A comprehensive examination of the formability-natural ageing stability time paradox in Al-Mg-Si alloys and developing mitigating pathways

Abstract

The present work scrutinizes the trade-off between natural ageing stability time and formability achieved via preageing and two-step quenching in Al-Mg-Si alloys. Experimental analysis via resistivity and yield strength measurements and differential scanning calorimetry revealed that a prolonged isothermal hold period of 4 h at 100 °C is required to achieve natural aging stability via preageing. In contrast, an isothermal hold period of 1 h at 100 °C is sufficient to impart natural ageing stability via two-step quenching. Vacancy simulations revealed that the free vacancy concentration decreases during the isothermal hold period and equates the free vacancy concentration at 100 °C after an isothermal hold period of 4 h and 1 h for the preaging and two-step quenching route, respectively. The decrease in free vacancy concentration during the optimized isothermal hold periods causes reduced mobility of the solute atoms and suppression of natural ageing during the subsequent room temperature storage. Formability analysis revealed that on all forming indicators such as work hardening ability, strain rate sensitivity and fracture strains, the preaged sample outperforms the two-step quenched sample. An obstacle characteristics-based approach revealed that the higher dynamic recovery rate in the two-step quenched sample due to a relatively coarse precipitate structure is responsible for the impairment of the forming potential of the two-step quenched sample compared to the preaged sample. Based on vacancy and work-hardening simulations, a suitable thermomechanical processing route is proposed to produce a fine-grained microstructure, which reduces the natural aging stability time by 75 % without compromising the formability.