Nickel alloy 718 is a heat-treatment hardenable alloy that has been used for decades in aeronautical, aerospace andnuclear industries due to its excellent fatigue and corrosion resistance up to 650◦C. Hence, this alloy has been usedearly in the development of additive manufacturing (AM) of metallic alloys . Although intense research has been devoted to achieve the best possible properties, controlling the grain structure established during solidification remains critical for that purpose . Solidification microstructure impacts strongly both properties and processing; primary dendrite arm spacing, grain size and grain texture affect the yield strength, fracture toughness and high cycle fatigue life, while having a grain refined equiaxed microstructure increases resistance to solidification cracking.

Different strategies have been proposed to control the grain structure ensuing from solidification. Contrary to conventional casting, it is possible with AM to play with the process parameters, such as the input energy, scanning velocity and building strategy in order to establish thermal conditions promoting equiaxed grains, i.e. low temperature gradients and fast solidification fronts. Along this line, the most successful solution relies on preheating the substrate,mainly in electron beam melting (EBM) technology , and predicted as potentially efficient in other technologies such as direct energy deposition (DED) and selective laser melting (SLM) . This solution is often associated with high energy inputs contributing to establish low temperature gradients. It seems more difficult to identify general trends for the scanning speed because the relation with the velocity of the solidification front is far from direct . The same statement can be done about the building strategies, although some success has been achieved for direct laser sintering . Enhancing the nucleation of new grains in the supercooled liquid pool is another promising route for controlling the grain structure. Hence, solutions similar to usual practices in conventional casting or welding have been identified, such as addition of grain refiners in Ti64 , dendrite fragmentation in high entropy alloys  and endogenous nucleants generated by the remelting of intermetallic in Al alloys with Sc. In some studies using DED process, it has been suggested that partially melted powder can act as heterogenous nucleation sites and promotes equiaxed grains.

In both welding and additive manufacturing technologies, fine equiaxed grains (typically ≤10 µm) have been observed by several authors for low to medium linear energy densities ranging from 0.11 J/mm (selective laser melting) to 117 J/mm (direct energy deposition) in AM processes such as SLM , DED , EBM and coaxial laser-wire processes. When this information is provided, the equiaxed microstructure is localised at the bottom of the layer in all the AM processes mentioned below and for a wide range of scanning velocities from 3 to 100027mm/s. Different explanations to the equiaxed regions have been proposed so far. Bambach et al. have attributed the formation of equiaxed grains to local recrystallisation of the solidification microstructure . Mostafa et al. and Choi et al. both have explained the formation of equiaxed grains by a higher concentration of impurities in remelted areas that could act as heterogeneous nucleation sites for equiaxed grains . Other authors have pointed the effect of thermal gradient G and solidification velocity V, particularly the ratio G/V evolution during solidification that could change the growth morphology . In addition, Parimi et al. have underlined the possible local fluctuations in G due to Marangoni effect and in nucleation sites due to powder injection. They have also shown that increasing the power prevents the equiaxed microstructure to appear. Finally, they have demonstrated that the building strategy does not play any role in the appearance of equiaxed microstructures, whereas increasing the power lead to a fully columnar microstructure.

In this work, the fine equiaxed microstructure observed in AM Inconel 718 was reproduced using a coaxial laser wire process. Based on detailed EBSD analysis, it is shown that the fine equiaxed grain regions consists of several assembly of nearest-neighbour grains having multiple-twins orientation relationship with a common h110 idirection leading to a 5-fold symmetry and compatible with an icosahedron. This is the signature of the Icosahedral Short Range Order (ISRO) mediated nucleation mechanism revealed by Kurtuldu et al. in Al-based and Au-based alloys . This makes nickel the third fcc metal where such ISRO-mediated nucleation mechanism can operate, leading to a drastic reduction of the solidification grain size. It opens new prospects for obtaining fully equiaxed additively manufactured parts.