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Combined joint method of brazing and diffusion bonding, for between tungsten (W) and Reduced Activated Ferritic/Martensitic (RAFM) steel (W/RAFM steel) with a pure-Cu intermediate layer, was developed. Flat plate type joint sample of W/pure-Cu/RAFM steel with the joint area of 20 × 20 mm² was selected in this experiment, in which W and the 1 mm thick pure-Cu was jointed by brazing with Ni-11%P filler material under the 960°C, 10 min, as a primary process. Then, pure-Cu and RAFM steel was jointed by simple diffusion bonding with 40 MPa uniaxial compressive load under the 720°C, 1 hour, as a secondary process. All joint procedures were conducted in the vacuum furnace.
Thickness of W, pure-Cu, RAFM steel and Ni-11%P filler were 3, 1, 5 mm and 38 micrometer, respectively. The most challenging point in this study was whether sufficient joint quality was able to obtain between Pure-Cu and RAFM steel joint under the relatively low bonding temperature of 720°C, that is below the tempering temperature (750°C) for RAFM steel and is the specified temperature for post-welded heat treatment (PWHT). Therefore, obtaining a suitable joint quality at this temperature would lead to advantage from the viewpoint of utilizing the RAFM steel to design the DEMO reactors. To confirm a joint quality for W/pure-Cu/RAFM steel sample, inspections for the joints were performed by using digital microscope, scanning electron microscope (SEM) and transmission electron microscope (TEM). To understand the thermal transfer properties of the joints, heat loading experiments were also performed by using an electron-beam device (ACT2) with actively cooled condition of the backside of the RAFM steel.
Although some micro-level voids were observed in the W/pure-Cu joint interface, they did not show any negative effects for thermal transfer properties during electron-beam heating. The pure-Cu/RAFM steel interface showed almost no joint defects such as voids and cavities in nanometer level, and inter diffusion of Fe and Cu elements were confirmed with the range of 1.0~2.0 micrometer. Thermal transfer property of W/pure-Cu/RAFM steel sample did not change from the initial condition up to the heat loading value of ~2.4 MW/m² with cyclic heat loading. On the other hand, thermal transfer properties degraded in the range of 2.4~2.8 MW/m² of repeated heat load due to the crack formation in the W bulk.
The combined joint method demonstrated one of the superior advantages to manufacturing the diverter heat removal component below PWHT temperature of the RAFM steel.