Page 103 - MSAM-4-3

Materials Science in Additive Manufacturing Heterostructures of A131 steel by DED

A B C

D E F

Figure 9. Broken surface of the tensile sample of AB A131 steel: (A-C) SEM images of defects at the edge; and (D-F) SEM images of defects in the middle.
(C, inset) EDS spectrums of the particle defect. Scale bars: 500 µm (A and D); 50 µm (B and E); 20 µm (C); 10 µm (F)
Abbreviations: AB: As-built; SEM: Scanning electron microstructure; EDS: Energy dispersive spectrometer; O: Oxygen; Al: Aluminum; N: Nitrogen; Mn:
Manganese

along ND. The grains were relatively small and comprised Figure 10G displays the microstructure at the interface
columnar grains, fine acicular martensite, and some between the single-pass track and the 304L steel substrate,
equiaxed grains, attributed to dendritic growth during the consisting of equiaxed grains and acicular martensite.
consolidation of the melt pool under rapid cooling rates. A distinctive interface could be observed, where fine
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Discontinuous columnar crystals were formed along ND A131 steel grains are deposited on the top of coarse
and surrounded by fine martensite. The IPF mappings 304L substrate grains. Compared to the microstructure
(Figure 10B) further confirmed dendritic crystal formation of the single-pass track at the top (Figure 10A-C) and
along the build direction, consisting of columnar and fine- middle (Figure 10D-F) regions, the grains near the
equiaxed grains with a weak (101) crystal orientation and a substrate (Figure 10H) resemble the fine-grained regions
huge misorientation angle of 5.24° (Figure S4). The average (Figures 4A; 5D and E; and 6), and the misorientation
equivalent grain size was 6.33 µm, with a maximum of significantly decreased (Figure S6). The average grain size
83.6 µm due to dendritic growth of the columnar grains. is approximately 5.45 µm (Figure 10I), slightly smaller
Figure 10D displays the microstructure of the single- than that of AB A131 steel (6.8 µm), with a maximum
pass track in the middle region, featuring coarse columnar diameter of 31.6 µm. This suggests that laser thermal
grains and acicular martensite. Compared to the top cycling promotes submicron-scale grain growth, leading
region (Figure 10A-C), the columnar grains are more to a slight increase in the average size. Such effects
evident (Figure 10E), indicating significant coarsening due facilitate the formation of alternating fine- and coarse-
to limited thermal conduction and a lower temperature grain regions during one-pass deposition. Subsequent
gradient at the center of the melt pool. This results in passes remelt the surface with similar thermal gradients
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a discrepancy in grain size from the center to the edge, to those near the 304L substrate, promoting the formation
accompanied by increased crystal misorientation of 6.03° of fined equiaxed crystals. These repeated thermal cycles
(Figure S5). The average grain size slightly decreased contributed to the uniform distribution of heterogeneous
to 5.93 µm, but the maximum grain size significantly structures at alternating layers.
increased to 146.8 µm with a high area ratio (Figure 10F), To further investigate the heterostructure on
substantially greater than in AB A131 steel (Figure 6). anisotropic mechanical performance, an MD model of the
This suggests that alternating remelting and cyclic thermal sandwich structure was built, consisting of the fine- and
effects helped refine the coarse columnar grains. columnar-grain regions with a similar volume ratio

Volume 4 Issue 3 (2025) 10 doi: 10.36922/MSAM025220038