U.S. patent application number 10/702567 was filed with the patent office on 2004-05-13 for heat resistant magnesium alloy.
Invention is credited to Kato, Takayuki, Kishi, Eiji, Miyoshi, Manabu, Okamoto, Yuki, Tanaka, Katsufumi, Tanizawa, Motoharu.
Application Number | 20040091384 10/702567 |
Document ID | / |
Family ID | 32105521 |
Filed Date | 2004-05-13 |
United States Patent
Application |
20040091384 |
Kind Code |
A1 |
Tanaka, Katsufumi ; et
al. |
May 13, 2004 |
Heat resistant magnesium alloy
Abstract
A heat resistant magnesium alloy contains 1 to 6 percentage by
mass of aluminum, 0.5 to 3 by mass ratio of calcium to aluminum,
and the remainder made from magnesium and unavoidable
impurities.
Inventors: |
Tanaka, Katsufumi;
(Kariya-shi, JP) ; Kishi, Eiji; (Kariya-shi,
JP) ; Tanizawa, Motoharu; (Kariya-shi, JP) ;
Okamoto, Yuki; (Kariya-shi, JP) ; Miyoshi,
Manabu; (Kariya-shi, JP) ; Kato, Takayuki;
(Kariya-shi, JP) |
Correspondence
Address: |
MORGAN & FINNEGAN, L.L.P.
345 Park Avenue
New York
NY
10154
US
|
Family ID: |
32105521 |
Appl. No.: |
10/702567 |
Filed: |
November 5, 2003 |
Current U.S.
Class: |
420/407 |
Current CPC
Class: |
C22C 23/02 20130101;
C22C 23/00 20130101 |
Class at
Publication: |
420/407 |
International
Class: |
C22C 023/02 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 11, 2002 |
JP |
P2002-326825 |
Claims
What is claimed is:
1. A heat resistant magnesium alloy comprising: aluminum in an
amount by weight of about 1 to about 6 percent; calcium in an
amount such that the weight ratio of calcium to aluminum is from
about 0.5 to about 3; and the remainder magnesium and unavoidable
impurities.
2. The heat resistant magnesium alloy of claim 1, wherein aluminum
is present in an amount by weight of about 2 to about 4
percent.
3. The heat resistant magnesium alloy of claim 1, wherein calcium
is present in an amount such that the weight ratio of calcium to
aluminum is from about 1 to about 2.
4. The heat resisting magnesium alloy of claim 1 further comprising
manganese in an amount by weight of about 0.2 to about 1
percent.
5. The heat resistant magnesium alloy of claim 4, wherein the
manganese is present in an amount by weight of about 0.5 to about
0.7 percent.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to a low-cost heat resistant
magnesium (Mg) alloy.
[0002] Because it is a lightweight material, Mg alloy, which is
more lightweight than aluminum (Al) alloy, has recently attracted
attention. Mg alloy is the most lightweight among suitable
materials used to construct airplanes and automobiles. Mg alloy is
used, for example, for wheels and head covers of engines of the
automobiles.
[0003] In recent years, making automobiles more lightweight has
been environmentally important. Therefore, use of Mg alloy is
considered even in equipment and apparatus for use in
high-temperature environments. In this case, it is the heat
resistance of Mg alloy that matters. Prior art Mg alloy is
characterized by a lack of high-temperature strength and is
unsuitable for use in high-temperature environments. Additionally,
when prior art Mg alloy is used for a structural material to which
relatively large stress is applied, it is susceptible to creep
deformation.
[0004] An additional element, when added to Mg alloy, improves the
heat resistance of Mg alloy. The following publications disclose
such a heat resistant Mg alloy.
[0005] Japanese Unexamined Patent Publication No. 9-272945
discloses Mg--Al--Ca--Si series alloy where Ca and Si respectively
denote calcium and silicon. Also, Japanese Unexamined Patent
Publication No. 9-291332, which corresponds to U.S. Pat. No.
3,229,954, discloses Mg--Al--Ca--RE--Mn series alloy where RE and
Mn respectively denote a rare-earth element and manganese. In
addition, Japanese Unexamined Patent Publication No. 2002-157979
discloses Mg--Al--Zn series alloy where Zn denotes zinc. Further,
other publications or references disclose heat resistant Mg alloy
such as Mg--Al--Zn--Mn series alloy, Mg--Al--Si--Mn series alloy,
Mg--Zn--Ca series alloy, and Mg--RE--Zn series alloy in such a
manner that the heat resistant Mg alloy contains various elements
and amounts of the elements.
[0006] However, most prior art Mg alloys contain a plurality of
elements in large amounts. Some prior art Mg alloys contain costly
RE. Consequently, these prior art Mg alloys are expensive.
SUMMARY OF THE INVENTION
[0007] The present invention is directed to a heat resistant
magnesium alloy excellent in heat resistance, which is produced
using a low-cost element and by appropriately adjusting an amount
of the element.
[0008] The present invention has a following feature. A heat
resistant magnesium alloy contains about 1 to about 6 percentage by
mass of aluminum, about 0.5 to about 3 by mass ratio of calcium to
aluminum, and the remainder made from magnesium and unavoidable
impurities.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] The invention together with objects and advantages thereof,
may best be understood by reference to the following description of
the presently preferred embodiments together with the accompanying
drawings in which:
[0010] FIG. 1 is a graph illustrating results from a relaxation
test applied to specimens made of various magnesium alloys;
[0011] FIG. 2A is a picture of metallic formation of a specimen No.
3 observed by a metallographical microscope; and
[0012] FIG. 2B is a picture of metallic formation of a specimen No.
C2 observed by a metallographical microscope.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0013] A heat resistant magnesium (Mg) alloy according to a first
preferred embodiment of the present invention will now be
described. In the first embodiment, the Mg alloy contains 1 to 6
percentage (%) by mass of aluminum (Al), 0.5 to 3 by mass ratio of
calcium (Ca) to Al, and remainder being Mg and unavoidable
impurities.
[0014] In the first preferred embodiment, the heat resistant Mg
alloy essentially contains only Al and Ca together with Mg so as to
improve heat resistance of the Mg alloy. Since only a few kinds of
elements that are low cost and commonplace are used, not only the
material cost of the heat resistant Mg alloy but also the total
manufacturing cost is reduced. Therefore, a competitive heat
resistant Mg alloy is obtained.
[0015] A Mg alloy excellent in heat resistance is obtained by
restricting Ca content and Al content respectively in the above
ranges. Al is an element that is dissolved in the crystal grain of
Mg so as to improve the strength of the Mg alloy at room
temperature. Also, Al lowers the melting point of the Mg alloy and
improves the casting performance thereof. At the same time, Al
narrows the temperature range of coagulation of the Mg alloy and
reduces stress caused by solidification shrinkage of the Mg alloy,
and thereby prevents casting crack. Therefore, when the Mg alloy
can be formed not only by metal mold casting but also by die
casting whose cooling speed is relatively high, Al is an useful
element for improving the casting performance of the Mg alloy.
[0016] If the amount of Al is less than 1% by mass, the above
effect is not sufficiently achieved. Even if the amount of Al is
more than 6% by mass, the above effect is not improved and is not
economical. Mg alloy preferably contains 2 to 4% by mass of Al.
[0017] Meanwhile, if the amount of Al that is contained in the Mg
alloy becomes more than a predetermined amount of Al, Al is
dissolved in Mg matrix including dendritic cell and alpha crystal
grain to excessive saturation to form Al-rich phase. Since Al-rich
phase is thermally unstable, if temperature of Mg alloy is raised
to a predetermined value, Al-rich phase and Mg alloy change to
Mg--Al compound such as Mg.sub.17Al.sub.12 and separates from Mg
matrix and Mg crystal grain boundary. If Mg alloy is left to stand
in a range of high temperature for an extended period,
intermetallic compound coheres and becomes coarse. Thereby, creep
deformation of Mg alloy is increased. That is, heat resistance of
Mg alloy is reduced.
[0018] However, in the first preferred embodiment, a proper amount
of Ca is contained in the Mg alloy in accordance with Al content.
Ca inhibits deterioration of the heat resistance of the Mg alloy
accompanied by increase of Al content. The reason is considered as
follows. Ca reacts with the Mg--Al compound and the Mg alloy matrix
and thereby reduces the amount of Mg.sub.17Al.sub.12, which causes
an increase of creep deformation to Mg alloy, while forming Ca--Al
compound and Mg--Ca compound, which are stable in a range of high
temperature, together with Al and Mg.
[0019] These intermetallic components are crystallized or separated
mainly from grain boundary so as to form a network. Thereby, it is
considered that the intermetallic components serve as a wedge for
preventing transposition of Mg alloy. For these reasons, when
proper amounts of Al and Ca are contained in the Mg alloy according
to the first preferred embodiment of the present invention, it is
considered that the Mg alloy excellent in heat resistance with very
little creep deformation even in a range of high temperature is
produced.
[0020] If the mass ratio of Ca to Al is lower than 0.5, separation
of Mg.sub.17Al.sub.12, which causes an increase of creep
deformation of the Mg alloy, is not sufficiently inhibited.
Therefore, heat resistance of Mg alloy becomes insufficient. On the
other hand, even if the mass ratio of Ca to Al is more than 3,
improvement of heat resistance of Mg alloy is not achieved. In this
case, it is not economical either. Also, since an excessive
increase of the amount of Ca causes deterioration of castability,
casting crack, burning to die, and deterioration of extensibility,
it is not preferable. The mass ratio of Ca to Al is preferably 1 to
2.
[0021] A heat resistant Mg alloy according to a second preferred
embodiment of the present invention will now be described. The Mg
alloy contains the same elements of the first preferred embodiment
and the same amounts thereof. In the second embodiment, the Mg
alloy further contains 0.2 to 1% by mass of Manganese (Mn). More
preferably, the Mg alloy contains 0.5 to 0.7% by mass of Mn.
[0022] Mn is an element that is also dissolved in the crystal grain
of Mg so as to improve the strength of the Mg alloy. Mn also reacts
with Al so as to prevent separation of Mg.sub.17Al.sub.12, which
causes an increase of creep deformation of the Mg alloy while
forming a thermally stable intermetallic compound together with Al.
Thus, Mn is an element that improves not only the strength of the
Mg alloy at room temperature but also the strength thereof in high
temperature. Further, Mn settles out impurities such as iron (Fe),
which causes corrosion, in order to remove the impurities. If the
amount of Mn is less than 0.2% by mass, the above effect is not
sufficiently achieved. Even if the amount of Mn is more than 1% by
mass, the above effect is not improved. In this case, it is not
economical either.
[0023] In the present specification, a compositional range of each
element is indicated in a form of x to y% by mass of the element.
In this case, unless it is specifically noted, the compositional
range of the element includes a minimum value or x% by mass itself.
In a similar manner, the compositional range of the element also
includes a maximum value or y% by mass itself.
[0024] In the present invention, the heat resistance is estimated
by a mechanical property of Mg alloy in a high-temperature
environment. The heat resistance is estimated, for example, by
creep characteristics or high-temperature strength resulting from a
test such as a relaxation test or an axial force retaining
test.
[0025] In the Mg alloy according to the present invention, the
manufacturing process of the Mg alloy is not restricted. Therefore,
the Mg alloy may be obtained by any method of sand-cast, metal
mould cast and die-cast. The materials used are also not
restricted. That is, pure metallic materials such as Mg, Al, Ca and
Mn may be used. On the other hand, a relatively low-cost alloy such
as Mg--Al alloy may be used.
[0026] Mg alloy according to the present invention is used in
various fields, such as space, military affairs, aviation,
automobile and household electrical apparatus. It is further
preferable if Mg alloy according to the present invention is
applied to a product used in a high-temperature environment in
order to utilize its heat resistance. The product is, for example,
an engine, a transmission, a compressor for an air conditioner and
an associated product that is placed in an engine compartment.
[0027] Examples of the Mg alloy according to the first and second
embodiments of the present invention will now be described. In the
examples, specimens of the Mg alloy are produced in such a manner
that amounts of Al, Ca and Mn contained in or added to the Mg alloy
are varied, and properties of the specimen are measured by various
tests.
[0028] First, specimens of the Mg alloy are produced as follows. A
halide flux is applied to an inner surface of crucible made of iron
that is preheated in an electric furnace. Pure Mg metal, pure Al
and Mg-Mn alloy are selectively introduced into the applied
crucible by a predetermined amount and dissolved therein. The
molten metal is maintained at a temperature of 750.degree. C. and a
predetermined amount of Ca is added thereinto. TABLE 1 illustrates
the amount of each element in each specimen. After those elements
are completely dissolved in the molten metal by stirring the
elements into the molten metal, the molten metal including the
elements is cooled down and is maintained at a predetermined
temperature. While Ca is dissolved in the molten metal, the surface
of the molten metal is sprayed with mixed gas of carbon dioxide and
sulfur hexafluoride (SF.sub.6) gas and is sprayed with flux in
order to prevent combustion of Mg.
1TABLE 1 COMPOSOTION (% BY MASS: REMAINDER Mg) MASS TENSILE
SPECIMEN RATIO STRENGTH EXTENSION No. Ca Al Mn Ca/Al (MPa) (%) 1 2
4 0.2 0.5 -- -- 2 3 6 -- 0.5 128.5 1.75 3 1 1 -- 1 104.8 2.08 4 3 1
-- 3 136.3 2.46 C1 1 9 -- 0.1 158.0 2.53 C2 1 3 -- 0.3 131.9
2.38
[0029] Thus, an alloyed molten metal is obtained. The alloyed
molten metal is poured into a die and is solidified in the
atmosphere. A test piece is cut from the obtained ingot and
cylindrical specimens of .phi.10.times.10 mm are machined from the
test piece.
[0030] Then, the specimens are measured as follows. Referring to
the above specimens Nos. 1, 2, 3 and 4, which are shown in TABLE 1,
the relaxation test is performed in order to examine heat
resistance of the specimens Nos. 1, 2, 3 and 4, or creep
characteristics thereof. In the relaxation test, stress applied to
the specimens is relaxed in accordance with passage of time in such
a manner that displacement of each specimen is retained at a
predetermined value in the atmosphere of 150.degree. C.
Specifically, when compressive stress of 100 MPa is first applied
to each specimen and displacement of each specimen is a
predetermined value, the compressive stress is reduced in
accordance with passage of time in such a manner that displacement
of each specimen is retained at the predetermined value. At this
time, the relationship between stress, which is applied to each
specimen, and time is shown in FIG. 1.
[0031] For a comparative test, a similar relaxation test is applied
to specimens Nos. C1 and C2, which are made of various alloys in a
market. The result from the test is shown in FIG. 1. Note that the
alloys used are an Al alloy ADC12 (Al-11 Si-2.5Cu), Mg alloys AE42
(Mg-4Al-2.7R.E.), AS21 (Mg-2Al-1Si), and AZ91 (Mg-9Al-0.9Zn).
[0032] Subsequently, normal tensile test is applied to each
specimen. Also, mechanical property of each specimen is measured at
room temperature. The results from the test and the measurement are
also shown in TABLE 1.
[0033] Further, metallic formation of the specimens Nos. 3 and C2,
which are shown in TABLE 1, is observed using a metallographical
microscope with a magnifying power of 500. Pictures of the metallic
formation are shown in FIGS. 2A and 2B.
[0034] The result from the relaxation test, which is shown in FIG.
1, is now analyzed. In view of the result of FIG. 1, since the Mg
alloy whose mass ratio of Ca to Al is equal to or more than 0.5 has
a relatively small rate of diminution of stress, it is understood
that the Mg alloy has sufficient heat resistance. In addition, as
the mass ratio of Ca to Al increases, the rate of diminution of
stress becomes small. The Mg alloy whose mass ratio is equal to or
more than 1.0 is equivalent to Al alloy (ADC12) in heat
resistance.
[0035] Further, even if the mass ratio of Ca to Al is equal to 0.5,
it is understood that the Mg alloy, which includes a proper amount
of Mn, is substantially equivalent to the above Al alloy in heat
resistance. Further, Mg alloys according to the present invention
are superior to prior art heat resistant Mg alloys in terms of
creep characteristics.
[0036] This reason is also understood from the pictures of metallic
formation, which is shown in FIGS. 2A and 2B. Specifically, as
shown in FIG. 2B, a relatively large amount of Mg.sub.17AL.sub.12,
which reduces the creep characteristics of Mg alloy, separates from
metallic formation of Mg alloy of the specimen No. C2 whose mass
ratio of Ca to Al is equal to 0.3. In contrast, as shown in FIG.
2A, every Mg.sub.17AL.sub.12 of Mg alloy of the specimen No. 3
whose mass ratio of Ca to Al is equal to 1.0 is replaced by Al--Ca
compound, which is thermally stable, or by Mg--Ca compound, which
is also thermally stable.
[0037] The present examples and preferred embodiments are to be
considered as illustrative and not restrictive, and the invention
is not to be limited to the details given herein but may be
modified within the scope of the appended claims.
* * * * *