Abstract: Owing to the special layer-by-layer deposition process of directed energy deposition (DED), columnar coarse grains, produced by cyclic reheating with intrinsic directional heat flow along the building direction, are difficult to avoid. These grains result in strong anisotropic characteristics with poor mechanical properties, which restrict the application of DED products. This work proposes a novel fabrication strategy based on the cyclic-phase-transformation behavior, which can reduce the anisotropy and improve the mechanical properties of DED-printed stainless steel. Using this fabrication strategy, 316 L powder (austenitic stainless steel) and 17-4PH powder (martensitic stainless steel) were mixed in different mass ratios to fabricate five types of DED-printed stainless steels. Among the five samples, P70 (mixture of 70 wt% 17-4PH powder and 30 wt% 316 L powder) showed the weakest anisotropy and the best mechanical properties, which can be attributed to the cyclic phase transformation under cyclic reheating treatment and the transformation-induced plasticity (TRIP) effect over a wide range of strains, respectively. Compared with the pure 316 L printed material, the tensile test results of P70 showed that the yield strength (YS) and ultimate tensile strength (UTS) increased by 35.4% and 34.5%, respectively, whereas the uniform elongation (UE) and total elongation (TE) were improved by 63.9% and 31.4%, respectively. In addition, the strength-ductility balance (UTS × UE) increased by as much as 120.4%. The proposed fabrication strategy is expected to reduce the anisotropy in other materials that undergo cyclic-phase-transformation phenomena during additive manufacturing.

Key words: Directed energy deposition, Anisotropy reduction, Cyclic phase transformation, Mechanical properties