Mitigation of scan strategy effects and material anisotropy through supersolvus annealing in LPBF IN718

摘要

The nickel-based superalloy Inconel 718 (IN718) is an excellent candidate among the existing aerospace alloys for laser powder bed fusion (LPBF) manufacturing. LPBF IN718 has a preference for (001) growth, resulting in a non-uniform, anisotropic microstructure which translates into orthotropic mechanical behavior. The most common heat treatment applied to IN718 is AMS 5662. This treatment was developed 60 years ago for wrought and cast metal forming processes. The small grains and columnar grain structure of LPBF IN718 are not affected by treatments per AMS 5662. Recrystallization and the removal of scan strategy effects have big implications for the acceptance of AM parts. If parts can be heat treated to remove any OEM-related microstructural differences, then parts fabricated on different machines can be printed in any orientation and possess the same properties. This research studies the microstructure of LPBF IN718 as it evolves under an annealing treatment of 1160 degrees C for up to 8 hours. It was hypothesized and later confirmed that this higher-temperature annealing would mitigate the scan strategy effects and anisotropy resulting from the LPBF process. The grain size, shape, and re crystallization 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. An equiaxed microstructure was successfully achieved through recrystallization and grain growth, resulting in isotropic microstructure for each scan strategy that was considered. Isotropic mechanical properties were achieved following a modified annealing treatment at 1160 degrees C for 4 hours and validated via nanoindentation and tensile testing.