Precipitation hardening in Al–Mg–Si alloys with and without excess Si

The aluminum alloys of the 6xxx series contain an excess of Si above that required to form stoichiometric Mg2Si, which is added to improve the age hardening due mostly to precipitation of metastable β″ precipitates. The excess Si is not believed to alter the precipitation sequence, structure and lattice parameters of the different metastable precursors, but rather promotes formation of additional particles/phases which do not contribute to hardening significantly. The presence of excess Si changes the composition and density of metastable β″ particles, although a systematic study of the Mg/Si ratio in particles from alloys of different composition is lacking. In this paper, it is shown that the precipitation sequence in the balanced alloy is independent of the composition and the strength increases with Mg2Si level. This is due primarily to both a higher volume fraction and a refined distribution of the β″ particles. Excess Si increases the effective amount of the hardening phases above ∼0.9 wt.% Si. It modifies the Mg/Si ratio in the clusters/zones and β″ precipitates and improves strength by altering their size, number density and distribution. In addition, the extent and rate of strengthening increases until the overall Mg to Si ratio in the alloy is close to approximately 0.4. The hardening precipitates with reduced Mg to Si ratio become less stable with aging and cause a decrease in strength during over aging.