Search

To enhance the efficiency and quality of grain refinement achieved by cold deformation, the structural state of the material before cold deformation should be changed radically. In this study, a FeCoCrNi high-entropy alloy fabricated via additive manufacturing (AM) was chosen as the initial material for investigating cold deformation. The microstructure and mechanical properties of compressed samples (50 % reduction along the building direction) annealed at 773–1373 K for 2 h are compared. As the annealing temperature increased, the rate of recrystallization increased continuously. After annealing for 2 h at a temperature lower than 973 K (recrystallization temperature), the work hardening from cold deformation was not released. As a result, the sample had high strength but low ductility. After annealing at temperatures ≥973 K, both the tensile strength and elongation at fracture became much higher than those of the as-printed sample. Annealing at 1173 K achieved the smallest average grain diameter and the narrowest grain diameter distribution, and many twins existed, which are the main reasons why the sample showed both high strength and ductility (tensile strength, 806.7±14.8 MPa; elongation at fracture, 55.8±2.1). Recrystallized grains were found to nucleate within the original columnar grain, indicating that a large amount of the deformation energy accumulated inside the grain. Dislocation networks, which constitute a unique structure of AM builds, significantly increased the storage rate of deformation energy during cold deformation. Therefore, in the subsequent annealing process after cold deformation, efficient recrystallization could be realized, significantly improving the mechanical properties of the alloy and increasing the potential of AM for industrial applications.

Metal matrix composites reinforced by carbon nanostructured material are beneficial in achieving a balance between strength and ductility. However, achieving this balance in stainless steel through the selective laser melting (SLM) process remains a challenge. In this study, SLM is used to fabricate 316 L stainless steel matrix composites, which are reinforced with 0.1 wt.% single-layer graphene nanoplatelets (GNPs). The optimal process parameters are obtained by combining the analyses of the relative density and mechanical properties. The results show that the GNPs in the 316 L/GNPs composites are dispersed at the grain boundary, either parallel or at a certain angle to the grain boundary. Based on the tensile test, the ultimate tensile strength of the 316 L/GNPs composites is 738 MPa, which was 17.9 % higher than that of 316 L. Furthermore, both SLM-printed 316 L and 316 L/GNPs composites have an elongation of ∼38 %. The interaction between the GNPs and the molten pool, and the redistribution of the GNPs leads to the formation of a lamellar and relatively high-density distribution area of GNPs in the molten pool. The strengthening mechanism of 316 L/GNPs composites is also elucidated. These results provide insights into enhancing the combined strength-ductility property of other SLM-printed GNPs reinforced metal matrix systems.