PSI - Issue 82

Tomáš Babinský et al. / Procedia Structural Integrity 82 (2026) 162–168 Tomáš Babinský et al. / Structural Integrity Procedia 00 (2026) 000–000

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increase in strength while maintaining high ductility. Further enhancement of mechanical properties can be achieved through optimized heat treatment. Multi-step heat treatment of superalloys is typically employed to relieve residual stresses and introduce the strengthening γ’ precipitates. Superior mechanical properties of additively manufactured IN939 to the cast counterpart have been demonstrated first at ambient temperature by Kanagarajah et al. (2013) who explored tensile properties, and later by Babinský et al. (2025) who explored low-cycle fatigue performance. In this work, we explore the feasibility of simplifying the heat treatment to a single step in the process called direct ageing. Our aim is to enhance mechanical properties by precipitating γ’ while preserving the beneficial cellular microstructure and reducing the overall cost of heat treatment. Recently Kumar et al. (2024) have shown that direct ageing yields very high strength at the cost of reduced ductility, ultimately resulting in inferior creep properties. However, the implications for fatigue design at lower temperatures (when creep is not significant) remain unclear and thus shall be studied. Nomenclature b Fatigue strength exponent c Fatigue ductility exponent k cr Fatigue life criterion ε ap Plastic strain amplitude ε f ‘ Fatigue ductility coefficient σ a Stress amplitude σ m Mean stress σ f ’ Fatigue strength coefficient IN939 superalloy is a high-chromium, high-cobalt nickel-base superalloy which is precipitation-hardened by a fine dispersion of coherent γ’ phase Ni 3 (Al, Ta, Ti) in the fcc γ matrix. It was processed additively by PBF-LB/M from a commercially available powder with an average particle size of 40±15 μm. Material was supplied in the form of rectangular blocks and flat dogbone specimens manufactured by Innomia a.s. using an EOS M 290 3D printer equipped with a 400 W Yb laser with a focus diameter of 100 μm, operating in Ar atmosphere. “Stripes” scanning strategy was employed with interlayer rotation of 67°, hatch spacing of 70 μm, and layer thickness of 40 μm. Chemical composition of the powder and the PBF-LB/M-ed specimens has been reported in our previous work, see Šulák et al. (2023) and Babinský et al. (2025). Direct ageing was performed post manufacturing at 800 °C/8 h/air. Similar direct ageing treatment at 800 °C/6 h had been independently investigated by Kumar et al. (2024). For reference, the direct aged PBF-LB/M IN939 superalloy was compared with Babinský et al. (2025) who investigated the same material in • cast, fully heat-treated state (solutionised and aged; no HIP treatment!), labelled as ‘cast’, • manufactured by PBF-LB/M with no post-processing heat treatment (as-built microstructure), labelled as ‘no HT’, • manufactured by PBF-LB/M with full heat treatment (solutionised and aged), labelled as ‘aged’. In this case, full heat treatment refers to a 3-step heat treatment consisting of solution annealing at 1160 °C/4 h/air and two-step precipitation hardening (ageing) 1000 °C/6 h/air + 800 °C/4 h/air suggested by Delargy et al. (1986) for conventional cast IN939. Cylindrical fatigue specimens were machined out of the rectangular blocks and ground to Ra 0.4 μm. Dogbone specimens intended for tensile testing were ground manually with SiC paper (P220) to reduce the surface roughness down to ca. Ra 0.8 μm. To investigate the influence of loading-to-building orientation, specimens were prepared with the loading axis either parallel or perpendicular to the building direction, being labelled as vertical (V) and horizontal (H), respectively, see Fig. 1c. 2. Experimental 2.1. Material and heat treatments