Prediction of Plastic Strain for Recrystallisation during Investment Casting of Single Crystal Superalloys

Abstract

Castings for single crystal aerofoils can be prone to recrystallisation during solution heat treatment; however quantitative information concerning the factors causing this phenomenon is lacking. In this paper, mathematical modelling and targeted experimentation are used to deduce the levels of localised plastic strain needed for recrystallisation to occur. The influences of differential thermal contraction against the shell, specimen geometry and stress concentration factor are quantified. The model predicts that the induced strain in the metal increased with the ceramic shell thickness, and in some geometries, with the solidification height. Negligible plastic strains were predicted in a solid casting with no stress concentration features. However, as the geometry became more complex by reducing the casting cross-section, by the insertion of a core and introduction of stress concentration features, the induced plastic strains increased significantly. 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.

Processing of directionally cast nickel-base superalloys: solidification and heat treatments

“…typically 6 hours at 1315 °C in case of CMSX-4 alloy [48]. But compressive and tensile tests were also tested at several temperatures, followed by isothermal annealing heat treatments for various parameters (holding time and duration) [48][49][50]. The presence of recrystallized grains is found to depend on i-the plastic strain, ii-the annealing temperature, and iii-the temperature at which the specimens were deformed.…”

“…The presence of recrystallized grains is found to depend on i-the plastic strain, ii-the annealing temperature, and iii-the temperature at which the specimens were deformed. Most of the experiments are conducted with plastic strains of 4% [48,50] and of 5% [49] when the annealing temperature and the deformation temperature are studied. But depending on the plastic strain, the annealing temperature seems to influence the formation of recrystallized grains.…”

“…But depending on the plastic strain, the annealing temperature seems to influence the formation of recrystallized grains. Similarly for CMSX-4, if the temperature at which 4% plastic strain is introduced is lower than 950 °C, no recrystallized grains were formed for annealing treatments below 1260 °C [50]. Finally, the recrystallized area is a direct function of the annealing temperature.…”