Optimization of the Post-Heat Treatment of Additively Manufactured IN718.

Inconel 718 has good fabricability, high strength at elevated temperatures, and high corrosion resistance and is widely deployed in many aerospace and other high-performance applications. With the molten pool's rapid solidification during laser powder bed fusion (LPBF), the resulting microstructures differ from those expected in equilibrium conditions. Residual stresses, microsegregation, anisotropy, undesirable phases, layered structure, and lower mechanical properties are the challenges that must be addressed before LPBF-ed Inconel 718 parts can be industrially implemented. Heat treatment of Inconel 718 after the LPBF process is widely discussed in the literature, and the proposed heat treatment processes do not address all the challenges mentioned above. For this reason, specific heat treatments should be designed to achieve desired mechanical properties. Four different high-temperature heat treatment procedures were developed and tested in recent work to study the effect of various heat treatment parameters on the type of precipitates, grain size, room and elevated temperature mechanical properties, and creep properties of LPBF-ed Inconel 718. Two heat treatment procedures were designed to study the effect of eliminating the stress relief cycle and increasing the soaking time of solution heat treatment at the same hot isostatic pressing (HIP) and aging conditions. The other two heat treatment procedures were designed to study the effect of stress relief and aging parameters, with similar HIP and solution annealing parameters. The investigated heat treatment procedures showed the formation of equiaxed grain size and a significant amount of γ″ particles at the grain boundary in addition to primary carbide types (MC). Three different grain sizes could be obtained, starting with the same as-built microstructure by controlling post-process heat treatment parameters. The first type, coarse grain size (ASTM grain size No. G 3), suitable for creep application, was achieved by increasing the soaking time at the aging cycle. The second type, the formation of serrated grain boundaries suitable for good fatigue and creep properties, was achieved by decreasing the stress relief cycle's soaking time and temperature. The third type, fine grain size (ASTM grain size No. G 6), which is preferable for fatigue properties, was achieved by decreasing the soaking time at the solution annealing cycle. Among the different heat treatment conditions, the heat treatment with lower stress relief temperature and longer soaking time at solution annealing steps showed compromised equiaxed grain size with serrated grain boundaries, and it was selected for mechanical testing. The mechanical properties at room and elevated temperature (650 °C) and creep properties under constant axial stress of 690 MPa at 650 °C until failure were evaluated. The obtained elongation is > 2 × higher than the available properties in the AMS 5663 for wrought Inconel 718, AMS 5383 for cast Inconel 718, and ASTM F3055 for LPBF while maintaining a higher tensile strength above the average level found in the standards. It can be concluded that the performed heat treatment achieves higher mechanical properties. The values of UTS, YS, elongation, and reduction of area percentages are similar in the Z and XY orientations, revealing the presence of isotropic microstructure and uniform distribution of MC and γ″ precipitates. [ABSTRACT FROM AUTHOR]