Micro-/defect-structure effects on the short and long fatigue crack growth behavior of additively manufactured metallic materials

Abstract

This work investigates the influence of microstructural and defect characteristics on the short and long fatigue crack growth (FCG) behavior of additively manufactured (AM) metallic materials. This work aims to establish a mechanistic understanding of how AM micro-/defect-structures govern fatigue crack initiation and propagation across multiple length scales. Fatigue crack growth behavior was evaluated in several laser powder bed fused (L-PBF) materials, including Ti-6Al-4V, several aluminum alloys (EOS AlSi10Mg, LPW AlSi10Mg, QuesTek Al, and Scalmalloy), and IN718, encompassing a range of microstructural length scales and defect sensitivities. These ranges were induced by adjusting the processing conditions during fabrication and through the application of post-fabrication thermal processing such as heat treatment. Fatigue crack growth tests were conducted on specimens under varying stress ratios and specimen orientations to assess long FCG behavior, while non-destructive characterization techniques, including X-ray computed tomography and diffraction contrast tomography, were employed to investigate crack initiation and short crack growth mechanisms. The results show that microstructural features play a dominant role in governing FCG behavior, particularly in the near-threshold regime and during crack initiation. Grain orientation and stress ratio affected near-threshold behavior through crack closure mechanisms. Higher threshold stress intensity factor ranges corresponded to larger microstructural length scales and reduced FCG rates due to increased crack path tortuosity and roughness-induced crack closure in IN718. In contrast, the influence of process-induced volumetric defects on FCG behavior was found to be highly dependent on the material system and relative defect population, with minimal impact observed in high-density materials such as L-PBF IN718. The stable crack growth regimes were largely unaffected in comparison to the near-threshold and unstable regimes. Investigations into crack initiation revealed that fatigue cracks predominantly nucleate at microstructural features rather than defects when the microstructural length scale is large. Crack initiation occurred along planes consistent with persistent slip band activity, with smaller grains exhibiting a higher propensity for initiation due to increased local constraint and cyclic damage accumulation.