Date of Award
Master of Science in Materials Science
Department of Aeronautics and Astronautics
Ryan O'Hara, PhD
Continuing improvement in the field of AM of metals provides the opportunity for direct fabrication of aerospace parts. AM was once used in large part for rapid prototyping but improvements in technology and increases in the knowledge base of AM materials has provided the opportunity for manufacture of AM parts for operational use. The ability to create low numbers of unique parts without having to invest in expensive tooling provides great economic incentive to utilize this technique. IN718 is the most common high temperature alloy used in the aerospace industry and lends itself readily to formation by LPBF. The superior strength of IN718 at temperatures up to 650°C and its excellent corrosion resistance make it the alloy of choice for compressor blades in jet turbines. Extensive data exists for conventionally produced IN718 but gaps in the data are present for AM material. Two of these gaps that are addressed by this research are FT and FCGR. A primary driver of the differences in behavior between conventionally manufactured and AM IN718 are the microstructural differences. Conventionally produced material has an equiaxed microstructure that provides isotropic material behavior. Conversely production by AM methods results in a highly directional microstructure related to the build direction of the part that drives anisotropy in material performance. It is hypothesized that a modified heat treatment that allows for re-crystallization of the columnar grains to grains that are more equiaxed will mitigate anisotropic material effects. This research will characterize the FCGR of AB, CHT, and MHT conditions. Results of FCGR indicate the the MHT is successful at reducing the overall FCGR of LPBF IN718. The standard heat treatment for wrought IN718 is shown to increase anisotropy in FCGR and give no improvement to the FCGR when compared to material in the AB condition.
DTIC Accession Number
Hohnbaum, Charles C., "Fracture Toughness and Fatigue Crack Growth Rate Characterization of Inconel 718 Formed by Laser Powder Bed Fusion" (2019). Theses and Dissertations. 2221.