Softening of Al during multi-axial forging in a channel die

Multi-axial forging of Al was carried out at room temperature in a confined channel die as a means of achieving severe plastic deformation (SPD). The microstructures of the 3, 6 and 9 pass samples were quantified using electron back scattered diffraction to obtain the distribution of boundary spacing and fraction of high angle boundaries. Subsequent compression tests at room temperature were carried out to determine their strength and strain rate sensitivity. Softening was observed beyond 6 passes both in terms of the pressing stress as well as the subsequent compression tests. Beyond 6 passes the fraction of high angle boundaries increased and the geometrically necessary dislocations (GND) density decreased. It was estimated that during SPD, Al had a higher hardening contribution due to GND and subgrain walls than that due to Hall-Petch type grain boundary strengthening. It was shown that as the fraction of high angle boundaries increased with increasing deformation, the dislocation strengthening term decreased and the Hall-Petch term increased, resulting in a net softening effect.