Abstract
Both Sn addition and pre-ageing are known to be effective in maintaining the artificial ageing potential after natural ageing of Al–Mg–Si alloys. In this study, the combined effects of Sn addition and pre-ageing at 100 °C or 180 °C on natural secondary ageing and subsequent artificial ageing of an alloy AA6014 were investigated using hardness, electrical resistivity, differential scanning calorimetry and transmission electron microscopy characterizations. It is found that pre-ageing can suppress natural secondary ageing and improve the artificial ageing hardening kinetics and response after 1 week of natural secondary ageing in both alloys with and without Sn addition. The effect of pre-ageing at 100 °C is more pronounced in the Sn-free alloy while the combination of pre-ageing at 180 °C and adding Sn shows superiority in suppressing natural secondary ageing and thus avoiding the undesired hardening before artificial ageing. Moreover, when natural ageing steps up to 8 h are applied before pre-ageing at 100 °C, the effect of pre-ageing in Sn-added alloy can be further improved. The influence of Sn on vacancies at different ageing temperatures is discussed to explain the observed phenomena.Graphical abstract
Highlights
The age-hardenability of Al–Mg–Si alloys is of great importance for their industrial application
A delay at room temperature (RT) after solutionising is inevitable and leads to a reduction in the hardening kinetics and the achievable strength during the following artificial ageing (AA) [3,4,5]. One reason for this negative effect of natural ageing (NA) lies in the fact that clusters formed during NA cannot transform into b’’ during AA [6] but rather reduce the solute supersaturation. Another possible explanation might be that the vacancy concentration, which plays a vital role in the diffusion of solute atoms, is lowered by the annihilation of vacancies by sinks [7] and the ‘‘vacancy-prison’’ effect of the formed NA clusters [8]
Sn addition delays hardening during NA
Summary
The age-hardenability of Al–Mg–Si alloys is of great importance for their industrial application. A delay at room temperature (RT) after solutionising is inevitable and leads to a reduction in the hardening kinetics and the achievable strength during the following AA [3,4,5] One reason for this negative effect of natural ageing (NA) lies in the fact that clusters formed during NA cannot transform into b’’ during AA [6] but rather reduce the solute supersaturation. Pre-ageing has been proved to be efficient in diminishing the negative effect of NA, the accompanying strength increase and the strengthening during following natural secondary ageing (NSA) reduces formability It has been suggested by some researchers [14, 22] that the PA time and temperature must be controlled in order to achieve a trade-off between an acceptable formability and a good AA response. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) imagining was performed on a spherical aberration probe corrected FEI Titan G2 80–200 ChemiSTEM operated at 200 kV
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