IMPROVED THICK WROUGHT 7XXX ALUMINUM ALLOYS, AND METHODS FOR MAKING THE SAME

Abstract

Disclosed are improved thick wrought 7xxx aluminum alloy products, and methods for producing the same. The new 7xxx aluminum alloy products may realize an improved combination of properties, such as an improved combination of two or more of environmentally assisted crack resistance, strength, elongation, and fracture toughness, among other properties. The new 7xxx aluminum alloy products may include 5.5-6.5 wt. % Zn, 1.3-1.7 wt. % Mg, and 1.7-2.3 wt. % Cu.

Claims

1. A wrought 7xxx aluminum alloy product comprising: 5.5-6.5 wt. % Zn; 1.3-1.7 wt. % Mg; 1.7-2.3 wt. % Cu; less than 0.15 wt. % Mn; up to 1.0 wt. % of grain structure control materials, wherein the grain structure control materials comprise at least one of Zr, Cr, Sc, and Hf; and up to 0.15 wt. % Ti; the balance being aluminum and unavoidable impurities; wherein the wrought 7xxx aluminum alloy product has a thickness of from 2.5 to 12 inches.

2. The wrought 7xxx aluminum alloy product of claim 1, wherein the wrought 7xxx aluminum alloy product includes not greater than 0.12 wt. % Mn.

3. The wrought 7xxx aluminum alloy product of claim 2, comprising from 0.05 to 0.15 wt. % Zr and not greater than 0.04 wt. % Mn.

4. The wrought 7xxx aluminum alloy product of claim 1, wherein the wrought 7xxx aluminum alloy product includes not greater than 6.4 wt. % Zn.

5. The wrought 7xxx aluminum alloy product of claim 4, wherein the wrought 7xxx aluminum alloy product includes at least 5.6 wt. % Zn.

6. The wrought 7xxx aluminum alloy product of claim 1, wherein the wrought 7xxx aluminum alloy product includes not greater than 2.25 wt. % Cu.

7. The wrought 7xxx aluminum alloy product of claim 6, wherein the wrought 7xxx aluminum alloy product includes at least 1.75 wt. % Cu.

8. The wrought 7xxx aluminum alloy product of claim 1, wherein the wrought 7xxx aluminum alloy product includes at least 1.35 wt. % Mg.

9. The wrought 7xxx aluminum alloy product of claim 8, wherein the wrought 7xxx aluminum alloy product includes not greater than 1.65 wt. % Mg.

10. The wrought 7xxx aluminum alloy product of claim 1, wherein the 7xxx aluminum alloy product realizes a typical L-S crack deviation resistance (K.sub.max-dev) of at least 25 ksi-sqrt-in.

11. The wrought 7xxx aluminum alloy product of claim 1, wherein the wrought aluminum alloy product passes HHSCC-G49 testing at 90° C. for 10 days.

12. An aerospace structural component made from the wrought 7xxx aluminum alloy product of claim 1.

13. A method for producing a wrought 7xxx aluminum alloy, the method comprising: (a) casting an alloy having the composition of claim 1 as an ingot or billet; (b) homogenizing the ingot or billet; (c) hot working the ingot or billet to an intermediate gauge product or final gauge product; (d) optionally cold working the intermediate gauge product into the final gauge product; (e) solution heat treating the final gauge product followed by quenching; (f) optionally stretching or compressing the solution heat treated and quenched product by 1-5%; (g) artificially aging the solution heat treated and quenched product.

14. The method of claim 13, wherein the homogenization temperature is at least T(homog.), wherein T(homog.) is calculated in degrees Fahrenheit from the formula 614.4+55.2*Cu+83.1*Mg−1.8*Zn, wherein the Cu, the Mg, and the Zn are the weight percent amounts of copper, magnesium and zinc, respectively, in the wrought 7xxx aluminum alloy.

15. The method of claim 13, wherein the artificial aging comprises first aging at a first aging temperature of from 200-300° F. followed by second aging at a second aging temperature of from 250-350° F., wherein the second aging temperature is at least 10° F. higher than the first aging temperature.

16. The method of claim 13, wherein the total equivalent artificial aging time is t(eq.), wherein t(eq.) is from 7 to 20 hours, wherein t(eq.) is calculated from the formula $t\left(\mathrm{eq}.\right)=\frac{\int \exp \left(-12800/T\right)\mathrm{dt}}{\exp \left(-12800/\mathrm{Tref}\right)}$ wherein T is the instantaneous temperature in ° K during the artificial aging, and wherein Tref is a reference temperature selected at 160° C. (433.15° K).

17. The method of claim 16, wherein t(eq) is not greater than 19 hours.

18. A rolled 7xxx aluminum alloy plate product comprising: 5.9-6.2 wt. % Zn; 1.4-1.7 wt. % Mg; 2.0-2.3 wt. % Cu; 0.05-0.15 wt. % Zr; up to 0.20 wt. % Cr; up to 0.15 wt. % Ti; and not greater than 0.04 wt. % Mn; the balance being aluminum and unavoidable impurities; wherein the rolled 7xxx aluminum alloy plate product has a thickness of from 3 to 12 inches; and wherein the rolled 7xxx aluminum alloy plate product realizes at least three of: (a) a typical tensile yield strength (ST) of at least 57 ksi; (b) a typical K.sub.IC plane-strain fracture toughness (S-L) of at least 20 ksi-sqrt-inch; (c) a typical elongation (ST) of at least 5%; and (d) passing of the HHSCC-G49 test at 90° C. for 15 days.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0049] FIG. 1 is a graph illustrating the strength versus K.sub.IC fracture toughness properties for the Example 1 alloys.

[0050] FIG. 2 is a graph illustrating the strength versus EAC resistance for the Example 1 alloys.

DETAILED DESCRIPTION

Example 1

[0051] Two aluminum alloys were cast as 6×18-inch (D×W) ingots, the compositions of which are provided in Table 1, below.

TABLE-US-00002 TABLE 1 Composition of Example 1 Alloys (wt. %) Alloy Si Fe Zn Mg Cu Zr Mn Cr Ti 1 0.05 0.05 5.88 1.60 2.17 0.10 0.02 0.02 0.02 2 0.02 0.06 5.98 1.50 2.12 0.10 — — 0.02
The ingots were then conventionally prepared for homogenization (e.g. by sawing and scalping). The first ingot was then processed to its final temper as per Japanese Patent No. H03-41540 (1991), Example 1, Alloy 4..sup.1 The second ingot was processed according to the inventive processes disclosed herein. .sup.1 Also published as JP01-290737 (1989).

[0052] Specifically, Alloy 1 was homogenized at 842° F. (450° C.) as per JPH03-41540. The alloy was then hot rolled to a final gauge of 1.75 inches (44.45 mm). Alloy 1 was then solution heat treated at 842° F. (450° C.) for 1 hour as per JPH03-41540, then quenched in 190° F. water (87.8° C.), and then stretched 1.5%. After stretching, Alloy 1 was artificially aged by first aging at 248° F. (120° C.) for 24 hours, heating to 302° F. and then aging at 302° F. (150° C.) for 24 hours as per JPH034-41540.

[0053] Alloy 2 was homogenized at 895° F. (479° C.) and then hot rolled to a final gauge of 1.75 inches (44.45 mm). Alloy 2 was then solution heat treated at 895° F. (479° C.) for 2 hours, quenched in 190° F. water (87.8° C.), and then stretched 2.25%. After stretching, some Alloy 2 was subjected to two different artificially aging practices: [0054] Practice 1: First aging at 250° F. (121° C.) for 6 hours, then heating to 320° F. (160° C.) and holding for 5.6 hours, air cooling to ambient, and then reheating to 250° C. (121° C.) and holding for 24 hours. [0055] Practice 2: First aging at 250° F. (121° C.) for 6 hours, then heating to 320° F. (160° C.) and holding for 9.75 hours, air cooling to ambient, and then reheating to 250° C. (121° C.) and holding for 24 hours.
The 190° F. quench temperature simulates the quench rate of the middle of a thick ingot (e.g., an eight-inch (203.2 mm) thick ingot).

[0056] Alloys 1-2 were metallographically examined and were found to be unrecrystallized, i.e., contained not greater than 45% recrystallized grains as determined using standard metallographic analysis procedures. In one embodiment, a new wrought 7xxx aluminum alloy product contains not greater than 35% recrystallized grains. In another embodiment, a new wrought 7xxx aluminum alloy product contains not greater than 25% recrystallized grains. In another embodiment, a new wrought 7xxx aluminum alloy product contains not greater than 15% recrystallized grains. In another embodiment, a new wrought 7xxx aluminum alloy product contains not greater than 5% recrystallized grains.

[0057] The alloys were then subjected to mechanical testing, the results of which are shown in Table 2, below. Test results relating to similarly produced conventional 7050 alloys are also provided, which results are from commonly-owned International Patent Application Publication No. WO2020/102441. Measurements are relative to the T/2 location for all alloys. Fracture toughness is relative to the S-L orientation.

TABLE-US-00003 TABLE 2 Mechanical Properties of Example 1 Alloys* Strength K.sub.IC Testing UTS TYS Elong. (ksi- Alloy Orientation (ksi) (ksi) (%) sqrt-in.)   1 L 76.8 70.2 12.1 —   1 ST 73.4 63.5 5.6 18.1   2 (AP1) L 78.0 72.1 12.6 —   2 (AP1) ST 75.5 65.8 9.4 20.7   2 (AP2) L 76.7 69.9 13.7 —   2 (AP2) ST 73.2 62.5 9.5 23.3 7050 L 72.3 61.6 11.7 30.7 (K.sub.Q) 7050 L 72.2 61.2 12.5 28.2 (K.sub.Q) 7050 ST 70.4 56.3 9.4 18.5 7050 ST 70.3 56.2 10.2 18.4 *AP1 = aging practice 1; AP2 = aging practice 2; sqrt = square root

[0058] The alloys were also subjected to EAC (environmentally assisted crack) resistance testing as per the HHSCC-G49 procedure provided above. Plant produced 7050-T7651 (3.9 inches thick) having a strength level similar to that of Alloys 1-2 was also tested. The HHSCC-G49 results are provided in Table 3, below.

TABLE-US-00004 TABLE 3 HHSCC-G49 Test Results Stress 90° C./85% RH (% Applied Days Days to failure TYS- stress in rep rep rep rep rep Alloy ST) (ksi) test 1 2 3 4 5 1 85 54 18 11 11 14 18 18 2 (API) 85 55.9 21 11 11 11 21 18 2 (AP2) 85 53.1 46 28 32 28 46 35 7050 85 53.0 55 28 55 55 19 41

[0059] As shown above and in FIGS. 1-2, Alloy 2 realizes an improved combination of properties over Alloy 1. As shown in FIG. 1, Alloy 2 realizes a much higher combination of strength and toughness over Alloy 1 and the conventional 7050 alloy. As shown in FIG. 2, Alloy 2 also realizes a much better combination of strength and EAC resistance over Alloy 1. Further, as shown in Table 2, the ST ductility of Alloy 2 is significantly higher than that of Alloy 1.

[0060] An analysis of the homogenization temperature for this alloy system was completed. It was determined that, for these particular alloys having 5.5-6.5 wt. % Zn, 1.3-1.7 wt. % Mg, and 1.7-2.3 wt. % Cu, the homogenization temperature should be at least as high as T(homog.), wherein T(homog.) is calculated in degrees Fahrenheit from the following formula:

T(homog.)=614.4+55.2*Cu+83.1*Mg−1.8*Zn

For the above formula, the Cu, the Mg, and the Zn are the weight percent amounts of copper, magnesium and zinc, respectively, in the wrought 7xxx aluminum alloy. The below table shows the calculation for Alloys 1 and 2.

TABLE-US-00005 TABLE 4 T(homog.) of Alloys 1-2 T(homog.) Alloy Zn Mg Cu (° F.) 1 5.88 1.60 2.17 856.5 2 5.98 1.50 2.12 845.3
As shown, the minimum homogenization temperature of Alloy 1 is 856.5° F. and the minimum homogenization temperature of Alloy 2 is 845.3° F.

[0061] Preferably, the homogenization temperature is higher than T(homog.) In one embodiment, the homogenization temperature is at least 5° F. higher than T(homog.), i.e., is ≥5° F+T(homog.). In another embodiment, the homogenization temperature is at least 10° F. higher than T(homog.), i.e., is ≥10° F+T(homog.). In yet another embodiment, the homogenization temperature is at least 15° F. higher than T(homog.), i.e., is ≥15° F+T(homog.). In another embodiment, the homogenization temperature is at least 20° F. higher than T(homog.), i.e., is ≥20° F+T(homog.). In yet another embodiment, the homogenization temperature is at least 25° F. higher than T(homog.), i.e., is ≥25° F+T(homog.). In another embodiment, the homogenization temperature is at least 30° F. higher than T(homog.), i.e., is ≥30° F+T(homog.). In yet another embodiment, the homogenization temperature is at least 35° F. higher than T(homog.), i.e., is ≥35° F+T(homog.). In another embodiment, the homogenization temperature is at least 40° F. higher than T(homog.), i.e., is ≥40° F+T(homog.). In yet another embodiment, the homogenization temperature is at least 45° F. higher than T(homog.), i.e., is ≥45° F+T(homog.). In another embodiment, the homogenization temperature is at least 50° F. higher than T(homog.), i.e., is ≥50° F+T(homog.). However, the homogenization temperature should be below the incipient melting temperature of the aluminum alloy. Preferably, the homogenization temperature is at least 10° F. below the incipient melting temperature of the aluminum alloy.

[0062] As it relates to solution heat treatment, all of the above teachings regarding homogenization apply equally to the solution heat treatment temperature. That is, the solution heat treatment temperature may be the same as T(homog.) and preferably is from 10-50° F. higher than T(homog.), as per above, but below the incipient melting temperature of the aluminum alloy, and preferably at least 10° F. below the incipient melting temperature of the aluminum alloy. Following solution heat treatment the alloy should be quenched in an appropriate medium, such as water or air. Preferably, the water is room temperature.

[0063] Based on the above data, an aging analysis was also completed. It was found that the alloys should be aged to a total equivalent aging time, t(eq.), of from 7 to 20 hours, the total equivalent artificial aging time being:

[00001]$t\left(\mathrm{eq}.\right)=\frac{\int \exp \left(-12800/T\right)\mathrm{dt}}{\exp \left(-12800/\mathrm{Tref}\right)}$

In the above formula, T is the instantaneous temperature in Kelvin (K) during the artificial aging, and Tref is a reference temperature selected at 160° C. (433.15K). The t(eq.) for Alloys 1-2 are shown in the below table.

TABLE-US-00006 TABLE 5 t(eq.) of Alloys 1-2 t(eq.) Alloy (hours) 1 14.58 2-AP1 10.57 2-AP2 14.57

[0064] As shown, both Alloy 1 and Alloy 2-AP2 were aged to generally the same total equivalent aging time. However, the aging practice of Alloy 2 is superior, at least partially contributing to its significantly improved properties. Accordingly, in one embodiment, t(eq.) is from 7 to 19 hours. In another embodiment, t(eq.) is from 7 to 18 hours. In yet another embodiment, t(eq.) is from 7 to 17 hours. In another embodiment, t(eq.) is from 7 to 16 hours. In yet another embodiment, t(eq.) is from 7 to 15 hours. In another embodiment, t(eq.) is from 7 to 14 hours. In yet another embodiment, t(eq.) is from 7 to 13.5 hours. In another embodiment, t(eq.) is from 7 to 13 hours. In yet another embodiment, t(eq.) is from 7 to 12.5 hours. In another embodiment, t(eq.) is from 7 to 12 hours. In yet another embodiment, t(eq.) is from 7 to 11.5 hours. In another embodiment, t(eq.) is from 7 to 11 hours.

[0065] It is believed that both a two-step and a three-step aging practice may be used with the presently disclosed wrought 7xxx aluminum alloys provided the proper homogenization and solution heat treatment practices are followed. Thus, in one embodiment, the artificial aging comprises first aging at a first aging temperature of from 200-300° F. followed by second aging at a second aging temperature of from 250-350° F., wherein the second aging temperature is at least 10° F. higher than the first aging temperature. In one embodiment, the second aging temperature is at least 20° F. higher than the first aging temperature. In another embodiment, the second aging temperature is at least 30° F. higher than the first aging temperature. In yet another embodiment, the second aging temperature is at least 40° F. higher than the first aging temperature. In another embodiment, the second aging temperature is at least 50° F. higher than the first aging temperature. In yet another embodiment, the second aging temperature is at least 60° F. higher than the first aging temperature. In another embodiment, the second aging temperature is at least 70° F. higher than the first aging temperature.

[0066] In one embodiment, the first aging temperature is not greater than 280° F. In another embodiment, the first aging temperature is not greater than 270° F. In yet another embodiment, the first aging temperature is not greater than 260° F. In another embodiment, the first aging temperature is not greater than 250° F. Multiple aging temperatures may be used within the first aging temperature range provided t(eq) is achieved.

[0067] In one embodiment, the second aging temperature is at least 305° F. In another embodiment, the second aging temperature is at least 310° F. In yet another embodiment, the second aging temperature is at least 315° F. In another embodiment, the second aging temperature is at least 320° F. Multiple aging temperatures may be used within the second aging temperature range provided t(eq) is achieved. After the second aging step, the product may be cooled to room temperature.

[0068] When a third aging step is used, it follows the second aging step. In one approach, the third aging step is similar or the same as the first aging step, such as by using an aging temperature of from 200-300° F. Multiple aging temperatures may be used within the third aging temperature range provided t(eq) is achieved. In one embodiment, the third aging temperature is at least 10° F. lower than second aging temperature. In another embodiment, the third aging temperature is at least 20° F. lower than second aging temperature. In yet another embodiment, the third aging temperature is at least 30° F. lower than second aging temperature. In another embodiment, the third aging temperature is at least 40° F. lower than second aging temperature. In yet another embodiment, the third aging temperature is at least 50° F. lower than second aging temperature. In another embodiment, the third aging temperature is at least 60° F. lower than second aging temperature. In yet another embodiment, the third aging temperature is at least 70° F. lower than second aging temperature.

[0069] In one embodiment, the third aging temperature is not greater than 280° F. In another embodiment, the third aging temperature is not greater than 270° F. In yet another embodiment, the third aging temperature is not greater than 260° F. In another embodiment, the third aging temperature is not greater than 250° F. Multiple aging temperatures may be used within the third aging temperature range provided t(eq) is achieved.

[0070] While various embodiments of the present disclosure have been described in detail, it is apparent that modifications and adaptations of those embodiments will occur to those skilled in the art. However, it is to be expressly understood that such modifications and adaptations are within the spirit and scope of the present disclosure.