Effect of heat treatment and hot isostatic pressing on the microstructure and mechanical properties of the directed energy deposition IN738LC and IN718: A comparative study

Additive manufacturing of nickel-based superalloys is challenging due to the presence of several alloying elements and their wide range of solidification temperatures, which causes residual stress, intermetallics formation, and in-process cracking. Moreover, upon achieving successful production, the as-built components generally possess poor ductility. In this paper, proper post-treatment strategies for IN738LC and IN718 are employed to investigate their effects on the microstructural and mechanical properties of the previously developed mixed IN738LC + IN718 (80/20) alloy. It is shown that the HIP can significantly increase the mass density of IN718 and the mixed IN738LC + IN718. The microstructure of mixed IN738LC + IN718 resembles the IN738LC gamma/gamma' microstructure with a few Laves and eutectics, which are common in IN718. Applying heat treatment or HIP cause the evolution of the gamma' phase, which directly affects the mechanical properties of this alloy. After the optimum post-treatment, the ultimate strength of the mixed IN738LC + IN718 is recovered to over 1300 MPa along with 17 % elongation at break. The hardness of this alloy can be improved to almost the same value for as-built IN738LC as well. Therefore, the addition of IN718 is not only harmless to the mechanical properties of IN738LC, it is an effective way to produce large components from this alloy without requiring special production conditions.