U.S. patent application number 10/419929 was filed with the patent office on 2004-03-25 for metallic materials for rheocasting or thixoforming and method for manufacturing the same.
Invention is credited to Hong, Chun Pyo, Itamura, Masayuki, Kim, Jae Min, Kim, Min Soo.
Application Number | 20040055734 10/419929 |
Document ID | / |
Family ID | 31999422 |
Filed Date | 2004-03-25 |
United States Patent
Application |
20040055734 |
Kind Code |
A1 |
Hong, Chun Pyo ; et
al. |
March 25, 2004 |
Metallic materials for rheocasting or thixoforming and method for
manufacturing the same
Abstract
A method for manufacturing a metallic material for rheocasting
or thixoforming and a metallic material formed using the method are
provided. The method includes: applying an electromagnetic field to
a vessel and loading a molten metal into the vessel; and cooling
the molten metal to form a metallic material for rheocasting or
thixoforming. The entire volume of molten metal is rapidly and
uniformly cooled throughout, from the wall toward the center of the
vessel, without generating latent heat caused by the formation of
solidification layers at the early stage of cooling. The molten
metal in the vessel is cooled rapidly below its liquidus
temperature within 1-10 seconds after the loading of the molten
metal into the vessel, so that numerous uniform crystal nuclei are
created throughout the entire volume of molten metal to form a
metallic material having uniform, micro, spherical particles.
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: |
31999422 |
Appl. No.: |
10/419929 |
Filed: |
April 22, 2003 |
Current U.S.
Class: |
164/499 ;
164/900 |
Current CPC
Class: |
C22C 1/005 20130101;
B22D 1/00 20130101; B22D 17/007 20130101; C22C 21/02 20130101 |
Class at
Publication: |
164/499 ;
164/900 |
International
Class: |
B22D 027/02 |
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 |
2003-13498 |
Claims
What is claimed is:
1. A method for manufacturing a metallic material for rheocasting
or thixoforming, comprising: applying an electromagnetic field to a
vessel and loading a molten metal into the vessel; and cooling the
molten metal to form a metallic material for rheocasting or
thixoforming.
2. The method of claim 1, wherein the electromagnetic field is
applied prior to the loading of the molten metal into the
vessel.
3. The method of claim 1, wherein the electromagnetic field is
applied simultaneously with the loading of the molten metal into
the vessel.
4. The method of claim 1, wherein the electromagnetic field is
applied in the middle of the loading of the molten metal into the
vessel.
5. The method of claim 1, wherein the application of the
electromagnetic field is stopped when the molten metal has a solid
fraction of 0.001-0.7.
6. The method of claim 1, wherein the application of the
electromagnetic field is stopped when the molten metal has a solid
fraction of 0.001-0.4.
7. The method of claim 1, wherein the application of the
electromagnetic field is stopped when the molten metal has a solid
fraction of 0.001-0.1.
8. The method of claim 1, wherein the metallic material is in the
form of slurries or billets.
9. The method of claim 1, wherein the molten metal is loaded into
the vessel in a temperature range between a liquidus temperature of
the molten metal and 100.degree. C. above the liquidus
temperature.
10. The method of claim 1, further comprising a secondary forming
process for the metallic material after cooling the molten
metal.
11. The method of claim 10, wherein the secondary forming process
for the metallic material includes die casting, squeeze casting,
forging, and pressing.
12. The method of claim 8, further comprising remelting the billets
back to semi-solid or semi-molten state for a secondary forming
process.
13. The method of claim 1, wherein the molten metal is cooled until
the molten metal has a solid fraction of 0.1-0.7.
14. The method of claim 1, wherein the molten metal is cooled at a
rate of 0.2-5.degree. C./sec.
15. The method of claim 1, wherein the molten metal is cooled at a
rate of 0.2-2.degree. C./sec.
16. The method of claim 1, wherein the molten metal is selected
from the group consisting of aluminum, magnesium, zinc, copper,
iron, and alloys of the forgoing metals.
17. A metallic material for rheocasting or thixoforming in the form
of slurries or billets manufactured according to the method of
claim 1, the metallic material having spherical particles with
uniform distribution.
18. The metallic material of claim 17, wherein the spherical
particles of the metallic material have an average diameter of
10-60 .mu.m.
Description
[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
2003-13498 filed on Mar. 4, 2003, in the Korean Intellectual
Property Office, the disclosures of which are incorporated herein
in their entireties by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to metallic materials for
rheocasting or thixoforming, and a method for manufacturing the
same.
[0004] 2. Description of the Related Art
[0005] Semi-solid or semi-molten metal processing combines casting
and forging processes and can be further divided into two
categories--rheocasting and thioxforming. In the rheocasting
process, a slurry prepared in a semi-solid state is directly cast
into final products. In the thixoforming process, billets which has
been formed from its semi-solid state is reheated to a semi-molten
state and then cast into final products through forging or die
casting.
[0006] Metal slurry for rheocasting or thixoforming refers to a
metallic material consisting of solid particles suspended in a
liquid phase in an appropriate ratio at temperature ranges for
semi-solid state, changing its form easily even by a small force
due to its thixotropic properties, and being cast like a liquid due
to its high fluidity. Billet can easily be processed back to a
metal slurry in a semi-molten state by reheating and, therefore, is
very useful metallic material for rheocasting or thioxforming.
[0007] Rheocasting or thixoforming, which uses metallic slurries or
billets, is more advantageous than processes which use liquid metal
alloys of the same composition. For example, metallic slurries have
fluidity at a temperature lower than the temperature at which
liquid metal alloys of the same composition completely melt, so
that the die casting temperature can be lowered, thereby ensuring
an extended lifespan of the die. In addition, when a metallic
slurry is extruded, turbulence does not occur and less air is
incorporated during a casting process, thereby preventing formation
of air pockets in final products. Therefore, the final product can
be subjected to a subsequent thermal process for improving
mechanical properties thereof. Besides, the use of metallic
slurries or billets leads to reduced shrinkage during
solidification, improved working efficiency and anti-corrosion, and
lightweight products. Therefore, such metal slurries can be used as
new materials in the productions of automobiles, airplanes, and
information communications equipment.
[0008] In conventional semi-solid alloy manufacturing methods,
dendritic particles are broken up into spherical particles suitable
for rheocasting, mainly by stirring molten metal at a temperature
lower than its liquidus temperature. Stirring methods include
mechanical stirring, electromagnetic stirring, gas bubbling,
electric shock agitation, and low-frequency, high-frequency, or
electromagnetic wave vibration and the like.
[0009] As an example, U.S. Pat. No. 3,948,650 discloses a method
for manufacturing a liquid-solid mixture. In this method, alloys
are heated to a temperature at which most alloys reach a liquid
phase, and the resulting molten metal is cooled while being
vigorously stirred. Specifically, by stirring and cooling the
molten metal until the percentage of solids in the molten metal
reaches 40-65%, the formation of dendritic particles is prevented
or dendritic particles on primary solid particles are eliminated or
reduced.
[0010] U.S. Pat. No. 4,465,118 discloses a method for manufacturing
a semi-solid alloy slurry. In this method, a molten metal in a
vessel is mixed electromagnetically by a moving, non-zero magnetic
field provided over substantially all of a solidification zone
within the vessel. The magnetic field causes the shearing of
dendrites formed in the solidification zone at a desired shearing
rate.
[0011] U.S. Pat. No. 4,694,881 discloses a method for manufacturing
a thixotropic material. In this method, an alloy is heated to a
temperature above its liquidus 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.
[0012] Japanese Patent Laid-open Application N0 11-33692 discloses
a method for producing a metallic slurry for rheocasting. In this
method, a molten metal is poured into a slurry manufacturing
container 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 the metallic slurry having spherical
particles for rheocasting.
[0013] In particular, dendritic particle structures, which are
considered to be grown from discrete nuclei at the initial stage of
solidification, are broken into separate particles by applying an
appropriate force near its liquidus temperature and then slowly
cooled to form a spherical shape of particles without an
interaction between the nuclei. This method also uses a physical
force, such as ultrasonic vibration, to break up the dendritic
particle structures grown at the early stage of solidification. In
this method, if the pouring temperature is greatly higher 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 pouring a molten metal into a slurry manufacturing
container and vibrating the molten metal using a vibrating bar
dipped in the molten metal to directly transfer its vibrating force
to the molten metal. In particular, an alloy of a liquid phase
having crystal nuclei at temperatures above its liquidus
temperature or a semi-solid thixotropic alloy containing crystal
nuclei in a temperature range between its liquidus temperature and
forming temperature is formed first. Next, the molten metal in the
container is cooled down to a temperature at which it has a
predetermined liquid fraction and held from 30 seconds to 60
minutes to allow micronuclei in the alloy to grow larger, thereby
resulting in a semi-molten metal. This method provides relatively
large particles of about 100 .mu.m and requires 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 a molten metal to form
the 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 techniques use
shear force to break the previously formed dendritic structures
into spherical structures during a cooling cycle. 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 caused by the formation of initial solidification
layers is generated. As a result, there are many disadvantages such
as reduced cooling rate and increased manufacturing time. In
addition, there is a need to precisely control the temperature
during loading the molten metal into the vessel. Otherwise,
dendritic structures are inevitably formed at the early stage of
solidification near the inner vessel wall due to a temperature
difference between the inner wall and the center of the vessel.
Therefore, the prior art necessitates the precise control of the
loading temperature and the cooling processes.
SUMMARY OF THE INVENTION
[0017] The present invention provides metallic materials for
rheocasting or thixoforming and a method for manufacturing the
same, with the advantages of finer spherical particles, improved
energy efficiency, reduced manufacturing costs, improved mechanical
properties, convenient casting process, and reduced manufacturing
time, compared to conventional methods.
[0018] According to an aspect of the present invention, there is
provided a method for manufacturing metallic materials for
rheocasting or thixoforming, comprising: applying an
electromagnetic field to a vessel and loading a molten metal into
the vessel; and cooling the molten metal to form a metallic
material for rheocasting or thixoforming.
[0019] According to another aspect of the present invention, there
is provided a metallic material for rheocasting or thixoforming in
the form of slurries or billets manufactured according to the above
method, the metallic material having spherical particles grown from
uniform crystal nuclei.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] 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:
[0021] FIG. 1A is a graph illustrating a process for manufacturing
a metallic material for rheocasting or thixoforming according to an
embodiment of the present invention, and FIG. 1B is a photograph
showing the microstructure of the metallic material manufactured
according to the process shown in FIG. 1A;
[0022] FIGS. 2 through 5 are photographs showing the
microstructures of metallic materials for rheocasting or
thixoforming manufactured at various pouring temperatures of a
molten metal using the method according to the present
invention;
[0023] FIGS. 6 through 9 are photographs showing the
microstructures of metallic materials for rheocasting or
thixoforming manufactured at various cooling rates of a molten
metal after terminating the application of an electromagnetic
field, using the method according to the present invention;
[0024] FIGS. 10 through 12 are photographs showing the
microstructures of metallic materials for rheocasting or
thixoforming manufactured at various termination point of the
application of the electromagnetic field, using the method
according to the present invention;
[0025] FIGS. 13 through 16 are photographs showing the
microstructures of metallic materials for rheocasting or
thixoforming manufactured at various cooling end temperatures of
the molten metal, using the method according to the present
invention;
[0026] FIG. 17 is a photograph showing the microstructure of the
metallic material for rheocasting or thixoforming manufactured by
pouring molten metal and applying an electromagnetic field at the
same time according to the present invention;
[0027] FIG. 18 is a photograph showing the microstructure of the
metallic material for rheocasting or thixoforming manufactured by
applying an electromagnetic field in the middle of pouring a molten
metal according to the present invention;
[0028] FIGS. 19A and 19B are photographs of the surface and core
regions, respectively, of a metallic material manufactured
according to another embodiment of the present invention;
[0029] FIGS. 20A and 20B are photographs of the surface and core
regions, respectively, of a metallic material manufactured
according to yet another embodiment of the present invention;
[0030] FIGS. 21A and 21B are photographs of the surface and core
regions, respectively, of a metallic material manufactured
according to a conventional method; and
[0031] FIGS. 22A and 22B are photographs of the surface and core
regions, respectively, of a metallic material manufactured
according to another conventional method.
DETAILED DESCRIPTION OF THE INVENTION
[0032] In a method for manufacturing metallic materials for
rheocasting or thixoforming according to the present invention, a
molten metal in a vessel has a uniform temperature. In particular,
since the temperature of the entire vessel containing the molten
metal is uniform throughout; at the center, inner wall, and upper
and lower regions, latent heat caused by a solidification in a
particular region is not generated at the early stage of cooling,
thereby enabling the molten metal to be cooled rapidly within a
short time. As a result, the density of crystal nuclei in the
molten metal markedly increases, leading to the formation of micro,
spherical particles.
[0033] Hereinafter, the present invention will be described in
greater detail.
[0034] According to the present invention, an electromagnetic field
is applied to a vessel before the completion of loading a molten
metal into the vessel, i.e., before, simultaneously, or in the
middle of loading of the molten metal into the vessel. Ultrasonic
waves instead of the electromagnetic field may be used. Suitable
metals which can be used in the method according to the present
invention include any metals available for rheocasting or
thixoforming, in which preferable metals are selected from the
group consisting of aluminum, magnesium, copper, zinc, iron, and
alloys of the forgoing metals. Such alloys may contain various
kinds of optional metals depending on the physical properties
required for final molded products.
[0035] It is preferable that the temperature of the molten metal be
maintained in a range from its liquidus temperature to 100.degree.
C. above the liquidus temperature (melt superheat=0-100.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 100.degree. C. above its
liquidus temperature, without the need to cool the temperature of
the molten metal to near its liquidus temperature.
[0036] On the other hand, in conventional methods, an
electromagnetic field is applied to a vessel after the completion
of loading a molten metal into the 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 at the inner wall of the vessel at the early stage of
cooling. Because the latent heat 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.
[0037] However, according to the present invention, since an
electromagnetic field is applied to a vessel before the completion
of loading a molten metal into the vessel, the entire vessel
containing the molten metal has a uniform temperature throughout,
i.e., at the inner wall, center region, and upper and lower regions
of the vessels. As a result, the molten metal does not solidify
near the inner wall of the vessel, which occurs in conventional
methods, and the entire molten metal in the vessel can be cooled
down rapidly below its liquidus temperature, thereby enabling
simultaneous formation of numerous crystal nuclei. In the present
invention, such a uniform temperature throughout the vessel is
directly related with the electromagnetic field applied to the
vessel before the completion of loading the molten metal into the
vessel. The electromagnetic field applied to the vessel before the
completion of loading the molten metal into the vessel induces the
entire molten metal to be vigorously stirred in the space between
the inner wall and the center of the vessel and facilitates heat
transfer throughout the molten metal in the vessel, thereby
suppressing the formation of solidification layers of the molten
metal near the inner vessel wall at the early stage of cooling. In
addition, while the molten metal is being thoroughly stirred,
conductive heat transfer from the molten metal to the comparatively
low-temperature inner vessel wall is facilitated, so that the
temperature of the entire molten metal is rapidly lowered. In the
present invention, as the molten metal is loaded into the vessel
and simultaneously stirred by the electromagnetic field, solid
particles in the molten metal scatter as crystal nuclei throughout
the vessel. As a result, a temperature disparity in the molten
metal at the various regions of the vessel does not occur. However,
in conventional methods, as a molten metal is loaded into a
low-temperature vessel, conductive heat transfer from the molten
metal to the vessel abruptly occurs, thereby resulting in the
formation of dendritic particles at the early stage of
solidification.
[0038] The principles of the present invention will become more
apparent when described in connection with latent heat of
solidification. In a method for manufacturing metallic materials
for rheocasting or thixoforming 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 is generated from
solidification. 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. Therefore, the temperature of the molten metal
can be lowered within a short time, uniformly throughout the
vessel, without the formation of dendritic particles at the early
stage of solidification. It takes merely about 1-10 seconds to
lower the temperature to a desired temperature from the point of
time at which the molten metal is loaded. As a result, numerous
crystal nuclei are created and dispersed uniformly throughout the
entire molten metal in the vessel, and the increased density of
crystal nuclei shortens the distance between the crystal nuclei,
thereby resulting in the growth of spherical particles instead of
dendritic particles.
[0039] The application of the electromagnetic field is stopped when
the temperature of the molten metal in the vessel reaches near its
liquidus temperature. However, the application of the
electromagnetic field may be stopped at any point between the
completion of nucleation of the molten metal and the cooling
process. The application of the electromagnetic field is stopped
when the solid fraction of the molten metal reaches, preferably,
0.001-0.7, more preferably, 0.001 to 0.4, and most preferably,
0.01-0.1 for energy efficiency.
[0040] After the application of the electromagnetic field to the
vessel is stopped, the molten metal is cooled until the solid
fraction of the molten metal reaches, preferably, 0.1-0.7.
[0041] In the cooling process, the molten metal is cooled,
preferably, at a rate of 0.2-5.0.degree. C./sec, and more
preferably, 0.2-2.0.degree. C./sec for more uniform distribution of
nuclei and smaller particle formation.
[0042] According to the present invention, after the loading of
molten metal into a vessel, a metallic material as slurry with a
solid fraction of 0.1-0.7 can be manufactured shortly in 30-60
seconds. This metal slurry can be processed into billets by rapid
cooling.
[0043] Metallic materials in the form of slurries or billets
according to the present invention may be subjected to secondary
molding, such as die casting, squeeze casting, forging, and press,
etc. Alternatively, metallic materials in the form of billets
according to the present invention may be cut to proper length to
form slugs. This slug is melted back to semi-solid state by
reheating for secondary forming.
[0044] A metallic material for rheocasting or thixoforming
manufactured using the method according to the present invention
contains metal particles that are spherical and have an average
diameter of 10-60 .mu.m and uniform distribution.
[0045] Hereinafter, the present invention will be described in
greater detail with reference to the following examples. The
following examples are for illustrative purposes and are not
intended to limit the scope of the invention.
EXAMPLE 1
[0046] An aluminum alloy, A356, was used for a molten metal. 500 g
of A356 alloy was melted using a graphite crucible in an electrical
furnace (10 kW) by heating at about 750.degree. C. for 1 hour. The
temperature of the resulting molten metal was measured at a K-type
thermal conduction sheath equipped with a digital thermometer to
maintain temperature of 100.degree. C. above the liquidus
temperature (about 615.degree. C. for A356 alloy) of the molten
metal.
[0047] FIG. 1A is a graph illustrating a working process for
manufacturing a metallic material according to the present
invention. An electromagnetic field was applied to a vessel using
an electromagnetic stirrer (EMS), which was manufactured by the
inventors, at a voltage of 250V, a frequency of 60 Hz, and an
intensity of 500 Gauss. Before pouring the molten metal into the
vessel, power was supplied to the EMS to operate and generate an
electromagnetic field. When the temperature of the molten metal
reached a pouring temperature (Tp) of 650.degree. C. (see FIG. 1A),
the molten metal was poured into the vessel.
[0048] After pouring the molten metal into the vessel with the
electromagnetic field to induce stirring of the molten metal, EMS
was shut off when the temperature of the molten metal reached near
its liquidus temperature (point "a" in FIG. 1A). The EMS was
operated only for the time interval "p" of FIG. 1A. Next, the
molten metal was cooled at a rate of 1.degree. C./sec to a
temperature at which the molten metal had a solid fraction of 0.6
(point "b" of FIG. 1A, corresponding to about 586.degree. C.) to
obtain a metal slurry. It took about 40 seconds from the pouring of
the molten metal into the vessel until the solid fraction of the
metal slurry became 0.6.
[0049] After point "b" of FIG. 1A, the metal slurry was subjected
to secondary forming process, such as die casting, squeeze casting,
forging, press, etc.
[0050] To observe the microstructure of the metallic material
manufactured according to the method of Example 1, sliced samples
were prepared as follows. The metal slurry was rapidly cooled, and
sliced using a bandsaw, polished, and etched in Keller solution (20
mL of H.sub.2O, 20 mL of HCL, 20 mL of HNO.sub.3, and 5 mL of HF),
and used as sliced samples for image analysis. The structure of the
sliced sample was observed using an image analyzer (LEICA DMR). The
result is shown in FIG. 1B. As is apparent from the image of FIG.
1B, the metallic material manufactured using the method according
to the present invention has a structure of micro, spherical
particles whose size is uniform, from the core to surface regions
of the cross-section.
EXAMPLES 2 THROUGH 5
[0051] Metallic materials were manufactured in the same manner as
in Example 1, except that the pouring temperature Tp of the molten
metal was varied to 720.degree. C. (Example 2), 700.degree. C.
(Example 3), 650.degree. C. (Example 4), and 620.degree. C.
(Example 5), the operation of the EMS was stopped when the solid
fraction of the molten metal became 0.05 (slightly above the
liquidus temperature), and the molten metal was cooled to obtain a
metal slurry having a solid fraction of 0.6. The metal slurries
were rapidly cooled, sliced samples were prepared according to the
same method as used in Example 1, and the microstructures thereof
were observed. The total time spent for manufacturing metallic
materials was less than 1 minute. FIGS. 2 through 5 show images
obtained from the image analysis for the samples of Examples 2
through 5, respectively. As shown in FIGS. 2 through 5, metal
alloys of micro, uniform particles that are spherical and have an
average diameter of 10-60 .mu.m can be manufactured with the range
of pouring temperatures of the molten metal from 720-620.degree.
C., within a short time of less than 1 minute. The high density of
crystal nuclei results in narrow distances between the particles
formed at the early stage of stirring and is believed to enable the
formation of semi-solid materials having particles of uniform size
and shape at a higher cooling rate than conventional methods.
EXAMPLES 6 THROUGH 9
[0052] Metallic materials were manufactured in the same manner as
in Example 1, except that the cooling rate of the molten metal was
varied to 0.2.degree. C./sec (Example 6), 0.4.degree. C./sec
(Example 7), 0.6.degree. C./sec (Example 8), and 2.0.degree. C./sec
(Example 9) to obtain metallic slurries. The resulting metal
slurries were rapidly cooled, sliced samples were prepared
according to the same method as used in Example 1, and the
microstructures thereof were observed. The results are shown in
FIGS. 6 through 9.
[0053] As shown in FIGS. 6 through 9, metallic materials of
spherical particles can be manufactured at the various cooling
rates of the molten metal. The spherical particles are fine with an
average particle diameter of 10-60 .mu.m and have uniform
distribution.
EXAMPLES 10 THROUGH 12
[0054] Metallic materials were manufactured in the same manner as
in Example 1, except that the applications of the electromagnetic
field were terminated when the solid fractions of the molten metal
were 0.2 (Example 10), 0.6 (Example 11), and 0.7 (Example 12). The
resulting metal slurries were rapidly cooled, sliced samples were
prepared according to the same method as used in Example 1, and the
microstructures thereof were observed. The results are shown in
FIGS. 10 through 12.
[0055] As is apparent from the images of FIGS. 10 through 12,
although the termination point of the application of the
electromagnetic field is varied, metal alloys of micro, spherical
particles can be manufactured with uniform distribution.
EXAMPLES 13 THROUGH 16
[0056] Metallic materials were manufactured in the same manner as
in Example 1, except that the cooling end temperature of the molten
metal was varied to 610.degree. C. (Example 13, equivalent to a
solid fraction of about 0.2), 600.degree. C. (Example 14),
590.degree. C. (Example 15), and 586.degree. C. (Example 16,
equivalent to a solid fraction of about 0.6) to obtain metallic
slurries. The resulting metal slurries were rapidly cooled, sliced
samples were prepared according to the same method as used in
Example 1, and the microstructures thereof were observed. The
results are shown in FIGS. 13 through 16.
[0057] As is apparent from the images of FIGS. 13 through 16,
although the cooling end temperature of the molten metal is varied,
metal alloys of micro, spherical particles can be manufactured with
uniform distribution. In other words, when the electromagnetic
field is applied to the vessel prior to the loading of the molten
metal and the electromagnetic stirring is continued until the
temperature of the molten metal reaches its liquidus temperature,
according to the method of the present invention, metal alloys of
uniform, micro, spherical particles can be manufactured regardless
of the changes of the cooling end temperature.
EXAMPLE 17
[0058] A metallic material was manufactured in the same manner as
in Example 1, except that the pouring temperature was 630.degree.
C., and the pouring of the molten metal and the application of the
electromagnetic field were performed simultaneously. The resulting
metal slurries were rapidly cooled, sliced samples were prepared
according to the same method as used in Example 1, and the
microstructures thereof were observed. The results are shown in
FIG. 17.
[0059] As is apparent from the images of FIG. 17, although the
pouring of the molten metal and the application of the
electromagnetic field were performed simultaneously, a metal alloy
of micro, spherical particles can be manufactured with uniform
distribution. In other words, the microstructure of the metallic
material prepared by applying the electromagnetic field
simultaneously with the pouring of the molten metal was
substantially the same as the one prepared by applying the
electromagnetic field prior to the pouring of the molten metal.
EXAMPLE 18
[0060] A metallic material was manufactured in the same manner as
in Example 1, except that the pouring temperature was 630.degree.
C., and the application of the electromagnetic field was performed
in the middle of (50% of the pouring process completed) pouring the
molten metal. The resulting metal slurries were rapidly cooled,
sliced samples were prepared according to the same method as used
in Example 1, and the microstructures thereof were observed. The
results are shown in FIG. 18.
[0061] As is apparent from the images of FIG. 18, although the
electromagnetic field was applied in the middle of the pouring of
the molten metal, a metal alloy of micro, spherical particles can
be manufactured with uniform distribution. In other words, the
microstructure of the metallic material prepared by applying the
electromagnetic field in the middle of the pouring process was not
much different from the ones prepared by the above-described
examples, even though the effect of applying the electromagnetic
field can be varied or reduced depending on the point of time at
which the electromagnetic field is applied.
EXAMPLE 19
[0062] A metallic material was manufactured in the same manner as
in Example 1, except that, the pouring temperature of the molten
metal was set to 650.degree. C., and the molten metal after being
stirred by the electromagnetic field was cooled at a rate of
1.5.degree. C./sec until the solid fraction reached 0.6. It took 35
seconds from the loading of the molten metal to the point of time
at which the metal slurry had a solid fraction of 0.6. Sliced
samples were prepared using the same method as in Example 1 for
microstructure observation, and the surface and core regions on
their cross-section were observed. The results are shown in FIGS.
19A and 19B.
EXAMPLE 20
[0063] A metallic material was manufactured in the same manner as
in Example 1, except that the pouring temperature of the molten
metal was set to 700.degree. C., and the molten metal after being
stirred by the electromagnetic field was cooled at a rate of
1.5.degree. C./sec until the solid fraction reached 0.6. It took 40
seconds from the loading of the molten metal to the point of time
at which the metal slurry had a solid fraction of 0.6. Sliced
samples were prepared using the same method as in Example 1 for
microstructure observation, and the surface and core regions on
their cross-section were observed. The results are shown in FIGS.
20A and 20B.
COMPARATIVE EXAMPLE 1
[0064] For comparison, a metallic material was manufactured in the
same manner as in Example 19, except that, after the molten metal
was loaded into the vessel, an EMS was operated at a temperature
slightly lower than the liquidous temperature of the molten metal
for 10 seconds, and the molten metal was cooled at a rate of
0.8.degree. C./sec until the solid fraction reached about 0.6. It
took 75 seconds from the loading of the molten metal to the point
of time at which the metal slurry had a solid fraction of 0.6.
Sliced samples were prepared using the same method as in Example 1
for microstructure observation, and the surface and core regions on
their cross-section were observed. The results are shown in FIGS.
21A and 21B.
COMPARATIVE EXAMPLE 2
[0065] For comparison, a metallic material was manufactured in the
same manner as in Example 20, except that, after the molten metal
was loaded into the vessel, an EMS was operated at a temperature
slightly lower than the liquidous temperature of the molten metal
for 10 seconds, and the molten metal was cooled at a rate of
1.0.degree. C./sec until the solid fraction reached about 0.6. It
took 85 seconds from the loading of the molten metal to the point
of time at which the metal slurry had a solid fraction of 0.6.
Sliced samples were prepared using the same method as in Example 1
for microstructure observation, and the surface and core regions on
their cross-section were observed. The results are shown in FIGS.
22A and 22B.
[0066] Comparing the results of Examples 19 and 20 and Comparative
Examples 1 and 2, the metallic materials manufactured in Examples
19 and 20 contain spherical particles that are fine and uniform in
average diameter at the core and surface regions of the
cross-section. However, for the metallic materials manufactured
using conventional methods in Comparative Examples 1 and 2, where,
after the molten metal was loaded into the vessel and the
temperature of the molten metal dropped below its liquidus
temperature, an electromagnetic field was applied to stir the
molten metal, there was a difference in the microstructure of the
core and surface regions of the cross-section, wherein spherical
particles appear at the core region and dendritic particles appear
at the surface region. Also, by using the method according to the
present invention, the manufacturing time of metallic materials for
rheocasting or thixoforming was greatly reduced. This is because
the initial density of crystal nuclei created from the molten metal
increases so that a predetermined solid fraction can be reached
through the growth of the crystal nuclei for a short time.
[0067] As is apparent from the above-described examples and
comparative examples, in a method for manufacturing metallic
materials for rheocasting or thixoforming according to the present
invention, it is possible to load the molten metal into a vessel at
a temperature about 100.degree. C. above its liquidus temperature,
and metallic materials for rheocasting or thixoforming having
micro, spherical particles can be manufactured in the form of
slurries or billets from alloys through electromagnetic stirring
for a short time.
[0068] Although manufacture of metallic materials for rheocasting
or thixoforming from commercially available A356 alloy has been
described in the above examples according to the present invention,
the present invention is not limited to this alloy, and other
various metals and alloys, for example, aluminum, magnesium, zinc,
copper, iron, and alloys of the forgoing metals can be used
according to the present invention.
[0069] As described above, in a method for manufacturing metallic
materials for rheocasting or thixoforming according to the present
invention, the entire volume of molten metal in the vessel can be
rapidly cooled below the liquidus temperature of the molten metal
uniformly throughout the center, peripheral, upper and lower
regions of the vessel, without generating latent heat caused by the
formation of solidification layers at the early stage of cooling.
As a result, the density of crystal nuclei is markedly increased,
so that alloys of uniform, micro, spherical particles of even
distribution can be manufactured with improved mechanical
properties.
[0070] A method for manufacturing metallic materials for
rheocasting or thixoforming according to the present invention is
simple and easy to control the overall procedure and can save the
time and energy for electromagnetic stirring. Therefore, the total
time and cost for manufacturing final products can be saved.
[0071] 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.
* * * * *