Determination of the Onset of the Dynamic Recystallization of a 7075 Al Alloy

Abstract

This paper presents an investigation about the occurrence of the dynamic recrystallization (DRX) during hot forming. Two 7075 aluminum alloy samples in different initial states were examined by compression tests at temperatures between 573 K and 723 K and constant strain rates ranging from 0.002 to 2 (s-1) with the maximum strain of 0.5 mm/mm. The activation energies of the examined aluminium were calculated, being 137.7 kJ/mol and 142.4 kJ/mol. The critical stresses and strains for the initiation of the dynamic recrystallization were determined using a numerical method based on the changes of the work hardening rate (θ) as a function of the flow stress (σ) or a strain (ε), respectively. Based on the micrographs taken after the deformations it was revealed that in some cases only dinamyc recovery (DRV) whereas in others DRX occurred under the applied examination conditions. The critical stress which belongs to the onset of the DRX or the DRV depend on the temperature, the strain rate, and the initial grain size. The critical and maximal stresses and the corresponding strain values at different conditions were determined. In the examined range of deformation, temperature and strain rate, the critical rations are σ_c/σ_p= (0.93-0.99) and εc/ εp = (0.3-0.74), respectively.