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

the melt pool profile generated from a single laser scan track and the thermal profile of the melt pool region
derived from a finite element analysis (FEA) method. The influence of grain size, composition of formed
phases, and residual strain on the hardness of the as-built alloy were investigated according to the
information gathered from XRD and EBSD analyses, and then compared with the reference as-cast alloy
prepared using the arc-melting technique.

MATERIALS AND METHODS
Sample preparation
The Fe and Si powders used in the LPBF fabrication of Fe-Mn-Si alloy were gas atomised and provided by
TLS Technik, Germany, while the Mn powder was from Merelex Corp, USA. Both the Fe and Si powders
had a purity of > 99 wt.%, and the purity of Mn was > 98 wt.%, as estimated using the Malvern Panalytical
Epsilon ED X-ray fluorescence spectroscopy (XRF), Supplementary Table 1. Figure 1A-C shows the
scanning electron microscope (SEM) micrographs of the Fe, Mn, and Si powder. Their particle size and
cumulative size distributions were measured using the Malvern Mastersizer 3000 and are shown in
Figure 1D-F, respectively. A nominal powder composition of Fe-30Mn-6Si (wt.%) was homogenised for 4 h
using the Turbula® T2F 3D mixer and then used as powder precursor. Meanwhile, the nominal
concentration of Fe-30Mn-6Si reference as-cast alloy was prepared using the arc-melting technique from Fe,
Mn, and Si high purity (> 99.9%) chips from Sigma-Aldrich. The arc-melted product was subsequently hot-
rolled at ~800 °C and then homogenised at 1100 °C for 14 h in an argon-purged furnace. Homogenisation
was performed by loading the sample at room temperature, heating it at 5 °C/min to 1100 °C, and followed
by furnace cooling. The resulting sample is referred to hereafter as reference as-cast alloy and its properties
were treated as a reference in the following investigations.

The LPBF fabrication was carried out using the Mlab Cusing 200R from Concept Laser GmbH equipped
with 200 W Yb:YAG fibre laser and the print chamber atmosphere was maintained up to 0.2 vol.% O using
2
a high purity Ar gas. Only freshly homogenised powder was used, and all printed parts were built on a
3
sandblasted stainless steel substrate. In identifying the optimum LBPF parameters, a 125 mm cube model
was prepared using the Materialise Magics v24 software. The influence of both laser power and laser scan
speed on the density of the built part was investigated by varying the laser power (P) from 100 W to 175 W
and the laser scan speed (ν) from 400 mm/s to 600 mm/s while keeping a constant laser hatch spacing, layer
thickness, and scan strategy at 45 μm, 50 μm, and island scan strategy, respectively. The scan strategy is
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unique to Concept Laser , where each island was maintained at 5 × 5 mm and was scanned by the laser in
[32]
one direction. The laser scan direction was rotated by 90° between the neighbour islands, and finally, this
whole pattern was rotated by 45° in the subsequent layer. A laser re-scan strategy was also included. This
was done by scanning the solidified layer again at a varying percentage of laser power (0%, 50%, and 100%)
that was applied in the first scan, laser scan speed from 400 mm/s to 600 mm/s, and a similar scan strategy
to increase the laser linear energy density (LED). LED is a simplified energy parameter defined as the P/ν
ratio and was considered when the layer thickness and laser space hatching were unchanged [33-35] . Table 1
summarises the parameters that were investigated.

LPBF product quality assessment
The density of the LPBF built parts was measured by applying the Archimedes method and using the
Mettler Toledo XS105 balance with a density kit. All surfaces of the samples were ground down to 1200 SiC
paper and then dried. Measurements were done on three replicates. The measured density was then divided
by the theoretical density (7.408 g/cm ) and reported as relative density. In addition, the surface of the
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LPBF-built parts along the build direction was viewed under the Hitachi TM4000Plus bench-top SEM
coupled with a Bruker X-Flash 630Hc EDS detector to further evaluate the product quality.