Effects of Heat Treatment on the Morphology of Primary α Phase and Hardness of B-Refined Al-7 wt% Si Alloy

The effects of heat treatment on morphology of primary α and hardness of B-refined Al-7 wt% Si alloy are investigated. In addition, the optimum heat treatment procedure is also determined. It is shown that, at 450°C with different holding time from 3h to 12h, morphology of primary α phase is converted from irregular dendrite to granular homogeneous equiaxed grains then to rosette structure. The primary α phase is gradually converted from dendrite into the rosette dendrite and then into particle crystal with increasing of temperature by holding 8h at 300~540°C. One can find fine and uniform distributed particle α phase at 400°C or 450°C. However, the primary α phase has changed into large and irregular shape when the temperature increases up to 450°C. The orthogonal analysis shows that the optimum heat treatment parameters are 540±10°Cand 6h.