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Materials Science in Additive Manufacturing Heterostructures of A131 steel by DED

1. Introduction rendering the mechanical performance insufficient to
match that of commercial aerospace alloys.
Additive manufacturing (AM) has demonstrated ultra-high
efficiency in producing heavy-load, complex-geometry, Constructing a heterogeneous microstructure with a
and large-volume equipment, exemplified by the successful dual-phase or dual-grain-size structure offers a promising
launch of Terran 1, a launch vehicle entirely manufactured strategy for enhancing the strength of A131 steel without
by 3D printing, primarily using directed energy deposition reducing plasticity. 19,20 Due to the high freedom and
(DED) technology. The 2-month production cycle of the the ability to deposit multimaterial parts with different
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3D-printed rocket set a new record for the fastest rocket powders, 21-23 the DED technique enables the development
fabrication in history. Large-format metallic structural of special heterostructures to improve the performance
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components developed using the DED method have been of the materials. 24-26 For example, Dan et al. developed
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widely employed in alloys, such as titanium, aluminum, heterogeneous TiAl alloys by alternately depositing
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copper, iron, and their matrix composites. Since the Ti and TiAl layers using DED methods, achieving a
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rapid commercialization and development of the aviation significant improvement in plasticity compared to that
industry, difficult-to-machine alloys (e.g., titanium alloys) of the heterogeneous TiAl alloy in a submicron scale.
have struggled to meet the demands of resource-efficient However, the heterogeneous structure still exhibited the
manufacturing. The outstanding performance of the inherent trade-off between strength and plasticity. Wu
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Falcon heavy-launch vehicle outlined that conventional et al. successfully fabricated a heterogeneous structure
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high-strength steel could also be used in space equipment in lamella-structured titanium alloys, consisting of
with remarkable economic effects. As a result, conventional alternating coarse and fine-grain regions, through post-
iron materials are gaining increasing attention for future treatment processes. This design led to a remarkable
applications in the commercial aerospace industry. improvement in tensile strength with a limited reduction
in elongation. Similarly, Li et al. reported the fabrication
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A131 steel, a distinctive low-carbon structural steel of heterostructures in titanium alloys composed of
developed over the past century, typically exhibits a alternating coarse and fine grains via a heat-treatment
standard yield strength of 235~390 MPa and an elongation (HT) process. The results demonstrated a noticeable
of 19~23%, occupying a significant share in the steel market improvement in strength when maintaining elongation
due to its comprehensive applications in both civilian comparable to that of the control group. Gao et al. 20,30
and defensive industries, especially in marine vessels. 5,8,9 fabricated heterostructures in SS316L steel via selective
However, the structural strength of conventional A131 steel laser melting using an alternating remelting method.
lacks the strength of conventional aerospace titanium alloy Their resulting heterostructure comprised nanoscale
(e.g., 800~1200 MPa for Ti-6Al-4V [Ti64]). 10,11 Leveraging alternating layers of recrystallized and non-recrystallized
the rapid cooling rates associated with laser melting to regions, achieving a notable improvement in ultimate
introduce the formation of martensite phases offers a tensile strength with a limited decrement in elongation.
promising route to enhancing the strength of the A131 Su et al. designed DED processes incorporating
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steel. 5,12,13 Wu et al. 14,15 employed a selective laser melting interlayer pauses to fabricate maraging steels with
method, combined with post-treatment, to fabricate heterogeneous layers composed of multiple phases in
A131 steel. Their results demonstrated a significant alternating layers, achieving a super-high strength of
enhancement in strength, with the as-built (AB) A131 1.5 GPa in maraging steel. These findings suggest that
steel achieving a tensile strength of 1030 MPa, comparable if a heterogeneous structure consisting of alternating
to that of Ti64 alloy. However, this enhancement was coarse and fine-grain layers could be controlled by an AM
accompanied by a reduction in plasticity, as the elongation method, the A131 steel could also potentially overcome
of the A131 steel dropped sharply to just 3%. Post-process the trade-off effects of strength and plasticity in a cost-
heat treatment led to a partial recovery of elongation, effective manner. Although selective laser melting has
albeit with a notable decrement in strength. Similarly, demonstrated the feasibility of constructing nanoscale
Wang et al. 16-18 also manufactured A131 steels using a heterogeneous structures to overcome the trade-off
selective laser melting method with various scanning effects, it has limited capabilities in fabricating complex
strategies. The results revealed the formation of substantial geometries on a large scale. Therefore, developing DED
martensite microstructures, with an improved tensile methods to construct a submicron heterostructure with
strength of 937 MPa and a notable reduction in elongation varied grain sizes is a promising strategy for achieving
(i.e., <6%). Although steel strength can be significantly simultaneous improvements in both strength and
improved through selective laser melting methods, it is plasticity, particularly for future applications in large-
often accompanied by a considerable loss in elongation, scale engineering equipment.

Volume 4 Issue 3 (2025) 2 doi: 10.36922/MSAM025220038