Room and high temperature high-cycle fatigue properties of Inconel 718 superalloy prepared using laser directed energy deposition

The high-cycle fatigue (HCF) properties of Inconel 718 (IN718) superalloy prepared using laser directed energy deposition (LDED) were investigated at room temperature (RT) and 650 °C. A specified heat treatment scheme, namely homogenization + solution + aging (HSA), was employed for the as-deposited LDED IN718 alloy. The results indicate that the grains of the HSA LDED IN718 alloy have a typical bimodal grain (BG) distribution consisting of coarse grains (CGs) and fine grains (FGs). The fatigue limit (σ_w) of the HSA IN718 alloy at RT was higher than that at 650 °C (~385 and ~280 MPa, respectively), and their σ_w values were only ~77% and ~63% those of wrought IN718, respectively. Fractographic analysis revealed that the fatigue failure model changed from surface slip failure to large crystallographic facet failure with increasing testing temperature. The HCF properties of the HSA IN718 alloy with BG microstructure are strongly affected by CGs in the material, which eclipses the deleterious effects of the initial pore defects in the deposit. Finally, the Basquin model was fitted to extract the stress-life curves of the HSA IN718 alloy. A certain degree of deviation was observed when predicting the σ_w value of the HSA IN718 alloy at 650 °C using the Murakami model.