Investigation on enhanced strength in W/(Ti/Cu) composite interlayer/steel diffusion bonding joint based on controlled diffusion mechanism

The development of W/steel composite structures has become one of the focuses in the advancing nuclear fusion reactor cladding subassembly, leveraging the complementary strengths of both materials. The interlayers currently used in diffusion joining either fail to establish effective metallurgical bonding with the substrates or induce excessive reactions and leading to the formation of large number of brittle compounds. This makes it difficult to meet the strength requirements of the W/steel composite structure. Considering a controlled chemical reaction at the interface is beneficial for improving metallurgical bonding, the “controlled diffusion” concept was proposed in this work. Specifically, a Ti/Cu composite interlayer with thin Ti foil and thick Cu foil was employed for diffusion bonding steel and W with the double-layer sandwich structure of steel substrate/Cu/Ti/W substrate. The Ti interlayer diffuses through the Cu interlayer and reacts with the steel substrate under appropriate interlayer thickness design and processing conditions to promote metallurgical bonding at the Cu interlayer/steel substrate interface. The activity of Ti is fully utilized to address the challenge of forming a strong bond between the Cu layer and the steel substrate, while reducing thermal stresses in the joint and preventing excessive formation of brittle intermetallic compounds. Finally, the high-performance W/steel composite structure (tensile strength of 280.7 MPa) with a Ti/Cu composite interlayer were successfully prepared based on controlled diffusion mechanism. This work provides a novel interlayer design concept and mechanistic insight for improving the diffusion bonding quality of dissimilar materials in high-performance structural applications.