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

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 −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.