Field-assisted sintering of nickel-based superalloy powder for high temperature hybrid turbine disk applications

Open Access
Author:
Lin, Charis I
Graduate Program:
Aerospace Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
November 23, 2018
Committee Members:
  • Namiko Yamamoto, Thesis Advisor
  • Jogender Singh, Thesis Advisor
  • Anil Kulkarni, Committee Member
  • Amy R. Pritchett, Committee Member
Keywords:
  • field assisted sintering
  • nickel-based superalloy
  • hybrid
  • turbine disk
  • FAST
  • hybridization
  • solid state joining
Abstract:
Turbine disks of aerospace engines are made of nickel-based superalloys, the only materials that can withstand the extreme operating conditions with their high strength and creep resistance at high temperatures. To optimize performance and robustness, advanced turbine disks consist of hybrid or dual-microstructures: the outer section of the disk, including the blades, is typically single-crystalline to withstand high temperatures up to 1400°C while the inner section of the disk is typically polycrystalline to withstand high shear loadings from the ~10,000 rpm rotation. However, one-step fabrication of such disks with enough of a microstructure gradient is difficult to achieve. Joining two (or more) parts with different microstructures has been attempted using mechanical fir-tree connectors or using friction welding but results in a heavy joint weight or weak bonding due to localized melting from welding. Here, this thesis studies the effectiveness and evaluates the potential of field assisted sintering technology (FAST) as a novel, weight-saving method to join turbine disk components with different microstructures and to provide strong bonding interfaces, without altering the performance of original parts. First, the capability of FAST was experimentally studied and confirmed for sintering of pre-alloyed Ni-based superalloy CM247LC powder into disks of scalable size (up to 200 mm). An increase in sintering temperature led to porosity migration away from the core of the sintered disks. After processing parameter optimization, the sintered material resulted in high relative density (>99%) and mechanical properties. To potentially improve melting temperature and mechanical properties of the sintered product, sintering of CM247LC powder with HfC additives, rarely studied in the past, was also conducted. HfC additives were expected to improve the performance of CM247LC disks due to grain growth inhibition and the higher melting temperature of HfC, but room-temperature performance improvement was found to be marginal, mostly due to poor diffusion of HfC into CM247LC grains. Because of the lack of performance improvement, HfC additions were not included during the manufacturing of hybrid disks using FAST. Second, FAST joining of hybrid disks, consisting of a powder-sintered CM247LC rim and a solid Inconel 718 core, was demonstrated, achieving an interface strength comparable with the solid core component without the formation of a heat affected zone. A new FAST die set-up was designed to provide uniform stress distribution when powders for the rim section are sintered (and thus compacted) and joined to the core section simultaneously. Characterization of the hybrid disk interface and the parent materials was performed via optical microscopy, scanning electron microscopy, ultrasonic sound velocity measurements, and room-temperature tensile testing. The CM247LC powder-sintered rim, without HfC additives, exhibited mechanical properties that are comparable with parent material (monolithic CM247LC disks). Meanwhile, the FAST-processed Inconel 718 core disks exhibited lower mechanical strengths than those of as-received reference Inconel 718 disks. The core-rim interfaces were observed to be oxidized, but homogeneous and free of defects and porosity, unlike the interfaces heat-affected by friction welding in past studies. Despite the high interface strength, a combination of interface effects and the effect of the weaker Inconel 718 material led to tensile fracture near the material interface, so further optimization of processing parameters is needed to improve interface bonding. This work may be a starting point for the joining of dissimilar nickel-based superalloys in turbine disk and blade assemblies using FAST. Future research can modify FAST sintering parameters to minimize weakening of solid cores and improve the rim-core interfaces. Current studies are limited to joining of two polycrystalline parts, but by joining polycrystalline and single-crystalline parts, interface formation is expected to be more complex due to the set crystalline orientation. In addition to improvement of hybrid disk components and interfaces, characterization of fabricated hybrid disks at high operating temperatures is needed.