Undercooling Related Casting Defects in Single Crystal Turbine Blades

Abstract

During the single crystal solidification of turbine blades casting defects such as stray grains are often formed in the inner and outer shroud due to a curvature of the liquidus isotherm and a resulting thermal undercooling of the melt. An intensive study of the thermal conditions in typical shroud regions for two different blade geometries was carried out using macroscopic solidification modeling and temperature measurements during casting experiments. It was tried to reduce undercooling related defects by temporarily changing the withdrawal velocity of the mold and by positively influencing the heat flux with ceramic insulation. In addition to that the application of grain continuators was investigated in order to eliminate stray grains while casting large blades for use in stationary turbines.