Recently, the need for the design and development of materials for the parts and components used at high temperatures, such as gas turbines for power generation, is increasing. Currently, although key components of turbine engines for power generation have been manufactured through precision casting and post-processing, it is difficult to design more complex-shaped internal parts with a response to the need for higher cooling efficiency. The additive manufacturing(AM) technology, which produces products by stacking materials into multiple layers, has recently drawn attention as it provides manufacturing of complex-shaped components omitting unnecessary parts, which also leads to efficiency with weight reduction. For this reason, AM can be used to solve low cooling efficiency problems of vane parts for gas turbines manufactured by conventional precision casting and machining methods.
In this paper, the feasibility study has been made to produce the vane parts of a power generation gas turbine using the Electron Beam Melting(EBM) method. Considering suitability for the application to the vane parts, Rene80 superalloy has been selected as the materials in terms of better AM performance, creep properties, and powder commercialization than existing materials used in vane parts, such as GTD111, IN738LC, and CM247LC. The cast Rene80 has a high fraction of γ'' 55%, which is known for on the one hand contributing to mechanical properties at high temperature, but on the other hand leading to inferior weldability, which is associated with processibility of AM. First, the alloy modification of Rene80 was attempted to achieve better AM processibility maintaining good creep properties. The addition of 1 wt.% Hf was adopted to alleviate grain boundary cracking through increasing grain boundary ductility, and Ti was reduced from 5 wt.% to 3 wt.% replacing it with the addition of 2.5 wt.% Ta to reduce evaporation and preserve the fraction of γ''. Based on this, the AM suitable Modified Rene80(M.Rene80) alloy of Ni-9.5Co-14Cr-4Mo-4W-3Al-3Ti-2.5Ta-1Hf was designed. M.Rene80 showed greatly improved processibility of AM with reduction of grain boundary cracking, while original Rene80 was subjected to poor processibility (balling, high fraction of porosities and grain boundary cracking). The as-EBMed M.Rene80 exhibited stable <100> directional columnar grains parallel to the building direction. In order to reduce grain boundary cracking further, scan speed was reduced from 2,000mm/s to 1,000mm/s. It resulted in a significant stabilization of grain boundary without cracking and similar distribution of γ'' to cast Rene80. On the other hand, it was confirmed that M.Rene80 showing spherical morphology of γ'', exhibited comparable tensile properties at high temperature, whereas Rene80 showed cuboidal shape. After aging heat treatment, the morphology of γ'' was changed to closely-spaced cuboidal shape, showing regular arrangement. On the deformation mechanism at high temperature, TEM analysis provided that γ'' particles were sheared by stacking faults and Anti-Phase Boundary in both as-built and aged M.Rene80. To control the characteristics of γ'' particles (fraction, size and shape in both primary and secondary γ'' particles) more favorable to better mechanical properties, post heat treatment conditions were investigated. First, the 1st aging heat treatment time was extended from 4 hr to 8, 32 hr to stabilize the γ'' shape as a cuboidal shape and increase the size of γ''. In addition, the size of particles such as carbide or oxide has increased. As a result, although morphology of γ'' was changed to cuboidal shape with increase of size, it didn’t contribute to the enhancement of mechanical properties. Second, the solid solution heat treatment temperature was raised from 1,204 ℃ to 1,230 ℃, 1,250 ℃, 1,270 ℃ and 1,300 ℃ for the stabilization of γ'' as a cuboidal shape and the dissolution of carbides. As a result, not only γ'' was changed to cuboidal shape, but also carbides were dissolved and hardness was increased, which probably attribute to strengthening effect. The effect of heat treatment condition was discussed in terms of MC dissolution, γ'' precipitation, and their contribution to dislocation interactions.