U.S. patent application number 10/103273 was filed with the patent office on 2003-03-20 for method for increasing the strength and/or corrosion resistance of 7000 series al aerospace alloy products.
Invention is credited to Chakrabarti, Dhruba J., Denzer, Diana K., Liu, John, Oswald, Lynn E., Westerlund, Robert W..
Application Number | 20030051784 10/103273 |
Document ID | / |
Family ID | 23060708 |
Filed Date | 2003-03-20 |
United States Patent
Application |
20030051784 |
Kind Code |
A1 |
Denzer, Diana K. ; et
al. |
March 20, 2003 |
Method for increasing the strength and/or corrosion resistance of
7000 series Al aerospace alloy products
Abstract
This invention relates to a method for artificially aging 7000
Series A1 aerospace alloys to impart improved strength and/or
corrosion resistance performance thereto. The method purposefully
adds a second aging step or stage to a one-step tempering, or a
third step/stage to a low-high, two-step aging operation. The added
step/stage extends at about 225-275.degree. F. for about 3-24
hours. More preferably, the added stage extends at about
250.degree. F. for about 6 hours or more.
Inventors: |
Denzer, Diana K.; (Lower
Burrell, PA) ; Chakrabarti, Dhruba J.; (Export,
PA) ; Liu, John; (Murrysville, PA) ; Oswald,
Lynn E.; (Harrison City, PA) ; Westerlund, Robert
W.; (Bettendorf, IA) |
Correspondence
Address: |
SKJERVEN MORRILL
MACPHERSON LLP
25 METRO DRIVE
SUITE 700
SAN JOSE
CA
95110
US
|
Family ID: |
23060708 |
Appl. No.: |
10/103273 |
Filed: |
March 20, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60277403 |
Mar 20, 2001 |
|
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|
Current U.S.
Class: |
148/698 |
Current CPC
Class: |
C22F 1/053 20130101 |
Class at
Publication: |
148/698 |
International
Class: |
C22F 001/04 |
Claims
What is claimed is:
1. A method for imparting improved strength at about the same
corrosion resistance performance level to a 7000 Series, aluminum
aerospace alloy product that has been artificially aged at one or
more temperatures between about 290-330.degree. F. for about 2-30
hours, said method comprising: (a) performing an additional aging
equivalent to about 225-275.degree. F. for about 3-24 hours after
the preceding, higher temperature artificial aging.
2. The method of claim 1 wherein said preceding, artificial aging
includes heating the alloy product between about 295-310.degree. F.
for about 4-18 hours.
3. The method of claim 1 wherein said preceding, artificial aging
includes a typical "T79" tempering.
4. The method of claim 1 wherein said preceding, artificial aging
is, itself, preceded by a first heat treatment at about
225-275.degree. F. for about 3 -28 hours.
5. The method of claim 4 wherein said first heat treatment is
followed by an air or cold water quenching.
6. The method of claim 4 wherein said first heat treatment ramps up
controllably through the artificial aging that precedes step (a)
above.
7. The method of claim 1 wherein step (a) includes heating the
alloy product for at least about 6 hours at about 250.degree.
F.
8. The method of claim 1 wherein step (a) is preceded by an air or
cold water quenching.
9. The method of claim 1 wherein said preceding, artificial aging
ramps down controllably through additional aging step (a).
10. The method of claim 1 wherein said alloy product is sheet or
plate.
11. The method of claim 1 wherein said alloy product is an
aerospace extrusion.
12. The method of claim 1 wherein said 7000 Series alloy is 7055
aluminum (Aluminum Association designation).
13. The method of claim 1 wherein step (a) is performed with the
alloy product in a forming die.
14. A method for imparting improved corrosion resistance
performance at about the same strength level to a 7000 Series,
aluminum aerospace alloy product artificially aged at one or more
temperatures between about 290-330.degree. F., said method
comprising: (a) performing an additional aging equivalent to about
225-275.degree. F. for about 3-24 hours after the preceding, higher
temperature artificial aging.
15. The method of claim 14 wherein the preceding, artificial aging
includes heating the alloy product between about 295-310.degree. F.
for about 4-18 hours.
16. The method of claim 14 wherein the preceding, artificial aging
is, itself, preceded by a first heat treatment at about
225-275.degree. F. for about 4-28 hours.
17. The method of claim 16 wherein said first heat treatment ramps
up controllably through the higher temperature, artificial aging
that follows it.
18. The method of claim 14 wherein step (a) includes heating the
alloy product for at least about 6 hours at about 250.degree.
F.
19. The method of claim 14 wherein said higher temperature,
artificial aging step ramps gradually down and through said
additional aging step (a).
20. The method of claim 14 whrein the preceding high temperature,
artificial aging ramps down controllably through step (a).
21. The method of claim 14 wherein said 7000 Series alloy contains
about 5-10 wt. % Zn, about 1-3 wt. % Mg and about 1-3 wt. % Cu.
22. The method of claim 21 wherein said 7000 Series alloy is 7055
aluminum (Aluminum Association designation).
23. The method of claim 14 wherein step (a) is performed in a
forming die.
24. In a method for artificially aging a 7000 Series aluminum
aerospace alloy product to a "T79" type temper, the improvement for
increasing the yield strength and/or corrosion resistance
performance of said alloy comprises: (a) performing an artificial
aging equivalent to about 225-275.degree. F. for about 3-24 hours
after the last T79 type tempering step.
25. The improvement of claim 24 wherein step (a) includes heating
the alloy product for at least about 6 hours at about 250.degree.
F.
26. The improvement of claim 24 wherein step (a) is affected by
controllably ramping down from the last T79 type tempering
step.
27. The improvement of claim 24 wherein said alloy product is sheet
or plate.
28. The improvement of claim 27 wherein said alloy product is an
aircraft wing component.
29. The improvement of claim 24 wherein said alloy product is made
from 7055 aluminum (Aluminum Association designation).
30. A method for improving the strength and/or corrosion resistance
performance of a 7000 Series aluminum alloy plate product
containing about 5-10 wt. % Zn, about 1-3 wt. % Mg and about 1-3
wt. % Cu, said method comprising: (a) artificially aging said plate
product at one or more temperatures between about 290-330.degree.
F. for about 2-30 hours, and (b) performing an additional aging on
said plate product equivalent to about 225-275.degree. F. for about
3-24 hours.
31. The method of claim 30 wherein said 7000 Series alloy is 7055
aluminum (Aluminum Association designation).
32. The method of claim 30 wherein step (b) is performed in a
forming die.
33. The method of claim 30 wherein step (b) includes heating the
plate product for at least about 6 hours at about 250.degree.
F.
34. The method of claim 30 wherein step (a) includes heating the
plate product between about 295-310.degree. F. for about 4-18
hours.
35. A method for improving the strength and/or corrosion resistance
performance of a 7000 Series aluminum alloy plate product
containing about 5-10 wt.% Zn, about 1-3 wt. % Mg and about 1-3 wt.
% Cu, said method comprising: (c) artificially aging said plate
product to an equivalent of about 225-275.degree. F. for at least
about 6 hours; (b) artificially aging said plate product at one or
more temperatures between about 290-330.degree. F. for about 2-30
hours, and (c) performing a further artificial aging on said plate
product equivalent to about 225-275.degree. F. for about 3-24
hours.
36. The method of claim 35 wherein said 7000 Series alloy is 7055
aluminum (Aluminum Association designation).
37. The method of claim 35 wherein step (b) includes heating the
plate product between about 295-310.degree. F. for about 4-18
hours.
38. The method of claim 35 wherein step (c) is performed in a
forming die.
39. The method of claim 35 wherein step (c) includes heating the
plate product for at least about 6 hours at about 250.degree. F.
Description
RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Patent Application Ser. No. 60/277,403 filed on Mar. 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 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
during 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 not 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-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.degree. F. for about 3 to 12 hours. The
second step of that two-step practice targets one or more
temperatures between about 280-350.degree. 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.degree. 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 averaging. 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. Among these are the prior art
disclosures of U.S. Pat. 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.degree. F. with a second step
above about 350 or 360.degree. F. That second step is then followed
by a third step similar to their first step temperature of about
250.degree. 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. Pat. 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.degree. 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.degree.+ 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.degree. F. for about 3-24 hours, or more
preferably at about 250.degree. F. for about 6 hours or more. 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.degree. F.
for about 3-24 hours, or more preferably at about 250.degree. 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.degree. F. for about 3-24 hours,
or more preferably at about 250.degree. 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
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] FIGS. 1(a) through (c) are graphic representations of three,
2-step aging schemes according to the invention;
[0013] FIGS. 2(a) through (g) are graphic representations of seven
representative 3-step aging schemes according to the invention;
[0014] FIG. 3 is a graph depicting the relative improvement in
strength, particularly longitudinal tensile yield strength (TYS),
versus electrical conductivity (in % IACS) both measured at T/2 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
purposefully added to the aforesaid known practices (shown with
solid circular data points);
[0015] FIG. 4 is the same graph of FIG. 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] FIG. 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 FIG. 4 above;
[0017] FIG. 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] FIG. 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] FIG. 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 FIGS. 1 and 2. Particularly,
FIGS. 1(a) through (c) are 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 FIG. 1(b), there is shown a
representative 2-step method that has a controlled, furnace ramping
down between first and second steps/stages. Finally, FIG. 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] FIGS. 2(a) through (g) are graphic representations of seven
representative 3-step aging schemes according to the invention. In
FIG. 2(a), a 3-step or staged method is shown with a partial (air)
cooling between controlled steps 2 and 3. FIG. 2(b) illustrates a
3-step method that has a controlled, furnace ramping down to
achieve the same effect as the isothermal 3.sup.rd step described
earlier. FIG. 2(c) represents a variation on 2(b) with a controlled
temperature ramping up as step 1. In FIG. 2(d), a variation on 2(a)
is shown with a controlled interrupted cool down between steps 1
and 2. Similarly, FIG. 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 3.sup.rd step
described earlier. FIG. 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 FIG. 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 foregoing aging examples,
both FIGS. 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 first- 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.sup.st Step"
time and temperature as "None". That, in effect, makes the Table 1
"2.sup.nd Step" so listed a 1.sup.st step of two, which is then
followed by the 40-50.degree. F. lower, second (of two) steps or
stages per the present invention.] 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 "3.sup.rd
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.sup.st step aging was performed thereon.
[0023] Tensile yield strength, electrical conductivity and
exfoliation corrosion 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 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.
1TABLE 1 Effect of Invention (added Aging Practice) on Strength
& Exfoliation Resistance 7055, 0.75 inch plate at T/2 EXCO Avg.
Depth of .DELTA. TYS (ksi) Identification Long. Attack with
invention step of 1.sup.st Step 2.sup.nd Step 3.sup.rd Step EC TYS
After 48 hrs Visual Rating minus w/o invention Experiments .degree.
F./hr .degree. F./hr .degree. F./hr (% IACS) (ksi) (inch) after 48
hours step 1 250/3 300/10 None 36.3 88.0 0.0090 EC 1A 250/3 300/10
250/6 36.7 89.1 0.0083 EC 1.1 2 250/3 300/17.5 None 37.4 87.2
0.0037 EB 2A 250/3 300/17.5 250/6 37.6 88.0 0.0047 EC 0.8 3 250/3
310/5.5 None 36.0 87.6 0.0063 EC 3A 250/3 310/5.5 250/6 36.4 89.9
0.0057 EC 2.3 4 250/3 310/9.6 None 37.5 86.4 0.0030 EB 4A 250/3
310/9.6 250/6 38.2 87.7 0.0023 EB 1.3 5 None 300/10 None 36.2 87.3
0.0040 EC 5A None 300/10 250/6 36.7 88.4 0.0060 EC 1.1 6 None
300/17.5 None 37.8 86.5 0.0023 EB 6A None 300/17.5 250/6 37.9 87.3
0.0017 EB 0.8 7 None 310/5.5 None 36.6 87.1 0.0030 EC 7A None
310/5.5 250/6 36.3 88.4 0.0063 EC 1.3 8 None 310/9.6 None 38.0 84.8
0.0003 EA 8A None 310/9.6 250/6 38.0 86.9 0.0030 EB 2.1 9 None
300/14 None 37.4 86.9 0.0043 EC 9A None 300/14 250/6 38.1 87.9
0.0027 EC 1.0 10 None 305/16 None 38.6 84.4 0.0030 EB 10A None
305/16 250/6 39.2 84.0 0.0027 EB -0.4 11 250/3 305/14 None 37.9
86.0 0.0030 EC 11A 250/3 305/14 250/6 38.5 86.5 0.0027 EB 0.5 12
250/3 310/14 None 38.5 84.4 0.0020 EB 12A 250/3 310/14 250/6 39.1
85.0 0.0017 EB 0.6 13 250/3 302/10 None 37.0 88.2 0.0057 EC 13A
250/3 302/10 250/3 37.0 89.5 0.0080 EC 1.3 13 250/3 302/10 None
37.0 88.2 0.0057 EC 14A 250/3 302/10 250/6 37.0 89.5 0.0057 EC 1.3
13 250/3 302/10 None 37.0 88.2 0.0057 EC 15A 250/3 302/10 250/12
37.4 89.3 0.0070 EC 1.1 13 250/3 302/10 None 37.0 88.2 0.0057 EC
16A 250/3 302/10 250/24 36.7 90.3 0.0070 EC 2.1 17 250/6 302/10
None 36.4 88.9 0.0070 EC 17A 250/6 302/10 250/6 36.6 90.2 0.0077 EC
1.3 17 250/6 302/10 None 36.4 88.9 0.0070 EC 18A 250/6 302/10
250/24 36.8 90.0 0.0070 EC 1.1 19 None 302/10 None 36.2 88.0 0.0057
EC 19A None 302/10 250/3 36.8 88.9 0.0083 EC 0.9 19 None 302/10
None 36.2 88.0 0.0057 EC 20A None 302/10 250/6 36.6 89.5 0.0080 EC
1.5 19 None 302/10 None 36.2 88.0 0.0057 EC 21A None 302/10 250/12
37.4 88.3 0.0063 EC 0.3 19 None 302/10 None 36.2 88.0 0.0057 EC 22A
None 302/10 250/24 36.9 89.2 0.0077 EC 0.2 19 None 302/10 None 36.2
88.0 0.0057 EC 23A None 302/10 275/3 36.5 88.8 0.0057 EC 0.8 19
None 302/10 None 36.2 88.0 0.0057 EC 24A None 302/10 275/6 37.0
88.3 0.0070 EC 0.3 19 None 302/10 None 36.2 88.0 0.0057 EC 25A None
302/10 275/12 37.0 87.7 0.0063 EC -0.3 19 None 302/10 None 36.2
88.0 0.0057 EC 26A None 302/10 225/6 36.5 89.3 0.0083 EC 1.3 19
None 302/10 None 36.2 88.0 0.0057 EC 27A None 302/10 225/24 37.1
89.3 0.0073 EC 1.3
[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 FIGS. 3 through 7 facilitate
such a comparison. At any given electrical conductivity value, it
was noted from FIG. 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/FIG. 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/FIG. 3 was
based on tests performed after the filing of the U.S. provisional
from which this application claims priority. In accompanying FIGS.
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):
2TABLE 2a Summary of Fit Quadratic Equation Adusted R.sup.2 86.12%
Root Mean Square Error 0.614 ksi
[0026]
3 2b: Analysis of Variance Source DF Sum of Squares Mean Square F
Ratio Model 3 96.926 32.309 85.829 Error 38 14.304 0.376 Prob >
F C.Total 41 111.230 <.0001
[0027]
4 2C: Parameter Estimates Term Estimate Std Error t Ratio Prob >
.vertline.t.vertline. Intercept -633.1809 189.995 -3.33 0.0019
Invention With 0.8392 0.099 8.46 <.0001 Without -0.8392 0.099
8.46 <.0001 EC Slope 39.9710 10.135 3.94 0.0003 EC.sup.2 Slope
-0.55335 0.135 -4.10 0.0002
[0028] Predicted
TYS=-632.3417+39.9710.multidot.EC-0.55335.multidot.EC.sup- .2 With
Invention
[0029] Predicted
TYS=-634.0201+39.9710.multidot.EC-0.55335.multidot.EC.sup- .2
Without
[0030] TYS Increase due to Inv. 1.678 ksi over range of EC (36.0 to
39.2% IACS)
[0031] The 95% confidence intervals for these quadratically
predicted strength versus EC curves, items A-A and B-B in FIG. 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 distancing between quadrically
calculated curves for flat 7055 plate product further evidences the
IMPROVEMENT over the prior art observed through the practice of
this invention.
[0032] Using the A-A and B-B curves of FIG. 4, accompanying FIG. 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. FIG. 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 FIGS. 5 and 6 is found in Table 3 that follows:
5TABLE 3 Predicted Increase in Tensile Yield Strength due to
Invention Quadratic Model EC (% IACS) (Numerical ksi) (Percentage
Increase) 36 1.91 36.5 1.91 37 1.92 37.5 1.678 1.93 38 1.96 38.5
1.98 39 2.02
[0033] Using electrical conductivity ("EC") as the standard for
side-by-side comparative statistical analyses, FIG. 7 shows the
numerical EC improvement predicted (in % IACS values) for the
invention over its known (1- and 2-step aged) counterparts. FIG. 8
predicts that same improvement in strength as a function of
electrical conductivity by percentage rather than in actual EC (%
IACS) values observed. Note that for both FIGS 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 FIGS. 7 and 8 is found
in Table 4 that follows:
6TABLE 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
[0034] 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 Al alloys having relatively lower SCC resistance
levels. Within that same wing box assembly, the spar members that
get subjected to greater tensile stress than compressive stresses.
Such spar members are traditionally made from more corrosion
resistant but lower strength temper materials such as those aged by
known T74-type practices.
[0035] 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.
[0036] 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 and/or
Sc, and grain refining additives like Ti, B and/or C added
thereto.
[0037] 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, first 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.
[0038] 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.degree. F. for about 3-24 hours.
[0039] 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.
* * * * *