Continuous cooling precipitation diagram of high alloyed Al-Zn-Mg-Cu 7049A alloy

The precipitation behaviour during cooling from solution annealing of high alloyed 7049A aluminium alloy was investigated, covering the complete cooling-rate-range of technical interest. This ranges from slow cooling rates close to equilibrium up to rates above complete supersaturation and is covering seven orders of magnitude in cooling rate (0.0005 to 5000 K/s). The continuous cooling precipitation behaviour of 7049A alloy was recorded by combining different differential scanning calorimetry (DSC) techniques and microstructure analysis by SEM and Vickers hardness testing. The high alloyed, high strength and quench sensitive wrought aluminium alloy 7049A was investigated during quenching from solution annealing by conventional DSC in the cooling rate range of 0.0005 to 4 K/s. In this range at least two exothermal precipitation reactions were observed: a high temperature reaction in a narrow temperature interval of 450-430 degrees C, and a low temperature reaction in a broad temperature interval down to about 200 degrees C. Intensities of both reactions decreased with increasing cooling rate. Quenching from solution annealing with rates up to 1000 K/s was investigated by differential fast scanning calorimetry (DFSC) and the differential reheating method (DRM). A critical quenching rate to suppress all precipitation reactions of 100-300 K/s was been determined.