Hardening of Al–Mg–Si alloys: Effect of trace elements and prolonged natural aging

Abstract

This study first provides a concise review of natural aging in Al–Mg–Si alloys and its effect on artificial aging. The second part investigates prolonged natural aging at different temperatures for >500days of commercial and trace element added alloys. Together, the two parts improve the picture of underlying mechanisms and refine suggestions regarding the five stages of natural aging. Trace Sn- or Sn+In-added alloys show a trend towards higher activation energies of clustering, and a higher temperature dependency of natural aging than commercial alloys. This is attributed to an additional contribution of a thermally activated vacancy release from Sn- and In-vacancy pairs. Sn and In additions are suggested to decrease cluster number density while increasing cluster size. Prolonged natural aging increasingly retards artificial aging kinetics. This is interpreted according to increasingly slower cluster dissolution kinetics and slower preferential growth of β´´ needles. The reachable strength however seems to strongly depend on the ratio and size of coarse β´´ needles, preferentially grown, and the number of fine re-precipitated β´´ needles. Artificial aging after prolonged natural aging at 45°C increases the artificial aging peak hardness due to a lower density of larger clusters than at lower temperatures.