Note: Descriptions are shown in the official language in which they were submitted.
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1 '
1
DESCRIPTION
ALUMINUM DIE CAST PRODUCT AND METHOD FOR MANUFACTURING SAME
Technical Field
[0001]
The present invention relates to an aluminum die cast
product that can be advantageously used, e.g., for a pressure
container that requires high airtightness, as in a compressor
housing, and to a method for manufacturing the aluminum die
cast product.
Background Art
[0002]
A solidification shrinkage space is known to occur in
aluminum die cast products as a result of solidification
shrinkage of a melt injected into a die during die cast
molding. In the case of aluminum alloys, a volume shrinkage
of about 6% occurs during solidification. Shrinkage cavities
occur due to inclusion of such shrinkage spaces in the
product, and the shrinkage cavities become casting defects.
Shrinkage cavities tend to expand to a wide range where the
cast portion has a large thickness or a low cooling rate.
In die cast products, the cooling rate of a portion
located close to the surface (casting surface) that is in
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2
contact with a die is generally high and, therefore, a region
with a small number of defects that is called a chill layer is
formed at the casting surface. However, because the cooling
rate is low inside thick portions, shrinkage cavities
occurring due to solidification shrinkage are formed therein.
In most cases, the shrinkage cavities are formed via three-
dimensional expansion.
[0003]
For example, in the case of a compressor housing, which
is a pressure container, a port hole for taking in and
discharging a cooling gas and a screw hole for attaching a
fitting serving to connect a pipe to the port hole are
provided in a thick portion that is thicker than other
portions of the housing. Cast holes obtained by die casting
using core pins usually serve as rough holes for the port hole
and screw hole, and because the dimensional accuracy of as-
cast products is low, machining is typically performed with a
cutting allowance of about 0.5 mm.
In this case, although no defects are present on the
casting surface and the appearance is beautiful, the shrinkage
cavity defects sometimes appear inside, as described
hereinabove. As a result, where the thickness of the region
without shrinkage cavity defects is less than the cutting
allowance, the shrinkage cavity defects are exposed on the cut
surface after the cutting, thereby creating a risk of the
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3
inside and outside of the housing, or the port hole and screw
hole being linked to each other. This can cause pressure leak
or cooling gas leak and becomes an important reason for the
decrease in the product yield.
[0004]
On the other hand, a local pressurization method and a
local cooling method (see, for example, Patent Citation 1) are
known as methods for inhibiting the occurrence of shrinkage
cavity defects in the thick portions, and it has been
empirically established that the occurrence of shrinkage
cavity defects can be inhibited by intensifying local cooling.
Further, a core pin for die casting a cast holes has been
suggested (see, for example, Patent Citation 2), the pin
having a configuration in which a cooling water path is formed
by fitting a cooling water pipe into the core pin body and
cooling water is forced to flow in the cooling water path, and
a process of casting a rough hole for a bolt hole provided in
a cylinder block has been described as an example.
[0005]
Patent Citation 1: Japanese Unexamined Patent
Application, Publication No. 2004-223610.
Patent Citation 2: Japanese Unexamined Patent
Application, Publication No. Hei 9-323149.
Disclosure of Invention
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[0006]
As described above, the effectiveness of local cooling
intensification in inhibiting the occurrence of shrinkage
cavity defects has been recognized by those skilled in the
art.
However, it is presently not clear how to intensify the
cooling to inhibit the occurrence of shrinkage cavity defects,
to what degree a region without shrinkage cavity defects is
formed due to intensification of cooling, what is the
correlation between the intensification of cooling and the
growth of dendrites during solidification shrinkage of the
melt, and whether the shrinkage cavity defects causing
pressure leak or gas leak do not occur if the growth of
dendrites is somehow controlled, and there is a need for
establishing a technology that can reduce a pressure leak or
gas leak ratio and increase the product yield in the
manufacture, e.g., of pressure containers that require a high
degree of airtightness, as in compressor housings.
[0007]
The present invention was created with the foregoing in
view, and it is an object of the present invention to provide
an aluminum die cast product, e.g., a pressure vessel that
requires high airtightness, in which pressure leak or gas leak
ratio can be reduced and the product yield can be increased
and also to provide a method for manufacturing the product.
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[0008]
The aluminum die cast product and method for manufacture
thereof in accordance with the present invention utilize the
following means to resolve the above-described problems.
Thus, the aluminum die cast product in accordance with
the present invention is an aluminum die cast product in which
a cast hole is formed by a core pin in a thick portion that is
made thicker than other portions, wherein a chill layer with a
secondary dendrite arm spacing of at least 5.5 m or less is
provided at the surface of the cast hole.
[0009]
The tests conducted by the inventors demonstrated that
the growth of dendrites during solidification shrinkage of a
melt is inhibited by the intensification of local cooling, and
where the secondary dendrite arm spacing is at least 5.5 m or
less, a chill layer can be formed in which shrinkage cavity
defects that cause pressure leak or gas leak do not appear on
the casting surface. In accordance with the present
invention, a chill layer with a secondary dendrite arm spacing
of at least 5.5 m or less is provided at the surface of cast
holes in the aluminum die cast product in which cast holes are
formed by core pins. Therefore, where the cast holes serve as
rough holes, the shrinkage cavity defects are not exposed on
the surface thereof and the occurrence of pressure leak or gas
leak via the cast holes can be reliably inhibited. Therefore,
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when a pressure vessel or the like is manufactured, the yield
thereof can be greatly increased.
The secondary dendrite arm spacing is a distance between
the centers of the adjacent secondary dendrite arms and
represented by an average value of secondary dendrite arm
spacing in a portion where four or more secondary dendrite
arms are continuously arranged side by side. More
specifically, a portion in which four or more secondary
dendrite arms are arranged side by side is selected, a total
of n (n is 4 or more) spacings 1 of secondary dendrite arm
spacings are measured, and 1/(n - 1) is found. The
measurements are performed in three or more locations and an
arithmetical average of the results is found.
[0010]
Further, the aluminum die cast product in accordance with
the present invention is the above-described aluminum die cast
product in which a thickness of a chill layer with a secondary
dendrite arm spacing of at least 5.5 m or less is made larger
than a cutting allowance of the cast hole that is set in
advance.
[0011]
In accordance with the present invention, the thickness
of the chill layer with a secondary dendrite arm spacing of at
least 5.5 m or less is made larger than a cutting allowance
of the cast hole that is set in advance. Therefore, even when
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the cast hole is machined as a rough hole by cutting, the
machining surface thereof can be confined within a range of
the chill layer with a secondary dendrite arm spacing of at
least 5.5 m or less in which the shrinkage cavity defects
have not occurred. Therefore, the shrinkage cavity defects
are not exposed on the cutting surface, and the occurrence of
pressure leak or gas leak directly from the cast hole portion
or via a screw hole, or the like, adjacent to the cast hole
portion can be reliably inhibited.
[0012]
Further, the aluminum die cast product in accordance with
the present invention is the above-described aluminum die cast
product in which the chill layer with a secondary dendrite arm
spacing of at least 5.5 m or less remains after cutting
performed with the aforementioned cutting allowance.
[0013]
In accordance with the present invention, because the
chill layer with a secondary dendrite arm spacing of at least
5.5 m or less remains after cutting performed with the
cutting allowance, the chill layer in which no shrinkage
cavity defects have occurred remains at the cast hole surface
in the product after cutting. Therefore, the exposure of
shrinkage cavity defects on the cutting surface can be
reliably prevented and the occurrence of pressure leak or gas
leak directly from the cast hole portion or via a screw hole,
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or the like, adjacent to the cast hole portion can be reliably
inhibited.
[0014]
Further, the aluminum die cast product in accordance with
the present invention is any of the above-described aluminum
die cast products in which the thickness of the chill layer
with a secondary dendrite arm spacing of at least 5.5 m or
less is at least 0.5 mm or more.
[0015]
In accordance with the present invention, because the
thickness of the chill layer with a secondary dendrite arm
spacing of at least 5.5 m or less is at least 0.5 mm or more,
the thickness of the chill layer with a secondary dendrite arm
spacing of at least 5.5 pm or less can be made larger than the
cutting allowance of aluminum die cast products that is
generally set to 0.5 mm or less. As a result, the chill layer
in which no shrinkage cavity defects have occurred can be
reliably left at the surface of cast hole after cutting.
Therefore, the shrinkage cavity defects are not exposed on the
cutting surface and the occurrence of pressure leak or gas
leak directly from the cast hole portion or via a screw hole,
or the like, adjacent to the cast hole portion can be reliably
inhibited.
[0016]
Further, the aluminum die cast product in accordance with
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the present invention is any of the above-described aluminum
die cast products in which the aluminum die cast product is a
pressure vessel.
[0017]
In accordance with the present invention, because the
aluminum die cast product is a pressure vessel, a pressure
vessel can be obtained in which no internal fluid leak occurs
from the cast hole via the shrinkage cavity defects.
Therefore, when an aluminum die cast pressure vessel is
manufactured, the yield thereof can be greatly increased and a
high-quality pressure vessel can be obtained at a low cost.
[0018]
Further, the aluminum die cast product in accordance with
the present invention is the above-described aluminum die cast
product in which the pressure vessel is a compressor housing.
[0019]
In accordance with the present invention, because the
pressure vessel is a compressor housing, it is possible to
obtain a compressor housing with high airtightness in which no
internal cooling gas leaks from the cast hole via the
shrinkage cavity defects. Therefore, when an aluminum die
cast compressor housing is manufactured, the yield thereof can
be greatly increased and a high-quality compressor housing can
be obtained at a low cost.
[0020]
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The method for manufacturing an aluminum die cast product
in accordance with the present invention is a method for
manufacturing an aluminum die cast product in which a cast
hole is formed by a core pin in a thick portion that is made
thicker than other portions, wherein a chill layer with a
secondary dendrite arm spacing of at least 5.5 m or less is
formed to a thickness equal to, or larger than a predetermined
thickness at the cast hole surface by passing a flow of a
cooling medium through the core pin and locally cooling the
cast hole.
[0021]
In accordance with the present invention, by passing a
flow of a cooling medium through the core pin and increasing
the local cooling capacity with the core pin during die
casting, the temperature gradient during melt solidification
shrinkage can be increased. Therefore, the growth of
dendrites can be inhibited and the chill layer with a
secondary dendrite arm spacing of at least 5.5 .m or less can
be formed to a thickness equal to, or larger than a
predetermined thickness at the cast hole surface. As a
result, no shrinkage cavity defects are exposed on the surface
of the cast hole even when it is used as a rough hole, the
occurrence of pressure leak or gas leak via the cast hole can
be reliably inhibited, and a yield in the manufacture of
aluminum die cast products such as pressure vessels can be
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11
greatly increased.
[0022]
Further, the method for manufacturing an aluminum die
cast product in accordance with the present invention is the
above-described method for manufacturing an aluminum die cast
product in which a core pin with a double-pipe structure in
which a central pipe is inserted into a hollow core pin body
is used as the core pin, the cooling medium is supplied from
the central pipe to a distal end portion of the core pin, the
cooling medium is forced to flow into a cooling medium channel
between the central pipe and the pin body, and the cast hole
is locally cooled.
[0023]
In accordance with the present invention, because a core
pin with a double-pipe structure in which a central pipe is
inserted into a hollow core pin body is used as the core pin,
the cooling medium is supplied from the central pipe to a
distal end portion of the core pin, and the cooling medium is
forced to flow into a cooling medium channel between the
central pipe and the pin body, a uniform cooling medium flow
channel is formed along the entire periphery, the flow rate of
the cooling medium can be stabilized, and local cooling of the
cast hole can be performed uniformly. As a result, the chill
layer with a secondary dendrite arm spacing of at least 5.5 m
or less can be reliably formed to a thickness equal to, or
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larger than a predetermined thickness at the cast hole surface
with uniformity over the entire periphery. Therefore, the
quality of the aluminum die cast product obtained by casting
can be stabilized and the yield thereof can be increased.
[0024]
Further, the method for manufacturing an aluminum die
cast product in accordance with the present invention is the
above-described method for manufacturing an aluminum die cast
product, wherein the cooling medium is caused to flow at a
flow rate of 12 cc/s or more.
[0025]
In accordance with the present invention, because the
cooling medium is caused to flow at a flow rate of 12 cc/s or
more, the temperature gradient during melt solidification
shrinkage is sufficiently increased and the dendrite growth is
inhibited. As a result, the chill layer with a secondary
dendrite arm spacing of at least 5.5 m or less can be
reliably formed to a thickness equal to, or larger than a
predetermined thickness at the cast hole surface.
[0026]
Further, the method for manufacturing an aluminum die
cast product in accordance with the present invention is the
above-described method for manufacturing an aluminum die cast
product, wherein a core pin of a partition plate structure in
which a partition plate arranged along a pin axial direction
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~ =
13
is inserted into a hollow core pin body is used as the core
pin, the cooling medium is supplied from one of cooling medium
channels partitioned by the partition plate, the cooling
medium is caused to flow into the other cooling medium
channel, and the cast hole is locally cooled.
[0027]
In accordance with the present invention, because a core
pin of a partition plate structure in which a partition plate
arranged along a pin axial direction is inserted into a hollow
core pin body is used as the core pin, the cooling medium is
supplied from one of cooling medium channels partitioned by
the partition plate, and the cooling medium is caused to flow
into the other cooling medium channel, a uniform cooling
medium flow channel can be formed, the flow rate of the
cooling medium can be stabilized, and the cooling performance
can be improved. As a result, the chill layer with a
secondary dendrite arm spacing of at least 5.5 m or less can
be reliably formed to a thickness equal to, or larger than a
predetermined thickness at the cast hole surface. Therefore,
the quality of the aluminum die cast product obtained by
casting can be stabilized and the yield thereof can be
increased.
[0028]
Further, the method for manufacturing an aluminum die
cast product in accordance with the present invention is any
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14
of the above-described methods for manufacturing an aluminum
die cast product, wherein the flow of the cooling medium is
initiated simultaneously with, or immediately prior to the
completion of melt injection.
[0029]
In accordance with the present invention, because the
flow of the cooling medium is initiated simultaneously with,
or immediately prior to the completion of melt injection, no
adverse effect, such as melt overcooling and melt flow
disruption, is produced on melt injection, and rapid cooling
can be performed simultaneously with completion of injection.
As a result, the temperature gradient during melt
solidification shrinkage can be sufficiently increased and
dendrite growth can be inhibited. Therefore, the chill layer
with a secondary dendrite arm spacing of at least 5.5 m or
less can be reliably formed to a thickness equal to, or larger
than a predetermined thickness at the cast hole surface.
[0030]
With the aluminum die cast product and method for
manufacturing same in accordance with the present invention,
the chill layer with a secondary dendrite arm spacing of at
least 5.5 m or less can be reliably formed to a thickness
equal to, or larger than a predetermined thickness at the cast
hole surface. As a result, where the cast hole serves as a
rough hole, the shrinkage cavity defects are not exposed on
CA 02631229 2008-05-29
the surface thereof and the occurrence of pressure leak or gas
leak via the cast hole can be reliably inhibited. Therefore,
when a pressure vessel or the like is manufactured, the yield
thereof can be greatly increased and high-quality aluminum die
cast products can be manufactured at a low cost.
Brief Description of Drawings
[0031]
[FIG. 1] FIG. 1 is a perspective view illustrating the
external appearance of a compressor housing that is an
aluminum die cast product of one embodiment of the present
invention.
[FIG. 2] FIG. 2 is a cross-sectional view along arrows
Y-Y of the compressor housing shown in FIG. 1.
[FIG. 3] FIG. 3 is a partial simplified cross-sectional
view illustrating a die casting state of the compressor
housing shown in FIG. 1.
[FIG. 4] FIG. 4 is a cross-sectional view illustrating
an example of a core pin employed for die casting of the
compressor housing shown in FIG. 1.
[FIG. 5] FIG. 5 is a cross-sectional view illustrating
another example of a core pin employed for die casting of the
compressor housing shown in FIG. 1.
[FIG. 6] FIG. 6 is a conceptual diagram illustrating
solidification shrinkage of the melt in the case of a small
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temperature gradient during die cooling.
[FIG. 7] FIG. 7 is a conceptual diagram illustrating
solidification shrinkage of the melt in the case of a large
temperature gradient during die cooling.
[FIG. 8] FIG. 8 illustrates observations performed with
an optical microscope for DAS measurements.
[FIG. 9] FIG. 9 is a graph illustrating the relationship
between a cooling water flow rate V (cc/s) and a thickness t
(mm) of the region free of shrinkage cavity defects.
[FIG. 10] FIG. 10 is a graph illustrating the
relationship between a depth d (mm) from the casting surface
in which shrinkage cavity is observed and DAS ( m).
Explanation of Reference:
[0032]
1: compressor housing
2: port hole
2A: cast hole
2B: chill layer
4: thick portion
5, 50: core pin
5A: pin body
5E: central pin
5F, 50I, 50J: cooling medium channel
50E: partition plate
20: dendrite arm
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21: secondary dendrite arm
DAS: secondary dendrite arm spacing
Best Mode for Carrying Out the Invention
[0033]
An embodiment of the present invention will be described
below with reference to the drawings.
FIG. 1 is a perspective view of a compressor housing 1 of
one embodiment of the present invention. FIG. 2 is a cross-
sectional view along arrows Y-Y in FIG. 1. In the present
embodiment, the compressor housing 1, which is a pressure
vessel requiring airtightness, is explained as an example of
an aluminum die cast product, but the aluminum die cast
product in accordance with the present invention is not
limited thereto.
[0034]
The compressor housing 1 constitutes the outer shell of
the compressor, and a compression mechanism (not shown in the
figure) is contained inside thereof. The compression
mechanism compresses a low-pressure cooling gas sucked in via
a port hole 2 from the outside of the compression housing 1
and discharges the resultant high-pressure cooling gas to the
outside via a port hole (not shown in the figure) provided in
the compression housing 1. The compression housing 1 forms a
compressor accommodation space that covers and seals the outer
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periphery of the compression mechanism so as to prevent leak
to the outside as the cooling gas is taken in, compressed, and
discharged, and the compression housing acts as a pressure
vessel.
[0035]
The aforementioned port hole 2 is provided so as to pass
through the compression housing 1, as shown in FIG. 2, and a
cooling medium pipe (not shown in the figure) is connected to
the port hole 2.
A thick portion 4 that is thicker than other portions is
provided in the outer peripheral portion of the compression
housing 1, and the port hole 2 is provided together with a
screw hole 3 for attaching a fitting for connecting the
cooling medium pipe in this thick portion 4. However, the
screw hole 3 is not necessarily provided and may be omitted.
The compression housing 1 is produced from an aluminum
alloy in order to reduce weight and is manufactured by die
casting. The rough holes of the port hole 2 and screw hole 3
are formed as cast holes 2A and 3A with core pins during die
casting and then finished by machining to predetermined
dimensions after die casting.
[0036]
FIG. 3 is a schematic sectional view illustrating a state
in which the cast holes 2A and 3A for the port hole 2 and
screw hole 3 are die cast with core pins 5 and 6. A molding
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space 9 of the thick portion 4 for molding the cast holes 2A
and 3A is formed by a fixed die 7 and a movable die 8. The
core pins 5 and 6 for molding the cast holes 2A and 3A are
introduced from the side of the fixed die 7 into the molding
space 9 and disposed therein. During die casting, the molding
space 9 is filled with a melt, thereby molding the cast holes
2A and 3A serving as rough holes for the port hole and screw
hole 3 with the core pins 5 and 6.
[0037]
The thick portion 4 where the port hole 2 and screw hole
3 are provided is thicker than other portions, and during die
casting, a solidification shrinkage space is produced by
solidification shrinkage of the melt and shrinkage cavity
defects easily appear therein. As described above, because
the cast holes 2A and 3A molded by the core pins 5 and 6 are
finished to obtain the port hole 2 and screw hole 3, where a
chill layer 2B on the casting surface is cut, the shrinkage
cavity defects can be exposed on the cutting surface and the
inside and outside of the compressor housing 1 or the port
hole 2 and screw hole 3 can be lined together via the
shrinkage cavity defects. As mentioned hereinabove, this can
result in a leak of pressure or cooling medium gas from inside
the compressor housing 1 to the outside.
[0038]
In order to prevent the leak of pressure or cooling
CA 02631229 2008-05-29
medium gas, the chill layer 2B in which no shrinkage cavity
defects have appeared is formed to a thickness above a
predetermined thickness at the casting surface, and when the
cutting is performed with a predetermined cutting allowance,
no shrinkage cavity defects are exposed on the cutting
surface. Furthermore, locally cooling the cast holes with the
core pins would be also effective for forming the chill layer
2B in which no shrinkage cavity defects have occurred to a
thickness exceeding the cutting allowance. From among the
core pins 5 and 6, the core pin 6 for the screw hole 3 has a
small pin diameter, and providing a cooling medium flow
channel inside thereof is difficult. For this reason, a flow
channel for the cooling medium (water) is provided inside the
core pin 5 with a comparatively large diameter that serves to
mold the port hole 2, local cooling capacity with the core pin
5 is enhanced, and a region (chill layer 2B) where no
shrinkage cavity defects occur is made thicker.
[0039]
FIG. 4 shows a configuration of the core pin 5 provided
with a flow channel for a cooling medium (water) inside
thereof. The core pin 5 is a core pin 5 of a double pipe
structure composed of a hollow pin body 5A, a holder 5C
connected via a thread 5B to the upper portion of the pin body
5A, a plug 5D screwed into the end portion of the holder 5C, a
central pipe 5E having one end thereof held in an up-down
CA 02631229 2008-05-29
21
partition portion within the holder 5C and another end opened
at the distant end portion inside the hollow pin body 5A, a
cooling medium flow channel 5F formed between the hollow pin
body 5A and the central pipe 5E and serving to pass the
cooling medium (water) therethrough, a cooling medium supply
pipe 5G connected to a space in the upper portion inside the
holder 5C, and a cooling medium discharge pipe 5H connected to
a space in the lower portion inside the holder 5C. The
cooling medium (water) can flow at a preset flow rate from a
water supply device 10 via the cooling medium supply pipe 5G
and cooling medium discharge pipe 5H inside the core pin S.
[0040]
The core pin 5 can be replaced with a core pin 50 shown
in FIG. 5. The core pin 50 has a partition plate structure in
which the central pipe 5E of the core pin 5 is replaced with a
partition plate 50E, one of the flow channels partitioned by
the partition plate 50 is a cooling medium supply channel 501
and the other flow channel is a cooling medium discharge
channel 50J. Other components are identical to those of the
core pin 5 and assigned with identical symbols. The
explanation thereof is omitted.
[0041]
A method for forming the chill layer 2B with a thickness
above a predetermined value in which no shrinkage cavity
defects have occurred at the surface of the cast hole 2A
CA 02631229 2008-05-29
22
serving as a rough hole for the port hole 2 by using the core
pin 5 or 50 and a configuration of the compressor housing 1
provided with the chill layer 2B will be described below.
First, the relationship between the temperature gradient and
growth of dendrite arms during solidification shrinkage of the
melt that is shown in FIG. 6 and FIG. 7 will be explained.
As shown in FIG. 6 and FIG. 7, the solidification of melt
proceeds while the melt is supplied from a liquid phase side
so as to compensate for the volume shrinkage that accompanies
solidification between the dendrite arms 20 extending from the
solid phase side. In order to prevent the occurrence of
solidification shrinkage space (shrinkage cavity), it is
necessary to supply a sufficient amount of melt between the
dendrite arms 20. For this purpose, it is important to
inhibit the growth of dendrite arms 20. In other words, where
the dendrite arms 20 grow, a sufficient amount of melt cannot
be supplied therebetween and a solidification shrinkage space
X (see FIG. 6) that will become a shrinkage cavity appears
between the dendrite arms.
[0042]
The growth of dendrite arms 20 can be inhibited by
increasing the cooling capacity of the die and increasing the
temperature gradient G. The temperature gradient G can be
represented by the following formula:
G = (TL - Ts) /L
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23
In this formula TL stands for a liquidus temperature, Ts -
a solidus temperature, and L - a length of dendrite arms 20.
Where the temperature gradient G is small (the case shown
in FIG. 6), the dendrite arms 20 grow and the length L thereof
increases to Ll. On the other hand, by increasing the
temperature gradient G (the case shown in FIG. 7), it is
possible to inhibit the growth of dendrite arms 20 and make
the length L thereof as short as L2 (Ll > L2).
[0043]
Further, the spacing between the secondary dendrite arms
21 (dendrite arm spacing, referred to hereinbelow as "DAS")
can be decreased by increasing the temperature gradient G. As
shown in FIG. 8, the DAS can be easily checked by using an
optical microscope after completion of solidification. The
DAS is a distance between the centers of adjacent secondary
dendrite arms and is taken as an average value of secondary
dendrite arm spacing in a portion where four or more of
secondary dendrite arms are arranged continuously side by
side. More specifically, a portion where four or more of
secondary dendrite arms are arranged continuously side by side
is selected, a total of n (n is 4 or more) secondary dendrite
arm spacings 1 are measured, and a value of 1/(n - 1) is
found. The measurements are preformed in three or more places
and an arithmetical average of the results is found.
[0044]
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24
The temperature gradient G can be increased by increasing
the cooling capacity of the mold. Therefore, apparently the
DAS can be decreased and the thickness of the region (chill
layer 2B) where no shrinkage cavities have occurred can be
increased by increasing the flow rate of the cooling medium
that cools the mold, thereby raising the cooling rate of the
mold, and inhibiting the growth of dendrite arms 20 during
solidification. Accordingly, the following test was
performed.
[0045]
In the test, the core pin 5 shown in FIG. 4 was used, the
cast holes 2A and 3A for the port hole 2 and screw hole 3 were
die cast with the core pins 5 and 6 (see FIG. 3), while
varying the flow rate V (cc/s) of the cooling medium (water),
the DAS ( m) in a location at a predetermined depth from the
casting surface (surface of cast product) was measured by a
linear intercept method, and the thickness t (mm) of the
region (chill layer 2B) without he shrinkage cavity defects
was measured. The measurement results are shown in Table 1.
[0046]
CA 02631229 2008-05-29
[Table 1]
Sample Cooling Secondary dendrite arm spacing Thickness
No. water [DAS] ( m) of region
flow rate Depth from Depth from Depth from without
(cc/s) casting casting casting shrinkage
surface: surface: surface: cavities
0.25 mm 1.0 mm 2.0 mm (mm)
a-1 0 5.8 9.3 9.7 0.34
a-2 0 5.1 8.0 10.1 0.3
b-1 1.6 4.7 9.4 11.5 0.42
b-2 1.6 4.5 7.5 10.5 0.39
c-1 12 4.1 7.7 8.8 1.1
c-2 12 4.0 7.3 7.8 0.86
c-3 12 4.0 7.5 7.6 0.99
d-1 20.7 3.2 7.3 8.0 0.92
d-2 20.7 3.3 6.6 7.4 1.36
[0047]
The casting conditions in the above-described process
were as follows: melt temperature 665 C, die temperature 200 C,
injection rate 0.2 m/s (low rate) and 2.3 m/s (high rate).
These casting conditions are identical to a melt temperature
of 640-690 C and mold temperature of 150-300 C that are typical
molding conditions for casting the aluminum die cast housing
1. As for the conditions relating to the flow of cooling
medium (water), the flow of cooling medium (water) was
initiated simultaneously with the completion of injection and
the flow passage time was 3 sec.
[0048]
The test results were analyzed and the relationship
between the flow rate V (cc/s) of the cooling medium (water)
and the thickness t (mm) of the region without shrinkage
cavity defects was plotted as shown in FIG. 9. A thick solid
CA 02631229 2008-05-29
26
line in FIG. 9 connects the minimum values of the thickness t
(mm) of the region without shrinkage cavities obtained at each
flow rate V (cc/s) of cooling water. These results also
demonstrated that the thickness t (mm) of the region without
shrinkage cavities can be obtained by increasing the flow rate
V (cc/s) of cooling water flowing in the core pin 5,
increasing the cooling capacity with the core pin 5, and
increasing the temperature gradient G during solidification.
In particular, it was confirmed that the thickness t (mm) of
the region without shrinkage cavities can be increased to 0.8
mm or more by raising the flow rate V (cc/s) of cooling water
to 12.0 cc/s or more.
[0049]
Further, a graph representing the relationship between
the depth d(mm) from the casting surface (surface of cast
product) where shrinkage cavities are observed and DAS ( m) is
shown in FIG. 10, the region shown by thick solid lines in
FIG. 10 indicates a region in which the depth d (mm) from the
casting surface (surface of cast product) where shrinkage
cavities are observed, that is, the thickness of the region
without shrinkage cavities assumes a minimum value (tmin) in
the molded products obtained at various flow rates V (cc/s) of
cooling water shown by broken lines. This region could be
considered as a threshold (5.5-6.9 m) of DAS at which
shrinkage cavities were observed, and it was made clear that
CA 02631229 2008-05-29
27
the condition for DAS at which no shrinkage cavities occur was
5.5 m or less.
[0050]
The above-described results demonstrated that by
inhibiting the growth of dendrites during melt solidification
shrinkage and making the DAS (secondary dendrite arm spacing)
at least 5.5 m or less, it is possible to form the chill
layer 2B (see FIG. 2) in which shrinkage cavity defects
causing pressure leak or gas leak in the compressor housing do
not occur and which has a thickness equal to or larger than a
predetermined thickness.
[0051]
In the present embodiment, the chill layer 2B with a DAS
of 5.5 m or less was formed to a thickness more than a
machining allowance in cutting after die casting at the
surface of cast hole 2A for the port hole 2 cast with the core
pin 5. Thus, in the case where the cutting allowance has a
usual value of 0.5 mm, the chill layer 2B with a DAS of 5.5 m
or less was formed to a thickness of 0.5 mm or more, and the
chill layer 2B with a DAS of 5.5 m or less remained on the
cut surface after cutting.
[0052]
As a result, even when the cast hole 2A is used as a
rough hole and subjected to cutting with the usual cutting
allowance, no shrinkage cavity defects are exposed on the cut
CA 02631229 2008-05-29
28
surface and the occurrence of pressure leak or gas leak via
the cast hole 2A can be reliably prevented. Therefore, when
the compressor housing 1 is manufactured, the production yield
thereof can be greatly increased and the high-quality
compressor housing 1 can be manufactured at a low cost.
In particular, by increasing the flow rate V (cc/s) of
the cooling medium (water) flowing in the core pin 5 to 12.0
cc/s or more, it is possible to increase the thickness t (mm)
of the region without shrinkage cavities in which DAS is 5.5
m or less to be 0.8 (mm) or more and to eliminate reliably
the occurrence of pressure leak or gas leak.
[0053]
Further, in the above-described embodiment, the flow of
the cooling medium in the core pins 5, 50 is initiated during
die casting at the same time as the melt injection is
completed, but performing the two actions simultaneously is
not necessary. Thus, the cooling medium can be caused to flow
immediately before the completion of melt injection, for
example, from about 1 s before the completion of injection.
In sum, where the cooling medium is left to flow or the
cooling medium is caused to flow too early, the die is
overcooled and an adverse effect is produced on the melt
injection. Thus, part of the melt starts solidifying during
loading or abnormal melt flow is induced. Therefore, the
cooling medium may be caused to flow before the completion of
CA 02631229 2008-05-29
29
injection, provided that no such adverse effect is produced.
Further, in the present embodiment, an example of
applying the present invention to the compressor housing 1 is
explained, but it goes without saying that the present
invention is not limited to this application and can be
applied to a wide variety of aluminum die cast products such
as other pressure vessels, aluminum cylinder blocks for
engines, and transmission cases. In these cases, the leak of
fluid sealed inside the product also can be reliably
eliminated.
