U.S. patent application number 10/447233 was filed with the patent office on 2004-03-25 for method and apparatus for manufacturing semi-solid metallic slurry.
Invention is credited to Hong, Chun Pyo, Itamura, Masayuki, Kim, Jae Min, Kim, Min Soo.
Application Number | 20040055735 10/447233 |
Document ID | / |
Family ID | 31999425 |
Filed Date | 2004-03-25 |
United States Patent
Application |
20040055735 |
Kind Code |
A1 |
Hong, Chun Pyo ; et
al. |
March 25, 2004 |
Method and apparatus for manufacturing semi-solid metallic
slurry
Abstract
The present invention provides a method and apparatus for
manufacturing a high-quality semi-solid metallic slurry containing
fine, uniform spherical particles that can be readily and
conveniently applied to a subsequent process, with improvements in
energy efficiency and mechanical properties, cost reduction,
convenience of casting, and shorter manufacturing time. The
semi-solid metallic slurry manufacturing method includes applying
an electromagnetic field to a space containing a slurry vessel,
loading a molten metal into the slurry vessel in a state where the
electromagnetic field is applied to the space, and drawing the
slurry vessel out from the space. The semi-solid metallic slurry
manufacturing apparatus includes at least one slurry vessel, at
least one stirring unit having a space for the at least one slurry
vessel and applying an electromagnetic field to the space, a
driving unit moving the slurry vessel at least up and down to place
the slurry vessel in the space, and a loading unit loading a molten
metal in liquid state into the slurry vessel.
Inventors: |
Hong, Chun Pyo; (Seoul,
KR) ; Kim, Jae Min; (Goyang-City, KR) ; Kim,
Min Soo; (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: |
31999425 |
Appl. No.: |
10/447233 |
Filed: |
May 29, 2003 |
Current U.S.
Class: |
164/499 ;
164/113; 164/900 |
Current CPC
Class: |
B22D 1/00 20130101; B22D
17/007 20130101; C22C 1/005 20130101; C22C 21/02 20130101 |
Class at
Publication: |
164/499 ;
164/900; 164/113 |
International
Class: |
B22D 027/02; B22D
017/08 |
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 |
2003-13517 |
Claims
What is claimed is:
1. A method of manufacturing a semi-solid metallic slurry, the
method comprising: applying an electromagnetic field to a space
containing a slurry vessel; loading a molten metal into the slurry
vessel in a state where the electromagnetic field is applied to the
space; and drawing the slurry vessel out from the space.
2. The method of claim 1, wherein applying the electromagnetic
field to the space is performed prior to loading the molten metal
into the slurry vessel.
3. The method of claim 2, wherein applying the electromagnetic
field to the space is performed prior to placing the slurry vessel
in the space.
4. The method of claim 1, wherein applying the electromagnetic
field to the space is performed at the start of loading the molten
metal into the slurry vessel.
5. The method of claim 1, wherein applying the electromagnetic
field to the space is performed in the middle of loading the molten
metal into the slurry vessel.
6. The method of claim 1, wherein applying the electromagnetic
field to the space is sustained until a slurry in the slurry vessel
has a solid fraction in the range of 0.001-0.7.
7. The method of claim 6, wherein applying the electromagnetic
field to the space is sustained until the slurry in the slurry
vessel has a solid fraction in the range of 0.001-0.4.
8. The method of claim 7, wherein applying the electromagnetic
field to the space is sustained until the slurry in the slurry
vessel has a solid fraction in the range of 0.001-0.1.
9. The method of claim 1, further comprising cooling the slurry
vessel containing the molten metal after loading the molten metal
into the slurry vessel.
10. The method of claim 9, wherein cooling the slurry vessel
containing the molten metal is sustained until a slurry in the
slurry vessel has a solid fraction in the range of 0.1-0.7.
11. The method of claim 9, wherein cooling the slurry vessel
containing the molten metal is performed at a rate of approximately
0.2-5.0.degree. C./sec.
12. The method of claim 11, wherein cooling the slurry vessel
containing the molten metal is performed at a rate of approximately
0.2-2.0.degree. C./sec.
13. An apparatus for manufacturing a semi-solid metallic slurry,
the apparatus comprising: at least one slurry vessel; at least one
stirring unit having a space for the at least one slurry vessel and
applying an electromagnetic field to the space; a driving unit
moving the slurry vessel at least up and down to place the slurry
vessel in the space; and a loading unit loading a molten metal in
liquid state into the slurry vessel.
14. The apparatus of claim 13, wherein the at least one stirring
unit applies the electromagnetic field to the space prior to
loading the molten metal into the at least one slurry vessel.
15. The apparatus of claim 13, wherein the at least one stirring
unit applies the electromagnetic field to the space at the start of
loading the molten metal into the at least one slurry vessel.
16. The apparatus of claim 13, wherein the at least one stirring
unit applies the electromagnetic field to the space in the middle
of loading the molten metal into the at least one slurry
vessel.
17. The apparatus of claim 13, wherein the driving unit moves the
slurry vessel up after a predetermined time from loading the molten
metal into the at least one slurry vessel to draw the at least one
slurry vessel out from the space.
18. The apparatus of claim 13, wherein the driving unit laterally
shifts the at least one slurry vessel.
19. The apparatus of claim 18, wherein the driving unit comprises a
rotary plate supporting the at least one slurry vessel at an edge,
moving the at least one slurry vessel down after a predetermined
time from loading the molten metal into the at least one slurry
vessel, and rotating the rotary plate to draw the at least one
slurry vessel out from the space.
20. The apparatus of claim 18, wherein the driving unit is
laterally moveable along a rail, moves the at least one slurry
vessel down after a predetermined time from loading the molten
metal into the at least one slurry vessel, and is moved along the
rail to draw the at least one slurry vessel out from the space.
21. The apparatus of claim 13, wherein the at least one stirring
unit applies the electromagnetic field to the space until a slurry
in the at least one slurry vessel has a solid fraction in the range
of 0.001-0.7.
22. The apparatus of claim 21, wherein the at least one stirring
unit applies the electromagnetic field to the space until the
slurry in the at least one slurry vessel has a solid fraction in
the range of 0.001-0.4.
23. The apparatus of claim 22, wherein at least one stirring unit
applies the electromagnetic field to the space until the slurry in
the at least one slurry vessel has a solid fraction in the range of
0.001-0.1.
24. The apparatus of claim 13, wherein the at least one slurry
vessel includes a temperature control element.
25. The apparatus of claim 24, wherein the temperature control
element comprises at least one of a cooler installed in the at
least one slurry vessel and an external electric heater.
26. The apparatus of claim 24, wherein the temperature control
element cools a slurry in the at least one slurry vessel to reach a
solid fraction of approximately 0.1-0.7.
27. The apparatus of claim 24, wherein the temperature control
element cools the slurry in the at least one slurry vessel at a
rate of approximately 0.2-5.0.degree. C./sec.
28. The apparatus of claim 27, wherein the temperature control
element controls the slurry in the at least one slurry vessel at a
rate of approximately 0.2-2.0.degree. C./sec.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority from Korean Patent
Application Nos. 2002-58163 filed on Sep. 25, 2002; 2002-63162
filed on Oct. 16, 2002; 2003-3250 filed on Jan. 17, 2003; and
2003-13517 filed on Mar. 4, 2003, the disclosures of which are
incorporated herein in their entirety by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a method and apparatus for
manufacturing a semi-solid metallic slurry, and more particularly,
to a method and apparatus for manufacturing a semi-solid metallic
slurry, in a solid and liquid combined state, containing fine,
uniform spherical particles.
[0004] 2. Description of the Related Art
[0005] Semi-solid metallic slurries refer to metallic materials in
a solid and liquid combined phase for use in a rheocasting or
thioxocasting process. 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. Rheocasting refers to a process of
manufacturing billets or mold products from metallic slurries
having a predetermined viscosity through casting or forging.
Thixocasting refers to a process involving reheating billets
manufactured through rheocasting back into a metal slurry and
casting or forging the metal slurry to manufacture final
products.
[0006] Such rheocasting orthixocasting is more advantageous than
general forming 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.
[0007] As described above, semi-solid slurries solidified from
molten metal by a predetermined method are used in rheocasting, and
semi-molten slurries obtained by reheating solid billets are used
in thixocasting. Throughout the specification of the present
invention, the term "semi-solid metallic slurry" means a metallic
slurry in a solid and liquid combined stage at a temperature range,
between the liquidus temperature and the solidus temperature of the
metal, which can be manufactured by rheocasting through
solidification of molten metal.
[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 a
subsequent process.
[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 unit, 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 unit to break up dendritic
crystalline structures formed in the solidification zone and to
pull an ingot from the slurry through the 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 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.degree. C. 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 .mu.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 B1 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 method and apparatus for
manufacturing a semi-solid metallic slurry containing fine, uniform
spherical particles, with improvements in energy efficiency and
mechanical properties, cost reduction, convenience of casting, and
shorter manufacturing time.
[0018] The present invention also provides a method and apparatus
for manufacturing a high-quality semi-solid metallic slurry within
a short time, which can be readily and conveniently applied to a
subsequent process.
[0019] In accordance with an aspect of the present invention, there
is provided a method for manufacturing a semi-solid metallic
slurry, the method comprising applying an electromagnetic field to
a space containing a slurry vessel; loading a molten metal into the
slurry vessel in a state where the electromagnetic field is applied
to the space; and drawing the slurry vessel out from the space.
[0020] According to a specific embodiment of the semi-solid
metallic slurry manufacturing method, applying the electromagnetic
field to the space is performed prior to loading the molten metal
into the slurry vessel. In this case, the slurry vessel is placed
in the space after applying the electromagnetic field to the
space.
[0021] Further, applying the electromagnetic field to the space may
be performed at the start of loading the molten metal into the
slurry vessel. Alternatively, applying the electromagnetic field to
the space may be performed in the middle of loading the molten
metal into the slurry vessel.
[0022] Further, applying the electromagnetic field to the space may
be sustained until a slurry in the slurry vessel has a solid
fraction of 0.001-0.7, preferably, 0.001-0.4, more preferably,
0.001-0.1.
[0023] The method for manufacturing a semi-solid metallic slurry
may further comprising cooling the slurry vessel containing the
molten metal after loading the molten metal into the slurry vessel.
In this case, cooling the slurry vessel containing the molten metal
is sustained until a slurry in the slurry vessel has a solid
fraction of approximately 0.1-0.7. Cooling the slurry vessel
containing the molten metal may be performed at a rate of
0.2-5.0.degree. C./sec, preferably, 0.2-2.0.degree. C./sec.
[0024] In accordance with another aspect of the present invention,
there is provided an apparatus for manufacturing a semi-solid
metallic slurry, the apparatus comprises at least one slurry
vessel; at least one stirring unit having a space for the at least
one slurry vessel and applying an electromagnetic field to the
space; a driving unit moving the slurry vessel at least up and down
to place the slurry vessel in the space; and a loading unit loading
a molten metal in liquid state into the slurry vessel.
[0025] According to specific embodiments of the semi-solid metallic
slurry manufacturing apparatus, the at least one stirring unit
applies the electromagnetic field to the space prior to loading the
molten metal into the at least one slurry vessel. The at least one
stirring unit applies the electromagnetic field to the space at the
start of loading the molten metal into the at least one slurry
vessel. The at least one stirring unit applies the electromagnetic
field to the space in the middle of loading the molten metal into
the at least one slurry vessel.
[0026] Further, the driving unit may move the slurry vessel up
after a predetermined time from loading the molten metal into the
at least one slurry vessel to draw the at least one slurry vessel
out from the space.
[0027] Further, the driving unit may laterally shift the at least
one slurry vessel. In this case, the driving unit may comprise a
rotary plate supporting the at least one slurry vessel at an edge.
The driving unit with the rotary plate moves the at least one
slurry vessel down after a predetermined time from loading the
molten metal into the at least one slurry vessel, and rotates the
rotary plate to draw the at least one slurry vessel out from the
space.
[0028] Further, the driving unit may be constructed to be laterally
moveable along a rail, so that it moves the at least one slurry
vessel down after a predetermined from loading the molten metal
into the at least one slurry vessel, and is moved along the rail to
draw the at least one slurry vessel out from the space.
[0029] According to a specific embodiment of the semi-solid
metallic slurry manufacturing apparatus, the at least one stirring
unit applies the electromagnetic field to the space until a slurry
in the at least one slurry vessel has a solid fraction of
approximately 0.001-0.7, preferably, 0.001-0.4, more preferably,
0.001-0.1.
[0030] In specific embodiments, the at least one slurry vessel used
in the semi-solid metallic slurry manufacturing apparatus may
include a temperature control element. This temperature control
element may comprise at least one of a cooler installed in the at
least one slurry vessel and an external electric heater. The
temperature control element cools a slurry in the at least one
slurry vessel to reach a solid fraction of approximately 0.1-0.7.
The temperature control element cools a slurry in the at least one
slurry vessel at a rate of approximately 0.2-5.0.degree. C./sec,
preferably, 0.2-2.0.degree. C./sec.
BRIEF DESCRIPTION OF THE DRAWINGS
[0031] 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:
[0032] FIG. 1 is a graph of temperature profile applied in a method
for manufacturing a semi-solid metallic slurry according to the
present invention;
[0033] FIGS. 2 and 3 illustrate a structure of an apparatus for
manufacturing a semi-solid metallic slurry according to an
exemplary embodiment of the present invention;
[0034] FIG. 4 is a sectional view of an example of a slurry vessel
used in a semi-solid metallic slurry manufacturing apparatus
according to the present invention;
[0035] FIG. 5 illustrates a structure of a semi-solid metallic
slurry manufacturing apparatus according to another exemplary
embodiment of the present invention;
[0036] FIG. 6 illustrates a structure of a semi-solid metallic
slurry manufacturing apparatus according to still another exemplary
embodiment of the present invention;
[0037] FIGS. 7(a)-(e) illustrate the operation of the semi-solid
metallic slurry manufacturing apparatus of FIG. 6; and
[0038] FIG. 8 illustrates a structure of a semi-solid metallic
slurry manufacturing apparatus according to yet still another
exemplary embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0039] The present invention will be described more fully in the
following exemplary embodiments of the invention with reference to
the accompanying drawings.
[0040] Unlike the above-described conventional techniques, a method
of manufacturing a semi-solid metallic slurry according to the
present invention includes stirring molten metal by applying an
electromagnetic field prior to the completion of loading the molten
metal into a vessel. In other words, electromagnetic stirring is
performed prior to or at the start or in the middle of loading the
molten metal into the vessel, to prevent formation of dendritric
structures. Ultrasonic waves instead of the electromagnetic field
can be applied for stirring.
[0041] In particular, an empty vessel is positioned in a space of a
semi-solid metallic slurry manufacturing apparatus. An
electromagnetic field is applied to the space, and molten metal is
loaded into the vessel. The intensity of the applied
electromagnetic field is strong enough to stir the molten
metal.
[0042] FIG. 1 is a graph of temperature profile applied in a method
for manufacturing a semi-solid metallic slurry according to the
present invention. As shown in FIG. 1, molten metal is loaded into
the vessel at a temperature Tp. As described above, an
electromagnetic field may be applied to the vessel prior to loading
molten metal into the vessel. Alternatively, the vessel may be
positioned in the space after the application of an electromagnetic
field into the space. 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 vessel.
[0043] Due to the electromagnetic stirring performed prior to the
completion of loading molten metal into the vessel, the molten
metal does not form dendritic structures near the inner wall of the
vessel at the early stage of solidification, and numerous
micronuclei are concurrently generated throughout the vessel
because the temperature of the entire molten metal is rapidly
dropped to a temperature lower than its liquidus temperature.
[0044] Applying an electromagnetic field to the vessel prior or
simultaneously to loading molten metal into the vessel leads to
active stirring of the molten metal in the center and the inner
wall regions of the vessel and rapid heat transfer throughout the
vessel, thereby suppressing the formation of solidification layers
near the inner wall of the vessel 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 vessel wall, so that the
molten metal can be cooled rapidly. Due to the electromagnetic
stirring, particles contained in the molten metal scatter upon
loading into the vessel and are dispersed throughout the vessel as
nuclei, so that a temperature difference in the vessel hardly
occurs during cooling. However, in conventional techniques where
molten metal is stirred after the completion of loading into a
vessel, the temperature of the molten metal suddenly drops as soon
as it contacts the low temperature inner vessel wall, so that
dendritic crystals grow from solidification layers formed near the
inner wall of the vessel at the early stage of cooling.
[0045] The principles of the present invention will become more
apparent when described in connection with latent heat of
solidification. In a method for manufacturing a semi-solid metallic
slurry according to the present invention, molten metal does not
solidify near the inner vessel 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 {fraction (1/400)} of the latent heat of
solidification. Therefore, dendrites, which are generated
frequently when using conventional methods near the inner vessel
wall at the early stage of cooling, are not formed, and the entire
molten metal in the vessel 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 dispersed uniformly
throughout the entire molten metal in the vessel. The increased
density of nuclei shortens the distance between the nuclei, and
spherical particles instead of dendritic particles are grown.
[0046] The same effects can be achieved even when an
electromagnetic field is applied in the middle of loading the
molten metal into the vessel. In other words, solidification layers
are hardly formed near the inner vessel wall even when
electromagnetic stirring begins in the middle of loading the molten
metal into the vessel.
[0047] It is preferable that the temperature, Tp, of the molten
metal be maintained in a range from its liquidus temperature to
100.degree. C. above the liquidus temperature (melt
superheat=0.about.100.degree. C.) at the time of being loaded into
the vessel. According to the present invention, since the entire
vessel containing the molten metal is cooled uniformly, it allows
for the loading of the molten metal into the vessel at a
temperature of 100.degree. C. above its liquidus temperature,
without the need to cool the temperature of the molten metal to
near its liquidus temperature.
[0048] On the other hand, in conventional methods, an
electromagnetic field is applied to a vessel after the completion
of loading molten metal into a vessel and a portion of the molten
metal reaches 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.degree. C. above its liquidus temperature.
[0049] 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 vessel reaches a temperature lower than its
liquidus temperature T.sub.1, 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
molten metal in the vessel throughout all the cooling process of
the molten metal, but prior to a subsequent forming process such as
die casting or hot forging. This is because, once nuclei are
distributed uniformly throughout the vessel, the electromagnetic
stirring does not affect the growth of crystalline particles from
the nuclei in the metallic slurry. Therefore, the electromagnetic
stirring can be sustained until the solid fraction of the molten
metal reaches at least 0.001-0.7. However, the electromagnetic
stirring is sustained until the solid fraction of the molten metal
reaches the range of, preferably, 0.001-0.4, and more preferably,
0.001-0.1, for energy efficiency.
[0050] After the electromagnetic stirring is completed, the vessel
containing the metallic slurry is drawn out from the
electromagnetic field application space for a continuous subsequent
process, for example, die casting, hot forging, billet formation.
The vessel containing the metallic slurry may be drawn out from the
electromagnetic field application space irrespective of the
termination of the electromagnetic stirring, i.e., after or during
the application of the electromagnetic field.
[0051] According to the present invention, the application of an
electromagnetic field performed prior to the completion of loading
the molten metal into the vessel to form and uniformly distribute
nuclei in the molten metal is followed by cooling to facilitate the
growth of the nuclei. This cooling process may be performed
simultaneously to loading the molten metal into the vessel.
[0052] As described above, the application of the electromagnetic
field may be sustained throughout all the cooling process. In other
worlds, cooling may be performed when the vessel containing the
metallic slurry stays in the electromagnetic field application
space, i.e., prior to the vessel being drawn out from the
electromagnetic field application space. As a result, a semi-solid
metallic slurry is manufactured in the electromagnetic field
application space and readily subjected to a following forming
process.
[0053] The cooling process may be sustained just prior to a
subsequent forming process, preferably, until the solid fraction of
the molten metal reaches approximately 0.1-0.7, i.e., up to time
t.sub.2 of FIG. 1. The molten metal may be cooled at a rate of
approximately 0.2-5.0.degree. C./sec, preferably, 0.2-2.0.degree.
C./sec depending on a desired distribution of nuclei and a desired
size of particles.
[0054] A semi-solid metallic slurry containing a predetermined
amount of solid is manufactured through the above-described
processes and readily subjected to billet formation, by rapid
cooling, for thixocasting or die casting, forging, or pressing to
form final products.
[0055] According to the present invention described above, a
semi-solid metallic slurry can be manufactured within a short time,
merely in 30-60 seconds from loading the molten metal into the
vessel for a metallic slurry with a solid fraction of approximately
0.1-0.7. In addition, formed products having a uniform, dense
spherical crystalline structure can be manufactured from the
semi-solid metallic slurry formed by the method according to the
present invention.
[0056] The above-described method for manufacturing a semi-solid
metallic slurry according to the present invention can be performed
using an apparatus according to an embodiment of the present
invention illustrated in FIGS. 2 and 3.
[0057] Referring to FIG. 2, a semi-solid metallic slurry
manufacturing apparatus according to an embodiment of the present
invention includes a space 13 to which an electromagnetic field is
applied, a stirring unit 1 equipped with a coil 11 for applying an
electromagnetic field to surround the space 13, at least one slurry
vessel 2 that can be accommodated in the space 13, a driving unit 3
for moving the slurry vessel 2 at least up and down, a loading unit
4 via which molten metal is loaded into the slurry vessel 2, and a
controller 5.
[0058] The stirring unit 1 is mounted on the top of a base plate 14
having a hollow member 14a. The base plate 14 is installed at a
predetermined height from the ground while supported by a support
member 15. The coil 11 for applying an electromagnetic field is
mounted on the base plate 14 around the hollow member 14a, while
supported by a frame 12 having an inner space for the space 13. The
coil 11 is electrically connected to the controller 5 and applies a
predetermined intensity of electromagnetic field to the space 13 to
electromagnetically stir the molten metal contained in the slurry
vessel 2 placed in the space 13. Although not illustrated in FIG.
2, the stirring unit 1 may be implemented as an ultrasonic
stirrer.
[0059] The slurry vessel 2 may be formed of a metallic material or
an insulating material and may have any size and any shape that can
be accommodated in the space 13 surrounded by the stirring unit 1.
However, it is preferable that the slurry vessel 2 is formed of a
material having a higher melting point than the molten metal to be
loaded thereinto. The slurry vessel 2 may have a lower stepped
portion 21 that fits a vessel receiver 33, to be described later,
to lock the slurry vessel 2 into the space 13. Although not
illustrated in FIG. 2, a thermocouple may be installed in the
slurry vessel 2 and connected to the controller 5 to provide
temperature information on the slurry vessel 2 to the controller
5.
[0060] The slurry vessel 2 may be formed with a simple structure
for containing molten metal, as illustrated in FIGS. 2 and 3.
However, the slurry vessel 2 may further comprise a temperature
control element 20, as illustrated in FIG. 4. The temperature
control element 20 is comprised of a cooler and/or a heater. In the
embodiment of FIG. 4, a cooling water pipe 23 is embedded in a
vessel body 22. Although not illustrated, alternatively, an
electric heater with a heating coil may be further installed
outside the slurry vessel 2. In addition, the cooler may be
implemented as a water jacket additionally attached outside the
slurry vessel 2, instead of the cooling water pipe 23. The cooler,
the heater, or a combination of the two may be installed in the
slurry vessel 2 to cool the molten metal contained in the slurry
vessel 2 at an appropriate rate. It will be obvious that such a
slurry vessel 2 can be applied to all of the following embodiments
of a semi-solid metallic slurry manufacturing apparatus according
to the present invention.
[0061] The driving unit 3 moves the slurry vessel 2 to place it in
the space 13 and to draw it out from the space 13. The driving unit
3 is implemented with a driving motor and a gear or a hydraulic
cylinder, etc. For example, the driving unit 3 may comprise a power
system 31 electrically connected to the controller 5, a piston 32
connected to and actuated by the power system 31 to move up and
down in the space 13, and a vessel receiver 33 attached to an end
of the piston 32 near the space 13 to support the slurry vessel 2
therein. The slurry vessel 2 is placed in the space 13 to fit the
vessel receiver 33.
[0062] In a state where the driving unit 3 is operated to raise the
piston 32 to place the slurry vessel 2 in the space 13, the loading
unit 4 supplies a molten metal in liquid state into the slurry
vessel 2. The loading unit 4 may be implemented with a general
ladle, which is electrically connected to the controller 5. Any
device for loading molten metal into the slurry vessel 2 can be
used for the loading unit 4.
[0063] In the embodiment of a semi-solid metallic slurry
manufacturing apparatus according to the present invention
illustrated in FIG. 2, after the driving unit 3 is operated to
place the slurry vessel 2 in the space 13, an electromagnetic field
having a predetermined frequency is applied to the space 13 at a
predetermined intensity by the coil 11 of the stirring unit 1.
Alternatively, the slurry vessel 2 may be placed in the space 13
after the application of the electromagnetic field to the space 13.
Next, a metal molten in a separate electrical furnace is loaded via
the loading unit 4 into the slurry vessel 2 under the
electromagnetic field. Applying an electromagnetic field to the
space 13 may be performed at the start or in the middle of loading
molten metal into the vessel 2, in addition to prior to the
loading, as described above.
[0064] After a predetermined period of time from the loading of the
molten metal into the slurry vessel 2, the driving unit 3 is
operated to raise the slurry vessel 2, as illustrated in FIG. 3, to
draw the slurry vessel 2 out from the space 13 and to load another
empty vessel into the space 13, for example, using a transfer unit
such as a robot. Next, the slurry vessel 2 drawn out from the space
13 is subjected to cooling at a predetermined rate until the solid
fraction of a resulting semi-solid metallic slurry reaches the
range of 0.1-0.7. The molten metal in the slurry vessel 2 may be
cooled at a rate of approximately 0.2-5.0.degree. C./sec,
preferably, 0.2-2.0.degree. C./sec. Alternatively, the molten metal
in the slurry vessel 2 may be cooled prior to being drawn out from
the space 13 by the driving unit 3. In this case, after the
completion of cooling the molten metal in the slurry vessel 2, the
slurry vessel 2 is drawn out from the space 13 and replaced with
another empty vessel.
[0065] The application of an electromagnetic field to the space 13
may be sustained throughout all the cooling process, i.e., prior to
drawing the slurry vessel 2 out from the space 13 by the operation
of the driving unit 3, until the solid fraction of the resulting
semi-solid metallic slurry reaches the range of approximately
0.001-0.7. However, the application of an electromagnetic field to
the space 13 by the coil 11 is sustained after loading the molten
metal into the slurry vessel 2 until the solid fraction reaches,
preferably, at least 0.001-0.4, more preferably, 0.001-0.1, for
energy efficiency, as described above. The time required for the
solid fraction to reach such a level can be experimentally
measured. It is obvious that cooling can be performed while the
electromagnetic field is applied to the space 13, as described
above.
[0066] As illustrated in FIG. 5, a semi-solid metallic slurry
manufacturing apparatus according to another embodiment of the
present invention includes at least two slurry vessels 2a and 2b
for simultaneous slurry formations. The basic structure of the
semi-solid metallic slurry manufacturing apparatus in this
embodiment is the same as in the previous embodiment, and thus, a
detailed description thereon will be omitted here. In the
embodiment of FIG. 5, two vessel receivers 33a and 33b for the at
least two slurry vessels 2a and 2b are mounted on a receiver plate
34. It is preferable that the height of the vessel receivers 33a
and 33b is substantially equal to the height of the spaces 13a and
13b such that the slurry vessels 2a and 2b fitted into the vessel
receivers 33a and 33b can be raised up to the top of the spaces 13a
and 13b to be drawn out therefrom, respectively.
[0067] FIG. 6 illustrates a semi-solid metallic slurry
manufacturing apparatus according to still another embodiment of
the present invention, which differs from the previous embodiments
in that the driving unit 3 is constructed to be able to laterally
shift the slurry vessel 2. The following description will be
focused on this difference from the previous embodiments.
[0068] Referring to FIG. 6, a semi-solid metallic slurry
manufacturing apparatus according to still another embodiment of
the present invention includes a rotary plate 35 attached to an end
of the piston 32 of the driving unit 3. The piston 32 is attached
to nearly the center of the rotary plate 35. At least two vessel
receivers 33a and 33b are mounted at the edge of the rotary plate
35, and the slurry vessels 2a and 2b are fitted into the vessel
receivers 33a and 33b, respectively. The power system 31 is
constructed to be able to move up and down and rotate the piston
32. As the rotary plate 35 is rotated by the power system 31, the
slurry vessel 2a is laterally shifted away from the space 13, as
shown in FIG. 6. The operation of the semi-solid metallic slurry
manufacturing apparatus of FIG. 6 will be described detail with
reference to FIG. 7.
[0069] FIG. 7 sequentially illustrates the operation of the
semi-solid metallic slurry manufacturing apparatus of FIG. 6. As
illustrated in FIG. 7(a), the piston 32 is raised to place the
first slurry vessel 2a in the space 13, and an electromagnetic
field is applied to the space 13 by the coil 11 of the stirring
unit 1. Alternatively, the first slurry vessel 2a may be placed in
the space 13 after the application of an electromagnetic field.
[0070] Next, as illustrated in FIG. 7(b), molten metal is loaded
into the first slurry vessel 2a from the loading unit 4 and left in
the electromagnetic field for a predetermined time. As described
above, the electromagnetic field may be applied at the start or in
the middle of loading the molten metal into the first slurry vessel
2a.
[0071] Next, as illustrated in FIG. 7(c), the piston 32 is moved
down to draw the first slurry vessel 2a out from the space 13.
Next, as illustrated in FIG. 7(d), the piston 32 is rotated to
switch the first slurry vessel 2a and the second slurry vessel 2b,
which is empty. The slurry in the first slurry vessel 2a is cooled
at an appropriate rate to form a semi-solid metallic slurry
containing a predetermined amount of solid. The piston 32 is raised
again, as illustrated in FIG. 7(e), to repeat the above processes
on the second slurry vessel 2b. The first slurry vessel 2a
containing the semi-solid metal is transferred by a transfer unit,
such as a robot 6, for a subsequent forming process.
[0072] A large amount of semi-solid metallic slurry can be
manufactured continuously using an apparatus according to the
present invention described above with more convenience when
applied to a subsequent process and enhanced overall processing
efficiency.
[0073] Laterally shifting a slurry vessel can be achieved in
various other ways, in addition to the method described in the
above embodiment. For example, the driving unit 3 may be
constructed to be laterally movable along a rail 36, as shown in
FIG. 8.
[0074] The methods and apparatuses for manufacturing a semi-solid
metallic slurry 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,
for rheocasting.
[0075] Semi-solid metallic slurries manufactured according to the
present invention contain micro spherical particles of even
distribution with an average diameter of 10-60 .mu.m and provide
improved mechanical properties, even for alloys. According to the
present invention, such uniform spherical particles can be formed
within a short time through electromagnetic stirring initiated at a
temperature above the liquidus temperature of a source metal to
generate more nuclei throughout the slurry vessel.
[0076] When using a semi-solid metallic slurry manufacturing
apparatus according to the present invention, the overall slurry
manufacturing process can be simplified, and the duration of
electromagnetic stirring and forming (casting) time can be greatly
shortened, thereby saving energy for the stirring and costs. The
semi-solid metallic slurry manufacturing apparatus according to the
present invention makes it convenient to perform a subsequent
process and increases the yield of formed products.
[0077] 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.
* * * * *