Effects of pre-stretching and baking treatment on microstructural evolution and mechanical properties of 7A09 aluminum alloy subjected to pre-hardening forming

Abstract

In this study, a novel pre-hardening forming process is applied to 7A09 aluminum alloy. Then, the effects of pre-stretching and baking treatment on mechanical properties and microstructural evolution are investigated. Uniaxial tensile tests are conducted to evaluate the evolution law of the mechanical properties of alloys treated using different parameters. Microstructural evolution is revealed by transmission electron microscopy (TEM), electron backscatter diffraction (EBSD), and small-angle X-ray scattering (SAXS). Results show that the yield strength (YS) of the alloys pre-aged at 110 ℃ for 12 h (PA) reaches 612–694 MPa after they are pre-stretched by 2–6 %. This strength is 26–43 % higher than that of T6-temper alloy according to the ASTM B221M-21 standard. The elongation of PA alloys with 6 % deformation increases from 2.7 % to 10 % after baking at 215 °C for 1 min. Hence, the subsequent baking treatment compensates for the loss in ductility. The TEM result shows that numerous GPII zones and few η΄ phases co-exist in the matrix of the PA alloys. Simultaneously, η΄ phases serve as the main strengthening phases after pre-stretching the PA alloys by 6 %. Moreover, pre-stretching can increase the density of dislocations and the volume fraction of precipitates. In addition, the average size of precipitates has increased from 6.4 nm to 10 nm with the increase of baking time from 1 min to 10 min, and the transformation of η΄ phases to coarse η phases has appeared during this process. All results indicate that the strength–ductility of the foregoing alloys is better than that of T6-temper alloy.