Al-Li, Aluminum Lithium Alloys

Lithium is the least dense elemental metal. When combined with aluminum to produce Al-Li alloys every 1% increase in lithium to the mix results in a 3% reduction in the 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 stiffness, high elastic modulus, fatigue and cryogenic durability(the 3rd generation of Space Shuttle external fuel tanks are made of Al-Li alloy), however the increased stiffness 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 of the jagged path cracks must follow through the alloy.

Some Al-Li alloys that are commercially available are: Alloy 2090 developed as a replacement for 7075-T6, offering 8% lower density and 10% higher stiffness than the conventional alloy that is used heavily in aircraft structures. The 2090 alloy also has a higher corrosion resistance in salt-spray (marine) environment than 7075-T6. Alloy 2091 developed as a replacement for conventional aluminum alloy 2024-T3, offering 8% lower density and 7% higher modulus as well as superior damage tolerance. Alloy 8090 developed as a replacement for some of the most long serving of the commercial aluminum alloys, namely 2014 and 2024. Alloy 8090 has 10% lower density and 11% higher modulus than these conventional counterparts, and 8090 exhibits superior mechanical properties at cryogenic temperatures. Weldalite 049 a weldable Al-Li alloy designed to replace 2219 and 2014 in spacecraft launch systems. The density of Weldalite 049 is 2.7 g/cm3 (about the same as its conventional counterparts), it has about 5% higher modulus than 2024, and tensile strengths of forged parts in excess of 700 MPa have been reported.

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 handling costs. Al-Li alloys are typically used in the construction of aircraft wing edges and access covers. Military and space applications are also common as main wing boxes and main fuselage. Al-Li alloys should not be combined with some other aluminum alloys as there is a potential for explosive reactions.