Investment casting of turbine components — practical implications of emerging concepts and ideas for solidification of Ni based superalloys

This paper presents applications of materials characeterisation methods and computational models in addressing the origins of defects encountered in investment casting of turbine components using Ni based superalloys. Some typical case studies are illustrated. In single crystal seeding, the role of thermal transients arising after mould soak and initial withdrawal has been assessed and shown to account for the nucleation propensity for stray grains and their subsequent competitive growth. In defects pertaining to latter stages of solidification; in the case of hot tears the role of minor alloy chemistry in altering the local freezing kinetics was identified, while the occurrence of non-equilibrium phases in high refractory containing newer generation alloys was studied by understanding the solidification sequence. Applications in the foundry arising from these studies in relation to operating process windows and physically based criteria for assessing alloy chemistry from the view of castability have been briefly discussed.