Effect of 3D printing on the microstructure and mechanical properties of an Inconel 718 Nickel-alloy (UNS N07718)

Abstract

Flow measuring instruments are important for the oil and gas industry. These instruments are exposed to hostile environments during operation. These hostile environments demands high quality materials with excellent properties, such as superalloys. Inconel 718 is a superalloy which can be manufactured by additive manufacturing. Additive manufacturing is threedimensional printing of metals. The technology uses laser or electron beam to melt thin layers of powder material. The method allows manufacturing of complex shapes that are hard or impossible to manufacture with conventional methods. The objective of this thesis is to investigate how the additive manufacturing process affects the microstructure and mechanical properties of an additive manufactured Inconel 718 superalloy. Additive manufacturing produces useful mechanical properties. However, yield and tensile strength is considerably lower than for commercially produced material. Therefore, heat treatments are necessary to obtain the mechanical properties achievable by the alloy composition. The heat treatment conducted in this experiment provided hardness values and tensile properties exceeding those of commercially produced material. However, this was at the cost of properties associated with ductility, elongation at break and contraction at fracture. These properties were considerably reduced and comparatively lower than with commercially produced material. Tensile tests show that the building direction of the additive manufactured material in the asprinted condition parallel to the pull direction of the tensile test, horizontally built, is stronger than the building direction normal to the pull direction, vertically built. EBSD analysis of the additive manufactured material in the as-printed condition shows grain size differences in the various planes of the specimens. Grains are smaller in the direction of laser motion than in the direction normal to it. This means that the horizontally built material has smaller grains than the other building directions in the pull direction. This is believed to be the main contribution to the tensile property differences. The EBSD analysis also shows tendencies of grains orientating in specific directions. The additive manufactured specimen in the as-printed condition built horizontally seems to have grains orientating in preferred directions, while the vertically built seem to have grains randomly orientated, with no preferred direction. This may also contribute to the differences in tensile properties of the various build directions