Influence of impurities, strontium addition and cooling rate on microstructure evolution in Al-10Si-0.3Fe casting alloys

The effect of impurities, Sr additions and cooling rate on the microstructural evolution of high-purity Al-10Si-0.3Fe and corresponding impure commercial alloys is examined by optical microscopy and in situ by X-ray synchrotron 3D tomography. At fast cooling rate (∼470 K min−1) the presence of impurities decreases the growth rate of primary Al dendrites and enables formation of the β phase. Besides a modification of the eutectic Si, the addition of Sr prevents the formation of the β phase and increases the growth rate of Al dendrites. A low cooling rate (∼1 K min−1) leads to the formation of α, γ, δ and β intermetallic phases. In all four alloys, the dominant phase is the δ phase, regardless of commercial impurities or the Sr level. Intermetallic phases formed during slow cooling rates are much coarser than those formed during fast cooling and they have different morphologies. Our results suggest that the velocity of growth and the final morphology and size of the intermetallic phases are mainly determined by diffusional processes which in turn are controlled by the cooling rate.