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Materials Science in Additive Manufacturing Heterostructures of A131 steel by DED

A B

Figure 3. The density (n = 5) (A) and XRD patterns (B) of A131 samples subjected to different processes
Abbreviations: AB: As-built; HT: Heat treatment; HR: Hot rolling

A B Figure 5D-F displays the AB A131 steel microstructure
along the TD section at different magnifications.
Figure 5D presents the transitional interface between
the fine and coarse grains, where the boundary is not
distinctive. The coarse-grain part mainly consisted of
three types of grains: Coarse columnar grain, middle-
size equiaxed crystal, and fine acicular martensite. The
maximum length of the continued columnar reached
83.4 µm, surrounded by other small-sized grains. The
dimension of equiaxed grains was measured as 16.5 µm,
smaller than that of columnar grains (Figure 5D, inset).
Figure 4. 3D-reconstructed microstructure of A131 steel: (A) AB; and The size of the fine acicular martensite was the same
(B) HT. Black arrows in (B) indicate pearlite as the grains in the fine grain region (Figure 5E). The
Abbreviations: AB: As-built; HT: Heat treatment; ND: Nominal direction; boundary between the coarse- and fine-grain regions
TD: Transverse direction; RD: Rolling direction depended on the distribution of the columnar grains,
and the fine grains primarily consisted of equiaxed grains
is most likely due to the HT process, which provided and acicular martensite (Figure 5E, inset) in a submicron
sufficient kinetic and thermodynamic conditions for size. The magnified image (Figure 5F) reveals randomly
both phase transformation and grain growth. In addition, distributed submicron defects and columnar grains
the anisotropy of the microstructure was significantly as discontinuous lamellae along ND. This observation
reduced, which could be beneficial to the isotropic suggested that the heterogeneous structure was related
mechanical properties. to dendritic growth in the melt pools, resulting in partial
Figure 5A-C displays the magnified image of the AB grain coarsening.
A131 steel microstructure in the ND section. Figure 5A To observe the AB A131 steel heterostructure, EBSD
presents an OM image of the fine acicular martensite analysis was performed on the TD section. Figure 6A
structure predominated with some coarse equiaxed grains features an inverse pole figure (IPF) with grain orientations
(black arrows), and the acicular martensite possessed a in different colors, revealing a non-uniform distribution
submicron size without specific orientation (Figure 5A, and an optimal grain orientation consistent with the XRD
inset). Figure 5B presents an SEM image of the acicular results. According to grain size and grain profiles, the IPF
martensite-equiaxed grain structure with notable particle image was divided into three parts (resembling a sandwich
defects (white arrows). These particles (300~800 nm) were structure): Fine grain region (#1), coarse grain region (#2),
randomly distributed (Figure 5B, inset), most likely due to and fine grain region (#3). The distribution of equivalent
oxidation during the deposition process. As these particles circle diameter for the relative regions is displayed in
were relatively small (<1 µm), their influence on density Figure 6B. The fine-grain region (#1) primarily consisted
is negligible. The magnified SEM image (Figure 5C) of acicular martensite and small equiaxed grains (average
highlights the submicron equiaxed grains (~2 µm) and diameter: 6.91 µm), and the maximum diameter was
acicular martensite blocks (~5 µm). <20 µm. Compared to region #1, grains in coarse grain

Volume 4 Issue 3 (2025) 6 doi: 10.36922/MSAM025220038