Abstract: High-performance 6xxx aluminium alloys are critical materials for automotive lightweighting, but they inevitably undergo natural ageing during storage and transportation, during which solute aggregates form and influence subsequent precipitation behavior. In this study, three-dimensional atom probe (3DAP) was used to classify solute aggregates into five size categories, and their impact on early-stage artificial ageing was investigated through microstructural characterisation and model-based property analysis. Results show that natural ageing decreases artificial ageing kinetics, i.e., large aggregates (>225 atoms) form rapidly within 0.13 h in samples without prior natural ageing, which are significantly delayed to about 2 h in samples with prior natural ageing. Aggregates containing 10-22 atoms provide the greatest contribution to strengthening, while aggregates of intermediate size (23-75 atoms) contribute the least during the early ageing. Additionally, smaller aggregates (5-9 atoms) have the greatest impact on resistivity, and as natural ageing time increases, the number fraction of these aggregates increases, leading to a decrease in electrical conductivity. A refined precipitation sequence in under-aged conditions is proposed as: SSSS → co-clusters _5-9 → co-clusters _10-22 → co-clusters _23-75 → zones _76-225 → β″ precipitates_>225. These insights highlight the importance of aggregate control in tailoring ageing response and mitigating the adverse effects of natural ageing in automotive aluminium alloys.

Key words: Natural ageing, Aggregates, Precipitation, Strength, 6xxx Aluminium, Atom probe