U.S. patent application number 09/982941 was filed with the patent office on 2002-07-11 for aluminum die casting alloy, aluminum die cast product and production process.
This patent application is currently assigned to Nissan Motor Company, Ltd. Invention is credited to Asakuno, Syuichi, Ikari, Takaaki, Kambe, Hiroshi, Kitaoka, Sanji, Kuramasu, Yukio, Shioda, Masahiko, Sumi, Shinichiro, Tsushima, Kenji.
Application Number | 20020088512 09/982941 |
Document ID | / |
Family ID | 26602756 |
Filed Date | 2002-07-11 |
United States Patent
Application |
20020088512 |
Kind Code |
A1 |
Kitaoka, Sanji ; et
al. |
July 11, 2002 |
Aluminum die casting alloy, aluminum die cast product and
production process
Abstract
An aluminum alloy for die casting, contains, in terms of mass
percentage: Si in the range of 1.0.about.3.5%; Mg in the range of
2.5.about.4.5%; Mn in the range of 0.3.about.1.5%; Fe in the range
equal to or less than 0.15%; Ti in the range of equal to or less
than 0.20%; and the balance of aluminum and inevitable impurities.
A die cast product having good strength and elongation in a high
strain rate region is produced from the aluminum alloy without the
need for solution heat treatment.
Inventors: |
Kitaoka, Sanji; (Shizuoka,
JP) ; Kuramasu, Yukio; (Shizuoka, JP) ;
Asakuno, Syuichi; (Kuala Lumpur, JP) ; Tsushima,
Kenji; (Yokohama, JP) ; Shioda, Masahiko;
(Yokohama, JP) ; Kambe, Hiroshi; (Kanagawa,
JP) ; Sumi, Shinichiro; (Shizuoka, JP) ;
Ikari, Takaaki; (Shizuoka, JP) |
Correspondence
Address: |
McDERMOTT, WILL & EMERY
600 13th Street, N.W.
Washington
DC
20005-3096
US
|
Assignee: |
Nissan Motor Company, Ltd
|
Family ID: |
26602756 |
Appl. No.: |
09/982941 |
Filed: |
October 22, 2001 |
Current U.S.
Class: |
148/549 ;
420/544 |
Current CPC
Class: |
C22F 1/047 20130101;
C22C 21/08 20130101 |
Class at
Publication: |
148/549 ;
420/544 |
International
Class: |
C22C 021/08 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 25, 2000 |
JP |
2000-325756 |
Aug 7, 2001 |
JP |
2001-238947 |
Claims
What is claimed is:
1. An aluminum alloy for die casting, comprising, in terms of mass
ratio: silicon in the range of 1.0.about.3.5%; magnesium in the
range of 2.5.about.4.5%; manganese in the range of 0.3.about.1.5%;
iron in the range equal to or less than 0.15%; a further
constituent comprising titanium in the range of equal to or less
than 0.20%; and the balance of aluminum and inevitable
impurities.
2. The aluminum alloy as claimed in claim 1, wherein the further
constituent consists of titanium, and the aluminum alloy consists
essentially, in terms of mass ratio, of: silicon in the range of
1.0.about.3.5%; magnesium in the range of 2.5.about.4.5%; manganese
in the range of 0.3.about.1.5%; iron in the range equal to or less
than 0.15%; titanium in the range of equal to or less than 0.20%;
and the balance of aluminum and inevitable impurities.
3. The aluminum alloy as claimed in claim 2, wherein the Ti content
is 0.10% or more.
4. The aluminum alloy as claimed in claim 1, wherein the further
constituent consists of titanium and boron, and the aluminum alloy
consists essentially, in terms of mass ratio, of: silicon in the
range of 1.0.about.3.5%; magnesium in the range of 2.5.about.4.5%;
manganese in the range of 0.3.about.1.5%; iron in the range equal
to or less than 0.15%; titanium in the range of 0.05.about.0.20%;
boron in the range of 0.001.about.0.10%; and the balance of
aluminum and inevitable impurities.
5. The aluminum alloy as claimed in claim 1, wherein the further
constituent consists of titanium and zirconium, and the aluminum
alloy consists essentially, in terms of mass ratio, of silicon in
the range of 1.0.about.3.5%; magnesium in the range of
2.5.about.4.5%; manganese in the range of 0.3.about.1.5%; iron in
the range equal to or less than 0.15%; titanium in the range equal
to or smaller than 0.20%; zirconium in the range of
0.05.about.0.30%; and the balance of aluminum and inevitable
impurities.
6. The aluminum alloy as claimed in claim 1, wherein the Mn content
is equal to or more than 1.0%.
7. The aluminum alloy as claimed in claim 1, wherein the Si content
is equal to or more than 2.5%.
8. The aluminum alloy as claimed in claim 1, wherein the Si content
is equal to or less than 2.0%.
9. The aluminum alloy as claimed in claim 1, wherein a ratio of the
Mg content in mass % to the Si content in mass % is equal to or
smaller than 2.0.
10. The aluminum alloy as claimed in claim 1, wherein a ratio of
the Mg content in mass % to the Si content in mass % is equal to or
greater than 1.8.
11. A product comprising an aluminum die casting of the aluminum
alloy as claimed in claim 1.
12. The product as claimed in claim 11, wherein the product is a
part of a motor vehicle.
13. A production process comprising: preparing an aluminum alloy
comprising, in terms of mass ratio, silicon in the range of
1.0.about.3.5%, magnesium in the range of 2.5.about.4.5%, manganese
in the range of 0.3.about.1.5%, iron in the range equal to or less
than 0.15%, a further constituent comprising titanium in the range
equal to or less than 0.20%, and the balance of aluminum and
inevitable impurities; and forming the aluminum alloy into a fixed
shape by die casting operation.
14. The production process as claimed in claim 13, wherein the die
casting is vacuum die casting.
15. The production process as claimed in claim 13, wherein a ratio
of the Mg content in mass % to the Si content in mass % of the
aluminum alloy is equal to or greater than 1.8; and the production
process further comprises heat-treating a die-casting obtained by
the die casting operation, in the temperature range of 130.degree.
C..about.300.degree. C. after the die casting operation.
16. The production process as claimed in claim 15, wherein the
operation of heat-treating is one of an artificial aging operation
and a stabilizing operation.
17. The production process as claimed in claim 15, wherein the
operation of heat-treating comprises at least one of an annealing
operation, and a baking operation of baking a coating layer.
18. The production process as claimed in claim 13, wherein a ratio
of the Mg content in mass % to the Si content in mass % of the
aluminum alloy is equal to or smaller than 2.0; and the production
process further comprises stabilizing treatment in the temperature
range of 150.degree. C..about.400.degree. C. of stabilizing a
die-casting obtained by the die casting operation.
19. A product comprising an aluminum die casting produced by the
production process as claimed in claim 13.
20. The product as claimed in claim 19, wherein the product is one
of A pillar, B pillar, C pillar, roof, space frame node, and
fitting for suspension of a motor vehicle.
21. The product as claimed in claim 19, wherein the product is one
of suspension arm, sub-frame and link for suspension.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to aluminum die casting alloy
providing superior mechanical properties and superior castability,
specifically resistance to cracking during casting, without
requiring solution heat treatment after die casting, and to die
cast product and production process based on such an aluminum
alloy.
[0002] Die casting process is widely used for producing parts of
automotive engines and transmissions because of its capability of
casing into thin wall shapes, high dimensional accuracy, high
productivity and flexibility in design of shape. Recently, nodes of
space frame structures, and center pillars of vehicle bodies, for
example, are enrolled as parts which can be made from aluminum
alloy die castings improved in mechanical properties such as
tensile strength, 0.2% offset yield strength, and elongation, by
heat treatment after vacuum die casting process.
[0003] Published Japanese Patent Application Publication (Kokai)
No. 8-41575 discloses die casting aluminum alloy of Al--Si--Mg--Mn
system usable for parts of motor vehicles. Furthermore, Published
Japanese Patent Applications Publication (Kokai) Nos. 5-9638,
11-293375, and 11-80875; and document "Use of Low Iron Die Casting
Alloys for the Automotive Industry", Properties and Applications,
DIE CASTING ENGINEER January/February 1998) disclose die casting
aluminum alloys requiring no solution heat treatment after die
casting.
SUMMARY OF THE INVENTION
[0004] The solution heat treatment and aging process performed
after die casting to achieve desired mechanical properties are
problematical in that the further need for an operation for
correcting distortion produced during water cooling after solution
heat treatment tends to deteriorate productivity and increase
production cost. Moreover, Al--Mg--Mn(--Zr) system alloys recited
in the above documents are susceptible to cracking during casting
even though the need for post-casting heat treatment is
eliminated.
[0005] For vehicle parts such as A pillar, B pillar, C pillar,
roof, space frame joint, and suspension mount, important features
are stable high strength and high elongation even in a high speed
deformation region to ensure the safety in case of collision. In
such applications, however, conventional technology deals merely
with static strength and elongation, and there remain problems in a
high strain rate region at a level of 1000/s.
[0006] It is an object of the present invention to provide
compositions of material, manufactures and production processes
about aluminum die casting alloys which are advantageous in
production, and superior in mechanical properties especially in
high strain rate region.
[0007] It is another object of the present invention to provide
compositions, products and production processes of aluminum die
casting alloys which can be used as final product without requiring
solution heat treatment after die casting process, which can
provide high strength and elongation stably especially in high
strain rate region as in collision of vehicles, to a level adequate
as material for vehicle body parts such as A, B and C pillars,
roofs, joints for space frames, and mounting parts for suspension,
and which is superior in castability, especially in resistance to
cracking during casting.
[0008] According to the present invention, an aluminum alloy for
die casting, comprises, in terms of mass ratio: Si in the range of
1.0.about.3.5%; Mg in the range of 2.5.about.4.5%; Mn in the range
of 0.3.about.1.5%; Fe in the range equal to or less than 0.15%; a
further constituent comprising Ti in the range of equal to or less
than 0.20%; and the balance of Al and inevitable impurities. The
further constituent may consist of titanium, and the aluminum alloy
may consist essentially, in terms of mass ratio, of: silicon in the
range of 1.0.about.3.5%; magnesium in the range of 2.5.about.4.5%;
manganese in the range of 0.3.about.1.5%; iron in the range equal
to or less than 0.15o%; titanium in the range of equal to or less
than 0.20%; and the balance of Al and inevitable impurities. The
further constituent may consist of Ti and B, and the aluminum alloy
may consist essentially, in terms of mass ratio, of: Si in the
range of 1.0.about.3.5%; Mg in the range of 2.5-4.5%; Mn in the
range of 0.3.about.1.5%; Fe in the range equal to or less than
0.15%; Ti in the range of 0.05.about.0.20%; B in the range of
0.001.about.0.10%; and the balance of Al and inevitable impurities.
The further constituent may consist of Ti and Zr, and the aluminum
alloy may consist essentially, in terms of mass ratio, of Si in the
range of 1.0.about.3.5%; Mg in the range of 2.5.about.4.5%; Mn in
the range of 0.3.about.1.5%; Fe in the range equal to or less than
0.15%; Ti in the range equal to or smaller than 0.20%; Zr in the
range of 0.05.about.0.30%; and the balance of Al and inevitable
impurities.
[0009] According to the present invention, a production process
comprises: preparing an aluminum alloy comprising, in terms of mass
ratio, Si in the range of 1.0.about.3.5%, Mg in the range of
2.5.about.4.5% Mn in the range of 0.3.about.1.5%, Fe in the range
equal to or less than 0.15%/, a further constituent comprising Ti
in the range equal to or less than 0.20%, and the balance of Al and
inevitable impurities; and forming the aluminum alloy into a fixed
shape by a die casting operation.
[0010] The other objects and features of this invention will become
understood from the following description with reference to the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 is a perspective view showing a cast product
according to the present invention, used for evaluation of cracking
in a first test.
[0012] FIG. 2 is a perspective view illustrating the positions of
cracks formed in a casting as shown in FIG. 1.
[0013] FIG. 3 is a graph for illustrating the influence of Si
content on resistance to cast cracking.
[0014] FIG. 4 is a plan view showing the shape of die cast material
formed by casting in a second test according to the present
invention.
[0015] FIG. 5 is a plan view showing the shape of a test piece used
in static tensile test of the die cast material shown in FIG.
4.
[0016] FIG. 6 is a plan view showing the shape of a test piece used
in dynamic tensile test of the die cast material shown in FIG.
4.
[0017] FIG. 7 is a schematic view showing a one-bar method high
speed tensile test machine used in the dynamic tensile test.
[0018] FIG. 8 is a graph showing a typical stress-strain curve in
the dynamic tensile test.
[0019] FIG. 9 is a graph showing the influence on the strength of
each alloy, of the Mg/Si ratio in Test 2.
[0020] FIG. 10 is a graph showing the influence on the elongation
of each alloy, of the Mg/Si ratio in Test 2.
[0021] FIG. 11 is a graph showing the influence on the strength of
each alloy, of the Mg/Si ratio in Test 3.
[0022] FIG. 12 is a graph showing the influence on the strength of
each alloy, of the Mg/Si ratio in Test 3.
[0023] FIG. 13 is a graph showing variation in hardness of the
alloys of practical example 13 and practical example 16 according
to the present invention while being held constantly at 250.degree.
C.
DETAILED DESCRIPTION OF THE INVENTION
[0024] The following is explanation on conditions for alloy
compositions and heat treatment according to the present
invention.
[0025] (1) Si: 1.0.about.3.5%
[0026] Silicon is an element effective in improving resistance to
cracking at the time of die casting. However, the effect is
insufficient when the Si content is lower than 1.0%. When, on the
other hand, the silicon content is beyond 3.5%, the elongation
becomes lower in the high strain rate region because of
crystallization of intermetallic compounds of Mg--Si system in
great quantities. Therefore, the Si content is in the range equal
to or more than 1.0%, and equal to or less than 3.50%. The Si
content range of 2.5.about.3.5% is preferable when the emphasis is
on the improvement in resistance to cracking. When the emphasis is
to be laid on the improvement in elongation in the high strain rate
region, the Si content range of 1.0.about.2.0% is preferable.
[0027] (2) Mg: 2.5.about.4.5%
[0028] Magnesium is an element effective for improving the strength
by being dissolved in a matrix of aluminum alloy and by forming
Mg2Si with coexisting Si. The improvement in the strength is
insufficient when the Mg content is less than 2.5%. When the Mg
content is more than 4.5%, the tendency to stress corrosion
cracking increases. Therefore, the Mg content in the range equal to
or more than 2.5%, and equal to or less than 4.5% is effective in
improving the strength and at the same time restraining the stress
corrosion cracking.
[0029] (3) Mn: 0.3.about.1.5%
[0030] Manganese is an element effective for improving the high
temperature strength by forming crystals of Al6Mn during
solidification, restraining deformation of a die cast product at
the time of removal of the product after die casting, and for
restraining the occurrence of soldering (or sticking) to the die
casting dies. The effects for the high temperature strength and
prevention of soldering to dies are insufficient when the Mn
content is less than 0.3%. When the Mn content is more than 1.5%,
the elongation especially in the high strain rate region becomes
smaller because of crystallization of coarse-grained intermetallic
compound of the AL-Mn system. Therefore, the Mn content in the
range equal to or more than 1.0%, and equal to or less than 1.5% is
effective. The Mn content range of 1.0.about.1.5% is preferable for
ensuring these effects of the addition of Mn.
[0031] (4) Fe: 0.15% or less
[0032] Iron is an element effective in preventing soldering (or
sticking) to metal die during die casting. However, an increase in
the Fe content increases the amount of crystallization of needle
intermetallic compound of Fe system, and thereby decreases the
elongation and toughness. Therefore, the Fe content range is equal
to or less than 0.15%.
[0033] (5) Ti: 0.20% or less; or Ti: 0.05.about.0.20% and B:
0.001.about.0.10%
[0034] Titanium, or titanium and boron is additive effective in
reducing a grain size of primary .alpha. (Al) crystal phase and
thereby improving mechanical properties of die castings. The effect
is insufficient when the Ti content is less than 0.1% in the case
of Ti alone, and when the Ti content is less than 0.05% and the B
content is less than 0.001% in the case of combined addition of Ti
and B. When the Ti content is more than 0.20% or the B content is
more than 0.10%, TiAl3 particles or TiB2 particles become coarse,
and the grain size refinement effect is reduced. Therefore, to
ensure these effects, it is desirable to set the Ti content in the
range equal to or more than 0.10%, and less than or equal to 0.20%,
or to set the Ti content in the range more than or equal to 0.05%,
and less than or equal to 0.20%, and the B content in the range
more than or equal to 0.001% and less than or equal to 0.10%.
[0035] (6) Zr: 0.05.about.0.30%
[0036] Zirconium is also effective in grain size refinement of the
primary .alpha. (Al) crystal phase and thereby improving mechanical
properties of die castings. Accordingly, it is possible to add Zr
according to the need. The effect is insufficient when the Zr
content is less than 0.05%. When the Zr content is more than 0.30%,
the tendency to coarse Al--Zr intermetallic compound increases and
the elongation (especially in the high strain rate region) becomes
lower. Therefore, if Zr is added, the Zr content is set in the
range of 0.05%.about.0.30% .
[0037] (7) Mg/Si ratio: 2.0 or less
[0038] The mechanical properties and age hardening are influenced
by the Mg/Si ratio which is the ratio of the Mg content to the Si
content (Mg content/Si content). When the Mg/Si ratio is smaller
than the ratio corresponding to the composition of Mg2Si (that is,
the Si content is great as compared to the Mg content), Si
particles tend to crystallize at the time of solidification, and
the strength is increased. Therefore, the ratio of the Mg content
to the Si content is set equal to or less than 2.0 to achieve a
high strength in the as-cast state.
[0039] (8) Mg/Si ratio: 1.8 or more
[0040] When the Mg/Si ratio is greater than the ratio corresponding
to the composition of Mg2Si (that is, the Si content is small as
compared to the Mg content), an excess amount of Mg acts to
decrease the solubility of Mg2Si. Therefore, the strength decreases
whereas the ductility improves. In order to achieve high ductility
in the as-cast state, the Mg/Si ratio is set equal to less than
1.8.
[0041] Artificial aging or stabilizing operation: 130.degree.
C..about.300.degree. C.
[0042] When the Mg/Si ratio is greater than the balanced ratio of
the composition (that is, the Si content is relatively low as
compared to the Mg content), the solid solubility of Mg2Si is low
and hence the amount of excess Mg2Si over saturation is high, so
that the alloy exhibits the age hardening effect by heat input
after die casting.
[0043] Accordingly, when higher strength is demanded, it is
possible to obtain a desired characteristic by heat treatment in
the temperature range equal to higher than 130.degree. C., and
equal to or lower than 300.degree. C. for a predetermined time
duration. This age hardening effect is achieved by the application
of heat to a die cast product. Therefore, it is possible to attain
the same effect by annealing operation to remove internal stresses
after die casting or by baking in coating process, instead of the
artificial age hardening operation or the stabilizing
operation.
[0044] Stabilizing operation: 150.degree. C..about.400.degree. C.
When the Mg/Si ratio is smaller than the balanced ratio (that is,
the Si content is relatively high with respect to the Mg content),
the solid solubility of Mg2Si is hardly decreased, and hence the
age hardening effect hardly appears. By the stabilizing operation,
however, it is possible to control the configuration of crystals
and thereby to improve the ductility. When a high ductility is
desired, it is possible to achieve a desired characteristic by
performing the stabilizing operation in the temperature range equal
to or higher than 150.degree. C., and equal to or lower than
400.degree. C.
[0045] The die casting aluminum alloy according to the present
invention having the above-mentioned composition can provide
sufficient strength due to solid solution strengthening by the
addition of Mg and Mn and precipitation by the addition of Mg and
Si. Besides, sufficient ductility is achieved by the limitation of
Fe detrimental to elongation within predetermined limits is
effective in ensuring elongation, and the addition of Mn to
minimize the undesired influence of Fe. Therefore, this die casting
aluminum alloy enables stable production of aluminum die cast
products having high strength and elongation even in high strain
rate condition, without the need for solution heat treatment after
casting. Accordingly, this die casting aluminum alloy is adequate
for automobile body parts such as A pillar, B pillar, C pillar,
roof, space frame joint, suspension fitting part, suspension arm,
sub-frame, and suspension link.
[0046] In producing aluminum die cast products by using this die
casting aluminum alloy, a high vacuum die casting process is a
preferable process. The high vacuum die casting process is
advantageous in reducing involvement of gases in casting, so that
it is possible to make full use of superior mechanical properties,
specifically elongation and toughness.
[0047] Moreover, it is optional to employ artificial aging process
or stabilizing treatment after the die casting. It is possible to
replace the artificial aging or stabilizing treatment by either or
both of annealing operation and coating layer baking operation.
[0048] The aluminum die casting alloy according to the present
invention can provide superior static and dynamic strength and
elongation without the need for heat treatment. However, in a case
where further improvement in strength or balance between strength
and elongation is desired, the application of artificial aging
process is effective in adjusting desired mechanical
properties.
[0049] An aluminum alloy according to the present invention
comprises, as alloying elements, 1.0.about.3.5% Si; 2.5.about.4.5%
Mg; 0.3.about.1.5% Mn; 0.15% or less Fe; and 0.20% or less Ti.
Therefore, the aluminum alloy can provide sufficient strength by
the solid solution strengthening by the addition of Mg,
precipitation hardening and natural ageing. At the same time the
aluminum alloy can ensure superior elongation by reduction of Fe
detrimental to elongation, to the minimum level. Moreover, the
addition of Ti further improves the strength and elongation by the
grain size refinement of primary a (Al) crystals.
[0050] Titanium is an ingredient for further improving mechanical
properties of die castings, such as strength and elongation, by
grain-refinement of the primary .alpha. (Al) phase of the alloy.
This ingredient may contain Ti only in the range of
0.10.about.0.20% (mass ratio); or may contain Ti in the range of
0.05.about.0.20% (mass ratio) and B in the range of
0.001.about.0.10% (mass ratio); or may contain Ti in the range of
Ti in the range of 0.20% or less (mass ratio) and Zr in the range
of 0.05.about.0.30% (mass ratio). Therefore, this ingredient
further improves mechanical properties securely by the grain size
refinement.
[0051] By the limitation of the Mn content to the range of
1.0.about.1.5% (mass ratio), it is possible to ensure the function
of Mn in the alloy, and thereby to further improve the strength at
high temperatures and elongation especially in the high strain rate
region. The Si content may be limited to the range of 2.5.about.3.5
(mass ratio). This range is effective in ensuring the function of
Si, and improving the resistance to cast cracking. The further
limited Si content range of 1.0.about.2.0% (mass ratio) is
effective in improving the elongation in the high strain rate
region.
[0052] The Mg/Si ratio range equal to or less than 2.0 is effective
in promoting crystallization of Si particles at the time of
solidification and thereby to provide sufficient as-cast
strength.
[0053] The Mg/Si ratio range equal to or more than 1.8 is effective
in obtaining high as-cast ductility though the strength is
decreased slightly.
[0054] Practical Examples
[0055] Practical examples according to the present invention are as
follows:
[0056] TEST 1
[0057] Molten aluminum alloy samples of practical examples were
prepared at 750.degree. . The compositions are listed in TABLE 1.
Thereafter, bubbling operation was performed with argon gas for
removal of inclusions and degassing. In Table 1, samples are alloy
samples of practical examples 1.about.5, and comparatives are alloy
examples of comparative examples 1.about.4.
1 TABLE 1 CHEMICAL COMPOSITION (MASS %) ALLOY Si Mg Mn Fe Ti Al
Mg/Si SAMPLE 1 1.1 4.0 0.9 0.13 0.04 BAL. 3.63 2 1.5 4.1 1.1 0.12
0.05 BAL. 2.73 3 2.0 4.0 1.1 0.13 0.04 BAL. 2.00 4 2.5 4.0 1.0 0.12
0.05 BAL. 1.60 5 3.5 3.9 1.1 0.14 0.05 BAL. 1.11 COMPARA- 1 0.4 4.2
1.1 0.13 0.06 BAL. 10.5 TIVE 2 0.8 4.1 1.1 0.12 0.05 BAL. 5.13 3
5.0 4.1 1.2 0.11 0.06 BAL. 0.82 4 9.5 4.2 1.0 0.15 0.04 BAL.
0.44
[0058] Then, the molten aluminum alloy samples were self-cooled to
720.degree. C., respectively, and thereafter cast into a form as
shown in FIG. 1 by gravity casting with an iron mold for evaluating
the characteristic of cast cracking.
[0059] After the gravity casting, each sample was cooled in the
mold until the temperature of the obtained casting was decreased to
about 100.degree. C. or lower. Then, the cast cracking
characteristic was evaluated by counting the numbers of cracks
formed as schematically shown in FIG. 2. FIG. 3 shows the results
arranged in the form of the number of cracks with respect to the Si
content. The results of FIG. 3 reveal that by increasing the Si
content, it is possible to restrain the number of cracks, and to
improve the tendency to cracking. When the Si content is increased
to 1.1%, the number of cracks is decreased to 1/2 of the number of
cracks at an Si content of 0.4%. When the Si content is 2.5% or
more, the number of cracks is decreased to zero. In FIG. 3, samples
1.about.5 stand for practical examples 1.about.5 according to the
present invention, respectively, and comparatives 1.about.4 stand
for comparative examples 1.about.4.
[0060] TEST 2
[0061] Molten aluminum alloy samples were prepared at 750.degree. .
The compositions are listed in TABLE 2. Samples 1, 2 and 4.about.10
stand for practical examples 1, 2 and 4.about.10 according to the
present invention, and comparatives stand for comparative examples
1, 2, 3, 5 and 6. Thereafter, bubbling operation was performed with
argon gas for removal of inclusions and degassing.
2 TABLE 2 CHEMICAL COMPOSITION (MASS %) ALLOY Si Mg Mn Fe Ti B Zr
Al Mg/Si SAM- 1 1.1 4.0 0.9 0.13 0.04 -- -- BAL. 3.63 PLE 2 1.5 4.1
1.1 0.12 0.05 -- -- BAL. 2.73 4 2.5 4.0 1.0 0.12 0.05 -- -- BAL.
1.60 5 3.5 3.9 1.1 0.14 0.05 -- -- BAL. 1.11 6 2.5 4.0 1.1 0.11
0.15 -- -- BAL. 1.60 7 2.7 4.2 1.0 0.10 0.11 0.03 -- BAL. 1.56 8
2.7 4.0 1.1 0.01 0.03 -- 0.20 BAL. 1.48 9 1.5 4.0 0.6 0.12 0.05 --
-- BAL. 2.67 10 1.4 3.9 0.3 0.13 0.06 -- -- BAL. 2.79 COM- 1 0.4
4.2 1.1 0.13 0.06 -- -- BAL. 10.5 PARA- 2 0.8 4.1 1.1 0.12 0.05 --
-- BAL. 5.13 TIVE 3 5.0 4.1 1.2 0.11 0.06 -- -- BAL. 0.82 5 2.0 5.2
0.6 0.12 0.15 -- -- BAL. 2.60 6 1.5 3.9 0.1 0.13 0.04 -- -- BAL.
2.60
[0062] then, each aluminum alloy was die-cast by using a high
vacuum die casting machine having a clamping force of 320 tons
after the application of powder parting agent to the die under the
conditions of a casting pressure of 60 MPa, a high injecting speed
of 3.5 m, a degree of vacuum in sleeve of 0.96 atmosphere, and a
degree of vacuum at vacuum valve of 0.95 atmosphere. The
temperature of molten alloy at the time of casting was 680.degree.
C. This test employed a die having a cavity shaped like a flat
plate of 50 mm.times.130 mm.times.2 mm as shown in FIG. 4.
[0063] From the thus-produced plate-shaped die casting, a test
piece of JIS 13B as shown in FIG. 5 was cut out and subjected to
static tensile test. The static tensile test was carried out with
Instron universal testing machine at a strain rate of 0.001/s.
[0064] From the above-mentioned plate-shaped die casting, a test
piece shown in FIG. 6 was cut out and subjected to dynamic tensile
test. The dynamic tensile test was carried out with one-bar method
high speed tensile testing machine as shown in FIG. 7 at a strain
rate of 1000/s. TABLE 3 shows the results of the static tensile
test and the dynamic tensile test. In both of the static and
dynamic tensile tests, the number of repetitions was five, and each
result was an average of five values obtained by the five
repetitions. In the dynamic tensile test, the stress varies as
shown in a stress-strain diagram of FIG. 8. Accordingly, the
strength of each die casting was evaluated in terms of a value of
the post-yielding peak stress (max stress after yield) as indicated
in FIG. 8.
3 TABLE 3 DYNAMIC TENSILE TEST STATIC TENSILE TEST MAX TENSILE 0.2%
YIELD ELONGATION STRESS AFTER ELONGATION ALLOY STRENGTH (MPa)
STRENGTH (MPA) (%) YIELD (MPa) (%) SAMPLE 1 245 124 24.6 277 26.8 2
244 126 24.3 279 25.9 4 249 134 23.8 284 23.9 5 255 144 21.2 302
20.9 6 251 140 24.1 291 24.0 7 255 143 24.2 305 23.9 8 250 139 23.8
298 23.4 9 242 125 24.7 283 26.1 10 242 122 25.1 280 26.4
COMPARATIVE 1 246 121 25.2 272 27.3 2 246 122 24.6 275 27.0 3 253
145 16.8 271 15.5 5 261 158 20.5 312 19.5 6 240 122 25.5 273
26.7
[0065] As shown by the results in TABLE 3, the aluminum die casting
material according to the practical examples of the present
invention can provide superior static and dynamic mechanical
properties. Presumably, this effect is attributable to the
following factors. The strength of solid solution is increased by
addition of Mg and Mn. The addition of Mg and Si improves the
strength by precipitation. The elongation is improved by minimizing
adverse influence of Fe by reduction of Fe content and addition of
Mn. Moreover, it is confirmed that the additive of Ti, Ti+B or Zr
improves the mechanical properties in both of the static and
dynamic conditions. The results of TABLE 3 show that in the range
of Si addition quantity beyond 3.5%, there is a tendency for
elongation to decrease in the dynamic tensile test. No or little
influence is exerted on the strength by increase or decrease of the
Mn additive quantity, but the elongation tends to improve as the Mn
additive quantity decreases. However, in the samples in which the
Mn content is lower than 0.3%, there is a tendency to soldering, or
sticking or burning to die during die casting.
[0066] FIGS. 9 and 10 show the results of the tensile test shown in
FIG. 3 in the form of relationship with respect to an Mg/Si ratio
between the Mg content and the Si content. As shown in graphs of
FIGS. 9 and 10, there is a tendency to greater elongation and
smaller strength in a range of the Mg/Si ratio greater than a
boundary defined by an Mg/Si ratio of about 2.0. In a range smaller
than the boundary of the Mg/Si ratio of about 2.0, there is a
tendency to smaller elongation and greater strength. In this
example, the Si content was varied largely so that it is not
adequate to conclude that the Mg/Si ratio is causative. Therefore,
further test was carried out in detail.
[0067] TEST 3
[0068] Molten aluminum alloy samples were prepared at 750.degree.
as in the preceding practical examples. TABLE 4 shows the
compositions of samples in practical examples 11.about.16 according
to the present invention. Thereafter, bubbling operation was
performed with argon gas for removal of inclusions and
degassing.
4 TABLE 4 CHEMICAL COMPOSITION (MASS %) ALLOY Si Mg Mn Fe Ti B Zr
Al Mg/Si SAM- 11 1.6 2.88 1.25 0.04 0.05 -- -- BAL. 1.80 PLE 12 1.5
2.8 1.27 0.13 0.04 -- -- BAL. 1.87 13 2.0 3.5 1.29 0.04 0.05 -- --
BAL. 1.75 14 1.6 3.3 1.21 0.18 0.05 -- -- BAL. 2.06 15 1.6 3.5 1.30
0.18 0.04 -- -- BAL. 2.19 16 1.5 4.4 1.28 0.14 0.05 -- -- BAL.
2.27
[0069] Then, each aluminum alloy was die-cast by using a high
vacuum die casting machine having a clamping force of 320 tons
after the application of powder parting (or releasing) agent to the
die, under the conditions of a casting pressure of 60 MPa, a high
injecting speed of 3.5 m, a degree of vacuum in sleeve of 0.96
atmosphere, and a degree of vacuum at vacuum valve of 0.95
atmosphere. The temperature of molten alloy at the time of casting
was 860.degree. C. This test employed a die having a cavity shaped
like a flat plate of 50 mm.times.130 mm.times.2 mm as shown in FIG.
4. From the thus-produced plate-shaped die casting, a test piece of
JIS 13B as shown in FIG. 5 was cut out and subjected to static
tensile test. The static tensile test was carried out with Instron
universal testing machine at a strain rate of 0.001/s.
[0070] As shown in FIG. 11, the strength (0.2% offset yield
strength) of the aluminum alloy according to the present invention
exhibits a tendency to increase in the range of Mg/Si ratio lower
than or equal to 2.0 without regard to the amount of the Mg
content. In this range, the Si content is relatively high with
respect to the Mg content, so that the strength is increased by
crystallization of Si grains during solidification. On the other
hand, as shown in FIG. 12, the elongation of the aluminum alloy
according to the present invention exhibits a tendency to decrease
sharply in the range of Mg/Si ratio lower than 1.8 without regard
to the amount of Mg content. In this range, the Si content is
relatively low with respect to the Mg content, so that the
solid-solubility of Mg2Si decreases because of the existence of
excess Mg, and the strength decreases whereas the elongation
improves. Therefore, in order to achieve high strength in the
as-cast state, it is desirable to adjust the Mg content and the Si
content so as to make the Mg/Si ratio lower than or equal to 2.0.
In order to achieve high ductility in the as-cast state, it is
desirable to adjust the Mg content and the Si content so as to make
the Mg/Si ratio greater than or equal to 1.8.
[0071] FIG. 13 shows the results of measurement of the hardness of
the alloy of the practical example 13 having a relatively small
Mg/Si ratio and the alloy of the practical example 16 having a
relatively great Mg/Si ratio while held at constant temperature of
250.degree. C. As shown in FIG. 13, the hardness decreases
monotonically with time by the thermal load in the case of the
material of the practical example 13 having the smaller Mg/Si
ratio. However, the material of the practical example 16 having the
greater Mg/Si ratio exhibits such behavior of age hardening that
the hardness increases first by thermal load and then decreases. In
the case of the alloy having the greater Mg/Si ratio, the solid
solubility of Mg2Si is decreased by the existence of excess Mg, and
Mg2Si tends to precipitate from a solid solution supersaturated
with Mg2Si, by heat input. Thus, in the alloy according to the
present invention, it is possible to control the balance between
strength and elongation by controlling the Mg/Si ratio and the
input of heat after die casting.
[0072] This application is based on Japanese Patent Application No.
2000-325756 filed in Japan on Oct. 25, 2000, and Japanese Patent
Application No.2001-238947 filed in Japan on Aug. 7, 2001. The
entire contents of these Japanese Patent Applications are hereby
incorporated by reference.
[0073] Although the invention has been described above by reference
to certain embodiments of the invention, the invention is not
limited to the embodiments described above. Modifications and
variations of the embodiments described above will occur to those
skilled in the art in light of the above teachings. The scope of
the invention is defined with reference to the following
claims.
* * * * *