The effects of fluid flow on secondary arm coarsening during dendritic solidification

Although dendrites are the result of diffusion limited growth it has long been appreciated that flow within the parent melt can have a dramatic effect on these structures. A free boundary model of dendritic solidification is used to assess the effects on the secondary arm coarsening processes of fluid flow within the parent melt. It is found that for solutally controlled coarsening realistic interdendritic flow velocities of the order 10^–3–10^–2 m s^–1 give rise to ripening rates which are comparable to diffusive transport. However, only flows with a component aligned from the secondary tip towards the root enhance the ripening rate. Oppositely aligned flows actually reduce the ripening rate. Thus, due to the four-fold symmetry of dendrites in cubic metals, the actual effect on the secondary arm spacing could be quite small. The results are shown to be in general agreement with recent microgravity experiments on dendritic coarsening.