Methods of preparing 7xxx aluminum alloys for adhesive bonding, and products relating to the same

Abstract

Methods of preparing 7xxx aluminum alloy products for adhesive bonding and products made therefrom are disclosed. Generally, the methods include preparing a 7xxx aluminum alloy product for anodizing, then anodizing the 7xxx aluminum alloy product, and then contacting the anodized 7xxx aluminum alloy product with an appropriate chemical to create a functionalized layer. The new 7xxx aluminum alloy products may realize improved shear bonding performance.

Claims

1. A method comprising: (a) preparing a 7xxx aluminum alloy product for anodizing, wherein the 7xxx aluminum alloy product comprises an oxide layer on a base, and wherein the preparing step (a) comprises: (i) cleaning surfaces of the 7xxx aluminum alloy product; (ii) after the cleaning step (a)(i), exposing the 7xxx aluminum alloy product to a caustic; (iii) after the exposing step (a)(ii), contacting the 7xxx aluminum alloy product with an acid; and (iv) rinsing the 7xxx aluminum alloy product with water; wherein, due to the preparing step at least some of the oxide layer is removed and a prepared oxide layer is produced on the base, wherein, due to steps (a)(i)-(a)(iv), a prepared 7xxx aluminum alloy product is produced; (b) anodizing the prepared 7xxx aluminum alloy product in an acidic electrolyte solution and for a time sufficient to produce an anodic oxide layer, thereby producing an anodized 7xxx aluminum alloy product; (i) wherein a total thickness of the prepared oxide layer plus the anodic oxide layer is from 15 to 150 nanometers; and (c) after the anodizing step (b), contacting the anodized 7xxx aluminum alloy product with a phosphorous-containing organic acid to create a functional layer on the anodic oxide layer of the anodized 7xxx aluminum alloy product, wherein the contacting is selected from a group consisting of spraying, immersion, roll coating, and combinations thereof.

2. The method of claim 1, wherein the base of the 7xxx aluminum alloy product comprises 2-12 wt. % Zn, 1-3 wt. % Mg, and 0-3 wt. % Cu.

3. The method of claim 2 comprising, after the contacting step (c), bonding at least a portion of the anodized 7xxx aluminum alloy product with a second material, thereby creating an as-bonded 7xxx aluminum alloy product.

4. The method of claim 1, wherein, when the anodized 7xxx aluminum alloy product is bonded so as to form a single-lap-joint specimen having a joint overlap of 0.5 inches, the as-bonded 7xxx aluminum alloy product achieves completion of 45 stress durability test (SDT) cycles according to ASTM D1002 (10).

5. The method of claim 4, wherein a residual shear strength of the single-lap-joint specimen after completing the 45 SDT cycles is at least 80% of an initial shear strength of the single-lap-joint specimen.

6. The method of claim 4, wherein a residual shear strength of the single-lap-joint specimen after completing the 45 SDT cycles is at least 85% of an initial shear strength of the single-lap-joint specimen.

7. The method of claim 4, wherein a residual shear strength of the single-lap-joint specimen after completing the 45 SDT cycles is at least 90% of an initial shear strength of the single-lap-joint specimen.

8. The method of claim 1, wherein the anodizing step (b) comprises applying a current to the acidic electrolyte solution at a current density sufficient and for a time sufficient to produce the anodic oxide layer, wherein a total thickness of the prepared oxide layer plus the anodic oxide layer is from 20 to 125 nanometers.

9. The method of claim 8, wherein the duration of the applying step is not greater than 120 seconds.

10. The method of claim 8, wherein the acidic electrolyte solution is selected from the group consisting of sulfuric acid, phosphoric acid, chromic acid, and oxalic acid.

11. The method of claim 10, wherein the anodizing acidic electrolyte solution is a 10-20 wt. % sulfuric acid solution.

12. The method of claim 11, wherein the current density is from 5 to 20 amperes per square foot (ASF).

13. The method of claim 10, wherein the anodizing acidic electrolyte solution comprises 5-20 wt. % phosphoric acid.

14. The method of claim 1, wherein the anodizing step (b) comprises applying a current to the acidic electrolyte solution at a current density sufficient and for a time sufficient to produce the anodic oxide layer, wherein a total thickness of the prepared oxide layer plus the anodic oxide layer is from 30 to 100 nanometers.

15. The method of claim 14, wherein the duration of the applying step is from 10 to 60 seconds.

16. The method of claim 1, wherein the anodic oxide layer consist essentially of aluminum oxide.

17. The method of claim 1, wherein a surface of the anodic oxide layer comprises pits.

18. The method of claim 1, wherein the preparing step (a) comprises removing at least some intermetallic particles from a surface of the 7xxx aluminum alloy product.

19. The method of claim 18, wherein the intermetallic particles comprise copper-bearing intermetallic particles.

20. The method of claim 1, wherein a total thickness of the prepared oxide layer plus the anodic oxide layer is not greater than 80 nanometers.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a cross-sectional schematic view of an 7xxx aluminum alloy product (1) (e.g., an as-received 7xxx aluminum alloy product) having a base (10) and surface oxides thereon (20) (not to scale; for illustration purposes only).

(2) FIG. 2 is a flow chart illustrating one embodiment of a method for producing 7xxx aluminum alloy products in accordance with the present disclosure.

(3) FIG. 3 is a flow chart illustrating one embodiment of the preparing step (200) of FIG. 2.

(4) FIG. 4 is a cross-sectional schematic view of a prepared 7xxx aluminum alloy product (1) having a base (10) with prepared surface oxides (30) thereon (not to scale; for illustration purposes only).

(5) FIG. 5 is a flow chart illustrating one embodiment of the anodizing step (300) of FIG. 2.

(6) FIG. 6 is a cross-sectional schematic view of a prepared and anodized 7xxx aluminum alloy product (1) having a base (10) with prepared surface oxides (30) and anodic oxides (40) thereon (not to scale; for illustration purposes only).

(7) FIG. 7 is a flow chart illustrating one embodiment of the creating step (400) of FIG. 2.

(8) FIG. 8A is a diagram illustrating a representative chemical bond structure of an as-functionalized 7xxx aluminum alloy product following the creating step (400) of FIG. 2.

(9) FIGS. 8B and 8C are diagrams illustrating chemical bond structures of a phosphoric acid anodizing 7xxx aluminum alloy product.

(10) FIG. 9 is a plot of X-ray photoelectron spectroscopy (XPS) oxide structure analysis results of a 7xxx aluminum alloy product treated according to one embodiment of the disclosure.

(11) FIG. 10 is a scanning electron micrograph (SEM) image of the surface topography of the 7xxx aluminum alloy product of FIG. 9.

DETAILED DESCRIPTION

Example 1

(12) Several samples of a 7xxx aluminum alloy (Al—Zn—Mg—Cu style) product were received and prepared as per step (200) of FIG. 2, above. After the preparing step (200) a native oxide layer (4-6 nm thick) was present on the surface of the sample. The 7xxx aluminum alloy products were not anodized, but, instead, were simply subjected to the creating step (400), as per FIG. 2, and in accordance with U.S. Pat. No. 6,167,609 to Marinelli et al. After the creating step, the samples were sequentially bonded and then subjected to an industry standard cyclical corrosion exposure test, similar to ASTM D1002, which continuously exposes the samples to 1080 psi lap shear stresses to test bond durability. All samples failed to complete the required 45 cycles in the bond durability test.

Example 2

(13) Several samples of a 7xxx aluminum alloy (Al—Zn—Mg—Cu style) were processed as per FIG. 2. The alloys were all anodized in a 15 wt. % sulfuric acid solution at 70° F. and 6 ASF for 10, 45, or 60 seconds. After anodizing, a functional layer was then created (400), per FIG. 2 and in accordance with U.S. Pat. No. 6,167,609 to Marinelli et al., on each of the materials, after which the materials were sequentially bonded and then subjected to an industry standard cyclical corrosion exposure test, similar to ASTM D1002.

(14) The samples anodized for 60 seconds successfully completed the required 45 cycles and produced retained lap shear strengths of 7253, 6600, 6851 and 7045 psi in the four replicate specimens (6937 psi, ave., with a stdev (σ) of 278 psi). These residual shear strength results are superior to the typical range of 4500-6000 psi typically observed for adhesively bonded 5xxx and 6xxx alloys prepared by another conventional industry practice. The four residual shear strength results are also consistent, as indicated by the low standard deviation. The samples anodized for only 10 or 45 seconds at 6 ASF did not successfully complete the bond durability testing. Only two of the 45 second anodized samples survived the 45 cycles, and none of the 10 second anodized samples survived the 45 cycle requirement.

(15) As a baseline, four of the same alloy samples were prepared similarly to above, but were held for 60 seconds in the 15 wt. % sulfuric acid anodizing bath at 70° F., without any current applied. The same functional layer was then created (400), per FIG. 2 and in accordance with U.S. Pat. No. 6,167,609 to Marinelli et al., on each of the materials, after which the materials were sequentially bonded and then subjected to an industry standard cyclical corrosion exposure test, similar to ASTM D1002. All four samples failed at either 2 or 3 cycles, confirming that the anodic oxide layer produced during anodization facilitates appropriate production of the functional layer and subsequent adhesive bonding.

Example 3

(16) Several samples of a 7xxx aluminum alloy (Al—Zn—Mg—Cu style) were processed as per FIG. 2. The alloys were all anodized in a 15 wt. % sulfuric acid solution at 70° F. and 15 ASF for 10, 20, 30, or 40 seconds. After anodizing, a functional layer was then created (400), per FIG. 2 and in accordance with U.S. Pat. No. 6,167,609 to Marinelli et al., on each of the materials, after which the materials were sequentially bonded and then subjected to an industry standard cyclical corrosion exposure test, similar to ASTM D1002. All four anodizing conditions resulted in the specimens completing the required 45 cycles, and with retained strength levels of from 3512 to 6519 psi. The average retained strengths were 5698 psi (stdev (σ) of 205 psi) (40 sec.), 5091 psi (30 sec.), 5665 psi (20 sec.), and 5167 psi (10 sec.). The higher current density (as compared to Example 2) facilitated production of an anodic oxide layer having an appropriate thickness for facilitating the creating step (400) and subsequent adhesive bonding.

(17) To verify oxide thickness, one of the 10 second anodized samples was analyzed by XPS. The analysis indicated that the anodic oxide layer had a thickness of 28 nm thick, and consisted essentially of aluminum oxides (e.g., Al.sub.2O.sub.3). See, FIG. 9. The surface of the oxide also includes a plurality of pits. See, FIG. 10. It is believed that these pits may at least assist in facilitating approved adhesive boding performance for the 7xxx aluminum alloy products.

(18) As per Example 2, baseline samples were also prepared using the same conditions as the anodized sample, but in the absence of anodizing—the samples, instead, were placed in the 15 wt. % sulfuric acid anodizing bath at 70° F. without any current applied. The same functional layer was then created (400), per FIG. 2 and in accordance with U.S. Pat. No. 6,167,609 to Marinelli et al., on each of the materials, after which the materials were sequentially bonded and then subjected to an industry standard cyclical corrosion exposure test, similar to ASTM D1002. All samples failed within a few cycles (3-6), again confirming that the anodic oxide layer produced during anodization facilitates appropriate production of the functional layer and subsequent adhesive bonding.

(19) To confirm that different anodizing conditions could be used with this same material, one additional sample of the material was prepared as per FIG. 2. The alloy was also anodized in a 15 wt. % sulfuric acid at 70° F., but at 6 ASF for 20 seconds. The same functional layer was then created (400), per FIG. 2 and in accordance with U.S. Pat. No. 6,167,609 to Marinelli et al., on each of the specimens, after which the materials were sequentially bonded and then subjected to an industry standard cyclical corrosion exposure test, similar to ASTM D1002. These specimens all completed the required 45 cycles, and with an average retained strength of 5032 psi.

Example 4

(20) Several additional 7xxx aluminum alloys (Al—Zn—Mg—Cu style) were processed as per FIG. 2. The alloys were all anodized in a 15 wt. % sulfuric acid solution at 70° F. and 12 ASF for 20, 40, or 60 seconds. After anodizing, a functional layer was then created (400), per FIG. 2 and in accordance with U.S. Pat. No. 6,167,609 to Marinelli et al., on each of the materials, after which the materials were sequentially bonded and then subjected to an industry standard cyclical corrosion exposure test, similar to ASTM D1002. In this example, the specimens anodized for 40 second and 60 second did not pass the testing—there was just one “survivor” out of each of the four specimens at each condition. However, in the set anodized for 20 seconds, three of the four specimens completed the required 45 cycles and produced retained shear strengths of 3765, 5294 and 6385 psi. The fourth specimen survived 44 of the 45 cycles, but failed at the 45th cycle.

(21) The anodic oxide layers of the 20 second and 40 second anodized sample were then analyzed by XPS. The 20 second anodized sample had an anodic oxide thickness of 72 nm, whereas the 40 second anodized sample has an anodic oxide thickness of 158 nm. These results indicate that the anodic oxide thickness must be maintained “thin” to facilitate subsequent functional layer preparation and adhesive bonding.

Example 5

(22) Several additional samples of a 7xxx aluminum alloy (Al—Zn—Mg—Cu style) were processed as per FIG. 2, except the alloys were anodized in a 10 wt. % phosphoric acid solution at 90° F. and 17.5V for 10 seconds. After anodizing, a functional layer was then created (400), per FIG. 2 and in accordance with U.S. Pat. No. 6,167,609 to Marinelli et al., on each of the materials, after which the materials were sequentially bonded and then subjected to an industry standard cyclical corrosion exposure test, similar to ASTM D1002. In this example, three out of four of the samples completed the required 45 cycles and produced retained shear strengths of 6011, 5932, and 5596, with an average of 5846 psi (stdev (σ) of 220 psi), showing the efficacy of the treatment using phosphoric acid anodizing.

(23) Without being bound to any particular theory, it is believed that the functionalization creates bonds between organic compounds and phosphorous in the anodic oxide layer, an example of which is FIG. 8a, wherein phosphorus atoms present in the functionalized layer covalently bond to an organic (R) group, in addition to being covalently bonded to oxygen atoms of the aluminum oxide. The “R groups” in the functionalized layer are generally organic groups containing 1-30 carbon atoms and/or hydrogen (i.e., R′), depending on the particular composition of the phosphorus-containing organic acid used during the creating (400) step. Phosphoric anodizing does not create such P—R boding. Instead, phosphoric anodizing generally creates P—O bonding, as illustrated in FIGS. 8b-8c. The identity of the chemical structures associated with phosphorus provides the ability to readily distinguish (e.g., using analytical methods such as Fourier-transform infra-red (FTIR) spectroscopy) between anodized and functionalized 7xxx aluminum alloy products (including, without limitation, 7xxx aluminum alloy products), as well as to characterize the compositions of the chemicals used for the various treatment steps and the degree to and conditions at which such steps have been completed.

(24) Whereas particular embodiments of this invention have been described above for purposes of illustration, it will be evident to those skilled in the art that numerous variations of the details of the present invention may be made without departing from the invention as defined in the appending claims.