Effect of variable rate non-isothermal aging on microstructure and properties of Al-Zn-Mg-Cu alloy

Variable rate non-isothermal aging (VR-NIA) treatment by changing the heating and cooling rates is proposed and applied to 7055 aluminum alloy to optimize the mechanical properties and corrosion resistance. The investigated 7055 aluminum alloy undergone two-stage solid solution is heated from 100 °C to 160 °C at a rate of 20 °C/h, then heated to 210 °C at a rate of 60 °C/h, and finally cooled to 100 °C according to the symmetrical temperature route, the hardness and tensile strength can reach 173.3 HV and 654.6 MPa, respectively, and the electrical conductivity is able to reach 38.6 %IACS. At the same time, improved corrosion resistance is obtained, the maximum depth of intergranular corrosion (IGC) is 29.4 μm, the 48 h exfoliation corrosion (EXCO) grade is EA, and the stress corrosion sensitivity index (ISSRT) is 1.8%. The results of electrochemical tests show that the open circuit potential (OCP) of VR-NIA alloy is −764.2 mV, the corrosion potential (Ecorr) is −715.3 mV, and the corrosion current density (Icorr) is 4.6 × 10−8 A/cm2. Microstructure observation shows that GPI zones, GPII zones and a small number of η’ phases are formed during the heating process of 100 → 160 °C, which enhances the mechanical properties of the alloy; in the process of 160 → 210 → 160 °C, small-sized precipitates dissolve back, and part of the large-sized η’ phases grow and transform into η phases. At the same time, the grain boundary precipitates (GBPs) change from continuous to discontinuous distribution, forming a precipitation-free zone (PFZ) with a suitable width, which greatly improves the electrical conductivity and corrosion resistance of the alloy. In the process of 160 → 100 °C, the GPI zones are re-precipitated, and more η phases are formed from the η’ phases without remarkable coarsening. Enhanced performance can be achieved in a relatively shorter time through VR-NIA, behaving with greater advantage over constant-rate non-isothermal aging.