Page 98 - MSAM-4-3

Materials Science in Additive Manufacturing Heterostructures of A131 steel by DED

TD, and RD corresponded to the x-, y-, and z-axes, Figure 3B displays the XRD patterns of the steels,
respectively. The entire configuration was box-shaped (43 with similar peak profiles across A131 steels subjected to
× 11 × 43 nm), packed with two kinds of grains with the different post-processing treatments. This suggests that the
same lattice parameter of 2.86 Å. These two types of grains crystal structure for the primary phase remained largely
formed a sandwich structure: Fine equiaxed crystals with unchanged, consisting mainly of the ferrite (α-Fe) and/
an average diameter of 2.86 nm at the edges, and coarse or martensite (α´), both of which possessed a BCC crystal
columnar grains in the center, measuring approximately structure. The α´-phase was preferentially formed in
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2.86 nm in diameter and 7 nm in length. the AB A131 steel due to non-equilibrium consolidation
30
4
6
The entire system was first thermalized and equilibrated and rapid cooling rates (10 – 10 K/s) in the melt pool.
to zero pressure using the isothermal-isobaric ensemble Subsequent treatments (HT or HR) facilitated the
5
(NPT) under 3D periodic boundary conditions. For the transformation of the α´-phase into α-Fe. In addition,
uniaxial tension simulation, all the samples were deformed the most prominent diffraction peak corresponded
either along the ND and TD axes at a constant strain rate to the crystal plane in all bulks, indicating a preferred
of 1 × 10 s . Periodic boundary conditions were applied crystallographic orientation, which might contribute to
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along RD. For ND and TD, the tensile direction was set as anisotropic mechanical performance.
“periodic,” while the non-loading direction was assigned as Figure 4 displays the 3D-reconstructed optical
“surface.” In addition, zero normal stress was maintained microscope (OM) image of the AB bulk microstructure in
along RD during deformation. Virial stresses in the an orthographic view. Figure 4A features the microstructure
tensile direction were calculated at each strain level. The of AB A131 steel, revealing minor visible defects, such as
engineering strain at each time step was calculated as: holes and unfused particles, further confirming that the
AB A131 steel was well-consolidated with full density.
ll− 0 (I) The grain size and grain profile were significantly different
l along ND and TD, which is due to dendritic growth during

where l and l represent the length of the present and the consolidation of the melt pool. 37,38 In addition, there
0
initial box along the tensile direction, respectively. The was a significant sandwich structure for the grain size in the
interaction between individual atoms in the Fe matrix was TD section, a novel heterogeneous structure was formed
modeled using a many-body embedded-atom method layer-by-layer, consisting of coarse columnar grains and
based on the interatomic potential, which can be expressed fine crystals, alongside the laser deposition direction.
as: The layer thickness of columnar grains was ~300 µm,
and fine acicular martensite and equiaxed crystals were
1 observed, likely resulting from the high cooling rate and
r
E 2 V() F() i thermal cycles induced by alternating laser deposition
tot
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ij
ij

i r () (II) layers. Therefore, the imbalanced consolidation provided
ij
ji insufficient kinetics for the phase transformation from
γ-Fe to α-Fe and pearlite, when promoting the formation
where V(r ) is the pair potential, r is the atomic of fine martensite. In addition, the thermal cycle offered
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ij
ij
separation between j and i, F is the embedded energy as a necessary thermodynamic advantages for the grain
function of ρ , and ρ()r is the atomic function. growth of fine crystals alongside ND, where a sandwich-
ij
i
heterostructure was formed, consisting of columnar grains
3. Results and fine crystals. This heterostructure contributed to the
3.1. Phase configuration and microstructure anisotropy in grain orientation, consistent with the XRD
results, and eventually induced variations in mechanical
Figure 3A displays the measured density and the relative performance.
density for the AB samples and control group, with
the corresponding data summarized in Table S2. The Following tempering, Figure 4B displays the
measured density of all the steel bulks exceeded 7.7 g/cm microstructure of HT A131 steel, with significant
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with minor variation, and the values of the AB steels were variations observed for grain sizes and crystal profiles.
slightly higher than those of the control group. Compared The sandwich heterostructure was visible but disappeared
to the theoretical density of A131 steel (7.85 g/cm ), all the when the grain size increased. The microstructure
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relative densities were above 98%, illustrating that both of HT A131 steel consisted of equiaxed grains with a
AB and HT steels were well-consolidated with minimal typical ferrite-pearlite structure, indicating negligible
defects. variation in grain profiles alongside ND and TD. This
Volume 4 Issue 3 (2025) 5 doi: 10.36922/MSAM025220038