Al-Li alloys
Aluminum-lithium alloys were developed primarily to reduce the weight of aircraft and aerospace structures. Initially, it was hoped that the new family of Al-Li alloys would be a direct weight–saving substitute for conventional aluminum alloys, their density being 8-12% lower than that of conventional 2xxx and 7xxx aluminum alloys with similar mechanical properties and manufacturability. Their modulus of elasticity is 10-12% higher. These advantages make them appear to be cost effective for a variety of applications. However, several property and processing differences were noted over the past 20 years, as the alloys underwent intense scrutiny by airframe manufacturers and university researchers. Problem areas that have been addressed include: ingot quality and product fabrication and supply; differences in materials testing; customer specifications; thermal stability of some products; assembly methods; forming, drilling, riveting, welding; super plastic forming; deoxidizing; finishing, and chemical milling.
Aluminum-lithium alloys are the most often selected aerospace components because of their low density, high strength and high specific modulus. For one-time or multi-use systems, the reduced weight--and higher modulus and in many cases, higher strength-- provided by Al-Li alloys completely overshadow any material cost differences between conventional aluminum alloys and Al-Li alloys. But these efficiencies of production and utilization of this more expensive material must outperform those of conventional aluminum alloys to offset the initial premium on material cost. The higher material price needs to be balanced by high “buy/fly ratio” for the finished products. Due to the forging technology the scrap rate is heavily reduced which results in better material utilization compared to machined parts.
The availability of new Al-Li alloys, and their optimized forging technology with cold deformation prior to aging, would help realize high “buy/fly ratio” components from Al-Li alloys.
The density of Al-Li alloys is 8-12% less than that of the present aerospace Al alloys. This advantage can be exploited in the manufacture of large-weight components including aircraft wheels, side frames, forged wing structures, doors and other thick-wall structures and components.
Besides the weight reduction, Al-Li alloys offer additional benefits to these applications, such as 12% higher stiffness and significant improvement in corrosion resistance.
The proposed benefits of Al-Li alloys have not been realized to date on a large commercial aircraft due to technological concerns with respect to fracture toughness, stress corrosion cracking resistance, thermal stability and short transverse ductility. Another important disadvantage has been a significant cost penalty. Alternatively, significant efforts over the past years to resolve these shortfalls made them viable candidates for currently considered applications.
Forging of Al-Li alloys
Although application of Al-Li extrusion and sheets and fuselage and wing structures may be considered as industrial technology, forging of Al-Li alloys still continues to be a research target.
The precise forging of 8090 and 2091 alloys, for example, has been investigated in European aerospace industry since 2000. The technology was not industrialized because of two major problems: formation of cracks in the material during forging operation, and degradation of material properties in short-transverse direction.
Another problem is he large volume of high-cost scrap after machining of forged components. Therefore, development of more precise forging technology is required.
SMWE has experience with the application of modified 1420 alloy for forging of automotive wheels. The wheel forged from the alloy was tested in comparison with the same wheel from AA 6061-T6 alloy. The results showed that ultimate momentum in rotation was 13% more for Al-Li wheel. The maximum load on Al-Li wheel was 8% more than on an aluminum one. The theoretical analysis showed that weight reduction of 20-25% is possible due to higher strength properties of 1420 alloy.
