Additive Manufacturing-Based Repair of In718 Superalloy and High-Cycle Fatigue Assessment of the Joint

Abstract

Room-temperature high-cycle fatigue (HCF) properties of IN718 repaired joint via laser direct energy deposition (DED) were studied with the fatigue axis perpendicular to the joint interface. Solution treated and aged (STA) joints were compared with directly aged (DA) joints. The wrought IN718 substrate showed equiaxed grains with a size of ~90 mm and a high fraction of annealing twins, whereas the DED deposit revealed a mixture of equiaxed and columnar grains with an average grain size of ~20 mm. There was little difference between the STA and DA conditions in the grain length-scale. Micro-hardness results highlighted the need for the heat treatment as it can remove the heat-affected zone and hardness dip, creating a uniform hardness profile across the joint. Although the monolithic DED deposit had a similar tensile strength compared to the wrought substrate, the DED joint exhibited an overall decreased HCF performance, regardless of the heat treatment conditions. When the fatigue stress was low, the STA condition had a better HCF performance than the DA, however, the opposite trend appeared for the high stress, resulting in a cross-over point on the stress-life S - N plot. Interrupted fatigue tests, combined with microscopy and fractography, revealed that the fatigue failure occurred in the substrate for the DED joint in the DA condition, whilst in the deposit zone for the STA condition due to the distribution and fracture of the Laves and δ phases. Grain boundary cracking in the substrate near the substrate-to-deposit interface can occur in both cases, probably due to the Nb-rich liquid films.

Keywords: additive manufacturing, Direct Energy Deposition, Inconel 718, microstructure, High Cycle Fatigue.