THE EFFECT OF COMPOSITION AND CASTING PRACTICE ON THE DYNAMIC RESTORATION PROCESSES AND HOT DUCTILITY OF ALUMINUM-ALLOYS

Abstract

An attemps has been made to establish and evaluate the influence of insoluble secondary phases formed by principal impurity elements (i.e. Fe and Si) on the dynamic restoration processes operating under high temperature deformation conditions of commercial purity aluminium. As the deformation temperature increases, dynamic softening processes facilitate dislocation climb and other relaxation mechanisms that reduce the stress concentration and thereby lead to greater hot ductility. However, the ease of relaxation and the temperature at which each alloy exhibits an elongation peak are dependent on the as-cast structure and the composition. Typically in permanent-mould cast Al-0.5% Fe alloy (solidified at a low cooling rate) dynamic recovery, which balances work hardening, occurs at relatively low temperatures leading to maxima in ductility; the converse occurs in rapidly solidified direct-chill cast Al-0.5% Fe alloy in which impurity elements are present largely as very fine particles after thermal and mechanical processing. In commercial purity aluminium the maximum ductility temperature corresponds to the temperature at which 100 % softening is achieved by dynamic softening and to the threshold temperature above which cavitation becomes effective.