In Situ Investigation of the Solidification of Al-20 wt.% Zn Alloys Inoculated by Al-5Ti-1B

In situ synchrotron x-ray radiography studies on the solidification sequences of non-refined and refined Al-20Zn alloys were carried out. The dendrite growth, porosity evolution, grain densities and growth rates were measured. Without and with 0.05% grain refiner, the solidification sequences appear as columnar dendrite. The average columnar dendrite growth rate of non-refined Al-20Zn alloy is larger than refined by 0.05% grain refiner. The burst growth is happened in columnar dendrite which is the first report and the time interval was about 1.2s. The columnar interdendritic region is prone to form porosities with solidification progress. The equivalent diameter of porosity with time is well fitted by Gaussian function, and the growth velocities of porosities of all sequences are faster in the first seven seconds and fluctuated with time. When the addition level of grain refiner is larger than 0.1%, the solidification sequences appear as equiaxed dendrite. The accumulation of grains of refined Al-20Zn alloys is well fitted by logistic function. The porosities disappeared during solidification with equiaxed grains. The equiaxed dendritic growth velocity, which is ~16μm/s at a cooling rate of 8K/min, is much lower than columnar.