Interfacial characteristics of Cu tube-embedded hybrid structures fabricated by directed energy deposition

Cu tube-embedded cooling channels have been considered a promising approach for mold applications because they can provide smoother internal surfaces than fully additively manufactured channels. Nevertheless, their practical application is limited by the difficulty of achieving stable bonding between the embedded Cu tube and H13 during directed energy deposition (DED), owing to the large thermal and metallurgical mismatch between the two materials. In this study, the effect of buffer layers on the interfacial stability of Cu tube-embedded H13 hybrid structures was investigated. Direct deposition of H13 on Cu resulted in lack of bonding above the Cu tube, whereas relatively sound bonding was obtained on the H13 substrate side. To improve the Cu–H13 interfacial condition, preheating and laser-power control were applied, and P21 and STS316L were introduced as buffer-layer materials. Cross-sectional and SEM–EDS analyses showed that the buffer layers promoted a more localized interfacial mixing and improved interfacial continuity. Microhardness measurements further showed that the buffer-layer specimens exhibited a more gradual hardness distribution than the non-buffer specimen. Thermal-property evaluation indicated that the P21-buffer specimen showed the highest thermal conductivity among the hybrid specimens, whereas the STS-buffer specimen showed the lowest value. In bending tests, the P21-buffer specimen exhibited higher fracture resistance and greater plastic deformation, while the non-buffer specimen fractured in a brittle manner and detached rapidly. These results demonstrate that an appropriate buffer layer is effective in improving the interfacial stability and mechanical reliability of Cu tube-embedded H13 hybrid structures fabricated by DED.