Numerical assessment of functionally graded tungsten/steel joints for divertor applications

In the current helium cooled divertor concept tungsten and its alloys are considered as refractory as well as structural materials. Due to the brittleness of tungsten at low temperatures its use as a structural material is limited to the high temperature region (>650 °C) of the component and a joint to another structural material, the ferritic martensitic steel EUROFER97-ODS, is necessary. However, the remarkable difference in thermal expansion between tungsten and steel causes a thermal mismatch between them resulting in stresses, which would yield failure of the joint. An idea to reduce these stresses is to introduce a functionally graded layer between the materials to be joined.In this work a functionally graded tungsten/steel joint is analyzed using the finite element method and considering a simplified model of the proper divertor component. Therefore elasto-plastic and elasto-viscoplastic simulations are performed varying the thickness of and the transition function within the graded layer and considering application relevant loadings. The resulting stresses and deformations are then evaluated to determine the optimal joining parameters. On the base of these parameters the realization of the joint is investigated by screening the methods applicable for the fabrication of functionally graded materials and selecting the most suitable ones.