Fatigue crack growth behavior and microstructural mechanisms at room and elevated temperatures in Inconel 718 manufactured by laser engineered net shaping

Laser engineered net shaping (LENS®) is a directed energy deposition additive manufacturing technique with the potential to fabricate safety-critical parts provided that their fatigue properties are suitably understood. The fatigue crack growth behavior of LENS-fabricated Inconel 718 was systematically investigated at room and elevated temperature (650 °C). The effects of several post-LENS heat treatments on the microstructure and fatigue crack propagation mechanisms were identified and compared. It was found that, when present, the Laves phase controls crack growth mechanisms and inhibits crack growth resistance by nucleating microvoids at the matrix-particle interface. Elimination of the Laves phase by hot isostatic pressing and aging heat treatment resulted in improved fatigue properties, having a 200% higher crack growth threshold compared to the as-fabricated LENS material and comparable fracture toughness (ΔK FT = 105.6 MPa√m) to the wrought material. This superior performance of the Laves-free material is maintained at elevated temperature with only slight increases in crack growth rates. The results contribute towards a comprehensive understanding of the process – microstructure – fatigue crack growth property relationships for this alloy, which brings significant contributions to the holistic component design process with additive manufacturing for this important material.