Microstructure, properties, and formation mechanisms of tungsten/steel hot isostatic pressing diffusion bonding joint utilizing a Ni-Si-B interlayer

In this study, tungsten and steel were successfully bonded utilizing hot isostatic pressing diffusion bonding with a Ni-Si-B interlayer. The interfacial structures and formation mechanisms of joints with interlayers of various thicknesses were systematically investigated. The results indicated that the interfacial structure of a joint with a thin (0.8 mu m) Ni-Si-B interlayer is a W/Fe-W inter-metallic compound (IMC) reaction layer/(Fe,Ni)ss diffusion layer/steel structure, where through cracks appear at the interface of the W/reaction layer, leading to poor shear strength. As the interlayer thickness increases (3.2-22.9 mu m), the brittle Fe-W IMC no longer appears and the interfacial structure is a W/Ni-W IMCs reaction layer/(Ni,Fe)ss diffusion layer/steel structure. When the interlayer thickness is 3.2 mu m, there are many large voids in the diffusion layer. When the interlayer thickness increases to 5.3 mu m, the most compact interfacial structure can be obtained with the maximum bonding strength of 363.5 MPa. However, when the interlayer thickness increases further to 12.6 and 22.9 mu m, many voids appear in the reaction layer, leading to a decrease in bonding strength. The formation mechanism of a joint with a thin (0.8 mu m) interlayer is a purely solid-solid reaction without the generation of a liquid phase. When the interlayer thickness is large (3.2-22.9 mu m), the formation process of joints includes three stages: solid-solid reaction, remaining interlayer liquefaction, and solid-liquid reaction. Electron probe microanalysis results reveal that the melting-point depressant element Si is segregated in the reaction layer or diffusion layer depending on the interlayer thickness, which is precisely controlled by the staged formation mechanism of the joints.