Aluminum - Lithium Alloys

Abstract:
Commercial aluminum-lithium alloys are targeted as advanced materials for aerospace technology primarily because of their low density, high specific modulus, and excellent fatigue and cryogenic toughness properties. The principal disadvantages of peak-strength aluminum-lithium alloys are reduced ductility and fracture toughness in the short transverse direction, anisotropy of in-plane properties, the need for cold work to attain peak properties, and accelerated fatigue crack extension rates when cracks are micro structurally small.

aluminum-lithium alloys have been developed primarily to reduce the weight of aircraft and aerospace structures. More recently, they have been investigated for use in cryogenic applications.

The major development work began in the 1970-1980, when aluminum producers accelerated the development of aluminum-lithium alloys as replacements for conventional airframe alloys. The lower-density aluminum-lithium alloys were expected to reduce the weight and improve the performance of aircraft.

Commercial aluminum-lithium alloys are targeted as advanced materials for aerospace technology primarily because of their low density, high specific modulus, and excellent fatigue and cryogenic toughness properties. The superior fatigue crack propagation resistance of aluminum-lithium alloys, in comparison with that of traditional 2xxx and 7xxx alloys, is primarily due to high levels of crack tip shielding, meandering crack paths, and the resultant roughness-induced crack closure. However, the fact that these alloys derive their superior properties from the above mechanisms has certain implications with respect to small crack and variable-amplitude behavior.

The principal disadvantages of peak-strength aluminum-lithium alloys are reduced ductility and fracture toughness in the short transverse direction, anisotropy of in-plane properties, the need for cold work to attain peak properties, and accelerated fatigue crack extension rates when cracks are micro structurally small.

Commercial Aluminum-Lithium Alloys

Development of commercially available aluminum-lithium-base alloys was started by adding lithium to aluminum-copper, aluminum-magnesium, and aluminum-copper-magnesium alloys. These alloys were chosen to superimpose the precipitation-hardening characteristics of aluminum-copper-, aluminum-copper-magnesium-, and aluminum-magnesium-base precipitates to the hardening of lithium-containing precipitates. Proceeding in this manner, alloys 2020 (Al-Cu-Li-Cd), 01429 (Al-Mg-Li), 2090 (Al-Cu-Li), and 2091 and 8090 (Al-Cu-Mg-Li) evolved. Besides these registered alloys, other commercial aluminum-lithium alloys include Weldalite 049 and CP276.

Weldalite 049
Chemical composition: Cu - 5.4, Li - 1.3, Ag - 0.4, Mg - 0.4, Zr - 0.14.

Weldalite 049 shows high strength in variety of products and tempers. Its natural aging response is extremely strong with cold work (temper T3), and even stronger without cold work (T4); in fact, it has a stronger natural aging response than that of any other known aluminum alloy. Weldalite 049 undergoes reversion during the early stages of artificial aging and its ductility increases significantly up to 24%. Tensile strengths of 700 MPa have been attained in both T6 and 18 tempers produced in the laboratory.

Weldalite 049 has very good weldability. For example, it displays no discernable hot cracking in highly restrained weldment made by gas tungsten arc, gas metal arc and variable polarity plasma arc (VPPA) welding. Extremely high weldment strengths have been reported using conventional 2319 filler, and even higher weldment strengths have been obtained with the use of proprietary Weldalite filler.

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