Abstract
During the single crystal (SX) solidification of turbine blades, grain defects can form in the platform regions which have abruptly varying cross-sections. A high undercoolability of a Ni-based alloy prevents the growth of stray grains in the thermally undercooled platform area. To evaluate the undercoolability of different Ni-based alloys, temperature measurements were conducted using the same thermal conditions in an Al2O3-SiO2 investment casting shell mold system. Furthermore, the results were compared with stray grains in directionally solidified components.
Highlights
Driven by the demand of high-efficiency gas turbines, Ni-based superalloys were developed over the last 40 years for high temperature strength, toughness and oxidation resistance
One side of the turbine blade is oriented towards the heaters and the opposite side faces the middle column of the shell mold
The curvature of TL depends on the process parameters, geometric factors and thermo-physical properties of the superalloy and the shell mold
Summary
Driven by the demand of high-efficiency gas turbines, Ni-based superalloys were developed over the last 40 years for high temperature strength, toughness and oxidation resistance. Re is an alloying element that has an especially high micro-segregation coefficient, and effects the solidification of Ni-based superalloys. As a consequence of this, superalloys with high undercoolability are less prone to stray grains and the reject rate of castings is lower. Chemical composition: Assuming homogeneous nucleation, the undercoolability of nickel alloys depends on the chemical composition of the melt [8,9,10,11]. In the industrial Bridgman process, undercooled zones are usually near the inner shell mold surface, and heterogeneous nucleation lowers the undercooling of the melt. Contact angle measurements between different ceramics and superalloys show limited chemical reactions between the melt and ceramics containing SiO2 [17,18]. Different Ni-based alloys were investigated under industrial process conditions to identify the relationship between melt undercooling and stray grains in three Ni-based alloys
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