shown in FIG. 11) is switched to its closed position.

[0147] The former half of ST11 of FIG. 3 will now be explained. Returning to FIG. 11, a sublimating heater 55 in a sublimating device 53 is placed in operation to heat the inside of a holding case 54 to a specific temperature (for example, at least 400.degree. C.). The heating of the inside of the holding case 54 causes the sublimation of a magnesium ingot 58 into a gaseous form. A magnesium valve 57 is switched to its open position so that argon gas may be introduced from the argon gas bottle 42 into the holding case 54 through a first magnesium introducing passage 51. The introduced argon gas causes gaseous magnesium to be introduced into the cavity 87 through a second magnesium introducing passage 52 and the introducing passage 41. When gaseous magnesium is introduced into the cavity 87, the second magnesium introducing passage 52 and the introducing passage 41 are preferably heated so that no magnesium may be deposited in the second magnesium introducing passage 52 or the introducing passage 41.

[0148] FIG. 13 is a diagram for explaining an example in which the cavity wall is heated to a specific temperature after the deposition of magnesium in the aluminum casting process according to the second embodiment of this invention, and it explains the latter half of Step ST11 and Step ST12. The gaseous magnesium introduced into the cavity 87 as shown by arrows has its temperature lowered to 150 to 250.degree. C. by contacting the wall of the cavity 87. Its drop in temperature to 150 to 250.degree. C. causes gaseous magnesium to be deposited on the wall of the cavity 87. The deposited magnesium is called a magnesium layer 102. After the deposition of the magnesium layer 102 on the wall of the cavity 87, the magnesium valve 57 (shown in FIG. 11) is switched to its closed position.

[0149] Step ST12 will now be explained. The heater (cartridge heater) 88 is heated after the magnesium layer 102 has been deposited on the wall of the cavity 25. It heats the area 87a (a part of the wall of the cavity 87) corresponding to the cylinder portion (portion of small thickness). The temperature of the area 87a corresponding to the cylinder portion (portion of small thickness) is detected by the thermocouple 89. When the temperature as detected by the thermocouple 89 has reached, for example, at least 400.degree. C., the heater (cartridge heater) 88 is so controlled as to maintain that temperature.

[0150] The Step ST13 shown in FIG. 3 will now be explained. The nitrogen valve 63 in the nitrogen gas introducing portion 60 shown in FIG. 11 is switched to its open position to allow nitrogen gas to flow from a nitrogen gas bottle 62 into a nitrogen introducing passage 61. As a result, nitrogen gas is introduced from the nitrogen gas bottle 62 into the cavity 87 through the nitrogen introducing passage 61.

[0151] FIG. 14 is a diagram for explaining an example in which magnesium nitride is formed on the cavity wall in accordance with the aluminum casting process according to the second embodiment of this invention. The wall of the cavity 87 has been heated by the heater (cartridge heater) 88 to, for example, at least 400.degree. C. in the area 87a corresponding to the cylinder portion of a cylinder block (portion of small thickness). As a result, the magnesium layer 102 in the area 87a corresponding to the cylinder portion (portion of small thickness) and nitrogen gas react with each other and form magnesium nitride (Mg.sub.3N.sub.2) 103 on the surface of the magnesium layer 102 in that area. When the area 87a corresponding to the cylinder portion (portion of small thickness) is heated to, for example, at least 400.degree. C. by the heater (cartridge heater) 88 as described, the magnesium layer 102 is heated and magnesium nitride 103 can be formed easily. This enables the efficient formation of magnesium nitride 103. After magnesium nitride 103 has been formed on the surface of the magnesium layer 102 in the area 87a, the nitrogen valve 63 is switched to its closed position.

[0152] For the formation of magnesium nitride 103, the magnesium layer 102 is first formed by magnesium deposited on the wall of the cavity 87, then the area 87a corresponding to the cylinder portion (portion of small thickness) is heated, and thereafter nitrogen gas is introduced into the cavity 87, as shown in FIGS. 13 and 14. As a result, magnesium nitride 103 is formed on the surface of the magnesium layer 102. Accordingly, it is possible to form magnesium nitride 103 on only the surface of the magnesium layer 102 and thereby shorten the time required for forming magnesium nitride 103. Moreover, it is possible to reduce the amount of nitrogen gas used, since it is sufficient to form magnesium nitride 103 on only the surface of the magnesium layer 102.

[0153] Step ST14 of FIG. 3 will now be explained with reference to FIGS. 15 and 16. FIGS. 15A and 15B are diagrams for explaining an example in which magnesium nitride is formed in accordance with the aluminum casting process according to the second embodiment of this invention. Referring to FIG. 15A, the pouring tank 97 in the casting apparatus proper 81 is tilted to supply molten aluminum 39 of the pouring tank 97 into the cavity 87 through the sprue 96 and the runner 95 as shown by arrows. Generally, molten aluminum 39 flows smoothly if the cavity 87 is a wide space, but it does hot flow smoothly if the cavity 87 is a narrow space. Accordingly, molten aluminum 39 flows smoothly along the area 87b of the cavity forming a wide space even if any oxide 39b may be formed on the surface 39a of molten aluminum 39. On the other hand, any oxide 39b formed on the aluminum surface 39a makes it difficult for molten aluminum 39 to flow smoothly along the area 87a of the cavity forming a narrow space which makes it relatively difficult for molten aluminum 39 to flow. In the area 87a of the cavity forming a narrow space, therefore, magnesium nitride 103 is formed on the wall of the cavity 87 to reduce any oxide 39b on the molten aluminum 39. This action will be explained with reference to FIG. 15B.

[0154] Referring to FIG. 15B, the molten aluminum 39 supplied into the cavity 87 has its surface 39a of the molten aluminum 39 contact magnesium nitride 103 upon reaching the area 87a corresponding to the cylinder portion of a cylinder block (portion of small thickness). It is likely that any oxide 39b may have been formed on the surface 39a of molten aluminum 39, and if any oxide 39b has been formed, its reaction with magnesium nitride 103 enables the removal of oxygen from the oxide 39b. This makes it possible to prevent the formation of any oxide film on the surface 39a of molten aluminum 39 and thereby restrain any increase in surface tension of molten aluminum 39. Accordingly, it is possible to maintain a good distribution of molten aluminum 39 along the area 87a corresponding to the cylinder portion of a cylinder block (portion of small thickness).

[0155] FIGS. 16A and 16B are diagrams for explaining an example in which an aluminum casting is molded in the cavity in accordance with the aluminum casting process according to the second embodiment of this invention. Referring to FIG. 16A, the pouring tank 97 is returned to its horizontal position after a specific amount of molten aluminum 39 has been supplied from the pouring tank 97 into the cavity 87. After molten aluminum 39 has solidified, the core 85 is lowered by the raising and lowering device 94 as shown by an arrow A and the movable mold member 84 is moved by the moving device 93 as shown by an arrow B, so that the casting mold 82 may be opened. Referring to FIG. 16B, the casting mold 82 is opened for the removal of an aluminum casting 105 obtained by the solidification of molten aluminum 39 (shown in FIG. 16A). The aluminum casting 105 is a product of higher quality owing to a good distribution of molten metal as poured. The aluminum casting 105 has its non-product portions 105a and 105b removed and has its product portion worked on to give an engine cylinder block.

[0156] Although the first and second embodiments have been described by the examples in which the wall of the cavity 25 or 87 is heated in the area 25a or 87a corresponding to the small thickness portion of the casting, those examples are not limitative, but it is also possible to arrange for heating the whole wall surface of the cavity 25 or 87. It is, however, to be noted that it is possible to reduce the amount of nitrogen as required if the area 25a or 87a corresponding to the small thickness portion of the casting is heated to have magnesium nitride 58b or 103 formed in only the area 25a or 87a.

[0157] Description will now be made of the third and fourth embodiments with reference to FIGS. 17 to 31.

Third Embodiment

[0158] FIG. 17 is an overall diagram showing an aluminum casting apparatus for carrying out the aluminum casting process (third embodiment) using a casting mold and embodying this invention. The aluminum casting apparatus 120 has a casting apparatus proper 121 having a casting mold 122, an inert gas introducing portion 140 for introducing argon (Ar) gas (inert (rare) gas) into the cavity 125 defined in the casting mold 122, a magnesium introducing portion 150 for introducing gaseous magnesium (Mg) into the cavity 125 into which the inert gas has been introduced, and a nitrogen gas introducing portion 160 for introducing nitrogen (N.sub.2) gas into the cavity 125 into which the gaseous magnesium has been introduced. The casting apparatus proper 121 includes a fixed plate 131 secured to a base 130, the casting mold 122 has a stationary member 123 secured to the fixed plate 131, guide rods 132 are secured to the fixed plate 131, a movable plate 133 is movably supported by the guide rods 132, and the casting mold 122 has a movable member 124 secured to the movable plate 133. A sprue runner 134 opening to the cavity 125 is formed in the stationary member 123 of the mold and the base 130 and holds a movable plunger 135 therein. A sprue 136 is formed vertically from the sprue runner 134 and has an upper end of the sprue 136 closed by a tenon 137, while a pouring tank 138 capable of communicating with the sprue 136 is situated above it. The stationary and movable members 123 and 124 constitute the casting mold 122.

[0159] According to the aluminum casting apparatus 120, the movement of the movable plate 133 in the directions of arrows by a moving device (not shown) enables the movable member 124 of the mold to move between a mold closing position (position shown in the drawing) and a mold opening position. The movable member 124 held in its mold closing position enables the stationary and movable members 123 and 124 to form the cavity 125. After molten aluminum 139 is supplied into the cavity 125, it can be pressed by the plunger 135 to mold an aluminum casting in the cavity 125.

[0160] The inert gas introducing portion 140 has an argon gas bottle 142 connected to the cavity 125 by an introducing passage 141 provided with an argon valve 143 midway. The argon valve 143 is a valve for switching the introducing passage 141 between its open and closed positions. The argon valve 143 enables argon to be introduced from the argon gas bottle 142 into the cavity 125 through the introducing passage 141 when it is switched to its open position.

[0161] The magnesium introducing portion 150 has a first magnesium introducing passage 151 and a second magnesium introducing passage 152 both connected with the introducing passage 141, a sublimating device 153 connected to the first and second magnesium introducing passages 151 and 152 and a magnesium valve 157 provided in the first magnesium introducing passage 151. The sublimating device 153 has a holding case 154 connected with the outlet end 151a of the first magnesium introducing passage 151 and the inlet end 152a of the second magnesium introducing passage 152 and a sublimating heater 155 surrounding the holding case 154. The sublimating heater 155 can heat the inside of the holding case 154 to a specific temperature (for example, at least 400.degree. C.) and thereby sublimate a magnesium ingot (magnesium) 158 in the holding case 154 into a gaseous form. The magnesium valve 157 is a valve for switching the first magnesium introducing passage 151 between its open and closed positions. The magnesium valve 157 makes it possible to introduce argon gas from the argon gas bottle 142 into the holding case 154 through the first magnesium introducing passage 151 when it is switched to its open position, so that the introduced argon gas may direct gaseous magnesium into the cavity 125 through the second magnesium introducing passage 152 and the introducing passage 141.

[0162] The nitrogen introducing portion 160 has a nitrogen gas bottle 162 connected with the cavity 125 through a nitrogen introducing passage 161 provided with a nitrogen valve 163 and a heater 164 midway. The heater 164 can heat nitrogen gas flowing in the nitrogen introducing passage 161 to a specific temperature (for example, at least 400.degree. C.). The nitrogen valve 163 is a valve for switching the nitrogen introducing passage 161 between its open and closed positions. The nitrogen valve 163 makes it possible to introduce nitrogen gas from the nitrogen gas bottle 162 into the cavity 125 through the nitrogen introducing passage 161 when it is switched to its open position.

[0163] Description will now be made of an example in which the casting process according to the third embodiment of this invention is carried out by the aluminum casting apparatus 120. FIG. 18 is a flowchart explaining the aluminum casting process according to the third embodiment of this invention, in which each ST-- indicates Step No.

[0164] ST20: The cavity of a closed mold is filled with an inert gas.

[0165] ST21: Gaseous magnesium is introduced into the inert gas-filled cavity to have magnesium deposited on the cavity wall.

[0166] ST22: Heated nitrogen gas is introduced into the magnesium-deposited cavity to have magnesium nitride formed on the cavity wall.

[0167] ST23: Molten aluminum is supplied into the cavity in which magnesium nitride has been formed, to mold an aluminum casting in the cavity, while the surface of molten aluminum is reduced with magnesium nitride.

[0168] Steps ST20 to ST23 of the aluminum casting process using a casting mold and embodying this invention will now be described in detail with reference to FIGS. 19 to 25. FIG. 19 is a diagram for explaining an example in which the cavity is filled with an inert gas in accordance with the aluminum casting process according to the third embodiment of this invention, and it shows ST20. The argon valve 143 is switched to its open position to introduce argon gas (shown in dots) from the argon gas bottle 142 into the cavity 125 through the introducing passage 141. The argon gas filling the cavity 125 expels air from the cavity 125 through, for example, any clearance between the stationary and movable members 123 and 124 of the mold. As a result, an argon gas atmosphere is created in the cavity 125. After an argon gas atmosphere is created in the cavity 125, the argon valve 143 is switched to its closed position.

[0169] FIG. 20 is a diagram for explaining an example in which gaseous magnesium is introduced into the cavity in accordance with the aluminum casting process according to the third embodiment of this invention, and it shows the former half of ST21. The sublimating heater 155 in the sublimating device 153 is placed in operation to heat the inside of the holding case 154 to a specific temperature (for example, at least 400.degree. C.). The heating of the inside of the holding case 154 causes the sublimation of the magnesium ingot 158 into a gaseous form. The gaseous magnesium in the holding case 154 is shown in dots. The magnesium valve 157 is switched to its open position so that argon gas may be introduced from the argon gas bottle 142 into the holding case 154 through the first magnesium introducing passage 151. The introduced argon gas causes gaseous magnesium (shown in dots) to be introduced into the cavity 125 through the second magnesium introducing passage 152 and the introducing passage 141. When gaseous magnesium is introduced into the cavity 125, the second magnesium introducing passage 152 and the introducing passage 141 are preferably heated so that no magnesium may be deposited in the second magnesium introducing passage 152 or the introducing passage 141.

[0170] FIG. 21 is a diagram for explaining an example in which gaseous magnesium is deposited on the cavity wall in accordance with the aluminum casting process according to the third embodiment of this invention, and it shows the latter half of ST21. The gaseous magnesium introduced into the cavity 125 as shown by arrows has its temperature lowered to 150 to 250.degree. C. by contacting the wall of the cavity 125. Its drop in temperature to 150 to 250.degree. C. causes gaseous magnesium to be deposited on the wall of the cavity 125. The deposited magnesium is called a magnesium layer 158a. After the deposition of the magnesium layer 158a on the wall of the cavity 125, the magnesium valve 157 (shown in FIG. 20) is switched to its closed position.

[0171] FIG. 22 is a diagram for explaining an example in which nitrogen gas is introduced into the cavity in accordance with the aluminum casting process according to the third embodiment of this invention, and it shows ST22. The heater 64 in the nitrogen gas introducing portion 60 is placed in operation and the nitrogen valve 63 is switched to its open position. The nitrogen valve 63 switched to its open position allows nitrogen gas to flow from the nitrogen gas bottle 62 into the nitrogen introducing passage 61. As a result, the nitrogen gas in the nitrogen gas introducing passage 16 is heated by the heater 64 and the heated nitrogen gas is introduced into the cavity 25 through the nitrogen introducing passage 61. The independent heating of nitrogen gas by the heater 164 makes it possible to heat nitrogen gas flowing in the nitrogen introducing passage 161 efficiently to a specific temperature (for example, at least 400.degree. C.).

[0172] FIG. 23 is a diagram for explaining an example in which magnesium nitride is formed in accordance with the aluminum casting process according to the third embodiment of this invention.

[0173] The temperature T (.degree. C.) of gas in the cavity 125 and the pressure P (atmosphere) in the cavity 125 are so selected as to maintain their relationship T.gtoreq.(130.times.P+270). If this condition is met, it is possible to have magnesium-nitride (Mg.sub.3N.sub.2) 158b formed on the surface of the magnesium layer 158a by the reaction of the magnesium layer 158a deposited on the wall of the cavity 125 and nitrogen gas. More specifically, their relationship T.gtoreq.(130.times.P+270) teaches that when the pressure P in the cavity 125 is, for example, 1 atmosphere, the temperature T of nitrogen gas in the cavity 125 may be set at 400.degree. C. for forming magnesium nitride 158b on the surface of the magnesium layer 158a. As the temperature T (.degree. C.) of nitrogen gas in the cavity 125 and the pressure P (atmosphere) in the cavity 125 are relatively easy to determine based on their relationship T.gtoreq.(130.times.P+270), it is possible to perform the adjustment of equipment within a short time. Moreover, nitrogen gas is heated and heated nitrogen gas is used for forming magnesium nitride 158b. This makes it possible to form magnesium nitride 158b efficiently, as it is possible to heat nitrogen gas to a temperature at which magnesium nitride 158b is easy to form. The nitrogen valve 163 is switched to its closed position after magnesium nitride 158b has been formed on the surface of the magnesium layer 158a.

[0174] For the formation of magnesium nitride 158b, the magnesium layer 158a is first formed by magnesium deposited on the wall of the cavity 125 and then, nitrogen gas is introduced into the cavity 125 to form magnesium nitride 158b on the surface of the magnesium layer 158a, as described with reference to FIGS. 21 and 23. Accordingly, it is possible to form magnesium nitride 158b on only the surface of the magnesium layer 158a and thereby shorten the time required for forming magnesium nitride 158b. Moreover, it is possible to reduce the amount of nitrogen gas used, since it is sufficient to form magnesium nitride 158b on only the surface of the magnesium layer 158a.

[0175] FIGS. 24A and 24B are diagrams for explaining an example in which molten aluminum is supplied into the cavity in accordance with the aluminum casting process according to the third embodiment of this invention, and they show the former half of ST23. Referring to FIG. 24A, the tenon 137 in the casting apparatus proper 121 is operated to open the sprue 136, so that molten aluminum 139 may be supplied from the pouring tank 138 into the cavity 125 through the sprue 136 and the runner 134 as shown by arrows. Referring to FIG. 24B, the molten aluminum 139 supplied into the cavity 125 has its surface 139a contact magnesium nitride 158b. It is likely that any oxide 139b may have been formed on the surface 139a of molten aluminum 139, and if any oxide 139b has been formed, its reaction with magnesium nitride 158b enables the removal of oxygen from the oxide 139b. This makes it possible to prevent the formation of any oxide film on the surface 139a of molten aluminum 139 and thereby restrain any increase in surface tension of molten aluminum 139. Accordingly, it is possible to maintain a good distribution of molten aluminum 139 in the cavity 125.

[0176] FIGS. 25A and 25B are diagrams for explaining an example in which an aluminum casting is molded in accordance with the aluminum casting process according to the third embodiment of this invention, and they show the latter half of ST23. Referring to FIG. 25A, the sprue 136 is closed by the tenon 137 after a specific amount of molten aluminum 139 has been supplied from the pouring tank 138 to the cavity 125. The plunger 135 is pushed forward toward the cavity 125 to fill the cavity 125 with molten aluminum 139. Referring to FIG. 25B, the casting mold 122 is opened for the removal of an aluminum casting 139c obtained by the solidification of molten aluminum 139 (shown in FIG. 25A). The aluminum casting 139c is a product of higher quality owing to a good distribution of molten metal as poured. The aluminum casting 139c is worked on to make the disk rotor 10 shown in FIG. 1.

Fourth Embodiment

[0177] Description will now be made of the fourth embodiment with reference to FIGS. 26 to 31. The reference numerals used for the third embodiment are used to denote like parts or materials for the fourth embodiment and no repeated description thereof is made.

[0178] FIG. 26 is an overall diagram showing an aluminum casting apparatus for carrying out the aluminum casting process (fourth embodiment) using a casting mold and embodying this invention. The aluminum casting apparatus 180 has a casting apparatus proper 181 having a casting mold 182, an inert gas introducing portion 140 for introducing argon (Ar) gas (inert (rare) gas) into the cavity 187 defined in the casting mold 182, a magnesium introducing portion 150 for introducing gaseous magnesium (Mg) into the cavity 187 into which the inert gas has been introduced, and a nitrogen gas introducing portion 160 for introducing heated nitrogen (N.sub.2) gas into the cavity 187 into which the gaseous magnesium has been introduced. The casting apparatus proper 181 includes a fixed plate 191 secured to a base 190, a stationary mold member 183 is secured to the fixed plate 191, a movable plate 192 is movably mounted on the base 190, a movable mold member 84 is secured to the movable plate 192, a device 193 for moving the movable plate 192 is mounted on the base 190 and a core 185 for the casting mold 182 is supported by the base 190 so as to be capable of being raised and lowered by a raising and lowering device 194. A sprue runner 195 opening to the cavity 187 is formed in the movable mold member 184, a sprue 196 is formed vertically from the sprue runner 195, while a pouring tank 197 holding molten aluminum 139 is situated above the sprue 196, and the casting mold 182 has an opening 198 formed at its top as a vent or feeder head. The stationary and movable mold members 183 and 184 and the core 185 constitute the casting mold 182. While FIG. 26 shows the sprue 196 and the opening 198 as being large relative to the cavity 187 to provide an easier understanding of the casting apparatus proper 181, the real sprue 196 and opening 198 are sufficiently small relative to the cavity 187 to enable the cavity 187 to keep a substantially completely closed state when the casting mold 182 is closed.

[0179] According to the aluminum casting apparatus 180, the movement of the movable plate 192 in the directions of arrows by the moving device 193 enables the movable mold member 184 to move between its mold closing position (position shown in the drawing) and its mold opening position. The movement of the core 185 in the directions of arrows by the raising and lowering device 194 enables the core 185 to move between its mold closing position (position shown in the drawing) and its mold opening position. The movable mold member 184 and the core 185 held in their mold closing positions enable the stationary and movable mold members 183 and 184 and the core 185 to form the cavity 187. If molten aluminum 139 is supplied into the cavity 187, it is possible to mold an aluminum casting in the cavity 187.

[0180] The casting apparatus proper 181 differs from the casting apparatus proper 121 according to the third embodiment in that it is so constructed as to allow molten aluminum 139 to flow into the cavity 187 by its own weight at the atmospheric pressure.

[0181] An example in which the casting process according to the fourth embodiment of this invention is carried out by the aluminum casting apparatus 180 will now be described with reference to FIGS. 18 and 26 to 31. The step ST20 of FIG. 18 will first be explained. The argon valve 143 shown in FIG. 26 is switched to its open position to introduce argon gas from an argon gas bottle 142 into the cavity 187 through an introducing passage 141. FIG. 27 is a diagram for explaining an example in which an argon gas atmosphere is created in the cavity in accordance with the aluminum casting process according to the fourth embodiment of this invention. The argon gas filling the cavity 187 expels air from the cavity 187 through, for example, the sprue 196 or the vent or feeder head opening 198. As a result, an argon gas atmosphere is created in the cavity 187. After an argon gas atmosphere is created in the cavity 187, the argon valve 143 (shown in FIG. 26) is switched to its closed position.

[0182] The step ST21 of FIG. 18 will now be explained. Returning to FIG. 26, a sublimating heater 155 in a sublimating device 153 is placed in operation to heat the inside of a holding case 154 to a specific temperature (for example, at least 400.degree. C.). The heating of the inside of the holding case 154 causes the sublimation of a magnesium ingot 158 into a gaseous form. A magnesium valve 157 is switched to its open position so that argon gas may be introduced from the argon gas bottle 142 into the holding case 154 through a first magnesium introducing passage 151. The introduced argon gas causes gaseous magnesium to be introduced into the cavity 187 through a second magnesium introducing passage 152 and the introducing passage 141. When gaseous magnesium is introduced into the cavity 187, the second magnesium introducing passage 152 and the introducing passage 141 are preferably heated so that no magnesium may be deposited in the second magnesium introducing passage 152 or the introducing passage 141.

[0183] FIG. 28 is a diagram for explaining an example in which magnesium is deposited on the cavity wall in accordance with the aluminum casting process according to the fourth embodiment of this invention. The gaseous magnesium introduced into the cavity 187 as shown by arrows has its temperature lowered to 150 to 250.degree. C. by contacting the wall of the cavity 187. Its drop in temperature to 150 to 250.degree. C. causes gaseous magnesium to be deposited on the wall of the cavity 187. The deposited magnesium is called a magnesium layer 202. After the deposition of the magnesium layer 202 on the wall of the cavity 187, the magnesium valve 157 (shown in FIG. 26) is switched to its closed position.

[0184] Step ST22 of FIG. 18 will now be explained. The heater 164 in the nitrogen gas introducing portion 160 shown in FIG. 26 is heated and the nitrogen valve 163 is switched to its open position. This enables nitrogen gas to flow from the nitrogen gas bottle 162 into the nitrogen introducing passage 161. As a result, the nitrogen gas in the nitrogen gas introducing passage 161 is heated by the heater 164 and the heated nitrogen gas is introduced into the cavity 187 through the nitrogen introducing passage 161. The independent heating of nitrogen gas by the heater 164 makes it possible to heat nitrogen gas flowing in the nitrogen introducing passage 161 efficiently to a specific temperature (for example, at least 400.degree. C.).

[0185] FIG. 29 is a diagram for explaining an example in which magnesium nitride is formed on the cavity wall in accordance with the aluminum casting process according to the fourth embodiment of this invention. The temperature T (.degree. C.) of nitrogen gas (shown in dots) in the cavity 187 and the pressure P (atmosphere) in the cavity 187 are so selected as to maintain their relationship T.gtoreq.(130.times.P+270). If this condition is met, it is possible to have magnesium nitride 203 formed on the surface of the magnesium layer 202 by the reaction of the magnesium layer 202 deposited on the wall of the cavity 187 and the nitrogen gas. More specifically, their relationship T.gtoreq.(130.times.P+270) teaches that when the pressure Pin the cavity 187 is, for example, 1 atmosphere, the temperature T of nitrogen gas in the cavity 187 may be set at 400.degree. C. for forming magnesium nitride 203 on the surface of the magnesium layer 202. As the temperature T of nitrogen gas in the cavity 187 and the third pressure P are relatively easy to determine based on their relationship T.gtoreq.(130.times.P+270), it is possible to perform the adjustment of equipment within a short time. Moreover, nitrogen gas is heated and heated nitrogen gas is used for forming magnesium nitride 203. This makes it possible to form magnesium nitride 203 efficiently, as it is possible to heat nitrogen gas to a temperature at which magnesium nitride 203 is easy to form. The nitrogen valve 163 (shown in FIG. 26) is switched to its closed position after magnesium nitride 203 has been formed on the surface of the magnesium layer 202.

[0186] For the formation of magnesium nitride 203, the magnesium layer 202 is first formed by magnesium deposited on the wall of the cavity 187 and then, nitrogen gas is introduced into the cavity 187 to form magnesium nitride 203 on the surface of the magnesium layer 202, as shown in FIGS. 28 and 29. Accordingly, it is possible to form magnesium nitride 203 on only the surface of the magnesium layer 202 and thereby shorten the time required for forming magnesium nitride 203. Moreover, it is possible to reduce the amount of nitrogen gas used, since it is sufficient to form magnesium nitride 203 on only the surface of the magnesium layer 202.

[0187] Step ST23 of FIG. 18 will now be explained. FIGS. 30A and 30B are diagrams for explaining an example in which molten aluminum is supplied into the cavity in accordance with the aluminum casting process according to the fourth embodiment of this invention. Referring to FIG. 30A, the pouring tank 197 in the casting apparatus proper 181 is tilted to supply molten aluminum 139 from the pouring tank 197 into the cavi