Impact of Three Additive Manufacturing Techniques on Microstructure and Creep Damage Development in Alloy 718

Additive Manufacturing (AM) has revolutionized traditional manufacturing processes by enabling the creation of complex and customized components. In this work, three prominent AM techniques: Laser-Based Powder Bed Fusion (PBF), Wire Direct Energy Deposition (DED), and Binder Jet (BJ) processes, were explored for the fabrication of Inconel 718 components for high temperature applications. Inconel 718 is a nickel-based superalloy known for its excellent combination of high temperature strength, corrosion resistance, and weldability. Samples produced from the three techniques underwent short-term creep tests, metallographic analysis, and were then compared to wrought material of the same alloy. Detailed electron microscopy unveiled equiaxed grains in both BJ and wrought samples while PBF samples displayed finer grain structures that were elongated in the build direction, characteristic of PBF. When assessing the three processes, the average grain size was found to be larger in the BJ samples, while the PBF samples exhibited the most significant variation in grain and sub-grain size. Number density, size, and shape of porosity present varied between samples from all three techniques. Post-creep test observations in PBF samples revealed the occurrence of wedge cracking at the failure point, accompanied by a preference for grain boundary creep void formation while BJ samples exhibited grain boundary creep void coalescence and cracking at the failure location. In contrast, the wrought sample showed void formation at the failure site with a preference for areas with primary carbide formation. Despite BJ samples demonstrating similar or even superior rupture life compared to other AM techniques, a noteworthy reduction in rupture ductility was observed. While a coarse, uniform grain size is generally linked to enhanced creep resistance and rupture life, the combination of pre-existing voids along grain boundaries and the formation of new voids during creep tests is hypothesized to accelerate rapid fracture, resulting in diminished ductility.