A novel method for forming “programmed” mechanical properties in local components of a part by controlling the microstructure of the alloy under additive production is suggested. It is shown that the thermal fields acting during the additive production may be used for manipulating the preferred orientation of the growing crystals and forming different-size grains, which affects the mechanical characteristics. A gradient material from a high-temperature nickel alloy Inconel 718 with coarse elongated grains in the core and fine-grained microstructure in the external shell is produced and shown to possess higher operating characteristics than the alloy fabricated by the conventional process.

Inconel 718 is a nickel-based superalloy commonly used in aircraft engine and nuclear applications, where components experience severe mechanical stresses. Due to the typical high temperature applications, Thermo-Mechanical Fatigue (TMF) and creep tests are common benchmarks for such applications. Additive manufacturing offers a unique way of manufacturing Inconel 718 with high degree of design freedom. However, limited knowledge exists regarding the resulting high temperature properties. The objective of this work is to evaluate creep and TMF behaviour of Inconel 718, produced by selective laser melting (SLM). A novel microstructural design, allowing for grain size control was employed in this study. The obtained functionally graded Inconel 718, exhibiting core with coarse and outside shell with fine grained microstructure, allowed for the best trade-off between creep and fatigue performance. The post heat-treatment regimens and resulting microstructures are also evaluated and its influence on creep and TMF is discussed.

Additive manufacturing offers a unique way of anisotropic microstructure control with a high degree of design freedom. This study demonstrates that application of suitable process parameters and laser sources in selective laser melting may favour either one sharp single component texture, more uniformly distributed crystal orientation, or a combination of the above in a preferred gradient, which influence the mechanical properties. It is shown that transitions in microstructure, texture, and properties in fabricated Inconel 718 functionally graded components can be obtained at relatively small or large length scales, depending upon the functional gradient desired in a particular application. Results obtained by electron backscatter diffraction showed distinct regions of coarse elongated grains with a strong (001) orientation uniformly embedded in randomly distributed fine grained matrix. Mechanical tests in the form of hardness, tensile and in-situ digital image correlation tests showed steep transitions in the developed Inconel gradients. The observed mechanical properties were found to be primarily dependent on the grain size and texture and are superior to the cast samples for both laser sources. The developed process strategy can be further applied to design functional gradients with selected tailored properties and to account for directional anisotropy of solidified components.

Additive manufacturing (AM) technologies are known to allow the production of parts with an extreme degree of complexity, enabling design and functional part optimization. However, the resulting microstructures and mechanical properties of AM materials are not well understood due to unique and complex thermal cycles observed during processing. This study aims to adjust the microstructure of Inconel 718 specimens produced by selective laser melting (SLM). The microstructural design was achieved through process parameters manipulation and post-process heat treatment. The effects of heat treatment on microstructure, process induced defects, deformation behaviour and failure mechanisms were studied. Directional columnar grained microstructure accompanied by interdendritic Laves phases and carbide particles was observed in as-processed material. Hot isostatic pressing (HIP) improved mechanical properties, which was attributed to dissolution of undesirable Laves and δ-phase as well as pore closure. All investigated samples maintained their intended tailored microstructural build up with distinct differences in mechanical properties. The results presented in this study show the capability of the SLM process to produce parts with mechanical properties better than conventional Inconel material. The microstructural design demonstrated here can be exploited in AM fabrication of complex components requiring challenging high-temperature mechanical performance.