The effect of local thermo-solutal convection on dendrite fragments at variable cross section

The dendrite fragments formation process of hypoeutectic Al–Cu alloy at variable cross section was observed by synchrotron radiation imaging. The results show that the variable cross section structure causes a series of chain reactions among the solidification parameters and dendrite characteristic parameters. It can induce the sudden decrease of concentration gradient, constitutional undercooling and dendrite growth rate. The solute enrichment process at the variable cross section can be divided into two stages: the rapid solute enrichment stage and the slow solute enrichment stage. The dendrite fragments occur in the rapid solute enrichment stage is satisfying the Rayleigh number based fragmentation criterion. The purpose of establishing the relationship between Rayleigh number and dendrite growth rate is to optimize the single crystal production process window. (paper)