Temperature evolution and grain defect formation during single crystal solidification of a blade cluster

In order to investigate the asymmetry of thermal conditions during directional solidification, the temperature evolution and correspondingly developed undercooling in a simplified single crystal blade cluster were numerically simulated. Simulation results demonstrate that the temperature distribution at the blade platforms is obviously asymmetrical. On the outside of the blade which directly faces the heating element, the liquidus(TL) isotherms progress relatively smoothly. On the inside of the blades facing the central rod, however, the TLisotherms are in concave shape and the slope goes upwards to the platform extremities. The average undercooling extent ?T and undercooling time ?t at the inside are much higher than those at the outside. It was then predicted that the inside platform extremities have significantly higher probabilities of stray grain formation compared to the outside ones. A corresponding experiment was carried out and the metallographic examination exhibited the same side-and height-dependence of stray grain formation in the blades as predicted. On the inside of the blades, all platforms are occupied by stray grains, while the platforms on the outside are nearly stray grain free. The simulation result agrees very well with the experimental observation.

Characterization of residual stresses and microstructural features in an Inconel 718 forged compressor disc

Residual stress plays an important part in fabricating commercial aero engine Inconel 718 components for their fatigue properties, reliability and durability. Due to the limitation of Chinese neutron diffraction instrument and lack of test practice and specifications, there is little systematic research on the residual stress of forged compressor disc. X-ray diffraction and neutron diffraction methods were used to measure the residual stress of Inconel 718 forged discs at the surface and in the interior, respectively. Scanning electron microscope and transmission electron microscope were used to characterize the microstructural features. The residual stress state at the disc is in near-surface compression, balanced by tension within the disc core. However, the surface residual stress of disc depends more on the rough machining than on the forging process. Also, the dislocation densities increase with decreasing distance to the surfaces of disc, and the residual stress accelerates dislocation generation and dynamic recrystallization.

Effect of nonuniform sintering on mechanical and thermal properties of silica-based ceramic cores

During sintering of the silica-based ceramic core of turbine blades,a phenomenon called"nonuniform sintering"occurs that negatively affects the thermal and mechanical properties of the core.Standard samples of silica-based core were prepared by an injection molding method and sintered with alumina backfilling powder with different sodium contents.The effect of sodium content on the nonuniform sintering of silica-based cores and the thermal and mechanical properties was evaluated.Results show that the sintering level and the content ofα-cristobalite in the surface layer are significantly higher than that of the sample interior.A considerable number of microcracks are found in the surface layer due to theβtoα-phase transition of cristobalite.As the sodium content in the alumina powder decreases,the level of the nonuniform sintering and the amount of crystallized cristobalite in the surface layer decrease,which is beneficial to the thermal expansion and flexural strength at ambient temperature.The flexural strength and thermal deformation at high temperature are improved by reducing the surface cracks,but deteriorated with the decrease of the cristobalite crystallization when the surface cracks are macroscopically invisible.

Systematic analysis of distinct flow characteristics and underlying microstructural evolution mechanisms of a novel fine-grained P/M nickel-based superalloy during isothermal compression

Isothermal forging(IF)is an effective method for forming difficult-to-deform materials like P/M superalloys.Understanding the isothermal compression microstructural evolution mechanism of a novel P/M s-peralloy provides the basis for its optimized IF planning.In this study,the isothermal compression tests of a novel fine-grained P/M nickel-based superalloy were carried out at 1000-1150℃with strain rates of 0.001-0.01 s^(−1).The results indicated that the alloy exhibits three distinct flow characteristics:continuous softening after reaching the peak stress,near-steady superplastic flow,and discontinuous hardening,corresponding to different strain rate sensitivity exponent(m)values.Varied microstructural evolution mechanisms,including grain boundary sliding(GBS),dynamic recrystallization(DRX),and grain growth,are dominated in different m-value domains.Meanwhile,different roles of primaryγ’play in microstruc-tural evolution were clarified.A moderate fraction of primaryγ’with 8.5%-14.2%can well coordinate the GBS and hinder excessive grain growth at a high m value domain(m>0.4).When 0.2<m<0.4,the role of the primaryγ’is changed to promote dislocation accumulation,accelerating the nucleation of DRXed grains.As the primaryγ’is dissolved at 1150℃,obvious grain growth was observed after compression.Work hardening effect by overgrown grains competed with DRX softening results in the discontinuous rising stress.

Novel casting processes for single-crystal turbine blades of superalloys

This paper presents a brief review of the current casting techniques for single-crystal （SC） blades, as well as an analysis of the solidification process in complex turbine blades. A series of novel casting methods based on the Bridgman process were presented to illustrate the development in the production of SC blades from super- alloys. The grain continuator and the heat conductor techniques were developed to remove geometry-related grain defects. In these techniques, the heat barrier that hinders lateral SC growth from the blade airfoil into the extremities of the platform is minimized. The parallel heating and cooling system was developed to achieve symmetric thermal conditions for SC solidification in blade clusters, thus considerably decreasing the negative shadow effect and its related defects in the current Bridgman process. The dipping and heaving technique, in which thin- shell molds are utilized, was developed to enable the establishment of a high temperature gradient for SC growth and the freckle-free solidification of superalloy castings. Moreover, by applying the targeted cooling and heating technique, a novel concept for the three-dimen- sional and precise control of SC growth, a proper thermal arrangement may be dynamically established for the microscopic control of SC growth in the critical areas of large industrial gas turbine blades.

Single Crystal Castability and Undercoolability of PWA1483 Superalloy

Both of the single crystal(SX) castability and undercoolability of PWA1383 superalloy were investigated during the directional solidifi cation and isothermal cooling. In all the six SX parts of a casting cluster, no stray grains were found, revealing a defect-free SX structure. This excellent SX castability of the superalloy was attributed to its good undercoolability. The melting point(T_L) and the critical nucleation temperature( T_N) of the alloy were measured to be 1327 °C and 1306 °C, respectively. The statistic average of the critical nucleation undercooling Δ T_N = T_L-T_N of the alloy was determined to be about 21 K, exhibiting a relatively great capacity to be deeply cooled to a temperature below the melting point without the onset of solidifi cation.