U.S. patent application number 11/629282 was filed with the patent office on 2007-11-08 for high-strength and high-toughness magnesium based alloy, driving system part using the same and manufacturing method of high-strength and high-toughness magnesium based alloy material.
Invention is credited to Katsuyoshi Kondoh.
Application Number | 20070258845 11/629282 |
Document ID | / |
Family ID | 35509686 |
Filed Date | 2007-11-08 |
United States Patent
Application |
20070258845 |
Kind Code |
A1 |
Kondoh; Katsuyoshi |
November 8, 2007 |
High-Strength and High-Toughness magnesium Based Alloy, Driving
System Part Using the Same and Manufacturing Method of
High-Strength and High-Toughness magnesium Based Alloy Material
Abstract
A high-strength and high-toughness magnesium based alloy
contains, by weight, 1 to 8% rare earth element and 1 to 6% calcium
and the maximum crystal grain diameter of magnesium constituting a
matrix is not more than 30 .mu.m. At least one intermetallic
compound (6) of rare earth element and calcium has a maximum grain
diameter of 20 .mu.m or less and it is dispersed in a crystal grain
boundary (5) and a crystal grain (4) of magnesium of the
matrix.
Inventors: |
Kondoh; Katsuyoshi; (Osaka,
JP) |
Correspondence
Address: |
CLARK & BRODY
1090 VERMONT AVENUE, NW
SUITE 250
WASHINGTON
DC
20005
US
|
Family ID: |
35509686 |
Appl. No.: |
11/629282 |
Filed: |
May 18, 2005 |
PCT Filed: |
May 18, 2005 |
PCT NO: |
PCT/JP05/09051 |
371 Date: |
December 13, 2006 |
Current U.S.
Class: |
420/405 ; 419/39;
420/403 |
Current CPC
Class: |
C22C 23/00 20130101;
B22F 2003/208 20130101; C22C 23/06 20130101; B22F 2998/10 20130101;
B22F 3/02 20130101; B22F 2998/10 20130101; B22F 3/20 20130101; C22C
1/0408 20130101; C22C 23/02 20130101 |
Class at
Publication: |
420/405 ;
419/039; 420/403 |
International
Class: |
C22C 23/06 20060101
C22C023/06; B22F 1/00 20060101 B22F001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 15, 2004 |
JP |
2004-177413 |
Claims
1. A high-strength and high-toughness magnesium based alloy
provided such that a plastic forming process is performed to
magnesium based alloy powder containing, by weight, 1 to 8% rare
earth element and 1 to 6% calcium to miniaturize a magnesium
crystal grain that constitutes a matrix and a compound grain
dispersed in the matrix, and immediately after heating a powder
solidified body provided from the miniaturized magnesium based
alloy powder, a warm extrusion process is performed to it,
characterized in that the maximum crystal grain diameter of
magnesium that constitutes the matrix is not more than 30 .mu.m,
said magnesium based alloy contains at least one intermetallic
compound of said rare earth element and said calcium, and when it
is assumed that the maximum grain diameter of said intermetallic
compound is "D" and the minimum grain diameter thereof is "d",
D/d.ltoreq.5 is satisfied.
2. The high-strength and high-toughness magnesium based alloy
according to claim 1, wherein the maximum grain diameter of the
intermetallic compound is not more than 20 .mu.m.
3. The high-strength and high-toughness magnesium based alloy
according to claim 2, wherein said intermetallic compound is a
compound of aluminum and rare earth element.
4. The high-strength and high-toughness magnesium based alloy
according to claim 2, wherein said intermetallic compound is a
compound of aluminum and calcium.
5. (canceled)
6. The high-strength and high-toughness magnesium based alloy
according to claim 2, wherein said intermetallic compound is
dispersed in said crystal grain boundary and crystal grain of
magnesium that constitutes said matrix.
7. The high-strength and high-toughness magnesium based alloy
according to claim 1, wherein the maximum crystal grain diameter of
magnesium that constitutes the matrix is not more than 20
.mu.m.
8. The high-strength and high-toughness magnesium based alloy
according to claim 1, wherein the maximum crystal grain diameter of
magnesium that constitutes the matrix is not more than 10
.mu.m.
9. The high-strength and high-toughness magnesium based alloy
according to claim 1, containing at least one kind, of element
selected from a element group consisting of, by weight, 0.5 to 6%
Zinc, 2 to 15% aluminum, 0.5 to 4% manganese, 1 to 8% silicon and
0.5 to 2% of silver.
10. The high-strength and high-toughness magnesium based alloy
according to claim 1, wherein a tensile strength (.sigma.) is not
less than 350 MPa and a breaking extension (.epsilon.) is not less
than 5%.
11. The high-strength and high-toughness magnesium based alloy
according to claim 1, wherein the product of the tensile strength
(.sigma.) and the breaking extension (.epsilon.) is such that
.sigma..times..epsilon..gtoreq.4000 MPa%.
12. The high-strength and high-toughness magnesium based alloy
according to claim 1, wherein said rare earth element contains at
least one kind of element selected from a group consisting of
cerium (Ce), lanthanum (La), yttrium (Y), ytterbium (Yb),
gadolinium (Gd), terbium (Tb), scandium (Sc), samarium (Sm),
praseodymium (Pr), and neodymium (Nd).
13. The high-strength and high-toughness magnesium based alloy
according to claim 1, containing, by weight, 1.5 to 4% manganese, 2
to 15% aluminum and iron of 10 ppm or less, wherein the maximum
grain diameter of an Al--Mn compound is not less than 20 .mu.m.
14. A driving system part for a car or a two-wheeled motor vehicle
using the high-strength and high-toughness magnesium based alloy
according to claim 1.
15. A manufacturing method of a high-strength and high-toughness
magnesium based alloy material comprising: a step of miniaturizing
a magnesium crystal grain that constitutes a matrix and
miniaturizing a compound grain dispersed in the matrix by
performing a plastic forming process to magnesium based alloy
powder containing, by weight, 1 to 8% rare earth element and 1 to
6% calcium; a step of manufacturing a powder solidified body from
said miniaturized magnesium based alloy powder by compression
molding; and a step of providing an alloy material by heating said
powder solidified body and immediately performing a warm extrusion
process to it.
Description
TECHNICAL FIELD
[0001] The present invention relates to a high-strength and
high-toughness magnesium based alloy and more particularly, to a
high-strength and high-toughness magnesium based alloy that is
superior in strength characteristics such as static tensile
characteristics, fatigue strength, and creep characteristics, and
superior in toughness such as breaking extension at room
temperature and at high temperature up to 200.degree. C. Such
high-strength and high-toughness magnesium based alloy is
advantageously applied to a car component and especially to an
engine part or a mission part used at a high temperature.
BACKGROUND ART
[0002] Since a magnesium alloy has low specific gravity and is
light in weight, it can be widely used in a package of a mobile
phone or a portable acoustic instrument, a car component, a machine
part, a structural material and the like. Especially, in order to
maximize the effect of light in weight, it is to be employed in a
part of motor system or an operating system and more particularly,
in a part of an engine system or driving system like a piston.
[0003] However, these parts and members require heat resistance
characteristics around 200.degree. C. in addition to the strength
and toughness at room temperature. According to a conventional
magnesium alloy, Mg--Al--Zn--Mn group alloy such as AZ91D alloy or
Mg--Al--Mn group alloy such as AM60B alloy defined in JIS standard,
for example, since its strength is lowered at a temperature above
120.degree. C., it cannot be used in the above part.
[0004] In order to answer the needs for reduction in weight, an
alloy in which the heat resistance characteristic of magnesium
alloy is improved has been aggressively developed. For example, in
"Materials Science Forum Vols. 419-422 (2003) pp. 425 to 432" in
"Magnesium Alloys 2003" at Magnesium International Conference
(Osaka International Conference Hall on Jan. 26 to 30, 2003), Mr.
Y. Guangyin and the like announced that they developed a
Mg--Al--Zn`Si--Sb--RE group alloy by a casting method and the alloy
had tensile strength of 178 MPa and a breaking extension of 14% at
150.degree. C. However, according to this alloy, since the average
crystal grain diameter of magnesium that constitutes a matrix is 70
.mu.m which is relatively large, its tensile strength is 235 MPa
and breaking extension is 9% at room temperature, so that it cannot
be applied to the above part.
[0005] Japanese Unexamined Patent Publication No. 2002-129272
discloses a Mg--Al--Zn--Ca--RE-Mn group magnesium alloy for
die-casting that is superior in creep characteristics at high
temperature around 150.degree. C. Since the magnesium alloy
disclosed in this document is manufactured by the casting method
similar to the case by Mr. Guangyin and the like, the following
problems are pointed out. [0006] (1) The crystal grain of magnesium
is as large as 60 to 150 .mu.m. [0007] (2) The compound such as
Al.sub.11RE.sub.3, Al.sub.2Ca, and Mg.sub.17Al.sub.12 deposited and
dispersed in the matrix grows to be coarse and becomes an acicular
compound having a length of 20 to 40 .mu.m or more. [0008] (3) The
acicular compound exist in a magnesium crystal grain boundary and
when it is excessively formed, it exists like a network along the
boundary.
[0009] As a result, there arises a problem such that it is inferior
in strength and toughness at room temperature. Furthermore, when
each element is added excessively in order to improve the tensile
characteristics at high temperature, a problem such as fluidity or
hot cracking is generated at the time of casting, so that the
content of an additive element is limited and further improvement
in heat resistance characteristics is not expected. For example, a
magnesium alloy provided by die-casting disclosed in the Japanese
Unexamined Patent Publication No. 2002-129272 is defined in its
appropriate content within a range containing, by weight, 1 to 3%
RE, 1 to 3% Ca, and 0.5 to 8% Al.
[0010] According to a high-strength magnesium alloy and a heat
treatment method of a magnesium alloy cast disclosed in Japanese
Unexamined Patent Publication No. 8-41576, it is described that a
cast alloy containing, by weight, 1 to 4% Al, 1 to 8% RE, 0.3 to
1.3% Ca, 0.1 to 2% Mn and the balance Mg has superior creep
characteristics. Furthermore, when a heat treatment such as
solution treatment or ageing treatment is performed to the Mg alloy
according to need, the characteristics are improved by enhancement
of solid solution of Al or Ca and enhancement of deposition of
Mg--Ca group compound.
[0011] However, since the magnesium alloy disclosed in the Japanese
Unexamined Patent Publication No. 8-41576 is manufactured by the
casting method, the Mg crystal grain is inevitably grown and
becomes coarse during its solidification. As a result, since its
tensile strength becomes 200 to 280 MPa at room temperature, it
cannot be applied to a car equipment or a machine part or a
structural material.
[0012] The inventor of the present invention found that the
following conditions were required to implement both high strength
and high toughness (extension) of the magnesium alloy within a
temperature range from room temperature up to around 200.degree. C.
[0013] (1) The crystal grain diameter of a magnesium alloy that
constitutes a matrix is to be miniaturized. [0014] (2) A compound
that is superior in heat resistance is to be uniformly deposited
and dispersed not as an acicular grain but as a fine grain. [0015]
(3) The above compound grain is to be dispersed in a magnesium
crystal grain as much as possible. [0016] (4) In order to deposit
and disperse the fine compound superior in heat resistance as much
as possible, it is effective to use a solid-phase (non-dissolved)
manufacturing method not using a conventional casting or
die-casting method but using a plastic forming method using powder
or chips as a starting raw material.
DISCLOSURE OF THE INVENTION
[0017] The present invention has been made in view of the above
findings and it is an object of the present invention to provide a
high-strength and high-toughness magnesium based alloy that is
superior in tensile strength, breaking extension and fatigue
strength at room temperature and at the same time has high heat
resistance characteristics at around 200.degree. C.
[0018] It is another object of the present invention to provide a
manufacturing method of a high-strength and high-strength magnesium
based alloy material having the above superior characteristics.
[0019] A high-strength and high-toughness magnesium based alloy
according to the present invention contains, by weight, 1 to 8%
rare earth element and 1 to 6% calcium, and characterized in that
the maximum crystal grain diameter of magnesium that constitutes a
matrix is not more than 30 .mu.m.
[0020] Preferably, the magnesium based alloy contains at least one
intermetallic compound of the rare earth element and the calcium,
in which the maximum grain diameter of the intermetallic compound
is not more than 20 .mu.m. One example of the intermetallic
compound is a compound of aluminum and rare earth element. Another
example of the intermetallic compound is a compound of aluminum and
calcium.
[0021] When it is assumed that the maximum grain diameter of the
intermetallic compound is "D" and the minimum grain diameter
thereof is "d", D/d.ltoreq.5 is satisfied. Further preferably, the
intermetallic compound is dispersed in the crystal grain boundary
and crystal grain of magnesium that constitutes the matrix. Here,
the maximum grain diameter means the maximum length of the compound
grain and the minimum grain diameter means the minimum length of
the compound grain.
[0022] Preferably, the maximum crystal grain diameter of magnesium
that constitutes the matrix is not more than 20 .mu.m. More
preferably, it is not more than 10 .mu.m.
[0023] According to one embodiment, the high-strength and
high-toughness magnesium based alloy contains at least one kind of
element selected from a group consisting of, by weight, 0.5 to 6%
Zinc, 2 to 15% aluminum, 0.5 to 4% manganese, 1 to 8% silicon and
0.5 to 2% silver.
[0024] Focusing on the mechanical characteristics of the
high-strength and high-toughness magnesium based alloy according to
the present invention, a tensile strength (.sigma.) is not less
than 350 MPa and a breaking extension (.epsilon.) is not less than
5%. In addition, focusing on another viewpoint, the product of the
tensile strength (.sigma.) and the breaking extension (.epsilon.)
is such that .sigma..times..epsilon..gtoreq.4000 MPa%.
[0025] Preferably, the rare earth element contains at least one
kind of element selected from a group consisting of cerium (Ce),
lanthanum (La), yttrium (Y), ytterbium (Yb), gadolinium (Gd),
terbium (Tb), scandium (Sc), samarium (Sm), praseodymium (Pr), and
neodymium (Nd).
[0026] According to one embodiment, the high-strength and
high-toughness magnesium based alloy contains, by weight, 1.5 to 4%
manganese, 2 to 15% aluminum and iron of 10 ppm or less and the
maximum grain diameter of an Al--Mn compound is not less than 20
.mu.m. Here, it is to be noted that the term "iron of 10 ppm or
less includes that iron is not included.
[0027] According to the high-strength and high-toughness magnesium
based alloy comprising the above constitution, since there is
provided a structure in which the matrix comprises magnesium having
fine crystal grain diameter and the fine granular intermetallic
compound is uniformly deposited and dispersed in the crystal grain,
it can be advantageously applied to a driving system part for a car
or a two-wheeled motor vehicle.
[0028] A manufacturing method of the high-strength and
high-toughness magnesium based alloy material according to the
present invention comprises the following steps. [0029] (1) A step
of miniaturizing a magnesium crystal grain that constitutes a
matrix and miniaturizing a compound grain dispersed in the matrix
by performing a plastic forming process to magnesium based alloy
powder containing, by weight, 1 to 8% rare earth element and 1 to
6% calcium. [0030] (2) A step of manufacturing a powder solidified
body from the miniaturized magnesium based alloy powder by
compression molding. [0031] (3) A step of providing an alloy
material by heating the powder solidified body and immediately
performing a warm extrusion process to it.
[0032] The working effect of the above-described present invention
will be described in the following "BEST MODE FOR CARRYING OUT
INVENTION" and "EXAMPLES")
BRIEF DESCRIPTION OF DRAWINGS
[0033] FIG. 1 is a view schematically showing the crystal structure
of a magnesium based alloy manufactured by a casting method;
[0034] FIG. 2 is a view schematically showing the crystal structure
of a magnesium based alloy manufactured by a solid-phase
manufacturing method using a plastic forming method;
[0035] FIG. 3 is a view showing manufacturing steps of a
high-strength and high-toughness magnesium based alloy material
according to the present invention;
[0036] FIG. 4 is a view showing one example of steps performed
repeatedly for starting raw material powder until a powder
solidified body is finally obtained;
[0037] FIG. 5A shows a structure photograph of a working example 9
shown in Table 1;
[0038] FIG. 5B shows a structure photograph of a working example 11
shown in Table 1;
[0039] FIG. 5C shows a structure photograph of a comparative
example 16 shown in Table 1;
[0040] FIG. 6A shows a structure photograph of an extruded material
(working example); and
[0041] FIG. 6B shows a structure photograph of an extruded material
(comparative example).
BEST MODE FOR CARRYING OUT THE INVENTION
[Effect of Each Additive Element]
[0042] (1) Rare Earth (RE) Element
[0043] A rare earth (RE) element component forms a Mg--RE compound
with magnesium that is a matrix, and forms Al--RE compound with
aluminum (Al) that is an example of an additive component. Since
the compound such as Al.sub.2RE or Al.sub.11RE.sub.3 is superior in
heat stability as compared with Mg--Al group compound such as
Mg.sub.2Al.sub.3 or Mg.sub.17Al.sub.12, when its fine powder is
diffused uniformly in the matrix, the heat resistance
characteristics of a magnesium alloy can be improved.
[0044] The appropriate range of a rare earth (RE) element content
is 1 to 8% by weight. When the rare earth (RE) element content is
less than 1%, the heat resistance characteristics are not
sufficiently improved. Meanwhile, when the rare earth (RE) element
content is more than 8%, the effect is not increased and on the
contrary, the deposited amount of the compound becomes excessive,
which causes a problem in the subsequent process. That is, when a
secondary process such as warm forging, rolling or drawing is
performed for the provided magnesium alloy, cracking is generated
due to lack of toughness. A more preferable rare earth element
content to provide both high strength and high toughness and
preferable secondary process workability is 3 to 5%.
[0045] By a normal casting method or a die-casting method, the
Mg--RE group compound and the Al--RE group compound are deposited
along a crystal grain boundary (.alpha. crystal grain boundary) of
magnesium and exist as acicular compounds or network-like compounds
formed with the connected acicular compounds as shown in FIG.
1.
[0046] FIG. 1 is a view schematically showing the crystal structure
of a magnesium based alloy manufactured by a casting method. A
magnesium crystal grain 1 that constitutes a matrix is coarse and
an acicular intermetallic compound 3 is provided along a crystal
grain boundary 2. When the acicular intermetallic compound 3 exists
along the crystal grain boundary 2 of the matrix in this way, the
mechanical characteristics of the magnesium based alloy is
lowered.
[0047] In view of the improvement in strength and toughness of the
magnesium based alloy, it is desirable that the intermetallic
compound is dispersed in the crystal grain as a fine granular
compound. FIG. 2 is a view. schematically showing the crystal
structure of a magnesium based alloy manufactured by a method of
the present invention that will be described below, that is, a
solid-phase manufacturing method using a plastic forming method. A
magnesium crystal grain 4 that constitutes a matrix is fine and a
fine granular intermetallic compound 6 is dispersed in a crystal
grain boundary 5 and the crystal grain 4. The magnesium based alloy
having the above structure provides superior characteristics in
strength and toughness.
[0048] Regarding the size of the intermetallic compound, a maximum
grain diameter is preferably not more than 20 .mu.m in view of
providing both high strength and high toughness, and more
preferably, it is not more than 10 .mu.m. When the maximum grain
diameter of the intermetallic compound is more than 20 .mu.m, the
toughness (breaking extension or an impact resistance value) of the
magnesium alloy at room temperature is lowered and especially when
it is more than 30 .mu.m, the strength is lowered with the lowering
of the toughness.
[0049] Regarding the configuration of the intermetallic compound,
it is more preferably granular than acicular. More specifically,
when it is assumed that the maximum grain diameter of the compound
grain is "D" and the minimum grain diameter thereof is "d", by
making an aspect ratio D/d below 5, both high strength and high
toughness can be provided. In view of the improvement of fatigue
strength, it is more preferably made below 3. Meanwhile, when the
ratio D/d is more than 5, the magnesium alloy becomes defective and
since a stress is concentrated at that part, the toughness is
lowered.
[0050] Since the ratio D/d of the acicular compound deposited along
the .alpha. crystal grain boundary by the casting method or
die-casting method is 5 to 20, it is difficult to provide high
strength and high toughness, and it is also difficult to provide
high fatigue strength.
[0051] In addition, as the rare earth element, cerium (Ce),
lanthanum (La), yttrium (Y), ytterbium (Yb), gadolinium (Gd),
terbium (Tb), scandium (Sc), samarium (Sm), praseodymium (Pr),
neodymium (Nd) and the like may be used. In addition, a misch metal
containing the above rare earth element may be used.
[0052] (2) Calcium (Ca)
[0053] Calcium (Ca) forms an Al--Ca group compound such as
Al.sub.2Ca with aluminum (Al) that is one example of the additive
component. Since this intermetallic compound is superior in heat
stability as compared with the Mg--Al group compound such as
Mg.sub.2Al.sub.3 or Mg.sub.17Al.sub.12 similar to the above Al--RE
group compound, when its fine compound grains are uniformly
dispersed in the matrix, the heat resistance characteristics of the
magnesium alloy can be improved. In addition, when Zn is contained,
a Mg--Zn--Ca group compound is formed and this contributes to the
improvement of the heat resistance characteristics similar to
Al.sub.2Ca.
[0054] An appropriate calcium content is 1 to 6% by weight. When
the calcium content is less than 1%, the effect of the improvement
of the heat resistance characteristics is not sufficiently
provided. Even when the calcium content is more than 6%, the effect
is not increased and on the contrary, the deposited amount of the
compound becomes excessive and a problem is raised in the
subsequent process. That is, when a secondary process such as warm
forging, rolling or drawing is performed for the provided magnesium
alloy, cracking is generated due to lack of toughness. A more
preferable calcium content to provide both high strength and high
toughness and preferable secondary process workability is 2 to
5%.
[0055] By a normal casting method or a die-casting method, the
Al--Ca group compound and the Mg--Zn--Ca group compound are also
deposited along a crystal grain boundary (.alpha. crystal grain
boundary) of magnesium and exist as acicular compounds or
network-like compounds formed with the connected acicular
compounds. As a result, the mechanical characteristics of the
magnesium based alloy is lowered. Hence, according to the present
invention, as described above, by applying strong processing strain
when a powdered or aggregated starting raw material is solidified
by the plastic forming method, the acicular or network-like Al--Ca
group compound and Mg--Zn--Ca group compound are finely ground and
uniformly dispersed in the magnesium crystal grain boundary and the
magnesium crystal grain as shown in FIG. 2.
[0056] Regarding the size of the intermetallic compound, a maximum
grain diameter is preferably not more than 20 .mu.m in view of
providing both high strength and high toughness, and more
preferably, it is not more than 10 .mu.m. When the maximum grain
diameter of the intermetallic compound is more than 20 .mu.m, the
toughness (breaking extension or an impact resistance value) of the
magnesium alloy at room temperature is lowered and especially when
it is more than 30 .mu.m, the strength is lowered with the lowering
of the toughness.
[0057] Regarding the configuration of the intermetallic compound,
it is more preferably granular than acicular. More specifically,
when it is assumed that the maximum grain diameter of the compound
grain is "D" and the minimum grain diameter thereof is "d", by
making an aspect ratio D/d below 5, both high strength and high
toughness can be provided. In view of the improvement of fatigue
strength, it is more preferably made below 3. Meanwhile, when the
ratio D/d is more than 5, the magnesium alloy becomes defective and
since a stress is concentrated at that part, the toughness is
lowered. Since the ratio D/d of the acicular compound deposited
along the .alpha. crystal grain boundary by the casting method or
die-casting method is 5 to 20, it is difficult to provide high
strength and high toughness, and it is also difficult to provide
high fatigue strength.
[0058] (3) Aluminum (Al)
[0059] Aluminum forms a Mg--Al group compound with magnesium of the
matrix and forms a Mg--Zn--Al group compound. Since the latter is
superior in heat resistance, when it is deposited and finely
dispersed in the matrix, it contributes to the improvement of the
heat resistance characteristics of the magnesium alloy. In order to
provide such effect, an Al content has to be not less than 2% by
weight. Meanwhile, when the Al content is more than 15%, a crack is
generated in an ingot in the course of manufacturing the ingot,
causing the productivity and yield to be lowered. Therefore, the
appropriate content of the Al component in the magnesium alloy in
the present invention is preferably in a range of 2 to 15% and in
view of providing both high strength and high toughness and the
above preferable secondary process workability, it is more
preferably in a range of 6 to 12%.
[0060] (4) Zinc (Zn)
[0061] Although zinc forms a Mg--Zn compound with magnesium of the
matrix, since this two-element compound is inferior in heat
stability, it lowers the heat resistance characteristics of the
magnesium alloy. However, as described above, when Al is added, a
Mg--Zn--Al group compound or Mg--Zn--Ca group compound that is
superior in heat resistance is formed and when solid solution
hardens the matrix as will be described below, it contributes to
the improvement of the heat resistance characteristics and
mechanical characteristics of the magnesium alloy at room
temperature. An appropriate Zn content in the magnesium alloy in
the present invention is 0.5 to 6% by weight. When it is less than
0.5%, the above effect is not sufficiently provided but when it is
more than 6%, the toughness of the magnesium alloy is lowered.
[0062] (5) Manganese (Mn)
[0063] Manganese (Mn) becomes solid solution in the magnesium
matrix and it contributes to the improvement of the mechanical
characteristics and especially resistance because of solid solution
hardening. An appropriate Mn content in the magnesium alloy in the
present invention is 0.5 to 4% by weight. When it is less than
0.5%, the above effect cannot be sufficiently provided but when it
is more than 4%, the toughness of the magnesium alloy is
lowered.
[0064] When the Mn content is 1.5 to 4%, a Fe content in the
magnesium based alloy is preferably not more than 10 ppm and more
preferably not more that 3 ppm, and at the same time the maximum
grain diameter of the Al--Mn compound is preferably not more than
20 .mu.m and more preferably not more than 10 .mu.m.
[0065] When a lot of Mn is added, the Fe content that lowers
corrosion resistance is reduced in the cast magnesium ingot, so
that corrosion resistance of the magnesium alloy is improved.
However, when Mn is added excessively (1% or more, for example),
the Al--Mn compound becomes coarse (about 20 to 80 .mu.m, for
example), which lowers the mechanical characteristic and
processability of the magnesium alloy.
[0066] However, when a mechanical grinding and miniaturizing
process according to the present invention that will be described
below is used, the above described structure in which the maximum
grain diameter of the Al--Mn compound is not more than 20 .mu.m and
more preferably not more than 10 .mu.m can be implemented, so that
the magnesium based alloy can provide balanced corrosion resistance
and mechanical characteristics.
[0067] (6) Silver (Ag)
[0068] Silver (Ag) becomes solid solution in the magnesium matrix
and it contributes to the improvement of the mechanical
characteristics and especially resistance because of solid solution
hardening. An appropriate Ag content in the magnesium alloy in the
present invention is 0.5 to 2% by weight. When it is less than
0.5%, the above effect cannot be sufficiently provided but when it
is more than 2%, the toughness of the magnesium alloy is
lowered.
[0069] (7) Silicon (Si)
[0070] Silicon (Si) forms magnesium silicide (Mg.sub.2Si) with
magnesium of the matrix. Since this magnesium silicide has high
rigidity, high hardness and high corrosion resistance, when it is
dispersed in the matrix, the above characteristics in the magnesium
alloy can be improved also. When a Si content is less than 1% by
weight, this effect is not sufficient but when it is more than 8%,
the toughness of the magnesium alloy, extension in the tensile
characteristics especially is considerably lowered and at the same
time, tool abrasion in the cutting process is generated and
material surface roughness is lowered associated with it.
[0071] [Maximum Crystal Grain Diameter of Magnesium of Matrix]
[0072] According to the magnesium alloy of the present invention,
both strength and toughness can be improved by miniaturizing the
magnesium crystal grain that constitutes the matrix. More
specifically, it has been found that when the maximum crystal grain
diameter of magnesium is not more than 30 .mu.m, the magnesium
alloy has high strength and high toughness such that tensile
strength is not less than 350 MPa and breaking extension is not
less than 5% at room temperature. Especially, when the maximum
crystal grain diameter is not less than 20 .mu.m, the magnesium
alloy has high strength above 400 MPa. Furthermore, it has been
found that when the maximum crystal grain diameter of magnesium is
below 10 .mu.m, during the process of plastic forming of Mg raw
material powder, since its texture is disordered, the Mg alloy
provides high toughness and improves its bending and pressing
processability at low temperature.
[Manufacturing Method of High-Strength and High-Toughness Magnesium
Based Alloy Material]
[0073] FIG. 3 shows manufacturing steps of a high-strength and
high-toughness magnesium based alloy material according to the
present invention. The method of the present invention will be
described in detail with reference to FIG. 3.
[0074] (1) Preparation of Raw Material Powder
[0075] A magnesium alloy ingot having a predetermined component
composition is manufactured by the casting method. The
predetermined component composition contains, by weight, 1 to 8%
rare earth element and 1 to 6% calcium and according to need, it
further contains at least one kind selected from an element group
consist of, by weight, 0.5 to 6% zinc, 2 to 15% aluminum, 0.5 to 4%
manganese, 1 to 8% silicon, and 0.5 to 2% silver.
[0076] Then, powder, aggregated grain, chip and the like is
provided from the magnesium alloy ingot manufactured by the casting
method through a machining process such as cutting or grinding
process, and used as starting raw material powder.
[0077] (2) Miniaturization of Crystal Grain and Miniaturization of
Compound Grain
[0078] Prior to manufacturing of powder solidified body, a plastic
forming process such as compression molding, extruding, casting, or
rolling is performed for the starting material powder to
miniaturize the magnesium crystal grain that constitutes the matrix
and miniaturize the compound grain dispersed in the matrix to
provide a crystal structure shown in FIG. 2.
[0079] When strong processing strain is applied to the starting raw
material, the acicular or network-like intermetallic compound (for
example, Mg--RE group compound or Al--RE group compound) can be
finely ground and uniformly dispersed in the magnesium crystal
grain that constitutes the matrix.
[0080] As the method applying the strong processing strain to the
magnesium alloy raw material powder, a method in which compression
molding or extruding are performed or a shearing process, bending
process, rotation shearing process and the like are performed for
the powder in a mold and the like, or a method of rolling the
powder, or a method in which a grinding process is performed with a
ball mill and the like are effective. In order to effectively
miniaturize the intermetallic compound and the magnesium crystal
grain, the plastic forming method is preferably performed in a warm
region at 100 to 300.degree. C.
[0081] FIG. 4 shows one example of the processes in which the
plastic forming processes are repeatedly performed for starting raw
material powder 10 until a powder solidified body 20 is finally
provided. One example of the method to apply the strong processing
strain will be described with reference to FIG. 4.
[0082] First, as shown in FIG. 4(a), a container comprising a mold
mill 11 and a lower punch 12 is filled with the powder 10. Then, as
shown in FIG. 4(b), a compression upper punch 13 is lowered in the
mold mill 11 to compress the raw material powder 10. Then, as shown
in FIG. 4(c) and 4(d), after the compression upper punch 13 has
been retreated, an indenting upper punch 14 is inserted into the
compressed raw material powder 10. The compressed raw material
powder 10 is extruded backward (a direction shown by an arrow B in
FIG. 4) by the indenting upper punch 14 and receives strong
processing strain.
[0083] Then, as shown in FIG. 4(e) and 4(f), after the indenting
upper punch 14 has been retreated, the compressed raw material
powder 10 having a U-shaped section is compressed by the
compression upper punch 13 again. The raw material powder 10
existing along the inner wall surface of the mold mill 11 is moved
inwardly (direction shown by an arrow C in FIG. 4) in the mold mill
1 by the above compression.
[0084] A series of processes as shown in FIG. 4(b) to 4(f) is
repeated to mechanically grind the raw material powder and
miniaturize the magnesium crystal grain of the matrix. At the same
time, the intermetallic compound is also finely ground and
dispersed in the magnesium crystal grain.
[0085] (3) Manufacturing of Powder Solidified Body
[0086] As shown in FIG. 4(g), after the miniaturizing process by
performing the necessary plastic forming process to the magnesium
based alloy raw material powder 10, the powder solidified body 20
is manufactured by compression molding.
[0087] (4) Heating and Warm Extruding
[0088] For example, the powder solidified body provided as
described above is heated up to 300 to 520.degree. C. and
maintained for 30 seconds and immediately processed by a warm
extrusion process under a condition that an extrusion rate is 37
and a mold temperature is 400.degree. C. to be a rod-like material.
The above warm extrusion process promotes the miniaturization of
the magnesium crystal grain and the compound grain. More
specifically, the compound grain is mechanically cut and further
miniaturized by the plastic process using the extrusion process,
and the magnesium crystal grain is dynamically recrystallized and
further miniaturized through the process and the heat
treatment.
[Mechanical Characteristics of Magnesium Based Alloy]
[0089] Since the magnesium based alloy according to the present
invention is superior in strength and toughness within a
temperature range from room temperature to about 200.degree. C., it
can be used as an engine part or a transmission part of a car or a
two-wheeled motor vehicle. When the magnesium alloy contains the
above appropriate component element defined by the present
invention, and the matrix magnesium has the crystal grain diameter
that satisfies the appropriate range, the tensile strength
(.sigma.) of 350 MPa or more and the breaking extension (.epsilon.)
of 5% or more at room temperature are implemented. More preferably,
the tensile strength is 400 MPa or more. In addition, the magnesium
alloy has high strength and high toughness in which the product of
the tensile strength (.sigma.) and the breaking extension
(.epsilon.) is such that .sigma..times..epsilon..gtoreq.4000 MPa%
.
[0090] Meanwhile, when the magnesium based alloy has the tensile
strength (.sigma.) of 350 MPa or more and breaking extension
(.epsilon.) of 5% or more at room temperature and/or satisfies that
.sigma..times..epsilon..gtoreq.4000 MPa%, it can be used as a
driving part used in a car or a two-wheeled motor vehicle such as a
piston, a cylinder liner, a con-rod and the like
EXAMPLE 1
[0091] Magnesium based alloy powder (grain diameter: 0.5 to 2mm)
having the alloy composition shown in Table 1 was prepared and a
mold was filled with it and then a powder solidified body was
manufactured by compression molding. This solidified body was
maintained at 400 to 480.degree. C. for 5 minutes in an inert gas
atmosphere and then immediately a warm extrusion process was
performed for it to provide an extruded material (diameter: 7.2 mm
.phi.).
[0092] The structure in the extruded direction of the above
material was observed after polishing and chemical etching and the
maximum crystal grain diameter of magnesium of the matrix was
measured by image analysis. In addition, a round rod extensile test
piece (diameter : 3mm .phi. and parallel part: 15 mm) was obtained
from the extruded material and tested at room temperature and
150.degree. C. The tensile speed was kept constant at 0.3 mm/min
and in the tensile test at 150.degree. C., a test piece was heated
and maintained at 150.degree. C. for 100 hours before the test and
tested.
[0093] These characteristic evaluation results are shown in Table
1. Regarding the crystal grain miniaturization of the matrix, while
the magnesium based alloy powder was heated and maintained at 100
to 300.degree. C., a plastic forming process (compression,
extrusion, shearing process and the like) was performed by press
molding or rolling, and magnesium based alloy powder having
different crystal grain diameters was manufactured. In addition,
according to a comparative example 19, an extruded material was
heat treated at 400.degree. C. for 20 hours in an inert gas
atmosphere to coarse the crystal grain.
[0094] According to the inventive examples 1 to 11, each extruded
material has the appropriate alloy composition and appropriate Mg
maximum crystal grain diameter defined by the present invention, so
that it has superior mechanical characteristics at room
temperature. Especially, when the maximum crystal grain diameter Mg
is below 10 .mu.m as shown in the inventive examples 10 and 11, the
extension (toughness) is improved as well as strength.
[0095] Meanwhile, according to the comparative examples 12 to 18,
since the extruded material does not have the alloy composition
defined by the present invention, it does not have enough strength.
Especially, in the comparative examples 14 and 15, since a RE or Ca
content exceeds an appropriate range, the toughness is lowered and
as a result, the tensile strength is also lowered. According to the
comparative example 19, since the Mg maximum crystal grain diameter
is as large as 66.8 .mu.m, the strength characteristics are not
sufficiently provided. TABLE-US-00001 TABLE 1 Maximum crystal grain
Tensile diameter of characteristics at UTS magnesium room
temperature (MPa) Chemical composition in weight matrix UTS
Extension .sigma. .times. .epsilon. at No. RE Ca Zn Al Mn Si Ag Mg
(.mu.m) (MPa) (%) (MPa. %) 150.degree. C. Inventive example 1 3.0
1.2 0.7 7.5 1.0 0.0 0.0 balance 22.1 383 14.4 5515 134 2 1.8 2.2
0.0 6.5 0.0 0.0 0.0 balance 18.0 376 13.8 5189 131 3 4.6 3.8 0.5
4.0 0.0 0.0 0.0 balance 14.3 388 15.2 5898 136 4 5.8 4.8 0.0 0.0
0.5 0.0 0.0 balance 17.2 368 14.2 5226 126 5 3.5 2.0 0.0 6.0 0.0
0.0 0.0 balance 15.2 398 11.2 4458 139 6 3.0 1.0 0.5 7.5 0.5 1.5
1.0 balance 16.5 412 9.8 4038 146 7 3.0 1.0 0.5 7.5 0.5 0.0 0.0
balance 14.0 418 9.6 4013 148 8 3.5 1.5 0.8 7.0 0.5 0.0 0.0 balance
26.2 365 16.2 5913 124 9 3.5 1.5 0.8 7.0 0.5 0.0 0.0 balance 15.4
394 11.1 4373 138 10 3.5 1.5 0.8 7.0 0.5 0.0 0.0 balance 9.3 406
12.6 5116 140 11 3.5 1.5 0.8 7.0 0.5 0.0 0.0 balance 3.7 426 14.8
6305 149 Comparative example 12 3.0 0.0 0.5 3.5 0.5 0.0 0.0 balance
20.1 324 19.3 6253 110 13 0.0 3.5 0.0 4.0 0.0 0.0 0.0 balance 15.5
319 18.8 5997 107 14 9.5 2.2 0.0 0.0 0.0 0.0 0.0 balance 16.4 289
2.7 780 102 15 2.5 7.2 0.0 0.0 0.0 0.0 0.0 balance 18.3 262 2.1 550
98 16 0.0 0.0 1.0 9.0 0.5 0.0 0.0 balance 27.3 336 15.6 5242 115 17
0.0 0.0 1.1 6.1 0.5 0.0 0.0 balance 24.8 305 17.2 5246 104 18 0.0
0.0 1.0 3.1 0.4 0.0 0.0 balance 28.2 280 16.9 4732 101 19 3.5 1.5
0.8 7.0 0.5 0.0 0.0 balance 66.8 318 18.8 5978 106
EXAMPLE 2
[0096] The structure photographs of the inventive examples 9 and 11
and the comparative example 16 shown in Table 1 are shown in FIG.
5. It is clearly found by observing and comparing those structure
photographs that the magnesium crystal grains of the extruded
materials of the inventive examples 9 and 11 are miniaturized.
EXAMPLE 3
[0097] An ingot containing, by weight, 3.5% RE, 1.5% CA, 0.8% Zn,
7% of Al, 0.5% Mn, and the balance Mg was manufactured by a casting
method and a magnesium based alloy powder (grain diameter: 0.5 to
1.5 mm) was obtained from the material. This Mg alloy powder was
heated up to 150.degree. C. and rolled to miniaturize the powder Mg
crystal grain and miniaturize the compound dispersed in the matrix.
The Mg alloy powder after such warm plastic forming process was
solidified by molding and heated up and maintained at 420.degree.
C. for 5 minutes in an inert gas atmosphere and then immediately a
warm extrusion process (extrusion ratio: 20) was performed for
it.
[0098] Meanwhile, according to the comparative example, Mg alloy
powder provided by a cutting process without the above rolling
process was directly formed by molding and it is processed by
heating and warm extrusion process in the same condition to be an
extruded material. According to the inventive example, the tensile
strength of the extruded material was 397 MPa and the breaking
extension thereof was 11.4% at room temperature, while according to
the comparative example, the tensile strength of the extruded
material was 316 MPa and the breaking extension thereof was
6.5%.
[0099] The structures of those extruded materials are shown in FIG.
6. According to the inventive example shown in FIG. 6(a), the
compound (here, Al.sub.2Ca and Mg.sub.17Al.sub.12) dispersed in the
matrix has a spherical shape or a shape close to it and uniformly
dispersed in the grain boundary and the grain of the Mg crystal
grain. As a result of image analysis, the ratio (D/d) of the
maximum grain diameter "D" to the minimum grain diameter "d" of the
compound is 1.2 to 2.4 and the maximum grain diameter is 3.8
.mu.m.
[0100] Meanwhile, according to the comparative example shown in
FIG. 6(b), a network-like compound (Al.sub.2Ca and
Mg.sub.17Al.sub.12) connected along the Mg crystal grain boundary
exists and as a result of the similar image analysis, it is found
that the intermetallic compound is coarse and have a high D/d value
exceeding 10 and its longest diameter is more than 30 .mu.m.
EXAMPLE 4
[0101] Magnesium based alloy powder (grain diameter: 0.5 to 2 mm)
having the alloy composition of each of samples No. 1 to 4 and 8
shown in Table 2 was prepared and each powder was heated up to
about 150.degree. C. to be processed by shearing and compression
processes so that the Mg crystal grain and the deposited and
dispersed compound in the powder material were miniaturized. Then,
a mold was filled with the powder and then a powder solidified body
was manufactured by compression molding. This solidified body was
maintained at 400.degree. C. for 5 minutes in an inert gas
atmosphere and then immediately a warm extrusion process was
performed for it to provide an extruded material (diameter: 7.2 mm
.phi.).
[0102] Magnesium based alloys of the samples 5 to 7 are ingot
materials manufactured by the casting method.
[0103] The structure in the extruded direction of the above
material was observed after polishing and chemical etching and the
maximum crystal grain diameter of the Mg matrix and the maximum
grain diameter of the Al--Mn group compound were measured by image
analysis.
[0104] In addition, a round rod extensile test piece (diameter : 3
mm .phi. and parallel part: 15 mm) was obtained from the extruded
material and tested at room temperature and 150.degree. C. The
tensile speed was kept constant at 0.3 mm/min.
[0105] Furthermore, in order to evaluate the corrosion resistance
of each sample, a pillar sample having a diameter of 6.8mm .phi.
and a length of 80 mm was obtained from the extruded material and
this was immersed in NaCl aqueous solution having a concentration
of 5% and pH 10 (solution temperature; 35.degree. C.) for 72 hours
and its corrosion speed (mg/cm.sup.2) was calculated from a reduced
weight amount before and after the test. These characteristic
evaluation results are shown in Table 2.
[0106] According to each of the inventive examples 1 to 4, the
extruded material has the appropriate alloy composition and
appropriate Mg maximum crystal grain diameter defined by the
present invention, so that each has superior mechanical
characteristics and corrosion resistance at room temperature.
Especially, as the Mn content is increased within a range of 1.5%
or more, the Fe content in the Mg alloy is decreased and as a
result, the corrosion resistance is improved (corrosion speed is
lowered). In addition, the tensile strength is increased as the Mn
content is increased, which is because the dispersion of the Al--Mn
group compound miniaturized to 10 .mu.m or less is enhanced.
[0107] Meanwhile, according to the comparative examples 5 to 7,
since the extruded material was manufactured by the casting method
and does not have the Mg crystal grain diameter defined by the
present invention, it does not have enough strength. At the same
time, since the Al--Mn group compound becomes coarse such that its
grain diameter is beyond 30 .mu.m, which is one factor causing the
strength and toughness of the Mg alloy to be lowered.
[0108] Meanwhile, according to the comparative example 8, although
it has a Mg crystal grain diameter of 20 .mu.m or less and have
superior mechanical characteristics, since it does not contain Mn,
a Fe content is increased to 135 ppm. As a result, the corrosion
resistance of the Mg alloy is considerably lowered. TABLE-US-00002
TABLE 2 Maximum Grain Tensile crystal grain diameter of
Characteristics diameter of Al--Mn at room temperature Corrosion
Chemical composition in weight matrix Compound UTS Extension
.sigma. .times. .epsilon. Speed No. RE Ca Zn Al Mn Fe (ppm) Si Ag
Mg (.mu.m) (.mu.m) (MPa) (%) (MPa. %) (mg/cm2) Inventive 1 3.0 1.5
0.5 7.0 1.6 10 0.0 0.0 balance 12.2 5.2 397 13.1 5201 11.3 example
2 3.0 1.5 0.5 7.0 2.3 8 0.0 0.0 balance 13.4 8.2 402 11.2 4502 9.6
3 3.0 1.5 0.5 7.0 2.9 6 0.0 0.0 balance 12.6 6.4 408 10.8 4406 7.2
4 3.0 1.5 0.5 7.0 3.6 3 0.0 0.0 balance 11.5 7.5 418 9.9 4138 5.4
Comparative 5 3.0 1.5 0.5 7.0 1.6 10 0.0 0.0 balance 112.5 36.8 182
4.6 837 12.2 example 6 3.0 1.5 0.5 7.0 2.3 8 0.0 0.0 balance 126.6
45.8 412 3.4 1401 10.1 7 3.0 1.5 0.5 7.0 2.9 6 0.0 0.0 balance
121.8 57.7 418 2.6 1087 7.9 8 3.0 1.5 0.5 7.0 0.0 135 0.0 0.0
balance 12.4 none 365 13.8 5037 287.0
[0109] Although the embodiments of the present invention have been
described with reference to the drawings in the above, the present
invention is not limited to the above-illustrated embodiments.
Various kinds of modifications and variations may be added to the
illustrated embodiments within the same or equal scope of the
present invention.
INDUSTRIAL APPLICABILITY
[0110] The present invention is applied to a magnesium based alloy
having superior strength characteristics and superior toughness at
room temperature and at a high temperature up to 200.degree. C.
Especially, since a high-strength and high-toughness magnesium
based alloy according to the present invention comprises a
magnesium matrix having a fine crystal grain diameter and has a
structure in which a fine granular intermetallic compound is
uniformly deposited and dispersed in its crystal grain, it can be
advantageously applied to an engine or a driving part of a car or a
two-wheeled motor vehicle.
* * * * *