Prediction of plastic strain for recrystallisation during investment casting of single crystal superalloys

Superalloys 2012: The 12th International Symposium on Superalloys

Castings for single crystal aerofoils can be prone to re- crystallisation during solution heat treatment; however quantitative information concerning the factors causing this phenomenon is lacking. In this paper, mathemati- cal modelling and targeted experimentation are used to deduce the levels of localised plastic strain needed for recrystallisation to occur. The in uences of dierential thermal contraction against the shell, specimen geometry and stress concentration factor are quantied. The model predicts that the induced strain in the metal increased with the ceramic shell thickness, and in some geometries, with the solidication height. Negligible plastic strains were predicted in a solid casting with no stress concen- tration features. However, as the geometry became more complex by reducing the casting cross-section, by the in- sertion of a core and introduction of stress concentration features, the induced plastic strains increased signicantly. The predicted plastic strain for recrystallisation in a cored casting was in good agreement with experimental critical strain data. The model provides the foundation for a systems-based approach which enables recrystallisation to be predicted and thus avoided, prior to its occurrence in the foundry.