The influence of natural aging on the precipitation behavior of the low-alloy content Al-Zn-Mg aluminum alloys during subsequent artificial aging and related mechanisms

Abstract

The age hardening and precipitation behavior of the low-alloy content and widely used Al-Zn-Mg alloy during artificial aging was studied adopting methods of Vickers hardness testing and transmission electron microscopy. To meet with practical application of the production line conditions, the influence of natural aging (NA, at room temperature) on the precipitation behavior during subsequent artificial aging (AA, at 130 °C∼150 °C) and related mechanisms are discussed based on the classical nucleation theory. The results show that precipitates like solute clusters and GP zones will form during the NA process, which have a significant influence on the subsequent precipitation behavior during AA. These NA precipitates will dissolve at the initial stage of AA resulting in abundant localized supersaturated regions which are the favorable nucleation sites for η′ phase. This positive effect is highly dependent on the temperature adopted during the AA process and the duration of NA. Higher AA temperatures result in a more noticeable increase in hardness compared to the samples without NA but require a longer duration of AA to achieve a relatively high and stable peak-aging hardness. However, an excessive long NA period can lead to a significant drop in hardness in the over-aging status. In the low-alloy content Al-Zn-Mg alloys, the actual nucleation rates of precipitation during the early stages of AA vary significantly with the AA temperature, which serves as the underlying cause of the observed NA effect. These results provide rather an important guidance for the production process of the 7003alloy and other aluminum alloys with similar chemical composition.