High strength aluminum alloys and process for making the same

Abstract

High strength aluminum alloys based on the Al—Zn—Mg—Cu alloy system preferably include high levels of zinc and copper to provide increased tensile strength without sacrificing toughness. In addition, small amounts of scandium are also preferably employed to prevent recrystalization. Preferred ranges of the elements include by weight, 8.5-11.0% Zn, 1.8-2.4% Mg, 1.8-2.6% Cu, 0.05-0.30% Sc and at least one element from the group Zr, V, or Hf not exceeding about 0.5%, the balance substantially aluminum and incidental impurities. During formation of the alloys, a homogenization process is preferably employed after alloy ingot casting in which a slow rate of temperature increase is employed as the alloy is heated as near as possible to its melting temperature. For the last 20-30 F below the melting temperature, the rate of increase is limited to 20 F / hr. or less to minimize the amount of low melting point eutectic phases and thereby further enhance fracture toughness of the alloy.

Description

CROSS REFERENCE TO RELATED APPLICATIONS [0001] This application claims the benefit, under 35 U.S.C. 119(e), of U.S. Provisional Application No. 60 / 464,654, which was filed on Apr. 23, 2003.BACKGROUND OF THE INVENTION [0002] 1. Field of the Invention [0003] The present invention relates, in general, to a high strength aluminum alloy based on the Al—Zn—Mg—Cu alloy system and a process for forming the same. Although not limited thereto, the alloys are particularly suited for use in sporting goods and aerospace applications. [0004] 2. Description of the Background Art [0005] The highest strength aluminum alloys known at this time are based on the aluminum-zinc-magnesium-copper system. Commercial high-strength alloys currently being produced include AA7055 (nominally 8% Zn—2% Mg—2.2% Cu—0.10% Zr), AA7068 (nominally 7.8% Zn—2.5% Mg—2.0% Cu—0.10% Zr) and a Kaiser Aluminum alloy designated K749 (nominally 8% Zn—2.2% Mg—1.8% Cu—0.14% Zr). These alloys are shown graphically on the equilibrium...

AI Technical Summary

Benefits of technology

[0007] The present invention addresses the foregoing need in a number of ways. More particularly, there are three distinct avenues for increasing an alloy's strength while maintaining its toughness: rich alloy chemistries; processing to maximize alloying effectiveness; and preventing recrystallization. Rich alloys provide more solute, which is potentially available for age hardening to higher strength levels; effective processing ensures that the solute is available for strengthening and not out of solution as second phases, which detract from fracture toughness; and maintaining an unrecrystallized microstructure optimizes both strength and toughness.

[0009] To maximize alloying effectiveness during formation of the alloys, a homogenization process is preferably employed after alloy ingot casting in which a slow rate of temperature increase is employed as the alloy is heated as near as possible to its melting temperature. In particular, for the last 20-30° F. below the melting temperature, the rate of increase is limited to 20° F. / hr. or less to minimize the amount of low melting point eutectic phases and thereby further enhance fracture toughness of the alloy.

Problems solved by technology

In the latter case all the alloying elements are not in solid solution at 860° F., and are not only unavailable for age hardening, but the undissolved phases remaining after heat treatment detract from toughness.

Although solution heat treating at a higher temperature than 860° F. will dissolve more of the solute, care has to be taken to ensure that the alloy does not undergo eutectic melting, which is a common problem in commercially cast alloys that have locally enriched regions as a result of microsegregation that occurred during casting.

However, this requirement presents a problem because, in general, as the tensile strength of an aluminum alloy is increased, its toughness decreases.

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