In situ observation of columnar-to-equiaxed transition in directional solidification using synchrotron X-radiation imaging technique

A columnar-to-equiaxed transition (CET) was induced by controlling the cooling rate in the directional solidification of an Al-15 wt.% Cu alloy. The morphological evolution of the mush–liquid interface in CET was tracked and recorded by X-ray imaging. The dendrite growth and detachment, solute distribution at the growth front and excessive solute enrichment in the liquid pockets were clearly observed. Based on the observations, it was found that there is an obvious increase in melt density in the liquid pocket due to the excessive solute enrichment. The density difference between the melt and the solid is an important reason for dendrite detachment and fragment movement, which is considered as the key to the inducement of CET. Finally, the CET process and the growth of equiaxed dendrites at the mush–liquid interface were described from the viewpoint of dendrite detachment.