Precipitation kinetics in warm forming of AW-7020 alloy

The warm formability of the precipitation hardening AW-7020 (AlZn 4.5Mg1) alloy is investigated by testing extruded tubes. The precipitation kinetics of different conditions before and after warm deformation is studied by differential scanning calorimetry and transmission electron microcopy. The precipitation conditions are correlated with the results of hardness tests at room temperature and of tensile tests at temperatures between 200 and 350 degrees C at different strain rates. The yield strength decreases with increasing test temperature approaching that of samples in the annealed condition, while the strain at fracture increases. The overall influence of the strain rate on ductility is dominated by the corresponding time required for deformation. The formability of the starting condition T1 as well as the corresponding strain hardening exponent is particularly promising for high strain rates at 250 degrees C, where the metastable precipitates of the T1 condition are dissolved. The short exposure of about 30 s at 250 degrees C re-establishes the potential for precipitation strengthening by natural ageing after the warm deformation and a following paint baking heat treatment maintains the hardness level. (C) 2012 Elsevier B.V. All rights reserved.