Design of Solute Clustering During Thermomechanical Processing of AA6016 Al-Mg-Si Alloy

Solute clustering is a technologically important microstructural process in Al alloys. Exerting control over this process in order 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 coiling step, 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 sought to raise the strength of AA6016, which nominally contains little or no Cu, to levels equivalent to the higher Cu-bearing 6xxx grade alloys, whilst preserving excellent ductility. This novel combination of process design and alloy selection has been studied in detail using a combination of atom probe tomography (APT) and first principles density functional theory (DFT) simulations. The thermomechanical process described here invokes a strong cluster strengthening phenomenon. We report on the details of the single-species Si-Si and Mg-Mg clusters and those of the Mg-Si co-clusters and relate the APT observations to our DFT calculations. Mg-Si co-clusters ≥ 20 solute atoms are found to be responsible for the excellent properties in the material exposed to the pre-ageing and paint-baking process. Our results 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.