Introduction to aluminum-lithium alloys
Lithium is the least dense elemental metal. When combined with aluminum to make Al-Li alloys, every 1% increase in lithium in the mixture results in a 3% decrease in density and therefore the weight of the resulting alloy, the modulus also increases by approximately 5%. In addition to reduced weight, Al-Li alloys also have increased hardness, high modulus of elasticity, fatigue and cryogenic endurance (3rd generation Space Shuttle external fuel tanks are made of Al-Li alloy), but the increased hardness of Al-Li alloys means reduced ductility and fracture resistance in the short transverse direction. Al-Li alloys have a high resistance to fatigue crack growth because the serrated cracks must follow through the alloy.
Some commercially available Al-Li alloys are: Alloy 2090, developed as a replacement for 7075-T6, offering 8% lower density and 10% higher hardness than the conventional alloy, which is heavily used in structures on the planes. Alloy 2090 also has higher corrosion resistance in a salt spray (marine) environment than 7075-T6. Alloy 2091, developed as a replacement for conventional 2024-T3 aluminum alloy, offering 8% lower density and 7% higher modulus, as well as superior damage resistance. Alloy 8090, developed as a replacement for some of the longest-used commercial aluminum alloys, namely 2014 and 2024. Alloy 8090 has a 10% lower density and 11% higher modulus than these conventional analogs, and 8090 exhibits superior mechanical properties at cryogenic temperatures. Weldalite 049 is a weldable Al-Li alloy designed to replace 2219 and 2014 in spacecraft launch systems. Weldalite 049 has a density of 2.7 g/cm3 and has about 5% higher modulus than 2024 and reported forging tensile strengths in excess of 700 MPa.
Al-Li is typically 3-5 times more expensive than other aluminum alloys due to the high cost of lithium as well as high processing and machining costs. Al-Li alloys are commonly used in the construction of aircraft wing edges and access hatches. Military and aerospace applications are also common such as main wing boxes and main fuselage. Al-Li alloys should not be combined with some other aluminum alloys because of the potential for explosive reactions.
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