Fabrication, microstructure, and mechanical properties of Fe-16Mn-10Al-5Ni-0.86C (wt.%) lightweight steel manufactured by directed energy deposition

초록

Laser Directed Energy Deposition (L-DED), as an advanced Additive Manufacturing (AM) technology, provides an efficient approach for fabricating complex lightweight steel components. However, due to a lower cooling rate compared to Laser Powder Bed Fusion (L-PBF), the microstructure of L-DED-fabricated materials is relatively coarse, limiting their mechanical properties. Additionally, lightweight steels strengthened by x-carbide tend to undergo microstructural coarsening during heat treatment, further weakening their strengthening effect. Therefore, developing suitable strengthening mechanisms for the L-DED process is crucial for optimizing material properties. In this study, B2-phase-strengthened Fe-16Mn-10Al-5Ni-0.86C (wt.%) lightweight steel was fabricated using the L-DED process, and the effects of heat treatment on its microstructure and mechanical properties were systematically investigated. Microstructural analysis revealed that the as-deposited sample consists of an FCC matrix phase and a BCC/B2 phase, with minor DO3 precipitates, exhibiting columnar and dendritic structures. After heat treatment at 900 degrees C for 1 h, dislocation nucleation facilitated the precipitation of fine B2 phases, which were uniformly distributed within austenite grains and along grain boundaries, with an average size of 4.58 mu m, accompanied by a small amount of fine x-carbide. The dispersed distribution of precipitates significantly enhanced the lightweight steel's strengthening effect, optimizing the balance between strength and ductility. After heat treatment, the yield strength increased to 1058.4 MPa, the ultimate tensile strength reached 1423.5 MPa, and the elongation significantly improved to 16.3 %, closely approaching the properties of the same composition lightweight alloy in the cold-rolled and tempered condition. This study demonstrates that the L-DED process, combined with appropriate heat treatment, effectively improves the mechanical properties of lightweight steel and provides theoretical guidance and process insights for microstructural control and performance optimization in L-DED-fabricated lightweight steels.

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