The 6XXX alloys have been considered to replace 2024 on a number of US Navy programs and alloy 6013 is being used on the Boeing 777. The major problems with 2XXX alloys for fuselage skins is that they must be clad since they can be susceptible to intergranular corrosion. In addition, the 2XXX alloys cannot be fusion welded, a process that is being considered for reducing weight and costs of manufacturing. The 6XXX alloys are weldable and cheaper than 2XXX alloys, however, Cu-rich 6XXX, e.g. 6013-T6 and 6056-T6 are also susceptible to intergranular corrosion. This susceptibility is associated with the formation of precipitate free zones at grain boundaries, which are developed during artificial aging, are depleted in Si and Cu and are anodic with respect to the grains [10]. A new temper has been developed, designated T78 that has a controlled degree of overaging that desensitizes 6056 to intergranular corrosion, keeping the yield strength at an acceptable level with respect to the T6 temper. Other experimental tempers have been developed that prevent intergranular corrosion without any loss in strength [11]. An alternative to changing alloys to facilitate joining, is the use of a relatively new joining method known as friction stir welding (FSW). In this method, the alloys are not melted, but instead are joined in the solid state by mechanical working. In a sense, they simply are “kneaded together” with a rotating tool. The use of FSW creates the opportunity to join a wide range of Al alloys that cannot be fusion welded.