EFFECT OF PULLING VELOCITY ON DENDRITE ARM SPACING IN STEADY-STATE DIRECTIONALLY SOLIDIFIED TRANSPARENT ORGANIC ALLOY BY NUMERICAL SIMULATION

A modified cellular automaton (MCA) model is proposed for columnar dendritic growth in directional solidification. Many factors including temperatur e gradient, pulling rate, constitutional undercooling, curvature undercooling are considered in the model. The model is helpful to understand the fundamental s of the formation of the cell-dendrite transition, the evolution mechanism of primary dendrite arm spacing change, which significantly impact the formation of microsegregation. and the mechanical properties of the castings. The MCA model illustrates that the simulated microstructur e parameters (primary dendrite arm spacing lambda(1), and secondary dendrite arm spacing lambda(2)) exhibited a power function of pulling velocity (V), which has good agreement with the theoretical models. NH4Cl-H2O system has the characteristci of transparency, cubic < 100> habit, liquidus line near room temperature, lower enthalpy of fusion, which allows the in situ observation of directionally solidified dendritic growth. A series of experiments were carried out under a range of pulling velocities, but a constant thermal gradient. Both branching and overgrowth mechanism of dendrite arm spacing were observed in the experiments. The terms lambda(1) and lambda(2), derived from the MCA model, agreed well with the experimental data.