AS-CAST MICROSTRUCTURES AND THE SOLIDIFYING MECHANISM OF 7075 ALUNMINUM ALLOY CAST BY LSC

The observations of microstructures of 7075 alloy cast by liquidus semi-continuous casting (LSC) revealed that the border of billet cast from the normal temperature (720℃) consists of coarse dendritics, and the center microstructure is very inhomogeneous. When the billet was cast from the temperature (642℃) upside liquidus and the alloy melt was held for 30 min, the border and the center microstructures are good and the difference between them is small. When the billet was cast from the temperature (638℃) closely near liquidus and the alloy melt was held for 30 min, the border and the center microstructures are the best and all consist of fine, uniform and globular grains. Decreasing the first cooling and the casting velocity and increasing the second cooling are helpful to the formation of fine, uniform and globular grains. The formation of the globular microstructures is related to a lot of nucleus produced form the inner alloy melt and reduction of the undercooling at solid/liquid interface. The results show that LSC is an efficient way for making semi-solid slurries.