EP1373591A2

EP1373591A2 - Method for aging 7000 series aluminium

Info

Publication number: EP1373591A2
Application number: EP02713880A
Authority: EP
Inventors: Diana K. Alcoa Technical Center DENZER; Dhruba J. Alcoa Technical Center CHAKRABARTI; John Alcoa Technical Center LIU; LYNN E. Alcoa Technical Center OSWALD; Robert W. Alcoa Inc. WESTERLUND
Original assignee: Alcoa Inc
Current assignee: Howmet Aerospace Inc
Priority date: 2001-03-20
Filing date: 2002-03-20
Publication date: 2004-01-02
Also published as: CN1531603A; CA2441168A1; WO2002075010A2; US20030051784A1; WO2002075010A3; AU2002245705A1; BR0208271A

Abstract

This invention relates to a method for artificially aging 7000 Series Al 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° F for about 3-24 hours. More preferably, the added stage extends at about 250° F for about 6 hours or more.

Description

METHOD FOR INCREASING THE STRENGTH AND/OR

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 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°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. 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. 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 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 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] 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 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 3^r(^ 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

3^r(l 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 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 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^st Step" time and temperature as

"None". That, in effect, makes the Table 1 "2^nd Step" so listed a 1^st step of two, which

is then followed by the 40-50°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^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^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.

Table 1

Effect of Invention (added Aging Practice) on Strength & Exfoliation Resistance

7055, 0.75 inch plate at T/2

Table 1 (continued)

[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

2b: Analysis of Variance

2C: Parameter Estimates

Predicted TYS = -632.3417 + 39.9710-EC - 0.55335ΕC² With Invention

Predicted TYS = -634.0201 + 39.9710ΕC - 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 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

[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 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

[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 Al 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 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 and/or Sc, and grain

refining additives like Ti, B and/or C added thereto. [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, 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.

[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.

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°F for about 2-

30 hours, said method comprising:

(a) performing an additional aging equivalent to about 225-275°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°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°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 ranips 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°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°F, said method

comprising:

(a) performing an additional aging equivalent to about 225-275°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°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°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°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°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°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°F for about 2-30 hours, and

(b) performing an additional aging on said plate product equivalent to about

225-275°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 wherem 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°F.

34. The method of claim 30 wherein step (a) includes heating the plate

product between about 295-310°F for about 4-18 hours.

35. A method for improving the strength and/or corrosion resistance

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°F

for at least about 6 hours;.

(b) artificially aging said plate product at one or more temperatures between

about 290-330°F for about 2-30 hours, and

(c) performing a further artificial aging on said plate product equivalent to

about 225-275°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°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°F.