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The vacuum investment casting process for a large gas turbine component, Inner Preswirl Support (IPS), was simulated by using commercial FEM package ProCAST(TM) with view factor radiation method. The solid fraction in mushy zone was directly measured by Differential thermal analysis(DTA-DSC mode). Three types gating design. considering liquid flow and heat release through it. were proposed. Solidification had begun at the ribs or thin sections of the IPS casting and advanced further through the upper and lower gates. The computed temperature gradient G and G/R values at 70% solidified temperature were used for prediction of microshrinkage formation during casting. The effect of mold preheat on the thermal history of the casting displayed minute effect on the microshrinkage formation.
Effects of vibration(V) and inoculation(I) during casting on the microstructure and tensile properties of nickel-base superalloy CM247LC were investigated. The vibration treatment with a frequency of 10 ㎐ was effective to refine the grain size and decrease the microporosity level to some extent. The vibration treatment for the inoculated mold developed the grains of the casting into fine equiaxed grains compared with columnar grains of the casting with vibration free inoculated mold. The existing phases of CM247LC alloy such as finely dispersed cuboidal γ', MC, and grain boundary precipitates were not influenced by the vibration and/or inoculation treatment. The fine equiaxed grained CM247LC had excellent tensile properties compared with those of the columnar grains. The tensile behavior of CM247LC alloy at 760℃ showed different aspects with respect to the grain size change and which was carefully analyzed by fractographic investigations.
The solidification grain structure and texture evolutions during single crystal (SX) casting of the advanced Ni-base superalloy CMSX-4 have been investigated. In order to understand the development of the solidification grain structure, SX casting experiments were carried out with a specially designed grain selector in a Bridgman directional solidification (DS) furnace. In addition to casting trials, the SX casting process was simulated by a 3-D cellular automaton-finite element (CAFE) model. The predicted solidification grain structure and the texture evolutions were validated by comparison with the microstructural observation and the electron back scattered diffraction (EBSD) results. It was shown that the overall grain structure, crystallographic texture evolution, and the location where the final selection of the single crystal occurs can be predicted well by the present CAFE model. The axial texture evolution of the single crystal was found to be significantly influenced by the grain density at the chill surface. The CAFE predictions also revealed that the geometry of the grain selector plays a significant role in the final selection of the single crystal.
The influence of rhenium (Re) and ruthenium (Ru) addition on the solidification and solute redistribution behaviors in advanced experimental Ni-base superalloys has been investigated. A series of model alloys with different levels of Re and Ru were designed based on the composition of Ni-6Al-8Ta and were prepared by vacuum arc melting of pure metallic elements. In order to identify the influence of Re and Ru addition on the thermo-physical properties, differential scanning calorimetry analyses were carried out. The results showed that Re addition marginally increases the liquidus temperature of the alloy. However, the γ` solvus was significantly increased at a rate of 8.2℃/wt.% by the addition of Re. Ru addition, on the other hand, displayed a much weaker effect on the thermo-physical properties or even no effect at all. The microsegregation behavior of solute elements was also quantitatively estimated by an electron probe microanalysis on a sample quenched during directional solidification of primary γ with the planar solid/liquid interface. It was found that increasing the Re content gradually increases the microsegregation tendency of Re into the dendritic core and γ` forming elements, such as Al and Ta, into the interdendritic area. The strongest effect of Ru addition was found to be Re segregation. Increasing the Ru content up to 6 wt.% significantly alleviated the microsegregation of Re, which resulted in a decrease of Re accumulation in the dendritic core. The influence of Ru on the microstructural stability toward the topologically close-packed phase formation was discussed based on Scheil type calculations with experimentally determined microsegregation results.
The Ni-base superalloy CM247LC was directionally solidified (DS) using the Bridgman-type furnace to understand the effect of the chill plate on the microstructural evolution, such as dendrite arm spacing, microporosity, and MC-type carbide. The DS process was also modeled by the PROCAST to predict the solidification rate, thermal gradient, and resultant cooling rate in the entire length of the DS specimen. Due to the quenching effects of chill plate, four distinct areas were found to form in the specimen, in which the solidification rate was changed, during DS at a given withdrawal rate of 0.083 mm/s. Among the microstructural features investigated, the dendrite arm spacings and average size of the MC-type carbide near the chill plate were found to be influenced by the quenching effect of the chill plate. However, no significant influence was found on the size and volume fraction of microporosity, and the volume fraction of the MC-type carbide. The relationship between the microstructural features and the solidification variables was also analyzed and discussed on the basis of a combination of experimental and modeling results
Mechanical alloying(MA) powders were prepared by single shaft attritor mill under Ar gas atmosphere. Powders milled for more than 35hours showed Ni solid solution and unimodal particle size distribution having the mean powder size of 12 ㎛ in diameter. These powders milled for 35hours were used for flame spraying. The optimum condition for the flame spraying of Ni-based self-fluxing powders manufactured by mechanical alloying for 35hours was studied and the coating produced under the condition of a spraying distance of 15 ㎝ gave rise to the best results. Sliding wear behaviors of the coating produced by fame spraying on cast iron substrate were investigated by ball-ondisc type wear test using an AISI 52100 steel ball as a counter material. Volumetric wear rates of coatings and substrate were calibrated using the Habig's equation by measuring a wear volume of ball and disc. Compared with cast iron substrate, relative volumetric wear rate of coatings showed significant improvement on the wear resistance.