Functionally-Graded Cu-W Interlayers for Enhanced Divertor/Heatsink Bonding

Energy-positive fusion reactors require the development of divertor modules at the bottom of the vacuum vessel to extract heat and ash produced by the fusion reaction, minimize plasma contamination, and protect the walls from thermal/neutronic loads. To provide the high heat transfer capacity to handle the anticipated flux from the fusion plasma, copper alloy heatsinks combined with tungsten plasma-facing components are planned. The dramatic mismatch in coefficients of thermal expansion between tungsten and copper represents a significant heat transfer and durability challenge. Functionally-graded bonding interlayers, tungsten-included copper composite and iron/tungsten alloy, will be designed to afford high and low tungsten concentrations near the plasma-facing component- and heatsink-facing surfaces, respectively. Pulsed electrodeposition will enable these functionally graded interlayers to provide smooth gradients in the coefficient of thermal expansion, facilitating strong brazed joints between plasma-facing and heatsink components. Phase I demonstrated pulsed-waveform electrodeposition of interlayers with tungsten content up to 50%: (i) composites with submicron tungsten particles in a copper matrix, (ii) iron/tungsten alloys, (iii) copper with zirconium or tungsten-oxide inclusions. Copper, stainless steel and tungsten coupons, with and without interlayers, were brazed using standard vacuum-furnace protocols and braze filler. The mechanical properties of the brazed samples were tested by lap-shear pull testing and thermal cycling between 150–350°C in air, and all samples maintained braze joint integrity, indicating that samples with interlayers maintained performance compared to samples without interlayers. An economic analysis yielded amortized per-unit costs of interlayer application of $198/unit and $241/unit for the copper/tungsten composite and iron/tungsten alloy, respectively, at 1920 units/year; low compared to the costs of the tungsten and copper alloy heat sink components (≥ $4,000-5,000). Phase II will continue electrolyte and process development of the Cu/W composite and Fe/W alloy interlayer systems, culminating in fabrication of functionally-graded interlayers. The performance of the brazing interlayers will be evaluated primarily by high-heat flux tests, mimicking fusion-relevant thermal loads. An alpha-scale apparatus will be fabricated to produce parts for capstone testing at a scale comparable to divertor plasma-facing components. Economic analysis will be refined. While applications of this manufacturing technology for functionally-graded interlayers in the fusion power market are long-term, successful development of fusion power represents an incalculable boon. Gas and steam turbines in aerospace and power generation are exploiting or investigating the potential of functionally-graded materials for bonding. Alternative commercialization pathways exist beyond the fusion power market.