Vito Di Pietro
Senior Project Leader, TWI Ltd.
INTEGRAL COOLING CHANNELS Thermal management of power electronics by friction stir channelling
TWI has recently invented a new technique, derived from friction stir channelling called CoreFlow that allows for sub-surface networks of channels to be machined within monolithic metallic parts in a single step.
Friction stir channelling is an alternative and efficient manufacturing process for thermal management systems, such as cold plates for IGBTs, LEDs, CPUs/GPUs or integrated liquid cooling enclosures for electric motors, batteries, power supplies, etc.
The thermal management challenge
The trend to increase power densities in the electric and electronic market, is leading to an increased heat generation and driving a collective demand for low cost, compact, lightweight and efficient heat transfer solutions.
In the electric vehicle (EV) market, for example, there is an increasing need for faster-charging rates, improved speed and autonomy, increased power density, and a general ambition to make EVs affordable. Batteries and power electronics release heat during charging and driving, and this is further aggravated for higher intensity charges and discharges. Currently, serpentine pipe heat exchangers are incorporated to prevent excessive temperatures. These systems take up space, add weight and manufacturing complexity to the vehicle, therefore friction stir channelling represents a huge opportunity to solve these challenges by manufacturing batteries or power modules with integral cooling channels built into their metallic enclosures.
New technology derived from friction stir welding
Derived from friction stir welding (FSW), CoreFlow is an innovative solid-state process that integrates sub-surface networks within metal structural elements. Figure 1 shows a 10 mm thick plate of aluminum AA6082-T6 processed by CoreFlow in order to create a cold plate with a serpentine cooling channel under its surface.
TWI has recently invented and patented a new stationary shoulder, as shown in Figure 2, which is employed to confine the nugget of viscoplastic material, limiting the flow of material extracted by the probe. With the appropriate rotation direction, the geometrical features of the probe causes part of the nugget material to be conveyed upwards, inside the shoulder. The material extracted is then re-directed through a series of orthogonally positioned apertures within the shoulder. This allows for the extracted channel material to be extruded as a solid, continuous length of wire. As the tool assembly traverses along a pre-defined path, the process of extracting the material leads to the formation of a closed channel within the workpiece and the continuous production of the extruded wire.
The channel cross-section geometry varies from rectangular to triangular, depending on the parameters used. All channels feature a flat bottom surface, with well-defined edges, coincident with the probe outline, as shown in Figure 3. The dimensions of the channel, including its depth, mainly depends on the geometry of the probe. The ceiling thickness also depends on the process parameters used as well as on probe design. Increasing the probe rotation speed, for example, will increase the extrusion rate, leaving less material behind in the channel and therefore creating a thinner channel ceiling.
Demonstration of friction stir channelling and its potential impact
As shown in Figure 4, aluminium plates with a thickness from 5 to 50 mm and tubular demonstrators that feature channels along linear and helical trajectories have been successfully produced. The demonstrators passed both leak and pressure testing, with leak rates well below 10-8 mbar∙L/s and pressure up to 9 bar, without bursting.
Currently, friction stir welding (FSW) is one of the most promising technologies for manufacturing heat exchangers. Their complex geometry currently forces engineers to split production in two stages (see Figure 5a). Typically, a housing is machined from a solid block of metal that incorporates cooling features and channels to circulate the cooling fluid through the part. In the second stage, a lid is joined to isolate the cooling channels from the environment. FSW has become an attractive technology for joining aluminium devices. Alternatives to FSW are brazing, welding and mechanical fastening.
Friction stir channelling has overcome these challenges by machining the cooling channels in a single step (Figure 5b). By creating a channel below the surface of a structure, CoreFlow provides an integrated method to dissipate heat from a part without having to add pipework, gaskets or other complex and costly solutions. This creates a simpler, more efficient manufacturing method, using approximately 20 % less raw material, producing almost 80 % less waste (in form of wire), and weighing less than its conventional counterpart.
By machining and re-sealing the cooling channels in a single step, the new technique consolidates multiple manufacturing operations, inherently offering an advantage compared to traditional technologies (e.g. milling, followed by sealing via brazing/bolting/welding).
This video shows how the new sub-surface machining technique works: