Manufacturing of W-steel joint using plasma sprayed graded W/steel-interlayer with current assisted diffusion bonding

The differences in the thermophysical properties between tungsten and steel create thermal stress peaks at its interface when joined directly for the breeding blanket of a fusion reactor. In order to solve this problem, a graded interlayer made of several layers of W/steel-composites is considered to reduce these stress peaks. Plasma spraying under an argon shrouded chamber was employed as a cost efficient manufacturing technique for such composites and field assisted sintering technology/spark plasma sintering was used to diffusion bond them with bulk-W and bulk-steel. Firstly, thermophysical characterizations were performed on these composites. Secondly, two approaches have been investigated to join bulk-W and 75 vol% W composite: direct joining and using a vanadium foil. Only vanadium foil resulted in successful bond formation at all the three bonding temperatures of 800 degrees C, 900 degrees C and 1000 degrees C. Thirdly, investigation of diffusion bonding parameters (temperature and time) for the joining of 25 vol% W, 50 vol% W, 75 vol% W and bulk-steel were studied and optimum process parameter were identified. Finally, this optimized parameter (1000 degrees C; 30 min) was employed to manufacture a complete 12 mm x 12 mm W-steel joint consisting of this graded interlayer.