U.S. patent application number 09/949621 was filed with the patent office on 2002-02-21 for heat-resistant magnesium alloy member.
Invention is credited to Hirabara, Shoji, Sakamoto, Kazuo, Sakate, Nobuo, Yamamoto, Yukio.
Application Number | 20020020475 09/949621 |
Document ID | / |
Family ID | 13785388 |
Filed Date | 2002-02-21 |
United States Patent
Application |
20020020475 |
Kind Code |
A1 |
Sakamoto, Kazuo ; et
al. |
February 21, 2002 |
Heat-resistant magnesium alloy member
Abstract
A heat-resistant magnesium alloy member having specially
excellent molding property and elongation while keeping creep
resistance property, which comprises 2 to 6% by weight of aluminum
and 0.5 to 4% by weight of calcium, and the balance comprising
magnesium and inevitable impurities, having a Ca/Al ratio of no
more than 0.8, preferably no more than 0.6. The method of preparing
the heat-resistant magnesium alloy member is characterized in a
semi-solid injection molding at a range between a solidus
temperaaature of the alloy and a liquidus temperature of the
alloy.
Inventors: |
Sakamoto, Kazuo;
(Hiroshima-shi, JP) ; Yamamoto, Yukio;
(Hiroshima-shi, JP) ; Sakate, Nobuo;
(Hiroshima-shi, JP) ; Hirabara, Shoji; (Kure-shi,
JP) |
Correspondence
Address: |
NIXON PEABODY, LLP
8180 GREENSBORO DRIVE
SUITE 800
MCLEAN
VA
22102
US
|
Family ID: |
13785388 |
Appl. No.: |
09/949621 |
Filed: |
September 12, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09949621 |
Sep 12, 2001 |
|
|
|
08834805 |
Apr 3, 1997 |
|
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Current U.S.
Class: |
148/667 ;
420/407 |
Current CPC
Class: |
C22C 23/02 20130101;
B22D 17/00 20130101; B22D 21/007 20130101 |
Class at
Publication: |
148/667 ;
420/407 |
International
Class: |
C22C 023/02; C22F
001/06 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 4, 1996 |
JP |
08-082832 |
Claims
What is claimed is:
1. A method of molding a heat-resistant magnesium alloy member
having a creep resistance property of no more than
4.times.10.sup.-10/S of minimum creep rate under the test
temperature of 150.degree. C. and the test load of 50 MPa,
comprising the steps of; providing aluminum and calcium; preparing
an alloy composition comprising 2 to 6% by weight of said aluminum
and 0.5 to 4% by weight of said calcium, and no more than 2% by
weight of at least one element selected from the group consisting
of zinc, manganese, zirconium, and silicon, and/or no more than 4%
by weight of a rare earth metal, and the balance of magnesium and
inevitable impurities, wherein a Ca/Al ratio of said alloy
composition is no more than 0.8 and the average particle size of
the crystal is no more than 30 .mu.m, processing said alloy
composition by a cutting method so that said alloy composition is
in the form of metal particles or pellets into which internal
strain is introduced, and subjecting said alloy composition to a
semi-solid injection molding by using an injection molding machine
having a cylinder and a screw at a temperature range between a
solidus temperature of the alloy and a liquidus temperature of the
alloy, wherein a solid phase ratio in semi-solid state is no more
than 30% at the time of carrying out an injection molding.
2. The method of molding a heat-resistant magnesium alloy member
according to claim 1, wherein said Ca/Al ratio is no more than
0.6.
3. The method of molding a heat-resistant magnesium alloy member
according to claim 1, wherein the magnesium alloy further contains
0.03 to 0.15% by weight of Sr.
Description
TECHNICAL FIELD OF THE INVENTION
[0001] The present invention relates to a heat-resistant magnesium
alloy member having an excellent molding property and. an excellent
elongation property while keeping creep resistance property, its
starting alloy compound, and a method of preparing the
heat-resistant magnesium alloy member.
BACKGROUND OF THE INVENTION
[0002] Magnesium alloy is the most low density one of the metal
materials which are in practically use at present, and is strongly
expected as a lightweight material for automobiles in future. The
magnesium alloy which is most popularly used at present is
Mg--Al--Zn--Mn alloy (e.g., AZ91D alloy), and as it has a high
strength at a room temperature and a good corrosion resistance, it
is applied to transmission cases for an automobile, cylinder head
covers, and the like. However, it has such defects that, at a
temperature range exceeding 120.degree. C., it begins to show loss
of strength characteristics, and especially becomes inferior in
creep resistance, leading to a problem of yielding of bearing
surface of the screw tightening part on the level of the packaged
product.
[0003] On the other hand, as an aluminum alloy having an improved
heat-resistance, there is used Mg--Al--Si AS41 magnesium alloy.
However, though said alloy shows better creep resistance than the
above AZ91D, it shows insufficient characteristics in the
neighborhood of 150.degree. C. of the use temperature, and
moreover, as it shows low tensile strength characteristics at both
room temperature and high temperature, it is required to be of
thick wall to secure the required strength, thereby providing a
problem of lowering the weight lightening effect due to magnesium
materials.
[0004] Besides, there are alloys such as QE22 with addition of
silver or rare earth metals to improve a heat resistance thereof,
but they have defects of being expensive and not suited to die-cast
due to a poor casting property.
[0005] For the above reasons, there came to be newly proposed
Mg--Al--Ca--Mn alloy (Japanese Laid-open Patent Publication
HEI6-25790/1994) having excellent strength at high temperature.
Here, it is said that, especially when the Ca/Al ratio is set to be
more than 0.7, preferably more than 0.75, precipitates to be
crystallized in the magnesium alloy convert into Mg--Ca compounds
which crystallize, resulting in production of high temperature
strength characteristics.
[0006] However, it has been found that, in a case of die-casting a
member with a magnesium alloy having a high Ca/Al ratio, there
often occur hot cracks, and due to a high melting temperature there
easily occurs seizure to the metal mold.
SUMMARY OF THE INVENTION
[0007] In view of the problems held by the conventional techniques
as above, a first object of the present invention is to provide a
heat-resistant magnesium alloy member having excellent molding
property and elongation while maintaining the physical properties,
especially creep resistance, suited to the engine parts of
automobiles and the like.
[0008] A second object of the present invention is to provide a
pertinent molding method for preparing the above heat-resistant
magnesium alloy member in place of conventional die-cast
methods.
[0009] Further, a third object of the present invention is to
provide an alloy composition suited for producing a heat-resistant
magnesium alloy member having the excellent molding property and
elongation while maintaining the creep resistance.
[0010] As a result of the repeated reviews to solve the above
problems, the present inventors have found out that, in the Al--Ca
magnesium alloy, when a semi-solid molding method of injection
molding under the state of solid phase and liquid phase being
present in mixture is applied in place of the die-cast method, the
seizure of metal mold can be prevented, and also an excellent
strength can be imparted to the molded member. However, in order to
maintain the state of presence in mixture of solid phase and liquid
phase, it is necessary to increase the addition amount of aluminum
as large as possible.
[0011] On the other hand, aluminum dissolves in magnesium in solid
state and shows age-hardening, and it is added to increase the
mechanical properties of alloy, but it is recommended to add
calcium so as to maintain the Ca/Al ratio to 0.7 or more to
strengthen the high temperature strength which is in a tendency to
be lowered by the addition of Al to Mg (Japanese Patent Laid-open
Publication HEI6-25790/1994). However, when the Ca amount is large,
casting cracks and seizure to metal mold often occur in molding,
and additionally, a large amount of Mg--Ca compound crystallizes,
with the result that there is a tendency of lowering of the
elongation of the molded product, and it has been found that it is
rather necessary to make the Ca/Al ratio no more than 0.8.
[0012] Accordingly, in the present invention, based on the above
two findings, there is provided a magnesium alloy molding member
comprising 2 to 6% by weight of aluminum and 0.5 to 4% by weight of
calcium, and the balance of magnesium and unavoidable impurities,
wherein a Ca/Al ratio is no more than 0.8, to have an excellent
anti-creep property, molding property, and elongation.
[0013] In general, in the magnesium alloy, in order to obtain solid
phase dissolution in magnesium, to exhibit age-hardening, and to
elevate mechanical strength, it has been understood to be
preferable to add 2-10% by weight of aluminum. While it is
necessary in the present invention to add more than 2% by weight of
aluminum, when the amount of addition exceeds 6% by weight, it has
been found that the elongation is lowered even if the semi-solid
injection molding would be carried out. Accordingly, in order to
obtain the designed effect while carrying out the semi-solid
injection molding, it has been found that the addition amount
should be limited to no more than 6% by weight. On the other hand,
calcium is added to increase the high temperature strength which is
in a tendency to be lowered by the addition of aluminum to
magnesium, but it has been found that it is necessary to suppress
the Ca/Al ratio to no more than 0.8% by weight to prevent lowering
of the molding property and elongation of the molding member, and
in addition, the Ca amount should be limited to 0.5-4% by
weight.
[0014] Strontium is used as a micronizing agent in the casting of
magnesium, and as it can display the micronizing effect in solid
phase in the semi-solid injection molding of the present invention,
it is preferably added. The suitable addition amount is no more
than 0.15% by weight.
[0015] The above molding member shows the crystal particle size of
no more than 30 .mu.m with the tensile strength of 180 Mpa
(298.degree. K.; ref. FIG. 9) or more, and excellent creep
resistance of the minimum creep rate of no more than
4.times.10.sup.-10/S under the test temperature of 150.degree. C.
and the test load of 50 MPa (according to JIS Z 2271 "method of
tensile creep test of metal material"). Accordingly, it is suitable
for the transmission part or engine part for automobiles.
Especially, when the Ca/Al ratio is no more than 0.6, the molding
member shows a more excellent creep resistance.
[0016] The present invention is to provide a heat-resistant
magnesium alloy material to be molded by a semi-solid injection
molding while maintaining excellent creep resistance property with
the excellent molding property and elongation, comprising as an
alloy material to be used for molding the above magnesium alloy
molding part heat-resistant magnesium comprising 2 to 6% by weight
of aluminum and 0.5 to 4% by weight of calcium, and the balance of
magnesium and unavoidable impurities, and preferably further Sr of
no more than 0.15% by weight, with adjustment, if necessary, of a
Ca/Al ratio of no more than 0.8, preferably a Ca/Al ratio of no
more than 0.6.
[0017] Especially, as for the alloy material, in case of molding by
a semi-solid injection molding method, it has been found that the
material in the form of metal particles or pellets into which
internal strain is introduced is effective for micronizing the
crystals (ref. FIG. 10). As for the processing method for the metal
particles or pellets, a cutting method is advantageous
costwise.
[0018] Further, in case of applying a semi-solid injection molding
wherein an injection molding is carried out in the state of a solid
phase and a liquid phase being present in mixture, practice can be
made at a temperature lower than a liquidus temperature.
Accordingly, the present invention is to provide a method for
molding a heat-resistant magnesium alloy member characterized by
carrying out a semi-solid injection molding, while maintaining an
excellent creep resistance property with having an excellent
molding property and elongation.
[0019] Against the fact that the die-cast method is in general to
make injection into the metal mold at a temperature of
30-50.degree. C. above a melting temperature, in the semi-solid
injection molding of the present invention, injection can be made
at a temperature higher than the solidus temperature of the alloy
and lower than the liquidus temperature, and accordingly the
injection temperature is lowered by at least 30-60.degree. C., so
that the seizure to the metal mold can be prevented.
[0020] It can be understood that, since solidification takes place
from a semi-solid state in the present invention, and coagulation
stress therewith becomes small, generation of hot cracking can be
prevented due to this method mechanism.
[0021] Especially, in the semi-solid molding method, in the range
of no more than 30% by weight of the solid phase rate, these
prevention and effect on a flow length become remarkable (ref. FIG.
8), and the generation of hot cracking can be effectively
prevented. Accordingly, in case of carrying out the semi-solid
molding, the solid phase ratio in the semi-solid state is
preferably no more than 30%. In general, it has been understood
that a higher solid phase ratio is more advantageous for the
seizure and coagulation stress, but in the present invention
method, when the solid phase rate is high, the fluidity is lowered
to give a tendency of lowering in filling property and generation
of cold shut, thereby making it difficult to obtain a sound molding
member.
[0022] It has been found that, especially when the average particle
size of these coagulation textures is no more than 30 .mu.m, the
elongation amount shows specially large improvement.
[0023] The above magnesium alloy may further contain no more than
2% by weight of at least one element selected from the group
consisting of zinc, manganese, zirconium, and silicon, and/or no
more than 4% by weight of a rare earth metal (e.g., yttrium,
neodymium, lanthanum, cerium, misch metal). These are to improve
the strength or high temperature strength of the above magnesium
alloy effectively in the range no more than the upper limit
thereof.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] FIG. 1 is a schematic diagram showing the constitution of
the molding machine to be used for the semi-solid molding process
and injection molding process according to the present
invention.
[0025] FIG. 2 is a graph for making comparison of the creep
characteristics of various magnesium alloy molding members.
[0026] FIG. 3 is a graph to show the relations between the Ca/Al
ratio and the elongation at room temperature in various magnesium
alloy molding members.
[0027] FIG. 4 is a schematic diagram showing a metal mold for
testing casting cracks.
[0028] FIG. 5 is a graph showing the relation between the solid
phase diameter and the staying time.
[0029] FIG. 6 is a graph showing the minimum creep strain rates of
various magnesium alloy molding members.
[0030] FIG. 7 is a schematic diagram showing the metal mold for
evaluating the flowing properties of various magnesium alloys.
[0031] FIG. 8 is a graph showing the relations between the solid
phase ratio and the flowing length in the alloy composition in
Example 2 measured by using a metal mold of FIG. 7.
[0032] FIG. 9 is a graph showing the relations between the average
crystal particle size and the tensile strength of the member molded
from the alloy composition of Example 3.
[0033] FIG. 10 is a schematic diagram showing the solid phase
growth stages in the cases of using the metal particles having no
work strain and those having the work strain.
DETAIED EXPLANATION OF THE INVENTION
[0034] In FIG. 1, there is shown the whole constitution of the
molding machine 1 to be used for the semi-solid molding method
according to the present invention. In the molding method of the
present invention, the material 3 of magnesium alloy metal
particles or pellets (more than 3 mm in diameter) manufactured by
the method of cutting or the like is charged into the hopper 8 in
the drawing. The material 3 is supplied to the cylinder 4 from the
hopper 8 through the inlet 7 of argon atmosphere. In this cylinder
4, the material 3 is heated while being sent forward by the screw
2. This heating zone is shown by the mark 10. At an approximate
liquidus temperature of heating, the magnesium alloy material 3
shows a molten state, but at a level lower than the liquidus
temperature the material becomes semi-solid condition in which the
solid phase and the liquid phase are present in mixture, as
illustrated. Also, in the magnesium alloy which is in a semi-solid
condition, its shearing force acts to separate the solid phase
finely as illustrated by agitation by the screw rotation. Here,
when the screw 2 is pushed forward with the rear high speed
injection mechanism 5, the molten material in which the solid phase
has been finely cut under the semi-solid state is injected at high
speed from the nozzle 9 as illustrated and filled in the metal mold
6. Here, the contents in the metal mold are held under pressure
until solidification, and thereafter the metal mold is opened to
take out the molding product.
EXAMPLES 1-7 AND COMPARATIVE EXAMPLES 1-5
[0035] An iron crucible is installed in a low frequency furnace,
and while flowing 1% of the SF.sub.6 gas (rest is dry air) on the
surface of the molten material, the alloys having the components of
Examples and Comparative Examples were prepared by melting. The
resulting alloys were cast on a plate to prepare 3-5 mm diameter
pellets by milling. Using these as raw materials, semi-solid
molding was carried out by using the above molding machine.
1TABLE 1 Chemical Composition (Wt. %) Al Ca Si Mn Sr Mg Example 1
Mg-3Al-2Ca 2.98 2.05 0.30 0.25 -- Remainder Example 2 Mg-4Al-2Ca
3.95 2.02 0.30 0.32 -- .Arrow-up bold. Example 3 Mg-4Al-3Ca 4.02
3.06 0.25 0.28 -- .Arrow-up bold. Example 4 Mg-6Al-3Ca 5.97 3.10
0.28 0.30 -- .Arrow-up bold. Example 5 Mg-4Al-2Ca-0.03Sr 3.87 2.06
0.25 0.25 0.03 .Arrow-up bold. Example 6 Mg-4Al-2Ca-0.09Sr 4.02
1.98 0.30 0.23 0.09 .Arrow-up bold. Example 7 Mg-4Al-2Ca-0.15Sr
4.05 2.10 0.23 0.25 0.15 .Arrow-up bold. Comparative ASTM AS41 4.39
-- 0.45 0.28 -- .Arrow-up bold. Example 1 Equivalent Comparative
Mg-9Al-0.5Ca 8.70 0.49 0.90 0.21 -- .Arrow-up bold. Example 2
Comparative ASTM AZ91D 8.84 -- 0.02 0.22 -- .Arrow-up bold. Example
3 Equivalent Comparative Mg-4Al-4Ca 4.02 3.96 0.32 032 -- .Arrow-up
bold. Example 4 Comparative Mg-3Al-3Ca 2.75 2.71 0.27 0.36 --
.Arrow-up bold. Example 5
[0036] For the semi-solid molding, a machine having the clamping
force of 450 t was used under the conditions of injection speed at
the metal mold gate part of 50 m/s, injection pressure of about 700
kg/cm.sup.2, and the temperature of the alloy at the nozzle part
was set to be lower than the liquidus level of 550-580.degree. C.
Under the above molding conditions a tensile test piece (JIS No.4
test piece) was prepared, with which the creep property at
150.degree. C., 50 MPa was examined by the tensile creep test
method based on JIS Z 2271. The results are shown in FIG. 2. It can
be seen that the magnesium alloy of the present invention is more
excellent in creep resistance characteristic than AS41 which is
commented as being superior in creep resistance to Az91D of
Comparative Example 3.
[0037] Further, the breaking strength and breaking elongation were
measured with an instron tensile tester at a cross head rate of 10
mm/min. and at a measuring temperature of 25.degree. C. The results
are shown in Table 2. It can be seen that, in comparison with
Comparative Example 2 in which the aluminum content exceeds the
present invention range of 2-6% by weight and Comparative Example 4
in which the aluminum and calcium contents lie within the present
invention range but the Ca/Al ratio exceeds by 0.8, the Examples
containing 2-6% by weight of aluminum and 0.5-4% by weight of
calcium and having the Ca/Al ratio of no more than 0.8 show
excellent elongation.
2 TABLE 2 Al amount Ca amount Elongation (Wt. %) (Wt. %) (%)
Example 2 3.95 2.02 6.7 Example 3 4.02 3.06 7.0 Example 4 5.97 3.10
5.2 Comparative Example 2 8.70 0.49 0.8 Comparative Example 4 4.02
3.96 1.2
[0038] Then, in Examples and Comparative Examples, the relations
between the Ca/Al ratio and the above elongation are illustrated in
FIG. 3, from which it can be seen that when the Ca/Al ratio exceeds
0.8, the elongation is sharply lowered. In this connection, when
the relations between the Ca/Al ratio and the minimum creep rate of
strain are observed, as shown in FIG. 6, in case of the Ca/Al ratio
being no more than 0.6 (Example 2), the smaller creep rate of
strain is shown, and it can be seen that the creep resistance
property becomes more excellent.
[0039] Further, when the semi-melting molding was carried out by
using the metal mold for test as shown in FIG. 4 and securing the
illustrated running, there were obtained the results as shown in
Table 3. As a result, it-was seen that when the Ca/Al ratio
approached 1, casting cracks were formed on the overflow side at
the top end of the cylindrical part, but at the Ca/Al ratio of no
more than 0.8, no such casting crack was formed at all.
3 TABLE 3 Ca/Al weight Casting crack ratio formed or not Example 1
0.69 No Example 2 0.51 No Example 3 0.76 No Example 4 0.52 No
Comparative Example 1 0.99 Yes Comparative Example 5 0.99 Yes
[0040] In general, when the staying time in casting is extended,
the solid phase diameter is sharply increased (Example in FIG. 5),
but it can be seen that, when strontium is added, the crystal
micronizing effect is actuated to suppress the increment in the
solid phase diameter attributed to the staying time.
[0041] Using the alloy material of Example 2, the semi-solid
molding temperature was varied in the metal mold for evaluating
flowing property as shown in FIG. 7, the molten material was
introduced in the illustrated direction, and its flowing property
was evaluated. The results are shown in FIG. 8. From the results it
can be seen that, when the solid phase rate exceeds 30%, the flow
length is sharply lowered, and as this flow gives effect on the
particle size of the texture crystals of the molding material,
desirably the molding is made under the solid phase condition of no
more than 30% in the semi-solid molding method.
[0042] In the semi-solid molding, the magnesium alloy material is
used in the form of the metal particles or pellets. When work
strain is given inside the metal particles by cutting work or the
like, the metal particles form the nuclei of recrystallization
shortly after the heating, and increase the solid phase diameter.
Therefore, when comparison is made between the case of using the
metal particles having no work strain and that of using the metal
particles having work strain, it can be understood that the growth
rates of the solid phase are different as shown in FIG. 10, and the
latter is superior to the former in the point of micronization of
the crystal particle size of the molding member.
[0043] As will be apparent from the above description, according to
the present invention, it is possible to obtain a molding member
having excellent creep resistance characteristic at high
temperature by controlling Ca/Al ratio in Mg--Al--Ca heat resistant
magnesium alloy member. Therefore, it is possible to produce the
transmission parts for automobiles such as clutch piston and clutch
drum and engine parts such as rocker arm with the lightweight
magnesium alloy to give a sufficient durability.
[0044] Further, according to the present invention, by carrying out
semi-solid molding at a temperature lower than the liquidus level,
the problems of hot crack and seizure to the metal mold which had
been remarkable in the conventional die-cast process are dissolved,
and on the other hand, the strength at room temperature and high
temperature along with elongation equivalent to or higher than
those of the conventional process can be retained.
* * * * *