Note: Descriptions are shown in the official language in which they were submitted.
CA 02564078 2006-10-23
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Heat Treatable AI-Zn-Mg Alloy for Aerospace and Automotive Castings
Cross Reference to Related Applications
[0001] This application claims the benefit of U.S. Provisional Application
Serial No.
60/564,813 filed on April 22, 2004, which is fully incorporated herein by
reference thereto. It
is also closely related to the patent application "Heat Treatable Al-Zn-Mg-Cu
Alloy for
Aerospace and Automotive Castings" filed concurrently with this application,
and which is
also incorporated herein by reference thereto.
Field of the Invention
[0002] This invention is an aluminum alloy for aerospace and automotive shaped
castings, castings comprised of the alloy, and methods of making cast
components of the alloy.
Background of the Invention
[0003] Cast aluminum parts are used in structural applications in automobile
suspensions to reduce weight. The most commonly used group of alloys, Al-Si7-
Mg, has well
established strength limits. In order to obtain lighter weight parts, higher
strength material is
needed with established material properties for design. At present, cast
materials made of
A356.0, the most commonly used Al-Si7-Mg alloy, can reliably guarantee
ultimate tensile
strength of 290 MPa (42,060 psi), and tensile yield strength of 220 MPa
(31,908 psi) with
elongations of 8% or greater.
[0004] A variety of alternate alloys exist and are registered that exhibit
higher strength
than the Al-Si7-Mg alloys. However, these exhibit problems in castability,
corrosion potential
or fluidity that are not readily overcome. The alternate alloys are therefore
less suitable for
use.
[0005] Where high strength is required, forged products are often used. These
are
usually more expensive than cast products. There exists the potential for
considerable cost
savings if cast products can be used to replace forged products with no loss
of strength,
elongation, corrosion resistance, fatigue strength, etc. This is true in both
automotive and
aerospace applications.
[0006] Casting alloys exhibiting higher tensile strength and fatigue
resistance than the
Al-Si7-Mg material are desirable. Such improvements could be used to reduce
weight in new
parts or in existing parts which can be redesigned to use the improved
material properties to
great advantage.
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Introduction to the Invention
[0007] The alloy of the present invention is an AI-Zn-Mg base alloy for low
pressure
permanent or semi-permanent mold, squeeze, high pressure die, pressure or
gravity casting,
lost foam, investment casting, V-mold, or sand mold casting with the following
composition
ranges (all in weight percent):
Zn: about 3.5-5.5%,
Mg: about 0.8-1.5%,
Si: less than about 1.0%,
Mn: less than about 0.30%,
Fe and other incidental impurities: less than about 0.30%.
[0008] Silicon up to about 1.0% may be einployed to improve castability. Lower
levels of silicon may be employed to increase strength. For some applications,
manganese up
to about 0.3% may be employed to improve castability. In other applications,
manganese is to
be avoided.
[0009] The alloy may also contain grain refiners such as titanium diboride,
TiB2 or
titanium carbide, TiC and/or anti-recrystallization agents such as zirconium
or scandium. If
titanium diboride is employed as a grain refiner, the concentration of boron
in the alloy may be
in a range from 0.0025% to 0.05%. Likewise, if titanium carbide is employed as
a grain
refiner, the concentration of carbon in the alloy may be in the range from
0.0025% to 0.05%.
Typical grain refiners are aluminum alloys containing TiC or TiB2.
[0010] Zirconium, if used to prevent grain growtli during solution heat
treatment, is
generally employed in a range below 0.2%. Scandium may also be used in a range
below
0.3%.
[0011] The purpose of the present invention is to provide a range of aluminum
alloys
having good strength, good castability for forming shaped castings, good
corrosion resistance
and good thermal shock resistance. A fine grain size is often desirable for
strength and for
appearance, particularly for components which are anodized and then coated
with a clear
finish layer.
Summary of the Invention
[0012] In one aspect, the present invention is an aluminum alloy including
from about
3.5-5.5% Zn, from about 0.8-1.5%Mg. It contains less than about 1% Si, less
than about
0.30% Mn; and less than 0.30% Fe and other incidental impurities.
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[0013] In another aspect, the present invention is a heat treatable shaped
casting of an
aluminum alloy including from about 3.5-5.5% Zn, from about 0.8-1.5% Mg, and
less than
about 1% Si, less than about 0.30% Mn, and less than 0.30% Fe and other
incidental
impurities.
[0014] In another aspect, the present invention is a method of preparing a
heat
treatable aluminum alloy shaped casting. The inethod includes preparing a
molten mass of an
aluminum alloy including from about 3.5-5.5% Zn, from about 0.8-1.5% Mg, and
less than
about 1% Si, less than about 0.30% Mn, and less than 0.30% Fe and other
incidental
impurities. The method further includes casting at least a portion of the
molten mass in a mold
configured to produce the shaped casting, permitting the molten mass to
solidify, and
removing the shaped casting from the mold.
Brief Description of the Drawinp-
[0015] The figure presents the results of ASTM G44 stress corrosion test on AI-
Zn-
Mg alloys with and without copper.
Detailed Description of Preferred Embodiments and Comparison with Prior Art
Alloys
[0016] When referring to any numerical range of values herein, such ranges are
understood to include each and every number and/or fraction between the stated
range
minimum and maximum. A range of about 3.5 to 5.5 wt% zinc, for example, would
expressly
include all intermediate values of about 3.6, 3.7, 3.8 and 3.9%, all the way
up to and including
5.3, 5.35, 5.4, 5.475 and 5.499% Zn. The same applies to each other numerical
property
and/or elemental range set forth herein.
[0017] Alloys according to the present invention were tested in comparison
with
similar AI-Zn-Mg alloys containing copper. The samples were directionally
solidified at a
cooling rate of 0.1 C/sec. The results are presented in Table 1.
Table 1
DS Casting T5 and T6 Properties
T5 T6
Alloy Tensile Yield E % Tensile Yield E%
Al-4.5Zn-1.2Mg 266 192 10 275 223.5 10
265.5 198 8 268 222.5 10
A1-4.5Zn-1.2Mg-0.4Si 238 174.5 10 309 230.5 16
238.5 173.5 10 312.5 233 16
A1-4.5Zn-1.2Mg-0.25Cu 285.5 207 10 269.5 210.5 10
287 205 12 276 210.5 14
A1-4.5Zn-1.2Mg-0.25Cu- 295 229 4 338.5 278.5 12
0.12Zr 298.5 227.5 4 329.5 266 10
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[0018] The first alloy shown in Table 1 was A1-4.5Zn-1.2Mg. Two samples were
tested, each in T5 and T6 tempers. The tensile strength and yield strength are
presented in
megapascals, and the elongation in percent is presented, for two samples of
the alloy, in both
T5 and T6 tempers. This alloy is an example of the present invention.
[0019] The second alloy shown in Table 1 also has a composition in the range
of the
present invention. It contains AI-4.5Zn-1.2Mg-0.4Si. This shows lower values
for tensile and
yield strength than the previous alloy in T5 temper. However, it has
significantly higher values
for tensile strength, yield strength and elongation in T6 temper than did the
previous alloy.
[0020] The third alloy shown in Table 1 is not within the composition range of
the
present invention. It is presented for comparison. The third alloy has higher
values for tensile
and yield strength and higher elongation values in T5 temper than the second
alloy in T5
temper, but lower values for tensile and yield strength and lower value for
elongation than the
second alloy in T6 temper.
[0021] The fourth alloy shown in Table 1 is also not within the composition
range of
the present invention. It, also, is presented for comparison. The data
presented illustrate the
effect of zirconium, probably for preventing grain growth. The results for the
T6 temper show
very high values for tensile strength, yield strength and elongation.
[0022] Mechanical properties of shaped castings of an alloy according to the
present
invention were tested in a first plant trial, and the results are presented in
Table 2.
Table 2
First Plant Trial Al-3.5Zn-0.97M
Tem er Sample # Tensile Yield E %
T5 1 219 175 14.6
2 211 169 8.6
As-cast 3 185 147 15.03
4 189 152 15.95
[0023] The composition for the first plant trial was A1-3.5Zn-0.97Mg. The
table
presents tensile strength and yield strength in megapascals, as well as
elongation in percent.
Two samples were tested in T5 temper, and two samples of the as-cast material
were tested.
It is noted that the elongation for the as-cast material had the extraordinary
values of 15.03
and 15.95%.
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[0024] Tests were also made in a second plant trial on an alloy containing
slightly
more magnesium than the alloy of Table 1. Data for the second plant trial are
presented in
Table 3.
Table 3
Second Plant Trial Al-3.5 Zn-1.1Mg
Tensile Yield E% Temper
210 161 16.7 160 C/1 hr
215 145 6.5 160 C/6 hrs
246 175 10.5 143 C/32 hrs
[0025] The data in Table 3 are for an alloy containing Al-3.5 Zn-1.1 Mg. This
is an
alloy according to the present invention. Data are presented for three
different heat
treatments. The first was 160 C for 1 hour, the second was 160 C for six
hours and the third
was 143 C for 32 hours. The tensile strength and yield strength values in
this table are
expressed in megapascals, and the elongation is expressed in percent.
[0026] Table 4 presents data for the same alloy as the samples in Table 3. The
samples
reported in Table 4 were subjected to a T6 heat treatment that consisted of
471 C for 3 hours,
and then 527 C for 10 hours followed by cold water quench. The samples were
then aged as
reported in Table 4, and the stress results in Table 4 were then obtained. The
first line in the
table is for a sample which was naturally aged only.
Table 4
Al-3.5 Zn-1.1Mg after T6 Heat Treatment
Ageing temp/time Tensile Yield E%
Natural age only 274 MPa 138 MPa 24.5
160 C / 6 hours 272 MPa 177 MPa 16.5
160 C /12 hours 287 MPa 201 MPa 16.0
160 C/18hours 309MPa 230MPa 15.0
143 C / 12 hours 270 MPa 175 MPa 17.5
143 C/32hours 288MPa 197MPa 15.5
143 C / 64 hours 311 MPa 239 MPa 11.5
[0027] Corrosion tests were also performed employing the ASTM G44 test which
is
the "Standard Practice for Exposure of Metals and Alloys by Alternate
Immersion in Neutral
3.5% Sodium Chloride Solution". In this test, stressed specimens are subjected
to a 1-hour
cycle which includes immersion in 3.5% NaCI solution for 10 minutes and then
in lab air for 50
minutes. The samples were stressed at 75% of their yield strength, and the
test was run for
180 days.
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[0028] The figure shows the results of this test. It is seen that at high
magnesium
levels, copper is needed to prevent stress corrosion cracking. However, for
the low
magnesium levels of the present invention (about 1.2% Mg), copper is not
required.
[0029] Presently preferred embodiments of the present invention having been
presented above, it is to be understood that the invention may be otherwise
embodied within
the scope of the appended claims.
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