Surface and internal microstructure driven fatigue crack propagation from artificial defects in Laser Powder Bed Fusion Inconel 718

• Use of artificial defects to compare surface and internal fatigue crack propagation in Inconel 718 L-PBF. • Reconstruction of the FCGR of internal cracks coupling X-ray microtomography, direct current potential drop technique and finite element analysis. • Crack path analyses by EBSD and TEM with site-specific FIB lamellae. • Evidence of environment impact in terms of deformation mechanisms. • Air exposure yields multiple slip activity, L-PBF atmosphere limits slip activity and FCGR. L-PBF generates specific defects, namely lack of fusion (LoF), which constitute fatigue crack initiation sites. LoF being mostly located in the volume of material, it is difficult to access their impact on the fatigue crack growth rate (FCGR). This study proposes to compare FCGR of internal crack with that of surface crack. To control the crack initiation site, internal artificial defects have been introduced directly by L-PBF in test specimens made of IN718. To determine the FCGR of internal crack, a methodology coupling X-ray microtomography, direct current potential drop technique and finite element analysis was developed. Surface crack FCGR was assessed by striation counting and optical measurement. The FCGR for internal cracks appears slower than for surface cracks. The influence of the microstructure on fatigue crack growth mechanism was analyzed by EBSD and TEM. Given the large grain size, a strong influence of the microstructure on the crack path is observed. For internal initiation, the crack grows along the slip bands, resulting in a faceted fracture surface and changes of direction on grains boundaries. This means that grain boundaries act as obstacles, resulting in a slower FCGR for internal cracks. This crack propagation mechanism appears similar to wrought IN718 tested in vacuum/inert atmosphere. These results appear to be related to the atmosphere of internal defects. In this case, the “inert” atmosphere inherited from L-PBF process promotes reversible slips in the crack tip plastic zone, and consequently the crack grows on a limited number of slip planes.