Effect of Processing Parameters and Shape of Blade on the Solidification of Single-Crystal CMSX-4 Ni-Based Superalloy
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elopment of turbine aircraft engines is focused on the improvement of their efficiency, which mainly depends on the temperature of exhaust gases before the turbine. Reed[1] reported that the maximum temperature of gases at the turbine inlet depended on the properties of materials applied in the engine hot section elements, mainly those of blades of high-pressure turbines. Kubiak et al.[2] described Bridgman and liquid metal cooling (LMC) methods for the directional solidification of single-crystal turbine blades made of nickel superalloy, applied in the aircraft engines and industrial gas
DARIUSZ SZELIGA is with the Department of Materials Science, Faculty of Mechanical Engineering and Aeronautics, Rzeszow University of Technology, 12, Powstancow Warszawy Str., 35-959 Rzeszow, Poland and also with the Research and Development _ Laboratory for Aerospace Materials, 4, Zwirki i Wigury Str., 35-036 Rzeszow, Poland. Contact e-mail: [email protected] Manuscript submitted April 29, 2017.
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turbines. The manufacturing process of these castings consists of pouring liquid metal into the preheated ceramic shell mold and its withdrawal at a specially selected velocity from the heating to the cooling area of the furnace. The industrial version of the furnace is equipped with cylinder-shaped heaters and chill rings in most cases. The manufacturing technique and value of the withdrawal velocity of the ceramic shell mold influence the parameters of the solidification process, i.e., the temperature gradient and solidification rates of casting, to a large extent.[3] Cooling the ceramic shell mold with liquid metal (LMC) makes increasing the withdrawal velocity and temperature gradient possible compared with the Bridgman method in which the cooling technique is performed by radiation in vacuum.[4] The temperature gradient, solidification and cooling rates determine the shape of the solid/liquid interface and microstructure of single-crystal castings.[5] An increase of the cooling rate leads to the favorable refinement of primary dendrite arm spacing and the decrease of eutectic islands (c+c’) and c’ precipitation as well as microsegregation, which result in the reduction of the duration and cost of the heat treatment process.[6]
Temperature gradient G and solidification rate v affect the formation of defects (freckles, high-angle grain boundaries, stray grains), especially in the case of large single-crystal castings made of nickel superalloy.[7] The lateral growth of dendrites and nucleation of unfavorable grains appear for the values of G/v = 1000 and 3500 °C s/cm2, respectively.[8] The parameters of the solidification process are usually established by performing numerical simulation for casting rods. Szeliga et al.[9] presented the solidification process with fixed parameters values, which started to change slightly above a certain height of rods. Their values for blades changed to a greater extent because of a sudden change of cross section.[10] The solidification process of blades is more complic
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