CoreFlow® is a sub-surface machining technique developed at TWI. This solid-state process is a development of friction stir channelling (FSC) and friction stir welding (FSW), which was also invented at TWI.
The CoreFlow® process allows for the introduction of sub-surface networks of channels to be integrated into two- and three-dimensional monolithic parts in a single manufacturing step. These channels can be used for a variety of applications, including heat exchange.
The process involves a tool (Figure 1) traversing along a pre-determined path, extracting material from the workpiece to create both a closed channel and the production of extruded wire. CoreFlow® uses a rotating probe that is plunged to a set depth into the workpiece, at which point it begins to traverse along the workpiece, conveying plasticised material upwards via rotating probe threads until it is expelled from the shoulder of the tool (Figure 2). This not only creates a closed channel in the workpiece but, through a process called ‘ForgeWire,’ can also be used to produce unlimited lengths of wire from a plate or pipe. This is of particular interest for producing spools of wire from materials with poor extrudability, such as alloys of aluminium or magnesium (Figure 3).
CoreFlow® delivers ready applications for the manufacture of heat exchangers, cooling systems, integrated fluid management and light-weighting of structures. This includes use in electric vehicles, aircraft, communication platforms, electronics, satellites and ships.
One area where the process has been investigated is for use within nuclear fusion reactors. Because plasma facing components (PFCs) used in magnetic confinement fusion systems are the first point of contact between the plasma used to create fusion and the chambers in which the plasma is contained, the PFCs are subjected to high heat flux on the top surface that needs to be actively cooled. Sub-structural cooling pipework is used to dissipate the heat, but traditional methods of creating and embedding these systems are time-consuming and costly while any joints need to be able to withstand large stresses caused by differential thermal expansion between the materials used. Early investigations into the use of CoreFlow® for PFCs has shown that copper tiles can be processed with this technique (Figures 4-6), although further work was required to mature the process for fusion energy system developments.
Our experts conducted parameter development trials to create preliminary process parameters that delivered repeatability in the production of a relatively rectangular channel cross-section approximately 14mm-wide and 8mm-tall, with a 2mm-thick channel ceiling across the top, yielding a cross-sectional area of around 112mm2 (Figure 7).
With these parameters identified, we then worked to achieve longer channel lengths, resulting in the production of two serpentine channels within a 750x350mm plate, with each channel being over 2,500mm-long (Figure 8), proving that the scale required by the fusion industry was achievable as well as de-risking further development.
CoreFlow® was also integral to the delivery of a prototype liquid-cooled heat sink for the Innovate UK-funded SubSurface project. Designed for power electronics applications, the prototype was based upon an existing product that required four manufacturing operations; machining an open channel in the aluminium substrate, bending a copper pipe for coolant circulation, pressing or swaging the copper pipe into the pre-machined channel along with thermal conductive paste to enable conduction at the interface between the two materials, and finally skimming the top surface of the assembly to ensure an even contact between the electronic devices and the aluminium and copper parts. The use of the CoreFlow® process allowed the work to be completed in a single step, using fewer raw materials and taking a fraction of the time. The finished component was also lighter and easier to recycle than with the traditional approach (Figure 9).
To find out more about the CoreFlow® process, please email contactus@twi.co.uk.