Comparison of Elevated Temperature Tensile Behavior of SLM and Wrought Inconel 718: Strain Rate, Test Temperature, and Heat Treatment Dependence

Selective laser melting of Inconel 718 provides many advantages over traditional manufacturing methods: reductions in part count, decreased lead time, and complex geometries that are not attainable through subtractive methods. However, a comparison between the high-temperature properties of traditional wrought and SLM Inconel 718 reveals significant differences between the two production techniques even after the SLM version has been fully HIP’ed and heat treated. The current work focuses on the high temperature tensile testing of SLM and wrought material given typical industry standard heat treatments. Testing was conducted in the 550 °C - 750 °C range, with strain rates between 10^-5/s and 10^-7/s. Smooth and notched samples were tested to evaluate the sensitivity of these properties in the presence of a stress concentration. Significant differences in ductility between wrought and SLM are observed at elevated temperature; for example, testing at 550 °C resulted in a 72% decrease in the elongation of SLM samples compared to wrought. Substantial decreases in elongation are observed across the entire testing temperature range despite minimal differences in strength. The ductility drop in SLM material is attributed to the microstructural features that arise from the SLM process. Fractography indicates that brittle intergranular cracking in SLM samples may be related to carbides precipitated along grain boundaries which originate from high degrees of Nb segregation during solidification. Wrought samples, with significantly less NbC content, fail via a ductile dimpled fracture that features high amounts of localized necking. Optimized post-build heat treatments of SLM 718 have been developed based on the identified microstructural differences.