Reversion of natural ageing and restoration of quick bake-hardening response in Al-Zn-Mg-Cu alloy

Abstract

The high-strength Al-Zn-Mg-Cu alloy provides much better dent resistance and weight-reduction potential compared to the conventional Al alloys used for the automobile body panels. However, natural ageing (NA) significantly reduces the formability of Al-Zn-Mg-Cu alloy. The reversion of natural ageing has been systematically investigated by hardness test, tensile test, differential scanning calorimetry (DSC) and transmission electron microscopy (TEM). Substantial reversion of natural ageing and thus hardness decrease occurs immediately upon thermal treatment at 120–210 °C in an Al-Zn-Mg-Cu alloy. Although the hardness of the most reverted state decreases with increasing temperature, the lowest hardness is still higher than the as-quenched state by 30HV. As revealed by the complementary DSC and TEM observations, this is ascribed to the synchronization of the dissolution and the re-precipitation of the solutes in the NA clusters during reversion ageing. Reversion at 180–210 °C for less than 30 s leads to a hardness decline of 40HV. The hardening kinetics during NA after reversion is slower than that during first-time NA due to the reduced vacancy concentration. Artificial ageing at 180 °C for 30 min after secondary NA of less than 24 h induces intensive precipitation of plate-like pre-η phases and a giant strength increase of 188–204 MPa. Potential use of high strength Al-Zn-Mg-Cu alloy in automobile body panels could be realized by appropriate reversion treatment improving the formability and the quick bake hardening response.