Analysis of the mechanical deformation arising from investment casting of directionally solidified nickel-based superalloys

Abstract

To provide insight into the factors causing recrystallisation of nickel-based single crystal superalloys, analysis of the thermal–mechanical deformation caused by investment casting of these components is presented. Three-dimensional thermal–mechanical finite element analysis is first used to demonstrate that the reaction of the casting and mould—at least in the aerofoil section—can be approximated as one-dimensional. One-dimensional models are then built based upon static equilibrium for plasticity on the microscale caused by differential thermal contraction of metal, mould and core, using temperature dependent material properties. The models take various forms to study the mechanical response under different situations relevant to practical applications. The results indicate that the plastic strain causing recrystallisation is likely to be induced during cooling at temperatures above 1000°C. The relative importance of thicker and stiffer ceramic shells is studied. Our analysis indicates that it is important to account for creep deformation for such applications.

The role of stress relaxation and creep during high temperature deformation in Ni-base single crystal superalloys – Implications to strain build-up during directional solidification

“…The elastic strain is given by Hook's law of elasticity, [16,24]; here it suffices to cover the salient aspects. Since high temperature deformation is time-dependent, creep and/or stress relaxation must be taken into account, which mitigates to some extent the stress calculated from the elasto-plastic model.…”

“…To a lesser extent, re-crystallisation is also dependent on the alloy composition [17], as well as the presence of the non-equilibrium γ/γ / eutectic; the latter has been proposed to act as pinning sites for the grain boundary and thereby retards growth of the re-crystallised grains [18]. Some key questions persist on the nucleation mechanism of these re-crystallised grains [19 -23] and the critical "inelastic" strains and their temperature dependence [16,24,25]. Cox et al.…”

Relating micro-segregation to site specific high temperature deformation in single crystal nickel-base superalloy castings

“…Accordingly, thermo-elasto-plastic models may be employed. These models implicitly assume a constitutive law where stress is a function of time-dependent plastic strain rate, plastic strain and temperature [14,26,27]. Consequently, the high fidelity thermo-mechanical data measured in this study by overcoming the drawbacks in in-situ or post-mortem approaches can be used in a numerical model, where the issues of history-dependent deformation are addressed by building on a previous study [28].…”

“…to the gauge length;LGauge This gauge length is almost constant during heating up to 1300C under zero-load, since T  0.02 << 1, where  = 15 × 10-6 K-1[14,27] and T  1300 C. Therefore, it is reasonable to approximate "constant gauge length under zero-load".…”

Prediction of recrystallisation in single crystal nickel-based superalloys during investment casting

“…And at what strain levels? [10] Ideally, the material response to the plastic strain in high temperature range should be studied and rationalised to provide guidance for the turbine blade designers to avoid recrystallisation in specific geometries. Moreover, the sensitivity to the geometrical variations, processing MATEC Web of Conferences parameters and materials properties should be researched in a rigorous manner.…”