Abstract: The effects of different direct chilling casting speed and mischmetal (i.e.，Ce and La elements) addition on microstructure and mechanical properties of Al-23%Si were investigated. The results showed that when the casting speed increases from 100mm/min to 250mm/min， the segregation of primary Si is restricted effectively， the distribution of primary Si becomes more uniform in different positions of the ingot， the size of primary Si is greatly decreased from 71.4μm to 40.9μm and the segregation of eutectic Si is also reduced. With an addition of 0.5% mischmetal， the size of eutectic Si refines and its morphology changes from flake-like shape to short rod-like one. Both hardness and tensile strength increase with the increase of DC casting speed and the addition of mischmetal.

Key words: casting speed, hypereutectic Al-Si alloy, primary Si, segregation, mischmetal, mechanical properties