Development and optimization of functionally graded Tungsten/Steel joints for divertor applications

Full text of publication follows: The divertor, which function is to exhaust He particles and impurities from the plasma, has to withstand very high surface erosion and heat loads of up to 15 MW/m2. To overcome these extreme operating conditions and so extending the life time of divertor components refractory tungsten and tungsten alloys are considered in the current helium cooled divertor concept developed at Forschungszentrum Karlsruhe, Germany. However the brittleness of tungsten at low temperatures limits its use to high temperature region (> 650 deg. C) of the component and so a joint to another structural material ODS EUROFER, a ferritic martensitic steel, will be necessary. Joining both components shows a major drawback: Thermal mismatch between tungsten and steel. It results in high stresses and deformations yielding failure of the joint. The aim of the current work is to present a functionally graded Tungsten/Steel joint as a solution to reduce misfit stresses and strains. furthermore the proposed Functionally Graded Material will be analysed using the Finite Element Method and the actual design of the proper divertor component. Therefore elastic, elasto-plastic and elasto-viscoplastic simulations are performed varying several joining parameters of the joint, such as the thickness of, the transition function within and the shape of the graded layer. The main focus will be the ratcheting behaviour and the failure of the joint under thermal cyclic loading. In addition to the FEM analysis and in order to determine the most adequate manufacturing technique of the functionally graded joint promising fabrication methods are screened and discussed. (authors)