METHOD OF MANUFACTURING A 7XXX-SERIES ALUMINIUM ALLOY PLATE PRODUCT HAVING IMPROVED FATIGUE FAILURE RESISTANCE

Abstract

The invention relates to a method of manufacturing an 7xxx-series aluminium alloy plate product having improved fatigue failure resistance, the method comprising the steps of (a) casting an ingot made of an aluminium alloy of the 7xxx-series comprising (in wt. %): Zn 5 to 9, Mg 1 to 3, Cu 0 to 3, balance aluminium and incidental elements and impurities; (b) homogenizing and/or preheating the cast ingot; (c) hot rolling the ingot into a plate product by rolling the ingot with multiple rolling passes, characterized in that when the intermediate thickness of the plate is between 80 and 220 mm, at least a high reduction hot rolling pass is carried out with a thickness reduction of at least 25%, wherein the plate product has a final thickness of less than 75 mm. The invention is also related to an aluminium alloy plate product and an aerospace structural member produced by this method.

Claims

1. A method of manufacturing a 7xxx-series aluminium alloy plate product having improved fatigue failure resistance, the method comprising the following steps: (a) casting an ingot of an aluminium alloy of the 7xxx-series, the aluminium alloy comprising (in wt. %): Zn 5 to 9, Mg 1 to 3, Cu 0 to 3, Fe up to 0.20, Si up to 0.15, Zr up to 0.5, balance aluminium and impurities; (b) homogenizing and/or preheating the cast ingot; (c) hot rolling the ingot into a plate product by rolling the ingot with multi-pie rolling passes, characterized in that, when at an intermediate thickness of the plate between 80 and 220 mm, at least one high reduction hot rolling pass is carried out with a thickness reduction of at least 25%; wherein the plate product has a final thickness of less than 75 mm.

2. The method according to claim 1, wherein the method further comprises the steps of (d) solution heat treating the plate product; (e) cooling of the solution heat treated plate product; (f) optionally stretching the solution heat treated and cooled plate product, and (g) artificial ageing the solution heat treated and cooled plate product.

3. The method according to claim 1, wherein the method does not comprise a cold rolling step to final gauge.

4. The method according to claim 1, wherein the high reduction hot rolling pass is carried out with a thickness reduction of at least 30%.

5. The method according to claim 1, wherein a deformation rate during the high reduction hot rolling pass is <1 s.sup.−1.

6. The method according to claim 1, wherein the intermediate thickness of the plate before the high reduction hot rolling pass is be-tween 100 mm and 200 mm.

7. The method according to claim 1, wherein the Si-content and/or the Fe-content of the aluminium alloy is equal to or more than 0.05 wt. %.

8. The method according to claim 1, wherein the aluminium alloy has a composition consisting of Zn 5% to 9%, Mg 1% to 3%, Cu 0% to 3%, Si up to 0.15%, Fe up to 0.20%, one or more elements selected from the group consisting of: Zr up to 0.5%, Ti up to 0.3% Cr up to 0.4% Sc up to 0.5% Hf up to 0.3% Mn up to 0.4% V up to 0.4% Ag up to 0.5%, balance being aluminium and impurities.

9. The method according to claim 1, wherein the aluminium alloy has a composition in accordance with AA7055.

10. The method according to claim 1, wherein the final thickness of the plate product is less than 45 mm.

11. The method according to claim 1, wherein the final thickness of the plate product is more than 10 mm.

12. The method according to claim 1, wherein in the method step (c), a hot rolling mill entry temperature is more than 380° C.

13. The method according to claim 1, wherein the plate product is artificially aged to a T7 temper.

14. An aluminium alloy plate product manufactured according to claim 1 and having improved fatigue failure resistance.

15. An aerospace structural member manufactured from the aluminium alloy plate product obtained by the method according to claim 1.

16. Use of an aluminium alloy plate product manufactured according to claim 1, for the manufacture of an aircraft structural member.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0074] Embodiments of the invention will now be described by way of non-limiting examples, and comparative examples representative of the state of the art will also be given.

[0075] FIG. 1 is graph of maximum net stress versus cycles to failure for plates prepared according to the method of this invention and plates prepared by conventional methods.

[0076] FIG. 2 is a graph showing the average logarithmic fatigue life of the plates prepared according to the method of this invention and the plates prepared by conventional methods, with lines connecting the data points corresponding to the averages.

EXAMPLE

[0077] Rolling ingots have been DC-cast of the aluminum alloy AA7055, with a composition as given in Table 1

TABLE-US-00001 TABLE 1 Lot Si Fe Cu Mg Zn Zr A, B, C, D, E 0.07 0.07 2.35 1.94 8.05 0.12

[0078] The rolling ingots had a thickness of about 400 mm. Homogenization of the ingots was carried out in a two-step homogenization procedure at 465° C. (first step) and 475° C. (second step) followed by cooling to ambient temperature. After scalping, the ingots were pre-heated to 410° C. for hot rolling. Hot rolling was carried out on a hot rolling mill having a work roll radius of about 575 mm. Lots A and B were processed in accordance with the invention, i.e. both lots receive a high reduction pass during the hot rolling process. During the high reduction rolling pass, lot A received a thickness reduction of about 30% (167 mm to 117 mm) and lot B received a thickness reduction of about 28% (165 mm to 118 mm). The rolling speed during this high reduction pass was about 25 m/min giving a deformation rate of about 0.53 s.sup.−1. Lots C, D, and E were processed according to a conventional hot rolling method (a thickness reduction between 9% to 18% for each hot rolling pass between 220 mm and 80 mm thickness). The rolling speed during the standard hot rolling passes was about 105 m/min giving a deformation rate of between 1.61 s.sup.−1 (entry thickness 188 mm) and 2.27 s.sup.−1 (entry thickness 123 mm). Plate A received 27 hot rolling passes, wherein the high reduction pass was pass number 19. Plate B received 25 hot rolling passes, wherein the high reduction pass was pass number 17.

[0079] The plates A, C, and E had a final thickness of 19 mm after the hot rolling process, and the plates B and D had a final thickness of 25.4 mm after the hot rolling process. After hot rolling, all the plates in final thickness were solution heat treated at a temperature of about 470° C., quenched and stretched for about 2%. An artificial ageing step was applied, bringing the plate products in a T7951 condition.

[0080] Fatigue testing was performed according to DIN EN 6072 by using a single open hole test coupon having a net stress concentration factor Kt of 2.3. The test coupons were 150 mm long by 30 mm wide, by 3 mm thick with a single hole 10 mm in diameter. The hole was countersunk to a depth of 0.3 mm on each side. The test coupons were stressed axially with a stress ratio (min load/max load) of R=0.1. The test frequency was 25 Hz and the tests were performed in high humidity air (RH≥90%). The individual results of these tests are shown in Table 2 and FIGS. 1 and 2. The lines in FIG. 2 are an interpolation between the calculated log average data points.

TABLE-US-00002 TABLE 2 Alloy A B C D E Temper T7951 T7951 T7951 T7951 T7951 final thickness 19 25.4 19 25.4 19 of plate (mm) inventive yes yes no no no method Cycles to Cycles to Cycles to Cycles to Cycles to failure failure failure failure failure max net 260 36.563 28.657 23.550 15.159 23.550 stress [MPa] 260 32.981 29.170 230 246.521 48.278 58.323 35.999 58.323 175 470.421 4.884.359 142.655 175.668 142.655 175 231.925 4.357.253 212.585 390.098 676.780 155 2.222.325 8.572.813 625.048 531.594 625.048 155 10.000.000 4.244.568 676.780 1.652.762 212.585

[0081] FIG. 1 illustrates that by using the method of this invention, it is possible to significantly improve the fatigue life and thus the fatigue failure resistance with respect to AA7055 alloy plates prepared by conventional methods. For example, at an applied net section stress of 175 MPa, plate A has a lifetime of 470421 cycles representing a 3.2 times improvement in life time compared to an AA7055 alloy, i.e. alloys C and E which have a life time of 142655 cycles. Accordingly, in the alloy prepared by the method of this invention and having a final thickness of 19 mm, a life time of 200000 cycles (see the log average curve in FIG. 2) corresponds to a maximum net stress of about 210 MPa for the invention, compared to 175 MPa in a 7055 alloy according to conventional standard. This represents an improvement of more than 20%, which could be utilized by an aircraft manufacturer to increase design stress of an aircraft, thereby saving weight, while maintaining the same inspection interval for the aircraft.

[0082] FIG. 2 shows the logarithmic average of lots A and B manufactured according to the method of this invention compared to the logarithmic average of lots C, D, and E manufactured according to a conventional method of the same alloys as given in FIG. 1, with lines showing the interpolation between the calculated log average data points. From this figure, it is evident that the method of this invention leads to an improvement of the fatigue live over conventional methods by using the same alloy composition.

[0083] The invention is not limited to the embodiments described before, which may be varied widely within the scope of the invention as defined by the appending claims.