Study On Joining Of Tungsten As Plasma Facing Materials With Heat Sink Materials

The development of fusion energy depends strongly not only on the progress in materials selection, but also on the technology for their manufacturing and joining. Designing and preparing of plasma facing materials and plasma facing components (PFM/PFC) are a difficult issue in nuclear fusion device. Tungsten (W) and its alloys are the promising candidates as PFMs, while copper (Cu) and low activation steels are the promising candidates as heat sink materials. Both of them compose the vital PFC. However, there are large differences in the coefficient of thermal expansion (CTE) and elastic modulus between W and Cu, which may cause high thermal stress during fabrication and service, and lead to cracking, delaminating and a reduced lifetime of the components by rapid detachment failure. As a result, the design and preparation of the joint of PFC are the difficult issue.The major aim of this research work is focusing on the research on the reliable joining between PFM (W) and heat sink materials (Cu and low activation steel). The research contents are as follows:Be aimed at current water-cooled divertor, the fabrication and evaluation of W/Cu mock-up by means of improved atmospheric plasma spraying (APS). The plasma spheroidization process was investigated. Properties of thick tungsten coating were characterized; finite element analysis (FEA) of W/Cu functionally graded coating materials including steady and transient state; the research on W coated on copper alloy and stainless steel by cold gas dynamic spraying (CGDS) was carried out. W particle deformation was simulated. The properties of W coating were investigated; the joining and evaluation of W/Steel mock-up by vacuum electron beam brazing and simulation of its joint for helium cooled reactor. The following innovative results are achieved.(1) The spherical W powders were preparated by plasma spheroidization technology by means of commercial APS equipment with water cooled system. The results indicated that percent of spheroidization of W powder was more than 90%. Content of oxygen in spherical W powder was less than 0.2 wt%. The flowability of the powder by Hall flowmeter was less than 6 s/50g. The yield of spherical W powder was more than 80%. The plasma spheroidization power, the distance from surface of the water and the morphology of starting powder mainly affected the basic properties of spherical W powder including percent of spheroidization, oxygen content of tungsten powder, morphology and size distribution under certain fixed powder feed rate.(2) The APS technology was improved through starting spraying powder, design of coating and spraying condition. The W coating with 4 mm thickness was fabricated on the CuCrZr alloy with area of 110 mm by 130 mm. The spherical W carbonyl powder by decomposing W hexacarbonyl (W(CO)6) was used for starting spraying powder. CuMo/MoW with 1 mm thickness was used for interlayer. There was no large vacancy at interior and interface of coating. The porosity of the W coatings was about 2%. The maximal bonding strength of the coating was about 10 MPa. The highest thermal conductivity of pure W coating was only 12.52 W/(m·K).(3) The FEA of W/Cu functionally graded coating materials indicate that the maximal equivalent stress first decreased quickly and then decreased slowly with the increasing thickness of gradient layer, while surface temperature of tungsten increases linearly. The maximal equivalent stress effectively relieved when the thickness of pure W and gradient layer was 2 mm and 240μm, respectively. The equivalent stress distribution in entire mock-up with different thickness of gradient layer changed, while entire mock-up had the same equivalent stress distribution under different heat flux density. The surface temperature of W increased sharply under off-normal high heat flux. The surface of W with high heat flux of 800 MW/m2 and the duration of 5 ms began to melt.(4) The research on W coated on copper alloy and stainless steel by CGDS was carried out theoretically and experimentally. The results from ANSYS/LS-DYNA indicated that the deformation of W powder increased with the increasing velocity under low velocity condition. The embedded phenomenon occurred in the flat W substrate when high velocity reached a certain degree. There was no the formation of a jet-type flow of W powder impacting onto a flat Cu substrate compared to that of Cu powder impacting onto a flat Cu subtract because W powder deformation ability was inferior to copper powder. W coating coated on copper alloy and was fabricated by CGDS and nitrogen (purity 99.9%) gas was used as the carrier gas and the accelerating gas at the pressure of 33 bar. A W coating with 5μm was prepared with the mean size grain (D50) of 2μm as starting spraying at a stand-off distance of 30 mm and the gas temperature of 730℃. No oxidation in the W coating was observed. The strating power with too big or small didn't deposit. The spraying distance was also the key factor of W powder deposition. The scratching experiment indicated strengthen of the tungsten coated on steel was higher than that of the tungsten coated on Cu alloy.(5) The joining of W and low activation steel was done with Ni based amorphous foiltype filler metals by means of vacuum electron beam brazing. The results indicated that the interface of the joint was sound. There was no crack in the joint after the process was optimized. There were solid solutions in W/filler interface and steel/filler interface. The shear strength value of the joint was 365 MPa. Along with the increase of brazing time, the shear strength value first increased and then decreased. The increasing brittle phases and resulted in the low shear strength of the joint.(6) The joining of W and low activation steel was done with Ti based amorphous foiltype filler metals by means of vacuum electron beam brazing. The results indicated that the interface of the joint was sound. There was no crack in the joint after the process was optimized. The pure Ti interlayer increased effectively the shear strength of the joint. The shear strength of W/Ti/Steel was 233 MPa. The finite element model indicated that pure Ti interlayer decreased the resuidal stress. The morphology of the W/Ti/Steel showed the interface of filler and pure Ti fuse. Along with the incresse of brazing time, the shear strength value decreased because of the amount of brittle phase and overflow of filler.