Electrodeposition of Functionally Graded Brazing Interlayers for Enhanced Joint Strength between Fusion Plasma-Facing Materials and Heat Sinks

Abstract

In response to the significant plasma science and engineering breakthroughs witnessed over the past decade, the fusion research community has developed an ambitious roadmap to achieve large-scale, energy-positive fusion reactors that will enable economically competitive fusion electricity. Critical to the success of commercial fusion energy is the development of materials able to serve as plasma facing components (PFCs) that can withstand the unprecedented high heat, particle, and neutron fluxes within the fusion reactor. Tungsten is a leading candidate for the plasma-facing portion of these PFCs because of its high melting point, high sputtering resistance, and low tritium retention. Copper or copper-based alloys (e.g., copper-chromium-zirconium alloy C18150) and reduced activation ferritic/martensitic (RAFM) steels have been proposed for the heat sink materials behind the tungsten armor due to their high thermal conductivity. Brazing is considered a promising method for joining the tungsten armor and heat sink layer. However, the extreme mismatch in the coefficient of thermal expansion (CTE) between tungsten and these potential heat sink materials makes directly joining these two layers challenging. This thermal stress can be reduced by incorporating a joining layer of functionally graded materials between the tungsten and heat sink that imparts gradual changes in CTE. Copper/tungsten or copper/tungsten carbide composites are leading candidates for these functionally graded materials. For these composites, gradual changes in the CTE are achieved by varying the volume fraction of tungsten/tungsten carbide in the composite, from tungsten-rich at the tungsten armor to tungsten-poor at the heat sink.This talk will discuss the development of a scalable, electrochemical approach for fabricating interlayers with functionally graded CTE that can decrease the thermal mismatch between tungsten PFCs and copper or RAFM-based heat sinks. In this approach, the copper/tungsten or copper/tungsten carbide composite are electrodeposited onto the tungsten PFC. This component is then bonded to the heat sink via brazing. A key objective of the work is to achieve compositional control during electrodeposition through the use of highly tunable pulse and pulse-reverse electric fields. Engineering of the electric field enables control of the mass transport and crystallization phenomena in the deposition process and thus enables control of composite properties such as composition, porosity, and surface roughness. Results from electrodeposition trials and mechanical testing of brazed joints under ambient conditions as an initial screen for interlayer performance will be discussed, in anticipation of high-heat flux testing of joint performance under fusion-relevant conditions.