The effect of fluid flow on the dendritic microstructure in two AlSiCu alloys

Abstract

Directional solidification experiments were performed with AlSi6Cu4 and AlSi9Cu4 alloys. The solidification velocity was varied by a factor of ten while the temperature gradient was kept constant at 3 K/mm over the processing length. Artificial fluid flow was induced in the samples by a rotating magnetic field device with flow velocities up to 10 mm/s being able to interact with the mushy zone. After processing, the microstructures were analyzed with respect to the primary dendrite stem spacing, the secondary dendrite arm spacing (SDAS), the specific surface area of the primary aluminium phase, and the local distribution of Si and Cu to quantify the flow-induced segregation of solute species. We generally observe that fluid flow leads to a reduction of the primary stem spacing, while the functional dependence on velocity does not change from the well-known models described in the literature. The SDAS, however, reacts differently. The usually observed cube root dependence of the spacing on solidification time is replaced by a square root dependence in agreement with newer theories on accelerated coarsening by fluid flow. The rotating magnetic field induces so-called secondary flows, which promote segregation of Si and Cu towards the sample center. The EDX analysis shows a pronounced segregation of Si and Cu, in agreement with numerical modeling.