The effect of conditions of directional crystallization on the structure of cast blades made of nickel superalloys

Abstract

The characteristics of gas turbine engines depend on the structural state of the material of the turbine blades and are largely determined by the technology of their manufacture. Identification of the specific defects of casting is an important goal aimed at improving the quality of blade castings since the castings of turbine blades with a monocrystalline structure made of high-alloyed nickel superalloys (NS) are subject to increased requirements for macro- and microstructure. We present the results of studying the effect of directional crystallization parameters on the morphology of the dendritic structure of NS and their tendency to form casting defects of the single-crystal structure. The structure of alloys in single-crystal blade castings and the effect of technological parameters on the dispersion of the structural components of NS were analyzed upon casting of turbine blades with <001> axial orientation on installations with liquid-metal cooling. It is shown that the parameters of the dendritic cells of castings vary in height from 220 to 330 μg and volume fraction of micropores changes from 0.02 to 0.2%. Electron probe microanalysis revealed that the phase and local elemental compositions of the alloy in the areas of the blade tip and isolated grains are identical. The appearance of isolated grains that violate the monocrystalline structure of castings is observed when the rate of NS cooling decreases. To increase the temperature gradient and reduce the probability of formation of extraneous crystals in castings ahead of the crystallization front, optimized temperature-speed modes of casting and directional crystallization are proposed. The obtained results can be used in the development of temperature-velocity parameters of the process of directional crystallization of blades of promising turbines made of new superalloys.