Sintering and Solid-State Joining of Nickel-based Superalloys using Field-Assisted Sintering Technology
Open Access
- Author:
- Lin, Charis I
- Graduate Program:
- Aerospace Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- September 21, 2021
- Committee Members:
- Namiko Yamamoto, Chair & Dissertation Advisor
Anil Kulkarni, Outside Unit & Field Member
Edward Smith, Major Field Member
Amy Pritchett, Program Head/Chair
Jacob Langelaan, Major Field Member
Jogender Singh, Special Member - Keywords:
- nickel
field-assisted sintering
spark plasma sintering
solid state joining
microstructure
mechanical properties
diffusion bonding
superalloy
powder metallurgy - Abstract:
- In Ni-based superalloys, careful selection of alloying elements and heat treatment produces an orderly precipitate microstructure, which provides high strength at elevated temperatures. For this reason, Ni-based superalloys are used in the high-pressure turbine in gas turbine engines, which experience high stress at elevated temperatures. Further improvements to mechanical and thermal properties by modifying the alloying elements are limited, as excessive alloying element additions produce unwanted topologically close-packed (TCP) phases which reduce mechanical strength. Thus, additional methods to reduce the cost and increase the effectiveness of the high-pressure turbine blade-disk assembly are needed, whether by improving the physical properties through the formation of a metal matrix composite (MMC), by decreasing the weight through replacement of the heavy blade-disk mechanical joint with a direct bonding interface, or by lengthening the operating lifecycle through blade repair. However, thermal stability improvement with particle additions in Ni-based superalloy MMCs has not yet been demonstrated, current joining methods for integrated blade-disk (blisk) manufacturing such as friction welding produce undesirable interface microstructure features, and previously proposed methods for complete blade repair are not reproducible. In this thesis, field-assisted sintering technology (FAST) is investigated as an alternative and novel processing method for both 1) net-shape production and strengthening of Ni-based superalloys by powder sintering with additives and also 2) solid-state joining of polycrystalline and/or single crystal Ni-based superalloys, as needed for blisk formation and single crystal blade repair. First, hafnium (Hf) and hafnium carbide (HfC) particle additives (micron-sized, 3-5 vol.%) were co-sintered with pre-alloyed Ni-based superalloy CM247LC powder using FAST to form particle-reinforced MMCs. The additives were expected to act as grain growth inhibitors, provide mechanical strengthening, and improve thermal stability through the diffusion of Hf into the nickel matrix. The fabricated MMCs were inspected for their microstructures (grain size and phase morphology), mechanical properties (tensile strength, elongation, and hardness), and thermal properties (oxidation resistance and critical phase change temperatures) using optical and electron microscopy, room-temperature tensile testing, Vickers hardness test, and simultaneous differential scanning calorimetry and thermogravimetric analysis. Second, FAST was utilized to join Ni-based superalloys of various joining angle orientations, elemental compositions, and grain structures (single crystal vs. polycrystalline). The FAST-prepared specimens’ interphase microstructures and tensile properties were characterized using optical microscopy, electron microscopy, and room- and elevated-temperature tensile testing. Such data were used to understand the diffusion behaviors and structure-property relationships during FAST processes, with a focus on the interface bonding strength. Through these studies, the FAST process was shown to provide strong fabrication and bonding of Ni-based superalloys with controlled microstructures, resulting in improved thermal stability with additives and strong bonding interphases without grain recrystallization or a heat-affected zone. 3 vol.% HfC or Hf additions to CM247LC decreased the tensile strength by 23% and 17%, respectively, compared to the CM247LC baseline specimens, due to crack initiation at the Hf-rich grain boundary phase. Despite the decrease in room-temperature strength, oxidation with 3 vol.% HfC or Hf addition decreased with respect to the baseline CM247LC by ~25% and 80% at 1490 °C, respectively, indicating that the additions can improve oxidation resistance compared to the base CM247LC material. Lower oxidation and greater thermal stability with Hf additions compared to HfC additions corresponded to greater diffusion of the additive into the Ni matrix, identifying diffusion as a major factor for the thermal stability of these particle-reinforced MMCs. Meanwhile, during FAST joining of a solid CM247LC component to Inconel 718 (IN718) powder at various joining angles with respect to the electric current and pressing force direction (15, 30, and 45°), electric current was shown to have a volumetric rather than directional effect, as the interphase formation and interphase strength were comparable for all three joining angles. Furthermore, during FAST solid-state joining of Ni-based superalloys, compositional gradient and crystal structure impacted interface diffusion. High bonding strengths were confirmed with joined polycrystalline Ni-based superalloys (CM247LC-IN718) or joined polycrystalline to single crystal superalloys (CM247LC-PWA1429) due to a diffusion bonding zone consisting of γ' precipitates of varying morphology and local volume fraction. On the other hand, FAST-joined single crystal components (PWA1429-PWA1429 and PWA1429-PWA1480) fractured at the interface plane, though ultimate tensile strength (UTS) was only marginally lower than that of the monolithic baseline materials (within 3%). Finally, to mitigate the weakening effect of FAST exposure during solid-state joining, recovery of the desired cuboidal γ' morphology and a corresponding increase in tensile strength after FAST joining was demonstrated using a post-bonding heat treatment, but further development of post-bonding heat treatment procedures are needed to regain high bonding strengths. The processes developed in this work to manufacture and join Ni-based superalloy components for high-temperature and high-stress environments can be applied in applications outside of turbomachinery, such as rocket engine combustion chambers or piping in nuclear power plants. Future work includes creep testing of FAST-joined Ni-based superalloys, characterization of FAST-joined specimens with a wider range of compositions, further development of post bonding heat treatments, and studying the effect of crystal orientation during solid-state joining of single crystal superalloys.