Al-Zn-Mg-Cu alloys and products with high mechanical characteristics and structural members suitable for aeronautical construction made thereof

Abstract

The present invention further relates to 7xxx alloys and products produced therewith that can be flat rolled, extruded or forged, as well as associated methods. Al—Zn—Mg—Cu alloys of the present invention preferably comprise (in mass percentage): a) Zn 8.3−14.0 Cu 0.3−2.0 Mg 0.5−4.5 Zr 0.03−0.15 Fe+Si<0.25 b) at least one element selected from the group consisting of Sc, Hf, La, Ti, Ce, Nd, Eu, Gd, Th, Dy, Ho, Er, Y and Yb, where the content of each of the elements, if selected, is between 0.02 and 0.7%, c) remainder aluminum and inevitable impurities. The present invention further is directed to products wherein Mg / Cu>2.4 and (7.9−0.4 Zn)>(Cu+Mg)>(6.4−0.4 Zn). The disclosed products can be used for example, as structural members in aeronautical construction, especially as stiffeners capable for use in fuselages of civilian and other aircrafts as well as in related applications.

Description

CLAIM FOR PRIORITY The present invention claims priority under 35 U.S.C. § 119 from French Patent Application No. 02 04250 filed Apr. 5, 2002, the content of which is incorporated herein by reference in its entirety. BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates generally to Al—Zn—Mg—Cu type alloys with high mechanical characteristics, typically having a Zn content greater than 8.3%, as well as to products and structural members suitable for aeronautical products and / or constructions manufactured from such products. 2. Description of the Related Art Al—Zn—Mg—Cu alloys (belonging to the family of 7xxx alloys) are currently in use in aeronautical construction, and are particularly used in the construction of civilian aircraft wings. For the wing exterior, a skin of plate made in alloys such as 7150, 7055, 7449 is often used, and optionally includes stiffeners (also called stringers) made from profiles in 7150, 7055, or 7449 alloys. 7150, 7050...

AI Technical Summary

Benefits of technology

A problem which the present invention attempted to resolve was therefore to obtain a novel alloy and associated novel wrought Al—Zn—Mg—Cu type products with a high zinc content, (i.e. greater than 8.3%) and especially extruded products, as well as their associated methods. Products of the present invention preferably possess a very high ultimate tensile strength (UTS), a very high tensile yield strength (TYS) as well as adequate resistance to corrosion and a high formability, and also are capable of being manufactured industrially under conditions of highest reliability compatible with the severe requirements of the aeronautical industry.

Problems solved by technology

The alloying elements of this alloy are very high in zinc, magnesium and copper, and as such, are difficult to put into solid solution because the temperature of solution heat treatment is limited by the melting point of intermetallic phases, which have the lowest incipient melting point.

Thus, such a product has a low formability.

These alloys are difficult to cast and the alloying elements are difficult to put into solid solution, which favours the presence of coarse precipitates, which are not welcome.

It is also well known (see for example U.S. Pat. No. 5,221,377) that discloses that when the zinc content of an alloy of the 7xxx family is increased beyond around 7 to 8%, then problems associated with insufficient resistance to exfoliation corrosion and stress corrosion will arise.

More generally, it is known that the most charged Al—Zn—Mg—Cu alloys are likely to pose corrosion problems.

But such treatments can then cause a corresponding drop in the static mechanical characteristics.

Whichever approach is used, the manufacture and use of such products poses at least two problems.

On one hand, alloys with a high zinc and magnesium content are difficult to cast and to transform, especially by extrusion, rolling or forging.

For example, the maximum force that can be supplied by an extrusion press can be a limiting factor.

Therefore, up until now, alloys that could be developed having a mechanical strength still higher than 7349 and 7449 alloys would likely be difficult to cast and to transform, and products made therefrom would tend to have a low formability.

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