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

Abstract

In this study the influence of 1‐ and 2‐step pre‐ageing 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 pre‐aged conditions using atom probe tomography (APT) revealed a broad distribution of Mg‐ and Si‐clusters, with some instances of GP I zones present. Additionally, the high alloyed system exhibited a growth of precipitates towards a needle‐like shaped. These clusters formed during the pre‐ageing treatment serve as nucleation sites in the subsequent artificial ageing treatment, leading to an increase in hardness. Microstructural analysis conducted using transmission electron microscopy (TEM) and APT demonstrate a reduction in the average precipitate length and a more narrow size distribution for T6 condition. This indicates that the pre‐ageing 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 lead 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 pre‐ageing treatment at lower temperatures on the subsequent artificial ageing treatment.This article is protected by copyright. All rights reserved.