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Dela Cruz et al. Microstructures 2023;3:2023012 https://dx.doi.org/10.20517/microstructures.2022.33 Page 11 of 25

Figure 6. The single laser scan melt pool profile on the polished Fe-30Mn-6Si reference alloy at varying LEDs of (A) 0.25 J/mm, (B)
0.29 J/mm, (C) 0.44 J/mm, and (D) 0.88 J/mm.

technique follows the solidification theory [65-67] , where the morphology is affected by the extent and direction
of the temperature gradient and the solidification rate of the melt pool . Likewise, the cooling rate, a
[68]
[70]
product of temperature gradient and the solidification rate , dictates the size of the solidified structure .
[69]
Therefore, the temperature gradient and the solidification and cooling rates are computed, and the
temperature profiles are also derived.

Figure 7A shows the variation in the temperature gradient within the melt pool for different LEDs. A low
temperature gradient is initially observed from the surface of the melt pool, and it increases as solidification
4
proceeds, leaving the bottom layer with the highest temperature gradient values of over 10 K/m. The LPBF
process has a typical temperature gradient range of 10 to 10 K/m . Temperature gradients between 10 to
7
4
[71]
4
5
[72]
10 K/m were associated with large melt pools , and such were observed in the melt pool profile
[Figure 6C and D]. The 0.25 J/mm has the highest temperature gradient at the surface at 2.32 × 10 K/m as
3
2
compared to the 1.03 × 10 K/m, 9.83 × 10 K/m, and 7.72 × 10 K/m for 0.29 J/mm, 0.44 J/mm, and 0.88
3
2
J/mm, respectively. Moreover, the 0.25 J/mm has the steepest slope in the temperature gradient, followed by
0.29 J/mm. The temperature gradient of 0.44 J/mm and 0.88 J/mm are almost constant up to 50 μm melt
pool depth and it increased gradually afterwards. The low temperature gradient for 0.44 J/mm and
0.88 J/mm at 0-50 μm was caused by their comparatively wide melt pool size in this area. Therefore, the
temperature of the surrounding material is high, and the heat sink effect is low. As the distance from the top
of the melt pool is increased, the melt pool achieves a lower width and lower surrounding temperature.