Optimisation of Mg₂Si Phase for Extrudability of AA 6063 and AA 6005 Alloys

The influence of various cooling conditions after homogenisation and preheating conditions before extrusion have been investigated on AA 6063 and AA 6005 alloys with special emphasis on the Mg2Si particle development during these billet treatments and their effect on flow stress and final hardness after ageing. The results have shown that two different Mg2Si phases may form in Al-Mg-Si alloys depending on the cooling conditions. At high temperatures, between 400-500 degrees C, Mg2Si precipitates as cubic beta-phase and below 400 degrees C as metastable beta'-phase with an hexagonal crystal structure. In water quenched or fast cooled billets most of magnesium and silicon are retained in solid solution which results in high flow stress at the extrusion temperature. During slow cooling, coarse beta-Mg2Si precipitates tend to form, which are stable to high temperatures and thus difficult to dissolve during extrusion. Step cooling in the temperature range of beta'-Mg2Si, forces the precipitation of the metastable beta'-platelets which remain stable at the reheating temperature but redissolve immediately above 500 degrees C. The step-cooling gives a microstructure which combines low flow stress at the extrusion temperature together with maximum dissolution effect at the extrusion exit temperature.