JP2809017B2 - Differential pressure casting method and aluminum alloy wheel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a differential pressure casting method.
In particular, it relates to a differential pressure casting method capable of improving the quality of a cast product.

[0002]

2. Description of the Related Art When casting a casting from a material that is easily oxidized such as aluminum by a low-pressure casting method or the like, it is necessary to prevent oxidation of molten aluminum in the stalk or to remove aluminum oxide in the stalk. Conventionally, low-pressure aluminum casting has been generally performed by removing a metal oxide by arranging a metal net in a lower mold gate. Although the above conventional method can remove large oxides,
The fine oxide passes through the wire mesh and enters the cavity,
In addition, it comes into contact with air in the cavity and is newly oxidized. Further, in this method, it is necessary to replace the wire mesh each time casting is performed, and since the wire mesh is attached to the scrap material cut off from the sprue portion of the product, the scrap material cannot be effectively used. There was a point.

As means for eliminating the oxidation of molten metal, for example, JP-A-48-49625 and JP-A-3-71964 disclose a differential pressure for supplying an inert gas into a cavity before pouring it into the cavity. Casting methods have been proposed. According to this method, without arranging a wire mesh in the lower mold gate,
The oxidation of the molten metal can be prevented. Also, Japanese Patent Laid-Open No. 1-1
No. 04457 discloses an inner cylinder in which the upper end is connected to the gate of the mold and the lower end is kept higher than the level of the molten metal in the holding furnace, and the upper end is sealed between the outer surface of the inner cylinder and the lower end is molten. Using a differential pressure casting stalk having an outer cylinder immersed and having a hole communicating between an annular area formed between the inner and outer cylinders and the outside, an inert gas heavier than air is supplied from the hole. After replacing the air in the molten metal surface and cavity with an inert gas,
A method for introducing a molten metal into a cavity by a differential pressure is disclosed.

[0004]

In recent years, lightening of automobile parts has been promoted from the viewpoint of energy saving, and aluminum wheels have come to be used for disc wheels of automobiles instead of conventional steel wheels. However, the demand for weight reduction has become even more severe, and it has become necessary to consider reducing the thickness of aluminum wheels. Here, simply reducing the thickness of the aluminum wheel will result in insufficient mechanical strength, and therefore, improving the mechanical strength is an essential requirement for reducing the thickness. JP-A-48
According to the differential pressure casting method of supplying an inert gas into a cavity before pouring as proposed in JP-A-49625, JP-A-3-71964, etc., the amount of oxides in the casting is reduced. Can be improved in mechanical strength. However, this is not enough to cope with further thinning.

Further, in the technique disclosed in Japanese Patent Application Laid-Open No. 1-104457, there is a problem that oxidation of the molten metal is still unavoidable because air exists on the surface of the molten metal before the supply of the inert gas. That is, this publication discloses that when the pressure difference between the molten metal and the cavity is released, the unsolidified molten metal descends and returns into the holding furnace, and the surface of the molten metal cannot be vacuum at that time. There is no other way than air exists. Therefore, at this point, even if an inert gas heavier than the air is supplied immediately to replace the air on the surface of the molten metal with the inert gas, a certain time is required for the replacement due to the difference in specific gravity. Is not exposed to the air and cannot escape a certain degree of oxidation of the molten metal. Even if the pressure difference between the molten metal and the cavity is released, there is molten metal that has not completely solidified at the upper part of the gate, and this seals the gate, so that the unsolidified molten metal immediately Does not descend. Therefore, while the molten metal does not descend, the cooling of the casting in the vicinity of the sprue is delayed, which not only impairs the production efficiency but also hinders the miniaturization of the metal structure.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a differential pressure casting method capable of improving mechanical strength corresponding to weight reduction of aluminum wheels and other castings. Another object of the present invention is to provide an efficient differential pressure casting method for replacing the air on the surface of a molten metal with an inert gas and always covering the surface of the molten metal with an inert gas to prevent oxidation of the molten metal. That is. Still another object of the present invention is to provide an aluminum alloy wheel having high mechanical strength.

[0007]

The inventor of the present invention has conducted various studies on a differential pressure casting method for supplying an inert gas into a cavity before pouring it into a cavity. It has been found that the flow of hot water in the inside can be improved. The first invention has been made by paying attention to the improvement of the flowability of the molten metal, in which a cavity is formed by a mold that can be opened and closed, and a sprue communicating with the cavity is opened at a lower portion of the mold. The upper end of the stalk is connected to the gate, the lower end of the stalk is immersed in a molten metal of an active metal or an active metal alloy in a holding furnace, and the molten metal is pressurized or the cavity is depressurized to reduce the molten metal in the cavity. In the differential pressure casting method in which the molten metal in the cavity is solidified and cast into a casting, the stalk and the interior of the cavity are maintained in an inert gas atmosphere before the molten metal is poured, and the molten metal is introduced into the cavity. The differential pressure application speed at the time of introduction is 0.001 to 0.006 kgf / c.
A differential pressure casting method characterized by casting at m ² / s.

The second invention is also directed to improving the flowability of molten metal, and maintains the stalk and the interior of the cavity in an inert gas atmosphere before pouring the molten metal to reduce the degree of superheat to 50 degrees.
When pouring a molten metal at ~ 80 ° C, or when targeting aluminum or an aluminum alloy, the stalk and the inside of the cavity are kept in an inert gas atmosphere before pouring the molten metal, and the temperature of the mold is set at 350-430. This is a differential pressure casting method characterized in that casting is performed while the temperature is maintained at ° C.

According to a third aspect of the present invention, a cavity is formed by a mold that can be opened and closed, a sprue communicating with the cavity is opened at a lower portion of the mold, and an upper end of a stalk is connected to the sprue. Is immersed in the molten metal in a holding furnace, and the pressure of the molten metal is raised above the pressure of the cavity (regardless of whether the molten metal is pressurized or the pressure of the cavity is reduced), and the molten metal is cast into the cavity. Introduced into the tee,
In a differential pressure casting method in which the molten metal in the cavity is solidified and cast into a casting, and the mold is opened to take out the casting.
By supplying an inert gas into the Stoke, the casting and the molten metal are separated from each other, and the level of the molten metal in the Stoke separated from the casting is kept higher than the level of the molten metal in the holding furnace. It is characterized by cooling and removing castings.

According to a fourth aspect of the present invention, there is provided an aluminum alloy wheel obtained by the above differential pressure casting method, wherein (1) mechanical strength which satisfies the following conditions (a) and (b), and (a) Tensile strength of rear flange (σ
_BR ), tensile strength of the front flange (σ _BF ) and spoke tensile strength (σ _BS ) are all 24K
g / mm ² or more, and σ _BR > σ _BF > σ _BS , and (b) elongation of the rear flange (ε _R ), elongation of the front flange (ε _F ), and elongation of the spokes (ε _S )
Is 3% or more, and ε _F > ε _R > ε _S , and (2) the secondary arm spacing is 2 mm or less, thereby achieving the above object. Here, the secondary arm spacing is desirably 0.8 mm or less.

[0011]

By the way, as is well known, the mechanical strength of a metal material is improved by making its structure finer. In addition, when the speed of pouring the molten metal is reduced, directional solidification remarkably occurs, and the structure can be refined. However, according to the study of the present inventor, when casting is performed in the atmosphere, an oxide film is formed at the tip of the molten metal that enters the cavity, and the flow of the molten metal is hindered by the action of surface tension due to the oxide film. It is thought that there is. The portion where the cavity is thin has a remarkable deterioration in molten metal flowability due to the oxide film as compared with other portions, and in this case, it is necessary to cast at a molten metal speed capable of securing the molten metal flowability. That is, in casting in the atmosphere, the flow rate of the molten metal cannot be sufficiently reduced due to low flowability of the molten metal. On the other hand, the first invention uses the effect of improving the flowability of molten metal to reduce the speed of molten metal as compared with the prior art, that is, to reduce the speed of increasing the differential pressure when introducing molten metal into the cavity. Has features. In other words, the use of an inert gas in the casting atmosphere suppresses the formation of an oxide film at the tip of the molten metal to improve the flowability of the molten metal. Therefore, the speed of the molten metal can be reduced as compared with the conventional method. Therefore, the structure of the cast product is refined, and the mechanical strength is improved. In the first invention, the differential pressure application speed is 0.001 to
0.006 kgf / cm ² / s is defined as 0.006 kgf / cm ² / s.
If it exceeds Kgf / cm ² / s, it is not possible to obtain a sufficient strength improvement by refining the structure.
If it is less than f / cm ² / s, there is a possibility that a poor running of the molten metal may occur due to a substantial decrease in the molten metal flowability.

As described above, the mechanical strength of a metal material can be improved by making its structure finer, but the structure can also be made finer by increasing the solidification rate of the molten metal. However, when casting is performed in the atmosphere, an oxide film is formed on the tip of the molten metal that enters the cavity as described above, so that the flowability of the molten metal is low. As a result, the temperature of the molten metal or the mold temperature is sufficiently reduced. Could not be reduced. Specifically, the degree of superheat of the molten metal is 100 to 150 ° C., and the mold temperature in the case of aluminum or aluminum alloy is 480 to 500.
° C. On the other hand, the second aspect of the invention is characterized in that the degree of superheat or the mold temperature is reduced as compared with the prior art by utilizing the effect of improving the flowability of the molten metal. That is, since the casting atmosphere is made of an inert gas to suppress the formation of an oxide film at the tip of the molten metal and improve the flowability of the molten metal, it is possible to lower the temperature of the molten metal or the temperature of the mold as compared with the prior art. Therefore, the structure of the cast product is refined, and the mechanical strength is also improved. In the second invention, the degree of superheat is 50 to
The reason why the temperature is set to 80 ° C. is that if the temperature is higher than 80 ° C., sufficient strength cannot be improved due to the refinement of the structure. Because there is. The degree of superheating means a difference between the temperature of the molten metal and the melting point of the metal or alloy. When aluminum or aluminum alloy is used in the second invention, the mold temperature is set to 35 for the same reason as described above.
0 to 430 ° C.

According to the casting method of the third invention, since the inert gas is supplied into the stalk at the appropriate time after the molten metal is introduced into the cavity, the inert gas enters the boundary between the solidified portion and the unsolidified portion. To separate them. Therefore, even if the mold is opened later, the surface of the molten metal is covered with the inert gas, and oxidation of the molten metal is prevented. Further, the molten metal can be quickly lowered, and the cooling in the vicinity of the gate is promoted, so that the casting cycle can be shortened. In addition, the molten metal separated by the inert gas is lowered only to a position higher than the surface of the molten metal in the holding furnace. In other words, the rising and falling width of the molten metal is reduced. And shorten the casting cycle. In order to keep the level of the molten metal in the stalk separated from the cast product higher than the level of the molten metal in the holding furnace, a predetermined pressure may be applied to the molten metal in the holding furnace. When this pressure is applied, the solidified portion and the non-solidified portion of the molten metal are difficult to be separated from each other. Therefore, there is a great need to allow an inert gas to enter and separate. Further, if the supply of the inert gas into the stalk for separating the casting from the molten metal is performed after the pressure of the molten metal is reduced after the molten metal is introduced into the cavity, the gas can be supplied at a low pressure.

According to the casting method of the present invention, an aluminum wheel having a high mechanical strength and a fine structure can be obtained. In particular, as in the fourth invention, the tensile strength of each part is 24K.
g / mm ² or more, and high elongation, and a high-strength integrated aluminum wheel having an extremely fine structure with a secondary arm spacing of 2 mm or less can be obtained. These aluminum wheels not only satisfy security standards, but also have high performance and high safety.

[0015]

Embodiment 1 The present invention will be described with reference to the drawings. FIG. 1 shows an example of a low pressure casting machine to which the method of the present invention is applied. A cavity 1 conforming to the shape of a casting to be cast is formed by a mold 2. The mold 2 is formed by an upper mold 2a, a lower mold 2b, and a horizontal mold 2c, and a gate 5 communicating with the cavity 1 is opened in the lower mold 2b. Stoke 10
Has an inner cylinder 11 and an outer cylinder 12. The upper end of the inner cylinder 11 is connected to the gate 5 of the mold, and the lower end is kept higher than the surface 8 of the molten metal 7 in the holding furnace 6. . The upper end of the outer cylinder 12 is hermetically sealed with the outer surface of the inner cylinder 11, and the lower end is immersed in the molten metal 7, thus forming an annular area 13 opened downward between the inner and outer cylinders 11 and 12. Have been. Outer cylinder 12
Is formed with a hole 14 for communicating the upper part of the annular region 13 with the outer surface of the outer cylinder. The hole 14 is connected to an argon gas cylinder 20 via a shutoff valve 21 and a check valve 22.

In order to cast a casting using this low-pressure casting machine, first, the shut-off valve 21 is opened, and the air in the stalk 10 is replaced with argon gas through the hole 14. Stoke 10
Until the air inside was replaced with argon gas, the molten metal surface 8 of the molten metal was exposed to air, so the molten metal 7 near the molten metal surface 8 was oxidized. When the air in the stalk 10 is replaced with argon gas and the shutoff valve 21 is closed to pressurize the holding furnace 6, the molten metal surface 8 rises and contacts the lower end of the inner cylinder 11, and thereafter the molten metal 7 is separated into two. As shown in FIG. 2, one enters the cavity 1 while excluding the argon gas, and the other enters the annular region 13 while compressing the argon gas 23. The volume of the space from the check valve 22 to the lower end of the annular region 13 is determined so that the molten metal surface 8 in the annular region 13 does not rise higher than the opening on the inner surface side of the hole 14, and the hole 14 and the piping connected thereto are determined. Is preferably protected from the molten metal 7. After a certain period of time has passed after the cavity 7 has been filled with the molten metal 7, the molten metal 7 solidifies and becomes a cast product. However, since the first supply was the oxidized molten material, the first cast product was regarded as a defective product. Become.

After the first casting has solidified, the shut-off valve 21
Is opened to supply the argon gas 23 into the annular region 13, and as shown in FIG.
The molten metal surface 8 in the inner cylinder 11 is gradually pushed down, and finally moves from the annular region 13 into the inner cylinder 11 and rises in the molten metal in the inner cylinder 11,
It accumulates between the casting 9 and the molten metal surface 8 to separate them. That is, the unsolidified molten metal 7 is a casting 9
After being in contact with the casting 9, it is separated from the casting 9 by the argon gas 23 to form a free surface. Therefore, whenever the molten metal surface 8 is formed, the molten metal surface 8 is covered with the argon gas 23. After the cast product 9 and the molten metal surface 8 are separated by the argon gas 23, and after the pressurization of the holding furnace 6 is released, the mold 2 is opened to discharge the first cast product, that is, a defective product, and the mold 2 is discharged. Close. Thereafter, the shutoff valve 21 is closed, and the holding furnace 6 is pressurized to cast a second casting. 2
Since the molten metal 7 for the individual casting was completely shut off from the air, only good products can be cast thereafter.

FIG. 4 shows an aluminum wheel for automobile (JIS AC4C) cast by the casting method of the first invention. As shown in FIG. 4, the ASTM, ASTM, B55
7, and a test piece according to E8 was taken to measure the mechanical strength. FIG. 5 shows the measurement results, in which the temperature of the mold was set to 480 ° C.
And the temperature of the hot water was kept at 710 ° C., and the measurement results of the mechanical properties when the pressurization rate, that is, the rate of increase of the pressure when the molten metal was introduced into the cavity, were changed. As is clear from the figure, when the pressing speed decreases, both the tensile strength and the elongation increase, and particularly, the pressing speed is reduced to 0.006 kgf / cm ² /
It has been found that the mechanical strength of the cast product can be significantly improved by setting it to s or less. It is also noted that the strength improvement effect at the spoke position S is remarkable. However, when the pressurization rate is less than 0.001Kgf / cm ² / s, because it may cause hot water around bad, it is preferably kept in 0.001~0.006Kgf / cm ² / s and eventually pressurization rate I understand. In this example, the casting of aluminum was performed, but other than Mg, Zn, Ni,
The same applies to activated metals such as Ti and Cu or alloys thereof.

FIG. 6 and FIG. 7 show the results of the dye flaw inspection performed on the aluminum wheel for automobiles, and FIG. 6 shows the results obtained by casting under an argon gas atmosphere at a pressing speed of 0.004 kgf / cm ² / s. FIG. 7 shows a case of casting in a conventional example at a pressurization rate of 0.009 kgf / cm ² / s and in an air atmosphere. As is clear from the comparison between the two figures,
In the conventional casting method, casting defects A and burrs B are scattered, but in the present embodiment, the casting defects are suppressed because the turbulence of the molten metal filling the cavity 1 is reduced, and the molten metal is Since the impact pressure when the dynamic pressure of the molten metal changes to the static pressure upon filling is suppressed, it is understood that the occurrence of burrs is also suppressed.

FIGS. 8 and 9 are micrographs of the sample taken at the spoke position S. FIG.
Kgf / cm ² / s, in the case of casting in an argon gas atmosphere. FIG. 9 shows a conventional pressurization rate of 0.009 kg.
f / cm ² / s, in the case of casting under air atmosphere. As is apparent from a comparison between the two figures, it is clear that the directional solidification is apt to occur in the casting method of the present example, and as a result, the metal structure is significantly densified.

[0021]

Embodiment 2 With respect to an aluminum wheel for automobile (JIS AC4C) cast by the casting method of the second invention, ASTM, B557, E are obtained from the spoke position S, front flange position F, and rear flange position R shown in FIG.
A test piece according to No. 8 was taken and the mechanical strength was measured. 10 and 11 show the results of this measurement. First, FIG. 10 shows that the temperature of the mold was kept at 480 ° C. and the pressing speed was 0.009 kg.
The measurement results of mechanical properties when the superheat degree of the molten metal was changed to f / cm ² / s are shown. As can be seen from the figure, when the degree of superheat decreases, both the tensile strength and the elongation increase, and it is found that the mechanical strength of the cast product can be significantly improved by setting the degree of superheat to 80 ° C. or less. It is also noted that the strength improvement effect at the spoke position S is remarkable. However, if the degree of superheat of the molten metal is less than 50 ° C., there is a possibility of causing poor running of the molten metal. Therefore, it has been found that it is preferable to maintain the degree of superheat of the molten metal at 50 to 80 ° C. In this example, the casting of an aluminum alloy was performed.
In addition, the same effect can be obtained for activated metals such as Mg, Zn, Ni, Ti, and Cu or alloys thereof.

FIG. 11 shows the measurement results of the mechanical properties when the temperature of the hot water was kept at 710 ° C., the pressurizing speed was 0.009 kgf / cm ² / s, and the mold temperature was changed. As is clear from the figure, when the mold temperature decreases, both the tensile strength and the elongation increase, and it was found that the mechanical strength of the cast product can be significantly improved by setting the mold temperature to 430 ° C. or less. However, if the mold temperature is lower than 350 ° C., there is a risk of causing poor running of the hot water. Therefore, it was found that it is preferable to maintain the mold temperature at 350 to 430 ° C.

FIGS. 12 and 13 are photomicrographs of the sample taken at the spoke position S. FIG.
FIG. 13 shows a case of casting in a conventional atmosphere at a mold temperature of 480 ° C. in an atmosphere of air. As is clear from the comparison between the two figures, it can be seen that the metal structure is significantly densified by the casting method of this example.

[0024]

Embodiment 3 The details of the third invention will be described with reference to the drawings.
You. In FIG. 1, first, the shutoff valve 21 is opened and the stalk 10
The air in the cavity and cavity 1 with argon gas 23
Replace and pressurize the holding furnace 6 further to increase the level of
H The initial value h₀Higher than the level of the molten metal in the holding furnace 6
And the height h of the lower end of the inner cylinder 11₁Slightly lower than
One. Argon gas pressure P in Stoke₁Initial value P of
_TenIs the pressure drop of the argon gas flow passing through the cavity.
Loss ΔP_COnly atmospheric pressure P₀Higher than P_Ten= P₀+ ΔP_C It has become. The pressure P of the argon gas cylinder 20_Ar
Flows from the argon gas cylinder 20 to the Stoke 10
Pressure drop ΔP of the flowing argon gas stream_ArOnly P_TenHigher than
, P_Ar= P_Ten+ ΔP_Ar It has become. However, differential pressure ΔP_C, ΔP_ArAre both small.
The pressure P of the holding furnace_TwoInitial value P of₂₀Is the height h₀Molten metal
P by the weight of_TenHigher than P₂₀= P_Ten+ Ρgh₀(Ρ is the density of aluminum, g is
(Gravitational acceleration). On the other hand, the air in Stoke 10 is
Until it is replaced by the metal 23, the surface 8 of the molten metal in the Stoke
Since it was exposed to air, the molten metal 7 near the molten metal surface 8 was oxidized.
Have been.

[0025] After the air in the stalk 10 and the cavity 1 is filled with argon 23 and further pressurizing than originally pressure P ₂₀ the holding furnace 6 as shown in FIG. 14,
The molten metal surface 8 further rises from the initial height h ₀ , and when the pressure P _{21 of the} holding furnace becomes P ₂₁ = P ₁₀ + ρgh ₁ , the molten metal surface 8 contacts the lower end of the inner cylinder 11. Thereafter, the molten metal 7 is divided into two parts, one of which penetrates into the cavity 1 while eliminating the argon gas 23 and the other compresses the argon gas 23 and forms the annular region 13 as shown in FIG.
Invade. The latter, that is, the molten metal 7 that is going to enter the annular region 13 is initially prevented from entering by the argon gas supplied from the argon gas cylinder 20,
Holding furnace further increases the pressure P ₂₂ of, when the pressure P ₁ = P ₂₂ -ρgh ₁ of argon gas resulting in stalk becomes more rises and P ₁ = P _Ar, ie, P ₂₂ = P _Ar When the pressure becomes + ρgh ₁ , the flow of the argon gas 23 from the argon gas cylinder 20 to the stalk 10 stops, and the check valve 22 closes.

When the pressure P ₂ of the holding furnace 6 is further increased, the argon gas 23 confined on the annular region 13 side of the check valve 22 is compressed so that the product of the absolute pressure and the volume becomes constant. When the pressure P _{2 of the} holding furnace reaches the maximum pressure P ₂₃ , the pressure P ₁ of the argon gas in the stalk becomes P ₁ = P ₂₃ −ρgh ₂ , that is, the level of the molten metal in the annular region 13 is up to a height h _2. This maximum height h ₂ is the hole 1
The volume of the space from the check valve 22 to the lower end of the annular region 13 is determined so as not to be higher than the opening on the inner surface side. On the other hand, when a certain period of time has passed after the cavity 7 has been filled with the molten metal 7, the molten metal 7 solidifies to form a cast product 9. Since the first supplied was the oxidized molten metal,
The first casting is rejected.

Even if the pressure of the holding furnace 6 is released after the first casting 9 has solidified, the contact portion between the casting 9 and the molten metal 7 is sealed, so that the molten metal 7 in the stalk 10 is immediately Instead of dropping, only the pressure of the molten metal 7 and the pressure P _{1 of the} argon gas trapped in the annular region 13 decrease. As a result, the argon gas 23 confined in the annular region expands in a process reverse to that of the compression, and therefore, when P ₁ = P _Ar , the surface level h in the stalk decreases to h ₁ , that is, When the pressure P ₂₂ of the holding furnace 6 becomes P ₂₂ = P _Ar + ρgh ₁ , the surface of the molten metal in the annular region reaches the lower end of the inner cylinder 11.

When the pressure P _{2 of the} holding furnace is further reduced, and as a result, the pressure P ₁ of the argon gas in the stalk attempts to _satisfy P ₁ <P _Ar , the check valve 22 is opened, and the pressure of the argon gas cylinder 20 changes to the stalk 10. The argon gas 23 flows toward it, and the pressure P ₁ of the argon gas in the stalk eventually becomes P ₁ = P
_An argon gas flow is generated while being kept at _Ar . As a result, as shown in FIG. 3, the argon gas 23 moves from the annular region 13 into the inner cylinder 11, rises in the molten metal in the inner cylinder 11, and collects between the casting 9 and the molten metal surface 8. Is separated. That is, when the unsolidified molten metal 7 is in contact with the casting 9 which has been the molten metal, it is separated from the casting 9 by the argon gas 23 to form a free surface. The molten metal surface 8 is always covered with the argon gas 23.

The pressure P ₁ of the argon gas in the Stoke is changed to P ₁
= P _Ar while the argon gas 23 is being supplied into the stalk, the pressure P _{2 of the} holding furnace is still maintained at the initial pressure P _20.
, The upper portion of the stalk is finally filled with argon gas 23, the flow of the argon gas from the argon gas cylinder to the stalk stops, and the check valve 22 closes. Molten metal surface height h ₃ of the stalk in that state, the height given by _{P 20 = P Ar + ρgh 3} , that is, _{h 3 = (P 20 -P Ar} ) / ρg. Note that the initial bath height h ₀ in the Stoke = (P ₂₀
-P ₁₀₎ / ρg and h ₃ and the difference h ₀ -h ₃ of is _{h 0 -h 3 = ΔP Ar /} ρg, namely by the amount of flow of argon gas reaching the stalk from the argon gas cylinder is stopped The bath surface height h slightly decreases.

After the casting 9 and the molten metal surface 8 are separated by the argon gas 23 and the casting 9 is cooled, the mold 2 is opened to discharge the first casting, that is, a defective product.
When the casting 9 is discharged, the pressure of the molten metal in the Stoke becomes almost the atmospheric pressure P ₀ , so that the height of the molten metal in the Stoke h
₄ rises to h ₄ = (P ₂₀ −P ₀ ) / ρg. the difference between h ₄ and h ₃ is only h ₄ -h ₃ = because it is _{(ΔP C + ΔP Ar) /} ρg. Then closing the mold 2, molten metal surface height h in the stalk leads to original value h ₀ is lowered. h ₄
The difference between the h ₀ is only because it is _{h 4 -h 0 = ΔP C /} ρg. Thereafter, the air in the cavity 1 is replaced with argon gas, and the holding furnace 6 is pressurized to cast a second casting. Since the melt 7 for the second casting is completely shut off from the air and the cavity 1 is filled with argon gas, only good products can be cast thereafter.

As described above, the casting method of this embodiment separates the casting 9 from the molten metal 7 by supplying the argon gas 23 into the stalk 10,
Is always covered with argon gas 23, the oxidation of the molten metal 7 is completely prevented, and the cooling of the casting 9 is performed in a state separated from the molten metal 7, so that the casting is cooled early and the casting cycle is shortened. Can be planned. Further, since the rise and fall width of the molten metal is small, the molten metal in the holding furnace is not agitated. In this respect, the molten metal is not oxidized and the casting cycle can be shortened.

FIGS. 5 and 9 show that the aluminum wheel of the present invention has mechanical strength satisfying the above-mentioned conditions (a) and (b). (The tensile strength of a normal aluminum wheel is about 21-22 kg /
mm ² . 8 and 12, the aluminum wheel of the present invention
It can be seen that there is an organization that satisfies the above conditions.
(Note that the secondary arm spacing of a normal aluminum wheel is 2 to 3 mm.) In each of the above embodiments, the pressure casting method has been described, but the present invention can be similarly applied to a reduced pressure casting method.

[0033]

【The invention's effect】

(1) The first invention is a casting method in which casting is performed under an inert gas atmosphere at a low differential pressure application rate, so that the metal structure can be remarkably densified, and therefore, the mechanical strength of the cast product can be reduced. As a result, the weight of the cast product can be reduced, and casting defects and burrs can be reduced. (2) The second invention is a casting method for casting under a low hot water temperature or a low mold temperature in an inert gas atmosphere.
The metal structure can be remarkably densified, and therefore, the mechanical strength of the cast product can be improved. As a result, the weight of the cast product can be reduced. Further, since the width of the temperature change of the mold is reduced, the amount of expansion and contraction is also reduced, so that the burr of the cast product is reduced, and the life of the mold can be extended. (3) According to the third aspect of the invention, the oxidation of the molten metal in the stalk and the stirring of the molten metal in the holding furnace can be prevented, so that the quality of the cast product can be improved and the casting cycle can be shortened. it can.

[Brief description of the drawings]

FIG. 1 is a schematic longitudinal sectional view of an example of a low pressure casting machine to which the method of the present invention is applied.

FIG. 2 corresponds to FIG. 1 when the holding furnace is pressurized.

FIG. 3 is a view corresponding to FIG. 1 when a cast product and a molten metal are separated by introducing argon gas.

FIG. 4 is a diagram showing a sampling position of a test piece from an aluminum wheel for a vehicle.

FIG. 5 is a diagram showing a change in mechanical properties of a casting with respect to a differential pressure application speed.

FIG. 6 is a schematic front view of a left half section and a right half section showing an example of a result of dyeing inspection of a cast product by the method of the present invention.

FIG. 7 is a view corresponding to FIG. 6 according to a conventional casting method.

FIG. 8 is a photograph of a metal structure of a test piece from an aluminum wheel for automobiles.

FIG. 9 is a photograph of a metal structure corresponding to FIG. 8 by a conventional casting method.

FIG. 10 is a diagram showing a change in mechanical properties of a cast product with respect to a degree of superheating of a molten metal.

FIG. 11 is a diagram showing a change in mechanical properties of a cast product with respect to mold temperature.

FIG. 12 is a photograph of a metal structure of a test piece from an aluminum wheel for automobiles.

FIG. 13 is a photograph of a metal structure corresponding to FIG. 12 by a conventional casting method.

FIG. 14 is a diagram showing the timing of pressurizing the holding furnace and supplying / stopping argon gas.

[Explanation of symbols]

1. Cavity 2. Mold 2
a: upper mold 2b: lower mold 2c: horizontal mold 5
... Gate 6 ... Holding furnace 7 ... Metal melt 8
… Fluid surface 9… Casting product 10… Stoke 1
DESCRIPTION OF SYMBOLS 1 ... Inner cylinder 12 ... Outer cylinder 13 ... Annular area 1
4 ... hole 20 ... argon gas cylinder 21 ... shut-off valve 2
2: Check valve 23: Argon gas A: Casting defect B
... Burr S ... Spoke F ... Front flange R
… Rear flange

Continuation of front page (56) References JP-A-5-131261 (JP, A) JP-A-4-200852 (JP, A) JP-A-5-123851 (JP, A) JP-A-5-96356 (JP) , A) JP-A-53-103930 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) B22D 18/04 B22D 18/06 B22D 18/08

Claims (6)

(57) [Claims] 1. A cavity is formed by a mold that can be opened and closed, a sprue communicating with the cavity is opened at a lower part of the mold, an upper end of a stalk is connected to the sprue, and a lower end of the stalk is held. Immerse in the molten metal of the activated metal or activated metal alloy in the furnace, pressurize the molten metal or depressurize the cavity, introduce the molten metal into the cavity, solidify the molten metal in the cavity In a differential pressure casting method for casting into a casting, the inside of the stalk and the inside of the cavity are maintained in an inert gas atmosphere, and the rate of applying a differential pressure when introducing the molten metal into the cavity is 0.001 to 0.006 kgf /. cm ² /
s and casting. 2. A cavity is formed by a mold that can be opened and closed, a sprue communicating with the cavity is opened at a lower part of the mold, an upper end of a stalk is connected to the sprue, and a lower end of the stalk is held. Immerse in the molten metal of the activated metal or activated metal alloy in the furnace, pressurize the molten metal or depressurize the cavity, introduce the molten metal into the cavity, solidify the molten metal in the cavity A differential pressure casting method for casting into a cast product, wherein the stalk and the cavity are maintained in an inert gas atmosphere, and the molten metal is cast at a superheat degree of 50 to 80 ° C. Method. 3. A cavity is formed by a mold that can be opened and closed, a sprue communicating with the cavity is opened at a lower portion of the mold, an upper end of a stalk is connected to the sprue, and a lower end of the stalk is held. Immerse in the molten aluminum or aluminum alloy in the furnace, pressurize the molten metal or depressurize the cavity and introduce the molten metal into the cavity,
In a differential pressure casting method for solidifying a molten metal in a cavity and casting it into a casting, casting is performed by maintaining the inside of the stalk and the inside of the cavity in an inert gas atmosphere and maintaining the temperature of the mold at 350 to 430 ° C. A differential pressure casting method. 4. A cavity is formed by a mold that can be opened and closed, a sprue communicating with the cavity is opened at a lower part of the mold, an upper end of a stalk is connected to the sprue, and a lower end of the stalk is held. Immersing the molten metal in a furnace, pressurizing the molten metal or depressurizing the cavity, introducing the molten metal into the cavity, solidifying the molten metal in the cavity and casting it into a casting, In the differential pressure casting method of extracting a casting by opening the casting, an inert gas is supplied into the stalk to separate the casting from the molten metal, and a surface level of the molten metal in the stalk separated from the casting. A differential pressure casting method characterized in that the casting is cooled and taken out while maintaining the temperature higher than the level of the molten metal in the holding furnace. 5. The differential pressure casting method according to claim 4, wherein the supply of the inert gas into the stalk for separating the casting from the molten metal is performed after the pressure of the molten metal is reduced. 6. An aluminum alloy wheel obtained by the differential pressure casting method according to any one of claims 1 to 5, wherein (1) mechanical strength satisfying the following conditions (a) and (b): (A) The tensile strength of the rear flange (σ _BR ), the tensile strength of the front flange (σ _BF ), and the tensile strength of the spoke (σ _BS ) are all 24 kg / mm ² or more. And σ _BR > σ _BF > σ _BS , and (b) elongation of the rear flange portion (ε _R ), extension of the front flange portion (ε _F ), and extension of the spoke portion (ε _S ) are 3
%, And ε _F > ε _R > ε _S , and (2) a secondary arm spacing of 2 mm or less.