Note: Descriptions are shown in the official language in which they were submitted.
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METHOD FOR INCREASING THE STRENGTH ANDIOR
CORROSION RESISTANCE OF 7000 SERIES
AL AEROSPACE ALLOY PRODUCTS
Related Applications
[0001] This application claims the benefit of U.S. Provisional Patent
Application
Ser. No. 60/277,403 filed on March 20, 2001 and entitled "Age Forming Practice
for
Increasing Tensile Yield Strength of 7xxx-''T79" Product", the disclosure of
which is
fully incorporated by reference herein.
Field of the Invention
[0002] This invention relates to the field of aluminum alloys for aerospace
applications, typically 7000 Series or 7xxx alloys as designated by the
Aluminum
Association. More particularly, this invention relates to an improved method
for
imparting better yield strengths to 7000 Series aluminum alloys tempered in a
known,
preferred manner. , This method achieves such strength improvements without
detrimentally effecting corrosion resistance, particularly exfoliation
corrosion resistance.
Conversely, the method of this invention can be used to impart better
corrosion resistance
performance in these 7000 Series aluminum aerospace alloys at or about the
same yield
strength levels. For the sheet and plate varieties of these products, the
invention may be
practiced on products situated in their respective dies for further achieving
some age
forming improvements thereon. It is to be understood that analogous
improvements in
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the strength/corrosion properties of 7000 Series extrusions and forgings
should also take
place.
Background of the Invention
(0003] The manufacturers of large commercial jetliners have been attempting to
improve the performance of their current and future lines of passenger
aircraft for some
time. They are currently considering new plate and extrusion products for the
upper wing
portions of these plane models. One manufacturer has been actively seeking to
improve
the strength and corrosion performance of next generation materials,
especially over
incumbent 7150-"T79" plate products. That temper, "T79", is produced by age-
forming
individual pre-machined panels, typically to the desired contour part shape
dug i~zg .
artificial aging.
[0004] A typical age forming practice for large aircraft wing panels usually
involves starting with a W51 tempered (solution heat treated and stress
relieved) plate
product. Alternately, that same W51-tempered part may be subjected to the
first of
several multiple step tempering practices while still flat, either by the
material supplier,
an intermediate distributor/handler, or the end user/customer, i.e. the
ultimate aircraft
manufacturer/assembler. Note that this first artificial aging step is riot
typically
performed while the alloy material is kept in its ultimate forming die.
Instead, the latter
plate product is sawed and machined to a desired shape and thickness for a
making given
wing panel component part therefrom. That machined panel is then aligned over
a
forming die whereupon pressure is applied to force said panel to assume its
final or near-
2
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final shape, that of the die itself. The die and panel may then be
artificially aged together
per prescribed practices. Alternately, this first tempering in a multiple step
aging practice
could take place with a sawed and machined part situated "in" its forming die,
after which
both part and die are further artificially aged together.
[0005] A typical 7xxx age forming practice entails one or two steps. If a two
step
practice is used, the first step is usually performed at a lower temperature
than the second.
That first step is typically about 200-250°F for about 3 to 12 hours.
The second step of
that two-step practice targets one or more temperatures between about 280-
350°F for
about 6 to 24 hours, and in some instances for as high as 30 hours. If only a
one step
practice is used, that typically transpires at one or more target temperatures
between about
280-320°F for about 6 to 24 hours.
[0006] For the upper wing panels of most large aircraft, both high strength
and
exfoliation corrosion resistance are critical. In the typical age form
practice, exfoliation
corrosion resistance is known to improve with progressive overaging. There is
a
corresponding decrease, or trade-off, in strength, however. As such, there is
a clear
industry-driven need for an improved aging practice that would provide higher
strengths
at about the same level of corrosion resistance, or a higher level of
corrosion resistance
performance at about the same strength level. This invention addresses both
such
industry needs.
[0007] Numerous 3-step aging practices are known for enhancing corrosion
resistance without degrading the strength of 7000 Series aluminum aerospace
alloys.
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Among these are the prior art disclosures of U.S. Patent Nos. 3,856,584,
3,957,542;
4,477,292; 4,863,528 and 5,108,520. For some of these disclosures, a first
aging step was
performed at about 250°F with a second step above about 350 or
360°F. That second step
is then followed by a third step similar to their first step temperature of
about 250°F.
Some of these references state that their observed benefits diminish at lower,
second step
temperatures. A two-step practice of note is also shown and described in U.S.
Patent No.
3,881,966. By contrast, the preferred first of two, or second of three, aging
practice steps
of this invention proceed at a significantly lower, first or second step
temperature as
compared to the prior art temperings described above, lower by about 40 to
50°F. As
such, the results of this invention were even more surprising since strength
increases were
not expected using a lower temperature aging treatment following the
300°+ practices of
the preferred embodiments herein.
Summary of the Invention
[0008] Briefly stated, this invention relates to an improved method for
artificially
aging 7000 Series aluminum aerospace alloys. This method imparts improved
strength
performance at the same corrosion resistance performance level, or improved
corrosion
resistance performance at the same strength level. It accomplishes these
property
improvements by purposefully adding a second aging step or stage to a typical
one-step
tempering process, or a purposeful third step/stage to a known two-step aging
operations.
The purposefully added step/stage (second of two or third of three) extends at
about 225-
275°F for about 3-24 hours, or more preferably at about 250°F
for about 6 hours or more.
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The invention especially imparts improved combinations of strength and
exfoliation
corrosion resistance to 7055 aluminum alloy products (Aluminum Association
designation) in sheet, plate, extrusion or even forged product forms.
[0009] One commercial jetliner manufacturer's specification for 7xxx age
formed
upper wing panels refers to the "-T7951" temper. As of the filing date for
this patent
application, that temper is still not officially registered with the Aluminum
Association.
The standard practice for "-T7951", described above, involves a one- or two-
step aging
practice. In the present invention, a second step is purposefully added to the
known,
typical one-step aging practice for "-T79". That second step extends at about
225-275°F
for about 3-24 hours, or more preferably at about 250°F for about 6
hours. With the
addition of that second aging step, the inventors herein observed a surprising
and
significant increase in strength at the same level of corrosion resistance,
especially
exfoliation corrosion resistance. Another way or restating this observed
improvement is
that the addition of the second aging step above imparted a significant
increase in
corrosion resistance, especially exfoliation corrosion resistance, at about
the same
strength level.
[0010] Alternately, this same invention entails adding a third step to the two-
step
aging practice for "-T7951". That third step likewise extends at about 225-
275°F for
about 3-24 hours, or more preferably at about 250°F for about 6 hours.
With the addition
of that third aging step following a lower than usual second temperature aging
practice, a
surprising and significant increase in strength was observed at the same level
of corrosion
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resistance, especially exfoliation corrosion resistance. Or restated once
more, the addition
of this third aging step above imparts a significant increase in corrosion
resistance,
especially exfoliation corrosion resistance, at about the same strength level.
[0011) In either instance, adding a second step to a one-step aging practice
for
7000 Series aluminum alloys, or adding a third step to a known two-step aging
practice, it
should be duly noted that the "additional step" of this invention is: (1)
always lower than
the aging step that it follows; AND (2) that preceding step, itself, whether
the first of now
TWO aging steps; or the second of now THREE aging steps, takes place at
temperatures
lower than what is otherwise known to be practiced for other T77 aging
practices for
7000 Series alloys.
Brief Description of the Drawings
[0012] Figures 1 (a) through (c) are graphic representations of three, 2-step
aging
schemes according to the invention;
[0013] Figures 2 (a) through (g) are graphic representations of seven
representative
3-step aging schemes according to the invention;
[0014] Figure 3 is a graph depicting the relative improvement in strength,
particularly longitudinal tensile yield strength (TYS), versus electrical
conductivity (in
IACS) both measured at Tl2 as representative of exfoliation corrosion
resistance
performance, for various samples of 0.75 inch thick, 7055 plate after
artificial aging by
known 1- and 2-step practices (hollow triangular data points) versus the
preferred aging
practice of this invention to which a controlled second or third step, as
appropriate was
6
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purposefully added to the aforesaid known practices (shown with solid circular
data
points);
[0015] Figure 4 is the same graph of Figure 3 through which solid curves A-A
and
B-B were drawn using a quadratic statistical equation approach for predicting
the
strength/EC slopes of the Invention versus known (1- and 2-step aged) 7055
plate product
and around which 95% confidence bands were drawn in dotted lines;
[0016] Figure 5 is a graph depicting the numerical increase in tensile yield
strength (ksi ) values predicted for 7055 Plate aged by the invention over its
known
(1- and 2-step aged) counterparts per the quadratic curves in Figure 4 above;
[0017] Figure 6 is a graph depicting the increase in tensile yield strength
values
predicted (by percent improvement) for 7055 Plate aged by the invention over
its known
(1- and 2-step aged) counterparts;
[0018] Figure 7 is a graph numerically depicting the improvement in electrical
conductivity (% IACS) predicted for 7055 Plate aged by the invention over its
known (1-
and 2-step aged) counterparts; and
(0019] Figure 8 is a graph depicting that same improvement in predicted
electrical
conductivity values (by percentages) for 7055 Plate aged by the invention
versus its
known (1- and 2-step aged) counterparts.
Detailed Description of the Invention
[0020] Numerous variations of aging practices according to the invention are
depicted in accompanying Figures 1 and 2. Particularly, Figures 1 (a) through
(c) are
7
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graphic representations of three, 2-step aging schemes according to the
invention, with
1 (a) representing a 2-step or staged method with a partial (air) cooling
between controlled
steps/stages. In Figure 1 (b), there is shown a representative 2-step method
that has a
controlled, furnace ramping down between first and second steps/stages.
Finally, Figure
1(c) schematically depicts a 2-step or staged method with a distinct, fully
separated .
cooling (via air or cold water quenching "CWQ") between steps/stages.
[0021] Figures 2 (a) through (g) are graphic representations of seven
representative
3-step aging schemes according to the invention. In Figure 2(a), a 3-step or
staged
method is shown with a partial (air) cooling between controlled steps 2 and 3.
Figure
2(b) illustrates a 3-step method that has a controlled, furnace ramping down
to achieve
the same effect as the isothermal 3rd step described earlier. Figure 2(c)
represents a
variation on 2(b) with a controlled temperature ramping up as step 1. In
Figure 2(d), a
variation on 2(a) is shown with a controlled interrupted cool down between
steps 1 and 2.
Similarly, Figure 2(e) depicts a variation on 2(b) with a full cool down
between steps 1
and 2 and a controlled, furnace ramping down to achieve the same effect as the
isothermal
3rd step described earlier. Figure 2(f) illustrates a variation on the 3 step
practice of 2(c)
above, but with a distinct, fully separated cooling (via air or cold water
quenching
"CWQ") between steps 2 and 3. Finally, representative Figure 2(g) shows still
another
variation on 2(f) with distinct, fully separated cooling (via air or cold
water quenching
"CWQ") between each of steps 1, 2 and 3. It is important to note that in each
of the
CA 02441168 2003-09-15
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foregoing aging examples, both Figures 1 and 2, that the latter stages of any
such practice
according to the invention can be performed either in or out of a forming die.
[0022] The following examples illustrate the relative TYS strength increases
observed in the practice of this invention on 7055 plate product. Samples of
0.75-inch
thick 7055 plate were given various combinations of frst- and second-step
aging
practices. [Note that when only a one step practice was supplemented per this
invention,
the data in Table 1 that follows actually lists a "1 st Step" time and
temperature as
"None". That, in effect, makes the Table 1 "2nd Step" so listed a 1st step of
two, which
is then followed by the 40-50°F lower, second (of two) steps or stages
per the present
invention.I Some of the Table 1 samples were given an additional aging step
for
performance comparison purposes. Those treated samples always list this added
step in
the "3rd Step" column of accompanying Table 1. But that step is meant to be
the second
of two, or third of three aging treatments, depending on whether a true 1 st
step aging was
performed thereon.
[0023] Tensile yield strength, electrical conductivity and exfoliation
coiTOSion
resistance (or "EXCO") values were measured for each Table 1 sample, the
latter EXCO
data per ASTM Standard No. G-34, the disclosure of which is incorporated
herein. With
respect to that table, it should be noted that electrical conductivity "EC"
serves as an
indicator of corrosion resistance, i.e., the higher the EC value measured (as
a % IACS
value), the more corrosion resistant that product ought to be. Ultrasonic
depth of attack
data gathered in conjunction with EXCO corrosion testing is also listed in
accompanying
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Table 1. A small (or shallow) depth of attack indicates improved corrosion
resistance. In
almost all cases, both strength and corrosion resistance improved with the
added aging
practice of this invention.
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[0024] One main means for evaluating the data of Table 1 is to compare
relative
sample strengths at a constant electrical conductivity EC value. Accompanying
Figures 3
through 7 facilitate such a comparison. At any given electrical conductivity
value, it was
noted from Figure 3 that TYS values ran about 1.5 ksi higher when another step
(the
second of two or third of three steps) was employed per the present invention.
An
alternative evaluation from Table 1/Figure 3 leads to another conclusion about
this
invention, namely that at a constant TYS value, relatively higher electrical
conductivity
values (and hence, relatively improved corrosion resistance performances) were
observed
per the added step or stage of this invention (again, the second of two or
third of three
steps).
[0025] Some of the data included in accompanying Table 1/Figure 3 was based on
tests performed after the filing of the U.S. provisional from which this
application claims
priority. In accompanying Figures 4 through 8, all of the foregoing
comparative data was
plotted for performing statistical analyses thereon using the quadratic
statistical
methodology commonly referred to as Analysis of Covariance (ANCOVA). The fit
for
this quadratic equation evaluation is summarized in the following Tables 2(a)
through (c):
Table 2a: Summary of Fit Quadratic Equation
Adusted R2 86.12%
Root Mean Square p.614
Error ksi
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2b: Analysis of Variance
Source DF Sum of SquaresMean SquareF Ratio
Model 3 96.926 32.309 85.829
Error 3 14.304 0.376 Prob
g > F
C. Total41 111.230 <.0001
2C: Parameter Estimates
Term Estimate Std Errort RatioProb>~t~
Intercept -633.1809 189.995 -3.33 0.0019
With 0.8392 0.099 8.46 <.0001
Invention Without-0.8392 0.099 8.46 <.0001
EC Slope 39.9710 10.135 3.94 0.0003
EC2 Slope -0.55335 0.135 -4.10 0.0002
Predicted TYS =-632.3417 + 39.9710~EC - 0.55335~EC2 With Invention
Predicted TYS =-634.0201 + 39.9710~EC - 0.55335~EC2 Without
TYS Increase due to Inv. 1.678 ksi over range of EC (36.0 to 39.2 % IACS)
[0026] The 95% confidence intervals for these quadratically predicted strength
versus EC curves, items A-A and B-B in Figure 4, were then drawn with dotted
lines in
that Figure. Statistically noteworthy from those two predicted curves, A-A
(and its 95%
band) for the Invention versus curve B-B for the known 1- and 2-step
comparative data
(and its 95% band) is the lack of overlap between 95% confidence bands. That
CA 02441168 2003-09-15
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distancing between quadratically calculated curves for flat 7055 plate product
further
evidences the IMPROVEMENT over the prior art observed through the practice of
this
invention.
[0027] Using the A-A and B-B curves of Figure 4, accompanying Figure 5 shows
the numerical increase in tensile yield strength (ksi) values predicted for
7055 Plate aged
by the invention over its knOWn (1- and 2-step aged) counterparts. Figure 6
predicts that
same improvement in strength as a function of electrical conductivity by
percentage
rather than in actual ksi values observed. The data supporting Figures 5 and 6
is found in
Table 3 that follows:
Table 3
Predicted Increase in Tensile Yield Strength due to Invention
Quadratic
Model
EC (%IACS) (Numerical (Percentage Increase)
ksi)
36 1.91
36.5 1.91
37 1.92
37.5 1.678 1.93
3g 1.96
38.5 1.98
39 2.02
[0028] Using electrical conductivity ("EC") as the standard for side-by-side
comparative statistical analyses, Figure 7 shows the numerical EC improvement
predicted
(in % IACS values) for the invention over its known (1- and 2-step aged)
counterparts.
Figure 8 predicts that same improvement in strength as a function of
electrical
16
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conductivity by percentage rather than in actual EC (% IACS) values observed.
Note that
for both Figures 7 and 8, EC increases could not be determined over the entire
range of
tensile yield strengths due to the mathematical consequence of inverting
quadratic
calculations. The data supporting Figures 7 and 8 is found in Table 4 that
follows:
Table 4
Predicted Increase in Electrical Conductivity due to Invention
Quadratic Model
TYS (ksi) (Numerical %IACS)(Percentage Increase)
85 0.595 1.55
85.5 0.642 1.68
86 0.703 1.85
86.5 0.787 2.09
87 0.913 2.45
87.5 1.152 3.13
87.8 (max) 1.663 4.59
[0029] In aerospace, marine, or other structural applications, it is customary
for
structural and materials engineers to select a material for a particular part
based on a
"weakest link" failure mode. For example, the upper wing alloy of a large
aircraft is
predominantly subjected to compressive stresses. There, then, stress corrosion
cracking
(or "SCC") resistance is not as big a design issue. As such, upper wing skin
alloys are
usually made from higher strength A1 alloys having relatively lower SCC
resistance
levels. Within that same wing box assembly, the spar members that get
subjected to
greater tensile stresses than compressive stresses. Such spar members are
traditionally
17
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made from more corrosion resistant but lower strength temper materials such as
those
aged by known T74-type practices.
[0030] Wing skins are typically made from thinner gauge plates as compared to
the
wing spars made from thick plate products. Thinner gauge plate products
possess thin,
narrow width grains brought about by greater rolling reductions. Such grains
tend to be
highly laminated. Unfortunately, corrosion induces delamination along these
grain
boundaries during service. Hence, resistance to exfoliation corrosion is an
important
requirement for the upper wing skins of today's larger aircrafts. As with SCC,
exfoliation
resistance improves with progressive overaging. This invention attempts to
maintain
exfoliation corrosion resistance performance while still managing to improve
strength
values, particularly those of a TYS variety. Alternately, this invention will
impart
improved exfoliation corrosion resistance performance at or about the same
strength
value levels.
[0031] While most of the data herein was performed on 7055 aluminum
(Aluminum Association designation), particularly that artificially aged per
known "T79"
practices, the method of this invention is also suitably practiced on still
other 7xxx or
7000 Series, aluminum aerospace alloys, including but not limited to: 7050,
7150, even
7075 aluminum. Restated, this invention would best be practiced on an aluminum
alloy
containing about 5 to 10 wt.% Zn, about 1 to 3 wt.% Mg and about 1 to 3 wt.%
Cu as its
main alloying constituents, with supporting elements, like Zr, Cr andlor Sc,
and grain
refining additives like Ti, B and/or C added thereto.
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CA 02441168 2003-09-15
WO 02/075010 PCT/US02/08538
[0032] It should be further noted that when the method of this invention
includes
adding a third aging step to a known two step aging practice, like "T79"
tempering, it is
not always necessary to practice the invention in separate, distinct stages.
In other words,
the method of this invention may just as easily be practiced on an aging
operation that
includes slowly ramping up, in a controlled manner, through one or more, f rst
stage
temperatures without any true stopping, or holding point. By gradually passing
through
the first "stage", one may still accomplish the effects of a first heat
treatment temperature
without really imposing a separately distinct furnace operation thereon.
[0033] Conversely, the same effect of this method may be achievable by slowly,
yet controllably, ramping down from the first of two, or second of three heat
treatment
steps/stages without having a purposeful cooling off period or quench (air,
cold water or
otherwise) thereafter. The same relative property improvements may be observed
ramping controllably down from the higher, preceding heat treatment (either
the first of
two; or second of three) stage and through the preferred added heat treatment
times and
temperatures of THIS invention ultimately achieving a total, cumulative effect
of 7000
Series aluminum alloy product exposure of about 225-275°F for about 3-
24 hours.
[0034] Having described the presently preferred embodiments, it is to be
understood that the invention may be otherwise embodied within the scope of
the
appended claims.
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