Study on the Nucleating Process and Liquidus Semi-solid Casting Microstructures of Al 356

The nucleating process and the variation of the chilling microstructures were studied when 356 alloy was held at the temperature near its liquidus in this paper. From the temperature (625℃) above liquidus, the melt had little nucleation. The primary grains were big and the number of grains per area was little. From the temperature (612℃) slightly under its liquidus, the nucleation number was most in the melt, the primary grains were fine and global, the number of grains per area was more, and the optimum holding time was near 30min at 612℃. From the temperature (600℃) below its liquidus, the nucleation number was more, but some nucleuses congregated and grew dendritically. The Liquidus Semi solid Casting was used to validate the nucleating mechanism. When the alloy melt was semi continuously cast after being held for 30min at the liquidus temperature (612℃), the as cast microstructures, average diameter of which was smaller than 40μm, were fine, uniform, near global and very suitable for thixoforming.