THE INFLUENCE OF FORCED-CONVECTION ON SOLIDIFICATION INTERFACES

The tendency of growing columnar dendrites to incline into the flow di rection under the influence of some shear flow at the solidification f ront is a well-known phenomenon in directionally solidifying alloys. A ccording to literature this effect is due to a deformation of the solu te layer around a dendrite tip. It is assumed that solute-rich liquid is swept away from the upstream side of the dendrite tip to the downst ream side, leading to a local enrichment of the solute and lowering of the solidus temperature on the downstream side. This gives preferenti al growth conditions on the upstream side, thus causing the crystal ti ps to incline upstream. On the basis of stirring experiments carried o ut at the Giesserei-Institut the deformation of the solute layer by fo rced convection at a 2-dimensional sine-wave-modulated interface and a 3-dimensional paraboloid of revolution was studied using a FEM-code ( FIDAP) to solve the momentum and species transport equations under ste ady state conditions. The sine-wave-modulated interface could not acco unt for the solute pile-up at the downstream side, especially after in corporating the influence of a solute partition coefficient at the int erface. After adapting the 3-dimensional paraboloid to the analytic Iv antsov-solution describing the growth of an isoconcentrate dendrite ti p a higher solute concentration could be observed at the downstream si de of the dendrite tip. The results indicate that the sweep away effec t actually occurs at the dendrite tip as long as the diffusion layers around the dendrite tips do not influence each other.