Synchrotron X-ray in-situ observation of the effect of gravity on the solidification microstructure of Al-30wt%Zn

The effects of gravity on the solidification structure of Al-30wt%Zn alloy were investigated using synchrotron Xray imaging techniques. The experiments were conducted using a Bridgman-type directional solidification furnace. The results indicate that gravity significantly affects the solidification structure. Experimental results show that during vertical upward solidification, the formation of coarse dendritic structures is accompanied by solute segregation. In contrast, vertical downward solidification produces finer and more elongated grains with no noticeable segregation observed in the field of view. These differences result from the combined effects of gravity and solute diffusion on the solidification microstructure. Increased cooling rates result in higher undercooling within the melt, promoting the formation of additional nucleation sites and leading to the presence of more fine grains in the melt. During downward solidification, at higher cooling rates, the "effect" of solute diffusion is suppressed, leading to solute enrichment at the dendrite tips and an increase in constitutional undercooling. This effect promotes the transition of grains from a columnar to an equiaxed structure.