Design of solute clustering during thermomechanical processing of AA6016 Al–Mg–Si alloy

Abstract

Solute clustering is a technologically important microstructural process in Al alloys. Exerting control over this process to enhance the alloy properties and reduce the energy costs of production is a major scientific and technological focus, with automotive sheet applications serving as a key driver. In this work, we detail changes in the state of clustering arising from the insertion of a thermomechanical pre-ageing process, via a coil-cooling step in a commercial production line, immediately after solution treatment. This pre-ageing step effectively mitigates the negative effects of the natural ageing on the mechanical properties that would otherwise occur after solution treatment and ahead of the final paint-bake step. Our work was focussed on a short duration/low temperature single-step pre-ageing process on a Cu-lean grade of AA6016. Using a combination of atom probe tomography and first principles density functional theory simulations, we are able to reveal and explain the atomic-scale microstructure arising from this thermomechanical process. Mg–Si co-clusters ≥ 20 solute atoms were found responsible for the excellent properties in the material exposed to the pre-ageing plus paint-baking process. Our simulations revealed that vacancies can effectively stabilise single-species Si clusters enabling them to attract further solute and serving as a pathway to formation of the larger Mg–Si co-clusters that are so beneficial to alloy properties. Our simulations also revealed that the nearest neighbour configurations are a critical aspect to the stability of the clusters.