[0001] The present invention relates to an aluminum alloy for high pressure die-casting
(hereinafter simply referred to as "Al alloy"), a subordinate frame for an automobile,
and a high pressure die-casting method, and more particularly, to the Al alloy for
die casting available for high pressure die casting without thermal seizure or soldering
of the alloy to a metal mold, and to the subordinate frame for the automobile produced
by the high pressure die-casting with the Al alloy, and to the high pressure die-casting
method using the Al alloy.
[0002] Relatively high strength and elongation are required in an Al alloy used as a raw
material of a subordinate frame available for a wheel suspension member and a structural
member of an automobile. Further, such material should provide corrosion resistance
at low temperature because the subordinate frame is in the corrosive atmosphere at
a low temperature. Conventionally, JIS-AC4CH alloy is recognized as a material capable
of meeting with such requirements and is used as the casting material for the subordinate
frame. Further, casting defect should preferably be minimized for casting the wheel
suspension member as the subordinate frame. To this effect, a low speed injection
casting method such as gravity die-casting, low pressure die-casting and squeeze die-casting
has been employed. Furthermore, the cast product was subjected to T6 treatment in
order to provide a predetermined mechanical property of the subordinate frame.
[0003] However, because of the requirement of light weight product for the reduction of
fuel consumption, a further reduction in thickness of the subordinate frame has been
required. High pressure die-casting method is the uppermost method for producing a
thin product. However, AC4CH alloy is not available for the high pressure die-casting
method because thermal seizure of the alloy to a metal mold may occur due to insufficient
content of Fe. On the other hand, the subordinate frame produced by gravity die casting
must be subjected to T6 treatment after casting, which lowers productivity.
[0004] Japanese patent publication No.Sho-59-43539 discloses a high toughness alloy available
for high pressure die-casting. However, this alloy is not preferable as material of
the subordinate frame used under corrosive atmosphere because of its extremely high
Fe content such as from 0.2 to 0.4 mass %.
[0005] It is therefore, an object of the present invention to provide an Al alloy for die-casting
capable of being used in the high pressure die-casting method, and capable of providing
a product having a sufficiently high strength and elongation without T6 treatment
after casting and endurable under a corrosive atmosphere, and to provide a subordinate
frame produced through the high pressure die-casting method while using the Al alloy,
and to provide a high pressure die-casting method for producing such product.
[0006] In order to attain the above object, the present invention provides an aluminum alloy
for high pressure die-casting containing from 8.0 to 9.0 mass % of Si, from 0.35 to
0.45 mass % of Mg, from 0.3 to 0.4 mass % of Mn, from 0.002 to 0.008 mass % of Be,
less than 0.20 mass % of Fe, not more than 0.2 mass % of Cu, not more than 0.1 mass
% of Zn, not more than 0.1 mass % of Ni, not more than 0.1 mass % of Sn, and remainders
of Al and inevitable impurities.
[0007] With this arrangement, high pressure die-casting can be performed with the Al alloy
in spite of the fact that the Al alloy provides the composition similar to that of
AC4CH alloy unavailable for high pressure die-casting. Accordingly, resultant cast
product provides high strength and high elongation those being required for a cast
product used under the severe working condition such as a subordinate frame of an
automobile in spite of elimination of T6 treatment after casting, and a thin product
can result with high productivity at low cost.
[0008] In another aspect of the invention, there is provided a subordinate frame for an
automobile produced through high pressure die-casting method, the subordinate frame
being made from an aluminum alloy containing from 8.0 to 9.0 mass % of Si, from 0.35
to 0.45 mass % of Mg, from 0.3 to 0.4 mass % of Mn, from 0.002 to 0.008 mass % of
Be, less than 0.20 mass % of Fe, not more than 0.2 mass % of Cu, not more than 0.1
mass % of Zn, not more than 0.1 mass % of Ni, not more than 0.1 mass % of Sn, and
remainders of Al and inevitable impurities.
[0009] The subordinate frame can be produced by high pressure die-casting, because the Al
alloy, which is the raw material of the subordinate frame, contains suitable composition
available for high pressure die-casting, even though the composition is similar to
that of AC4CH alloy incapable of being used in high pressure die-casting. Because
the subordinate frame is produced by high pressure die-casting, a thin product can
result with high productivity at low cost.
[0010] In still another aspect of the invention, there is provided a high pressure die-casting
method including the steps of using an aluminum alloy containing from 8.0 to 9.0 mass
% of Si, from 0.35 to 0.45 mass % of Mg, from 0.3 to 0.4 mass % of Mn, from 0.002
to 0.008 mass % of Be, less than 0.20 mass % of Fe, not more than 0.2 mass % of Cu,
not more than 0.1 mass % of Zn, not more than 0.1 mass % of Ni, not more than 0.1
mass % of Sn, and remainders of Al and inevitable impurities, injecting the aluminum
alloy into a mold cavity, and evacuating the mold cavity by means of highly vacuum
gas vent means to a level not more than 10 kPa during the injection step.
[0011] With this arrangement, high pressure die-casting can be performed with using Al alloy
whose composition is similar to that of AC4CH alloy unavailable for high pressure
die-casting because Al alloy contains proper composition. Because the used Al alloy
contains composition similar to that of AC4CH alloy, a product providing high strength
and high elongation can be produced in spite of elimination of T6 treatment after
casting. Further, because high vacuum level can be provided in the mold cavity during
injection, a stabilized quality results in the resultant cast product at low cost.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] In the drawings;
Fig. 1 is a graph representing a change in sludge factor in accordance with the change
in Mn content in the alloy when segregation amount of Fe and Mn at a surface of a
cast product becomes three times as high as that at an interior thereof at a casting
temperature of 680°C;
Fig. 2 is a graph representing a change in sludge factor in accordance with the change
in Mn content in the alloy when segregation amount of Fe and Mn at a surface of a
cast product becomes three times as high as that in an interior thereof at Fe content
of 0.15 mass %;
Fig. 3 is microscopic photographs showing surface structures of test pieces 1 and
2;
Fig. 4 is a cross-sectional view showing a die-casting machine used in a high pressure
die-casting method according to the present embodiment;
Fig. 5 is a photographic view showing a surface, after blister test, of a subordinate
frame produced by the high pressure die-casting method according to the present embodiment;
Fig. 6 is a photographic view showing a surface, after blister test, of a subordinate
frame produced by a conventional vacuum high pressure die-casting method with a conventional
vacuum level; and
Fig. 7 is a photographic view showing a surface, after welding test, of a subordinate
frame produced by the high pressure die-casting method according to the present embodiment.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0013] A subordinate frame for an automobile in accordance with one embodiment of the present
invention will be described. The subordinate frame is coupled to a body of an automobile
for reinforcing a part of the body in order to enhance rigidity at a portion where
a suspension mechanism is attached. The subordinate frame for the automobile according
to the present embodiment can be produced by high pressure die-casting, because the
Al alloy provides proper composition ratio. Further, the subordinate frame can provide
high strength and elongation those required in the automobile even without T6 treatment
and, a thin product can be produced at high productivity to lower production cost
because of high pressure die-casting.
[0014] The Al alloy in accordance with the present embodiment will next be described, the
Al alloy being used for raw material of the subordinate frame and used in high pressure
die-casting. The Al alloy contains from 8.0 to 9.0 mass % of Si(silicon), from 0.35
to 0.45 mass % of Mg(magnesium), from 0.3 to 0.4 mass % of Mn(manganese), from 0.002
to 0.008 mass % of Be(beryllium), less than 0.20 mass % of Fe(iron), not more than
0.2 mass % of Cu(copper), not more than 0.1 mass % of Zn(zinc), not more than 0.1
mass % of Ni(nickel), not more than 0.1 mass % of Sn(tin), and remainders of Al(aluminum)
and inevitable impurities.
[0015] If the amount of Si is less than 8.0 mass %, fluidity of the alloy will be reduced
to cause misrun. Therefore, the amount of Si is set not less than 8.0 mass %. On the
other hand, if the Si amount is more than 9.0 mass %, elongation and toughness of
a resultant alloy will be lowered. Therefore, the Si amount is set not more than 9.0
mass %. By setting the Si amount ranging from 8.0 to 9.0 mass %, castability can be
enhanced so that a thin product can be produced.
[0016] If the amount of Mg is less than 0.35 mass %, strength and load at the 0.2% proof
stress will be lowered, and therefore, the resultant alloy will not meet with requirements
for the subordinate frame. Thus, the Mg amount is set not less than 0.35 mass %. On
the other hand, if Mg amount is more than 0.45 mass %, elongation will be reduced,
and therefore the alloy will not provide sufficient strength required for the subordinate
frame. Thus, Mg amount is set not more than 0.45 mass %. Amount of Mn will be described
later.
[0017] Function of Be is to prevent the concentration of Mg in the alloy from being lowered
due to oxidation of Mg during melting and holding of the molten metal in the furnace.
If Be amount is less than 0.002 mass %, such function does not exhibit. Therefore,
Be amount is set not less than 0.002 mass %. On the other hand, if Be amount is more
than 0.008 mass %, crystallization of unwanted compound occurs to lower the strength.
Therefore, Be amount is set not more than 0.008 mass %.
[0018] If Fe amount is not less than 0.20 mass %, corrosion resistance of the alloy will
be lowered. Therefore, Fe amount is set less than 0.20 mass %. If Cu amount exceeds
0.2 mass %, corrosion resistance of the alloy will be lowered. Therefore, Cu amount
is set not more than 0.2 mass %. If Zn amount exceeds 0.1 mass %, corrosion resistance
of the alloy will be lowered. Therefore, Zn amount is set not more than 0.1 mass %.
If Ni amount exceeds 0.1 mass %, crystallization of unwanted compound will occur to
lower the mechanical strength of the alloy. Therefore, Ni amount is set not more than
0.1 mass %. If Sn amount exceeds 0.1 mass %, hot cracking may occur in the cast product.
Therefore, Sn amount is set not more than 0.1 mass %. Incidentally, Fe, Cu, Zn, Ni,
and Sn are impurities inevitably exist in the alloy. Accordingly, these are not requisite
elements in the alloy.
[0019] Next, Mn content will be described. As described above, in the Al alloy for the high
pressure die-casting according to the present invention, Fe content is limited to
a low level such as less than 0.20 mass % in order to enhance corrosion resistance
of the alloy. In order to avoid thermal seizure of the alloy at the metal mold due
to the shortage of Fe content, one conceivable method is the formation of crystallization
of sludge (which is Al-Si-Fe-Mn compound) at a surface of the cast product. However,
if Mn amount is less than 0.3 mass %, crystallization of the sludge at the surface
of the cast product does not occur, which cannot obviate thermal seizure of the alloy
to the metal mold. Accordingly, Mn amount is set not less than 0.3 mass %. On the
other hand, if Mn amount exceeds 0.4 mass %, crystallization amount of the sludge
becomes excessive, and sludge are also generated in an interior of the cast product.
Sludge themselves are rigid and high hardness particles. Therefore, mechanical strength
of the cast product may be lowered if the sludge exists in the interior of the cast
product. Consequently, the crystallization amount of sludge should be restrained to
a low level. Thus, Mn content is set not more than 0.4 mass %.
[0020] Next, the relationship between Mn content and crystallization of sludge will be described
in detail. SF value (sludge factor) can be obtained by the following equation (1)
in order to acknowledge the fact whether or not crystallization of sludge appears
during casting with the alloy.

[0021] Here, "%Fe" designates Fe content (mass %) in the alloy, "%Mn" designates Mn content
(mass %) in the alloy, and "T" designates casting temperature (°C). If the sludge
factor exceeds a sludge formation limit SF
H, sludge will be crystallized. The sludge formation limit SF
H is computed by the following equation (2):

[0022] Table 1 shows various sludge factors computed by the equation (1) provided that amounts
of Mn and Fe are varied at a casting temperature of 680 °C . Unit of all values is
mass %. Here, sludge formation limit SF
H at the casting temperature of 680°C is computed to 2.72 mass % according to the equation
(2). Thus, it is understood that no sludge is formed in the interior of the cast product
even if the Mn content in the alloy is 0.6 mass %.

[0024] As described above, the sludge formation limit SF
H at the casting temperature of 680°C is 2.72 mass %. As is apparent from Table 2,
the sludge factor exceeds 2.72 mass % when the Mn amount is not less than 0.4 mass
% in a case where the segregation of Fe and Mn at the surface is twice as high as
that in the interior at the casting temperature of 680°C. And as is apparent from
Table 3, the sludge factor exceeds 2.72 mass % when the Mn amount is not less than
0.3 mass % in a case where the segregation of Fe and Mn at the surface are three times
as high as that in the interior at the casting temperature of 680°C.
[0025] In order to provide crystallization of the sludge at the surface of the cast product,
the sludge factor should exceed the sludge formation limit SF
H. In this connection, Mn content must be set so that the sludge factor can exceed
the sludge formation limit SF
H. On the other hand, if Mn amount is excessive, crystallization amount of the sludge
becomes excessive to undesirably form the sludge even in the interior of the cast
product. Thus, the Mn amount is set to from 0.3 to 0.4 mass %, where the sludge factor
just exceeds the sludge formation limit SF
H.
[0026] Incidentally, according to the equation (1), the sludge factor is not only dependent
on Mn content but also dependent on Fe content. However, as is apparent from Tables
2 and 3, the sludge factor does not exceed the sludge formation limit SF
H 2.72 mass % at the casting temperature of 680°C if the Fe amount is less than 0.2
mass %, but exceeds the sludge formation limit SF
H when the Mn amount exceeds 0.4 mass % in a case where the segregation of Fe and Mn
at the surface is twice as high as that in the interior and when the Mn amount exceeds
0.3 mass % in a case where the segregation of Fe and Mn at the surface is three times
as high as that in the interior. Therefore, Mn content is the predominant factor to
determine the sludge factor in case where Fe amount is less than 0.2 mass %.
[0027] Fig. 1 shows the graphical representation showing the relationship between the sludge
factor and the Mn content in the alloy in a case where the segregation amount of Mn
and Fe at the surface of the cast product becomes three times as high as that at the
interior thereof at the casting temperature of 680 °C. The sludge factors with the
parameter of Fe content of 0.10 mass %, 0.15 mass % and 0.20 mass % are almost the
same, and therefore, it is understood that the sludge factor is not hardly dependent
on Fe content. Further, it is understood that sludge factor does not exceed the sludge
formation limit SF
H in case where Mn amount is not more than 0.2 mass % despite the fact that Fe amount
is 0.20 mass %. On the other hand, sludge factor exceeds the sludge formation limit
SF
H in case where Mn amount is not less than 0.3 mass % despite the fact that Fe amount
is 0.10 mass %. Accordingly, it is apparent from Fig. 1 that sludge factor is not
dependent on Fe content in the alloy, and Mn content ranging from 0.3 to 0.4 mass
% is preferable. Incidentally, the sludge is made up from Al-Si-Fe-Mn compound. However,
a sludge not including Fe can also be crystallized as Al-Si-Mn compound. Therefore,
Fe can be dispensed with in the resultant alloy. This is the case where %Fe is zero
in the theoretical equation (1).
[0028] As is understood from the equation (1) sludge factor is also dependent on the casting
temperature T. However, in case of the preferable range of the die-casting temperature
ranging from 680°C to 700°C, sludge factor is not greatly changed in spite of the
change in the casting temperature T within the above range. Table 4 shows the sludge
factors provided by the change in casting temperature T such as 660°C, 680°C and 700°C
and Mn amount in a condition where Fe content is 0.15 mass % and Fe and Me segregation
amount at the surface of the cast product becomes twice as high as that in the interior
thereof. Further, Table 5 shows the sludge factors provided by the change in casting
temperature T such as 660°C, 680°C and 700°C and Mn amount in a condition where Fe
content is 0.15 mass % and Fe and Me segregation amount at the surface of the cast
product becomes three times as high as that in the interior thereof. Units in Tables
4 and 5 are mass %.

[0029] Upon computation from the equation (2), sludge formation limit SF
H is slightly changed such as 2.59 mass %, 2.72 mass %, and 2.85 mass %, when the casting
temperature is 660°C, 680°C, and 700°C, respectively. However, as is apparent from
Table 4, the sludge factor exceeds respective sludge formation limit SF
H when the Mn amount is not less than 0.4 mass % in a case where the segregation of
Fe and Mn at the surface is twice as high as that in the interior. And as is apparent
from Table 5, the sludge factor exceeds respective sludge formation limit SF
H when the Mn amount is not less than 0.3 mass % in a case where the segregation of
Fe and Mn at the surface is three times as high as that in the interior.
[0030] Fig. 2 shows the graphical representation showing the relationship between the sludge
factor and the Mn content in the alloy in a case where the segregation amount of Mn
and Fe at the surface of the cast product becomes three times as high as that in the
interior thereof with the constant Fe amount of 0.15 mass %. The sludge factors with
the parameter of casting temperature of 660°C, 680°C, and 700°C are almost the same,
and therefore, it is understood that the sludge factor is not greatly dependent on
the casting temperature T. Further, even though the sludge formation limit SF
H is slightly changed by the change of the casting temperature, it is understood that
sludge factor does not exceed the sludge formation limit SF
H even in a case where Mn content is not more than 0.2 mass % and the casting temperature
is set to 660 °C , which provides a minimum sludge formation limit. On the other hand,
sludge factor exceeds the sludge formation limit SF
H in a case where Mn content is not less than 0.3 mass % and the casting temperature
is set to 700°C, which provides a maximum sludge formation limit. Accordingly, Mn
content ranging from 0.3 to 0.4 mass % is preferable for any casting temperature of
high pressure die-casting.
[0031] Several test pieces in accordance with ASTM standard for tensile strength test were
produced by high pressure die-casting with an alloy X which is the Al alloy in accordance
with the present embodiment and with an alloy Y in which Mn content is greater than
Mn content of the present embodiment. Test piece 1 was the alloy X in accordance with
the present embodiment, test piece 2 was the alloy Y containing Mn amount greater
than that of the Al alloy of the present embodiment, test piece 3 was made from the
material the same as Test piece 1 (alloy X) , and test piece 4 was made from the material
the same as Test piece 2 (alloy Y) . Table 6 shows compositions of the alloy X and
alloy Y. For casting the test pieces 1 through 4, a metal mold for simultaneously
casting two test pieces, one for tensile strength test piece and another for impact
strength test piece, was used, and 90 tons cold chamber type die-casting machine was
used. Casting temperature was 700°C, and injection speed was 1.2 m/s. The test pieces
3 and 4 were subjected to T5 treatment at temperature of 180°C for 3 hours after casting.
Table 6
|
Cu |
Si |
Mg |
Zn |
Fe |
Mn |
Ni |
Be |
Sn |
Alloy X |
0.0 |
8.0 |
0.43 |
0.0 |
0.12 |
0.40 |
0.0 |
0.005 |
0.0 |
Alloy Y |
0.0 |
9.0 |
0.36 |
0.0 |
0.15 |
0.47 |
0.0 |
0.005 |
0.0 |
[0032] Table 7 shows test results of tensile strength test with respect to test pieces 1
through 4. The test piece 2 made from the alloy Y provided tensile strength and 0.2%
proof stress higher than those of the test piece 1 made from the alloy X, but provided
elongation lower than that of the test piece 1. Judging from these facts, crystallization
of the sludge occurred even in the interior of the test piece 2 and the breakage of
the test piece 2 may be easily started from the sludge portion. Because higher elongation
is considered to be more important than the higher tensile strength and higher 0.2%
proof stress for the subordinate frame such as suspension components of an automobile,
the alloy X can be more suitable for the suspension components of the automobile than
the alloy Y can. Incidentally, even though the test piece 1 (alloy X) provided the
tensile strength and 0.2% proof stress lower than those of the test piece 2 (alloy
Y), these are sufficient for the suspension components such as the subordinate frame
of the automobile. If much improvement on the tensile strength and 0.2% proof stress
are required, T5 treatment appears to be effective. If comparison is made between
the test pieces 3 and the test piece 4 both being subjected to T5 treatment, reduction
in elongation can be restrained to a small level in case of the test piece 3 made
from the alloy X, whereas elongation was greatly reduced in case of the test piece
4 made from the alloy Y. Accordingly, alloy X appears to be much suitable than the
alloy Y as the material of the subordinate frame of the automobile such as the suspension
component.
Table 7
|
Tensile Strength(MPa) |
Load at the 0.2% proof stress(MPa) |
Elongation (%) |
Test Piece 1 |
270 |
138 |
9.4 |
Test Piece 2 |
310 |
151 |
8.1 |
Test Piece 3 |
314 |
197 |
8.4 |
Test Piece 4 |
340 |
228 |
6.0 |
[0033] Test pieces 1 and 2 were cut to observe internal structure thereof. Microscopic views
at cut surfaces of the test pieces 1 and 2 are shown in Fig. 3. Left upper photograph
is 200 times magnification of the cut surface of the test piece 2, right upper photograph
is 500 times magnification thereof. Left lower photograph is 200 times magnification
of the cut surface of the test piece 1, and right lower photograph is 500 times magnification
thereof. As is understood from these photographs, the cut surface of the test piece
2 shown in upper left and right views contains large crystallization volume of sludge
those appearing in black. On the other hand, no sludge appears in the cut surface
of the test piece 1 shown in lower left and right views.
[0034] Further, ASTM test pieces were produced with the compositions shown in Table 8. Unit
in Table 8 is mass %. Thermal seizure of the alloy to the metal mold did not occur
with the Mn content of 0.3 mass %.
Table 8
Cu |
Si |
Mg |
Zn |
Fe |
Mn |
Ni |
Ti |
Be |
0.0 |
8.9 |
0.40 |
0.0 |
0.15 |
0.30 |
0.0 |
0.04 |
0.003 |
[0035] A high pressure die-casting method and a die-casting machine performing the method
in accordance with the present embodiment will be described with reference to Fig.
4. In the embodiment, casting is performed using the above-described Al alloy according
to the above-described embodiment.
[0036] A die-casting machine 1 includes a movable platen 2 and a fixed platen 3. The movable
platen 2 has a side in confrontation with the fixed platen 3, the side being fixed
with a movable holder 4. The fixed platen 3 has a side in confrontation with the movable
platen 2, the side being fixed with a fixed holder 5.
[0037] The movable holder 4 is formed with a recessed portion open to the fixed holder 5,
and a movable die 6 is fixed in the recessed portion. The movable holder 4 and the
movable die 6 provide a gap therebetween where a liquidized gasket 7 made from a silicone
rubber is injected from the surface confronting the fixed holder 5, thereby enhancing
sealability. A recess 6a is formed at a surface of the movable die 6, the surface
being in confrontation with the fixed holder 5. A flow passage for directing the molten
metal to a mold cavity is defined by the recess 6a and the fixed holder 5 when the
movable holder 4 is brought into abutment with the fixed holder 5.
[0038] A recess open to the movable holder 4 is formed in an interior of the fixed holder
5, and a fixed die 8 is fixed in the recess. The fixed holder 5 and the fixed die
8 provide a gap therebetween where a liquidized gasket 9 made from a silicone rubber
is injected from the surface confronting the movable holder 4, thereby enhancing sealability.
A recess 8a is formed at a side of the fixed die 8, the side being in confrontation
with the movable holder 4. A mold cavity is defined between the recess 8a and the
movable die 6 when the movable holder 4 is brought into abutment with the fixed holder
5. Incidentally, in Fig. 4, the flow passage for directing the molten metal to the
mold cavity is delineated as being in non-communication with the mold cavity. This
is due to the fact that Fig. 4 is the cross-sectional diagram. The flow passage is
in communication with the mold cavity at other cross-sectional portions.
[0039] The fixed platen 3 and the fixed holder 5 form a molten metal supplying passage 10
for supplying a molten metal to the flow passage defined by the recess 6a. Further,
a plunger tip 11 is slidably disposed in the supplying passage 10 for injecting the
molten metal into the mold cavity. The plunger tip 11 has a sliding surface provided
with a ring 11A formed from a stainless steel in order to maintain hermetic seal between
the supplying passage 10 and the tip 11.
[0040] The fixed platen 3 and the fixed holder 5 are formed with a suction passage 12 in
communication with the recess 8a for discharging air in the mold cavity to outside.
The suction passage 12 is connected to a vacuum tank 13 which is connected to a vacuum
pump 14. Thus, air in the mold cavity is sucked by the vacuum pump 14 by way of the
suction passage 12 and the vacuum tank 13. A rotary pump is available as the vacuum
pump 14.
[0041] A sealing rubber 15 is attached at the surface of the fixed holder 5, the surface
confronting the movable holder 4. The sealing rubber 15 has a configuration to surround
the recess 8a and the recess 6a formed at the movable die 6 of the movable holder
4. The sealing rubber 15 is made from a silicone rubber. The sealing rubber 15 can
maintain airtight seal of the mold cavity, the suction passage 12, the molten metal
flow passage and the molten metal supply passage 10 when the movable holder 4 is brought
into abutment with the fixed holder 5.
[0042] Ejector pins 16 are slidably disposed in the movable holder 4 for ejecting the cast
product from the metal mold. The ejector pins 16 extend through the movable holder
4 and the movable die 6. Each one end of each ejector pin 16 is fixed to a pin fixing
plate 17 positioned close to the movable platen 2. Further, each another end of each
ejector pin 16 is protrudable from the surface of the movable die 6 toward the fixed
holder 5. A pin seal plate 18 is provided at another side of the movable holder 4
so as to surround the ejector pins 16, the other side being opposite to the one side
confronting the fixed holder 5. The pin seal plate 18 is adapted for maintaining air-tightness
at a gap between the ejector pins 16 and the movable die 6. Incidentally, in Fig.
4, the lowermost ejector pin 16 is not in confrontation with the mold cavity, but
in confrontation with the molten metal flow passage for directing the molten metal
to the mold cavity. Therefore, the lowermost ejector pin 16 is adapted to eject a
runner portion formed at the molten metal flow passage.
[0043] In the high pressure die-casting method according to the present embodiment, the
die-casting machine 1 is used, and air in the mold cavity is sucked by the vacuum
pump 14 during injection of the molten metal in the mold cavity. High vacuum level
of 4kPa is applied to the mold cavity for casting, which vacuum level is 1/10 or less
than the conventional vacuum level in die-casting machine. Vacuum level of not more
than 10kPa is necessary in the cavity to avoid lowering of the mechanical strength
of cast product. In the die-casting machine 1, sealability of the mold cavity is enhanced
by the ring 11A of the plunger tip 11, the liquidized gaskets 7, 9, the sealing rubber
15 and the pin seal plate 18. With these arrangement, vacuum level can be enhanced
in comparison with the conventional arrangement, and gas entrapment in the molten
metal can be obviated. A combination of the ringed tip 11, the liquidized gaskets
7, 9, the sealing rubber 15, the pin seal plate 18, the suction passage 12, the vacuum
tank 13 and the vacuum pump 14 constitute a highly vacuum gas vent means.
[0044] The subordinate frame produced by the above-described high pressure die-casting method
according to the present embodiment was held at the casting temperature of 500°C for
3 hours for blister test where generation of blister was investigated. Further, similar
blister test was performed with respect to another subordinate frame produced by a
conventional vacuum high pressure die-casting method providing the conventional vacuum
level in the cavity while using the alloy the same as the alloy of the subordinate
frame according to the present embodiment. Fig. 5 shows a surface of the subordinate
frame according to the present embodiment after the blister test, and Fig. 6 shows
a surface of the subordinate frame produced by the conventional high pressure die-casting
method with the conventional vacuum level after the blister test.
[0045] Judging from Fig 6, blisters are observed at the surface of the subordinate frame
produced by the conventional vacuum high pressure die-casting method with the conventional
vacuum level. On the other hand, judging from Fig. 5, no blisters are observed at
the surface of the subordinate frame produced in accordance with the casting method
of the present embodiment. Consequently, in accordance with the high pressure die-casting
method of the present embodiment, casting defect due to gas involvement during casting
can be greatly reduced while using the Al alloy according to the present embodiment.
Therefore, variation in mechanical strength of the cast product can be greatly reduced.
On the other hand, as a result of the conventional vacuum high pressure die-casting
method with the conventional vacuum level while using the Al alloy, casting defect
occurred due to gas involvement, which does not attain a sufficient mechanical strength
required in the cast product such as the subordinate frame used under the severe working
condition.
[0046] Further, welding test was performed with respect to the subordinate frame produced
by the high pressure die-casting method of the present embodiment. Fig. 7 shows a
surface of the subordinate frame after welding test. Only slight blisters were generated
due to the welding, and this fact revealed that variation in mechanical strength of
the cast product due to gas involvement can be greatly reduced. Further, it is found
that the cast product produced by the high pressure die-casting method of the present
embodiment can undergo welding with no problem.
[0047] Advantage of the subordinate frame attendant to the high pressure die-casting method
will be described. For a tensile strength test, the subordinate frame was produced
by high pressure die casting method with the alloy of the present embodiment, and
the cast subordinate frame was cut out into a shape of a test piece regulated by JIS
No. 7. The test piece will be referred to as test piece 5. Further, another cut-out
test piece regulated by JIS No. 7 was produced by a conventional gravity die-casting
method and a subsequent T6 treatment using a conventional AC4CH alloy for the tensile
strength test. The latter test piece will be referred to as test piece 6. Incidentally,
in order to make use of the characteristic of high pressure die-casting, the surface
of the test piece 5 was not subjected to machining but as-cast surface remained. Result
of tensile strength test is shown in Table 9.
Table 9
|
Strength (MPa) |
Load at the 0.2% proof stress (MPa) |
Elongation (%) |
Test piece 5 |
255 |
145 |
8 |
Test piece 6 |
270 |
230 |
8 |
[0048] The test piece 5 provided the tensile strength lower than that of the test piece
6, and the elongation approximately the same as that of the test piece 6. That is,
even though the product made from the A1 alloy according to the present embodiment
and produced by high pressure die-casting method was not subjected to T6 treatment,
the product provided the elongation approximately the same as that of the conventional
product subjected to T6 treatment. Consequently, the subordinate frame according to
the present embodiment is available for mass production because of the application
of high pressure die-casting method, and can also reduce cost because of elimination
of T6 treatment.