Effect of grain size inhomogeneity of ingot on dynamic softening behavior and processing map of Al-8Zn-2Mg-2Cu alloy

The effect of grain size inhomogeneity of large ingot on the hot deformation behavior of Al-8Zn-2Mg-2Cu alloy were investigated using hot compression tests over a temperature range from 300 A degrees C to 450 A degrees C with strain rate from 0.1 s(-1) to 10 s(-1). It was found that the grain size (100 mu m) in surface layer of the ingot is finer than that of the central layer (> 200 mu m). The surface specimen exhibited lower flow stress than that of central one in the temperature range from 350 A degrees C to 450 A degrees C and the strain rate range from 0.1 s(-1) to 1 s(-1). The softening mechanism is dynamic recovery, together with a partial dynamic recrystallization (DRX) at high temperature and low strain rate condition regardless the grain size. The nucleation mechanism of DRX is the strain induced grain boundary migration. Therefore, the DRX fraction is higher in surface specimen because the higher grain boundary density provides more nucleation sites. Based on processing maps, the applicable hot deformation parameters for the large size ingot are determined to be at the temperature range of 440-450 A degrees C and the strain rate of 0.1-0.3 s(-1) when taking the grain size inhomogeneity into account.