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, but modest levels of magnesium, to provide increased tensile strength without sacrificing toughness. Preferred ranges of the elements include by weight, 8.5-10.5% Zn, 1.4-1.85 % Mg, 2.25-3.0% Cu and at least one element from the group Zr, V, or Hf not exceeding about 0.5%, the balance substantially aluminum and incidental impurities. In addition, small amounts of scandium (0.05-0.30%) are also preferably employed to prevent recrystalization. During formation of the alloys, homogenization, solution heat treating and artificial aging processes are preferably employed.

Description

CROSS REFERENCE TO RELATED APPLICATIONS [0001] This application contains subject matter that is related to the subject matter set forth in U.S. application Ser. No. 10 / 829,391, which was filed on Apr. 22, 2004.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). From the published phase relatio...

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. Once the ingot is formed into finished shape using extrusion and rolling steps, for example, the product is preferably solution heat treated at 870 to 900 degrees F. and then artificially aged. The aging process can be carried out by exposing the product to a one, two or three step heat treatment process. In the first step, the product is exposed to a temperature range of 175-310 degrees F. for 3 to 30 hours. In the optional second step, the first step is followed by heating at 310 to 360 degrees F. for 2 to 24 hours. Finally, in the third optional step, the product is heated at 175 to 300 degrees F. for 1 to 30 hours. As a still further option, the second and third aging steps can be used without the first aging step.

[0010] The foregoing alloys and processing operations enhance the properties of the Al—Zn—Mg—Cu alloy system, such that they can be more effectively employed in numerous applications. Specific products or items in which the subject alloys can be employed include, among others, sporting goods including baseball and soft ball bats, golf shafts, lacrosse sticks, tennis rackets, and arrows; and aerospace application including aerospace components such as wing plates, bulkheads, fuselage stringers, and structural extrusions and forgings; and ordnance parts such as sabots and missile launchers.

Problems solved by technology

Additionally, it has been found that toughness decreases as the total weight percentage of magnesium and copper increases.

Images