Ageing behavior and stress corrosion cracking resistance of a non-isothermally aged Al–Zn–Mg–Cu alloy

A non-isothermal ageing (NIA) process was proposed for an Al–Zn–Mg–Cu alloy aiming to accommodate heating and/or cooling procedures in large components. The precipitation was investigated systematically via TEM observation and DSC analysis. The age-hardening and the conductivity variation were examined to evaluate the potential in the strength and the corrosion resistance, respectively. Double cantilever beams (DCB) experiments were carried out to evaluate the stress corrosion cracking (SCC) resistance. A secondary precipitation occurs during the cooling procedure of the NIA process, which leads to an increase in the precipitates number density and thus induces an extra hardening. The secondary precipitation also contributes to the increase of conductivity of the alloy in the most part of the cooling procedure, but leads to a slight decrease at the terminal stage. The alloy׳s resistance to SCC improves persistently as the NIA proceeds. The evolution in microchemistry in grain boundary and adjacent regions is supposed to contribute to the improvement of corrosion resistance. The current study suggests that the NIA process (40 °C→190 °C→100 °C, 20 °C/h) is capable of enduing higher mechanical performances and comparable corrosion resistance to Al–Zn–Mg–Cu alloys, as compared with that of the T74 condition, which can be technically significant for ageing treatment of large components.