Integration eines funktionell gradierten W/Cu-Übergangs für Divertorkomponenten von Fusionsanlagen

摘要

One of the most difficult topics in the design and development of future fusion devices, e.g. ITER (Latin for "the way") is the field of plasma facing components for the divertor. In steady-state mode these will be exposed to heat fluxes up to 20 MW/m2. The favored design-option is a component made out of tungsten and copper-alloys. Since these materials differ in their thermal expansion coefficient and their elastic modulus a temperature gradient within the component, caused by thermal loads, results in stresses at the interface. An alternative design-option for divertor-components deals with the insertion of a functionally graded material (FGM) between tungsten and copper. This establishes a continuous change of material properties and therefore minimize the stresses and optimize the thermal behavior of the component. Low pressure plasma-spraying and direct laser-sintering are introduced as possible production-methods of graded W/Cu-composites. Based on preliminary investigations both are used for fabricating W/Cu-composite materials with different mixing ratios. Thermo-mechanical and thermo-physical material properties will be determined on these composites and extrapolated to all mixing ratios. For laser-sintering these are limited to Cu-contents of ~20 to 100 Vol%. Therefore the plasma-spraying process is favored. In finite-element-analyses the graded material and its material properties will be implemented into a 2-D simulation-model of a divertor component. The composition and the design of the graded W/Cu-composite will be optimized. Best results are obtained by high contents of tungsten within the graded layer, which are still improved by a macro-brush design with dimensions of 4.5 x 4.5 mm2. This results in a transfer of critical stresses from the mechanical bonded interface between the plasma facing and the graded material to the diffusion bonded interface between the graded material and copper. The joining of tungsten, a plasma-sprayed graded W/Cu-interlayer, OFHC-Cu (Oxygen Free High Conductivity) and CuCrZr will be done by Hot Isostatic Pressing. Parameters are a temperature of 550øC an a pressure of 195 MPa. Electrochemical deposited copper and nickel are added. Copper is used as surface layer of the graded W/Cu-composite and nickel for the strengthening of the diffusion bonding. Ultra-sonic-testing revealed narrow areas with inhomogeneous bonding at the interface, mainly nearby the outer surface of the module. The module containing the macro-brush has been tested at the electron-beam test facility JUDITH. It survived power loads at steady state operation of 23.8 MW/m2 and 150 cycles at 20 MW/m2 during thermal fatigue experiments. These results verify, that the insertion of a graded W/Cu-interlayer increases the resistance against thermal loads. Especially in the combination with the castellated structure.