Abstract
The influence of quench rate after solution heat treatment on the microstructure in the as-quenched state and subsequent ageing kinetics of alloy AA6014 was investigated by means of transmission electron microscopy, positron annihilation lifetime spectroscopy and hardness measurements. Various ageing temperatures and stages were taken into consideration. Consistent with previous studies, we found that solute and vacancy supersaturation decrease during slow quenching due to precipitation and annihilation, respectively. Additionally, we observed cluster formation during cooling below 200 °C. As for the influence on ageing behaviour we observe different behaviour for high and low ageing temperature: Artificial ageing is more affected than pre-ageing and natural secondary ageing. The detrimental effect of natural ageing on paint-bake hardening also depends on the quench rate. Possible interpretations are associated with cluster formation during natural ageing and also during quenching. The influence of pre-ageing at different temperatures on subsequent ageing kinetics is similar for slower industrial-type quenching and for fast quenching, thus allowing to apply the findings from idealised quenching conditions to situations closer to real application.
Highlights
Precipitation hardening is the main method for strengthening the Al-Mg-Si alloys (6000 series)
The influence of quench rate after solution heat treatment on the microstructure in the asquenched state and subsequent ageing kinetics of alloy AA6014 was investigated by means of transmission electron microscopy, positron annihilation lifetime spectroscopy and hardness measurements
As the first step after solution heat treatment (SHT), quenching plays a crucial role as it ensures the high level of dissolved solute atoms and trapped excess vacancies necessary for precipitate formation during the ensuing natural or artificial ageing (NA/AA)
Summary
Precipitation hardening is the main method for strengthening the Al-Mg-Si alloys (6000 series). Studies of quench sensitivity have shown that slower quenching can limit the maximum hardenability of the Al-Mg-Si alloys during direct AA [1,2,3]. PA suppresses hardening during subsequent natural secondary ageing (NSA) and enhances the final paint-bake (PB) response [7, 8], the effect of which is determined by the PA temperature [9]. Whether the multi-stage ageing behaviour will be influenced by the quench conditions has been studied only recently, when the combined effect of quench rate and NA on AA was investigated on two different alloys [10, 11], but the mechanisms still remain elusive. In this study we aim to elucidate the influence of quench rate on the microstructure as well as on the following single-, two-, and multi-stage ageing treatments
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