METHOD OF MANUFACTURING A 2XXX-SERIES ALUMINIUM ALLOY PLATE PRODUCT HAVING IMPROVED FATIGUE FAILURE RESISTANCE

Abstract

A method of manufacturing an AA2xxx-series aluminium alloy plate product having improved fatigue failure resistance and a reduced number of flaws, the method comprising the following steps (a) casting an ingot of an aluminium alloy of the 2xxx-series, the aluminium alloy comprising (in wt. %): Cu 1.9 to 7.0, Mg 0.3 to 1.8, Mn up to 1.2, balance aluminium and impurities, each 0.05 max., total 0.15; (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 at an intermediate thickness of the plate between 100 and 200 mm, at least one high reduction hot rolling pass is carried out with a thickness reduction of at least 15%; wherein the plate product has a final thickness of less than 60 mm. The invention is also related to an aluminium alloy product produced by this method.

Claims

1. A method of manufacturing an AA2xxx-series aluminium alloy plate product having improved fatigue failure resistance and a reduced number of flaws, the method comprising the following steps: (a) casting an ingot of an aluminium alloy of the AA2xxx-series; (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 at an intermediate thickness of the plate between 100 and 200 mm, at least one high reduction hot rolling pass is carried out with a thickness reduction of at least 15%; and wherein the plate product has a final thickness of less than 60 mm.

2. The method according to claim 1, wherein the method further comprises the steps of (d) optionally pre-stretching or applying a skin pass by cold rolling of the plate product after the hot rolling; (e) solution heat treating the plate product; (f) cooling of the solution heat treated plate product; (g) optionally stretching the solution heat treated plate product; and (h) natural ageing or artificially aging the solution heat treated and cooled plate product.

3. The method according to claim 1, wherein the high reduction hot rolling pass is carried out with a reduction of at least 20%.

4. The method according to claim 1, wherein a deformation rate during the high reduction pass is <0.77 s.sup.−1.

5. The method according to claim 1, wherein the intermediate thickness of the plate before the high reduction pass is carried out between 120 and 180 mm.

6. The method according to claim 1, wherein the 2xxx aluminium alloy has a composition comprising, in wt. %: TABLE-US-00007 Cu 1.9 to 7.0, Mg 0.3 to 1.8, Mn up to 1.2, balance aluminium and impurities.

7. The method according to claim 1, wherein the 2xxx aluminium alloy has a composition comprising, in wt. %: TABLE-US-00008 Cu 1.9 to 7.0, Mg 0.3 to 1.8, Mn up to 1.2, Fe up to 0.40, Si up to 0.40, Ti up to 0.15, Zr up to 0.25, Zn up to 1.0, Li up to 2.0, Ni up to 2.5, Ag up to 0.80, V up to 0.25, Cr up to 0.35, balance aluminium and impurities.

8. The method according to claim 1, wherein the 2xxx aluminium alloy has a Cu-content of 3.0% to 6.8%, and preferably 3.8% to 5.0%.

9. The method according to claim 1, wherein the 2xxx aluminium alloy has a Mg-content of 0.35% to 1.6%.

10. The method according to claim 1, wherein the 2xxx aluminium alloy has a Mn-content of 0.2% to 1.2%.

11. The method according to claim 1, wherein the Ti-content is within a range of 0.01% to 0.10 wt. %.

12. The method according to claim 1, wherein the aluminium alloy has a composition in accordance with AA2024.

13. The method according to claim 1, wherein the final thickness of the plate is less than 50 mm.

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

15. The method according to claim 1, wherein in the method step (c) the hot rolling mill exit temperature is more than 385° C.

16. The method according to claim 1, wherein the plate product is naturally aged to a T3 temper.

17. An aluminium plate product manufactured from the aluminum alloy product obtained by the method according to claim 1 and having improved fatigue failure resistance and less flaws in an ultrasonic inspection.

18. An aircraft skin product manufactured from the aluminium alloy plate product obtained by the method according to claim 1.

19. Use of an aluminium alloy product manufactured according to claim 1 for the manufacture of an aircraft skin.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0079] 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.

[0080] 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.

[0081] FIG. 2 is a graph showing the number of ultrasonic indications versus the plate thickness from plates prepared according to the method of this invention and plates prepared by conventional methods.

EXAMPLES

Example 1

[0082] Rolling ingots have been DC-cast of the aluminium alloy AA2024, with a composition (in wt. %, balance aluminium and impurities) as given in Table 1.

TABLE-US-00003 TABLE 1 Ingot Lot No. Si Fe Cu Mn Mg Zn Ti A, B 0.07 0.03 4.0 0.5 1.3 0.02 0.03

[0083] The rolling ingots have a thickness at the start of about 330 mm. Homogenization and pre-heating of the ingots were carried out in a two-step procedure, the first step at 495° C. for 18-24 hours and the second step at 485° C. for 1 to 16 hours (pre-heat). Then the ingots were hot rolled to an intermediate thickness of 100-140 mm (first hot rolling), wherein ingot A was processed according to the invention, i.e. this ingot received a high reduction pass during the first hot rolling. At about 170 mm ingot A was reduced in thickness with a reduction of about 26% (171 mm to 127 mm). The rolling speed during this high reduction pass was about 25 m/min giving a deformation rate of 0.52 s.sup.−1.

[0084] Ingot B was processed according to a conventional hot rolling method (a thickness reduction between 3% and 8% for each hot rolling pass between 300 and 120 mm). The rolling speed during the standard hot rolling passes was between 60 m/min (entry thickness 177 mm) and 100 m/min (entry thickness 131 mm) giving a deformation rate of between 0.77 s.sup.−1 and 1.56 s.sup.−1. The exit temperature after the first hot rolling series is above 400° C. At an intermediate thickness of 120 mm (lot A and lot B) both plates were heated to 490° C. for 24 to 30 hours and then set to 485° C. for 1 to 12 hours. After this re-heating the plates were hot rolled to the final thickness of 23 mm (second hot rolling series). The exit temperature after the second hot rolling is above 400° C.

[0085] Plate A received 24 hot rolling passes, wherein the high reduction pass was pass number 12. Plate B received 26 hot rolling passes without a high reduction pass. As already outlined above, both plates were first hot rolled to intermediate thickness between 100 and 140 mm. Plate A was subjected to the second pre-heating after pass No. 15 and Plate B was subjected to the second pre-heating after pass No. 17. Both plates have a final thickness of 23 mm after the hot rolling process. After the hot rolling steps both plates were solution heat treated at a temperature of about 495° C. and quenched. Then, they received a rolling skin pass for flatness improvement and were stretched for about 2-3%. A naturally ageing step was applied for at least 5 d, bringing the plate products to a T351 condition.

[0086] 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 30 Hz and the tests were performed in high humidity air (RH≥90%). The individual results of these tests are shown in Table 2 and FIG. 1.

TABLE-US-00004 TABLE 2 Alloy A B Temper T351 T351 final thickness of plate (mm) 23 23 High reduction pass yes no inventive method yes no Cycles to failure Cycles to failure max net stress 235 45.490 39.906 [MPa] 220 73.690 55.573 200 252.233 109.719 180 1.050.476 634.427 165 1.364.233 202.649 165 287.674 130 5.862.397 2.855.895 130 780.995

[0087] 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 AA2xxx alloy plates prepared by conventional methods. For example, at an applied net section stress of 200 MPa, plate A has a lifetime of 252.233 cycles representing a 2.3 times improvement in lifetime compared to alloy B which has a life time of 109.719 cycles.

Example 2

[0088] An ultrasonic inspection of the alloy plates given in Table 3 have been carried out according to AMS-STD-2154. Test plates were used having a thickness of 16 mm or 23 mm. The composition (in wt. % and balance aluminium and impurities) is given below in Table 3.

TABLE-US-00005 TABLE 3 final Ingot thickness Si Fe Cu Mn Mg Zn Ti Lot A, B 23 mm 0.07 0.03 4.0 0.5 1.3 0.02 0.03 C, D, E, F 16 mm 0.07 0.03 4.0 0.5 1.3 0.02 0.03

[0089] The rolling ingots have a thickness at the start of about 330 mm. Plates A and B were produced as outlined above in Example 1, i.e. plate B received 26 hot rolling passes without a high reduction pass and plate A received 24 hot rolling passes including a high reduction pass at about 170 mm.

[0090] Regarding lots C, D, E and F the rolling ingots have a thickness at the start of about 330 mm. Homogenization and pre-heating, first hot rolling, second pre-heating and second hot rolling of the ingots were carried out as outlined in Example 1, i.e. at about 170 mm lots E and F were reduced in thickness with a reduction of about 26% (171 mm to 127 mm) and lots C and D were processed according to a conventional hot rolling method. All plates have a final thickness of 16 mm after the hot rolling process. After the hot rolling steps the plates were pre-stretched in a range of 0.5% to 1% to improve the flatness of the plates. Then these were solution heat treated at a temperature of 495° C., quenched and again stretched for about 2-3%. A naturally ageing step was applied, bringing the plate products to a T351 condition.

[0091] The following Table 4 shows the number of ultrasonic (US) indications that the plates show. The plates having a final thickness of 16 mm have a dimension of 16 mm×1000 mm×12000 mm and the plates having a final thickness of 23 mm have a dimension of 23 mm×1500 mm×17000 mm.

TABLE-US-00006 TABLE 4 High re- Number of US indications per size range LOT final thick- duction <1.2 1.2-1.9 ≥2.0 Sum of US Nos. ness pass mm mm mm indications B 23 mm no 18 6 0 24 A 23 mm yes 0 1 0 1 C 16 mm no 20 7 0 27 D 16 mm no 22 16 1 39 E 16 mm yes 0 0 0 0 F 16 mm yes 0 0 0 0

[0092] From this Table it is evident that the plate products of lots A, E and F prepared by the method of the present invention, i.e. receiving the high reduction pass, show a reduced number of flaws (see sum of US indications) detected with ultrasonic inspection according to AMS-STD-2154.

[0093] 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.