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CoreFlow™: A Sub-Surface Machining Process

Introduction

TWI has recently invented a highly disruptive, new sub-surface machining technique called CoreFlow™. This solid state process is a development of friction stir welding (FSW) and friction stir channelling (FSC) which allows for sub-surface networks of channels to be integrated into two-dimensional or three-dimensional monolithic parts in a single manufacturing step. These channels could then be used for heat exchange or other applications.

Figure 1 shows a cooling plate demonstrator recently produced in aluminium (AA6082-T6) and infrared thermal imaging evidencing the circulation of liquid coolant.

CoreFlow - A Novel Sub-Surface Machining Technology from TWI - CGI Video

History and Development of CoreFlow™

TWI is at the forefront of solid phase friction welding and processing technology. Active and innovative in welding research and development since the 1960s, TWI has been responsible for many key innovations and developments in solid phase joining. The most notable examples are friction stir welding (FSW), which was invented at TWI in 1991, and the development of linear friction welding (LFW) into a mature joining process for turbine blades.

FSW uses a rotating tool designed to generate frictional heating and soften the adjoining metal to a point where it can be viscoplastically deformed and displaced around the tool (Figure 2). The FSW tool traverses the joint line between two parts, where its stirring action enables the formation of a solid-state weld. The resulting joint has excellent tensile strength and fatigue and fracture properties. The tensile strength can be within a few percentage points of the parent material, with the fatigue strength very similar and fracture resistance at least as good as the parent material.

Friction stir channelling (FSC) is an innovative solid-state process, derived from FSW, for integrating sub-surface networks within metal structural elements. In 2005, the original FSC concept was patented by Rajiv S. Mishra. In his work, poor material consolidation during FSW was deliberately promoted in order to produce a continuous void along the tool path (Figure 3). However, despite its great potential, this technology has not yet been adopted by industry, mainly by not fulfilling the required repeatability, surface finish or design flexibility.

TWI has recently invented and patented a new stationary shoulder variant of FSC (SSFSC) which has proven to overcome many of the drawbacks of conventional FSC. As shown in Figure 4, a stationary shoulder 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 cause part of the nugget material to be conveyed into the shoulder. The material extracted is then re-directed towards a series of vents in the shoulder and expelled.

As the tool assembly traverses along a pre-defined path, the process of extracting the material leads to the formation of i) a closed channel within the workpiece and ii) the production of extruded wire. Process parameters and tool designs are combined such that the rate of material extraction per unit distance is limited to maintain a fully consolidated channel ceiling.

Figure 5 shows the four main stages in the process. The cycle is initiated by plunging the rotating probe into the plate (Figure 5b). Once the probe reaches its target plunge depth, the machine initiates its traverse motion (Figure 5c). As the tool traverses along the workpiece, the plasticised material is conveyed upwards by the rotating probe threads, into the shoulder and then expelled as extruded material (Figure 5d). This subtraction of material leads to the formation of a closed sub-surface channel.

It is interesting to note that the material extruded from the plate in the form of wire (see Figure 6) could be used as feedstock for other processes, like wire-based additive manufacturing, or could be used simply as welding wire. CoreFlow™, indeed, is capable of extruding an unlimited length of wire from a plate or a pipe.

This process, labelled ForgeWire, is of great interest for producing spool of wire from materials with poor extrudability, such as alloys of aluminium or magnesium. Wire manufacturers or additive manufacturing process developers now have a quick option for producing wire with a tailored chemical composition and from experimental alloy formulations (e.g. Aluminium-Lithium or Aluminium-Scandium), potentially even directly from rolled product or cast billets.

TWI successfully demonstrated the extrusion of AA1050-H14, AA6082-T6 and AA6082-T6 in diameters of 3 mm and 6 mm.

Figure 1. Cooling plate demonstrator produced by CoreFlow™ with an infrared thermal image of liquid coolant circulating through the heated plate demonstrator
Figure 1. Cooling plate demonstrator produced by CoreFlow™ with an infrared thermal image of liquid coolant circulating through the heated plate demonstrator
Figure 2. FSW process schematic
Figure 2. FSW process schematic
Figure 3. Schematic representation of the conventional FSC method
Figure 3. Schematic representation of the conventional FSC method
Figure 4. CoreFlow™ tooling description and main process parameters
Figure 4. CoreFlow™ tooling description and main process parameters
Figure 5. Main stages of CoreFlow™: (a) Start of probe spindle rotation; (b) Plunge into workpiece; (c) Shoulder contacts the workpiece; (d) Formation of sub-surface channel as the tool traverses
Figure 5. Main stages of CoreFlow™: (a) Start of probe spindle rotation; (b) Plunge into workpiece; (c) Shoulder contacts the workpiece; (d) Formation of sub-surface channel as the tool traverses
Figure 6. Spool of wire as by-product of CoreFlow with its cross and longitudinal sections
Figure 6. Spool of wire as by-product of CoreFlow with its cross and longitudinal sections

Demonstrators and Channel Cross Section

In the last few years, the CoreFlow™ concept has been demonstrated and further developed by TWI. AA6082-T6 and AA1050-H14 plates have been successfully processed, with a thickness varying from 5 to 50 mm. Moreover, flat and tubular demonstrators (see Figure 9) were successfully manufactured, featuring channels along linear, curved and helical trajectories. The demonstrators passed both leaking and pressure testing, with leak rates well below 10-8 mbar∙L/s and pressure up to 9 bar.

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 7. From the micrograph it is also possible to appreciate how the anisotropic grain orientation in the parent material, caused by the plate sample rolling process, transforms into a fully recrystallized microstructure in the channel ceiling due to the stirring action of the probe in the viscoplastic material.

 

Applications and Use Cases of CoreFlow™

CoreFlow™ already looks set to find revolutionary applications in the manufacture of heat exchangers, cooling systems, integrated fluid management and the general light-weighting of structures. Heat exchangers can be found everywhere from cars to aeroplanes, but also communication platforms, satellites and ships.

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 to the public. These expectations are resulting in increased heat generation and are driving a collective demand for low cost, compact, lightweight and efficient heat transfer solutions.

Battery packs release heat during charging and driving due to electric resistivity. This is aggravated for higher intensity charges and discharges (e.g. fast charging or high loads during operation). Currently, serpentine pipe heat exchangers are incorporated inside the battery pack to prevent excessive temperatures that could lead to cell chemical degradation or battery derating. These systems, however, take up space and add weight and manufacturing complexity to the vehicle, therefore, CoreFlow represents a big opportunity to solve these challenges by manufacturing battery trays with integral cooling channels built into the metallic structure.

Another interesting example is the thermal management of electric motors. These components heat-up mainly due to the resistivity of the copper windings and eddy-current losses, and this excess heat is detrimental to motor performance and may result in permanent physical damage. Cooling systems are usually incorporated into the casing based on natural or forced convection (of air, liquid or both). CoreFlow™ could be used to incorporate channels in thin-walled motor casings for active cooling, reducing overall weight, complexity and improving the performance of the motor.

Aerospace is another industry in which thermal management solutions could be implemented using the CoreFlow™ process. Aircraft engine cooling, for example, is usually performed by heat exchangers located between the engine and the nacelle, cooling the engine oil using air or fuel. These components, however, disrupt the airflow, creating drag and decreasing thrust output. With CoreFlow™ cooling networks could be incorporated directly in the nacelles. The same applies to hydraulic cooling systems, which are usually installed under the wing surface and could be replaced by integrated manifolds manufactured by CoreFlow™, rather than hoses, pipes and fittings.

The channels produced by CoreFlow™, could be used also for anti-icing systems or for embedding instrumentation in metallic surfaces or for reducing the aircraft heat signature.

Currently, 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 8). Typically, in the first stage, a housing is casted, extruded or machined from a solid block of metal (usually aluminium or copper) which incorporates cooling features and channels to circulate the cooling fluid through the part. In the second stage, a lid is joined and friction welded in order to isolate the cooling channels from the environment and seal the component. FSW has become the most effective choice to perform the second stage of this manufacturing process.

CoreFlow™ overcame these challenges by machining the cooling channels in the part in a single step. Helical serpentines can be successfully incorporated into aluminium piping or housings to create a thermal management functionality. By creating a channel below the surface of a structure, CoreFlow™ provides an integrated method to vent heat from a part without having to add extra pipework or other complex and costly solutions.

This translates not only to a simpler process, but also to a more efficient and environmental friendly manufacturing method, using approximately 20% less raw material, producing almost 80% less waste (in form of wire), and therefore weighing less than its conventional counterpart (see Figure 8).

Not only will this new technique allow for heat reduction, but can be used for the production of anti-icing features on wings and flight control surfaces for aircraft. The technique can also be used to create lubrication networks for hydraulics, to embed instrumentation into a structure, to perform cable management, or simply for additional light-weighting. Outside of transport, CoreFlow™ can be used for the cooling of data servers, communication infrastructures and radar installations or to manage the thermal load in manufacturing equipment, for example in the semiconductor or display manufacturing industry.

TWI is continuing to develop CoreFlow™ by defining guidelines for use with different workpiece materials, while working on a range of industrially-relevant technology demonstrators.

With a variety of applications having already been proposed for CoreFlow™, this new friction technique could soon be used in industries ranging from aerospace and automotive to electronics and sensors.

Figure 7. Micrograph of channel cross-section processed with CoreFlow™
Figure 7. Micrograph of channel cross-section processed with CoreFlow™
Figure 8. Comparison of current manufacturing practice versus CoreFlow™ for planar heat exchangers
Figure 8. Comparison of current manufacturing practice versus CoreFlow™ for planar heat exchangers
Figure 9. Tubular demonstrators
Figure 9. Tubular demonstrators
Avatar João Gandra Principal Project Leader – Friction and Forge Processes

João specialises in friction welding processes, including Friction Stir Welding. His current role is to support TWI Member companies seeking to adopt these technologies to manufacture new or existing products. He acts as a consultant during product development, design-for-manufacturing, prototyping, technology transfer and continuous improvement. Most of his experience was gathered in the aerospace, rail and automotive sectors. Before joining TWI, João completed a PhD in Manufacturing and Industrial Management at the Technical University of Lisbon, where he also worked as part-time lecturer and researcher. He has published over 20 peer-reviewed publications and conference papers, actively participating in international standards committees like the ISO 25239 for Friction Stir Welding.

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