Design of variable withdrawal rate for superalloy single-crystal blade fabrication

Abstract

Stray grains bring great challenges to the production of superalloy single-crystal blades using the Bridgman method. The height difference between the liquidus isotherms in the airfoil and platform, which is induced by the difference between liquidus isotherm advancing velocity and withdrawal rate, provides undercooling for the nucleation and the growth of stray grains. The height range of liquidus isotherm is used for the first time to describe the asymmetric distribution of temperature and radiation, and to estimate the probability of stray grain formation. A real-time adjustment of withdrawal rate method is proposed to design the withdrawal rate-time curve aiming to fabricate superalloy single-crystal blades with complex geometry. The obtained withdrawal rate curve is then validated by numerical simulation and hence applied to the fabrication of single-crystal blades. The simulation results show that with obtained withdrawal rate curve the distribution of radiation and temperature are significantly improved and the undercooling in the platform is largely reduced. The observed macrostructure in the experiments proves that the stray grains in the platform can be avoided through the designed withdrawal rate curve. The proposed method for designing withdrawal rate can provide a reliable strategy for the research and manufacture of stray-grain-free single-crystal blades.