Effect of thermal strain on the microstructure evolution and post-aging mechanical properties of Al-Zn-Mg-Cu alloy in simulating hot stamping process

Abstract

To identify the effect of thermal strain on the microstructure evolution and mechanical properties of Al-Zn-Mg-Cu alloys in simulating hot stamping process, the alloys were strained at elevated temperature and then artificially aged at 120 °C for different time. The post-aging mechanical properties of the alloys were examined, and the microstructures after aging were systematically characterized via using electron backscattered diffraction (EBSD), X-ray diffraction (XRD), differential scanning calorimetry (DSC), and transmission electron microscopy (TEM). Especially, the effect of dislocation on precipitation behavior and its mechanism on properties were analyzed in detail. The results show that an 8.96% rise of ultimate tensile stress (UTS) can be found in solid solution specimens and the peak-aging time shortens by 50% with the increase of thermal strain from 0% to 30%. The deformed grains are slightly refined from 12.62 μm to 10.99 μm when the strain increases to 30%, while aging time has no obvious effect on the grain size and orientation. Experiments demonstrate that dislocations definitely exist after thermal strain, and will not be completely devoid because of recovery. To a certain extent, dislocations can be used as the favorable nucleation core for the precipitates, promote the rapid formation of strengthening phases, and finally result in a shorter peak-aging time with mixed intragranular precipitates of a large amount of η' and a small amount of GPII zones.