U.S. patent application number 13/877679 was filed with the patent office on 2013-08-01 for high-strength magnesium alloy wire rod, production method therefor, high-strength magnesium alloy part, and high-strength magnesium alloy spring.
This patent application is currently assigned to NHK SPRING CO., LTD.. The applicant listed for this patent is Yuji Araoka, Yoshiki Ono, Tohru Shiraishi. Invention is credited to Yuji Araoka, Yoshiki Ono, Tohru Shiraishi.
Application Number | 20130195711 13/877679 |
Document ID | / |
Family ID | 45938409 |
Filed Date | 2013-08-01 |
United States Patent
Application |
20130195711 |
Kind Code |
A1 |
Araoka; Yuji ; et
al. |
August 1, 2013 |
HIGH-STRENGTH MAGNESIUM ALLOY WIRE ROD, PRODUCTION METHOD THEREFOR,
HIGH-STRENGTH MAGNESIUM ALLOY PART, AND HIGH-STRENGTH MAGNESIUM
ALLOY SPRING
Abstract
A high-strength magnesium alloy wire rod suitable for products
in which at least one of bending stress and twisting stress
primarily acts is provided. The wire rod has required elongation
and 0.2% proof stress, whereby strength and formability are
superior, and has higher strength in the vicinity of the surface.
In the wire rod, the surface portion has the highest hardness in a
cross section of the wire rod, the highest hardness is 170 HV or
more, and the inner portion has a 0.2% proof stress of 550 MPa or
more and an elongation of 5% or more.
Inventors: |
Araoka; Yuji; (Yokohama-shi,
JP) ; Shiraishi; Tohru; (Yokohama-shi, JP) ;
Ono; Yoshiki; (Yokohama-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Araoka; Yuji
Shiraishi; Tohru
Ono; Yoshiki |
Yokohama-shi
Yokohama-shi
Yokohama-shi |
|
JP
JP
JP |
|
|
Assignee: |
NHK SPRING CO., LTD.
Yokohama-shi, Kanagawa
JP
|
Family ID: |
45938409 |
Appl. No.: |
13/877679 |
Filed: |
October 14, 2011 |
PCT Filed: |
October 14, 2011 |
PCT NO: |
PCT/JP2011/073649 |
371 Date: |
April 4, 2013 |
Current U.S.
Class: |
419/28 ;
420/402 |
Current CPC
Class: |
B22F 2998/00 20130101;
B22F 3/20 20130101; C22C 23/06 20130101; B22F 3/24 20130101; B22F
2998/10 20130101; C22C 23/00 20130101; B22F 5/12 20130101; B22F
9/08 20130101; B22F 3/20 20130101; B22F 3/10 20130101; B22F 2303/01
20130101; B22F 2998/00 20130101; B22F 2301/058 20130101; B22F
2998/10 20130101; C22F 1/06 20130101 |
Class at
Publication: |
419/28 ;
420/402 |
International
Class: |
C22C 23/00 20060101
C22C023/00; B22F 3/24 20060101 B22F003/24 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 15, 2010 |
JP |
2010-232364 |
Claims
1. A high-strength magnesium alloy wire rod used for members in
which at least one of bending stress and twisting stress primarily
acts, the wire rod comprising: a surface portion having the highest
hardness in a cross section of the wire rod, the highest hardness
being 170 HV or more; an inner portion having a 0.2% proof stress
of 550 MPa or more and an elongation of 5% or more.
2. The high-strength magnesium alloy wire rod according to claim 1,
wherein the magnesium contains Mg as a main element and Ni and
Y.
3. The high-strength magnesium alloy wire rod according to claim 2,
wherein the magnesium consists of 2 to 5 atomic % of Ni, 2 to 5
atomic % of Y, and the balance of Mg and inevitable impurities.
4. The high-strength magnesium alloy wire rod according to claim 1,
wherein the portion having the highest hardness in the vicinity of
the surface has an average grain diameter of 1 .mu.m measured by an
EBSD method.
5. A production method for a high-strength magnesium alloy wire
rod, the method comprising: a step for yielding a raw material in a
form of foil strips, foil pieces, or fibers of a magnesium alloy by
a rapid solidification method, a sintering step for forming a
billet by bonding, compressing, and sintering the raw material, a
step for plastic forming the billet, thereby obtaining the wire rod
according to claim 1.
6. A production method for a high-strength magnesium alloy wire
rod, the method comprising: a step for forming fibers by a molten
metal extraction method, a sintering step for forming a billet by
bonding, compressing, and sintering the fibers, an extruding step
for directly charging the billet into a container of a press
machine and extruding the billet, thereby obtaining the wire rod
according to claim 1.
7. The production method for a high-strength magnesium alloy wire
according to claim 5, wherein the sintering step is performed in a
temperature of 350 to 500.degree. C. for 10 minutes or more at a
pressure of 25 MPa or more.
8. The production method for a high-strength magnesium alloy wire
according to claim 6, wherein the extruding step is performed in a
temperature of 315 to 335.degree. C. at an extruding rate of 5 to
13 with a speed of 2.5 mm/second or less of a press ram.
9. A high-strength magnesium alloy part produced from the
high-strength magnesium alloy wire according to claim 1.
10. A high-strength magnesium alloy spring produced from the
high-strength magnesium alloy wire according to claim 1.
11. The production method for a high-strength magnesium alloy wire
according to claim 6, wherein the sintering step is performed in a
temperature of 350 to 500.degree. C. for 10 minutes or more at a
pressure of 25 MPa or more.
Description
TECHNICAL FIELD
[0001] The present invention relates to a high-strength magnesium
alloy wire rod, production method therefor, high-strength magnesium
alloy part, and high-strength magnesium alloy spring, which are
suitable for products in which at least one of bending stress and
twisting stress primarily acts.
BACKGROUND ART
[0002] In various fields, such as aerospace, vehicles (automobiles,
motor cycles, trains), medical instruments, welfare devices, and
robots, low weights of parts are desired for improvement of
function, improvement of performance, and improvement of
operability. Specifically, in the field of vehicles such as
automobiles, emission amounts of carbon dioxide has been desired to
be reduced in view of the environment. Therefore, requirements for
lightweight for improvement of fuel consumption has become
increasingly stringent every year.
[0003] Development of lightweight parts has been active primarily
in the field of vehicles, and great strengthening of steels by
improvements in composition, surface modification, and combination
thereof in steels has been primarily researched. For example,
high-tension steels have been primarily used for springs, which are
typical strong parts, and fatigue strength thereof is further
improved by applying surface modification such as nitriding and
shot peening, thereby yielding lightweight springs. However, great
strengthening of steels by conventional improvements in composition
is nearing a limit, and great reductions in weight in the future
cannot be anticipated.
[0004] Therefore, lightweight alloys, typically having low specific
gravity, such as titanium alloys, aluminum alloys, and magnesium
alloys are desired for further reduction in weight. Magnesium
alloys have the lowest specific gravity in the practical metals,
which is about 1/4 of that of steels, about 1/2.5 of that of
titanium alloys, and about 1/1.5 of that of aluminum alloys.
Therefore, magnesium alloys have great advantages in being low in
weight and as a resource, and they are expected to be widely used
in the market.
[0005] However, conventional magnesium alloys are limited in use as
products. The main reason of this is that the strengths of the
conventional magnesium alloys are low. Therefore, in order to
obtain strength for parts, it is necessary to increase size of
parts compared to that of the conventional steel parts. That is,
the conventional magnesium alloys have not been accepted as strong
parts in the market since low weight and compact size are
incompatible.
[0006] Under such circumstances, research in high-strength
magnesium alloys for use as strong parts has been actively made.
For example, Japanese Patent Unexamined Publication No. 3-90530
discloses a technique in which a molten Mg--Al--Zn--Mn--Ca-RE (rare
earth) alloy is subjected to wheel casting, thereby forming a solid
member, which is drawn and densified, thereby obtaining a magnesium
alloy member having a 0.2% proof stress of 565 MPa.
[0007] Japanese Patent Unexamined Publication No. 3-10041 discloses
a technique in which a molten Mg--X-Ln (X is one or more of Cu, Ni,
Sn, and Zn, Ln is one or more of Y, La, Ce, Nd, Sm) alloy is
rapidly cooled and solidified, thereby obtaining an amorphous foil
strip composed of a magnesium alloy foil strip having a hardness of
200 HV or more.
[0008] Japanese Patent Unexamined Publication No. 2003-293069
discloses a technique in which a cast material or an extruded
material composed of a Mg--Al--Mn alloy is drawn, thereby obtaining
a magnesium alloy wire having a tensile strength of 250 MPa or more
and an elongation of 6% or more.
[0009] The techniques disclosed in the publications are effective
for greatly strengthening magnesium alloys. However, in the
magnesium alloy disclosed in Japanese Patent Unexamined Publication
No. 3-90530, mechanical properties for satisfying requirements of
the market as strong parts are not sufficient. For example, when it
is assumed that the alloy is applied to a spring in which at least
one of bending stress and twisting stress primarily acts, according
to estimates by the inventors, the magnesium alloy wire rod must
have a 0.2% proof stress of 550 MPa or more in an inner portion of
the wire rod and a 0.2% proof stress of 650 MPa or more in the
vicinity of the surface of the wire rod if the size of the wire rod
is the same as that of existing steel springs and light weight can
be achieved. Furthermore, in order to form a coiled spring, at
least an elongation of 5% or more in an inner portion is required.
However, in the invention product disclosed in Japanese Patent
Unexamined Publication No. 3-90530, which has the highest 0.2%
proof stress of 565 MPa, the ductility is low and the elongation is
merely 1.6%. On the other hand, in the invention product disclosed
in Japanese Patent Unexamined Publication No. 3-90530, which has
the highest ductility and an elongation of 4.7%, the elongation is
close to the value that is required in the present invention.
However, the strength is low in a 0.2% proof stress of 535 MPa, and
the requirement is not satisfied.
[0010] In the magnesium alloy disclosed in Japanese Patent
Unexamined Publication No. 3-10041, a hardness of 170 HV or more is
obtained. The hardness corresponds to 0.2% proof stress of 650 MPa
or more according to estimates by the inventors. However, in
Japanese Patent Unexamined Publication No. 3-10041, properties
related to ductility are not disclosed. The magnesium alloy
disclosed in this publication contains a large amount of rare earth
elements and 50% of amorphous phase, whereby the ductility is
extremely low, and it is easily assumed that the elongation that is
required in the present invention is not obtained. Furthermore,
amorphous phases show poor thermal stability and easily crystallize
by external causes such as environmental temperature. Since a
mix-phase alloy of amorphous phase and crystal phase greatly varies
the properties according to the proportion of the phases, it is
difficult to stably produce products having uniform properties, and
it is not suitable for applying to industrial products because of
difficulty of quality guaranty and safety guaranty in the
market.
[0011] In the magnesium alloy disclosed in Japanese Patent
Unexamined Publication No. 2003-293069, the elongation is 6% or
more and shows sufficient ductility. However, the tensile strength
is 479 MPa at most, and the above-mentioned 0.2% proof stress of
550 MPa or more in the inner portion of the wire rod is not
obtained.
DISCLOSURE OF THE INVENTION
[0012] Thus, the conventional magnesium alloys do not satisfy 0.2%
proof stress and elongation for strong parts (for example, springs)
to which at least one of bending stress and twisting stress
primarily acts. Therefore, an object of the present invention is to
provide a high-strength magnesium alloy wire rod, a high-strength
magnesium alloy part, and production method therefor, in which 0.2%
proof stress and elongation, which are in a trade-off, are both
satisfied, whereby strength and formability (ductility required for
bending and coiling) are improved, and higher surface strength is
provided, thereby being suitable for products in which at least one
of bending stress and twisting stress primarily acts.
[0013] The present invention provides a high-strength magnesium
alloy wire rod used for members in which at least one of bending
stress and twisting stress primarily acts, the wire rod comprising:
a surface portion having the highest hardness in a cross section of
the wire rod, the highest hardness being 170 HV or more, and an
inner portion having a 0.2% proof stress of 550 MPa or more and an
elongation of 5% or more.
[0014] The vicinity of a surface is defined as a range from the
surface of the wire rod to a depth of about d/10 (d is the diameter
of the wire rod). Since the wire rod has a surface portion having
the highest hardness in the cross section of the wire rod and the
highest hardness is 170 HV or more, 0.2% proof stress of 650 MPa or
more in the vicinity of the surface of the wire rod can be
achieved, as mentioned as above. In the present invention, although
strength (hardness) gradually decreases from in the vicinity of
surface to the center of the wire rod, the inner portion has a 0.2%
proof stress of 550 MPa or more and an elongation of 5% or more.
That is, the present invention is a high-strength magnesium alloy
having strength and formability suitable for products in which at
least one of bending stress and twisting stress primarily acts.
[0015] Thus, since the present invention has a high-strength and
high-ductile inner portion and a higher-strength portion in the
vicinity of the surface, 0.2% proof stress and elongation which are
in relation of trade-off can be satisfied for products in which at
least one of bending stress and twisting stress primarily acts by
providing suitable distribution of mechanical properties. In this
case, the outermost surface can be reformed by providing
compressive residual stress by shot peening, whereby fatigue
properties can further be improved for parts in which at least one
of bending stress and twisting stress primarily acts.
[0016] Next, the present invention provides a production method for
a high-strength magnesium alloy wire rod, the method comprising: a
step for yielding a raw material in a form of foil strips, foil
pieces, or fibers of a magnesium alloy by rapid solidification
method, a sintering step for forming a billet by bonding,
compressing, and sintering the raw material, a step for plastic
forming the billet, thereby obtaining the above-mentioned wire
rod.
[0017] In the present invention, a raw material having
below-mentioned compositions in a form of foil strips, foil pieces,
or fibers of a magnesium alloy by rapid solidification method is
preferably used. Therefore, special steps disclosed in Japanese
Patent Unexamined Publication No. 3-90530, in which a raw material
is charged in a container in a moment after forming or a raw
material is subjected to canning, are not needed, although such
steps are required for a powder having large specific surface area
or an alloy having active composition.
[0018] The present invention provides another production method for
a high-strength magnesium alloy wire rod, the method comprising: a
step for forming fibers by molten metal extraction method, a
sintering step for forming a billet by bonding, compressing, and
sintering the fibers, an extruding step for directly charging the
billet into a container of a press machine and extruding the
billet, thereby obtaining the above-mentioned wire rod.
[0019] In the present invention, a billet that is not subjected to
canning is directly extruded, whereby a high-strength and
high-ductile inner portion and a higher-strength portion in the
vicinity of the surface can be obtained. The high-strength and
high-ductile inner portion and the higher-strength portion in the
vicinity of the surface are gradually connected and do not have a
clear boundary of mechanical properties. This is greatly preferable
for fatigue in which cyclic stresses act. If the portions have a
clear boundary, the boundary may be an initiation of a crack due to
difference of hardness or elastic strain. Therefore, since the
portions do not have a clear boundary and are gradually connected,
there is no risk that a boundary will be an initiation of a crack.
In the present invention, since a billet is directly charged into a
container of a press machine, the number of steps can be reduced
and production cost can be lowered compared to the case in which
canning is performed.
EFFECTS OF THE PRESENT INVENTION
[0020] According to the present invention, a high-strength
magnesium alloy wire rod has high-surface strength and high
formability. Therefore, by applying the invention to formed parts
in which at least one of bending stress and twisting stress
primarily acts, great reduction in weight of parts'can be achieved
without increasing size of parts compared to conventional steel
parts. Specifically, the present invention has strength and
formability that are sufficient for, for example, automobile parts
such as seat frames which have higher proportion of weight and
springs (suspension springs, valve springs, clutch torsion springs,
torsion bars, stabilizers) which are required to have high
strength.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIGS. 1A and 1B show a production apparatus for metallic
fiber used in an embodiment of the present invention.
[0022] FIG. 2 shows an extrusion apparatus used in an embodiment of
the present invention.
[0023] FIG. 3 shows a graph showing a relationship between distance
from the center in a cross section and hardness of a wire rod in
each extruding temperature in the example of the present
invention.
[0024] FIG. 4 shows a graph showing a relationship between distance
from the center in a cross section and hardness of a wire rod in
each composition of a material in the example of the present
invention.
[0025] FIG. 5 shows a graph showing a relationship between distance
from the center in a cross section and hardness of a wire rod in
each inner diameter of a container and each extruding rate.
EMBODIMENT OF THE INVENTION
1. Composition of Materials
[0026] Zn has been conventionally added in magnesium alloys as a
primary additional element for improving strength and ductility.
However, Zn is not sufficient for compatibility of high strength
and ductility which are required in the present invention.
Therefore, Ni is preferably added in magnesium alloys as a primary
additional element. Ni has great function for improving high
strength and high ductility compared to Zn.
[0027] However, high strength, which is required in the present
invention, is not easy merely by adding Ni, which greatly
contributes to improving high strength and high ductility.
Therefore, Y is preferably added as a secondary additional element.
A high-strength compound phase of Mg--Ni--Y type is formed by
adding Y. Y has high solubility with respect to Mg and is effective
for solid-solution strengthening in an .alpha.-Mg phase.
Furthermore, by combining with yielding a raw material by a rapid
solidification method, as mentioned below, greater strengthening
can be achieved. The magnesium alloy in the present invention is
not limited to compositions composed of three elements of Mg, Ni,
and Y. The main elements are Mg, Ni, and Y, and a third element
such as Zr and Al can be added for refinement of crystal grain and
improvement of corrosion resistance.
[0028] When a magnesium alloy in which Ni and Y are added to Mg as
a main element is used, the alloy preferably consists of, by atomic
%, Ni: 2 to 5%, Y: 2 to 5%, and the balance of Mg and inevitable
impurities. If Ni is less than 2 atomic % and Y is less than 2
atomic %, the highest hardness in the vicinity of the surface is
not the hardness required in the present invention and the strength
is not sufficient for strength parts in which at least one of
bending stress and twisting stress primarily acts. On the other
hand, if Ni is more than 5 atomic % and Y is more than 5 atomic %,
formability is extremely deteriorated and breakage occurs in
extruding. In this case, amount of high-hardness compounds formed
by Ni and Y increases and the compounds become coarse, whereby the
deformation resistance of the alloy increases and the toughness of
the alloy is decreased, and thereby the alloy breaks.
2. Production of Raw Materials
[0029] A raw material of a magnesium alloy having the above
composition is produced. A rapid solidification method such as a
single roller method, a molten metal spinning method, and a molten
metal extraction method was used, and a raw material in a form of
foil strips, foil pieces, or fibers was produced. The amounts of
additional elements contained by solid solution in an .alpha.-Mg
phase of foil strips, foil pieces, or fibers which is yielded by
rapid solidification method is large compared to common casting
methods in which solidification rate is low. Therefore, even though
amounts of additional elements are the same as in the casting
method, the alloy is greatly strengthened by solid solution
strengthening. The crystal grain is fine in a rapid solidification
method. Fine crystal grain improves strength and elongation, and
combined with solid solution strengthening, all of the mechanical
properties are improved.
[0030] It should be noted that rapid solidification powders such as
atomized powder that is yielded by rapid solidification of a raw
material is not suitable for the present invention. Since Mg is
active, an extremely thin oxide film is easily formed on a surface
of the powder when Mg is exposed in air. In a powder having large
specific surface area, the total area of the oxide film is greatly
large compared to that of foil strips, foil pieces, or fibers. If
obtained powder is exposed in air and subjected to sintering, the
oxide film prevents bonding at the contacting surface of the
particles. Even though particles are bonded, oxides or oxygen
generated by resolution of the oxides is largely taken in the
particles. Thus, in powders having large specific surface area,
poor bonding and embrittlement caused by contamination of oxygen
and oxides easily occur, whereby the properties may be reduced
compared to the case in which foil strips, foil pieces, or fibers
are used. In order to avoid such disadvantages, powders must be
subjected to canning in a moment after forming the powder. As a
result, high strengthening in the vicinity of a surface of a wire
rod after plastic forming (for example, extruding) is difficult, as
mentioned below.
[0031] In a condition of a powder, there may be a concern that a
dust explosion may occur. Therefore, active magnesium alloy powder
cannot be handled in air. Specifically, if powder is used, powder
that is yielded in a vacuum or in an inert atmosphere must not be
exposed to air, and is charged into a metallic capsule such as
copper capsule in a sequential apparatus having a vacuum or an
inert atmosphere. When an inert atmosphere is used, the metallic
capsule is degassed and sealed. Thus, if powder is used, the
above-mentioned canning in a vacuum process or an inert atmosphere
process is required. In an apparatus for performing canning in a
vacuum or in an inert atmosphere, the sizes of products are
limited. Therefore, it is difficult to realize sequential processes
composed of a vacuum process or an inert atmosphere process using
powder in industrial mass-production with respect to parts having
such sizes as springs for automobiles (suspension springs, valve
springs, clutch torsion springs, torsion bars, stabilizers) and
seat frames.
[0032] FIGS. 1A and 1B show schematic structures of a production
apparatus for metallic fiber 100 (hereinafter referred to simply as
"apparatus 100") for performing a step for forming a fiber in an
embodiment of the present invention, FIG. 1A shows a cross
sectional view of the entire apparatus 100 and FIG. 1B shows a
cross sectional view of a circumferential portion 141a of a
rotating disk 141. FIG. 1B is a side sectional view in a direction
perpendicular to the plane of the paper.
[0033] The apparatus 100 is a production apparatus for metallic
fiber using a molten metal extraction method. In the apparatus 100
using a molten metal extraction method, an upper end portion of a
rod-shaped raw material M is melted, and a molten metal Ma contacts
the circumferential portion 141a of the rotating disk 141, a
portion of the molten metal Ma is extracted toward the direction of
the substantially tangential line of the circumference of the disk
141, and is rapidly cooled, thereby forming a magnesium alloy fiber
F. For example, a magnesium alloy such as Mg--Ni--Y type is used as
a raw material M, and a magnesium alloy fiber F having a diameter
200 .mu.m or less is produced. The diameter of the magnesium alloy
fiber F is not limited, and the diameter is selected according to
production efficiency and handling facility in a later process.
When diameter is 200 .mu.m or less, sufficient amounts of
additional elements can be contained in .alpha.-Mg phase by solid
solution, and the structure can be fine.
[0034] As shown in FIG. 1A, the apparatus 100 includes a chamber
101 which can be sealed. A raw material feeding portion 110, a raw
material holding portion 120, a heating portion 130, a
metallic-fiber forming portion 140, a temperature measuring portion
150, a high-frequency generating portion 160, and a metallic fiber
receiving portion 170 are provided in the chamber 101.
[0035] An inert gas such as argon gas is provided in the chamber
101 as an atmosphere gas, thereby inhibiting reaction of impurities
such as oxygen included in an atmosphere gas and a molten material
Ma. The raw material feeding portion 110 is located at the bottom
of the chamber 101, feeds the raw material M toward the direction
of the arrow B at predetermined speed, and provides the raw
material M to the raw material holding portion 120. The raw
material holding portion 120 prevents movement of the molten
material Ma toward a radial direction thereof and guides the raw
material M toward a suitable position of the fiber forming portion
140.
[0036] The raw material holding portion 120 is a tubular member and
is located between the raw material feeding portion 110 and the
metallic fiber-forming portion 140 and below the disk 141. The
heating portion 130 is a high-frequency induction coil which
generates magnetic flux for melting the upper portion of the raw
material M and forming the molten material Ma. As a material for
the raw material holding portion 120, a material which does not
react with the molten material Ma is preferable. Graphite is
preferable as a material for the raw material holding portion 120
for practical use.
[0037] The fiber forming portion 140 produces a magnesium alloy
fiber F from the molten material Ma by the disk 141 which rotates
around a rotating shaft 142. The disk 141 is made from copper or a
copper alloy having high thermal conductivity. As shown in FIG. 1B,
a V-shaped circumference 141a is formed on the circumferential
portion of the disk 141.
[0038] The temperature measuring portion 150 measures the
temperature of the molten material Ma. The high-frequency
generating portion 160 provides high-frequency current to the
heating portion 130. The power of the high-frequency generating
portion 160 is controlled based on the temperature of the molten
material Ma, which is measured by the temperature measuring portion
150, and the temperature of the molten material Ma is maintained to
be constant. The metallic fiber receiving portion 170 receives the
metallic fiber F which is formed by the metallic fiber forming
portion 140.
[0039] In the above apparatus, the raw material feeding portion 110
continually feeds the raw material M in a direction of the arrow B,
thereby supplying it to the raw material holding portion 120. The
heating portion 130 melts the upper portion of the raw material M
by induction heating, thereby forming the molten material Ma. Then,
the molten material Ma is continually fed to the circumference 141a
of the disk 141 rotating in the direction of the arrow A, the
molten material Ma contacts the circumference 141a of the disk 141,
a part thereof is extracted toward a direction of an approximate
tangential line of the circle of the disk 141 and is rapidly
cooled, thereby forming a magnesium alloy fiber F. The formed
magnesium alloy fiber F extends toward the direction of an
approximate tangential line of the circle of the disk 141 and
received by the metallic fiber receiving portion 170 which is
located in the direction in which the fiber F extends.
3. Sintering
[0040] The yielded raw material is formed to a billet for plastic
working by sintering. Sintering is performed by atmosphere
sintering, vacuum sintering, or discharge plasma sintering in a
non-pressurized or a pressurized condition. Properties and quality
of the billet after sintering affect properties and quality of
products after plastic working. Therefore, in order to form a
billet in which the cleanliness is high, the structure is uniform,
and number of pores is small, sintering is preferably performed by
a vacuum hot press (HP) apparatus which has a compressing mechanism
and enables sintering in a vacuum or an inert gas atmosphere. By
compressing heating in vacuum or an inert gas atmosphere, a billet
which has few pores can be obtained.
[0041] In an HP apparatus, a heating chamber is disposed in a
vacuum vessel, a mold is disposed in the heating chamber, a
cylinder is disposed in the upper portion of the vacuum vessel, a
press ram projected from the cylinder is vertically movable in the
heating chamber, and an upper punch installed at the press ram is
inserted into the mold. A magnesium alloy fiber F as a raw material
is charged into the mold of the HP apparatus constructed as above,
the vacuum vessel is evacuated or purged with an inert gas, and the
heating chamber is heated to a predetermined sintering temperature.
Then, the magnesium alloy fiber F is compressed by the upper punch
inserted into the mold, and is sintered.
[0042] The sintering is preferably performed at a temperature of
250 to 500.degree. C. for 10 minutes or more at a pressure of 25
MPa or more. By such conditions, a billet in which sintering is
sufficiently promoted at contacting points of the magnesium alloy
fibers can be obtained. More preferably, sintering is performed at
a temperature of 350 to 500.degree. C. for 30 minutes or more at a
pressure of 40 MPa or more. By such conditions, a densified billet
in which sintering is sufficiently promoted at contacting points of
the magnesium alloy fibers and the porosity thereof is less than
10% can be obtained. It should be noted that if the heating
temperature is less than 250.degree. C., sintering is not
sufficiently promoted at contacting points of the magnesium alloy
fibers and large numbers of pores are remained. In the products
after plastic working, contacting points which are not sufficiently
sintered and boundaries of magnesium alloy fibers which are not
sintered are remained, whereby the strength is lowered. Therefore,
the heating temperature is preferably 250.degree. C. or more. If
the heating temperature is more than 500.degree. C., sintering is
sufficiently promoted at contacting points of fibers and pores are
few. However, in this condition, the structure is coarse and
products after plastic working do not have required fine structure.
As a result, a magnesium alloy wire rod having desired strength
cannot be obtained. Therefore, the heating temperature is
preferably 500.degree. C. or less.
[0043] If the raw material is a powder, sintering must be performed
before sealing in canning. However, a big sequential apparatus for
providing a vacuum or an inert atmosphere is required, and it is
difficult to uniformly charge a powder into a mold or a metallic
capsule in a closed apparatus. As a result, it is difficult to
produce a densified compact. That is, if a powder is used, calming
must be performed before the powder is exposed to air, and
sintering of the particles in the compact in the capsule is
insufficient. Furthermore, the compact has large numbers of pores
and density thereof is not uniform. Since the compact has pores
communicated with the surface thereof, the inner portion thereof is
exposed to air after the metallic capsule is removed. Therefore,
the metallic capsule cannot be removed in a condition of a billet,
whereby next process of plastic working must be performed in a
condition of canning.
4. Plastic Working
[0044] Working from a billet to a wire rod is performed by warm
plastic working such as drawing, rolling, extruding, or forging.
Plastic working performed at a suitable temperature and a working
ratio (reduction ratio of cross section) generates refinement of
the structure caused by dynamic recrystallization and work
hardening, and is effective for high strengthening of the magnesium
alloy. In these plastic working, drawing and extruding are
preferable for wire rods in which at least one of bending stress
and twisting stress primarily acts. In the plastic working, a
uniform cross section, which is indispensable for a wire rod can be
obtained and greater strain can be introduced in the surface area
of the wire rod compared to the inner portion thereof. As a result,
the structure in the surface area is further refined and
strengthened compared to the inner portion.
[0045] Naturally, strength and elongation are in a trade-off.
Magnesium alloys in which the structure is refined and highly
strengthened have been researched by using powders. Although the
magnesium alloys had high-strength structure, they did not have
sufficient elongation and was not able to be formed to a shape of
part. Since the powder was charged into a metallic capsule and
worked, strain generated by the working is preferentially
introduced to the metallic capsule that was the outermost portion.
Therefore, high-strengthening of the portion in the vicinity of the
surface as obtained by the present invention could not be
obtained.
[0046] In the case in which a billet is produced by casting, high
strengthening cannot be obtained even if the magnesium alloy has
the same composition as in the present invention. The reason for
this is that the crystal grain of an .alpha.-Mg phase in a cast
metal is naturally coarse and precipitated compounds are also
coarse, deformation resistance is large, and accumulation of strain
is large, whereby the metal is shear fractured before obtaining
required fine structure. Furthermore, the amounts of additional
elements contained in the .alpha.-Mg phase by solid solution is
small, whereby high strengthening of the .alpha.-Mg phase by solid
solution is poor. In contrast, in the billet produced from foil
strips, foil pieces, or fibers having fine structure, by sintering
at a suitable temperature, the working resistance is small since
the structure after sintering is fine. Therefore, since the billet
has superior deformability, large strain can be introduced at a
lower temperature in plastic working, and large internal energy,
which is a driving force for recrystallization can be accumulated,
whereby further fine structure can be obtained. Furthermore, since
the amount of additional elements contained in the .alpha.-Mg phase
by solid solution is large, high strengthening is achieved as a
joint result of large effects of solid-solution strengthening and
the fine structure.
[0047] FIG. 2 shows an extruding apparatus 200 used when extruding
is applied as plastic working. In FIG. 2, reference numeral 205 is
an outer mold, reference numeral 210 is a container installed in
the outer mold. The container 210 has a tubular shape. A lower mold
220 is coaxially disposed at an end surface of the container 210. A
die 230 is disposed between the container 210 and the lower mold
220. A punch 240 is slidably inserted in the container 210. A
heater 260 is disposed in the outer circumference of the container
210.
[0048] In the extruding apparatus 200, a billet B which is heated
is charged into the container 210, the punch 240 moves downward,
thereby compressing the billet B. The diameter of the compressed
billet B is reduced by the die 230 and the billet B is extruded to
the space of the lower mold 220, thereby forming a wire rod.
[0049] The extrusion in the extruding apparatus is preferably
performed at a temperature of the billet B of 315 to 335.degree.
C., at an extrusion ratio of 5 to 13, and at a forwarding speed of
the punch 240 of 0.01 to 2.5 mm/second. By such conditions,
refinement of structure caused by dynamic recrystallization and
work hardening caused by introduction of strain are sufficient.
Therefore, a high-strength magnesium alloy wire rod in which the
inner portion thereof has high strength and high ductility and the
portion in the vicinity of the surface has higher strength, is
formed. Specifically, the portion in the vicinity of the surface
has the highest hardness of 170 HV or more and the inner portion
has a 0.2% proof stress of 550 MPa or more and an elongation of 5%
or more, whereby a magnesium alloy wire rod suitable for a strength
part in which at least one of bending stress and twisting stress
primarily acts is obtained.
[0050] It should be noted that if the heating temperature is less
than 315.degree. C., extruding is difficult since the deformation
resistance is large, thereby resulting breakage in extruding and
rough surface and cracking in the surface of the wire rod. Even
though a wire rod is formed, the hardness of the wire rod is too
high and elongation is deteriorated, whereby elongation of 5% or
more which is required to be formed cannot be obtained. On the
other hand, if the heating temperature is more than 335.degree. C.,
refinement of structure caused by dynamic recrystallization and
work hardening caused by introduction of strain are not sufficient.
As a result, required hardness in the vicinity of the surface
cannot be obtained, whereby the wire rod cannot be applied to parts
in which at least one of bending stress and twisting stress
primarily acts.
[0051] The conditions in the extruding are not limited to the
above-mentioned range and below-mentioned examples, and should be
decided in focusing on obtaining high strength and high elongation
in the inner portion and higher strength in the vicinity of the
surface. That is, introduction of strain and inducement of dynamic
recrystallization are affected by complex relationship of
composition of the material, working ratio, working temperature,
and so on, whereby the conditions should be suitably decided based
on theory, experience, and experimentation.
[0052] The average crystal grain diameter of .alpha.-Mg phase in
the portion having the highest hardness in the vicinity of the
surface of the high-strength magnesium alloy wire rod produced in
the above is preferably 1 .mu.m or less measured by an EBSD method.
It is well known that refinement of crystal grain greatly
contributes to high strengthening as well as the theory of
Hall-Petch. Refinement of crystal grain is effective for inhibiting
generation of initial crack on a surface of a fatigue part to which
repeated stress acts. In the below-mentioned practical examples of
the present invention having the highest hardness in the vicinity
of the surface and the highest hardness is 170 HV or more, the
average crystal grain diameter is greatly fine at 1 .mu.m or less,
whereby the examples are suitable for static strength and fatigue
strength.
EXAMPLES
[0053] The present invention will be explained in detail by way of
specific examples. Raw materials of each element for casting were
weighed such that required composition of a magnesium alloy and
required size of a cast metal were obtained, and raw materials were
melted in a vacuum and were cast. The compositions of the cast
metals are shown in Table 1. In the melting, a crucible made from
graphite and a die made from a copper alloy were used. Fibers were
produced using the apparatus shown in FIG. 1 according to a molten
metal extraction method. In production of fibers according to the
molten metal extraction method, a raw material holding portion made
from graphite and a disk made from a copper alloy were used, and
fibers having an average diameter of 60 .mu.m were produced in an
argon gas substituted inert atmosphere.
TABLE-US-00001 TABLE 1 Composition Sintering Inner diameter of
Extruding Extruding (at %) temperature container Extruding
temperature speed No. Mg Ni Y Form of billet (.degree. C.) (mm)
ratio (.degree. C.) (mm/second) Result Practical Example 1 93.5 3.0
3.5 Fiber sintered body 400 35 10 300 0.05 Bad Practical Example 2
.uparw. .uparw. .uparw. .uparw. .uparw. .uparw. .uparw. 305 .uparw.
Bad Practical Example 3 .uparw. .uparw. .uparw. .uparw. .uparw.
.uparw. .uparw. 310 .uparw. Not good Practical Example 4 .uparw.
.uparw. .uparw. .uparw. .uparw. .uparw. .uparw. 315 .uparw. Good
Practical Example 5 .uparw. .uparw. .uparw. .uparw. .uparw. .uparw.
.uparw. 320 .uparw. Good Practical Example 6 .uparw. .uparw.
.uparw. .uparw. .uparw. .uparw. .uparw. 325 .uparw. Good Practical
Example 7 .uparw. .uparw. .uparw. .uparw. .uparw. .uparw. .uparw.
330 .uparw. Good Practical Example 8 .uparw. .uparw. .uparw.
.uparw. .uparw. .uparw. .uparw. 335 .uparw. Good Practical Example
9 .uparw. .uparw. .uparw. .uparw. .uparw. .uparw. .uparw. 340
.uparw. Good Practical Example 10 .uparw. .uparw. .uparw. .uparw.
.uparw. .uparw. .uparw. 350 .uparw. Good Practical Example 11
.uparw. .uparw. .uparw. .uparw. .uparw. .uparw. .uparw. 375 .uparw.
Good Practical Example 12 .uparw. .uparw. .uparw. .uparw. .uparw.
.uparw. .uparw. 400 .uparw. Good Practical Example 13 98.0 1.0 1.0
.uparw. .uparw. .uparw. .uparw. 325 .uparw. Good Practical Example
14 96.0 2.0 2.0 .uparw. .uparw. .uparw. .uparw. .uparw. .uparw.
Good Practical Example 15 90.0 5.0 5.0 .uparw. .uparw. .uparw.
.uparw. .uparw. .uparw. Good Practical Example 16 88.0 6.0 6.0
.uparw. .uparw. .uparw. .uparw. .uparw. .uparw. Bad Practical
Example 17 93.5 3.0 3.5 .uparw. 300 .uparw. .uparw. .uparw. .uparw.
Good Practical Example 18 .uparw. .uparw. .uparw. .uparw. 350
.uparw. .uparw. .uparw. .uparw. Good Practical Example 19 .uparw.
.uparw. .uparw. .uparw. 450 .uparw. .uparw. .uparw. .uparw. Good
Practical Example 20 .uparw. .uparw. .uparw. .uparw. 500 .uparw.
.uparw. .uparw. .uparw. Good Practical Example 21 .uparw. .uparw.
.uparw. .uparw. 525 .uparw. .uparw. .uparw. .uparw. Good Practical
Example 22 .uparw. .uparw. .uparw. .uparw. 400 16 .uparw. 325
.uparw. Good Practical Example 23 .uparw. .uparw. .uparw. .uparw.
.uparw. .uparw. .uparw. 350 .uparw. Good Practical Example 24
.uparw. .uparw. .uparw. .uparw. .uparw. 35 3 325 .uparw. Good
Practical Example 25 .uparw. .uparw. .uparw. .uparw. .uparw.
.uparw. 5 .uparw. .uparw. Good Practical Example 26 .uparw. .uparw.
.uparw. .uparw. .uparw. .uparw. 13 .uparw. .uparw. Good Practical
Example 27 .uparw. .uparw. .uparw. .uparw. .uparw. .uparw. 15
.uparw. .uparw. Bad Practical Example 28 .uparw. .uparw. .uparw.
.uparw. .uparw. .uparw. 10 .uparw. 0.01 Good Practical Example 29
.uparw. .uparw. .uparw. .uparw. .uparw. .uparw. .uparw. .uparw. 0.5
Good Practical Example 30 .uparw. .uparw. .uparw. .uparw. .uparw.
.uparw. .uparw. .uparw. 2.5 Good Practical Example 31 .uparw.
.uparw. .uparw. .uparw. .uparw. .uparw. .uparw. .uparw. 5 Not good
Comparative Example 1 93.5 3.0 3.5 Cast metal -- .uparw. .uparw.
375 0.05 Bad Comparative Example 2 .uparw. .uparw. .uparw. .uparw.
-- .uparw. .uparw. 400 .uparw. Not good Comparative Example 3
.uparw. .uparw. .uparw. .uparw. -- .uparw. .uparw. 425 .uparw.
Good
[0054] The produced fibers were directly charged into a sintering
die made from graphite without canning, and sintered by HP method,
thereby obtaining a billet having a diameter of 15 mm and a length
of 50 mm and a billet having a diameter of 33 mm and a length of 50
mm. The sintering according to the HP method was performed at a
temperature of 300 to 525.degree. C. and at a pressure of 50 MPa in
an argon gas substituted inert atmosphere (atmosphere pressure of
0.08 MPa).
[0055] Next, the billet was formed to a wire rod using the
extruding apparatus shown in FIG. 2. Specifically, a graphite type
lubricant (provided by Japan Acheson, OILDAG-E) was used, the
extruding speed (forward speed of the punch 240) was 0.01 to 5
mm/minute, and the extruding temperature was 300 to 425.degree. C.
as shown in Table 1. The billet having a diameter of 15 mm was
extruded using a container 210 having an inner diameter of 16 mm
and a die 230 having a bore diameter of 5 mm (extruding ratio of
10), thereby obtaining a wire rod. The billet having a diameter of
33 mm was extruded using a container 210 having an inner diameter
of 35 mm, a die 230 having a bore diameter of 20 mm (extruding
ratio of 3), a die 230 having a bore diameter of 15.5 mm (extruding
ratio of 5), a die 230 having a bore diameter of 11 mm (extruding
ratio of 10), a die 230 having a bore diameter of 9.7 mm (extruding
ratio of 13), a die 230 having a bore diameter of 9 mm (extruding
ratio of 15), thereby obtaining a wire rod. A cast billet was
extruded for comparison.
[0056] Tensile test of the produced wire rod was performed. In the
tensile test, a test piece having a 1.6 mm thick parallel portion
was machined from the wire rod having a diameter of 5 mm, and a
test piece having a 3 mm thick parallel portion was machined from
the wire rod having a diameter of 9 mm or more. The test pieces
were subjected to tensile test at room temperature using a
universal testing machine (provided by Instron, No. 5586) at a test
speed of 0.5 mm/minute. The results of the tensile test are shown
in Table 2.
TABLE-US-00002 TABLE 2 Hardness (HV) 0.2% Proof Highest value
stress Elongation in the vicinity No. (MPa) (%) Center of surface
Practical Example3 670 4.0 169 168 Practical Example4 663 5.0 159
180 Practical Example5 643 5.2 158 182 Practical Example6 620 5.3
160 181 Practical Example7 613 5.9 154 178 Practical Example8 580
6.2 152 170 Practical Example9 563 6.4 146 168 Practical Example10
532 6.7 141 156 Practical Example11 510 10.3 138 155 Practical
Example12 493 13.3 137 140 Practical Example13 540 7.2 144 159
Practical Example14 582 6.0 154 173 Practical Example15 660 5.1 165
183 Practical Example17 422 3.8 164 165 Practical Example18 579 5.2
163 173 Practical Example19 623 5.5 161 183 Practical Example20 601
6.1 159 174 Practical Example21 483 8.2 143 152 Practical Example22
645 5.0 158 175 Practical Example23 633 5.1 144 159 Practical
Example24 483 7.7 136 142 Practical Example25 551 7.0 153 172
Practical Example26 655 5.0 159 177 Practical Example28 615 5.4 161
182 Practical Example29 622 5.2 158 176 Practical Example30 625 5.2
159 179 Practical Example31 600 5.1 158 155 Comparative Example2
408 10.6 109 129 Comparative Example3 399 10.0 102 130
[0057] In Table 1, the section specified by "Form of billet" shows
a production method of a billet before extruding, "Fiber sintered
body" shows a billet obtained by sintering fibers, and "Cast metal"
shows a billet as cast. In Table 1, "Bad" shows the case in which
breakage occurred in extruding and a wire rod could not be
obtained, "Not good" shows the case in which rough surface and
cracking in a surface of a wire rod was confirmed by visual
contact, although a wire rod was obtained, and "Good" shows the
case in which a good wire rod without rough surface and cracking
was obtained. The tensile test was performed to the test piece in
which the result of extruding was "Not good" and "Good".
[0058] Hardness was measured with respect to the wire rod in which
the result of extruding was "Not good" and "Good". The test piece
for measuring hardness was embedded in a resin so that the cross
section of the extruded wire rod is exposed and mirror finished by
mechanical polishing. Distribution of hardness of the cross section
of the extruded wire rod was measured using a Vickers hardness
testing machine (provided by Future-Tech, No. FM-600) at a testing
load of 25 gf. The result of the measuring hardness is shown in
Table 2 and FIGS. 3 to 5.
[0059] In Table 2 and FIGS. 3 to 5, the test pieces in which the
highest hardness in the vicinity of the surface of the wire rod was
170 HV or more and 0.2% proof stress of 550 MPa or more and
elongation was 5.0% or more in the inner portion were practical
examples of the present invention (Practical Examples Nos. 4 to 8,
14, 15, 18 to 20, 22, 25, 26, and 28 to 30). The strength in the
practical examples was very high compared to the Comparative
Examples No. 2 and 3, which were produced from the cast billets.
The inner portion of the wire rod had a high strength and high
ductility portion in which the 0.2% proof stress was 563 MPa or
more and the elongation was 5% or more was. In these practical
examples, since the highest hardness in the vicinity of the surface
was 170 HV or more, a higher strengthened portion in which the 0.2%
proof stress was 650 MPa or more was provided. The high strength
and high ductility portion in the inner portion and the higher
strengthened portion in the vicinity of the surface were gradually
connected and did not have clear boundary, whereby the whole wire
rod had superior strength and toughness and sufficient
formability.
[0060] As shown in Table 1, in Practical Examples Nos. 1 and 2,
deformation resistance was large since the extruding temperature
(heating temperature of the billet) was low, breakage occurred in
extruding and a wire rod was not obtained. In Practical Example No.
3, rough surface and cracks were generated in the surface layer,
although a wire was obtained, and the inner portion was
deteriorated in ductility as strengthening was promoted, whereby
elongation of 5% or more which was required for formability was not
obtained.
[0061] In Practical Examples Nos. 9 to 12 and 23, since the
extruding temperature was greater than 335.degree. C., refinement
of structure caused by dynamic recrystallization and work hardening
caused by introduction of strain were not sufficient. As a result,
the highest hardness in the vicinity of the surface was less than
170 HV. Therefore, the hardness in the vicinity of the surface was
insufficient for applying the wire rod to strong parts in which at
least one of bending stress and twisting stress primarily acts. In
Practical Example No. 13, since amounts of Ni and Y were small at
1.0 atomic %, solid solution strengthening in the .alpha.-Mg phase
and amount of precipitated high strength Mg--Ni--Y type compound
were small, the highest strength of 170 HV or more in the vicinity
of the surface was not obtained. In contrast, in Practical Example
No. 16, since amounts of Ni and Y were large at 6.0 atomic %,
high-strength Mg--Ni--Y type compounds were greatly precipitated
and coarse. As a result, deformation resistance was large and
toughness was low, whereby breakage occurred in extruding.
[0062] In Practical Example No. 27, since the extruding ratio was
more than 13, the wire rod was greatly strengthened and toughness
was low, and breakage occurred in extruding. In Practical Example
No. 21, since the sintering temperature was more than 500.degree.
C., phase effective for high strengthening was decomposed and the
crystal grain was coarse, whereby the hardness in the vicinity of
the surface was less than 170 HV. In Practical Example No. 17,
since the sintering temperature was less than 350.degree. C., a
densified billet was not obtained. In the billet, a large amount of
unbonded boundaries of fibers, which was difficult to eliminate by
the next process of plastic working and was a defect of a wire rod
after extruding, were present, and bonding strength at contacting
points of the magnesium alloy fibers was insufficient. As a result,
sufficient 0.2% proof stress and elongation were not obtained,
although the hardness was improved. In Practical Example No. 31,
since the extruding speed was more than 2.5 mm/second, lubrication
was insufficient, whereby rough surfaces such as scuffing in the
surface of the wire rod were formed. Deformation strain was
released by such a rough surface, the hardness in the vicinity of
the surface was less than 170 HV although a 0.2% proof stress of
600 MPa and an elongation of 5.1% in the inner portion were
obtained. In Comparative Examples Nos. 1 and 2, since the billet
was a cast metal, the .alpha.-Mg phase was coarse and the
precipitated compound phases were also coarse. As a result, the
deformation resistance and accumulation of strain were large.
Therefore, in Practical Example No. 1, breakage occurred in
extruding, and in Practical Example No. 2, rough surfaces and
cracks were formed in extruding. In practical Example No. 3,
required properties were not obtained, although breakage did not
occur since the extruding temperature was high.
[0063] Next, the relationship between the average crystal grain
diameter and the hardness in the .alpha.-Mg phase in the vicinity
of the surface in the practical examples of the present invention
and Comparative Example No. 3 was evaluated. The results are shown
in Table 3. Measurement of the average crystal grain diameter of
the .alpha.-Mg was performed on the test piece that was subjected
to the measurement of the hardness using an EBSD method (electron
beam backscattering diffraction apparatus, provided by TSL)
utilizing an FE-SEM (electrolysis emission type scanning electron
microscope, provided by JEOL, No. JSM-7000F) and. The measurement
was performed at the position in the vicinity of the surface at
which the highest hardness was obtained for practical examples at
analysis magnification of 10,000 times and for Comparative Example
No. 3 at analysis magnification of 2,000 times. In Table 3, the
highest hardness in the vicinity of the surface is shown
together.
TABLE-US-00003 TABLE 3 Highest hardness in Average crystal diameter
the vicinity of of .alpha.-Mg phase No. surface (HV) (.mu.m)
Practical Example4 180 0.21 Practical Example5 182 0.26 Practical
Example6 181 0.23 Practical Example7 178 0.59 Practical Example8
170 0.35 Practical Example14 173 0.27 Practical Example15 183 0.20
Practical Example18 173 0.36 Practical Example19 183 0.62 Practical
Example20 174 0.76 Practical Example22 175 0.33 Practical Example25
172 0.69 Practical Example26 177 0.33 Practical Example28 182 0.30
Practical Example29 176 0.19 Practical Example30 179 0.53
Comparative Example3 130 6.76
[0064] As shown in Table 3, the average crystal grain diameter of
the .alpha.-Mg was very fine at 0.19 to 0.76 .mu.m compared to 6.76
.mu.m of Comparative Example No. 3. It is apparent that the fine
crystal grain contributes to improvement of the hardness.
INDUSTRIAL APPLICABILITY
[0065] The magnesium alloy wire rod of the present invention is
suitable for a high-strength part in which at least one of bending
stress and twisting stress primarily acts. By using the magnesium
alloy wire rod of the present invention, great weight reduction can
be achieved without increase in size of parts compared to
conventional steel parts. The weight reduction is very effective
for, for example, automobile parts such as seat frames, which have
a higher proportion of weight, and springs (suspension springs,
valve springs, clutch torsion springs, torsion bars, stabilizers)
which are required to have high strength.
* * * * *