U.S. patent application number 14/513479 was filed with the patent office on 2015-09-03 for high strength, high stress corrosion cracking resistant and castable al-zn-mg-cu-zr alloy for shape cast products.
The applicant listed for this patent is AUTOMOTIVE CASTING TECHNOLOGY, INC.. Invention is credited to RICK A. BORNS, MICHAEL BRANDT, BOB R. FORS, JEN C. LIN, MOUSTAPHA MBAYE, JAMES P. MORAN, JOHN M. NEWMAN, RALPH R. SAWTELL, GERALD D. SCOTT, XINYAN YAN, WENPING ZHANG.
Application Number | 20150247229 14/513479 |
Document ID | / |
Family ID | 39092707 |
Filed Date | 2015-09-03 |
United States Patent
Application |
20150247229 |
Kind Code |
A1 |
LIN; JEN C. ; et
al. |
September 3, 2015 |
HIGH STRENGTH, HIGH STRESS CORROSION CRACKING RESISTANT AND
CASTABLE AL-ZN-MG-CU-ZR ALLOY FOR SHAPE CAST PRODUCTS
Abstract
The present invention provides an Al--Zn--Mg--Cu casting alloy
that provides high strength for automotive and aerospace
applications and optimized stress corrosion cracking resistance in
highly corrosive and tensile environments. The inventive alloy
composition includes about 3.5 wt. % to about 5.5 wt. % Zn; about
1.0 wt. % to about 3.0 wt. % Mg; about 0.5 wt. % to about 1.2 wt. %
Cu; less than about 1.0 wt. % Si; less than about 0.30 wt. % Mn;
less than about 0.30 wt. % Fe; and a balance of Al and incidental
impurities.
Inventors: |
LIN; JEN C.; (EXPORT,
PA) ; YAN; XINYAN; (MURRYSVILLE, PA) ; ZHANG;
WENPING; (MURRYSVILLE, PA) ; MORAN; JAMES P.;
(NORTH HUNTINGTON, PA) ; NEWMAN; JOHN M.; (EXPORT,
PA) ; SAWTELL; RALPH R.; (GIBSONIA, PA) ;
SCOTT; GERALD D.; (GIBSONIA, PA) ; BRANDT;
MICHAEL; (MURRYSVILLE, PA) ; FORS; BOB R.;
(FRUITPORT, MI) ; BORNS; RICK A.; (SPRING LAKE,
MI) ; MBAYE; MOUSTAPHA; (ADA, MI) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
AUTOMOTIVE CASTING TECHNOLOGY, INC. |
FRUITPORT |
MI |
US |
|
|
Family ID: |
39092707 |
Appl. No.: |
14/513479 |
Filed: |
October 14, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13449273 |
Apr 17, 2012 |
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14513479 |
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11856631 |
Sep 17, 2007 |
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13449273 |
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60826131 |
Sep 19, 2006 |
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Current U.S.
Class: |
148/549 ;
148/417 |
Current CPC
Class: |
C22F 1/053 20130101;
C22C 1/06 20130101; C22C 21/10 20130101 |
International
Class: |
C22F 1/053 20060101
C22F001/053; C22C 21/10 20060101 C22C021/10 |
Claims
1. A cast aluminum part comprising an Al--Zn--Mg--Cu alloy,
wherein: the alloy comprises about 4.0 wt. % to about 4.5 wt. % Zn;
about 1.2 wt. % to about 1.8 wt. % Mg; about 0.6 wt. % to about
0.85 wt. % Cu; less than about 1.0 wt. % Si; less than about 0.30
wt. % Mn; less than about 0.30 wt. % Fe; a total Mg and Zn content
of less than about 6%; and, incidental impurities; and, a cast
aluminum part is produced from a process comprising producing a
melt comprising the alloy having a fluidity which exceeds a length
of 7 cm in a spiral mold for the casting process; casting at least
a portion of the melt into a mold to provide the cast aluminum
part; and, heat treating the cast aluminum part to an overaged
condition, the process including a T6 heat treatment and aging at a
temperature greater than about 340.degree. F. for greater than
about four hours; wherein, the cast aluminum part has a
time-to-failure of greater than 96 hours under ASTM G103 testing
conditions for stress corrosion cracking.
2. The cast aluminum part of claim 1, wherein heat treating the
cast aluminum part to the overaged condition further comprises:
heating the cast aluminum part from about room temperature to a
temperature in a range of about 200.degree. F. to about 300.degree.
F. within a time period of one hour.
3. The cast aluminum part of claim 1, wherein the aluminum alloy
melt comprises about 0.65 wt. % to about 0.85 wt. % Cu., and the
aging of the cast aluminum part occurs at a temperature ranging
from about 340.degree. F. to about 380.degree. F. for greater than
about four hours
4. The cast aluminum part of claim 1, wherein the time-to-failure
under ASTM G103 testing conditions is greater than 96 hours.
5. The cast aluminum part of claim 1, wherein the magnesium
concentration ranges from a concentration of about 1.5 wt. % to 1.8
wt. %, and the ratio of Zn to Mg is less than about 3.0.
6. The cast aluminum part of claim 1, wherein the Cu concentration
ranges from 0.65 wt % to 0.85 wt % and the ratio of Zn to Mg ranges
from about 2.7 to about 3.8.
7. The cast aluminum part of claim 1, wherein the copper
concentration ranges from 0.65 wt % to 0.85 wt % and the ratio of
Zn to Mg ranges is about 2.7, about 3.3, or about 3.8.
8. The cast aluminum part of claim 1, wherein the Cu concentration
ranges from 0.65 wt % to 0.85 wt % and total Mg and Zn content
ranges from 5.2 wt % to 5.7 wt %.
9. A method of making a cast aluminum part comprising: creating an
aluminum alloy melt comprising about 4.0 wt. % to about 4.5 wt. %
Zn; about 1.2 wt. % to about 1.8 wt. % Mg; greater than about 0.5
wt. % to about 0.85 wt. % Cu; less than about 1.0 wt. % Si; less
than about 0.30 wt. % Mn; less than about 0.30 wt. % Fe; a total Mg
and Zn content of less than about 6%; and incidental impurities;
casting at least a portion of the melt into a mold to provide a
cast aluminum part; and, heat treating the cast aluminum part to an
overaged condition, the heat treating including a T6 heat treatment
and aging at a temperature greater than about 340.degree. F. for
greater than about four hours; wherein, the melt has a fluidity
which exceeds a length of 7 cm in a spiral mold for castability;
and, the cast aluminum part produced by the method has an
elongation of 8% or greater; and, a time-to-failure of greater than
96 hours under ASTM G103 testing conditions for stress corrosion
cracking.
10. The method of claim 9, wherein heat treating the cast aluminum
part to the overaged condition further comprises: heating the cast
aluminum part from about room temperature to a temperature in a
range of about 200.degree. F. to about 300.degree. F. within a time
period of one hour.
11. The method of claim 9, wherein the aluminum alloy melt
comprises about 0.65 wt. % to about 0.85 wt. % Cu., and the aging
of the cast aluminum part occurs at a temperature ranging from
about 340.degree. F. to about 380.degree. F. for greater than about
four hours.
12. The method of claim 9, wherein the ratio of Zn to Mg ranges
from about 2.66 to about 3.75.
13. An Al--Zn--Mg--Cu aluminum alloy, comprising: about 4.0 wt. %
to about 4.5 wt. % Zn; about 1.2 wt. % to about 1.8 wt. % Mg;
greater than about 0.5 wt. % to about 0.85 wt. % Cu; less than
about 1.0 wt. % Si; less than about 0.30 wt. % Mn; less than about
0.30 wt. % Fe; a total Mg and Zn content of less than about 6%;
and, incidental impurities; wherein, the alloy has a fluidity which
exceeds a length of 7 cm in a spiral mold for a shaped casting
process; and, when used in a casting process that includes T6 heat
treatment and aging at a temperature greater than about 340.degree.
F. for greater than about four hours, provides a shaped aluminum
casting having (i) an elongation of 8% or greater and (ii) a
time-to-failure of greater than 96 hours under ASTM G103 testing
conditions for stress corrosion cracking.
14. The aluminum alloy of claim 13, wherein the alloy comprises
about 0.65 wt. % to about 0.85 wt. % Cu.
15. The aluminum alloy of claim 13 further comprising at least one
grain refiner selected from a group consisting of boron, carbon and
combinations thereof.
16. The aluminum alloy of claim 1 further comprising at least one
anti-grain growth agent selected from the group consisting of
zirconium, scandium, manganese and combinations thereof.
17. The aluminum alloy of claim 1, wherein the magnesium
concentration ranges from a concentration of about 1.5 wt. % to 1.8
wt. %, and the ratio of Zn to Mg is less than about 3.0.
18. The aluminum alloy of claim 1, wherein said magnesium is at a
concentration of about 1.5 wt. % to 1.8 wt. %.
19. The aluminum alloy of claim 1, wherein the ratio of Zn to Mg is
less than about 3.0.
20. The aluminum alloy of claim 11, wherein said copper is at a
concentration of about 0.7 wt. % to 0.8 wt. %.
21. The aluminum alloy of claim 1, wherein the ratio of Zn to Mg
ranges from about 2.77 to about 3.0.
22. The aluminum alloy of claim 1, wherein the concentration of
copper is selected from the group consisting of about 0.6 wt. %,
about 0.7 wt. %, and about 0.8 wt. %.
23. The aluminum alloy of claim 1, wherein the ratio of Zn to Mg
ranges from about 2.66 to about 3.75.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. application Ser.
No. 13/449,273, filed Apr. 17, 2012, which is a continuation of
U.S. application Ser. No. 11/856,631, filed Sep. 17, 2007, which
claims the benefit of U.S. Provisional Application No. 60/826,131,
filed Sep. 19, 2006, each application of which is incorporated
herein by reference in its entirety.
FIELD OF THE INVENTION
[0002] This invention relates generally to aluminum alloys for
aerospace and automotive shaped castings having high tensile
strength and high resistance to stress corrosion cracking
(SCC).
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, Al7SiMg, 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 Al7SiMg
alloy, can reliably guarantee ultimate tensile strength of 290 MPa
(42 ksi). and tensile yield strength of 220 MPa (32 ksi) with
elongations of 8% or greater.
[0004] In applications where high strength is required forged
products are typically used. Forged products are disadvantageously
more expensive than cast products. Considerable cost savings may be
realized in both automotive and aerospace applications if cast
products can be used to replace forged products with little or no
loss of strength. elongation performance, general corrosion
resistance. stress crack corrosion resistance and fatigue
strength.
[0005] A variety of alternative casting alloys exist that exhibit
higher strengths than Al7SiMg alloys. However these exhibit
problems in castability, corrosion performance or fluidity, which
are not readily overcome. For example, U.S. patent application Ser.
No. 11/111,212, titled "Heat Treatable Al--Zn--Mg--Cu Alloy for
Aerospace and Automotive Castings", filed on Apr. 21, 2005,
discloses an Al--Zn--Mg--Cu alloy for shaped castings having high
fatigue resistance and high strength that is suitable for
automotive and aerospace applications. While the Al--Zn--Mg--Cu
alloy disclosed in U.S. patent application Ser. No. 11/111,212
provides good general corrosion resistance, it has been determined
that the alloy exhibits a less than optimum stress corrosion
cracking (SCC) resistance in environments of high tensile stress
and corrosion, which could limit it's application.
[0006] In light of the above, a need exists for a casting alloy
having strengths suitable for high strength automotive and
aerospace applications, while simultaneously providing high stress
corrosion cracking (SCC) resistance. It is further desired that the
alloy maintain acceptable levels of fatigue resistance and general
corrosion resistance and castability to be suitable for providing
shaped castings for aerospace and automotive applications.
SUMMARY OF THE INVENTION
[0007] Generally speaking, the present invention provides an
Al--Zn--Mg--Cu alloy for shaped castings having ultimate tensile
strengths greater than that achieved by comparable castings of
A356, while maintaining corrosion performance suitable for
automotive and aerospace applications, specifically including a
good resistance to stress corrosion cracking (SCC) in severely
corrosive environments of high tensile stress. Broadly, the
inventive alloy is composed of
[0008] about 3.5-5.5 wt. % Zn,
[0009] about 1.0-3.0 wt. % Mg,
[0010] about 0.5-1.2 wt. % Cu,
[0011] less than about 1.0 wt. % Si,
[0012] less than about 0.30 wt. % Mn,
[0013] less than about 0.30 wt. % Fe, and
[0014] a balance of Al and incidental impurities.
[0015] In one aspect of the present invention, the stress corrosion
cracking (SCC) resistance of the alloy was increased by optimizing
the amount of Zn and Mg, as well as the amount of Cu, Specifically,
to achieve optimized stress corrosion cracking (SCC) performance
the Mg and Zn content is to he limited to less than or equal to 6.0
wt. %, and the Cu content is incorporated in greater than or equal
to 0.5 wt. %. The corrosion character of the Al--Zn--Mg--Cu alloy
of the present invention when in an overaged condition exhibits
pitting corrosion, which is the preferred mode of corrosion in
comparison to intergranular corrosion.
[0016] In another aspect of the present invention, a method of
manufacturing a shaped casting is provided in which an
Al--Zn--Mg--Cu alloy provides strengths greater than that achieved
by comparable castings of A356, while maintaining corrosion
performance that is suitable for automotive and aerospace
applications, specifically including a good resistance to stress
corrosion cracking in severely corrosive environments of high
tensile stress. Broadly, the method includes the steps of:
[0017] preparing a molten mass of an aluminum alloy composed
of:
[0018] about 3.5-5.5 wt. % Zn,
[0019] about 1-3 wt. % Mg,
[0020] about 0.5-1.2 wt % Cu,
[0021] less than about 1.0 wt. % Si,
[0022] less than about 0.30 wt. % Mn,
[0023] less than about 0.30 wt. % Fe, and
[0024] incidental impurities;
[0025] casting the melt into a cast body; and
[0026] heat treating the cast body to an overaged temper;
[0027] casting at least a portion of the melt into a mold to
provide a shaped casting; and
[0028] heat treating the shaped casting to an overaged
condition.
[0029] In one embodiment, the optimum conditions for the alloy to
achieve high stress corrosion cracking (SCC) resistance and high
tensile strength includes an alloy composed of a Mg and Zn content
of 6.0 wt. % or less and a Cu content greater than 0.5 wt. % in
combination with casting and heat treating to an overaged temper.
In the overaged condition, the inventive Al--Zn--Mg--Cu alloy
exhibits only pitting corrosion mode (general corrosion), which is
the preferred corrosion mode in comparison to intergranular
corrosion (IG) when tested under ASTM G110 conditions.
[0030] The inventive Al--Zn--Mg--Cu alloy and method of producing a
shaped casting in addition to providing levels of strength and
corrosion performance that were previously not obtainable with
prior Al--Zn--Mg--Cu alloys, additionally provides acceptable hot
cracking performance and fluidity for casting shaped products.
BRIEF DESCRIPTION OF THE DRAWINGS
[0031] The following detailed description, given by way of example
and not intended to limit the invention solely thereto, will best
be appreciated in conjunction with the accompanying drawings,
wherein like reference numerals denote like elements and parts, in
which:
[0032] FIG. 1 is a plot illustrating the effect of Cu, Mg, and Zn
on the stress corrosion cracking (SCC) resistance of an
Al--Zn--Mg--Cu alloy in accordance with the present invention when
subjected to a 7 day boiling salt SCC test (ASTM G103) under a
tensile stress of 240 MPa.
[0033] FIG. 2 is a plot illustrating the effect of Cu, Mg, and Zn
on the Stress Corrosion Cracking (SCC) resistance of an
Al--Zn--Mg--Cu alloy in accordance with the present invention when
subjected to a 7 day boiling salt test (ASTM G 103) under a tensile
stress of 160 MPa.
[0034] FIG. 3 is a plot illustrating the effect of Cu on the
tensile yield strength of an Al--Zn--Mg--Cu alloy in accordance
with the present invention.
[0035] FIGS. 4a-4c are photographs of test specimens formed in
accordance with the present invention, and comparative examples,
evaluated for general corrosion performance by immersion in a
NaCl+H.sub.2O.sub.2 solution in accordance with ASTM G110.
[0036] FIGS. 5a-5c depict graphs of the depth of corrosion in
microns of test specimen evaluated by ASTM G110 corrosion testing,
wherein the test specimen include Al--Zn--Mg--Cu compositions
including greater than 0.5 wt. % Cu and comparative examples having
less than 0.5 wt. % Cu.
[0037] FIGS. 6a-6c are photographs of the side cross section of
test specimens formed in accordance with the present invention, and
comparative examples, illustrating the degree of intergranular
corrosive attack.
[0038] FIG. 7 is a graph of the hot cracking index verses the Cu
content of pencil probe castings of Al--Zn--Mg--Cu alloys, in
accordance with the present invention.
[0039] FIG. 8 is a graph of the fluidity verses the Cu content of
spiral mold castings of Al--Zn--Mg--Cu alloys, in accordance with
the present invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0040] The present invention provides an Al--Zn--Mg--Cu alloy
having yield strengths, fatigue strength, general corrosion
performance, and stress corrosion cracking (SCC) performance
suitable for automotive and aerospace applications. while
maintaining castability. All component percentages herein are by
weight percent unless otherwise indicated. When referring to any
numerical range of values, such ranges are understood to include
each and every number and/or fraction between the stated range
minimum and maximum. A range of about 0.5-1.2 wt. % Cu, for
example, would expressly include all intermediate values of about
0.6, 0.7, 0.8 and all the way up to and including 1.1 wt. % Cu. As
used herein, the term "incidental impurities" refers to elements
that are not purposeful additions to the alloy, but that due to
impurities and/or leaching from contact with manufacturing
equipment. trace quantities of such elements being no greater than
0.05 wt. % that may, nevertheless, find their way into the final
alloy casting or casting alloy ingot.
[0041] The Al--Zn--Mg--Cu casting alloy of the present invention is
composed of:
[0042] 3.5-5.5 wt. % Zn,
[0043] about 1.0-3.0 wt. % Mg,
[0044] about 0.5-1.2 wt. % Cu,
[0045] less than about 1.0 wt. % Si,
[0046] less than about 0.30 wt. % Mn,
[0047] less than about 0.30 wt. % Fe, and
[0048] a balance of Al and other incidental impurities.
[0049] In one aspect of the present invention, the Mg, Zn and Cu
content of the alloy is selected to provide increased strength and
stress corrosion cracking resistance. The stress corrosion cracking
performance of the alloy is effected by both chemical and physical
factors. From a physical standpoint, to provide an alloy having
good stress corrosion cracking performance the degree of
precipitation within the alloy must be of a level to provide
sufficient strength, but not be so great as to cause embrittlement
of the alloy. From a chemical standpoint, resistance to stress
corrosion cracking requires resistance to chemical attack by
corrosion.
[0050] Preferably, the Al--Zn--Mg--Cu alloy provides increased
stress cracking corrosion (SCC) resistance with a composition of
alloying elements and ratios including a Mg and Zn content of about
6.0 wt. % or less, and a Cu content greater than 0.5 wt. %,
preferably ranging from 0.5 wt. % to 1.2 wt. %, so long as the Cu
content is not so great to cause embrittlement of the alloy.
Specifically, in one preferred embodiment, high tensile yield
strengths on the order of about 300 MPa and Stress Corrosion
Cracking resistance at a stress of up to 240 MPa may be provided by
the Al--Zn--Mg--Cu alloy, in accordance with the present
invention.
[0051] In one aspect of the present invention, the Mg and Zn
content of the Al--Zn--Mg--Cu alloy is preferably selected to
provide strength enhancing MgZn.sub.2 precipitates. Preferably, the
Zn/Mg ration is about 3.3 or less and the total of the Mg and Zn
content is less than about 6.0 wt. % of the alloy composition.
During processing of the Al--Zn--Mg--Cu alloy of the present
invention, Mg and Zn from the alloy precipitate to form MgZn.sub.2
in the Al Matrix and at the grain boundary. Increasing the Mg and
Zn content to greater than 6.0 wt. % may produce excess MgZn.sub.2
at the grain boundaries, resulting in reduced stress corrosion
cracking (SCC) resistance. In one preferred embodiment, the Zn is
at a concentration of about 3.8 to 4.6 wt. %, most preferably
ranging from 4.0 wt. % to 4.4 wt. %. In one preferred embodiment,
the Mg is at a concentration of concentration of about 1.2 wt. % to
1.8 wt. %, most preferably ranging from 1.4 wt. % to 1.6 wt. %.
[0052] In another aspect of the present invention, the Cu content
of the Al--Zn--Mg--Cu alloy is select to substantially reduce
intergranular corrosion, while further providing precipitate
hardening mechanisms through the formation of Al.sub.2Cu
(.theta.-phase) precipitates or Al.sub.2CuMg (s-phase)
precipitates. Preferably, the Cu content is selected to increase
the corrosion potential of the precipitate free zone of the alloy
relative to the alloy's aluminum matrix. In one embodiment, the Cu
content is greater than about 0.5 wt. %, preferably ranging from
about 0.5 wt. % to about 1.2 wt. %, even more preferably ranging
from about 0.65 wt. % to about 1.0 wt. % and most preferably
ranging from about 0.7 wt. % to 0.8 wt. %. Increasing the Cu to
greater than about 1.2 wt. % may result in an excess of constituent
particles of Al--Fe--Cu at the grain boundary, which may decrease
the alloy's ductility, fatigue resistance and toughness.
[0053] The Al--Zn--Mg--Cu alloy of the present invention provides
increased stress corrosion cracking (SCC) resistance, wherein in
one aspect the Mg, Zn and Cu constituents are selected to
substantially reduce intergranular corrosion. As opposed to general
corrosion or pitting, intergranular corrosion can be particularly
troublesome as occurring below the surface of the casting, wherein
severe corrosion may occur without any indication by visual
inspection. Additionally, localized corrosion at the grain
boundaries by intergranular corrosion accelerates failure, as
opposed to the more homogenous corrosion provided by pitting.
[0054] Intergranular corrosion in prior Al--Zn--Mg alloys results
from differences in the corrosion potential between the Aluminum
Alloy Matrix, the precipitate free Zone (PFZ) and the grain
boundary of the casting alloy. The differences in corrosion
potential in prior alloys results from the incorporation of the
elements that are at least in part introduced for precipitate
hardening mechanisms, such as Mg and Zn.
[0055] Precipitation at the grain boundary after quench is
initially continuous, since diffusion at the grain boundary is very
fast relative to diffusion within the Al Matrix due to the grain
boundary's open structure. Therefore, precipitates such as
MgZn.sub.2 more readily form large continuous precipitates at the
grain boundary, whereas the Al matrix restricts diffusion and
growth of precipitates resulting in a more uniform fine dispersion
of precipitates. At the interface of the Al matrix and the grain
boundaries is the precipitate free zone (PFZ), being substantially
free of precipitates, in which the alloying elements, such as Mg
and Zn, are in solution.
[0056] In one instance, intergranular corrosion results from the
equivalent of a galvanic cell (micro-galvanic corrosion) formed
between the Aluminum Matrix and the precipitate free zone (PFZ) due
to the potential difference between the differing composition of
the aluminum matrix and the composition of the precipitate free
zone (PFZ). In prior Al--Zn--Mg alloys the corrosion potential
difference between the matrix and the precipitate free zone (PFZ)
can be significant, in which the casting is particularly
susceptible to intergranular corrosion and typically degrades. Such
degradation can results in decreased resistance to stress corrosion
cracking and premature failure of the casting. In prior Al--Zn--Mg
alloys the corrosion potential difference between the Aluminum
Matrix and the precipitate free zone (PFZ) typically results from
elements of Mg and Zn incorporated into the PFZ solution, wherein
the incorporation of Mg and Zn decreases the corrosion potential of
the precipitate free zone (PFZ) relative to the Al matrix.
[0057] In one aspect of the present invention the Cu content is
selected to increase the corrosion potential of the precipitate
free zone (PFZ) relative the Al matrix. Preferably, the
incorporation of Cu into the alloy, and hence the precipitate free
zone, offsets the decrease in corrosion potential resulting from
the Mg and Zn incorporated in the metal solution at the precipitate
free zone, preferably to provide uniformity in corrosion potential
between the precipitate free zone and the aluminum matrix. The
alloy of the present invention by specifying and controlling the
alloying amounts maintains a balance between the electrochemical
potential of the Al matrix and the precipitate free zone. By
employing an alloy chemistry that reduces or eliminates the
potential difference between the precipitate free zone and the Al
matrix, the present invention increases stress corrosion cracking
resistance in one aspect by significantly reducing or eliminating
the localized corrosion.
[0058] The alloy of the present invention may further include up to
about 1.0 wt. % Silicon, wherein Si may improve castability.
Further, lower levels of Si may be employed to increase strength.
For some applications, manganese in amounts up to about 0.3 wt. %
may be employed.
[0059] The alloy may also contain grain refiners such as titanium
diboride, TiB.sub.2 or titanium carbide, TiC and/or anti-grain
growth agents such as zirconium, manganese 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 wt. % to 0.05 wt. %.
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 wt. % to 0.05 wt. %. Typical grain refiners are aluminum
alloys containing TiC or TiB.sub.2.
[0060] Zirconium, if used to prevent grain growth during solution
heat treatment, is generally employed in a range below 0.2 wt. %,
preferably ranging from 0.05 wt. % to 0.2 wt. %.
[0061] Scandium may also be used in a range below 0.3 wt. %,
preferably ranging from 0.05 wt. % to 0.3 wt. %.
[0062] In another aspect of the present invention, a heat treatment
in conjunction with the alloying elements and ratio's optimizes
precipitation at the grain boundaries to increase stress corrosion
cracking resistance. Precipitates, such as MgZn.sub.2 and Cu
precipitates, form within the metal matrix and at the grain
boundary. The precipitates at the grain boundary are susceptible to
corrosive attack. Moreover, precipitation at grain boundary after
alloy quench initially includes a continuous distribution of fine
precipitates and disadvantageously results in localized continuous
corrosion at the grain boundary. Corrosion of the continuous and
fine precipitates at the grain boundary (intergranular corrosion)
disadvantageously decreases the alloy's stress corrosion cracking
(SCC) resistance.
[0063] The present invention provides a heat treatment to overage
the alloy, wherein the heat treatment results in coarsening of the
fine precipitates at the grain boundary to provide a discontinuous
distribution of large precipitates interrupted by Aluminum.
Aluminum has a greater resistance to corrosion than the grain
boundary precipitates. Therefore, the discontinuous distribution of
large precipitates at the grain boundary results in a discontinuous
mode of corrosion at the grain boundary, which advantageously
increases the stress crack corrosion resistance (SCC) of the
alloy.
[0064] For the purposes of this disclosure the term "overage" or
"overage temper" or "overaged condition" denotes that the time and
temperature of the heat treatment is selected to sacrifice a degree
of strength from the alloy's peak strength for improved stress
corrosion cracking (SCC) resistance. Applicants state that the term
"peak strength" or "peak condition" denotes the maximum tensile
strength or yield strength that may be achieved for a given
precipitate hardening composition, such as, but not limited to,
Al--Zn--Mg--Cu alloy system, wherein the strength is dependent on
the temperature and time of the heat treatment.
[0065] Preferably, the heat treatment to be used with the
Al--Zn--Mg--Cu alloy including a Mg and Zn content of 6.0 wt. % or
less, and a Cu content greater than 0.5 wt. % to provide increased
stress corrosion cracking performance includes at least one
treatment at a temperature of greater than 340.degree. F.,
preferably ranging from 340.degree. F. to 380.degree. F., for a
time period of 4.0 hours or greater. In one preferred embodiment,
the heat treatment includes two stages. In a first stage the
casting is heated from room temperature to 250.degree. F. within a
time period of one hour and heated from 250.degree. F. to greater
than 340.degree. F. within a time period of one hour. In a second
stage, the alloy is aged at greater than 340.degree. F. until
achieving an overaged temper, wherein the second stage is conducted
for greater than four hours.
[0066] In the overaged temper, the Al--Zn--Mg--Cu alloy system
demonstrates 50% higher tensile yield strength than is obtainable
from A356.0-T6, while maintaining similar elongations and providing
stress corrosion cracking (SCC) resistance at a stress of up to 240
MPa and is applicable to part designs requiring higher strength
than AlSiMg alloys that are readily available today. such as
A356.0-T6 or A357.0-T6. Fatigue performance in the T6 temper is
increased over the A356.0-T6 material by 45%. Specifically, high
tensile strengths on the order of about 300 MPa and stress
corrosion cracking (SCC) resistance at a stress of up to 240 MPa
are provided by the Al--Zn--Mg--Cu alloy of the present
invention.
[0067] In addition to providing increased resistance to stress
corrosion cracking (SCC) and providing strengths suitable for
automotive castings, the alloy of the present invention provides
acceptable general corrosion (pitting) performance. Further, the
castability of the inventive Al--Zn--Mg--Cu alloy is suitable for
providing shaped castings.
[0068] Although the invention has been described generally above,
the following examples are provided to further illustrate the
present invention and demonstrate some advantages that arise
therefrom. It is not intended that the invention be limited to the
specific examples disclosed.
[0069] Table 1 includes alloy compositions (Alloy composition
numbers 1-17) having Mg, Cu, and Zn in accordance with the present
invention and includes the composition of comparative examples.
Alloy composition numbers 1-5 represent some embodiments of the
alloy of the present invention having about 3.5 wt. % to about 5.5
wt. %. Zn, about 1.0 wt. % to about 3.0 wt. %. Mg. and about 0.5
wt. % to about 1.2 wt. % Cu, wherein the total Mg and Zn content
ranges about from 5.2 wt. % to about 5.7 wt. %, the Zn/Mg ratio
ranges from about 2.66 wt. % to about 3.75 wt. %, and the Cu
content ranges from about 0.65 wt. % to about 0.85 wt. %. Alloy
composition numbers 6-9 and 18-20 represent comparative examples of
alloys in which the Cu content is less than 0.5 wt. %. Alloy
composition numbers 10-13 represent comparative examples of alloys
in which the Mg and Zn content is equal to 6.0 wt. %. Alloy
composition numbers 14-17 represent comparative examples of alloys
in which the Mg and Zn content is greater than 6.0 wt. %. In each
of the compositions listed in Table 1, the Si content is less than
0.05 wt. %, the Fe content is less than 0.05 wt. %, the Mn content
is less than 0.05 wt. %, the Zr content is less than 0.09 wt. %,
the B content is less than 0.02 wt. %, and the Ti content is less
than 0.06 wt. %.
TABLE-US-00001 TABLE 1 ALLOY COMPOSITION Alloy Composition Alloy #
Zn Mg Cu Mg + Zn Zn/Mg 1 4 1.2 0.85 5.2 3.3 2 4 1.5 0.65 5.5 2.7 3
4 1.5 0.85 5.5 2.7 4 4.5 1.2 0.65 5.7 3.8 5 4.5 1.2 0.85 5.7 3.8 6
4 1.2 0 5.2 3.3 7 4 1.2 0.35 5.2 2.7 8 4 1.5 0.35 5.5 2.7 9 4.5 1.2
0.35 5.7 3.8 10 4.5 1.5 0.25 6 3 11 4.5 1.5 0.45 6 3 12 4.5 1.5
0.65 6 3 13 4.5 1.5 0.85 6 2.5 14 4.5 1.8 0.25 6.3 2.5 15 4.5 1.8
0.45 6.3 2.5 16 4.5 1.8 0.65 6.3 2.5 17 4.5 1.8 0.85 6.3 2.5 18 4
1.2 0.25 5.2 3.3 19 4 1.5 0 5.5 2.7 20 4.5 1.2 0 5.7 3.8
Stress Corrosion Cracking Resistance
[0070] Tables 2-4 provide the results of boiling salt stress
corrosion cracking testing for test samples having the alloy
compositions listed in Table 1, wherein the boiling salt stress
corrosion cracking testing under stress levels of 160 MPa
(representative of .about.50% of the alloy's tensile yield strength
target) and 240 MPa (representative of .about.75% of the alloy's
tensile yield strength) was conducted in accordance with the
"Standard Practice for Evaluating Stress-Corrosion Cracking
Resistance of Low Copper 7XXX Series Al--Zn--Mg--Cu Alloys" as
described in ASTM G103. In accordance with the guidelines of ASTM
G103, stressed specimens are totally and continuously immersed in
boiling solution containing about 6% sodium chloride for up to 168
hrs. The specimens are regularly checked for visual cracking. The
time to failure is used to indicate the stress corrosion cracking
(SCC) resistance of the aluminum alloys. A test specimen was
considered to have acceptable stress corrosion cracking (SCC)
resistance if it could survive the boiling salt test for a time
period of 96 hours. The boiling salt stress corrosion cracking
(SCC) test was conducted for seven days, wherein test samples that
did not fail during the seven day period were given a value of 168
hours.
[0071] Table 2 includes the stress corrosion cracking (SCC) data
provided for Al--Zn--Mg--Cu alloys (alloy composition numbers 1-5)
having alloying ranges within the scope of the present invention
and heat treated to an overaged condition. The alloy heat treatment
included two stages, in which the first stage included heating the
alloy from room temperature to 250.degree. F. within one hour. The
second stage is aging the alloy to the overaged condition, wherein
Table 2 includes data for aging at 340.degree. F. for 16 hours, and
aging at 340.degree. F. for 24 hours.
TABLE-US-00002 TABLE 2 BOILING SALT STRESS CORROSION CRACKING TEST
2nd state aging at 340 F. for 16 hrs 2nd state aging at 340 F. for
24 hrs Alloy Time (hours) at Time (hours) at Time (hours) at Time
(hours) at # 160 Mpa 240.0 MPa 60 Mpa 240.0 MPa 1 168 168 168 168
168 168 168 168 168 132 168 168 2 168 168 168 107 168 168 168 168
168 132 166 168 3 168 168 168 168 168 168 168 168 168 168 168 168 4
168 168 168 166 168 168 168 168 168 58 132 168 5 168 168 168 107
168 168 168 168 168 168 168 168
[0072] As indicated by Table 2, the Al--Zn--Mg--Cu alloys having
alloying ranges within the scope of the present invention (Alloys
#1-5) and heat treated to an overaged condition survived at least
96 hours of stress corrosion cracking (SCC) testing in accordance
with the boiling salt test. Generally, the test specimens survived
from 119 hours to the entire length of the test (168 hours).
[0073] SCC testing was not conducted for test specimens of alloy
composition numbers 1-9 being treated with a second stage heating
step of 340.degree. F. for four hours, since the intergranular
corrosion of these tests specimens as measured using the Standard
Practice for Evaluating Intergranular Corrosion Resistance of Heat
Treatable Aluminum Alloys by Immersion in Sodium Chloride+Hydrogen
Peroxide Solution in accordance with ASTM G110 indicated that this
period of aging was not suitable to provide sufficient SCC
resistance.
[0074] Table 3 includes the stress corrosion cracking (SCC) data
provided for Al--Zn--Mg--Cu alloys (alloy composition numbers 6-9)
similar to the alloy of the present invention except for having a
Cu content of less than 0.5 wt. %. Alloy composition numbers 6-9
were tested for stress corrosion cracking (SCC) resistance using
the same boiling salt test applied to alloy composition numbers
1-5. Alloy composition numbers 6-9 where aged using a heat
treatment that includes two stages, in which the first stage
included heating the alloy from room temperature to 250.degree. F.
within one hour. The second stage is aging the alloy to the
overaged condition, wherein Table 2 includes data for aging at
340.degree. F. for 4 hours, and aging at 340.degree. F. for 16
hours.
TABLE-US-00003 TABLE 3 BOILING SALT STRESS CORROSION CRACKING TEST
FOR AlZnMgCu ALLOY HAVING LESS TI IAN 0.5 wt. % Cu 2nd state aging
at 340 F. for 4 hrs 2nd state aging at 340 F. for 16 hrs Alloy Time
(hours) at Time (hours) at Time (hours) at Time (hours) at # 160
Mpa 240.0 MPa 160 Mpa 240.0 MPa 6 2 2 90 4 4 4 7 4 5 139 4 4 24 44
168 168 18 24 168 8 70 168 168 5 90 125 168 168 168 18 44 72 9 18
148 168 4 18 168 18 72 168 4 18 18
[0075] As indicated in Table 3, alloy composition numbers 6-9
having a Cu content of less than 0.5 wt. % displayed a high
incidence of failure before reaching 96 hours of under stress at
160 MPa or 240 MPa under boiling salt testing. Specifically, only
one test specimen having less than 0.5 wt. % Cu and aged for 16
hours at 340.degree. F. passed the boiling salt corrosion test
under a stress of 240 MPa, representing .about.75% of the desired
minimum yield strength. Typically, alloy composition numbers 6-9
failed within 4-72 hours of testing under boiling salt test.
[0076] Table 4 includes the stress corrosion cracking (SCC) data
provided for Al--Zn--Mg--Cu alloys (alloy composition numbers
10-14) similar to the alloy of the present invention except for
having a combined Zn and Mg content of 6.0 wt. % or greater. The
alloy heat treatment included two stages, in which the first stage
included heating the alloy from room temperature to 250.degree. F.
within one hour. The second stage includes aging the alloy to the
overaged condition, wherein Table 4 includes data for aging at
340.degree. F. for 4 hours, 340.degree. F. for 16 hours, and aging
at 340.degree. F. for 24 hours.
TABLE-US-00004 TABLE 4 BOILING SALT STRESS CORROSION CRACKING TEST
FOR AlZnMgCu ALLOY HAVING AT LEAST 6.0 wt. % Mg AND Zn. 2nd stage
aging 2nd stage aging at Alloy at 340 F. for 4 hrs 340 F. for 16
hrs 2nd stage aging at 340 F. for 24 hrs # 160 MPa 160 MPa 240.0
MPa 160 MPa 240.0 MPa 10 2 2 3 11 3 3 10 3 5 29 12 20 20 20 168 168
168 24 24 24 168 168 168 21 58 168 13 168 168 168 168 168 90 68 90
168 68 114 168 58 76 168 14 1 1 2 15 1 2 3 5 10 10 16 20 4 20 60
168 17 44 44 168
[0077] As indicated in Table 4, alloy composition numbers 10-17
having a total Mg and Zn content of 6.0 wt. % or greater displayed
a high incidence of failure before being subjected to 96 hours of
stress at 160 MPa or 240 MPa under boiling salt SCC testing. As
compared to stress corrosion cracking (SCC) performance of alloy
composition numbers 1-5 having a total Mg and Zn content of less
than 6.0 wt. % illustrated in Table 1, alloy composition numbers
10-17 having a total Mg and Zn or 6.0 wt. % or greater
disadvantageously exhibited reduced stress corrosion crack
resistance.
[0078] Increasing the Mg and Zn content to 6.0 wt. % or greater
introduces an excess of MgZn.sub.2 to the alloy, wherein the excess
MgZn.sub.2 decreases the chemical potential at the precipitate free
zone (PFZ) relative to the alumina matrix to a level that can not
be offset by the incorporation of Cu, without increasing the amount
of ALCuFe and AlCuFeSi at the grain boundary, which
disadvantageously reduces the alloys fracture toughness.
Specifically, Alloy composition numbers 14-17 having a total Mg and
Zn content of 6.3 wt. % exhibited decreased stress corrosion
cracking (SCC) resistance than alloy composition numbers 10-13
having a total Mg and Zn content of 6.0 wt. %.
[0079] The data included in Tables 1-4 has been plotted in FIGS. 1
and 2. FIG. 1 illustrates the alloy compositions that pass the 96
hour boiling salt water stress corrosion cracking (SCC) resistance
test under a stress of 240 MPa (.about.75% of the alloy's tensile
yield strength target), hence providing suitable stress corrosion
cracking (SCC) resistance. FIG. 2 illustrates the alloy
compositions that pass the 96 hour boiling salt water stress
corrosion cracking (SCC) resistance test under a stress of 160 MPa
(.about.50% of the alloy's tensile yield strength target), hence
providing suitable stress corrosion cracking (SCC) resistance.
Referring to FIGS. 1 and 2, reference lines 10a, 10b represents 96
hours of boiling salt SCC testing, wherein the area 15a, 15b under
the curve presented by reference line 10a, 10b indicates alloy
compositions having suitable stress corrosion cracking (SCC)
resistance.
[0080] To provide sufficient stress crack resistance to pass
boiling salt SCC testing an Al--Zn--Mg--Cu alloy requires that the
total Mg and Zn content be less than 6.0 wt. % and the Cu content
be greater than 0.5 wt. %, and that the alloy be treated to an
overaged condition, preferably including greater than four hours
aging at 340.degree. F., and even more preferably including 16 hrs
of aging at 340.degree. F.
Mechanical Properties
[0081] Tables 5-7 provide the results of mechanical testing for
test samples having the alloy compositions listed in Table 1,
wherein the mechanical properties measured included tensile yield
strength (TYS), ultimate tensile strength (UTS) and percent
elongation (E). Similar to the stress corrosion cracking (SCC)
evaluation, each test sample was treated to a two-step heat
treatment was used, in which the first stage including keeping the
heat treatment constant at 250.degree. F. for 3 hours. Following
the first stage, an aging stage was conducted, in which the furnace
temperature was raised to 340.degree. F. for soaking times ranging
from 4 to 32 hours. The alloy reached peak strength at about 4
hours at 340.degree. F. Overaged conditions were investigated at 16
hours, and 24 hours at 340.degree. F. Test specimens were
considered to have acceptable mechanical properties when providing
tensile yield strength (TYS) on the order of at least 300 MPa,
wherein at the lab scale, test specimens having a tensile yield
strength (TYS) being on the order of 320 MPa, were highly
preferred.
[0082] Table 5 includes the mechanical properties measured for
Al--Zn--Mg--Cu alloys (alloy composition numbers 1-5) having
alloying ranges and ratios within the scope of the present
invention and heat treated to an overaged condition.
TABLE-US-00005 TABLE 5 Ultimate Yield Tensile 2.sup.nd step aging
ALLOY Strength Strength Elongation time, hrs # (MPa) (MPa) % @340
F. 1 322.5 377.0 11.0 16.0 1 312.0 370.0 11.0 24.0 2 326.0 381.5
12.0 16.0 2 317.0 372.0 12.0 24.0 3 325.0 377.5 10.0 16.0 3 328.0
381.5 12.0 24.0 4 295.5 339.5 16.0 4.0 4 315.0 368.5 12.0 16.0 4
307.0 362.3 12.0 24.0 5 315.0 372.0 13.5 16.0 5 306.0 364.0 14.0
24.0
[0083] As indicated by Table 5, the Al--Zn--Mg--Cu alloys having
alloying ranges within the scope of the present invention (Alloys
#1-5) and heat treated to an overaged condition provided a tensile
yield strength (TYS) on the order of at least 300 MPa. In a
preferred embodiment, the Zn/Mg ratio is preferably less than
.about.3.0, since increasing the Zn/Mg ratio for a fixed amount of
Zn+Mg to greater than 3.0 typically results in a reduction of MgZn2
strengthening precipitates disadvantageously reducing tensile yield
strength.
[0084] For example, alloy composition numbers 1-3 having Zn/Mg
rations ranging from 2.77 to about 3.0 have a higher tensile yield
strength than alloy composition numbers 4-5 having a Zn/Mg ratio
being greater than 3.0. as illustrated in Table 5. The lab scale
test specimens having a Zn/Mg ratio from 2.77 to 3.0 (alloy
composition numbers 1-3) provided a tensile yield strength (TYS)
being on the order of 320 MPa or greater, whereas test specimen
having a Zn/Mg ratio on the order of 3.3 recorded lower tensile
yield strength (TYS) values being, in some instances being closer
to 300 MPa.
[0085] As discussed above, the alloy of the present invention
includes greater than 0.5 wt. % Cu to substantially minimize the
effect of the Mg and Zn on the difference in corrosion potential
between the Al matrix and the precipitate free zone (PFZ) to
provide an alloy having increased SCC resistance, while maintaining
tensile properties suitable for high strength applications. Table 6
illustrates that the increased Cu content of the Al--Zn--Mg--Cu
alloys of the present invention has a minimal effect on the alloy's
tensile properties when compared to alloys having lower Cu
contents. Specifically, Table 6 includes the tensile properties
measured for Al--Zn--Mg--Cu alloys (alloy composition numbers 6-9
and 18-20) having a Cu content of less than 0.5 wt. %.
TABLE-US-00006 TABLE 6 Yield Tensile 2nd step aging ALLOY Strength
Strength Elongation time, hrs # (MPa) (MPa) % @340 F. 6 263.5 318.0
16.0 16.0 7 299.5 352.0 13.0 16.0 8 305.0 354.0 14.0 4.0 8 309.0
361.0 15.0 4.0 8 310.0 362.5 15.0 16.0 9 311.0 356.0 11.0 4.0 9
304.5 355.0 13.0 16.0 18 304.5 351.0 14.5 4.0 18 297.0 345.0 14.0
16.0 19 293.5 341.0 16.0 4.0 19 270.0 324.5 16.0 16.0 20 299.0
342.0 18.0 4.0 20 264.0 316.0 18.0 16.0
[0086] As indicated by comparison of Tables 5 and 6, Al--Zn--Mg--Cu
alloys having alloying ranges within the scope of the present
invention have similar if not greater tensile properties than
similar Al--Zn--Mg--Cu alloys having less than 0.5 wt. % Cu. For
example, alloy composition number 3 having a Cu content of 0.85 wt.
% provides a tensile yield strength value of 325 MPa, while alloy
composition number 8 being of similar composition to alloy
composition number 1 provides a similar tensile yield strength of
about 310 MPa when similarly heat treated to an overaged condition
including a second stage heat treatment of 340.degree. F. for about
16 hours. The incorporation of Cu within the range of 0.5 wt. % to
1.2 wt. % has little to no disadvantageous effect on the tensile
yield strength of the alloy, as illustrated by Tables 5 and 6, yet
advantageously increases the alloy's stress corrosion cracking
(SCC) resistance, as illustrated in Tables 2 and 3.
[0087] Table 7 includes the mechanical properties measured for
Al--Zn--Mg--Cu alloys (alloy composition numbers 10-17) having a
Zn+Mg content of 6.0 or greater.
TABLE-US-00007 TABLE 7 Yield Tensile 2nd step aging ALLOY Strength
Strength Elongation time, hrs # (MPa) (MPa) % @340 F. 10 344.6
389.5 14.5 4.0 10 350.0 402.0 17.0 16.0 11 353.1 399.0 13.0 4.0 11
357.0 407.0 13.0 16.0 12 358.5 405.0 13.0 4.0 12 362.3 415.0 13.0
16.0 12 350.5 403.0 13.0 24.0 13 370.1 419.8 11.3 4.0 13 366.0
418.0 16.0 16.0 13 354.0 409.3 13.0 24.0 14 364.5 412.0 13.8 4.0 14
376.0 431.0 14.0 16.0 15 371.3 422.4 11.5 4.0 16 387.8 434.9 11.0
4.0 17 398.5 445.3 9.5 4.0
[0088] Referring to Tables 5 and 7, increasing the Mg+Zn content to
6.0 wt. % or greater provides increases the alloys tensile yield
strength, but disadvantageously decreases the alloy's resistance to
stress corrosion cracking (SCC), as indicated in Tables 2 and 4. As
explained above increasing the Mg and Zn content in a manner that
increases the Zn+Mg content to greater than 6.0 wt. % decreases the
corrosion potential of the precipitate free zone (PFZ) zone
relative to the Al matrix to a point that cannot be offset by the
addition of increased Cu without producing an excess of constituent
particles of AlFeCu at the grain boundary that decreases the
alloys' fatigue resistance and toughness. Further, the increased
Zn+Mg also produces higher amounts of MgZn2 at the grain boundary.
which also disadvantageously reduces the alloy composition's
resistance to stress corrosion cracking (SCC).
[0089] The tensile properties were also measured from automotive
steering knuckles cast using Vacuum Riserless Casting
(VRC)/Pressure Riserless Casting (PRC) methods and composed of
Al--Zn--Mg--Cu aluminum alloys in accordance with the present
invention and having greater than 0.5 wt. % Cu and up to and
including 0.9 wt. % Cu. FIG. 3 illustrates a plot depicting the
relationship between the Cu content of the alloy and the tensile
yield strength (data line 52), ultimate tensile strength (data line
51) and elongation of the alloy (data line 50). Each casting was
aged to an overaged condition using a two stage heat treatment
including a first stage at 250.degree. F. for 3 hrs and a second
stage at 340.degree. F. for 16 hrs. Increases in tensile yield
strength 52 and ultimate tensile strength 51 were recorded in
Al--Zn--Mg--Cu alloys with increasing Cu content from greater than
0.4 wt. % Cu to about 0.9 wt. % Cu.
General Corrosion
[0090] General corrosion (corrosion attack mode) was evaluated
using ASTM G110 corrosion testing, which is the "Standard Practice
for Evaluating Corrosion Resistance of Heat Treatable Aluminum
Alloys by Immersion in Sodium Chloride+Hydrogen Peroxide
Solution".
[0091] Referring to FIGS. 4a-4c depicting photographs of tests
specimen evaluated using ASTM G110 corrosion testing, wherein alloy
composition numbers 1, 2, and 4 represent alloy compositions in
accordance with the present invention, alloy composition numbers 8,
9, 18, and 19, represent comparative alloy compositions having less
than 0.5 wt. % Cu, and alloy composition number 13 represents a
comparative alloy composition having a Zn+Mg content of 6.0 wt. %.
The test specimens were cast from a directionally solidified (DS)
mold.
[0092] Similar to the evaluations for stress corrosion cracking
(SCC) performance and mechanical performance, each test sample was
treated to a two-step heat treatment, in which the first stage
including keeping the heat treatment constant at 250.degree. F. for
3 hours. Following the first stage, an aging stage was conducted,
in which the furnace temperature was raised to 340.degree. F. for
soaking times ranging from 4 to 32 hours. The alloy reached peak
strength at .about.4 hours at 340.degree. F. Overaged conditions
were investigated at 16, 24 and hours at 340.degree. F.
[0093] In accordance with the procedures detailed in ASTM G110, the
test specimens were immersed in a solution 3.5% NaCl+H.sub.2O.sub.2
for 24 hours. Once removed from the corrosive solution, the test
specimens were investigated using an optical microscope to
determine the mode of corrosion attack and depth of corrosive
attack.
[0094] As depicted in FIGS. 4a-4c, test specimen composed of alloy
compositions having a Cu content of greater than 0.5 wt. % (alloy
composition numbers 8. 9, 13, and 19) are generally more
susceptible to corrosive attack by pitting than test specimen
having less than 0.5 wt. % Cu (alloy composition numbers 8, 9, 18
and 19). More specifically, the frequency and depth of corrosion
sites increases with increasing Cu content.
[0095] The effect of the Cu content on the depth of corrosion
further illustrated with reference to FIGS. 5a-5c. FIGS. 5a-5c
depict graphs of the depth of corrosion in microns of test specimen
evaluated by ASTM G110 corrosion testing, wherein the test specimen
include greater than 0.5 wt. % Cu (alloy composition numbers 1, 2,
and 4) in accordance with the present invention; and comparative
examples having less than 0.5 wt. % Cu (alloy composition numbers
6, 7, 8, 9, and 18). The test specimens included castings from
directionally solidified (DS) molds. The data plotted includes the
maximum corrosion depth measured and an average of the depth for
five of the deepest corrosion sites. As indicated by FIGS. 5a-5c,
corrosion depth for alloy compositions having greater than 0.5 wt.
% Cu is greater than the corrosion depth for alloy compositions
having less than 0.5 wt. % Cu. Deeper corrosion depth was measured
from cast surfaces, as opposed to machined surfaces, which was
believed to result from microsegregation of Cu on the as cast
surface of the DS castings.
[0096] Referring to FIGS. 5a-5c, the corrosion depth decreased with
increasing aging time. Each of the test samples where aged using a
two stage heat treatment, in which the first stage including
keeping the heat treatment constant at 250.degree. F. for 3 hours
and the second stage included raising the furnace temperature to
340.degree. F. for soaking times ranging from 4 or 16 hours. The
alloy reached peak strength with a second stage heat treatment of
about 4 hours at 340.degree. F. Overaged conditions were
investigated at 16 hours at 340.degree. F. The degree of overaging
effects the corrosion mode, wherein greater degrees of overaging in
Al--Zn--Mg--Cu alloys in accordance with the present invention
result in corrosive attack having a greater degree of pitting, as
opposed to intergranular corrosion, and lesser degrees of overaging
in Al--Zn--Mg--Cu alloys result in corrosive attack having a
greater degree of intergranular corrosion, as opposed to pitting.
Intergranular corrosion results in a greater corrosion depth than
pitting.
[0097] Referring to FIGS. 6a-6c, the mode of corrosion was
evaluated by sectioning the test samples and visually inspecting
the samples cross section with an optical microscope. Referring to
alloy composition #18 having a Cu content on the order of 0.25 wt.
% in FIGS. 6a and 6b, when heat treated to peak strength
conditions, i.e. a first stage heat treatment at 250.degree. F. for
3 hours followed by a 4 hour second stage heat treatment at
340.degree. F., the corrosion mode of the alloy composition may be
characterized as pitting. Referring to FIG. 6a, corrosion depth
generally increases in cast surfaces of alloy compositions
including 0.5 wt. % or greater Cu contents greater, such as alloy
composition numbers 1 and 13 composed of 0.85 wt. % Cu, when
compared to alloy compositions having less than 0.5 wt. % Cu, such
as alloy composition number 18 having 0.25 wt. % Cu. Referring to
FIGS. 6b and 6c, increasing the Cu content to 0.35 wt. % or 0.45
wt. %, such as alloy composition numbers 7, 8, 9 and 11, changes
the mode of corrosion to at least partially being intergranular
corrosion. The mode of corrosion may be entirely intergranular
corrosion in Al--Zn--Mg--Cu compositions aged to peak strength
(i.e. 2.sup.nd step at 340.degree. F. for 4 hours) and having
greater than 0.5 wt. % Cu, such as alloy composition number 16
having a Cu content of 0.65 wt. %, as depicted in FIG. 6c.
[0098] FIG. 6c further illustrates at least one advantage of the
present invention, in which a heat treatment is provided to overage
the alloy resulting in coarsening of the fine precipitates at the
grain boundary to provide a discontinuous distribution of large
precipitates interrupted by Aluminum. As discussed above, Aluminum
has a greater resistance to corrosion than the grain boundary
precipitates. Therefore, the discontinuous distribution of large
precipitates at the grain boundary results in a discontinuous mode
of corrosion at the grain boundary, which advantageously increases
the stress crack corrosion resistance (SCC) of the alloy.
[0099] The heat treatment to provide the overaged condition
included two stages, in which the first stage included heating the
alloy from room temperature to 250.degree. F. within one hour. The
second stage is aging the alloy to the overaged condition, wherein
Table 4 includes data for aging at 340.degree. F. for 4 hours (peak
condition), 340.degree. F. for 16 hours (overaged condition), and
aging at 340.degree. F. for 24 hours (overaged condition). Alloy
composition #16, as depicted in FIG. 6c, clearly illustrates that
the heat treatment of the present invention, i.e. overaging for
greater than four hours at 340.degree. F. during the second stage
of the heat treatment, advantageously converts the mode of
corrosion from intergranular to pitting.
Castability
[0100] The castability of the Al--Zn--Mg--Cu alloy of the present
invention having greater than 0.5 wt. % Cu was assessed using
pencil probe for hot cracking index and spiral molds for fluidity.
The hot cracking index is the smallest diameter of the central
connection rod on the pencil probe casting that does not exhibit
cracking, wherein the lower the value for the hot cracking index
the better the hot cracking resistance of the alloy
composition.
[0101] FIG. 7 illustrates the effect of the Cu content on the hot
cracking index of an Al--Zn--Mg--Cu alloy, having 4.5 wt. % Zn,
0.09 wt. % Zr. and 1.8 wt. % Mg or 1.5 wt. % Mg. The Cu content hot
cracking resistance of the Al--Zn--Mg--Cu alloy of the present
invention is not substantially affected by the incorporation of Cu
being greater than 0.5 wt. %.
[0102] FIG. 8 illustrates the effect of the Cu content on the
fluidity of an Al--Zn--Mg--Cu alloy having 4.5 wt. % Zn, 0.09 wt. %
Zr, and 1.8 wt. % Mg or 1.5 wt. % Mg. Cu had no appreciable effect
on fluidity in Al--Zn--Mg--Cu compositions including 1.8 wt. % Mg,
and provides a slight increase in fluidity in Al--Zn--Mg--Cu
compositions including 1.5 wt. % fluidity.
[0103] While the present invention has been particularly shown and
described with respect to the preferred embodiments thereof, it
will be understood by those skilled in the art that the foregoing
and other changes in forms of details may be made without departing
form the spirit and scope of the present invention. It is therefore
intended that the present invention not be limited to the exact
forms and details described and illustrated, but fall within the
scope of the appended claims.
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