New Insights into the Origin of Fine Isometric Microstructures in Nickel Alloy 718 for Additive Manufacturing

(New Insights into the Origin of Fine Isometric Microstructures in Nickel Alloy 718 for Additive Manufacturing)

In 718 is a heat-treatable hardenable alloy. Because of its excellent fatigue
resistance and corrosion resistance at a high temperature of 650°C, it has been
used in the aerospace, aerospace and nuclear industries for decades. Therefore,
this alloy has been used in the early development of additive manufacturing (AM)
of metal alloys. Although intensive research has been conducted to achieve
optimal performance, it is still feasible to control the grain structure
established during solidification. This is essential. The solidified
microstructure has a great influence on performance and processing. The spacing
of the primary dendrite arms, grain size and grain texture will affect the yield
strength, fracture toughness and high cycle fatigue life, while the refined
equiaxed microstructure will increase the resistance to solidification
cracks.

Different strategies have been proposed to control the grain structure
produced by solidification. In contrast to conventional casting, AM can handle
process parameters such as input energy, scanning speed, and establishing
strategies to establish thermal conditions that promote equiaxed grains, namely
low temperature gradients and rapid solidification fronts. Along this route, the
most successful solutions rely on preheating the substrate, mainly in electron
beam melting (EBM) technology, and are predicted to be used in other
technologies such as direct energy deposition (DED) and selective laser melting
( SLM)) may be effective. This solution is usually associated with a high energy
input that helps to establish a low temperature gradient. It seems more
difficult to determine the overall trend of scanning speed, because the
relationship with the speed of the solidification front is far from direct.

In 718 is additively manufactured by laser line coaxial process. Multiple
isometric belts were observed in the sample. According to detailed EBSD
analysis, it has been shown that equiaxed grains are produced by ISRO-mediated
nucleation mechanism, which was previously only observed in Al-based and
Au-based fcc alloys. It is expected that the icosahedral symmetry derived from
the EBSD analysis of the fcc phase originates from the rapid remelting of the
previous layer, thereby providing a metastable configuration in the liquid,
which is conducive to the nucleation of the fcc phase. These results open up new
prospects for the additive manufacturing of nickel-based alloys.

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(New Insights into the Origin of Fine Isometric Microstructures in Nickel Alloy 718 for Additive Manufacturing)