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Dela Cruz et al. Microstructures 2023;3:2023012 https://dx.doi.org/10.20517/microstructures.2022.33 Page 15 of 25

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gradient = 9.83 × 10 K/m, cooling rate = 4 × 10 K/s, the slow solidification rate of R = 3.98 × 10 mm/s, and
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the melt pool temperature of ~1300 °C at 140 μm melt pool depth were sufficient to melt the solidified
equiaxed grains in the previous laser scan and then subsequently re-solidify into columnar grains. A similar
grain morphology holds for the laser re-scanned LPBF alloy, albeit grains were relatively fine and less
columnar when laser re-scanning was applied.

The re-scan strategy had been reported to improve surface quality , increase density , and reduce residual
[95]
[66]
stress in AM components. This additional step was included in this study to enhance the alloying of the
[96]
blended powders, and this resulted in a notably different microstructure from that of a non-re-scanned
alloy. The melt pool width, depth and overall area associated with the re-scan strategy [Figure 6D] were
considerably larger than after single scanning [Figure 6C], and this is caused by the higher thermal
conductivity of the solidified layer than the powder material . Hence, the enhanced heat transfer in the
[97]
solidified layer resulted in a more pronounced melt pool, which was reflected in the calculated thermal
profile. A coarse and columnar grain structure was still expected in the laser re-scanned LPBF alloy because
the parent grains in the non-re-scanned alloy have solidified into columnar grains. The relatively gentle
slope of solidification for 0.88 J/mm LED [Figure 7B] and its low cooling rate (3 × 10 K/s) promoted the
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epitaxial growth of columnar grains, but its temperature profile shown in Figure 7D suggested that
remelting of the previously solidified layer had occurred.

Completely remelting an alloy reshapes its microstructure, and such was evident in this work by the
decrease in the average grain size and aspect ratio in the remelted LPBF alloy. A region of coarse and refined
grains was apparent on close inspection in the re-solidified structure (marked areas in Figure 4J).
Xiong et al. reported a similar observation in pure tungsten . During re-scanning, the laser irradiated heat
[79]
[98]
initially remelted the surface and the convection current in the melt pool engulfed and remelted the
remaining solid within the melt pool. This caused some of the initially formed columnar grains to be
[66]
separated and these freed grains became the nuclei for growth . The fast-moving laser that drives the rapid
cooling rate (10 K/s) in the LPBF process curbs the growth of the newly nucleated grains and freezes them
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into a fine microstructure , thereby forming regions of non-uniform microstructure.
[99]
For the low LED (0.25 J/mm), the melt pool temperature at 50 μm pool depth was 1360 °C which was
enough to melt the blended powders and potentially melt the surface of the previously solidified layer.
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However, the high cooling rate of 9 × 10 K/s and the high solidification rate at this setting resulted in the
retention of the equiaxed grain morphology. Moreover, the chemical segregation [Figure 2C] preserved in
this LED suggests the presence of partially alloyed powder both in the melt pool and the solidified layer
when the next layer was melted. The bulk nucleation mechanism was favoured in the presence of partially
alloyed powder since they can act as heterogeneous nucleation sites and impede the epitaxial growth of the
previously solidified equiaxed grains at the bottom of the melt pool .
[100]

For the high laser power and fast scan speed (175 W, 600 mm/s, 0.29 J/mm), the melt pool depth of 110 μm
could get through an equivalent of two powder layers and had enough heat to sufficiently remelt the
previously solidified layer and re-solidify them into a full-columnar structure. However, the solidified grains
shown in Figure 4D were equiaxed and rather coarse (105 μm) compared to the finer grains (64 μm)
associated with the low LED (0.25 J/mm) in Figure 4A. The partially melted powder observed at this setting
could have induced the bulk nucleation of the grains and interrupted the epitaxial growth of grains.

The significant influence of the studied processing parameters on the resultant LPBF microstructure
presents an opportunity to control the microstructure and texture, and therefore the properties of any given