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Dela Cruz et al. Microstructures 2023;3:2023012 https://dx.doi.org/10.20517/microstructures.2022.33 Page 13 of 25

constant and then dropped at a particular depth depending on the LED. Its magnitude was also dependent
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on LED. The low LED (0.25 J/mm) had a stable cooling rate of 9.28 × 10 K/s up to ~30 μm melt pool depth,
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and for 0.29 J/mm LED, it was stable at 6.21 × 10 K/s until ~60 μm deep. For the LEDs of 0.44 J/mm and
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0.88 J/mm, the cooling rates were rather stable until ~120 μm deep at 3.93 × 10 K/s and 3.09 × 10 K/s,
respectively. Moreover, the percentage change in the cooling rates after varying the laser power and scan
speed was the same at ~58%. When the laser power was raised from 100 W to 175 W, the cooling rate
dropped from 9.28 × 10 K/s to 3.93 × 10 K/s, while the increase in scan speed from 400 mm/s to 600 mm/s
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increased the cooling rate from 3.93 × 10 K/s to 6.21 × 10 K/s.
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The maximum calculated temperature in the melt pool as a function of melt pool depth for different LEDs
derived from the FEA analysis is shown in Figure 7D. The melt pool temperature responds positively to the
increase in LED, and it is observed to decrease within the melt pool. For example, at the melt pool surface, a
temperature of roughly 2081 °C, 2173 °C, 3019 °C, and 3256 °C was computed for 0.25 J/mm, 0.29 J/mm,
0.44 J/mm, and 0.88 J/mm LEDs, and it decreased to 1360 °C, 1755 °C, 2484 °C, and 2716 °C at 50 μm melt
pool depth, respectively. As a guide for the melting of the powder, the melting points of the constituent
elements (Fe = 1535 °C, Si = 1410 °C, and Mn = 1245 °C) in the blended powder are likewise inscribed in
Figure 7D. The observed temperatures at the melt pool surface are beyond the melting temperature of the
powder, which may have likely evaporated some elements. In particular, the loss of manganese is expected
when an LED over 0.25 J/mm is applied because of its relatively low boiling temperature (Fe = 2750 °C,
Si = 2357 °C, and Mn = 2062 °C), Figure 3B.

The solidified surface of the LPBF-built alloy fabricated at 0.44 J/mm LED was re-scanned at 175 W and
400 mm/s (100% re-scan) to further promote the alloying of the blended powders. The added step doubled
the LED from 0.44 J/mm to 0.88 J/mm at each layer and consequently raised the temperature in the melt
pool, as shown in Figure 7E. At the surface, the maximum temperature for 0.44 J/mm LED reached 3019 °C,
and this was similar for 0.88 J/mm LED, as depicted by the first peak in the temperature profile on the
surface of the laser re-scanned LPBF-built alloy. The alloy was expected to have solidified after the first scan
because the temperature dropped to almost 270 °C. However, the additional re-scan step reached a much
higher temperature of 3256 °C, as seen in the second peak, than in the first scan because the re-scan started
at a relatively high temperature of 270 °C, and there is a difference in the thermal conductivity of the
powder and the alloy [78,79] . A similar pattern showing the two temperature peaks was observed at different
melt pool depths when the re-scan strategy was applied.

Microstructure evolution
Both the highly directional heat flow conditions and large temperature gradients generated during laser
melting of an outermost layer of metal powder, which usually also resulted in the partial remelting of the
already solidified grains of the underlying built substrate, favour epitaxial growth of these existing grains in
certain crystallographic directions towards the heat source (i.e., they grow antiparallel to the direction of
heat flow into the underlying substrate). For certain laser input conditions, a highly directional columnar
morphology and strong texture were frequently observed in LPBF-built alloys [80,81] . However, the EBSD
maps shown in Figure 4 revealed a gradual change in the microstructure from a nearly equiaxed to
columnar grain structure as the LED was increased.

The equiaxed-to-columnar transition in the grain structure was commonly observed within the melt pool,
and this transition depends on the alloy chemistry and the heat transfer conditions according to the LPBF
processing conditions [64,82-86] . A near-homogeneous grain structure with weak texture was achieved when
Attard et al. applied the island scan strategy or checkerboard style , a standard parameter unique to the
[87]