Relationship between internal defect size and fatigue limit in selective laser melted Inconel 718

Abstract

Inconel 718 is a Ni-based superalloy, which shows excellent mechanical properties such as tensile strength, fatigue strength, and creep strength at high temperatures up to 700 ℃. However, as Inconel 718 is a difficult-to-cut material, it causes severe wear of machining tools. Fabricating Inconel 718 by Selective laser melting (SLM), a type of additive manufacturing (AM), is therefore expected as the manufacturing method to deal with this problem. However, it is impossible to completely prevent the occurrence of the internal defects in SLM parts. These internal defects deteriorate the mechanical properties such as the fatigue strength of SLM parts. Though the effect of the internal defect size on the fatigue limit of SLM alloys such as Ti6Al4V and AlSi10Mg has been extensively investigated, its effect on SLM Inconel 718 has not yet been investigated. In this study, Inconel 718 specimens with various internal defect sizes were fabricated by SLM and the effect of the internal defect size on their fatigue strength was investigated. Internal defect size distribution in as built specimen can be approximated by Gumbel distribution. The specimens containing internal defects with a diameter of about 400 μm showed no significant decrease in the fatigue strength. In the case of the plane specimen, the fatigue limit predicted using the statistic of extremes and the √area parameter model was 40% higher than that obtained experimentally.