2014 Volume 55 Issue 5 Pages 774-778
High brilliance synchrotron X-ray radiography has been used to in situ study the solidification process of Al–20 mass%Cu alloy with nominal spatial and temporal resolutions of 7.4 µm and 300 ms, respectively. The block of favorably oriented dendrites by the unfavorably oriented ones was first observed in metal solidification, which related to solute interaction of neighbor dendrites. Solute interaction can also retard the growth of dendrites at the grain boundary, which makes the unusual overgrowth phenomenon possible. Fragmentation occurred when dendrite branches were detached from mother dendrite by local re-melting during the dendrite ripening. Fragmentations together with nucleated dendrites contributed to the columnar to equiaxed transition (CET) and the gravity played a significant role in the final microstructure due to the density difference between primary aluminum dendrites and the melt. In addition, the microporosity formation was observed in the inter-dendrite at the last stage of solidification.