Modeling-Based Processing of Al-Li Alloys for Delamination Resistance

Al-Li alloys are of interest for use in aerospace structures due to the desirable combination of high strength and low density. However, high strength Al-Li alloys exhibit an intergranular delamination fracture mode which has limited widespread structural applications. Previous work at NASA LaRC, under limited testing, found that the failures tend to be between grains of the two Brass texture component variants. Taylor Factor calculations show these to have severe strain incompatibility. These grain orientations are known to develop as a preferred orientation in Al-Li-X-Zr alloys during severe, hot rolling. This program will verify the results and work with Industry to develop an alternative, delamination-free process.

Designers have been reluctant to use Al-Li in a structural capacity on aircraft for the very problem we are attempting to solve with this program: delamination fracture. If this problem is solved, then a number of applications will benefit from the high strength and low density of these alloys, due to lower fuel costs and higher vehicle load capabilities. The higher cost of Al-Li-X alloys is worthwhile for applications which benefit from the improvements in specific strength and stiffness. These include, in order of fuel cost savings: spacecraft, aircraft, ground vehicles and shipping.

Aluminum-Lithium alloys are used for cryotanks by NASA. Structural applications could also benefit from the low specific weight and high specific stiffness available with these alloys, if the delamination problem can be solved. Fail safe design philosophy requires better reliability for structures, and this approach, if successful, will allow widespread use of these alloys in NASA hardware.

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