U.S. patent application number 14/456900 was filed with the patent office on 2014-11-27 for method for improving mouldability of magnesium-alloy sheet material, and magnesium-alloy sheet material produced thereby.
This patent application is currently assigned to Korea Institute of Machinery and Materials. The applicant listed for this patent is Korea Institute of Machinery and Materials. Invention is credited to Seong Gu Hong, Ha Sik Kim, Hyung Lae Kim, Young Min Kim, Chong Soo Lee, Jeong Hun Lee, Sung Hyuk Park, Chang Dong Yim, Bong Sun You.
Application Number | 20140348696 14/456900 |
Document ID | / |
Family ID | 48984404 |
Filed Date | 2014-11-27 |
United States Patent
Application |
20140348696 |
Kind Code |
A1 |
Park; Sung Hyuk ; et
al. |
November 27, 2014 |
METHOD FOR IMPROVING MOULDABILITY OF MAGNESIUM-ALLOY SHEET
MATERIAL, AND MAGNESIUM-ALLOY SHEET MATERIAL PRODUCED THEREBY
Abstract
A method for increasing formability of magnesium alloy sheet and
the magnesium alloy sheet prepared by the same are provided, in
which the method includes the following steps: forming {10-12}
twins in magnesium alloy sheet (step 1); and annealing the
magnesium alloy sheet of step 1 (step 2). The present invention
also provides the magnesium alloy sheet containing {10-12} twins
prepared by the method. Accordingly, the room temperature
formability and the warm formability are increased by forming
{10-12} twins through deformation on the magnesium alloy sheet and
subsequently performing annealing, because it is possible to
artificially form {10-12} twins without increasing dislocations in
the magnesium alloy sheet and accommodate the deformation generated
during the forming via annihilation of the twins.
Inventors: |
Park; Sung Hyuk;
(Changwon-si, KR) ; Kim; Young Min; (Changwon-si,
KR) ; Kim; Ha Sik; (Changwon-si, KR) ; Yim;
Chang Dong; (Changwon-si, KR) ; You; Bong Sun;
(Changwon-si, KR) ; Hong; Seong Gu; (Daejeon,
KR) ; Lee; Chong Soo; (Pohang-si, KR) ; Lee;
Jeong Hun; (Pohang-si, KR) ; Kim; Hyung Lae;
(Ulsan, KR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Korea Institute of Machinery and Materials |
Daejeon |
|
KR |
|
|
Assignee: |
Korea Institute of Machinery and
Materials
Daejeon
KR
|
Family ID: |
48984404 |
Appl. No.: |
14/456900 |
Filed: |
August 11, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
PCT/KR2012/010780 |
Dec 12, 2012 |
|
|
|
14456900 |
|
|
|
|
Current U.S.
Class: |
420/409 ;
148/667; 72/342.2 |
Current CPC
Class: |
B22D 11/0622 20130101;
C22F 1/06 20130101; C22C 23/02 20130101; C22C 23/00 20130101 |
Class at
Publication: |
420/409 ;
148/667; 72/342.2 |
International
Class: |
C22F 1/06 20060101
C22F001/06; C22C 23/02 20060101 C22C023/02 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 13, 2012 |
KR |
1020120014278 |
Claims
1. A method for increasing formability of magnesium alloy sheet
comprising the following steps: forming {10-12} twins in magnesium
alloy sheet (step 1); and annealing the magnesium alloy sheet of
step 1 (step 2), wherein the magnesium alloy sheet comprises
{10-12} twins present after the annealing and reduced dislocations
than before the annealing.
2. The method for increasing formability of magnesium alloy sheet
according to claim 1, wherein the forming of the {10-12} twins in
step 1 causes texture to develop, in which basal planes are aligned
perpendicular to the rolling direction.
3. The method for increasing formability of magnesium alloy sheet
according to claim 1, wherein the step 1 is performed by applying
compressive deformation to the magnesium alloy sheet.
4. The method for increasing formability of magnesium alloy sheet
according to claim 3, wherein the compressive deformation is
applied to the direction parallel to the rolling plane of the
magnesium alloy sheet.
5. The method for increasing formability of magnesium alloy sheet
according to claim 3, wherein the amount of compressive deformation
is 1.about.15%.
6. The method for increasing formability of magnesium alloy sheet
according to claim 1, wherein the annealing is performed at
200.about.550.degree. C. for 10.about.480 minutes.
7. The method for increasing formability of magnesium alloy sheet
according to claim 6, wherein the annealing is performed at
350.about.500.degree. C. for 60.about.240 minutes.
8. A magnesium alloy sheet with increased formability, which is
prepared by the method according to claim 1 and comprises the
{10-12} twins.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This is a continuation of International Application No.
PCT/KR2012/010780, filed Dec. 12, 2012, the entire disclosure of
which is incorporated herein by reference in its entirety.
FIELD
[0002] The present invention relates to a method for increasing
formability of magnesium alloy sheet and the magnesium alloy sheet
prepared by the same method.
DESCRIPTION OF RELATED ART
[0003] Magnesium is the lightest metal whose density is 1.74
g/cm.sup.3, the smallest density among all of structural metals
including aluminum and steel. Magnesium is also characterized by
high specific strength, excellent machinability, dampling capacity,
and electromagnetic wave shielding property.
[0004] For the properties of magnesium alloys as those mentioned
above, use of magnesium alloys has recently been growing, replacing
steel and aluminum alloys, to suit demands for lighter weighted
transport equipments with higher fuel efficiency. The magnesium
alloys also find increasing use in applications like mobile phones
or laptop computers where LTSS (light, thin, short and small),
excellent electromagnetic wave shieldability, etc. are required.
However, magnesium alloy has problems of comparatively lower
strength, ductility, and corrosion resistance than those of
aluminum alloy. Ductility of magnesium alloy can be improved by
raising process temperature. However, due to difficulty of
compression-molding products with complicated shapes or shaft
corners, commercialization of magnesium alloy is still
restricted.
[0005] Conventionally, magnesium alloy parts have been prepared
largely by die casting or squeeze casting. However, the parts
prepared by the said conventional methods demonstrated weakness in
mechanical properties including elongation and strength mostly
because of casting defects, for example gas cavity, compared with
the other parts prepared by plastic working including rolling,
extruding, forging, etc.
[0006] In general, magnesium alloy has the hexagonal closed packed
structure (HCP), suggesting that it has less slip systems than
other metals which results in the low formability at room
temperature.
[0007] Particularly, according to von Mises yield criterion, 5
independent slip systems must operate to deform magnesium alloy
randomly. However, because the critical resolved shear stress
(CRSS) of non-basal slip for magnesium and its alloys is much
larger than that of basal slip at room temperature, it is difficult
for non-basal slip to operate properly. Accordingly, only 2
independent basal slips operate at room temperature, resulting in
the low formability.
[0008] It is generally known that wrought magnesium alloys prepared
by the conventional method in the art develop strong texture. That
is, the wrought magnesium alloys have most basal planes of crystal
grains aligned parallel to the rolling plane of the sheet or
extrusion direction of the extruded bar, so that when subjected to
tensile stress applied in the direction parallel to processing
direction (i.e., rolling direction or extrusion direction), Schmid
factor of the basal slip approaches 0, and the basal slip, which is
the main slip system of magnesium alloy, is hardly activated.
[0009] According to the conventional method, the wrought magnesium
alloys are warm formed at 200.degree. C. at which the non-basal
slip is activated, in order to improve formability of the wrought
magnesium alloy.
[0010] However, the warm forming has following disadvantages
compared with the cold forming performed at room temperature; an
equipment to control molding temperature is additionally required;
and the production cost goes high because of the extended molding
time. In addition, die soldering between mold and magnesium alloy
and the difficulty in regulation of thermal strain of mold are also
known to be the problems of warm forming.
[0011] Following methods have been informed as the way to improve
formability of wrought magnesium alloys.
[0012] Korean Patent No. 10-0783918 (Registration date: Dec. 3,
2007) relates to the method to improve formability of magnesium
alloy sheet at room temperature by the texture control, in which
slab-shaped magnesium ingot in mushy state prepared from magnesium
alloy sheet by using twin roll casting undergoes 6-step hot-rolling
to control texture and improve formability at room temperature
(patent reference 1). However, KR10-0783918 is limited to the use
of magnesium alloy sheet twin roll casting, indicating the
limitation of applicability and problem of difficulty in
controlling the complicated rolling condition.
[0013] Korean Patent No. 10-0860091 (Registration date: Sep. 18,
2008) relates to magnesium alloy with reduced axial ratio and the
method for preparing the said magnesium alloy sheet, in which rare
earth elements or commercialized alloys composed of rare earth
elements are added to magnesium alloy to reduce the ratio of unit
cell height (c) to unit cell side length (a) of hexagonal closed
packed structure in order to prepare magnesium alloy with improved
formability at room temperature (patent reference 2). However, the
magnesium alloy prepared as suggested in patent reference 2 has a
problem of difficulty of mass-production because production cost
inevitably increased owing to need for the addition of the
expensive rare earth elements.
[0014] In the course of study to improve formability of wrought
magnesium alloy, i.e., the magnesium alloy sheet, at room
temperature, the present inventors found out that the formability
of magnesium alloy sheet at room temperature or at warm-forming
condition could be improved, by deforming magnesium alloy sheet to
develop {10-12} twins and removing the dislocations increased by
the deformation by annealing, that is, by forming {10-12} twins in
magnesium alloy without increasing dislocations and accommodating
the deformation induced during forming via the {10-12} twin
annihilation, and thus completed the present invention.
SUMMARY OF THE INVENTION
[0015] It is an object of the present invention to provide a method
for increasing formability of magnesium alloy sheet.
[0016] It is another object of the present invention to provide a
magnesium alloy sheet containing {10-12} twins prepared by the
above method.
[0017] To achieve the above objects, the present invention provides
a method for increasing formability of magnesium alloy sheet
comprising the following steps:
[0018] forming {10-12} twins in magnesium alloy sheet (step 1);
and
[0019] annealing the magnesium alloy sheet of step 1 (step 2).
[0020] The present invention also provides a magnesium alloy sheet
containing {10-12} twins prepared by the said method.
[0021] As explained hereinbefore, the method for increasing
formability of magnesium alloy sheet of the present invention is
characterized by forming {10-12} twins through the deformation of
the sheet which may be accomplished by compressive deformation in
the direction parallel to the basal plane of texture of the sheet
and removing, by annealing, dislocations increased due to the
deformation. Accordingly, the method provides advantageous effect
of improved formability of magnesium alloy sheet at room
temperature or at warm forming condition, by artificially forming
{10-12} twins without increasing dislocations in magnesium alloy,
and by easily accommodating the deformation induced during forming
via twin annihilation.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] The application of the preferred embodiments of the present
invention is best understood with reference to the accompanying
drawings, wherein:
[0023] FIG. 1 is a set of photomicrographs illustrating the changes
of microstructure according to compressive deformation in the
magnesium alloy sheet of the present invention;
[0024] FIG. 2 is a graph illustrating twin volume fractions
according to the compressive deformation in the magnesium alloy
sheet of the present invention;
[0025] FIG. 3 is a set of photographs illustrating the results of
electron back scattered diffraction (EBSD) displaying the changes
of texture according to compressive deformation in the magnesium
alloy sheet of the present invention;
[0026] FIG. 4 is a diagram illustrating the testing device for
Erichsen test;
[0027] FIG. 5 is a diagram illustrating the specimen used for
evaluating formability in Erichsen test;
[0028] FIG. 6 is a set of photographs illustrating the shape of the
specimen used for Erichsen test of the magnesium alloy sheet of the
present invention;
[0029] FIG. 7 is a load-deflection graph measured during Erichsen
test with the magnesium alloy sheet of the present invention
(Example 2);
[0030] FIG. 8 is a load-deflection graph measured during Erichsen
test with the magnesium alloy sheet of the present invention
(Comparative Example 1);
[0031] FIG. 9 is a photograph illustrating the result of EBSD
displaying the changes of microstructure in the magnesium alloy
sheet of the present invention (Example 2);
[0032] FIG. 10 is a photograph illustrating the result of EBSD
displaying the changes of microstructure in the magnesium alloy
sheet of the present invention over the forming (Example 2);
[0033] FIG. 11 is a photograph illustrating the result of EBSD
displaying the changes of microstructure in the magnesium alloy
sheet of the present invention over the forming (Example 2);
[0034] FIG. 12 is a photograph illustrating the result of EBSD
displaying the changes of microstructure in the magnesium alloy
sheet of the present invention (Comparative Example 1); and
[0035] FIG. 13 is a photograph illustrating the result of EBSD
displaying the changes of microstructure in the magnesium alloy
sheet of the present invention over the forming (Comparative
Example 1).
DESCRIPTION OF THE PREFERRED EMBODIMENTS As used herein,
"A.about.B" is defined as a range between at least A (A or more)
and up to B (B or under), unless defined otherwise.
[0036] As used herein, "room temperature" means the temperature
range of 0.about.50.degree. C.
[0037] The present invention provides a method for increasing
formability of magnesium alloy sheet comprising the following
steps:
[0038] forming {10-12} twins in magnesium alloy sheet (step 1);
and
[0039] annealing the magnesium alloy sheet of step 1 (step 2).
[0040] The present invention also provides a magnesium alloy sheet
comprising {10-12} twins prepared by the said method.
[0041] Hereinafter, the present invention is described in
detail.
[0042] Before explaining the invention in detail, the formability
of the magnesium alloy sheet of the present invention will be
explained below based on the following principals.
[0043] In the course of rolling or extruding, an intense texture
having a preferred crystal orientation to a specific direction is
formed on the processed magnesium alloy material. In the rolled
magnesium alloy material, the basal planes of crystal grains are
aligned parallel to the rolling direction. In the extruded
magnesium alloy material, the basal planes of crystal grains are
aligned parallel to the extrusion direction.
[0044] When magnesium alloy sheet is formed by stretch processing
or deep drawing processing, a multi-axial tensile strain is
developed parallel to the rolling plane and a compressive
deformation is developed along the thickness direction which is
perpendicular to the rolling plane.
[0045] In general, the basal planes of crystal grains in magnesium
alloy sheet are aligned parallel to the rolling plane of the
magnesium alloy sheet. So, the tensile strain parallel to the
rolling plane is accommodated by basal slip having the slip
direction to a-axis, making cold forming comparatively easy.
However, because the compressive deformation developed along the
thickness direction which is perpendicular to the rolling plane
requires the strain to the direction of c-axis, cold forming is
difficult because of limitation in deformation to the direction of
thickness caused by unaquirable deformation mode at room
temperature.
[0046] Therefore, according to the present invention, {10-12} twins
were artificially formed to control the texture in the magnesium
alloy sheet, to improve the formability of magnesium alloy sheet at
room temperature.
[0047] Unlike dislocations, the {10-12} twins are generated at a
specific stress direction and the band has the angle of
approximately 86.degree. to the initial grain. The {10-12} twins
are only generated when such tensile stress is loaded to the c-axis
and not generated when such compressive stress is loaded to the
c-axis.
[0048] Precisely speaking, in the wrought magnesium alloy in which
the basal planes of crystal grains are aligned parallel to the
rolling plane or the extrusion direction, when a compressive stress
is loaded to the direction of processing, the magnesium alloy
becomes stress state in which the c-axis of crystal grain is under
tension, and thus {10-12} twins are easily generated. On the other
hand, when a tensile stress is applied to the direction of
extruding or rolling, {10-12} twins are hardly generated.
[0049] Also, the {10-12} twins can accommodate deformation easily
during the generation and annihilation, thereby leading to the low
yield strength and low strain hardening rate. The {10-12} twins are
annihilated when the stress is applied to the opposite direction of
the stress applied in order to form the twins.
[0050] Based on the above principles, the present invention is
described in more detail.
[0051] The present invention provides a method for increasing
formability of magnesium alloy sheet comprising the following
steps:
[0052] forming {10-12} twins in magnesium alloy sheet (step 1);
and
[0053] annealing the magnesium alloy sheet of step 1 (step 2).
[0054] In the method of the present invention, step 1 is to form
{10-12} twins in the magnesium alloy sheet.
[0055] The magnesium alloy sheet as used herein can be selected
from the group consisting of ZM21, ZC63, AZ91, AZ91D, AM50A, AM608,
AZ31, and AZ80, but not always limited thereto and any commercial
magnesium alloy sheet acceptable by those skilled in the art can be
used herein without limitation.
[0056] In the method for increasing formability of magnesium alloy
sheet of the present invention, the {10-12} twins can be formed in
the magnesium alloy sheet by applying a compressive deformation
thereto. The compressive deformation herein can be applied in the
direction parallel to the rolling plane of the magnesium alloy
sheet.
[0057] As explained hereinbefore, {10-12} twins can be formed by
applying a compressive deformation to the direction parallel to the
rolling plane of the magnesium alloy sheet, but not always limited
thereto. Accordingly, any method possibly used for forming {10-12}
twins can be implemented.
[0058] For the compressive deformation, it is preferred to make
deformation in the range of 1.about.15% to form {10-12} twins and
more preferably in the range of 3.about.10%.
[0059] Under loading of the less than 1% compressive deformation,
it is difficult to form {10-12} twins, suggesting that the
formability of magnesium alloy sheet is hardly improved. Under
loading of more than 15% compressive deformation, {10-12} twins are
saturated during the magnesium alloy processing, suggesting that
the formability is not improved and instead {10-11} twins are
formed rather to reduce the formability of magnesium alloy.
[0060] In the method of the present invention, step 2 is to treat
the magnesium alloy sheet of step 1 by annealing.
[0061] The dislocations generated in the magnesium alloy sheet by
the compressive deformation of step 1 can be eliminated through the
annealing in step 2.
[0062] In the method for increasing formability of magnesium alloy
sheet of the present invention, the annealing is preferably
performed at 200.about.550.degree. C. for 10.about.480 minutes, and
more preferably at 350.about.500.degree. C. for 60.about.240
minutes.
[0063] When the annealing is performed at the temperature under
200.degree. C. for less than 10 minutes, the dislocations generated
by the compressive deformation of step 1 are not completely
eliminated, resulting in the decrease of formability of magnesium
alloy sheet. When the annealing is performed at the temperature
over 550.degree. C. for more than 480 minutes, the partial
dissolution occurs in the segregate or second phase of the
magnesium alloy sheet, also resulting in the decrease of
formability of magnesium alloy sheet.
[0064] Practical and presently preferred embodiments of the present
invention are illustrative as shown in the following examples.
[0065] However, it will be appreciated that those skilled in the
art, on consideration of this disclosure, may make modifications
and improvements within the spirit and scope of the present
invention.
[0066] In the following examples, {10-12} twins were formed under
loading of a compressive deformation to the rolling direction of
the magnesium alloy sheet of the invention.
EXAMPLE 1
[0067] After loading of compressive deformation on the rolled AZ31
magnesium alloy sheet (thickness: 50 mm, composition: 3.6%
aluminum, 1.0% zinc, 0.5% manganese, magnesium, and other
inevitable impurities) to the rolling direction at the amount of
deformation of 2%, annealing followed at 350.degree. C. for 60
minutes.
EXAMPLE 2
[0068] After loading of compressive deformation on the rolled AZ31
magnesium alloy sheet (thickness: 50 mm, composition: 3.6%
aluminum, 1.0% zinc , 0.5% manganese, magnesium, and other
inevitable impurities) to the rolling direction at the amount of
deformation of 5%, annealing followed at 350.degree. C. for 60
minutes.
EXAMPLE 3
[0069] After loading of compressive deformation on the rolled AZ31
magnesium alloy sheet (thickness: 50 mm, composition: 3.6%
aluminum, 1.0% zinc, 0.5% manganese, magnesium, and other
inevitable impurities) to the rolling direction at the amount of
deformation of 8%, annealing followed at 350.degree. C. for 60
minutes.
EXAMPLE 4
[0070] After loading of compressive deformation on the rolled AZ31
magnesium alloy sheet (thickness: 50 mm, composition: 3.6%
aluminum, 1.0% zinc, 0.5% manganese, magnesium, and other
inevitable impurities) to the rolling direction at the amount of
deformation of 8%, annealing followed at 250.degree. C. for 60
minutes.
EXAMPLE 5
[0071] After loading of compressive deformation on the rolled AZ31
magnesium alloy sheet (thickness: 50 mm, composition: 3.6%
aluminum, 1.0% zinc, 0.5% manganese, magnesium, and other
inevitable impurities) to the rolling direction at the amount of
deformation of 8%, annealing followed at 450.degree. C. for 60
minutes.
COMPARATIVE EXAMPLE 1
[0072] Neither the compressive deformation loading nor annealing
was performed for the rolled AZ31 magnesium alloy sheet (thickness:
50 mm, composition: 3.6% aluminum, 1.0% zinc, 0.5% manganese,
magnesium, and other inevitable impurities).
COMPARATIVE EXAMPLE 3
[0073] Compressive deformation was not loaded on the rolled AZ31
magnesium alloy sheet (thickness: 50 mm, composition: 3.6%
aluminum, 1.0% zinc, 0.5% manganese, magnesium, and other
inevitable impurities), but annealing was performed at 350.degree.
C. for 60 minutes.
COMPARATIVE EXAMPLE 4
[0074] Compressive deformation was not loaded on rolled AZ31
magnesium alloy sheet (thickness: 50 mm, composition: 3.6%
aluminum, 1.0% zinc ( ) 0.5% manganese, magnesium, and other
inevitable impurities), but annealing was performed at 450.degree.
C. for 60 minutes.
COMPARATIVE EXAMPLE 5
[0075] After loading of compressive deformation on the rolled AZ31
magnesium alloy sheet (thickness: 50 mm, composition: 3.6%
aluminum, 1.0% zinc, 0.5% manganese, magnesium, and other
inevitable impurities) to the rolling direction at the amount of
deformation of 2%, annealing was not performed.
COMPARATIVE EXAMPLE 6
[0076] After loading of compressive deformation on the rolled AZ31
magnesium alloy sheet (thickness: 50 mm, composition: 3.6%
aluminum, 1.0% zinc, 0.5% manganese, magnesium, and other
inevitable impurities) to the rolling direction at the amount of
deformation of 5%, annealing was not performed.
COMPARATIVE EXAMPLE 7
[0077] After loading of compressive deformation on the rolled AZ31
magnesium alloy sheet (thickness: 50 mm, composition: 3.6%
aluminum, 1.0% zinc, 0.5% manganese, magnesium, and other
inevitable impurities) to the rolling direction at the amount of
deformation of 8%, annealing was not performed.
[0078] Analysis
[0079] 1. Analysis of Changes in Microstructure of Magnesium Alloy
Sheet According to Compressive Deformation
[0080] To investigative the changes in microstructure of the
magnesium alloy sheet of the present invention according to amount
of compressive deformation, observation of the microstructure
before and after the compressive deformation was performed. The
results are shown in FIG. 1 and FIG. 2 (FIGS. 1 and 2 : Comparative
Example 1, Comparative Examples 5.about.7)
[0081] FIG. 1 illustrates the formation of twins, in which the
light region indirectly indicates the formation of twins. FIG. 2 is
a graph illustrating twin volume fractions according to the
compressive deformation in the magnesium alloy sheet of the present
invention.
[0082] Referring to FIG. 1 and FIG. 2, twins were not found in the
initial magnesium alloy sheet before loading of the compressive
deformation, but twins were increased according to the increase of
the compressive deformation.
[0083] 2. Analysis of Texture of Magnesium Alloy Sheet According to
Compressive Deformation
[0084] To analyze the texture of the magnesium alloy sheet of the
present invention according to compressive deformation, electron
back scattered diffraction (EBSD) was performed before and after
the compressive deformation, and the results are shown in FIG. 3
(FIG. 3 : Comparative Example 1, Comparative Examples
5.about.7).
[0085] The twinned region formed by compressive deformation was
reoriented at the angle of approximately 86.degree. to the initial
crystal grain. As shown in the (0002) pole figure in FIG. 3, the
basal planes of crystal grains of the initial magnesium alloy sheet
without compressive deformation were aligned parallel to the
rolling plane. In the meantime, in the sheet with compressive
deformation, the original texture was weakened because of the
reorientation caused by twins and the basal planes of crystal
grains were aligned perpendicular to the rolling direction. Such
changes in the texture grew bigger as the compressive deformation
increased.
[0086] From the above results, it was confirmed that the
compressive deformation parallel to the rolling plane induced the
formation of twins in the magnesium alloy sheet, by which the
texture of the alloy sheet was changed.
EXPERIMENTAL EXAMPLE 1
Analysis of Formability of Magnesium Alloy Sheet According to
Compressive Deformation and Heat Treatment 1
[0087] To analyze the formability of the magnesium alloy sheet of
the present invention, Erichsen test was performed as shown in FIG.
4 and the results are presented in Table 1 and FIG. 6 (FIG. 6 :
Comparative Example 1, Examples 2 and 3).
[0088] Particularly, the magnesium alloy sheet of the invention was
prepared in the processed specimens as shown in FIG. 5 (thickness:
1 mm, diameter: 50 mm) for the Erichsen test. The Erichsen test was
conducted with the specimens at 23.degree. C. (room temperature),
100.degree. C., 200.degree. C., and 300.degree. C. at the speed of
0.1 mm/s to obtain limit dome height (LDH). The presented LDH is
the mean value obtained from the experiment repeated at least three
times for each condition. At this time, as the LDH is bigger, the
formability becomes excellent.
TABLE-US-00001 TABLE 1 Com- pressive Annealing strain Temp Time
Limit Dome Height (%) (.degree. C.) (Min) 23.degree. C. 100.degree.
C. 200.degree. C. 300.degree. C. Example 1 2 350 60 3.2 4.2 4.4 6.6
Example 2 5 350 60 5.1 5.8 6.0 6.8 Example 3 8 350 60 6.3 7.1 8.1
8.8 Example 4 8 250 60 3.9 -- -- -- Example 5 8 450 60 6.4 -- -- --
Comparative -- -- -- 3.1 3.8 4.1 5.1 Example 1 Comparative -- 250
60 3.1 -- -- -- Example 2 Comparative -- 350 60 3.0 -- -- --
Example 3 Comparative -- 450 60 3.2 -- -- -- Example 4 Comparative
2 -- -- 2.9 -- -- -- Example 5 Comparative 5 -- -- 3.0 -- -- --
Example 6 Comparative 8 -- -- 2.9 -- -- -- Example 7
[0089] Referring to Table 1, it was suggested that the improvement
in the formability was hardly observed in the magnesium alloy sheet
treated with compressive deformation (or annealing) only, compared
with the formability of the conventional magnesium alloy sheet
which is not processed by any of the said processes (Comparative
Example 1 in Table 1).
[0090] Precisely, when the magnesium alloy sheet was subjected to
the compressive deformation only, the formability was not increased
because the dislocations generated by the compressive deformation
reduced elongation of the sheet even with the presence of twins.
When the magnesium alloy sheet was subjected to heat treatment
(i.e., annealing) only, again, the formability was not improved
because the texture of crystal grains in the magnesium alloy sheet
was not much changed.
[0091] On the contrary, the magnesium alloy sheet of the present
invention demonstrated maximum 103% increased room temperature
formability and maximum 98% increased warm formability, compared
with the conventional sheet, under every experimental temperature
condition.
[0092] Particularly, as the amount of compressive deformation in
the magnesium alloy sheet increases, twin volume fraction was
increased, indicating that the accommodatable deformation to the
thickness direction of the magnesium alloy sheet was increased.
Also, the dislocations generated in the magnesium alloy sheet were
eliminated through annealing, indicating the formability of the
magnesium alloy sheet was increased.
[0093] Therefore, the above results confirmed that the magnesium
alloy sheet of the present invention could be used widely in the
industry requiring energy efficiency improvement, owing to the
increased warm and room temperature formability obtained by forming
twins with compressive deformation and eliminating the dislocations
generated by the compressive deformation with annealing.
EXPERIMENTAL EXAMPLE 2
Analysis of Formability of Magnesium Alloy Sheet According to
Compressive Deformation and Heat Treatment 2
[0094] To analyze more precisely the causes of the improvement of
formability of the magnesium alloy sheet of the present invention,
electron back scattered diffraction (EBSD) was performed with the
thick region of the center of the specimen, which is experienced
the biggest deformation during forming, before and after the
forming as shown in the load-displacement graph obtained from
Erichsen test (FIG. 7 and FIG. 8). The results are shown in FIG.
9.about.FIG. 13 (FIG. 7, FIGS. 9.about.11: Example 2; FIG. 8, FIG.
12, FIG. 13: Comparative Example 1).
[0095] FIG. 9.about.FIG. 11 illustrate the results of EBSD
performed with the magnesium alloy sheet of Example 2 before
forming (FIG. 9), with the magnesium alloy sheet formed to the
thickness of 3 mm by Erichsen test (FIG. 10), and with the
magnesium alloy sheet formed to the thickness of 4 mm (FIG.
11).
[0096] FIG. 12 and FIG. 13 illustrate the results of EBSD performed
with the magnesium alloy sheet of Comparative Example 1 before
forming (FIG. 12) and with the magnesium alloy sheet formed to the
thickness of 3 mm (FIG. 13).
[0097] According to FIG. 7 and FIG. 8, the magnesium alloy sheet of
the present invention displays improved formability by compressive
deformation to the rolling direction and heat-treatment.
[0098] According to FIG. 9, twins were formed in a large scale in
the magnesium alloy sheet before forming by compressive deformation
to the rolling direction and FIG. 10 and FIG. 11 reveal that twins
decreased as the forming processed.
[0099] According to FIG. 12, twins were not formed in the magnesium
alloy sheet treated with neither compressive deformation nor
heat-treatment. Referring to FIG. 13, small amount of contraction
twins or {10-11}-{10-12} double twins were formed along the forming
process by the compressive deformation to the thickness direction,
which may cause fracture of the material.
[0100] Therefore, the above results indicate that the magnesium
alloy sheet of the present invention is characterized by improved
formability at room temperature as a result of compressive
deformation to the rolling direction and annealing, because the
deformations generated during forming are accommodated by
annihilating {10-12} twins massively formed without increasing
dislocations in the magnesium alloy sheet.
[0101] Those skilled in the art will appreciate that the
conceptions and specific embodiments disclosed in the foregoing
description may be readily utilized as a basis for modifying or
designing other embodiments for carrying out the same purposes of
the present invention. Those skilled in the art will also
appreciate that such equivalent embodiments do not depart from the
spirit and scope of the invention as set forth in the appended
Claims.
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