Comparison of long-term natural aging to artificial aging in Duralumin

TL;DR

This study compares natural long-term aging of an aircraft aluminium alloy with artificial aging, revealing differences in microstructure, reversibility of aging effects, and implications for material durability and testing methods.

Contribution

It provides insights into the microstructural evolution of aged aluminium alloys and highlights limitations of artificial aging protocols in replicating natural aging effects.

Findings

Old alloy shows unique nanostructure due to aircraft cycle heat exposure.

Aging effects are reversible with heat treatment.

Artificial aging did not replicate the natural aging nanostructure.

Abstract

The understanding of long-term aging of aeronautical materials, in particular aluminium alloys used in the fuselage and structure of aircraft is of extreme importance for airline fleets. In this work, a plate from an old aircraft (Breguet) was retrieved and studied in terms of microstructure and mechanical properties. A comparison was made between this naturally-aged alloy and a modern alloy on which different artificial aging conditions were applied. The old alloy exhibits a precipitation of $\theta$-Al2Cu at grain boundaries and of $\Omega$-Al2Cu on dispersoids. This non-expected nanostructure for an alloy in T4 state was attributed to the heat that the plate experienced during the aircraft cycles. However, it is shown that this aging is reversible (after a solution treatment). Moreover, the very long time of outdoors exposure seems to have caused intergranular corrosion causing the early failure during tensile tests on some of the specimens. The artificial aging (low temperature, 100{\textdegree}C for up to 10,000h) applied on the modern 2017A alloy did not allow to reproduce the nanostructure of the old plate, meaning that isothermal conditions for artificial aging might not be appropriate in this case.