Structural phenomena induced in the course of and post low-temperature KOBO extrusion of AA6013 aluminum alloy

Abstract AA6013 aluminum alloy in two states: supersaturated and precipitation hardened (T6 temper), was subjected to low-temperature KOBO extrusion with a high extrusion ratio (λ = 100) with alternating twisting frequency of 2.5 and 8 Hz. Higher twisting frequency resulted in lower extrusion force, which was linked to the high number of generated point defects. The same phenomenon is thought to be responsible for the lack of precipitation hardening in the wires extruded by the KOBO method from the T6 temper resulting either from tensile relaxation, derived from the deformations of crystal lattice around precipitations (interactions with point defects), or the change in solubility of the alloy's components in the overequlibrial concentration of point defects. The structural image of the phenomenon induced in the course of and post low-temperature KOBO extrusion of 6013 aluminum alloy, reveals the trans-grain rows of 'strips' (shear bands) saturated with point defects, with strong local disturbance of the crystal lattice. The images were interpreted as a stage of the 'strips' transformation (disintegration) into clusters of point defects, particularly interstitial atoms. With the use of TEM, HRTEM and HRSTEM techniques, the clusters were identified as nano-metric structural elements containing additional atomic planes. It was concluded, that if the number of generated point defects is higher than the number of defects taking part in the relaxation processes (decrease in, lack of precipitation hardening), during the ageing process, hardening clusters of point defects are formed.