Gas Phase Alloying and Sintering Kinetics of 3D Printed Ni-Based Structures

Doctoral dissertation, University of Cincinnati 2021
S. Khodabakhsh

Porous materials, including foams and lattice structures, are used in many applications such as biomedical implants, heat exchangers, catalysts, and batteries due to their light weight, high surface area and energy absorption properties. Lattice structures, specifically, are of great interest since their properties can be tailored by employing various design methodologies (e.g., topology optimization). On the other hand, Ni-based superalloys are used in many applications where high-temperature and oxidation/corrosion resistance are important such as in gas turbine components. The advantageous properties of these Ni-Cr-Al-based alloys with the geometry and tailored mechanical properties of lattice structures can be combined through a number of different additive manufacturing modalities (e.g., laser powder-bed fusion). However, this can result in residual stresses and potential crack formation that degrade mechanical properties. Therefore, there is a need for the development of an alternative approach. One such approach would be the decoupling of the printing and alloying steps. Beyond preventing residual stresses and crack formation, one extra benefit would be the ability to introduce a second level of porosity via the Kirkendall effect. This would further increase the surface area and reduce the weight per volume. This work focuses on this decoupled approach via studying i) the sintering kinetics of printed Ni strands to understand if the polymer binder and extrusion pressure affect the sintering behavior compared to pressure-less sintering of loosely poured and pressed Ni powders ii) the sintering kinetics of printed strands made from either pre-alloyed Ni-Cr or Ni and Cr mixed elemental powders to determine differences in sintering rates and final densification iii) alloying Ni-Cr strands with Al via pack cementation and homogenization to investigate how the microstructure influences Kirkendall pore development and evolution and iv) alloying Ni-Cr strands with Al via pack cementation and homogenization using a modified pack to be able to conduct in situ X-ray tomography such that the pack aluminization and homogenization process can be observed real time. To conduct this work, both ex-situ metallography and X-ray tomography were used to study the sintering behavior and the phase and Kirkendall pore formation and evolution. The results indicate that (i) the sintering behavior of the printed strands does not significantly differ from that of loosely poured powder, (ii) the pre-alloyed Ni-Cr powders sinter significantly better than the mixed elemental Ni and Cr powders, (iii) pack aluminization results in similar phases forming in the diffusion coating, but wildly different microstructures for a commercial wire as compared to the printed strands, particularly for longer aluminization times, due to the remnant porosity from the sintering process, and (iv) a lower activator concentration can be used to successfully conduct in situ X-ray tomography experiments of the pack aluminization and homogenization process and significantly reduces the deposition kinetics such that different phase constituents form in the diffusion coating layers. These results will be discussed in more detail in the following chapters. Further work remains to be able to produce full scaffold structures and study the evolution of pores within them.