Fatigue of Solid State Additive Manufactured Inconel 625

In this work, the fatigue behavior of additive friction stir (AFS) manufactured Inconel 625 is characterized.  The AFS method is a unique solid state manufacturing process that differs from traditional friction stir welding since solid rod is fed through a non-consumable rotating cylindrical tool generating heat and plastically deforming the feedstock material through controlled pressure as successive layers are built upon a substrate. In order to quantify the fatigue behavior of the AFS Inconel 625, strain-life experiments were conducted under low and high cycle conditions at various locations of build height. The cyclic behavior of this alloy displayed varying degrees of cyclic hardening depending on the strain amplitude. However, the fatigue performance of the AFS Inconel 625 appeared to be insensitive to build location height suggesting minimal effect of interlayer debonding. In order to further understand the effect of microstructure on the fatigue behavior, a multi-stage fatigue (MSF) model that incorporates incubation and crack growth regimes was implemented to capture the effect of dynamic recrystallization and grain refinement in the AFS process. The MSF model exhibited good correlation to the experimental results, suggesting that the multi-stage approach for modeling fatigue damage in AFS  inconel 625 captures the underlining mechanisms associated with damage in this type of additive manufacturing process.