Effect of Interlayer Cooling Strategies on the Microstructure and Mechanical Properties of DED‐Fabricated 308L Stainless Steel Deposited on 304L Substrate

The issue of thermal accumulation in directed energy deposition (DED) leads to grain coarsening, microstructural inhomogeneity, and deterioration of mechanical properties, which severely restricts the quality of large‐scale components. Therefore, effectively controlling interlayer temperature and mitigating heat accumulation have become critical challenges in optimizing the manufacturing quality of DED. In this study, the effects of interlayer active cooling on the microstructure and mechanical properties of DED‐fabricated 304L stainless steel were investigated. Three cooling strategies were comparatively examined: natural air cooling (A1), forced air cooling (A2), and substrate water‐based cooling (W1). Microstructural features were characterized using optical microscopy, scanning electron microscopy (SEM), and electron backscatter diffraction. Mechanical behavior was evaluated through tensile testing and fracture surface observation by SEM. The results revealed that active cooling (A2 and W1) significantly reduced interlayer cooling time, improved macroscopic morphology, minimized melt pool overflow, and enhanced the consistency of layer height and width. Compared with natural cooling, forced air cooling reduced the interlayer cooling time by ≈35–45%, refined the average grain size from ∼ 42 µm to ∼ 26 µm, and limited interlayer ultimate tensile strength (UTS) variation to ≤8% (≈682–688 MPa). Water‐based cooling further reduced grain size (∼22 µm) but induced pronounced strength anisotropy, with bottom‐layer UTS decreasing by ∼33% relative to natural cooling. The mechanical properties of water‐based cooling specimens exhibited pronounced nonuniformity along the build height: the middle layers showed the highest ultimate tensile strength, whereas the bottom layers displayed relatively low strength, primarily due to the development of strong textured columnar grains induced by a high temperature gradient‐to‐cooling rate ratio. Forced air cooling achieved the best uniformity of mechanical properties, demonstrating greater effectiveness in suppressing thermal accumulation and balancing the strength–ductility trade‐off throughout the component.