U.S. patent application number 10/448103 was filed with the patent office on 2004-03-25 for die casting method and apparatus for rheocasting.
Invention is credited to Hong, Chunpyo, Itamura, Masayuki, Kim, Jaemin, Kim, Minsoo.
Application Number | 20040055726 10/448103 |
Document ID | / |
Family ID | 31999423 |
Filed Date | 2004-03-25 |
United States Patent
Application |
20040055726 |
Kind Code |
A1 |
Hong, Chunpyo ; et
al. |
March 25, 2004 |
Die casting method and apparatus for rheocasting
Abstract
Provided are a die casting method and apparatus for rheocasting
that ensure the manufacture of products with a fine, uniform,
spherical particle structure, with improvements in energy
efficiency and mechanical properties, cost reduction, convenience
of casting, and shorter manufacturing time. The die casting method
involves applying an electromagnetic field to a slurry
manufacturing domain in a sleeve having an end through which a
plunger is inserted and the other end connected to a casting die
with a mold cavity, loading a molten metal into the slurry
manufacturing domain to manufacture a semi-solid metallic slurry,
and moving the plunger toward the casting die to push the metallic
slurry into the mold cavity.
Inventors: |
Hong, Chunpyo; (Seoul,
KR) ; Kim, Jaemin; (Goyang-City, KR) ; Kim,
Minsoo; (Seoul, KR) ; Itamura, Masayuki;
(Ube-shi, JP) |
Correspondence
Address: |
ROTHWELL, FIGG, ERNST & MANBECK, P.C.
1425 K STREET, N.W.
SUITE 800
WASHINGTON
DC
20005
US
|
Family ID: |
31999423 |
Appl. No.: |
10/448103 |
Filed: |
May 30, 2003 |
Current U.S.
Class: |
164/113 ;
164/312; 164/499; 164/900 |
Current CPC
Class: |
B22D 1/00 20130101; B22D
17/007 20130101; C22C 21/02 20130101; C22C 1/005 20130101 |
Class at
Publication: |
164/113 ;
164/312; 164/499; 164/900 |
International
Class: |
B22D 027/02; B22D
017/10; B22D 025/00 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 25, 2002 |
KR |
2002-58163 |
Oct 16, 2002 |
KR |
2002-63162 |
Jan 17, 2003 |
KR |
2003-3250 |
Mar 4, 2003 |
KR |
2003-13515 |
Claims
What is claimed is:
1. A die casting method for rheocasting, the method comprising:
applying an electromagnetic field to a slurry manufacturing domain
in a sleeve having an end through which a plunger is inserted and
the other end connected to a casting die with a mold cavity and
loading a molten metal into the slurry manufacturing domain to
manufacture a semi-solid metallic slurry; and moving the plunger
toward the casting die to push the metallic slurry into the mold
cavity.
2. The die casting method of claim 1, wherein the sleeve is
horizontally positioned, and the slurry manufacturing domain is
defined by a door installed near the other end of the sleeve and
the plunger inserted through the end of the sleeve.
3. The die casting method of claim 1, wherein the sleeve is
inclined such that the end through which the plunger is inserted
faces downward, and the slurry manufacturing domain is defined by
only the plunger inserted through one end of the sleeve.
4. The die casting method of claim 1, wherein at least a portion of
the sleeve is inclined at an angle such that the end through which
the plunger is inserted faces downward, and the slurry
manufacturing domain is defined by only the plunger inserted
through one end of the sleeve.
5. The die casting method of claim 1, wherein the sleeve is
vertically positioned such that the end through which the plunger
is inserted faces downward, and the slurry manufacturing domain is
defined by only the plunger inserted through one end of the
sleeve.
6. The die casting method of any one of claims 1 through 5, wherein
applying the electromagnetic field to the slurry manufacturing
domain is performed prior to loading the molten metal into the
sleeve.
7. The die casting method of any one of claims 1 through 5, wherein
applying the electromagnetic field to the slurry manufacturing
domain is performed at the start of loading the molten metal into
the sleeve.
8. The die casting method of any one of claims 1 through 5, wherein
applying the electromagnetic field to the slurry manufacturing
domain is performed in the middle of loading the molten metal into
the sleeve.
9. The die casting method of any one of claims 1 through 5, wherein
applying the electromagnetic field to the sleeve is sustained until
the molten metal in the slurry manufacturing domain has a solid
fraction of 0.001-0.7.
10. The die casting method of claim 9, wherein applying the
electromagnetic field to the sleeve is sustained until the molten
metal in the slurry manufacturing domain has a solid fraction of
0.001-0.4.
11. The die casting method of claim 10, wherein applying the
electromagnetic field to the sleeve is sustained until the molten
metal in the slurry manufacturing domain has a solid fraction of
0.001-0.1.
12. The die casting method of any one of claims 1 through 5,
further comprising cooling the molten metal loaded into the slurry
manufacturing domain under the electromagnetic field.
13. The die casting method of claim 12, wherein cooling the molten
metal is sustained until the molten metal in the slurry
manufacturing domain has a solid fraction of 0.1-0.7.
14. The die casting method of claim 12, wherein cooling the molten
metal is performed at a rate of 0.2-5.0/sec.
15. The die casting method of claim 14, wherein cooling the molten
metal is performed at a rate of 0.2-2.0/sec.
16. A die casting apparatus for rheocasting, the apparatus
comprising: a stirring unit which includes a space and applies an
electromagnetic field to the space; a sleeve which is accommodated
in the space of the stirring unit and into which a molten metal is
loaded; a plunger which is inserted through an end of the sleeve to
push a semi-solid slurry manufactured in the sleeve; and a casting
die connected to the other end of the sleeve, the casting die
including a movable die and a fixed die which form a mold cavity
when combined together and casting a product from the slurry pushed
into the mold cavity by the plunger.
17. The die casting apparatus of claim 16, wherein the sleeve is
horizontally positioned, and a door is further installed close to
the other end of the sleeve connected to the casting die so as to
close a through hole of the casting die during the manufacture of
the slurry and to open the through hole when the manufactured
slurry is pushed toward the casting die by the plunger.
18. The die casting apparatus of claim 16, wherein at least a
portion of the sleeve is inclined at an angle such that the end of
the sleeve through which the plunger is inserted faces
downward.
19. The die casting apparatus of claim 16, wherein the sleeve
comprises a first sleeve having the end through which the plunger
is inserted and being able to pivot downward and a second sleeve
horizontally positioned, and the first sleeve can be positioned at
an angel to be placed in the space of the stirring unit and can be
positioned to be aligned with the second sleeve.
20. The die casting apparatus of claim 16, wherein the sleeve is
vertically arranged to direct the end through which the plunger is
inserted downward, is movable up and down, and is raised together
with the plunger after the manufacture of the slurry to couple to
the casting die and allow the plunger to push the manufactured
slurry into the mold cavity of the casting die.
21. The die casting apparatus of claim 16, wherein the stirring
unit applies the electromagnetic field prior to loading the molten
metal into the sleeve.
22. The die casting apparatus of claim 16, wherein the stirring
unit applies the electromagnetic field at the start of loading the
molten metal into the sleeve.
23. The die casting apparatus of claim 16, wherein the stirring
unit applies the electromagnetic field in the middle of loading the
molten metal into the sleeve.
24. The die casting apparatus of claim 16, wherein the stirring
unit applies the electromagnetic file until the molten metal in the
sleeve has a solid fraction of 0.001-0.7.
25. The die casting apparatus of claim 24, wherein the stirring
unit applies the electromagnetic field until the molten metal in
the sleeve has a solid fraction of 0.001-0.4.
26. The die casting apparatus of claim 25, wherein the stirring
unit applies the electromagnetic field until the molten metal in
the sleeve has a solid fraction of 0.001-0.1.
27. The die casting apparatus of claim 20, wherein the sleeve
comprises a temperature control element.
28. The die casting apparatus of claim 27, wherein the temperature
control element comprises at least one of a cooler and an
electrical heater.
29. The die casting apparatus of claim 27, wherein the temperature
control element cools the molten metal in the sleeve to reach a
solid fraction of 0.1-0.7.
30. The die casting apparatus of claim 27, wherein the temperature
control element cools the molten metal in the sleeve at a rate of
0.2-5.0/sec.
31. The die casting apparatus of claim 30, wherein the temperature
control element controls the molten metal in the sleeve at a rate
of 0.2-2.0/sec.
Description
BACKGROUND OF THE INVENTION
[0001] This application claims priority from Korean Patent
Application No. 2002-58163 filed on Sep. 25, 2002, No. 2002-63162
filed on Oct. 16, 2002, No. 2003-3250 filed on Jan. 17, 2003 and
No. 2003-13515 filed on Mar. 4, 2003, in the Korean Intellectual
Property Office, the disclosures of which are incorporated herein
in their entirety by reference.
[0002] 1. Field of the Invention
[0003] The present invention relates to a die casting method and
apparatus for rheocasting, and more particularly, to a die casting
method and apparatus for rheocasting that ensure the manufacture of
products with a fine, uniform, spherical particle structure.
[0004] 2. Description of the Related Art
[0005] Rheocasting refers to a process of manufacturing billets or
mold products from semi-solid metallic slurries having a
predetermined viscosity through casting or forging. Semi-solid
metallic slurries consist of spherical solid particles suspended in
a liquid phase in an appropriate ratio at temperature ranges for
semi-solid state, and thus, they change form easily by a small
force due to their thioxotropic properties and can be cast easily
like a liquid due to their high fluidity.
[0006] Such rheocasting is closely related with thixocasting.
Thixocasting refers to a process involving reheating billets
manufactured through rheocasting back into a metal slurry and
casting or forging it to manufacture final products.
[0007] Such rheocasting and thixocasting are more advantageous than
general molding processes, such as casting or forging, using molten
metal. For example, semi-solid or semi-molten slurries used in
rheocasting or thixocasting have fluidity at a lower temperature
than molten metal, so that the die casting temperature can be
lowered in rheocasting or thixocasting, thereby ensuring an
extended lifespan of the die. In addition, when a semi-solid or
semi-molten metallic slurry is extruded through a cylinder,
turbulence is less likely to occur, and less air is incorporated
during casting, thereby preventing formation of air pockets in
final products. Besides, the use of semi-solid or semi-molten
metallic slurries leads to reduced shrinkage during solidification,
improved working efficiency, mechanical properties, and
anti-corrosion, and lightweight products. Therefore, such
semi-solid or semi-molten metallic slurries can be used as new
materials in the fields of automobiles, airplanes, and electrical,
electronic information communications equipment.
[0008] In conventional rheocasting, molten metal is stirred at a
temperature of lower than the liquidus temperature while cooling,
to break up dendritic structures into spherical particles suitable
for rheocasting, for example, by mechanical stirring,
electromagnetic stirring, gas bubbling, low-frequency,
high-frequency, or electromagnetic wave vibration, electrical shock
agitation, etc.
[0009] As an example, U.S. Pat. No. 3,948,650 discloses a method
and apparatus for manufacturing a liquid-solid mixture. In this
method, molten metal is vigorously stirred while cooled to be
solidified. A semi-solid metallic slurry manufacturing apparatus
disclosed in this patent uses a stirrer to induce flow of the
solid-liquid mixture having a predetermined viscosity to break up
dendritic crystalline structures or disperse broken dendritic
crystalline structures in the liquid-solid mixture. In this method,
dendritic crystalline structures formed during cooling are broken
up and used as nuclei for spherical particles. However, due to
generation of latent heat of solidification at the early stage of
cooling, the method causes problems of low cooling rate,
manufacturing time increase, uneven temperature distribution in a
mixing vessel, and non-uniform crystalline structure. Mechanical
stirring applied in the semi-solid metallic slurry manufacturing
apparatus inherently leads to non-uniform temperature distribution
in the mixing vessel. In addition, the apparatus is operated in a
chamber, thereby making it difficult to continuously perform
subsequent processes.
[0010] U.S. Pat. No. 4,465,118 discloses a method and apparatus for
manufacturing a semi-solid alloy slurry. This apparatus includes a
coiled electromagnetic field application portion, a cooling
manifold, and a vessel, which are sequentially formed inward,
wherein molten metal is continuously loaded down into the vessel,
and cooling water is flowed through the cooling manifold to cool
the outer wall of the vessel. In manufacturing a semi-solid alloy
slurry, molten metal is injected through a top opening of the
vessel and cooled by the cooling manifold, thereby resulting in a
solidification zone in the vessel. Cooling is sustained while a
magnetic field is applied by the electromagnetic field application
portion to break up dendritic crystalline structures formed in the
solidification zone and to pull an ingot from the slurry through a
lower end of the apparatus. The basic technical idea of this method
and apparatus is to break up dendritic crystalline structures after
solidification by applying vibration. However, many problems, such
as complicated processing and non-uniform particle structure, arise
with this method. In the manufacturing apparatus, since molten
metal is continuously supplied downward to grow an ingot, it is
difficult to control the state of the metal ingot and the overall
process. Moreover, the vessel is cooled using water prior to
applying an electromagnetic field, so that there is a great
temperature difference between the peripheral and core regions of
the vessel.
[0011] Other types of rheocasting and thixocasting described later
are available. However, all of the methods are based on the
technical idea of breaking up dendritic crystalline structures
after formation, to generate nuclei of spherical particles, and
arise such problems described in conjunction with the above
patents.
[0012] U.S. Pat. No. 4,694,881 discloses a method for manufacturing
thixotropic materials. In this method, an alloy is heated to a
temperature at which all metallic components of the alloy are
present in a liquid phase, and the resulting molten metal is cooled
to a temperature between its liquidus and solidus temperatures.
Then, the molten metal is subjected to a sufficient shearing force
to break dendritic structures formed during the cooling of the
molten metal, so that thixotropic materials are manufactured.
[0013] Japanese Patent Laid-open Application No. 11-33692 discloses
a method for producing a metallic slurry for rheocasting. In this
method, a molten metal is supplied into a vessel at a temperature
near its liquidus temperature or 50 above its liquidus temperature.
Next, when at least a portion of the molten metal reaches a
temperature lower than the liquidus temperature, i.e., the molten
metal is cooled below a liquidus temperature range, the molten
metal is subjected to a force, for example, ultrasonic vibration.
Finally, the molten metal is slowly cooled into a metallic slurry,
for rheocasting, containing spherical particles. This method also
uses a physical force, such as ultrasonic vibration, to break up
the dendrites grown at the early stage of solidification. In this
method, if the casting temperature is greater than the liquidus
temperature, it is difficult to form spherical particle structures
and to rapidly cool the molten metal. Furthermore, this method
leads to a non-uniformity of surface and core structures.
[0014] Japanese Patent Laid-open Application No. 10-128516
discloses a casting method of thixotropic metal. This method
involves loading a molten metal into a vessel and vibrating the
molten metal using a vibrating bar dipped in the molten metal to
directly transfer its vibrating force to the molten metal. A molten
alloy containing nuclei, which is a semi-solid and semi-liquid
state, at temperatures lower than its liquidus temperature is
formed and cooled to a temperature at which it has a predetermined
liquid fraction and held from 30 seconds to 60 minutes to allow
nuclei in the molten alloy to grow larger, thereby resulting in
thixotropic metal. This method provides relatively large particles
of about 100 m and takes a considerably long processing time, and
cannot be performed in a larger vessel than a predetermined
size.
[0015] U.S. Pat. No. 6,432,160 discloses a method for making a
thixotropic metal slurry. This method involves simultaneously
controlling the cooling and the stirring of molten metal to form a
thixotropic metal slurry. In particular, after loading a molten
metal into a mixing vessel, a stator assembly positioned around the
mixing vessel is operated to generate a magnetomotive force
sufficient to stir the molten metal in the vessel rapidly. Next,
the temperature of the molten metal is rapidly dropped by means of
a thermal jacket equipped around the mixing vessel for precise
control of the temperature of the mixing vessel and the molten
metal. The molten metal is continuously stirred during cooling
cycle in a controlled manner. When the solid fraction of the molten
metal is low, high stirring rate is provided. As the solid fraction
increases, a greater magnetomotive force is applied.
[0016] Most of the above-described conventional methods and
apparatuses for manufacturing semi-solid metal slurries use shear
force to break dendritic structures into spherical structures
during a cooling process. Since a force such as vibration is
applied after the temperature of at least a portion of the molten
metal drops below its liquidus temperature, latent heat is
generated due to the formation of initial solidification layers. As
a result, there are many disadvantages such as reduced cooling rate
and increased manufacturing time. In addition, due to a non-uniform
temperature between the inner wall and the center of the vessel, it
is difficult to form fine, uniform spherical metal particles. This
structural non-uniformity of metal particles will be greater if the
temperature of the molten metal loaded into the vessel is not
controlled.
SUMMARY OF THE INVENTION
[0017] The present invention provides a die casting method and
apparatus for rheocasting that ensure the manufacture of products
with a fine, uniform, spherical particle structure, with
improvements in energy efficiency and mechanical properties, cost
reduction, convenience of casting, and shorter manufacturing
time.
[0018] The present invention provides a method of manufacturing
quality products using a semi-solid slurry in a short time.
[0019] In accordance with an aspect of the present invention, there
is provided a die casting method for rheocasting, the method
comprising: applying an electromagnetic field to a slurry
manufacturing domain in a sleeve having an end through which a
plunger is inserted and the other end connected to a casting die
with a mold cavity and loading a molten metal into the slurry
manufacturing domain to manufacture a semi-solid metallic slurry;
and moving the plunger toward the casting die to push the metallic
slurry into the mold cavity.
[0020] According to specific embodiments of the above die casting
method, the sleeve may be horizontally positioned. In this case,
the slurry manufacturing domain is defined by a door installed near
the other end of the sleeve and the plunger inserted through the
end of the sleeve. Alternatively, the sleeve may be inclined such
that the end through which the plunger is inserted faces downward.
In this case, the slurry manufacturing domain is defined by only
the plunger inserted through one end of the sleeve. Alternatively,
at least a portion of the sleeve may be inclined at an angle such
that the end through which the plunger is inserted faces downward.
In this case, the slurry manufacturing domain is defined by only
the plunger inserted through one end of the sleeve. The sleeve may
be vertically positioned such that the end through which the
plunger is inserted faces downward. In this case, the slurry
manufacturing domain is defined by only the plunger inserted
through one end of the sleeve.
[0021] According to more specific embodiments of the above die
casting methods, applying the electromagnetic field to the slurry
manufacturing domain may be performed prior to, at the start, or in
the middle of loading the molten metal into the sleeve. Applying
the electromagnetic field to the sleeve may be sustained until the
molten metal in the slurry manufacturing domain has a solid
fraction of 0.001-0.7, preferably, 0.001-0.4, more preferably,
0.001-0.1.
[0022] An alternative die casting method according to the present
invention may further comprises cooling the molten metal loaded
into the slurry manufacturing domain under the electromagnetic
field. In this case, cooling the molten metal may be sustained
until the molten metal in the slurry manufacturing domain has a
solid fraction of 0.1-0.7. In addition, cooling the molten metal is
performed at a rate of 0.2-5.0/sec, preferably, 0.2-2.0/sec.
[0023] In accordance with another aspect of the present invention,
there is provided a die casting apparatus for rheocasting, the
apparatus comprising: a stirring unit which includes a space and
applies an electromagnetic field to the space; a sleeve which is
accommodated in the space of the stirring unit and into which a
molten metal is loaded; a plunger which is inserted through an end
of the sleeve to push a semi-solid slurry manufactured in the
sleeve; and a casting die connected to the other end of the sleeve,
the casting die including a movable die and a fixed die which form
a mold cavity when combined together and casting a product from the
slurry pushed into the mold cavity by the plunger.
[0024] According to specific embodiments of the above die casting
apparatus, the sleeve may be horizontally positioned. In this case,
a door is further installed close to the other end of the sleeve
connected to the casting die so as to close a through hole of the
casting die during the manufacture of the slurry and to open the
through hole when the manufactured slurry is pushed toward the
casting die by the plunger. Alternatively, at least a portion of
the sleeve may be inclined at an angle such that the end of the
sleeve through which the plunger is inserted faces downward.
Alternatively, the sleeve may comprise a first sleeve having the
end through which the plunger is inserted and being able to pivot
downward and a second sleeve horizontally positioned, wherein the
first sleeve can be positioned at an angel to be placed in the
space of the stirring unit or can be positioned to be aligned with
the second sleeve. Alternatively, the sleeve may be vertically
arranged to direct the end through which the plunger is inserted
downward, be movable up and down, and be raised together with the
plunger after the manufacture of the slurry to couple to the
casting die and allow the plunger to push the manufactured slurry
into the mold cavity of the casting die.
[0025] According to more specific embodiments of the die casting
apparatus, the stirring unit may apply the electromagnetic field
prior to, at the start, or in the middle of loading the molten
metal into the sleeve.
[0026] The stirring unit may apply the electromagnetic file until
the molten metal in the sleeve has a solid fraction of 0.001-0.7,
preferably, 0.001-0.4, more preferably, 0.001-0.1.
[0027] In another die casting apparatus according to the present
invention, the sleeve may comprise a temperature control element.
This temperature control element may include at least one of a
cooler and an electrical heater. The temperature control element
may cool the molten metal in the sleeve to reach a solid fraction
of 0.1-0.7. The temperature control element cools the molten metal
in the sleeve at a rate of 0.2-5.0/sec, preferably,
0.2-2.0/sec.
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] The above and other features and advantages of the present
invention will become more apparent by describing in detail
exemplary embodiments thereof with reference to the attached
drawings in which:
[0029] FIG. 1 is a graph of temperature profile applied in a die
casting method for rheocasting according to the present
invention;
[0030] FIGS. 2 and 3 illustrate the structure of a die casting
apparatus for rheocasting according to an embodiment of the present
invention;
[0031] FIG. 4 is a partial sectional view of an example of a sleeve
applicable to a die casting apparatus according to the present
invention;
[0032] FIG. 5 illustrates the structure of a die casting apparatus
for rheocasting according to another embodiment of the present
invention;
[0033] FIGS. 6 and 7 illustrate the structure of a die casting
apparatus for rheocasting according to another embodiment of the
present invention; and
[0034] FIGS. 8 and 9 illustrate the structure of a die casting
apparatus for rheocasting according to still another embodiment of
the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0035] The preset invention will be described more fully in the
following exemplary embodiments of the invention with reference to
the accompanying drawings.
[0036] Unlike the above-described conventional techniques, a die
casting method for rheocasting according to the present invention
involves manufacturing a semi-solid metallic slurry from a molten
metal in a sleeve and die casting products from the semi-solid
metallic slurry using a casting die. In particular, according to
the present invention, an electromagnetic field is applied prior to
the completion of loading the molten metal into the sleeve. In
other words, electromagnetic stirring is performed prior to, at the
start or in the middle of loading the molten metal into the sleeve,
to prevent formation of dendritric structures. Ultrasonic waves
instead of the electromagnetic field can be applied for
stirring.
[0037] In particular, an empty sleeve is located in a space of a
die casting apparatus. An electromagnetic field is applied to a
predetermined slurry manufacturing domain of the sleeve. The
intensity of the applied electromagnetic field is strong enough to
stir molten metal.
[0038] FIG. 1 is a graph of temperature profile applied in a die
casting method for rheocasting according to the present invention.
As shown in FIG. 1, molten metal is loaded into the slurry
manufacturing domain of the sleeve at a temperature Tp. As
described above, the molten metal may be loaded into the slurry
manufacturing domain while an electromagnetic field is applied to
the domain. However, the present invention is not limited to this,
and electromagnetic stirring may be performed at the start or in
the middle of loading the molten metal into the sleeve.
[0039] Due to the electromagnetic stirring initiated prior to the
completion of loading molten metal into the slurry, the molten
metal does not grow into dendritic structures near the inner wall
of the sleeve at the early stage of solidification, and numerous
micronuclei are concurrently generated throughout the slurry
manufacturing domain because the temperature of the entire molten
metal rapidly drops to a temperature lower than its liquidus
temperature.
[0040] Applying an electromagnetic field to the slurry
manufacturing domain prior to or at the start of loading molten
metal into the sleeve leads to active stirring of the molten metal
at the center and the inner wall regions of the sleeve and rapid
heat transfer throughout the entire molten metal in the sleeve,
thereby suppressing the formation of solidification layers near the
inner wall of the sleeve at the early stage of cooling. In
addition, such active stirring of the molten metal induces smooth
convection heat transfer between the higher temperature molten
metal and the lower temperature inner sleeve wall, so that the
entire molten metal can be cooled rapidly. Due to the
electromagnetic stirring, particles in the molten metal scatter
upon loading into the sleeve and disperse throughout the sleeve as
nuclei, so that there is rare a temperature difference in the
slurry manufacturing domain during cooling. However, in
conventional techniques where molten metal is stirred after the
completion of loading into a sleeve, the temperature of the molten
metal suddenly drops as soon as it contacts the low temperature
inner sleeve wall, so that dendritic crystals grow from
solidification layers formed near the inner slurry vessel wall at
the early stage of cooling.
[0041] The principles of the present invention will become more
apparent when described in connection with latent heat of
solidification. In a die casting method for rheocasting according
to the present invention, molten metal does not solidify near the
inner sleeve wall at the early stage of cooling, and no latent heat
of solidification is generated. Accordingly, the amount of heat to
be dissipated from the molten metal for cooling is equivalent only
to the specific heat of the molten metal that corresponds to about
1/400 of the latent heat of solidification. Therefore, dendrites,
which are generated frequently near the inner sleeve wall at the
early stage of cooling when using conventional methods, are not
formed, and the entire molten metal throughout the slurry
manufacturing domain can be uniformly cooled. It takes merely about
1-10 seconds from the loading of the molten metal. As a result,
numerous nuclei are created and disperse uniformly throughout the
entire molten metal in the slurry manufacturing domain. The
increased density of nuclei shortens the distance between the
nuclei, and spherical particles instead of dendritic particles are
grown.
[0042] The same effects can be achieved even when an
electromagnetic field is applied in the middle of loading the
molten metal into the sleeve. In other words, solidification layers
are hardly formed near the inner sleeve wall even when
electromagnetic stirring begins in the middle of loading the molten
metal into the sleeve.
[0043] It is preferable that the temperature, Tp, of the molten
metal be maintained in a range from its liquidus temperature to 100
above the liquidus temperature (melt superheat=0.about.100) at the
time of being loaded into the sleeve. According to the present
invention, since the entire slurry manufacturing domain containing
the molten metal is cooled uniformly, it allows for the loading of
the molten metal into the sleeve at a temperature of 100 above its
liquidus temperature, without the need to cool the temperature of
the molten metal to near its liquidus temperature.
[0044] On the other hand, in conventional methods, an
electromagnetic field is applied to a slurry vessel after the
completion of loading molten metal into the slurry vessel and a
portion of the molten metal has reached below its liquidus
temperature. Accordingly, latent heat is generated due to the
formation of solidification layers near the inner wall of the
vessel at the early stage of cooling. Because the latent heat of
solidification is about 400 times greater than the specific heat of
the molten metal, it takes much time to drop the temperature of the
entire molten metal below its liquidus temperature. Therefore, in
these conventional methods, the molten metal is loaded into the
vessel after the molten metal has cooled to a temperature near its
liquidus temperature or to a temperature of 50 above its liquidus
temperature. However, in practice, controlling the overall
manufacturing procedure is not easy when there is such a need to
wait for a temperature drop of the molten metal to a predetermined
level.
[0045] According to the present invention, the electromagnetic
stirring may be stopped at any point after at least a portion of
the molten metal in the sleeve reaches a temperature lower than its
liquidus temperature T.sub.l, i.e., after nuclei are created in the
molten metal at a solid fraction of about 0.001, as illustrated in
FIG. 1. For example, an electromagnetic field may be applied to the
slurry manufacturing domain of the sleeve throughout all processes
of loading molten metal into the domain, cooling the molten metal
into a semi-solid slurry, and pushing the semi-solid slurry into a
casting die. This is because, once nuclei are distributed uniformly
throughout the sleeve, the electromagnetic stirring does not affect
the growth of crystalline particles from the nuclei in the metallic
slurry.
[0046] Therefore, the electromagnetic stirring can be sustained
only during the manufacture of the metallic slurry, until the solid
fraction of the molten metal reaches at least 0.001-0.7. However,
the electromagnetic stirring may be sustained until the solid
fraction of the molten metal in the slurry manufacturing domain
reaches the range of, preferably, 0.001-0.4, more preferably,
0.001-0.1, for energy efficiency.
[0047] After loading a molten metal into the slurry manufacturing
domain and allowing nucleation of a uniform distribution in the
molten metal, the slurry manufacturing domain is cooled to
accelerate the growth of the nuclei. This cooling may be concurrent
with the loading of the molten metal into the slurry manufacturing
domain. As described above, the electromagnetic stirring may be
sustained throughout all the cooling process.
[0048] Alternatively, the cooling process may be sustained just
prior to pushing a resulting semi-solid metallic slurry into the
casting die, preferably, sustained until the molten metal has a
solid fraction of 0.1-0.7, this point of time being denoted as
t.sub.2 in FIG. 1. In this case, the molten metal may be cooled at
a rate of 0.2-5.0/sec. However, the cooling rate of the molten
metal may be varied in the range of 0.2-2.0/sec depending on a
desired nuclei distribution and granularity.
[0049] Immediately after the manufacture of a semi-solid metallic
slurry having a predetermined solid fraction according to the
above-described method, the semi-solid metallic slurry is pushed
into a mold cavity of a casting die for die casting.
[0050] According to the above-described method according to the
present invention, a semi-solid metallic slurry can be manufactured
within a short time, merely in 30-60 seconds from loading the
molten metal into the sleeve for a metallic slurry with a solid
fraction of 0.1-0.7. In addition, products having a uniform, dense
spherical particle structure can be manufactured through die
casting of the semi-solid metallic slurry formed by the method.
[0051] The above-described die casting method for rheocasting can
be applied to a horizontal sleeve, a slant sleeve, and a vertical
sleeve.
[0052] For example, in a horizontal sleeve, the slurry
manufacturing domain may be defined by a door and a plunger
installed at each end of the sleeve. In a slant sleeve, the slurry
manufacturing domain may be defined by only a plunger installed at
one end of the sleeve. In a vertical sleeve into which a plunger is
inserted through its bottom end to be perpendicular to the ground,
the slurry manufacturing domain may be defined by only the plunger.
These structural variations of die casting apparatuses depending on
the position of the sleeve will be described later.
[0053] The above-described die casting method for rheocasting can
be implemented using a die casting apparatus according to an
embodiment of the present invention illustrated in FIGS. 2 and
3.
[0054] Referring to FIG. 2, a die casting apparatus for rheocasting
according to an embodiment of the present invention includes a
stirring unit 1 having a space 12 and a coiled electromagnetic
field application portion 11 arranged around the space 12; a sleeve
2 accommodated in the space 12 of the stirring unit 1; a plunger 3
inserted into an end of the sleeve 2; and a casting die 4 connected
to the other end of the sleeve 2.
[0055] In the stirring unit 1, the space 12 and the coiled
electromagnetic field application portion 11 are fixed by means of
a frame (not shown). The coiled electromagnetic field application
portion 11 emanates a predetermined intensity of electromagnetic
field towards the space 12 so as to stir the molten metal loaded
into the sleeve 2 in the space 12 and is electrically connected to
a controller (not shown) which controls the intensity of the
electromagnetic field generated by the coiled electromagnetic field
application portion 11, its operating duration, etc. Any coiled
apparatus for electromagnetic stirring may be used for the coiled
electromagnetic field application portion 11 without limitations.
In addition, the stirring unit 1 may be implemented to be able to
apply ultrasonic waves, instead of the electromagnetic field, for
stirring.
[0056] As shown in FIG. 2, the coiled electromagnetic field
application portion 11 is installed below the sleeve 2 and around a
slurry funnel 22 formed to extend above a slurry loading hole 21 of
the sleeve 2. Accordingly, molten metal can be thoroughly stirred
prior to being loaded into the sleeve 2.
[0057] In the die casting apparatus according to the present
invention, the sleeve 2 serves as a slurry vessel in which a
semi-solid metallic slurry is manufactured from molten metal with
electromagnetic field stirring and as a channel along which the
manufactured semi-solid metallic slurry is readily guided into the
casting die 4.
[0058] The sleeve 2 is cylindrical and is accommodated in the space
12 of the stirring unit 1, wherein the plunger 3 is inserted into
an end of the sleeve 2, and the casting die 4 is connected to the
other end of the sleeve 2. The slurry loading hole 21 is formed on
the top of the sleeve 2, and the slurry funnel 22 extends from the
slurry loading hole 21 to above the stirring unit 1. The slurry
funnel 22 makes it easier to pour molten metal from a loading unit
5 via the slurry loading hole 21 into the sleeve 2.
[0059] The sleeve 2 may be made of a metallic material or an
insulating material, such as alumina or aluminum nitride. For a
metallic sleeve 2, a metal having a higher melting point than the
molten metal to be loaded therein is preferable. Although not
illustrated in FIG. 2, a thermocouple may be installed in the
sleeve 2 connected to the controller (not shown) to provide
temperature information on the sleeve 2 to the controller.
[0060] In the embodiment of the present invention illustrated in
FIGS. 2 and 3, the sleeve 2 is horizontally positioned, with a door
23 installed near the end connected to the casting die 4. The door
23 is shut while a semi-solid metallic slurry is manufactured in
the sleeve 2 and is opened when the resulting semi-solid metallic
slurry is pushed toward the casting die 4 by the plunger 2. When
both ends of the sleeve 2 are blocked by the plunger 3 and the draw
door 23, the sleeve 2 can serve as a slurry vessel for
manufacturing slurry.
[0061] Although the sleeve 2 is illustrated in FIGS. 2 and 3 as
having a simple structure only for containing molten metal, the
sleeve 2 may further comprise a temperature control element 24, as
illustrated in FIG. 4. The temperature control element 24 is
comprised of a cooler and/or a heater. A preferred cooler may be a
cooling water pipe 25 additionally attached to surround the sleeve
2, like a water jacket. A preferred heater may be an external
electrical heater (not shown). The cooling water pipe 25 may be
fitted into a support block 26 placed on an outer wall of the
sleeve 2. It is obvious that a thermocouple (not shown) can be
installed in the sleeve 2.
[0062] The molten metal loaded in the sleeve 2 can be cooled at an
appropriate rate by the cooling water pipe 25 and the electrical
heater (not shown). It will be obvious that the sleeve 2 and the
temperature control element 24 illustrated in FIG. 4 can be applied
to all of the following embodiments of a die casting apparatus for
rheocasting according to the present invention.
[0063] The plunger 3 inserted through an end of the sleeve 2 is
connected to an additional pressing apparatus (not shown) to be
able to reciprocate forward and backward like a piston. Once the
manufacture of a semi-solid slurry is completed in the sleeve 2,
the plunger 3 is moved toward the casting die 4 to push the
semi-solid slurry into the casting die 4.
[0064] The casting die 4 connected to the other end of the sleeve 2
includes a movable die 41 and a fixed die 42. The movable die 41
and the fixed die 42 form a mold cavity 43 when combined together.
The fixed die 42 has a through hole 44 via which the semi-solid
slurry is injected from the sleeve 2 into the mold cavity 43. The
movable die 41 and the fixed die 42 are supported by respective
support plates 45a and 45b which are connected to the die casting
apparatus via mechanical equipment. After casting is completed, the
movable die 41 is separated from the fixed die 42 to release a
product from the mold cavity 43.
[0065] The operation of the die casting apparatus for rheocasting,
having the above-described structure, according to the present
invention will be described with reference to FIGS. 1 through
3.
[0066] The coiled electromagnetic field application portion 11 of
the stirring unit 1, shown in FIG. 2, applies an electromagnetic
field having a predetermined frequency to the space 12 at a
predetermined intensity. As a nonlimiting example, a 60-Hz
electromagnetic field may be applied at a voltage of 250V and an
intensity of 500 Gauss.
[0067] In this state, a molten metal prepared in a separate furnace
(not shown) is transferred into a loading unit 5, for example, a
ladle, and loaded into the slurry manufacturing domain of the
sleeve 2 under an electromagnetic field. Alternatively, the furnace
may be connected to the sleeve 2 to directly load molten metal into
the sleeve 2. As described above, the molten metal can be loaded
into the sleeve 2 at a temperature of 100 above its liquidus
temperature,
[0068] When a fully molten liquid metal is loaded into the sleeve 2
with electromagnetic stirring, fine particles are uniformly
distributed over the sleeve 2 and grow fast without forming
dendritic structures.
[0069] Alternatively, the electromagnetic field may be applied at
the start or in the middle of loading the molten metal into the
sleeve 2, as described above.
[0070] In addition, the application of the electromagnetic field
may be sustained just prior to pushing a resulting semi-solid
slurry into the mold cavity 43, for example, sustained until the
solid fraction of the molten metal reaches at least 0.001-0.7,
preferably 0.001-0.4, more preferably 0.001-0.1, for energy
efficiency. The duration of applying an electromagnetic field can
be experimentally determined for practical application.
[0071] After the termination of applying the electromagnetic field
or while the electromagnetic field is applied, the molten metal in
the sleeve 2 is cooled at a predetermined rate into a semi-solid
metallic slurry having a solid fraction of 0.1-0.7. The cooling
rate is controlled by the temperature control element 24 (see FIG.
3) installed on the outer wall of the sleeve 2, for example, to be
0.2-5/sec, preferably, 0.2-2/sec.
[0072] After the manufacture of a semi-solid metallic slurry is
completed, the door 23 is opened, and the plunger 3 is moved toward
the casting die 4 to push the semi-solid metallic slurry via the
through hole 44 into the mold cavity 43 of the casting die 4, as
illustrated in FIG. 3, followed by rapid cooling to manufacture a
product having a shape conforming to the shape of the mold cavity
43.
[0073] Products having a fine, uniform particle structure can be
manufactured in a simple way when using the above-described die
casting apparatus for rheocasting according to the present
invention. In addition, due to a sharp reduction in time required
to manufacture a semi-solid metallic slurry, the overall processing
time required to manufacture products is reduced with energy saving
and higher productivity effects.
[0074] FIG. 5 illustrates the structure of a die casting apparatus
for rheocasting according to another embodiment of the present
invention, which differs from the previous embodiment in that the
sleeve 2 is inclined such that the end that serves as the entrance
of the plunger 3 faces downward. The following description will be
focused on this difference from the previous embodiment.
[0075] The die casting apparatus of FIG. 5 does not require a door
for blocking the flow of molten metal not completely processed into
a semi-solid slurry into the casting die 4 because the sleeve 2 is
inclined. Since a molten metal loaded via the slurry loading hole
21 flows downward toward the plunger 3, without the probability of
overflowing and entering the casting die 4, there is no need to
install a separate door in the sleeve 2. However, it is preferable
that an additional barrier for blocking the slurry loading hole 21
is installed to prevent a resulting semi-solid metallic slurry in
the sleeve 2 from flowing out through the slurry loading hole 21
when the plunger 3 is moved towards the casting die 4. The
inclination angle of the sleeve 2 may be varied according to design
requirements, but is limited to such a degree that molten metal
does not overflow and stays within the sleeve 2 during the
manufacture of a semi-solid metallic slurry.
[0076] As shown in FIG. 5, the casting die 4 connected to the other
end of the sleeve 2 away from the plunger 3 is also inclined as a
whole. However, this inclination of the casting die 4 causes
limitations when equipping other necessary die casting machines.
Accordingly, an alternative die casting apparatus for rheocasting
according to the present invention may be constructed, as
illustrated in FIGS. 6 and 7, where the casting die 4 is
horizontally arranged, and only the slurry manufacturing domain of
the sleeve can be positioned at an angle with respect to the
casting die 4.
[0077] In particular, referring to FIGS. 6 and 7, the sleeve 2 is
comprised of a first sleeve 23 which can be positioned at an angle
and a second sleeve 24 fixed to the casting die 4, wherein the
first sleeve 23 serves as a slurry manufacturing domain and is
positioned in the space 12 of the stirring unit 1. The first sleeve
23 is hinged to the second sleeve 24 at an angle , preferably, of
less than 90 degrees. When the first sleeve 23 is positioned at 90
degrees with respect to the ground, as shown in FIG. 6, molten
metal is loaded into the first sleeve 23 and processed into a
semi-solid metallic slurry therein. After the manufacture of the
semi-solid metallic slurry is completed, the first sleeve 23 is
positioned to be aligned with the second sleeve 24, and the plunger
3 is moved toward the casting die 4 to push the semi-solid metallic
slurry into the mold cavity 43 for casting, as shown in FIG. 7. The
die casting apparatus of FIGS. 6 and 7 does not require a separate
slurry loading hole.
[0078] FIGS. 8 and 9 illustrate the structure of a die casting
apparatus for rheocasting according to another embodiment of the
present invention. In the die casting apparatus of FIGS. 8 and 9,
the sleeve 2 is vertically positioned to be movable up and down in
connection with an additional driving apparatus (not shown). The
plunger 3 is inserted upward through a bottom end of the sleeve 2.
The sleeve 2 is separated from the casting die 4.
[0079] In particular, the sleeve 2 is comprised of a main sleeve
2a, a movable sleeve 2b, and a fixed sleeve 2c. The main sleeve 2a,
with the bottom end through which the plunger 3 is inserted upward
and a top open end through which molten metal is loaded, serves as
a slurry manufacturing domain. The bottom end of the main sleeve 2a
contacts the movable sleeve 2b connected to the driving apparatus
(not shown). The movable sleeve 2b pushes the main sleeve 2a up
after the manufacture of a semi-solid slurry has been completed, to
couple it to the fixed sleeve 2c attached to the fixed die 42. The
main sleeve 2a and the movable sleeve 2b may be formed as a single
body. The main sleeve 2a is positioned in the space 12 of the
stirring unit 1 installed on a supporting structure 13, with the
coiled electromagnetic field application portion 1 1 installed to
surround the space 12.
[0080] The casting die 4 is also vertically positioned such that
its through hole 44 faces the vertically positioned sleeve 2. The
fixed die 42 has a stepped bottom end portion, and the fixed sleeve
2c and a support member 46 are attached to the stepped bottom end
portion of the casting die 42. The main sleeve 2a is fitted into
the fixed sleeve 2a and tightly supported by the support member
46.
[0081] In operating the die casting apparatus having the
above-described structure, the main sleeve 2a is separated from the
casting die 4 and placed in the space 12 of the stirring unit 1, as
illustrated in FIG. 8. Next, an electromagnetic field is applied to
the space 12 by the coiled electromagnetic field application
portion 11, and a molten metal is loaded via the loading unit 5
into the main sleeve 2a. The main sleeve 2a can serve as a slurry
manufacturing domain due to the plunger 2a blocking its bottom
end.
[0082] The molten metal loaded into the main sleeve 2a is processed
into a semi-solid metallic slurry via a cooling process, as
described in the previous embodiments.
[0083] After the completion of manufacturing the semi-solid
metallic slurry, the movable sleeve 2b and the plunger 3 are raised
to fit the main sleeve 21 into the fixed sleeve 2c attached to the
casting die 4. Next, the plunger 3 is accelerated to reach the
fixed die 42 and push the semi-solid metallic slurry into the mold
cavity 43 of the casting die 4 for casting.
[0084] The above die casting apparatus for rheocasting according to
the present invention described with reference to FIGS. 8 and 9 has
a simplified structure without a door serving as a barrier between
the sleeve 2 and the casting die 4 and in which the sleeve 2 serves
as a slurry manufacturing domain.
[0085] As described above, a die casting method and apparatus for
rheocasting according to the present invention are compatible with
various kinds of metals and alloys, for example, aluminum,
magnesium, zinc, copper, iron, and alloys of the forgoing
metals.
[0086] A die casting method and apparatus for rheocasting according
to the present invention provide the following effects.
[0087] First, products having a uniform, fine, spherical particle
structure can be manufactured.
[0088] Second, densely populated, uniform spherical particles can
be formed with molten metal as a starting material in a short time
through electromagnetic stirring initiated at a temperature above
the liquidus temperature of a source metal to generate more nuclei
throughout the sleeve.
[0089] Third, products manufactured using the die casting apparatus
according to the present invention have improved mechanical
properties.
[0090] Fourth, the duration of electromagnetic stirring is greatly
shortened, thereby saving energy for the stirring.
[0091] Fifth, the simplified overall process and the reduced
casting duration improve productivity.
[0092] While the present invention has been particularly shown and
described with reference to exemplary embodiments thereof, it will
be understood by those of ordinary skill in the art that various
changes in form and details may be made therein without departing
from the spirit and scope of the present invention as defined by
the following claims.
* * * * *