Microstructure and mechanical properties stability of pre-hardening treatment in Al–Cu alloys for pre-hardening forming process

The stability of the microstructure and mechanical properties of the pre-hardened sheets during the pre-hardening forming (PHF) process directly determines the quality of the formed components. The microstructure stability of the pre-hardened sheets was investigated by differential scanning calorimetry (DSC), transmission electron microscopy (TEM), and small angle X-ray scattering (SAXS), while the mechanical properties and formability were analyzed through uniaxial tensile tests and formability tests. The results indicate that the mechanical properties of the pre-hardened alloys exhibited negligible changes after experiencing 1-month natural aging (NA). The deviations of ultimate tensile strength (UTS), yield strength (YS), and sheet formability (Erichsen value) are all less than 2%. Also, after different NA time (from 48 h to 1 month) is applied to alloys before pre-hardening treatment, the pre-hardened alloys possess stable microstructure and mechanical properties as well. Interestingly, with the extension of NA time before pre-hardening treatment from 48 h to 1 month, the contribution of NA to the pre-hardening treatment is limited. Only a yield strength increment of 20 MPa is achieved, with no loss in elongation. The limited enhancement is mainly attributed to the fact that only a limited number of clusters are transformed into Guinier-Preston (GP) zones at the early stage of pre-hardening treatment, and the formation of θ'' phase inhibits the nucleation and growth of GP zones as the precipitated phase evolves.