Abstract
The microstructure and mechanical properties of additively manufactured (AM) parts have been shown to be different from that of cast and wrought counterparts. In this study, electron beam powder bed fusion (EB-PBF) fabricated Alloy 718 was exposed to three different heat treatment routes followed by strain-controlled fatigue testing at 550 °C. The fatigue tests were performed with specimens built with their center axis parallel and transverse relative to the build direction. The microstructure showed saturated precipitation of δ-Ni3Nb after repeated solution treatment at 954 °C. In contrast, no δ-Ni3Nb precipitates could be observed after a single-step solution treatment at 1025 °C. However, the disparity of secondary phases showed no noticeable influence on the fatigue life. A significant difference in fatigue behavior was noted between the parallel and transverse directions. The specimens loaded parallel to the elongated grains showed on average ~5× greater life in comparison to the perpendicularly loaded specimens. Compared to corresponding heat-treated material conditions tested at ambient temperature, the specimens showed lower life at high strain amplitude and superior life at low strain amplitude. Moreover, competitive internal and surface failure modes were observed at the lower strain amplitudes while for the higher strain ranges, surface failure modes dominated.
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