Optimization of the heat treatment of additively manufactured Ni-base superalloy IN718

Additive manufacturing (AM) of Ni-base superalloy components can lead to a significant reduction of weight in aerospace applications. AM of IN718 by selective laser melting results in a very fine dendritic microstructure with a high dislocation density due to the fast solidification process. The complex phase composition of this alloy, with three different types of precipitates and high residual stresses, necessitates adjustment of the conventional heat treatment for AM parts. To find an optimized heat treatment, the microstructures and mechanical properties of differently solution heat-treated samples were investigated by transmission and scanning electron microscopy, including electron backs-catter diffraction, and compression tests. After a solution heat treatment (SHT), the Nb-rich Laves phase dissolves and the dislocation density is reduced, which eliminates the dendritic substructure. SHT at 930 or 954 degrees C leads to the precipitation of the delta-phase, which reduces the volume fraction of the strengthening gamma '- and gamma ''-phases formed during the subsequent two stage aging treatment. With a higher SHT temperature of 1000 degrees C, where no delta-phase is precipitated, higher gamma ' and gamma '' volume fractions are achieved, which results in the optimum strength of all of the solution heat treated conditions.