Simultaneous improvement of low-cycle and high-cycle fatigue performance of LPBF IN718 alloy via pre-aging laser polishing

Additive-manufactured superalloys face persistent fatigue limitations due to synergistic effects of surface defects, residual stresses, and anisotropic microstructures. Here, we develop a pre-aging laser polishing (PRAP) strategy that fundamentally restructures fatigue resistance mechanisms. Results reveal that PRAP enhances both low-cycle fatigue (LCF) (∼10⁴ cycles) and high-cycle fatigue (HCF) (>10⁶ cycles), with the HCF life extended by up to 70% while maintaining strength-ductility balance. Moreover, it preserves high yield strength (Δσ < 3% reduction) while boosting ductility (11% increase). Advanced microstructure characterisation demonstrates that PRAP uniquely achieves: (1) stabilised dislocation cell structures without detrimental tensile stresses, (2) elimination of columnar grain morphology, and (3) precipitate-mediated pinning that enhances cyclic stability. These nanoscale cells function as crack-blocking deformation units, as evidenced by in situ SEM fatigue testing. Remarkably, PRAP introduces a dual enhancement mechanism including suppression of crack nucleation in the HCF regime and stabilisation of cyclic plasticity under LCF large-strain deformation, overcoming the traditional strength-ductility-fatigue trade-off in AM superalloys.