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Page 14 of 25 Dela Cruz et al. Microstructures 2023;3:2023012 https://dx.doi.org/10.20517/microstructures.2022.33

[32]
[21]
setup , and when Ewald et al. heated the build platform to 500 °C . Attard’s group associated this with
the even distribution of heat in the island scan strategy. Meanwhile, the heated build platform in the 0.5 mm
sized product reduced the temperature gradient in Ewald et al.’s LPBF product, which also reduced the
[21]
temperature gradient and promoted a nearly homogeneous and equiaxed microstructure . The lack of
grain morphology transition in the melt pool in the present Fe-30Mn-6Si LPBF alloy may have been caused
by the island scan strategy with 45° scan rotation in the subsequent layers, leading to a homogeneous grain
morphology in each parameter setting.

The similar microstructures of LPBF processed parts and conventionally welded components make it
convenient to describe the solidified LPBF microstructure in terms of the well-established physical
metallurgy principles associated with fusion welding . Grain shape and scale were defined by the
[69]
solidification theory, and may be controlled by the temperature gradient G, solidification velocity R, the
temperature solidification range of an alloy ∆T, and the liquid diffusion coefficient DL [65,67] . The
relationships between these key solidification parameters are given below :
[66]

where the G/R ratio and the G·R product, which is the cooling rate, can predict the morphology and
dimensions of the solidified microstructure, respectively. For example, a low G/R value correlates to
equiaxed grains, with the morphology transitioning to columnar dendritic, cellular, and then to planar for
[70]
increasing values of G/R, and the high cooling rate resulted in a fine solidified grain structure .
Investigation of the thermal history of LPBF-processed alloy was necessary for understanding its expected
final microstructure, and in Figures 4 and 7, the select parameters showed that the different thermal profiles
affected both the morphology and dimensions of the solidified grains.

The solidification of grains in LPBF-processed alloys follows the well-established nucleation and growth
[88]
processes in solidified metals and alloys. Li and Tan provided the general grain characteristics of LPBF
alloys and summarised two possible nucleation mechanisms: (i) bulk nucleation; and (ii) epitaxial or surface
nucleation. Bulk nucleation occurs on the top side of the melt pool and at the head of a solidification
front . Nuclei also form from the partially melted powder in the melt pool , and they can survive given a
[88]
[89]
sufficient volume of surrounding undercooled liquid metal . These formed grains then assume an
[90]
equiaxed morphology due to the low G/R ratio on the top side of the melt pool . Epitaxial nucleation
[91]
[92]
occurs at the interface of the melt pool and the substrate, or at the previously solidified layer . A high LED
and a low solidification rate in the melt pool encouraged grains to grow in a preferred crystallographic
orientation , which was <100> for cubic and <1010> for hexagonal metals, respectively . Grains
[69]
[93]
possessing these favoured orientations outgrew grains with less favourable orientations , eventually
[65]
generating a highly textured, columnar microstructure .
[94]
The prevalence of a highly textured and columnar grain morphology at the high LED settings [Figure 4G]
suggested an epitaxial mechanism. Without an added and known potent nucleating particle in the elemental
mixture and because of the steep temperature gradient on melting and solidification, the previously
solidified layer would act as a suitable substrate for continued growth into the melt pool, whereby the partly
melted grains propagate by epitaxial “nucleation” towards the heat source. Equiaxed grains may form on the
top surface of the melt pool when the melt pool trail ended because of the low G/R ratio in this region, and
such was seen on the last fabricated layer in NiTi . In the Fe-30Mn-6Si, at 0.44 J/mm LED, a temperature
[86]