Microstructure and mechanical properties of directed energy deposition-arc/wire bimetallic hierarchical structures of hot-working tool steel and martensitic stainless steel

The hetero-structure with multi-material is believed to possess excessive potential to retard the strength-ductility trade-off. In this work, bimetallic hierarchical structures (BHSs) of hot-working tool steel RMD535 and martensitic stainless steel RMD650 with different proportions were fabricated, through directed energy deposition-arc/wire (DED-Arc/wire) with alternative deposition of two robots. The microstructure evolution and interfacial properties were investigated with the OM, SEM, XRD, EDS, and EBSD techniques. The mechanical properties were explored through the hardness test, loading-unloading-reloading tensile test (LUR) and fracture morphology analysis. Furthermore, the hetero-deformation mechanism affecting the strength and ductility was discussed with the digital image correlation (DIC) method, the calculation of hetero-deformation induced (HDI) stress and geometrically necessary dislocations (GNDs). The results indicate that different elements diffusion behaviors occurred at RMD650–535 and RMD535–650 interface due to the remelting effect, dilution, and discrepant thermal conductivity. The skeletal δ ferrite phases embedded in the fine low-carbon lathy martensite matrix are found in the hard region of RMD650 stainless steel, while the dominated coarser martensite with bainite appeared in the soft region of RMD535 hot-working tool steel. The yield and ultimate tensile strength decrease with the reducing proportion of hard material. The BHSs of 75% hard and 25% soft material reveal the highest yield strength of 890.97 MPa and a slightly decreased elongation of 7.41%. The HDI strengthening effect surpasses the softening effect when the proportion of soft material is less than 50%, and the HDI stress increases with the increasing proportion of hard materials. The strain distributes in the longer gauge length in BHSs because of the strain partition, resulting in extra strengthening and delayed strain/stress localization. These findings demonstrate the capability of DED-Arc/wire in constructing bimetallic hierarchical multi-material structures.