The Influence of Transient Change in Strain Rate on Subgrain Size During Deformation by Torsion of AA6063

During hot extrusion a extrusion billet s cross section is reduced, altering the metals micro- structure. As the billet is passing through the extrusion die the strain rate and temperature increases, resulting in a with a highly deformed extrusion profile with a high subgrain density, giving a great driving force for recrystallisation. The extruded profile recrystallises almost imme- diately after the extrusion die [1, 2]. However, recrystallisation after extrusion of non-dispersoid containing alloys like AA6063 is not fully understood. Such alloys tend to undergo uncontrolled grain growth in the surface area [2, 3]. This master project aims to give a better understanding of the recrystallisation process occurring under extrusion by investigating the subgrain size evolution during transient changing strain rates. Using torsion to deform the metal, the deformation starts with a high (1st) strain rate allowing the steady-state microstructure of the high strain rate to be developed, then immediately changing the strain rate to a lower (2nd) strain rate, effectively simulating the passing through the extrusion die. The objective was to identify the strain region in which the subgrain size is in a transient state between the 1st and the 2nd steady-state microstructure, referred to as the metals deformation memory. AA6063s deformation memory at 375°C was found to be in the higher strain region of shear strains between 0.18 − 0.26 (equivalent to strains between 0.10 − 0.15). In the investigated strain region it was found little texture evolution. Investigations of the texture found that the samples texture intensity increased with recovery due to annihilation of the orientation gradients. A correlation between the recorded stress and the deformation memory was also found, whereas the stress is in a transient state during the deformation memory