Interfacial structure and formation mechanism of tungsten/steel HIP diffusion bonding joints using Ni interlayer

Tungsten and steel were successfully bonded using hot isostatic pressing (HIP) diffusion bonding with Ni interlayer. The influences of surface roughness, post-weld heat treatment (PWHT) and interlayer thickness on the properties of W/steel joint were researched, and the interfacial structure as well as the formation mechanism was investigated systematically. The results indicated that, the interfacial structure of the joint is steel/diffusion layer/reaction layer/W. The number of voids at the interface will be decreased greatly by reducing the roughness of the surfaces to be joined, which leads to the compactness of joints improving obviously. For the joints with thin (5-20μm) interlayers, after PWHT, the brittle reaction layer will further grow into W substrate and reduce the joint strength. While for those joints with thick (30-80μm) interlayers, PWHT is beneficial to increase the thickness of the helpful diffusion layer which shows the positive effect on the joint strength. A surprising result in this work is that, due to the different element concentration gradients, the structure of reaction layers is various with different interlayer thickness. With the very thin (≤20μm) interlayer, the reaction layer is a double-layer structure including W2Ni(Fe) layer and sublayer. When the thickness of the interlayer is 30μm, it is a W2Ni(Fe) single-layer structure. As the interlayer thickness increases to 50μm, the reaction layer shows a WNi and W2Ni double-layer structure. While when the interlayer thickness further increases to and above 80μm, it is a W2Ni and sublayer structure. Among the layers mentioned above, the sublayer close to W substrate is the decisive area to influence joint strength.