Unexpected Long-term Room Temperature Hardening of AA7075 after Thermo-mechanical Processing and T6 Aging

Kurzfassung

Given the potential to tailor the microstructure and mechanical properties of precipitation-hardenable aluminum alloys, different hot forming processes are proposed to realize load-adapted, complex-shaped components suitable for the aerospace and automobile industry. Differential cooling, an approach inspired by hot stamping of steel, offers the opportunity to create varied microstructures and dissimilar mechanical properties within one part of high-strength aluminum alloys by locally controlled cooling rates. The concept of differential cooling is based on three sequential stages; namely solution heat treatment, hot forming using segmented forming tools with heated and cooled zones and artificial aging to attain the peak strength. However, since these formed components are used as structural elements, they are expected to remain in service for extended periods and must maintain their mechanical property and performance throughout long‑term operation.

Therefore, the present study aims to investigate the effect of the long-term room temperature (RT) aging on mechanical property and associated microstructural changes of hot formed AA7075. For this purpose, AA7075 sheet materials were solution heat-treated at 480 °C, formed using heated tool at 350 °C and held within closed forming tools for 10 s to 30 s, followed by artificial (T6) aging at 120 °C for 20 h. After T6 aging, the first set of samples was immediately subjected to mechanical testing, microstructure observation by differential scanning calorimetry (DSC) and transmission electron microscopy (TEM) techniques, whereas the other set was stored at RT for five‑years.

The results demonstrate that despite prior T6 aging, which is generally expected to stabilize the microstructure, an unexpected increase in strength following long-term storage at RT for 5 years occurs, compared to the tested condition immediately after T6 aging. Eddy-current-based measurements of electrical conductivity show a decrease in conductivity with increasing RT storage time, suggesting changes in the microstructure. To analyze these microstructural changes and understand the strengthening mechanism resulting from the RT storage, DSC and TEM were utilized to characterize phase evolution and the nano-sized matrix precipitates, respectively. The current observations provide new insights into the precipitation behavior and the strengthening mechanism of high-strength 7xxx alloys processed by the differential cooling technique.