R. Muzzolini, Alcan Aerospace, Kirkland, WA; M. Niedzinski, Alcan Aerospace, Chicago, IL; H. Ribes, Alcan Aerospace Transportation and Industry, Issoire Cedex, France; D. J. Chellman, E. Balmuth, M. Crill, Lockheed Martin Aeronautics, Fort Worth, TX; K. P. Smith, Alcan Rolled Products, Ravenswood, WV

Summary:

Much of the internal structure of modern fighters is made of bulkheads, ie stiffened structures of complex shape, which are usually machined from thick aluminum plates. These structural parts require a well-balanced set of properties (strength, fatigue, toughness and corrosion resistance) making 7050 the natural choice. Alcan Aerospace has developed an Aluminum- Lithium solution that provides a balance of properties equivalent to 7050 with the beneficial lower density and high fatigue resistance of Lithium-containing alloys. Significant weight savings of around 5% can thus be achieved through a simple “drop-in-replacement” strategy of 7050 by this newly registered 2050, with no additional re-design costs. This further enables reduced inspection and replacement costs where fatigue margins might be low and only marginally improved with local repairs.

This 2^nd generation Li-containing 2XXX alloy has a low Lithium content for improved damage tolerance and lower quench sensitivity. Weldalite alloys such as 2050 contain the minor alloying element Ag, whose addition has been demonstrated to increase peak strength of T1-hardened Al-Cu-Li alloys. Initial results of thermal exposure testing indicate that Yield Strength is stable and that toughness does not drop very fast, even after significant time at 100ºC (212 F). Since corrosion behavior is also excellent, there is some latitude regarding the aging treatment. Thus, it can be tailored for more toughness or higher static properties, while ensuring that properties stay above the 7050 level.

Since Al-Li alloys have a propensity for crack branching under mixed-mode loading, a large amount of testing is under way to evaluate the behavior of 2050. Fatigue testing on L-S hourglass tensile coupons as well as SENB coupons will provide comparative results with 7050. A more comprehensive study is also currently carried out by Lockheed-Martin Aeronautics to better understand the phenomenon and define criteria for crack branching prediction.