951 Aluminum Lithium Alloys

Lithium is the lightest metallic element, and since the density of an alloy is the weighted average of the density of its constituents, lithium is attractive as an alloying element. But lithium has additional benefits—in addition to a 3% decrease in density for every 1% of lithium added (up to the solubility limit of 4.2%), the elastic modulus increases by 5-6%. Aluminum-lithium alloys are also heat treatable. These advantages are offset by the reactivity of lithium, which necessitates the use of an inert gas atmosphere when adding the liquid metal to the alloy. Al-Li alloys are often alloyed with copper, magnesium, zirconium, or other elements to improve properties. Since there is no aluminum-lithium alloy series, when lithium is the greatest alloying element, the designation number is 8xxx (see Section When other alloying elements are in greater proportion than lithium, the designation number is based on the element in greatest proportion (such as 2195, which contains 4% copper and 1% lithium).

The Germans developed the first aluminum-lithium alloy in the 1920s, but the first Al-Li alloys to win commercial application were those developed for aircraft between the 1950s and 1970s. Alloy 2020 was used for compression wing skins of the RA5C Vigilante, but its registration was discontinued in 1974. Applications were hampered by low ductility and fracture toughness.

The second phase of Al-Li alloy development, which occurred in the 1980s, used relatively high levels of lithium (over 2%) in order to maximize property improvements. Alloys 2090 and 8090, typical of this phase, had some success but were limited by anisotropic behavior and relatively low corrosion resistance. Finally, in the late 1980s and 1990s, work done at Martin Marietta yielded the Weldalite Al-Li alloys, which appear destined to achieve significant success in aerospace and aircraft applications. These alloys are weldable, as the name implies, and use copper as the primary alloy, with modest amounts of lithium (slightly over 1%), and about 0.4% magnesium and 0.4% silver.

The most promising application for experimental, extremely light, and strong materials is space launch vehicles, where the cost of attaining low Earth orbit is about $8000/ kg and the number of reuses is limited. The U.S. space shuttle external fuel tank is a good example. The first application of the Weldalite-type alloys was the use of 2195 to replace 2219, a weldable aluminum-copper alloy, for the shuttle's liquid hydrogen and liquid oxygen tanks, producing a weight savings of 3500 kg. Alloy 2197 is now being used to refurbish F-16 fighter jet bulkheads, improving the range and performance of the aircraft. Commercial aircraft applications are anticipated next. As consumption has increased, Al-Li alloy material costs have fallen from a premium of 20 times that of common alloys to less than 4 times.

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