Comparison of Cast, Wrought, and LPBF Processed IN718 Concerning Crack Growth Threshold and Fatigue Crack Growth Behavior

Abstract

This study focuses on the influence of microstructure and ductility on the short crack behavior and the fatigue crack growth (FCG) threshold. For this, an additively manufactured batch of the nickel-based alloy IN718, made via Laser Powder Bed Fusion (LPBF) process, is compared to both conventionally wrought and cast IN718 material. An initial characterization revealed significant differences in grain structure, from regular fine grained wrought material to large grains of several millimeters in diameter in the cast variant and a chessboard-like grain structure typical for LPBF. The ductility parameters of IN718-LPBF and wrought IN718 are comparable at room temperature, but at 650 °C the LPBF-variant shows macroscopic brittle fracture. The fatigue crack behavior both in the short and long crack regime is investigated in air at 650 °C. To produce minimal crack growth increments of about 1 μm, a compressive precracking and subsequent threshold test procedure with stepwise load increase has been adapted for high temperature testing. To assess the short crack behavior, cyclic R-curves have been generated taking the influence of three different stress ratios (R = −1, 0, and 0.5) into account. As expected, increasing crack closure mechanisms at higher stress ratios lead to higher stress intensity amplitudes ΔKI necessary to initiate crack growth. The crack growth resistance of the LPBF-processed variant is higher compared to wrought IN718. In the long crack regime, the wrought alloy yields higher crack growth rates compared to LPBF and cast IN718. An expected R-ratio dependency is observed for all material states.