Aluminum alloy having superior strength-toughness combinations in thick gauges

Abstract

A 7XXX series aluminum alloy having reduced quench sensitivity suitable for use in aerospace structural components, such as integral wing spars, ribs, extrusions and forgings comprises, in weight %: 6 to 10 Zn, 1.3 to 1.9 Mg, 1.4 to 2.2 Cu, wherein Mg<=Cu+0.3, one or more of 0 to 0.4 Zr, up to 0.4 Sc, up to 0.2 Hf, up to 0.4 Cr, up to 1.0 Mn and the balance Al plus incidental additions including Si, Fe, Ti and the like plus impurities. By controlling the Mg content to 1.3 to 1.7 wt. %, limiting Mg<=Cu+0.3 and 6.5<=Zn<=8.5, the alloy provides significantly improved combined strength and fracture toughness in heavy gauges. For example, in a six-inch thick plate there is provided a combination of about 75 ksi quarter-plane tensile yield strength (L) with a fracture toughness (L-T) of about 33 ksi{square root}in which progresses by artificial aging/tempering to a combined strength and fracture toughness of about 67 ksi tensile yield strength (L) and a fracture toughness (L-T) of about 40 ksi{square root}in. The alloy product possesses equally attractive combinations of strength and fracture toughness when intentionally quenched slowly following solution heat treatment so as to lessen dimensional distortion, particularly in shapes of varying cross section.

Description

[0001] 1. Field of the Invention[0002] The present invention relates generally to aluminum alloys and, more particularly, to 7XXX series aluminum alloys having superior strength-toughness combinations suitable for thick gauge (e.g., 2-10 inches) structural parts in aerospace applications. Parts made from the alloy of the present invention find specific utility as structural components such as integral spar members and the like which are integrally machined from thick sections, including rolled plate, used in the construction of structural members for high capacity aircraft. The alloy may also be formed by other known hot forming techniques such as extrusion and forging. The forging process is particularly suitable for manufacturing high strength aircraft components, such as, for example, main landing gear beams.[0003] 2. Background of the Invention[0004] As the size of new aircraft, or modification of current models, gets larger to accommodate heavier payload and / or longer flight ra...

AI Technical Summary

Problems solved by technology

Obviously, the fasteners and fastener holes at the joints are structural weak links.

However, current alloys do not meet the desired property requirements.

For example, the lower strength of alloy 2024-T351, commonly used in lower wing applications, will not be able to safely carry the load transmitted from the highly loaded upper wing unless its section thickness is significantly increased, which adds undesirable weight to the aircraft.

Alternatively, to design the upper wing according to the 2XXX strength capability would result in overall weight penalty.

Therefore, if an integral spar with 60 ksi yield strength is used, the strength capability of the upper wing skin will not be able to be taken full advantage of for maximum weight efficiency.

As the product shape gets thicker, the quench rate experienced at the interior of the product cross section naturally decreases, which results in a loss of strength and fracture toughness in the product.

Up until now, no one has been able to solve the problem concerning how to avoid these conflicting phenomena and provide a stronger, thick alloy product combined with appropriate high fracture toughness.

However, the purported superior strength-toughness advantage was significantly reduced in this alloy when a quench rate simulating the mid-plane of a 6-inch product is used after SHT, showing only limited improvement in properties over that of 7050 alloy.

Thus, quench sensitivity in this alloy has been responsible for the loss in property advantages in thicker gauges.

While some property improvements over 7050 alloy were claimed in thicker gauges, the improvements of 7040 still fell short of those desired for newer commercial aircraft designs.

When multiple dispersoid elements are present, there may be synergistic effects for grain structure control.

It is not always feasible to solution heat treat and quench the finished structural products because the rapid cooling of the quenching step would induce residual stress and cause dimensional distortion.

The quench-induced residual stress could also possibly cause stress corrosion cracking, and re-work to straighten parts associated with dimensional distortion could render assembly impracticably difficult.

While it is much easier to obtain better mechanical properties in thinner cross sections because of the faster cooling which prevents unwanted precipitation of alloying elements, this cannot be done when quench distortion is present.

The latter occurrence producing the coarse precipitates results in a decline in mechanical properties.

In thick product cross sections, i.e., over 3 inches, and more particularly in heavier sections of 4 to 8 inches or more, the quenching medium acting on the exterior surfaces of the workpiece (such as a plate, forging or extrusion, for example) cannot efficiently extract heat from the center or mid-plane region of the material.

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