Containerless Solidification Of Hypereutectic Al-Si Alloys

The solidification of Al-50at.%Si is studied by electron beam melting, drop tube and electromagnetic levitation facility. The growth of the primary silicon is also studied with the modified LKT-BCT model.The top surface of the ingot from electron beam melting is observed. Typical faceted dendrites almost cover the entire surface developing from the edge to the center. The branches of dendrite also exhibit preferable growth directions. From the center to the edge, refinement of primary silicon dendrites and shorter and blunter of the needle-like or flake-like eutectics are observed because of different cooling rates.Solidified droplets with diameter 0.1~1.0 mm are achieved by drop tube. Phase morphologies of deeply etched sections with different diameters are examined. The microstructures are faceted primary silicon dendrites and Al-Si eutectics. There are stratified deposits of aluminum in primary silicon along the silicon lathing. With the decreasing of diameter of droplet, refinement of primary silicon, shorter and blunter of the eutectics and anomalous eutectics are observed, which are all influenced by large cooling rates. Crystal growth is restricted by the solute pile-up at the liquid-solid interface, which results in microstructural refinement.Undercooling and solidification of Al-50at.%Si is achieved by repeatedly heating and cooling in electromagnetic levitation facility. A maximum undercooling of 320 K is obtained. Phase morphologies of surfaces and deeply etched sections at different undercoolings are examined. The primary silicon shows faceted dendrites with stratified deposits of aluminum at small undercooling, but shows granular form without deposits of aluminum at large undercooling. The form of the Al-Si eutectic is also found changed into anomalous form at large undercoolings. The microstructural refinement is observed at large undercooling, as a result of solute restriction of crystal growth and fragmentation of the primary silicon dendrites. Calculation based on modified LKT-BCT model shows the growth of primary silicon is mainly influenced by the solute pile-up at the liquid-solid interface...