An age-hardenable aluminium alloy containing roughly four percent copper, along with small amounts of magnesium and manganese. It was one of the earliest high-strength aluminium alloys developed, offering a strength-to-weight ratio that made it standard for aerospace construction. Because the copper content makes it susceptible to corrosion, it is often clad with pure aluminium.
Heard on the job
The vintage aircraft frames were mostly riveted duralumin, which explains why we're finding so much intergranular corrosion on the unclad sections.