Thermal-solutal-fluid flow of channel segregation during directional solidification of single-crystal nickel-based superalloys

Channel segregation – known as freckle – is one of the most complex defects which severely limit mechanical properties of investment cast single-crystal turbine blades. The freckle phenomenon is induced by the thermal-solutal-fluid flow instability during solidification process; it is therefore important to gain better understanding of the fluid flow, heat transfer and species transport during the investment casting process. In this work, a three-dimensional Eulerian two-phase model is developed to investigate chimney behaviours and the evolution of channel segregation in the directional solidification of nickel-based single crystal superalloy CMSX-4. The results reveal that the channels are merely distributed on the surface of ingot backed up with targeted experiments, and the morphology of the predicted channel segregation is also consistent with the observed freckle chains. By ignoring lateral heat flux, deviation in the prediction of channel segregation using Rayleigh number criterion is anticipated, because not all the chimneys can be developed into channel segregation under the significant perturbation. In the mushy zone, the perturbation of the melt convection from the horizontal direction to the casting direction leads to the extinction of existing chimneys, thereby exacerbating channel segregation. Once the stable channel segregation is formed, it will be developed steadily under further solute enrichment caused by thermal-solutal convection. Channel segregations is difficult to develop continuously in the centre of the representative rod, while channels near the lateral walls are more likely to develop stable flow along the casting direction. It is suggested that the weaker lateral heat flux can suppress the formation of channel segregation, providing an effective process design criterion for the freckle mitigation during the single crystal casting.