Influence of a Preageing Treatment on the Cluster Formation and the Further Ageing Route in Al–Mg–Si Extrusion Alloys

Abstract

Herein, the influence of one‐ and two‐step preageing treatments prior to the artificial ageing step to peak age condition for three different 6xxx alloys, from low to high alloy content, is investigated. Analysis of the preaged conditions using atom probe tomography (APT) reveals a broad distribution of Mg and Si clusters, with some instances of Guinier‐Preston I zones present. Additionally, the high alloyed system exhibits a growth of precipitates toward a needle‐like shaped. These clusters formed during the preageing treatment serve as nucleation sites in the subsequent artificial ageing treatment, leading to an increase in hardness. Microstructural analysis conducted using transmission electron microscopy and APT demonstrates a reduction in the average precipitate length and a more narrow size distribution for T6 condition. This indicates that the preageing treatment results in a decreased portion of already overaged precipitates compared to the direct aged condition. Consequently, the more narrow size distribution of the precipitates leads to more effective hardening of the material, as a higher portion of precipitates is at or close to their optimum size. The investigations highlight the significant influence and positive effect of a preageing treatment at lower temperatures on the subsequent artificial ageing treatment.