Effect of Withdrawal Rate on Solidification Microstructures of DD9 Single Crystal Turbine Blade

Abstract

Single crystal superalloys are widely used in the manufacturing of turbine blades for aero-engines due to their superior performance at high temperatures. The directional solidification process is a key technology for producing single crystal turbine blades with excellent properties. In the directional solidification process, withdrawal rate is one of the critical parameters for microstructure formation and will ultimately determine the blade’s properties. In this paper, the as-cast microstructures in the typical sections of a DD9 single crystal (SX) superalloy turbine blade were investigated with 3 mm/min and 5 mm/min withdrawal rates during the directional solidification process. With increased withdrawal rate, the dendrite morphologies tended to become more refined, and the secondary dendritic arms tended to be highly developed. The dendrite in the blade aerofoil section was more refined than that in the tenon section, given the same withdrawal rate. Additionally, with increasing withdrawal rates, the size and dispersity of the γ′ precipitates in the inter-dendritic (ID) regions and dendritic core (DC) tended to decrease; furthermore, the size distributions of the γ′ precipitates followed a normal distribution law. Compared with the ID regions, an almost 62% reduction in the average γ′ sizes was measured in the DC. Meanwhile, given the same withdrawal rate, at the blade’s leading edge closest to the heater, the γ′ sizes in the aerofoil section (AS) were more refined than those in the tenon section (TS). As compared with the decreasing cross-sectional areas, the increased withdrawal rates clearly brought down the γ′ sizes. The sizes of the γ–γ′ eutectics decreased with increasing withdrawal rates, with the γ–γ′ eutectics showing both lamellar and rosette shapes.