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CoreFlow® Friction Stir Channelling for Fusion Reactors

Overview of CoreFlow® Friction Stir Channelling

TWI invented and patented a stationary shoulder variant of friction stir channelling (SS-FSC), which has subsequently been trademarked as CoreFlow®, and is a derivative technology of friction stir welding (FSW), a joining process invented at TWI in 1991. As shown in Figures 1 and 2, a stationary shoulder is employed to confine the viscoplastic nugget, limiting the flow of material extracted by the probe and preserving enough material to consolidate a roof over the channel. Therefore, an enclosed liquid cooling channel can be created in metallic material, such as aluminium or copper, in a single step.

Challenges with Fusion Reactor Thermal Management

The fusion industry is considering the possibility of using CoreFlow® friction stir channelling to manufacture components which manage the temperature of the “first wall” component in a nuclear fusion reactor.

Traditional approaches for associating or embedding cooling networks in fusion reactor structures include gun drilling, welding/brazing accessory pipework, and machining cooling networks followed by mechanically fastening or welding lids. In addition to being time-consuming and costly, these approaches incur a thermal efficiency penalty, as the heat needs to be dissipated through multiple multi-material interfaces and, in some cases, relatively limited contact areas. Whenever using gun-drilling or installing pipework there is also a design compromise that hinders performance. Fusion welding processes may not even be compatible with some of the materials under consideration, leading to the consideration of mechanically fastened sealing solutions, known for being bulky and adding to manufacturing complexity.

Innovation with CoreFlow® Friction Stir Channelling

CoreFlow® presents an innovative way to produce these embedded cooling channels in a single step, thereby:

  1. Enabling leaner, simplified manufacturing practices with reduced waste;
  2. Offering smarter design possibilities to improve thermal efficiency;
  3. Saving time and cost to manufacture and qualify components
Figure 1. SS-FSC technology developed by TWI, trademarked as CoreFlow®
Figure 1. SS-FSC technology developed by TWI, trademarked as CoreFlow®
Figure 2. CoreFlow® friction stir channelling of copper in action
Figure 2. CoreFlow® friction stir channelling of copper in action

Objectives

  1. Further mature the process for manufacturing optimised channel geometries within commercially-pure copper (C101) and copper alloys relevant to the fusion industry, including Copper Chromium Zirconium (CuCrZr)
  2. Scale-up the process alongside commercial partners according to the requirements and specific properties of the final application
  3. Explore the manufacture of channels aligned to fusion reactor design requirements
  4. Develop a suite of non-destructive testing (NDT) qualification options aligned to industry adoption and regulatory approval
  5. Investigation of CoreFlow® process feasibility in steel

This case study is part of the UK Atomic Energy Authority (UKAEA) Fusion Innovation Programme (FIP) Cycle 2 projects. In Phase 1, a feasibility study assessed CoreFlow® friction stir channelling in commercially-pure copper. Following the successful completion of this phase, Phase 2 furthered this research and explored CuCrZr copper alloy and steel material.

Results

The targeted channel cross-sectional area based on current “first wall” component design was 95mm2. Due to the aspect ratio currently used for CoreFlow® process probes, it was decided that a 14mm-diameter probe which was plunged 10mm-deep into the material could produce the targeted channel cross-sectional area with the development of suitable process parameters. A 14mm-diameter probe is expected to produce a 14mm-wide channel; therefore, a channel height of around 6.8mm would produce a channel with an approximate 95mm2 cross-sectional area.

A series of parameter development trials were conducted to optimise the channel cross-section and assess consistency and repeatability. As a result, preliminary process parameters were capable of repeatedly producing a relatively rectangular channel cross-section, which is approximately 14mm-wide and 8mm-tall, with a 2mm-thick channel ceiling across the top, yielding a cross-sectional area of around 112mm2, as shown in Figure 3.

Once suitable parameters were identified from the parameter development trials, the next focus was on achieving longer channel lengths that sustained suitable channel consistency. This culminated in the successful production of two serpentine channels within a 750x350mm plate, with each channel being over 2,500mm-long, as shown in Figure 4. This demonstrator proves the scale required for fusion industry applications is achievable and significantly de-risks further development.

Using TWI’s Crystal™ software, a non-destructive testing (NDT) procedure was developed which utilised ultrasonic testing (UT) to provide a quantitative measurement of channel ceiling thickness. The colour-coded heatmap in Figure 5 can be used to quantitatively assess the consistency of channel ceiling thickness along the length of the channel.

Thermal testing was conducted by Newcastle University and assessed the thermal resistance performance of CoreFlow® samples versus a milled and brazed reference sample. As shown in Figure 6, there is very close agreement between the CoreFlow® samples and the reference sample, with no significant difference in thermal resistance. These results therefore demonstrate that there is not an expected performance detriment by using a cooling plate manufactured via CoreFlow®. This is alongside the potential benefits of utilising the CoreFlow® for manufacture, which includes lower manufacturing costs, quicker qualification, and improved channel path and overall component geometry. 

Following the success with developing process parameters in copper, early-stage viability was proven in Copper Chromium Zirconium (CuCrZr), which is a fusion industry relevant copper alloy. Figure 7 details an example channel cross-section with this work progressing to successfully produce repeated serpentine samples. This achievement significantly de-risks future development, which will focus on further maturing the process in CuCrZr copper alloy.

It was previously unknown whether the CoreFlow® process fundamentals could be proven in steel, due to the harsh process environment and different material properties when compared with aluminium and copper. TWI collaborated with Element Six to procure preliminary toolsets to use for feasibility assessment trials. As demonstrated in Figure 8, the CoreFlow® process fundamentals were proven in the feasibility trials conducted. The rotating tool was capable of generating sufficient heating in the steel material to draw up and extrude steel wire whilst leaving behind an enclosed channel. These results de-risk future development and have identified the key focus areas as tool material development and tool design optimisation.

Conclusions

  1. Demonstration of successful channel manufacture at a large and fusion industry relevant scale aligned to an ITER-scale mock-up first wall panel component
  2. Proven feasibility of creating channels in Copper Chromium Zirconium (CuCrZr) copper alloy using the CoreFlow® process
  3. Significant progress made on understanding the process parameters
  4. Commercial solution proven for industry adoption
  5. Proven process fundamentals in steel

Future Research Direction

  1. Manufacture of TRL6 demonstrators for fusion industry applications
  2. Technology transfer and process adoption across to the fusion industry working towards qualified components
  3. Further maturity of the process for Copper Chromium Zirconium (CuCrZr) copper alloy
  4. Technology validation in relevant environment alongside commercial partners aligned to fusion reactor applications

Acknowledgement:

This project has been supported by the UK Atomic Energy Authority through the Fusion Industry Programme. The Fusion Industry Programme is stimulating the growth of the UK fusion ecosystem and preparing it for future global fusion powerplant market. More information about the Fusion Industry Programme can be found online: https://ccfe.ukaea.uk/programmes/fusion-industry-programme/

The author would like to acknowledge the support and contribution of the following TWI teams:

  • Friction Welding and Processing (FWP)
  • NDT Research and Technology (NRT)
  • Materials Performance Labs (MPL)
  • Materials Performance and Characterisation (MPC)
  • Thermal Processing Technologies (TPT)
  • Business Development (BUD)
  • Administrators (JTS)

The author would also like to acknowledge the support from external companies and collaborators:

Figure 3. Channel cross-section example in copper
Figure 3. Channel cross-section example in copper
Figure 4. CoreFlow® friction stir channelling demonstrator aligned to current fusion reactor design
Figure 4. CoreFlow® friction stir channelling demonstrator aligned to current fusion reactor design
Figure 5. Development of ultrasonic testing (UT) for non-destructive examination (NDE) to quantify channel ceiling consistency using TWI’s Crystal™ software
Figure 5. Development of ultrasonic testing (UT) for non-destructive examination (NDE) to quantify channel ceiling consistency using TWI’s Crystal™ software
Figure 6. Summary of thermal testing performed by Newcastle University
Figure 6. Summary of thermal testing performed by Newcastle University
Figure 7. Channel cross-section example in CuCrZr copper alloy
Figure 7. Channel cross-section example in CuCrZr copper alloy
Figure 8. Summary of feasibility trials in steel
Figure 8. Summary of feasibility trials in steel
Avatar Sam Holdsworth Project Engineer

Sam Holdsworth is a Project Engineer at TWI, specialising in Advanced Manufacturing Processes, namely Friction Welding and Processing. He is a recent first class MEng Materials Engineering graduate from the University of Birmingham, and is the technical lead for TWIs patented invention – CoreFlow™, which is a variant of friction stir channelling. He supports companies seeking to adopt this technology, acting as a consultant and project manager during product development, prototyping and technology transfer. He is also currently involved in the delivery of projects funded by Innovate UK and the European Commission, with both projects focussed on developing the CoreFlow™ process.

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