7449 alloy is basically aluminum + 8.1% zinc and a minor percentage of a few other elements. From what I have read, the molten alloy is cast in the form of large plates from which the ribs are milled to the desired configuration. Heat treating consists of a solution treatment, tempering treatment followed by an over-aging treatment. This gives the alloy higher tensile strength and modulus verses older aluminum alloys. These properties enable the panels to be designed and subsequently milled to thinner cross sections thereby reducing weight.
My guess would be that the cracks first found in holes of the feet could have been a manufacturing problem where the sharp edges were not radiused to the desired specification in some panels. Holes in structural components are significant stress risers points where small cracks can develop under stress and cycles at sharp corners or edges.
The initial assembly of these A-380 wings is a real engineering achievement!