U.S. patent application number 10/568775 was filed with the patent office on 2006-10-05 for heat resistant magnesium die casting alloys.
This patent application is currently assigned to TOYOTA JIDOSHA KABUSHIKI KAISHA. Invention is credited to Takumi Hijii, Tomoyasu Kitano, Harutoshi Matsuyama, Yusuke Nakaura, Koichi Ohori.
Application Number | 20060222556 10/568775 |
Document ID | / |
Family ID | 34380325 |
Filed Date | 2006-10-05 |
United States Patent
Application |
20060222556 |
Kind Code |
A1 |
Hijii; Takumi ; et
al. |
October 5, 2006 |
Heat resistant magnesium die casting alloys
Abstract
A heat resistant magnesium die casting alloy simultaneously
improved in heat resistance and castability and expanded in range
of application is provided. It comprises, includes, by wt %, the
following composition: Al: over 6% to not more than 10 %, Ca: 1.8
to 5%, Sr: 0.05 to 1.0%, Mn: 0.1 to 0.6%, and Bal: Mg and
unavoidable impurities, the ratio Ca/Al of the Ca content to the Al
content being 0.3 to 0.5. To improve the corrosion resistance, 0.1
to 3% of a RE may also be added.
Inventors: |
Hijii; Takumi; (Aichi,
JP) ; Kitano; Tomoyasu; (Aichi, JP) ; Ohori;
Koichi; (Shizuoka, JP) ; Nakaura; Yusuke;
(Shizuoka, JP) ; Matsuyama; Harutoshi; (Shizuoka,
JP) |
Correspondence
Address: |
OLIFF & BERRIDGE, PLC
P.O. BOX 19928
ALEXANDRIA
VA
22320
US
|
Assignee: |
TOYOTA JIDOSHA KABUSHIKI
KAISHA
1, Toyota-cho Toyota-shi
Aichi
JP
471-8571
|
Family ID: |
34380325 |
Appl. No.: |
10/568775 |
Filed: |
September 16, 2004 |
PCT Filed: |
September 16, 2004 |
PCT NO: |
PCT/JP04/13974 |
371 Date: |
February 21, 2006 |
Current U.S.
Class: |
420/405 ;
420/410 |
Current CPC
Class: |
C22C 23/02 20130101 |
Class at
Publication: |
420/405 ;
420/410 |
International
Class: |
C22C 23/02 20060101
C22C023/02 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 18, 2003 |
JP |
2003-326563 |
May 20, 2004 |
JP |
2004-150393 |
Claims
1. A heat resistant magnesium die casting alloy comprising, by wt
%, the following composition: Al: over 6% to not more than 10%, Ca:
1.8 to 5%, Sr: 0.05 to 1.0%, Mn: 0.1 to 0.6%, and Bal: Mg and
unavoidable impurities, the ratio Ca/Al of the Ca content to the Al
content being 0.3 to 0.5.
2. A heat resistant magnesium die casting alloy as set forth in
claim 1, wherein Ca: over 2% to not more than 5%.
3. A heat resistant magnesium die casting alloy as set forth in
claim 1, wherein Ca: 2.5 to 3.5%.
4. A heat resistant magnesium die casting alloy as set forth in
claim 1, further comprising a rare earth metal in an amount of 0.1
to 3 wt %.
5. A die cast product comprised of a magnesium alloy as set forth
in claim 1.
6. A heat resistant magnesium die casting alloy as set forth in
claim 2, further comprising a rare earth metal in an amount of 0.1
to 3 wt %.
7. A heat resistant magnesium die casting alloy as set forth in
claim 3, further comprising a rare earth metal in an amount of 0.1
to 3 wt %.
8. A die cast product comprised of a magnesium alloy as set forth
in claim 2.
9. A die cast product comprised of a magnesium alloy as set forth
in claim 3.
10. A die cast product comprised of a magnesium alloy as set forth
in claim 4.
Description
TECHNICAL FIELD
[0001] The present invention relates to a heat resistant magnesium
die casting alloy and a die cast product of that alloy.
BACKGROUND ART
[0002] In recent years, to deal with the demand for reduction of
the weight of vehicles, greater application of alloys of magnesium,
the lightest of the practical metals, has been desired. However,
conventional die casting magnesium alloys greatly deform at high
temperatures. Not much progress has been made for parts having
bolted portions exposed to high temperature environments
(120.degree. C. or more). Up until now, various heat resistant
magnesium die casting alloys have been developed, but it has not
been possible to simultaneously improve the heat resistance (high
temperature strength and creep resistance) and castability
(hot-cracking resistance and die-sticking resistance during die
casting) and therefore the range of application has been
limited.
[0003] Therefore, to achieve both heat resistance and castability,
JP-A-2001-316752 has proposed a die casting magnesium alloy
comprised of 2 to 6 wt % Al, 0.3 to 2 wt % Ca, 0.01 to 1 wt % Sr,
0.1 to 1 wt % Mn, and the balance of Mg and unavoidable impurities.
Due to this, it becomes possible to simultaneously improve the heat
resistance and castability and expand the range of application.
[0004] Even with the magnesium alloy of the above proposal,
however, it has not been possible to sufficiently cover the range
of applications required, so development of a heat resistant
magnesium die casting alloy with further improved combination of
heat resistance and castability has been desired.
DISCLOSURE OF INVENTION
[0005] The present invention has as its object to provide a heat
resistant magnesium die casting alloy simultaneously improved in
heat resistance and castability and expanded in range of
applications and a die cast product of the same alloy.
[0006] To achieve the above object, according to the present
invention, there is provided a heat resistant magnesium die casting
alloy comprising, by wt %, the following composition:
[0007] Al: over 6% to not more than 10%,
[0008] Ca: 1.8 to 5%,
[0009] Sr: 0.05 to 1.0%,
[0010] Mn: 0.1 to 0.6%, and
[0011] Bal: Mg and unavoidable impurities,
[0012] the ratio Ca/Al of the Ca content to the Al content being
0.3 to 0.5.
[0013] The present invention is characterized by limiting the ratio
Ca/Al of the contents of Al and Ca to within a predetermined range
so as to improve the combination of the heat resistance and
castability over the conventional limits without causing
deterioration of characteristics even if adding Al and Ca to high
contents considered unsuitable in the past.
[0014] For example, JP-A-2001-316752 sets the upper limit of the Al
content to 6 wt % and the upper limit of the Ca content to 2 wt %.
The reason for the limitations is explained as being that if the Al
content is over 6 wt %, the creep resistance rapidly deteriorates,
while if the Ca content exceeds 2 wt %, casting cracks easily occur
(see paragraph 0010 to 0012 of the publication).
[0015] As opposed to this, the inventors newly discovered that by
limiting the ratio Ca/Al of the Ca content to the Al content to the
range of 0.3 to 0.5, even if adding Al and Ca exceeding the upper
limits of the above publication, it is possible to simultaneously
achieve an improvement of the high temperature strength and
castability, which are the main effects of high Al, and an
improvement of the creep resistance, which is the main effect of
high Ca, without causing either a drop in the creep resistance due
to the higher Al or casting cracks due to the higher Ca. The
present invention was completed based on this novel discovery.
BRIEF DESCRIPTION OF DRAWINGS
[0016] FIG. 1 is a graph comparing the retained bolt loads of
various types of Mg alloys.
[0017] FIG. 2 is a graph of the relationship between the high
temperature retained bolt load and Ca/Al ratio.
[0018] FIG. 3 is a graph of the relationship between the casting
crack length and Ca/Al ratio.
[0019] FIGS. 4A and 4B are graphs of the (A) change in corrosion
weight loss and (B) change in corrosion rate with respect to the
test duration of a salt water spray test for Mg alloys with various
RE contents.
[0020] FIG. 5 is a graph of the change in the corrosion rate with
respect to the RE content for specific test durations (numbers of
days).
[0021] FIGS. 6A and 6B are graphs of the (A) 0.2% proof stress and
tensile strength and the (B) elongation in the temperature range of
room temperature to 250.degree. C.
[0022] FIG. 7 is a graph comparing the high temperature retained
bolt loads of a 0.44% RE material and no-addition material among
the alloys of the present invention and comparing them with the
conventional use alloy AZ91D.
BEST MODE FOR CARRYING OUT THE INVENTION
[0023] The composition of the heat resistant magnesium die casting
alloy of the present invention is limited due to the following
reasons. Note that in this description, unless otherwise specified,
the "%" in the indications of the content of the components mean
"wt %".
[0024] [Al: Over 6% to not More than 10%]
[0025] Al raises the strength at room temperature and high
temperature by dispersion strengthening (in particular grain
boundary strengthening) by forming Al--Ca-based, Al--Sr-based, and
Mg--Al-based intermetallic compounds. Further, it lowers the
melting point (liquidus line) of the alloy to raise the fluidity of
the melt and improve the castability. In the present invention, by
including Al over 6% under a predetermined range of Ca/Al ratio, it
is possible to increase the room temperature and high temperature
strength over the conventional limit and secure a good castability.
However, even if limiting the Ca/Al ratio to within the
predetermined range of the present invention, if Al is present in
excess, the creep resistance (high temperature retained bolt load)
drops, so the upper limit of the Al content is made 10%.
[0026] [Ca: 1.8 to 5%]
[0027] Ca improves the proof strength at room temperature and high
temperature by grain boundary strengthening by Al--Ca-based
intermetallic compounds and simultaneously particularly raises the
creep resistance (high temperature retained bolt load). In the
present invention, by making the Ca content 1.8% to 5% under a
predetermined range of the Ca/Al ratio, it is possible to improve
the proof strength and creep resistance over the conventional
limits in the copresence with Al. However, even if limiting the
Ca/Al ratio to a predetermined range of the present invention, if
Ca is presence in excess, hot-cracking and die-sticking easily
occur during die casting, so the upper limit of the Ca content is
made 5%. The Ca content is preferably over 2% and not more than 5%,
more preferably 2.5 to 3.5%.
[0028] [Ratio Ca/Al of Ca Content to Al Content: 0.3 to 0.5]
[0029] In the present invention, by limiting the Ca/Al ratio to
this range, it becomes possible to increase the Al content and Ca
content over the conventional limits without causing a drop in the
creep resistance due to the higher Al or a deterioration of the
castability due to the higher Ca and therefore possible to further
raise the high temperature strength and creep resistance over the
past and secure a good castability. To stably secure a high creep
resistance, it is necessary to make the Ca/Al ratio at least 0.3.
To stably suppress the occurrence of hot-cracking during die
casting, it is necessary to make the Ca/Al ratio not more than
0.5.
[0030] [Sr: 0.05 to 1.0%]
[0031] Sr is added to further improve the effect of prevention of
casting cracks and securing creep resistance. To obtain this
effect, it is necessary to add Sr to at least 0.05%. The effect
becomes greater with increasing the amount of addition. However,
even if added over 1.0%, the effect does not increase not much at
all.
[0032] [Mn: 0.1 to 0.6%]
[0033] Mn is added to secure a good corrosion resistance. To obtain
this effect, it is necessary to make the Mn content at least 0.1%.
However, if Mn is present in excess, free Mn precipitates and
embrittlement occurs, so the upper limit of the Mn content is made
0.6%.
[0034] The magnesium alloy of the present invention is remarkably
improved in corrosion resistance by further adding a rare earth
metal (RE) to the above composition in the range of 0.1 to 3%. To
realize this effect, it is necessary to make the RE content at
least 0.1%. However, if the RE content exceeds 3%, the castability
rapidly deteriorates, casting cracks and misruns end up occurring,
and a sound casting is not obtained, so the upper limit of the RE
content is made 3%.
[0035] The heat resistant magnesium alloy of the present invention
is particularly limited to one for die casting. By die casting, a
fine network comprised of Al--Ca-based or Al--Sr-based
intermetallic compounds is formed and a good heat resistance can be
secured.
[0036] The basic process for obtaining a product by applying the
alloy of the present invention to die casting is as follows:
[0037] Alloy metal.fwdarw.charging into crucible
(*1).fwdarw.melting.fwdarw.temperature adjustment.fwdarw.die
casting (*2).fwdarw.removal of product
[0038] *1) The crucible used is made of iron.
[0039] *2) The die casting is by a cold chamber, hot chamber,
etc.
[0040] The die casting heat resistance magnesium alloy of the
present invention is particularly advantageous when applied to
parts requiring heat resistance such as parts of automobile
engines, in particular, oil pans, headlight covers, etc. and also
transmission cases.
EXAMPLES
Example 1
[0041] The following experiment was performed to confirm the effect
of improvement of the castability and heat resistance by alloy
compositions of the present invention.
[0042] Mg alloys of the compositions of Table 1 were die cast under
the following conditions using a 135 ton cold chamber die casting
machine.
[0043] <Die Casting Conditions>
[0044] Shape and dimensions of die: 70 w.times.150 L (3, 2, and 1 t
from gate side) . . . flat plate [0045] 15.phi..times.120 L . . .
rod
[0046] Die preheating: 200.degree. C.
[0047] Casting temperature: 700 to 720.degree. C.
[0048] Casting atmosphere: 1% SF.sub.6+CO.sub.2
[0049] The obtained alloy samples were subjected to tensile tests
(test temperature: room temperature (RT), 150.degree. C.) and
measured for crack length at casting and bolt load retention. As
the bolt load retention, the retained bolt load was measured under
the following conditions. The measurement results are shown all
together in Table 2 and Table 3.
[0050] <Measurement Conditions of High Temperature Retained Bolt
Load>
[0051] Initial bolt load: 8 kN
[0052] Holding temperature: 150.degree. C.
[0053] Holding time: 300 h
[0054] Retained rate: bolt load before and after holding at a high
temperature measured at room temperature and calculated as retained
bolt load
[0055] Further, FIG. 1 is a graph showing the high temperature
retained bolt loads of different alloy samples, FIG. 2 the
relationship between the high temperature retained bolt load and
Ca/Al ratio, and FIG. 3 the relationship between the casting crack
length and Ca/Al ratio.
[0056] In particular, from the results of FIG. 2, it is clear that
the retained bolt load increases with increasing the Ca/Al ratio
and that to secure the practically required retained bolt load of
at least 70%, it is necessary that Ca/Al ratio.gtoreq.0.3.
[0057] From the results of FIG. 3, it is clear that the casting
crack length increases along with an increase in the Ca/Al ratio
and that to secure the actually required crack length of not more
than 600 mm, it is necessary that Ca/Al ratio.ltoreq.0.5.
[0058] From the above results, it is clear that only when the
contents of the components are in the range of the present
invention and the Ca/Al ratio is in the range of the present
invention can the strength (room temperature and high temperature)
and creep resistance (high temperature retained bolt load) be
improved while stably suppressing casting cracking. TABLE-US-00001
TABLE 1 Analysis values (wt %) No. Name Al Ca Sr Mn Ca/Al 1 M310101
3.03 1.01 0.11 0.11 0.33 2 M310203 2.95 0.96 0.22 0.31 0.33 3
M310506 3.16 1.02 0.51 0.62 0.32 4 M320103 3.10 2.04 0.13 0.30 0.66
5 M320206 3.24 2.06 0.23 0.64 0.64 6 M320501 3.09 1.99 0.50 0.11
0.64 7 M330106 3.30 2.87 0.12 0.64 0.87 8 M330201 3.10 3.09 0.22
0.12 1.00 9 M330503 3.18 3.13 0.54 0.31 0.98 10 M510206 5.19 1.04
0.11 0.31 0.20 11 M510501 5.31 1.04 0.25 0.64 0.20 12 M510501 5.13
1.02 0.52 0.11 0.20 13 M520106 5.34 2.06 0.11 0.62 0.39 14 M520201
4.99 2.05 0.22 0.10 0.41 15 M520503 5.12 2.09 0.54 0.33 0.41 16
M530101 5.26 3.22 0.12 0.13 0.61 17 M530203 5.00 3.03 0.22 0.32
0.61 18 M530506 5.32 3.11 0.54 0.63 0.58 19 M710106 7.28 1.06 0.12
0.58 0.15 20 M710201 7.16 1.10 0.23 0.13 0.15 21 M710503 7.08 1.09
0.51 0.33 0.15 22 M720101 7.22 1.98 0.12 0.12 0.27 23 M720203 6.99
2.06 0.23 0.29 0.29 24 M720506 7.33 2.10 0.54 0.57 0.29 25 M730103
6.98 3.08 0.12 0.29 0.44 26 M730206 7.32 3.08 0.22 0.58 0.42 27
M730501 7.19 3.13 0.52 0.11 0.44
[0059] TABLE-US-00002 TABLE 2 Proof strength Tensile strength (MPa)
(MPa) No. RT 150 RT 150 1 133 118 195 144 2 119 115 196 145 3 143
127 198 169 4 165 134 186 170 5 164 137 204 176 6 166 133 187 161 7
166 148 203 179 8 183 145 217 177 9 193 154 200 170 10 199 129 209
162 11 146 133 234 173 12 148 127 220 169 13 155 142 227 182 14 156
135 188 172 15 165 143 207 175 16 177 149 206 195 17 172 146 218
186 18 181 154 215 198 19 160 132 244 178 20 158 133 232 179 21 160
136 234 178 22 174 145 230 189 23 166 146 229 182 24 174 148 217
190 25 176 152 234 197 26 173 156 236 203 27 177 155 231 204
[0060] TABLE-US-00003 TABLE 3 Crack Retained length rate after No.
(mm) 300 h (%) 1 2770 55.90 2 3500 61.90 3 2310 63.43 4 2614 70.36
5 1174 70.26 6 1694 79.79 7 792 74.79 8 1852 81.62 9 3098 77.59 10
514 52.73 11 386 48.39 12 544 62.13 13 512 67.71 14 558 78.26 15
346 81.70 16 744 80.69 17 1020 77.39 18 842 80.16 19 0 15.70 20 10
21.43 21 8 30.42 22 300 62.34 23 548 61.38 24 314 68.00 25 456
79.83 26 134 81.61 27 230 88.89
Example 2
[0061] The following experiment was performed to confirm the effect
of improvement of the corrosion resistance by RE addition in the
alloy composition of the present invention.
[0062] The Mg alloys of the compositions of Table 4 were die cast
in the same way as in Example 1. The alloy compositions of No. 101
to 105 shown in Table 4 were basically comprised (target values) of
7% Al-3% Ca-0.5% Sr-0.3% Mn with amounts of RE added (target
values) of successively 0% (no addition), 0.1%, 0.5%, 2.0%, and
3.0% (analysis values of added RE elements of 0.08%, 0.44%, 1.77%,
and 2.68%). For the RE addition, a Ce-rich (50%) misch metal was
used.
[0063] The obtained alloy samples were subjected to salt water
spray tests under the following conditions to evaluate the
corrosion resistance.
[0064] <Salt Spray Test Method>
[0065] 1. Cut out test piece (width 70 mm.times.length 50
mm.times.thickness 3 mm) from the die cast product in the as-cast
state.
[0066] 2. Immerse the test piece in acetone and ultrasonically
clean it for 15 minutes, then measure its weight (initial
weight).
[0067] 3. Mask the parts of the surface of the test piece finished
being measured for weight other than the as-cast surface (test
surface).
[0068] 4. Perform the salt spray test by a 5% NaCl aqueous solution
under conditions defined in JIS Z2371.
[0069] 5. After the end of the test, boil and clean the test piece
by a 15% chromic acid aqueous solution for 1 minute so as to remove
the corrosion product on the surface of the test piece.
[0070] 6. Dry, then measure the weight of the test piece and use
the difference from the initial weight as the corrosion weight
loss. Further, divide the value of the corrosion weight loss by the
test area and the number of test days and use the result as the
corrosion rate.
[0071] FIG. 4A and FIG. 4B show changes in the corrosion weight
loss and corrosion rate for different test durations (numbers of
days). Compared with the no-RE material 101, the RE-added materials
102 to 105 all had small corrosion weight losses and small
corrosion rates. At FIG. 4A showing the change along with time of
the corrosion weight loss, the curves are convex upward. In FIG. 4B
converting this to the change along with time of the corrosion
rate, the curves are convex downward. Along with the elapse of the
test duration, there is a tendency for the corrosion to proceed
slower.
[0072] FIG. 5 is a graph of the effects of the RE content on the
progression of corrosion. The corrosion rate was plotted against
the RE content for a test duration of one day and 10 days. At both
test durations, the corrosion rate clearly decreases by the
addition of 0.08% of RE as compared with no RE (0%). With
increasing the amount of addition of 0.44% and 1.77%, the corrosion
rate further decreases. However, if increasing the amount of
addition to 2.68%, the corrosion rate conversely starts to
increase, but even so the corrosion rate is far smaller than with
no addition. By adding RE in a range of 0.1% to 3% according to the
present invention, it is learned that the corrosion resistance is
remarkably improved compared with no addition.
[0073] Next, the effects of the addition of RE on the strength
properties and creep resistance properties were investigated.
[0074] As a typical composition of the RE-added material, a
0.44%-added material (103) was compared with the non-addition
material (101). FIGS. 6A and 6B show the (A) 0.2% proof strength
and tensile strength and (B) elongation at the test temperature
from room temperature to 250.degree. C. At all test temperatures,
it was learned that the 0.44% RE material (.diamond-solid. plot)
was provided with similar strength characteristics to the
non-addition material (o plot).
[0075] FIG. 7 compares the high temperature retained bolt loads of
a 0.44% RE material (103), a non-addition material (101), and an
AZ91D (typical known heat resistant Mg die casting alloy). The test
procedure was the same as that in Example 1.
[0076] First, it is learned that the alloy of the present invention
is far larger in retained bolt load compared with the conventional
use alloy AZ91D regardless of the addition of RE.
[0077] Further, in the alloys of the present invention, the 0.44%
RE material (103) fell in retained bolt load by about 10% compared
with the non-addition material (101), but sufficiently secured the
practically required at least 70%, so was provided with both the
practically sufficient heat resistance and corrosion resistance.
Simultaneously, an excellent castability was also provided and it
was possible to die cast without any problem. TABLE-US-00004 TABLE
4 Analysis values (wt %) Basic alloy components Rare earth metal
No. Name Al Ca Sr Mn Total Ce La Nd Ca/Al 101 M730503 7.08 2.86
0.50 0.31 0 0 0 0 0.40 102 M73050301 6.75 3.24 0.54 0.16 0.08 0.04
0.03 0.01 0.48 103 M73050305 6.83 2.85 0.50 0.26 0.44 0.22 0.13
0.09 0.42 104 M73050320 6.85 2.89 0.48 0.25 1.77 0.91 0.55 0.31
0.42 105 M73050330 7.13 2.93 0.50 0.34 2.68 1.33 0.78 0.57 0.41
INDUSTRIAL APPLICABILITY
[0078] According to the present invention, a heat resistant
magnesium die casting alloy simultaneously improved in heat
resistance and castability and able to be used for a wider range of
applications than the past is provided.
[0079] Further, due to the RE addition, in addition to the heat
resistance and castability, the corrosion resistance may also be
simultaneously improved.
* * * * *