Influence of high initial porosity introduced by laser powder bed fusion on the fatigue strength of Inconel 718 after post-processing with hot isostatic pressing

Laser-based additive manufacturing (AM) of metallic materials can produce AM parts of high quality. The obtained density of metal parts manufactured by laser powder bed fusion (LPBF) is mostly greater than 99% and high mechanical strengths can be achieved; however, a long processing time is typically required to reach a high quality. Possibilities to shorten the LPBF processing time involve increasing either the scanning speed or the hatch distance, but both will result in greater porosity. In general, the porosity of AM parts can be reduced by subsequent post-densification of the materials by hot isostatic pressing (HIP). In this manner, not only can the LPBF process be accelerated but the fatigue strength of the parts can be increased by reliable reduction of porosity and the simultaneous homogenization of the microstructure. Nevertheless, argon porosity introduced by the LPBF process cannot be completely eliminated by HIP due to the insolubility of argon in metallic materials. In the present study, the influence of the initial porosity on the fatigue strength of IN718 specimens after HIP and different heat treatments were investigated with a view to understanding the impact of argon porosity. The fatigue strength under a rotating bending load was determined for four different test batches and one almost fully dense as-built state as a reference. For the test batches, two different initial porosities of 3.5% and 9.9% were introduced by increasing the hatch distance. These porous samples were subjected to the same HIP treatment but aged differently: half of the samples were annealed, quenched, and aged within the HIP vessel under pressure and the other half were annealed and quenched within the HIP vessel but aged in an ambient air atmosphere. HIP post-processing significantly improved the fatigue strength of IN718 samples produced by LPBF, even in the case of a high initial porosity and residual argon content. In the case of a 3.5% initial porosity, no difference was observed in fatigue behavior with respect to aging with or without pressure. In contrast, samples with an 9.9% initial porosity showed a large difference in fatigue strength according to different aging conditions.