Anisotropic tensile behavior of in situ precipitation strengthened Inconel 718 fabricated by additive manufacturing

Selective laser melting is an additive manufacturing method, where parts are built through the controllable melting of continuous metal powder layers with a laser beam. Selective laser melting adopts the selective densification of a pre-alloyed powder by melting a powder bed in layer-by-layer process via scanning a laser beam consistent with the information provided in a sliced CAD-file. Reference to the specific production principle of the selective laser melting, geometrically complex as well as hollow parts can be produced, whose production would be challenging and almost impossible for typical manufacturing methods.

Inconel 718 is among the many nickel-based super alloys, which is a suitable option for industrial applications. Inconel 718 is an age-hardenable Ni-Fe-Cr alloy, which combines high temperature strength up to 700 °C with superior fabricability, and high resistance against corrosion. In view of these properties, Inconel 718 is normally adopted for the production of components of aircraft turbines, high-speed airframe parts, high-temperature bolts, and nuclear engineering.

It has been reported that selective laser melting processed Inconel 718 shows superior mechanical attributes as opposed to one produced by the traditional methods. Columnar grains have been consistently observed to grow with strong anisotropy in the microstructure of the selective laser melting processed materials reference to the serious cooling gradient that exerts considerable effects on the performance in practical applications.

For this reason, systematic and precise characterization of anisotropic microstructure and the tensile behavior in selective laser melting processed IN718 is an appropriate path for promoting the frequent use selective laser melting of IN718 components manufacturing.

Mang Ni, Chao Chen, Ruidi Li, Xiaoyong Zhang, and Kechao Zhou at State Key Laboratory of Powder Metallurgy in Central South University in collaboration with Xiaojun Wang and Pengwei Wang at Hunan Farsoon High-Tech Co, Ltd in China investigated the microstructure as well as anisotropic mechanical attributes of selective laser melting processed Inconel 718 component. They focused on finding the anisotropy of microstructures as well as the various failure mechanisms between transverse and longitudinal samples for further improvement of mechanical attributes of IN718 for practical applications. Their research work is published in Materials Science & Engineering A.

The authors fabricated IN718 by selective laser melting with gas atomized powders under various laser parameters and transverse and longitudinal arrangements in relation to the build direction. They then investigated the microstructures as well as their evolution in the course of the selective laser melting process. Finally, they characterized the mechanical attributes between transverse and longitudinal directions of the selective laser melting processed IN718.

The authors observed the density of the selective laser melting IN718 to increase with the increase in input energy density. They realized the highest density of 99.52% at a power of 400W and a scanning velocity of 900mm/s. The selective laser melting processed IN718 exhibited columnar grained microstructure with high disperse precipitates that led to more excellent mechanical attributes as opposed to casting methods.

Owing to the highest cooling gradient parallel to the build direction, columnar grains grew along the build direction during the selective laser melting. The authors found that the build direction played a key role in the mechanical attributes of the selective laser melting IN718. Longitudinal specimens showed lower tensile strength but higher elongation than transverse samples in the IN718. This was reference to the columnar grains as well as accelerated damage in transverse specimens under mode I opening tension when tension was applied.