Impact of thermal fatigue on W-W brazed joints for divertor components

W-W brazed joints (tungsten block: 8 × 8 × 4 mm joined to an actively cooled copper heat sink) were exposed to steady state heat loads to study the effect of the thermal fatigue on their microstructure, mechanical integrity and heat transfer capability. Two different surface temperatures were tested (1000 °C and 1250 °C) varying the number of applied cycles (100 and 1000). The results indicated that a surface temperature of 1000 °C represents the limit condition for the joints to be used in the DEMO fusion reactor, which corresponds to a braze temperature in the range of 562-599 °C according to thermal simulation results. The use of that temperature caused a limited effect on the microstructure and heat transfer capability for both 100 and 1000 applied cycles. The increase of the surface temperature to 1250 °C caused degradation of the joint and a reduction of sustained cycles and accordingly lifetime.