5th International Conference on Advances in Solidification Processes (ICASP-5) 5th International Symposium on Cutting Edge of Computer Simulation of Solidification, Casting and Refining (CSSCR-5)

The growth dynamics of multiple equiaxed dendrites in a thin metallic sample is studied using *in situ* synchrotron X-ray imaging of Al-4 wt.%Cu alloy solidification experiments and corresponding full-scale three-dimensional (3-D) phase-field (PF) simulations. The tip growth velocity *V*, tip radius *ρ*, the secondary dendritic arm spacing (SDAS) *λ*$_2$ and the total solid fraction *f*$_s$, are analyzed systematically in a relatively large range of cooling rate *R*$_c$. Through an in-depth comparison of the growth dynamics with experiments, including the tip velocity and the primary arm length, the nucleation undercooling for each grain is estimated. Quantitative agreements in *V* of several selected grains and *f*$_s$ between experiments and simulations have been achieved. The tip velocity, followed by free growth from the melt that is cooled continuously at constant *R*$_c$, decreases first at the very beginning of solidification in both experiments and simulations. Subsequently, *V* increases up to a peak value until the grains interact with each other by solutal effect. Moreover, an oscillation growth of the dendritic tip is observed during the soft-impinged growth stage in both experiments and simulations, featured by an acceleration of *V* after it goes down to a minimum from the peak value. With increasing *R*$_c$, *V* will be enlarged while *ρ* will be reduced, due to the narrowed solute boundary layer ahead of the tip. Furthermore, *ρ* and *λ*$_2$ show good agreements between simulations and dendrite growth theories, with *ρ* $\sim$ *V*$^{-1/2}$ and *λ*$_2$ $\sim$ *R*$_c^{-1/3}$. However, experimental measurements of *λ*$_2$ are generally larger than *λ*$_2$ in PF simulations, and this discrepancy could be attributed to liquid convection in experiments, despite the thin sample configuration. We incorporate liquid flow dynamics into the computationally less-demanding 2-D PF simulations. It has been found that with the gravity-driven convection the sidebranches are more developed and coarsened, thereby enlarging the spacing between the secondary arms.