Process Parameters Control Fatigue Performance in AM Superalloys

The Challenge

Inconel 718 is the workhorse superalloy for aerospace turbine components, where very-high-cycle fatigue performance is non-negotiable. As laser powder bed fusion scales toward production of these safety-critical parts, manufacturers face an overlooked variable: the choice of shielding gas. Argon and nitrogen both prevent oxidation during printing, but their effects on long-term fatigue life remained poorly understood.

The Solution

This research compared L-PBF IN-718 produced under argon versus nitrogen shielding, characterizing defects via optical microscopy, Archimedes density, and X-ray CT, then subjecting specimens to VHCF testing at 20 kHz. The results revealed a critical tradeoff: nitrogen shielding produced finer grain structure but introduced more porosity and inclusions. These defects became crack initiation sites, degrading fatigue performance despite the microstructural refinement.

Argon-shielded specimens showed narrower fatigue life scatter and different failure mechanisms. Cracks initiated from microstructural features rather than defects, leaving characteristic facets at initiation sites.

Key takeaway: Nitrogen shielding refines microstructure but introduces porosity and inclusions that become crack initiation sites, making argon the safer choice for fatigue-critical L-PBF IN-718 components.

Results

The study demonstrates that process gas selection directly impacts fatigue reliability in AM superalloys. For fatigue-critical aerospace applications, argon shielding delivers more consistent performance. This finding gives manufacturers a concrete process parameter to control when qualifying L-PBF IN-718 for turbine components where billion-cycle durability is required.