D. Newell, A. Palazotto, R. O’Hara, G. Cobb, L. Burggraf, J. Hess
Air Force Institute of Technology,
Keywords: Inconel 718, LPBF, anisotropy, annealing, microstructure characterization
Summary:The nickel-based superalloy Inconel 718 (IN718) is an excellent candidate among the existing aerospace alloys for laser powder bed fusion (LPBF) manufacturing. IN718 possesses a unique resistance to post-weld heat treatment cracking, a common frailty of other age-hardened superalloys. Current heat treatments for IN718 were developed over 60 years ago for wrought and cast metal forming processes. The small grains and columnar grain structure of LPBF IN718 are not significantly affected by the traditional heat treatment. LPBF IN718 has a preference for (001) growth, resulting in a non-uniform, anisotropic microstructure which translates into orthotropic mechanical behavior. This research studies the microstructure of LPBF IN718 as it evolves under an annealing treatment of 1160 °C for up to 8 hours with the purpose to mitigate the scan strategy effects and anisotropy resulting from the LPBF process. The grain size, shape, and recrystallization are compared throughout the evolution. Additionally, the X--Y and X--Z planes are compared to find a point at which the annealing process results in equiaxed, isotropic grains and the scan strategy effects are mitigated. Specifically, an equiaxed microstructure was successfully achieved through recrystallization and grain growth, resulting in indistinguishable, hence isotropic, microstructure for each scan strategy that was considered. The modified annealing at 1160 °C for 4 hours is proposed for additional research into the effects on mechanical properties.