Microstructure, residual stresses and mechanical properties of diffusion bonded tungsten–steel joint using a V/Cu composite barrier interlayer

• A V/Cu composite barrier interlayer was designed for diffusion bonding of W to steel.
• The interface of W/V/Cu/steel without any intermetallic phase was achieved.
• The sound joint with a high strength of 402 MPa was obtained.
• The fracture occurred predominantly at the W substrate near the W/V interface.
• The maximum residual stress occurred on the W side near to the joining interface.

Diffusion bonding is a preferred method to join W and steel for divertor applications. To minimize the residual stress induced by the large mismatch of thermal expansion coefficients and to inhibit the formation of brittle intermetallic phases, a V/Cu composite barrier interlayer was designed and examined to produce a joint between W and steel. The diffusion bonding was carried out at 1050 °C for 1 h under a 10 MPa pressure in vacuum. Metallographic analysis revealed excellent bonding at all of the joining interfaces. Neither intermetallic compounds nor other brittle phases were found in the bonded region. Nano-indentation test across the joint interfaces demonstrated the effect of solid solution strengthening in the diffusion zone. The strength of the joint was as high as 402 MPa and the failure occurred predominantly at the W substrate near the W/V interface due to the residual stress concentration.