Enhanced strength of A131 steel via heterostructures induced by laser-directed energy deposition

Abstract

The trade-off between strength and plasticity has posed a challenge to the broader application of conventional metallic structural materials in high-speed, heavy-load, and extreme service environments. Heterogeneous structure designs could potentially overcome these limitations with their inherent superior combination of strength and plasticity. To harness this potential, this study employed a directed energy deposition additive manufacturing (AM) technology to fabricate a novel heterostructure in as-built (AB) A131 steel, consisting of alternating coarse and fine-grain layers along the building direction. In addition, a heat treatment process was applied to fabricate a near-homogeneous microstructure, allowing for the investigation of the role of crystal misorientation in tensile anisotropy. Compared to the performance of commercial hot-rolled ASTM A131 steel (yield strength [σYS]: 346.5 MPa; ultimate tensile strength [σUTS]: 545.0 MPa), the AB A131 steel achieved significant enhancements of 168.3% and 78.0% in σYS and σUTS, respectively, when maintaining a comparable elongation of 24.6% along the deposition direction similar to the ASTM A131 standard. Comprehensive experimental characterizations, combined with molecular dynamics simulations, were conducted to investigate the underlying formation mechanism of the heterostructure and the origins of mechanical anisotropy. It was found that single-pass deposition produced three distinct microstructure regions with different grain sizes owing to dendrite growth. With repeated thermal cycles, these evolved into a layered heterostructure consisting of alternating fine crystals and coarse-columnar grains. This heterostructure remarkably contributed to an exceptional improvement in strength, accompanied by only a minor reduction in plasticity. These findings present an efficacious avenue for substantially augmenting the mechanical properties of conventional iron-based alloys, offering useful references for overcoming the strength-plasticity trade-off in other alloys fabricated by AM.

Keywords

Additive manufacturing / A131 steel / Heterostructure / Mechanical performance / Molecular dynamics

Funding

This study was funded by the Guangdong Basic and Applied Basic Research Foundation (2023A1515110594; 2024A1515012049), Shenzhen Science and Technology Program (JCYJ20241202123701003; QTD20210811090146075), and Shenzhen Natural Science Fund (Stable Support Plan Program; GXWD20231129161359002).

Conflict of Interest

Yuchao Bai serves as the Guest Editor of the Special Issue, but was not in any way involved in the editorial and peer-review process conducted for this paper, directly or indirectly. Other authors declare they have no competing interests.

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