Nickel-based Inconel 625 is widely used for both low and high-temperature applications. It has several applications in aerospace, marine, chemical, and petrochemical industries due to its high strength, corrosion resistance, good formability, and weldability. 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 625 parts can be industrially implemented. Heat treatment of Inconel 625 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. Five different high-temperature heat treatment procedures were developed and tested in recent work in comparison with the standard heat treatment for wrought alloy (AMS 5599), to study the effect of various heat treatment parameters on the type of precipitates, grain size, room, and elevated temperature mechanical properties, and to develop an elevated-temperature tensile curve between room temperature (RT) and 760°C of LPBF-ed Inconel 625. Four heat treatment procedures showed complete recrystallization and the formation of equiaxed grain size containing annealed twins and carbide precipitates. However, either eliminating the stress relief cycle or conducting it at a lower temperature resulted in microstructures having the same pool deposition morphology with grains containing dendritic microstructure and epitaxial grains. Two 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 4.5), suitable for creep application, was achieved by applying hot isostatic pressing (HIP) followed by solution annealing. The second type, fine-grain size (ASTM grain size No. G 6), preferable for fatigue properties, was achieved by applying solution annealing followed by HIP. The mechanical properties at room and elevated temperature 540°C are higher than the available properties in the AMS 5599 for wrought Inconel 625 while maintaining a higher ductility above the average level found in the standards. It can be concluded that the performed heat treatment achieves higher mechanical properties. The values of ultimate tensile strength (UTS), yield strength (YS), elongation, and reduction of area percentages are similar in the XZ and XY orientations, revealing the presence of isotropic microstructure. The ultimate tensile strength values show an anomalous behavior as a function of the temperature. From the room temperature until around 500°C, there occurs a decrease in the yield strength and a slight increase up to 600°C, decreasing sharply at 700°C. An anomaly is also present in relation to the elongation, with a significant decrease in the elongation at temperatures after 600°C.

IN718 has good fabricability, high strength at elevated temperature, and corrosion resistance, and it is widely deployed in many aerospace and other high-performance applications. With the molten pool rapid solidification during laser powder bed fusion (L-PBF), the resulting microstructure is anisotropic and inhibits macro-segregation. The as-built condition usually exhibits lower mechanical properties. Four different heat treatment procedures were designed and tested to study the effect of different heat treatment parameters on the type of precipitates and grain size. The investigated heat treatment procedures showed the formation of equiaxed grain size and a significant amount of γ' and γ" particles at the grain boundary in addition to primary carbide types (MC). Three types of microstructure characteristics and grain size were achieved. Coarse grain size suitable for creep application was achieved by increasing the soaking time at the aging cycle. 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. Fine-grain size, which is preferable for fatigue properties, was achieved by decreasing the soaking time at the solution annealing cycle.

Laser powder bed fusion (L-PBF) is an additive manufacturing (AM) technique through which net shape/near-net components are built by selectively melting powder, one layer at a time, with a focused laser beam. The as-built microstructures have a great impact on the phase transformation and precipitation behavior during subsequent heat treatment. This study was directed to understand the effect of component thickness, in the case of complex shape components, on the microstructure, type of precipitates of L-PBF IN 718 in as-built and heat-treated conditions. Standard heat treatment cycles per ASTM F3055 and AMS 2774D were investigated. This work shows that microstructure, grain size, types of precipitates, and formed phases of components produced by L-PBF in the as-built condition and after heat treatment are profoundly different with different component thicknesses. In order to obtain the optimal microstructure and mechanical properties, specific heat treatments are necessary due to the complexity of the components produced.