Modelling the filling and solidification of single crystal nickel-based superalloy turbine blades to understand freckle formation

Although the investment casting process is well-known, components manufactured with this process still show anomalies. In this thesis the process has been investigated by means of numerical and analytical models to improve its modelling capability. Furthermore, special attention has been given to freckle chains and to convective plumes. First, pouring has been studied, considering penny melting and tilting crucible techniques. Both approaches have been simulated by means of CFD and analytical models. Both models were able to predict inlet conditions of mould filling. From these results, the sensitivity to inlet conditions of mould filling simulations has been investigated. Consequently, a systematic study has been performed, which highlighted the importance of considering the variation of mass flow rate, both in terms of diameter and velocity. In studying the directional solidification, an experimental investigation was performed to create freckling chains. A CFD model was also developed to investigate the origin of convective plumes during solidification. The model implemented a pseudo-binary alloy as a simplification of multi-components. Results agree with the literature but were not experimentally validated. In addition, the permeability of the mushy zone, which is strongly affected by the micro-structure, was assumed to be isotropic and constant. To overcome this lack of information, a novel method for tracking dendrite tips during the directional solidification, which is also capable of predicting the dendritic selection mechanism, was proposed and validated