Effects Of Melt States On The Solidificallon Characteristics Of Al-80%Si Alloy

By using the high speed camera (HSC) and scanning electron microscope (SEM), the effects of melt thermal history and cerium-rich mischmetal on the solidification behaviors of levitated Al-80% Si alloy under both triggered nucleation and free solidification condition have been studied in this thesis. The results are as follows.(1) Under free solidification condition, the undercooling of Al-80%Si alloy increases with increasing the superheat temperature and the holding time. When the undercooling is lower than 130K, the morphology of the growing crystal during the solidification process is flake-like; and the structure after the solidification process is composed of large flake grains with pronounced edges and faces. When the undercooling is greater than 130K, the recalescence interface and the growing crystal show dendritic features, after solidification, the structure is composed of refined equiaxed grains and flake grains.(2) Under triggered nucleation condition, the solidification behavior of primary silicon in Al-80%Si alloy was influenced by melt thermal history. At the same holding time for 2min, the size of primary silicon decreases with increasing the superheat temperature. At the same superheat temperature (50K), the size of primary silicon decreases with increasing the holding time.(3) The growth behavior, grain size and distribution of primary silicon depend on not only the nucleation undercooling but also RE addition level. The critical undercooling, for the growth-mode transition from lateral mode to intermediary one (△T1) and the critical undercooling for the growth-mode transition from intermediary mode to continuous one (△T2) decrease with increasing the RE addition level. When the RE content is increased to a certain value, △T1 and △T2 tend to be steady. The minimum values of △T1 and △T2 are 78K and 180K, respectively, which is obtained when the RE content is equal to 1.5%. Under the same critical undercooling △T2, the size of primary silicon decreases from 40μm to 20μm when RE content is increased from 1% to 1.5%.