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Abstract

The present invention relates to a method for producing a high strength Al--Zn--Cu--Mg alloy with an improved fatigue crack growth resistance and a high damage tolerance, comprising the steps of casting an ingot with the following composition (in weight percent) Zn 5.5-9.5, Cu 1.5-3.5, Mg 1.5-3.5, Mn<0.25, Zr<0.25, Cr<0.10, Fe<0.25, Si<0.25, Ti<0.10, Hf and/or V<0.25, other elements each less than 0.05 and less than 0.15 in total, balance aluminum, homogenizing and/or pre-heating the ingot after casting, hot working the ingot and optionally cold working into a worked product of more than 50 mm thickness, solution heat treating, quenching the heat treated product, and artificially ageing the worked and heat-treated product, wherein the ageing step comprises a first heat treatment at a temperature in a range of 105.degree. C. to 135.degree. C. for more than 2 hours and less than 8 hours and a second heat treatment at a higher temperature than 135.degree. C. but below 170.degree. C. for more than 5 hours and less than 15 hours. The invention concerns a weldable plate product of such high strength Al--Zn--Cu--Mg having a thickness of more than 50 mm and an aircraft structural member produced from such alloy.

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First Claim