Heat treatment and age hardening of Al–Si–Mg–Mn commercial alloy with addition of Sc and Zr

Abstract

Microstructure and precipitation processes in an Al–Si–Mg–Mn–Sc–Zr alloy were studied. The as-cast alloy contains fine Al3(Sc,Zr) particles with L12 structure, rods and/or needles of the Mn,Fe-containing phase and Si,Mg co-clusters. These co-clusters have a detrimental effect on precipitation hardening which is caused exclusively by fine needle-shaped β″ phase particles. The pronounced precipitation of the Al6(Mn,Fe)-phase was observed during isochronal annealing in the as-cast alloy. Nevertheless, the precipitation has a negligible effect on hardness. The apparent activation energy for the Al6(Mn,Fe)-phase precipitation was calculated as (170±20) kJ·mol−1. Heat treatment at 530°C for 45min dissolves Si,Mg co-clusters but causes a precipitation of the Al12(Mn,Fe)3Si phase particles while the Al3(Sc,Zr) particles remain unaffected. The Sc and Zr addition to the Al–Si–Mg–Mn alloy influences precipitation hardening in the course of isochronal annealing neither in the as-cast nor in the solution-treated alloy. The age hardening is very poor in the as-cast alloy (~4%) and it is more pronounced in the solution-treated alloy (~60%) due to more convenient size, aspect ratio and number density of the β″ and β′ phases. The apparent activation energy values of precipitation of the transient β″ and β′ phase were determined as ~90kJ·mol−1 and ~130kJ·mol−1, respectively.