Influence of retrogression and reaging heat treatment on calorimetric behavior of EN AW-7049a alloy

Aluminum alloy EN AW-7049A samples in artificial (T6 temper) and after multi-stage aging (T77 temper, i.e. retrogression and reaging treatment- RRA), were investigated by means of differentialscanning calorimetry (DSC).DSC was employed to distinguish the dissolution behavior of different metastable and Cu-enriched precipitate populations in EN AW-7049A alloy in the T6 and T77 tempers. DSC thermographs exhibited two consecutive endothermic peaks in the T6 temper, which correspond to the sequential dissolution of metastable semi-coherent phases. First peak correspondsto the main strengthening η' phase, the second peak is associated with the dissolution of thermally more stable Cu-enriched η′/incipient η precipitates. These Cu-rich precipitates enhance yield strength and hardness by increasing dislocation resistance and significantly improve thermal stability by retarding precipitate coarsening and overaging.In the T77 temper, only a single endothermic peak was observed, indicating a reduced concentration of metastable η′ precipitates and the predominance of coarser η′ or incoherent equilibrium η precipitates formed during the retrogression and reaging treatment. The dissolution and transformation of metastable semi-coherent precipitates during RRA promote the nucleation and growth of the stable incoherent η phase, particularly at grain boundaries, leading to strength reduction. Consequently, the increased fraction and coarsening of η precipitates account for the lower strength of the EN AW-7049A alloy in the T77 temper compared with the initial T6 temper.