Process optimization for diffusion bonding of tungsten with EUROFER97 using a vanadium interlayer

Abstract

Solid-state diffusion bonding is a selected joining technology to bond divertor components consisting of tungsten and EUROFER97 for application in fusion power plants. Due to the large mismatch in their coefficient of thermal expansions, which leads to serious thermally induced residual stresses after bonding, a thin vanadium plate is introduced as an interlayer. However, the diffusion of carbon originated from EUROFER97 in the vanadium interlayer during the bonding process can form a vanadium carbide layer, which has detrimental influences on the mechanical properties of the joint.For optimal bonding results, the thickness of this layer and the residual stresses has to be decreased sufficiently without a significant reduction of material transport especially at the vanadium/tungsten interface, which can be achieved by varying the diffusion bonding temperature and duration. The investigation results show that at a sufficiently low bonding temperature of 700°C and a bonding duration of 4h, the joint reaches a reasonable high ductility and toughness especially at elevated test temperature of 550°C with elongation to fracture of 20% and mean absorbed Charpy impact energy of 2J (using miniaturized Charpy impact specimens). The strength of the bonded materials is about 332MPa at RT and 291MPa at 550°C. Furthermore, a low bonding temperature of 700°C can also help to avoid the grain coarsening and the alteration of the grain structure especially of the EUROFER97 close to the bond interface.