Additive manufacturing of copper – H13 tool steel bi-metallic structures via Ni-based multi-interlayer

When selecting a material for dies and molds, strength is needed at high temperatures to hold the shape of the component along with a high thermal conductivity to decrease the solidification time of the components. This need has led to the investigation of Cu to H13 tool steel bi-metallic structures. Utilizing a directed energy deposition experimental setup, two fabrication approaches were used: direct deposition of Cu on H13 and utilizing an intermediate layer of Deloro 22 (D22, >95 wt.% Ni content). Three structures were fabricated: Cu-H13 direct joints (DJ), Cu-D22-H13 multi-metallic structures (MMS), and D22-H13 DJ. In order to characterize the structures, the following was performed: microstructure characterization, elemental distribution, tensile testing, hardness, and thermal conductivity measurements. Directly joining the Cu onto the H13 resulted in cracking at the interface. By introducing D22 buffer layers, defect-free Cu was successfully deposited on H13. A sharp transition of elemental contents was experienced at the D22-H13 interface due to very limited layer diffusion. Across the D22-Cu interface, a gradual transition of Cu and Ni was detected, indicating a successive elemental diffusion. Tensile testing revealed that the Cu-D22-H13 MMS specimens fractured in the Cu zone with a morphology indicating a ductile fracture. The D22-H13 DJ failed in the D22 region although elongation mostly happened in the H13 section. The interfaces of both Cu-D22-H13 MMS and D22-H13 DJ survived tensile testing, indicating a strong bonding strength. Microhardness measurements observed an increased hardness at the surface of H13 due to laser hardening. The material hardness dropped rapidly across the Cu-H13 DJ but gradually in the Cu-D22-H13 MMS as the Ni in D22 diffused in multiple layers of the Cu. Thermal conductivity test shows the overall thermal conductivity of the Cu-D22-H13 MMS increased by approximately 100 % when compared with pure H13. The volume fraction of Cu can significantly affect the overall thermal conductivity of the Cu-D22-H13 MMS.