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

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 degrees C. 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 mu m 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 root area parameter model was 40% higher than that obtained experimentally.

Automated summary

Inconel 718 is a high-temperature alloy with excellent mechanical properties, but internal defects during manufacturing may affect its fatigue strength. The study found that Inconel 718 specimens fabricated by SLM with internal defects of approximately 400 microns in diameter showed no significant decrease in fatigue strength. The predicted fatigue limit for the flat specimens was 40% higher than the experimental results.