Influence of rolling parameters on dynamically recrystallized microstructures in AZ31 magnesium alloy sheets

Conventional rolling experiments via the embedded pin in rolling sheet method were carried out at different reduction rates, starting rolling temperatures, and rolling speeds, and the effects of rolling parameters (i.e., temperature, equivalent strain, and rolling time) on dynamically recrystallized (DRX) microstructures of AZ31 alloy during hot rolling were studied quantitatively. The temperature-strain dependence of the high-angle grain boundary fraction (HAGB%) was examined through electron backscattered diffraction. Results showed that as-rolled microstructures with high HAGB% may be obtained under average rolling temperatures of 270-320 °C, equivalent strains higher than 0.8, and a rolling speed of 246 mm/s. These results may be related to the DRX kinetics and dynamic recovery which are controlled by deformation temperature and strain. HAGB% decreased with increasing rolling time (decreasing rolling speed), which is attributed to dynamic recovery, and the recrystallized grain size decreased as rolling time increased. However, further increases in rolling time increased average grain sizes but decreased mean subgrain sizes; these results are attributed to increases in the low-angle grain boundary (LAGB) length per unit area with rolling time. LAGB formation was controlled by dynamic recovery, which consistently follows polygonization or formation of new subgrains inside larger grains; hence, average subgrain sizes decreased with the rolling time. The effect of dynamic recovery on HAGB and LAGB formation and their related mechanisms over a wide range of strains and temperatures were discussed in detail.