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Abstract

Additive manufacturing of carbon fibre reinforced polymer composites enables the steering of individual fibre tows. This design flexibility has been successfully used to optimise carbon fibre composites for mechanical loading, but limited work has been carried out investigating how it could be used to enable processes that rely on the electrical properties. In this study, several carbon fibre samples are additively manufactured, and their induction heating behaviour is measured. The results indicate that a concentric layup pattern, made possible by tow steering, increases the maximum temperature reached by over 260%, compared to equivalent additively manufactured laminates with a traditional quasi-isotropic layup pattern. This demonstrates that tow steering of continuous carbon fibre can be used to alter electrical behaviour and enable new functionality. Induction welding is then used to demonstrate a practical application of this ability. Several additively manufactured samples are designed with their main structure consisting of unidirectional fibres for strength, and the joining area of the samples manufactured using a concentric pattern with improved inductance. The samples are then joined by induction welding. The results show that tow steering enables the manufacture of composite structures with one area designed for strength and another area designed to enable induction heating.

Keywords

Additive manufacturing carbon fibre electrical properties induction heating testing thermoplastic composites tow steering

Introduction

Background of induction welding of composites

Induction welding is a thermoplastic composite joining method that has been shown to offer a viable alternative to traditional adhesive bonding and fastening.^1,2 Induction welding is a process where heat is generated by an induced current, while pressure is applied to enable the intimate contact needed to join components together. Fokker Aerostructures (Netherlands) stated that the use of induction welding in the rudder of the Gulfstream G650 resulted in a 20% cost and 10% weight saving.² By enabling lightweight joining of structures, induction welding has the potential to enable more fuel-efficient commercial aviation and, hence, a reduction in emissions from aviation. Initially, induction welding was applied primarily to woven materials, however significant progress has also been achieved in induction welding of unidirectional (UD) materials.^3–6

Theory of induction heating of carbon fibre reinforced polymer composites

To heat carbon fibre reinforced polymer (CFRP) composites via induction, a global electrical loop must be formed in the workpiece and to do this the workpiece must be electrically conductive.^7,8 Figure 1 shows a schematic of the induction heating process. An alternating current in the coil induces closed eddy current loops in the sample. These closed loops generate heat in the sample in accordance with joule’s law where the heating power is given by:

P = I^{2} \frac{1}{σ}

Where

P

is the heating power,

I

is the magnitude of the eddy current and

σ

is the conductivity. When carbon fibres are embedded in polymer, the laminate’s electrical conductivity, much like the strength and stiffness, becomes highly anisotropic. The electrical conductivity depends on many factors such as the fibre conductivity, fibre distribution, fibre volume fraction (FVF) and fibre straightness.⁹ The heating mechanisms during induction heating of composites are joule heating, dielectric hysteresis when fibres are separated, and contact resistance when fibres are touching.⁷ Joule heating is heating due to the resistance of the carbon fibres and polymer. Dielectric hysteresis is heating at junctions between fibres when the fibres are separated by the fibre matrix. Contact resistance is the heating at the junction between fibres when the fibres are touching.^{7,10,11,12,13,14,15,16,17,18,10} While it has generally been accepted that joule heating is dominant in fabric materials, contact resistance between fibres was thought to be the dominant heating mechanism in unidirectional laminates,⁷ however recent modelling indicates that joule heating facilitated by current flowing through the resin from fibres in adjacent plies is the dominant mechanism.¹⁹

Figure 1.

Schematic of the induction heating process showing the induction coil, the samples and the global eddy current loop induced in the coil.

Challenges of induction Welding

A major challenge of induction welding is ensuring that the entire weld is adequately heated without overheating the surrounding composites. Induction heating heats the volume of the composite with the highest induced current occurring at the location nearest to the induction coil. Overheating can cause deconsolidation of the laminate leading to void formation, microbuckling and delamination.^1,20 To avoid deconsolidation during induction welding, process parameters such as pressure and induction current must be tightly controlled, and cooling may sometimes be used on the laminate surface.^21,22 Several studies have highlighted issues along the edge of the heat affected zone.^1,21,23–25 These edge effects occur due to difficulty in controlling the process parameters at the edge of the heat affected zone.

Laminates made from both woven and unidirectional materials have been induction welded, with induction welding of multiaxial laminates made from unidirectional plies generally considered more challenging due to issues generating and controlling heat.^3,26 To address these challenges, susceptor materials, traditionally manufactured from metals, have been used to allow sufficient heat to be induced at the weld,^1,27–29 , however, more recent work has demonstrated induction welding of unidirectional materials without the inclusion of additional susceptor materials.^3,4,6,30 Several studies have demonstrated that the heating profile can be tailored by altering the geometry of the metallic susceptor,^28,31 but each of these approaches also lead to increases in process complexity. The induction heating of woven CFRP materials has been widely experimentally investigated and modelled.^{21,23,32–38} The studies show that current can flow in closed loops in woven materials and in the case of one study, woven CFRP of 59% FVF, was successfully modelled using an isotropic conductivity value of 13,890 S/m.²¹ Unidirectional plies have all fibres running in a single direction only, and the conductivity in the fibre direction is orders of magnitudes higher (39,000 S/m) than that in the transverse or out-of-plane directions (8 S/m).39 These numbers will vary depending on what CFRP is considered and are given here for illustrative purposes only. For multiaxial laminates, manufactured using unidirectional plies, the flow of current is more difficult as the fibres are not interwoven. For current to flow in such a multiaxial laminate, the current must travel in the out of plane direction (commonly referred to as z-direction) to the next ply of the laminate or it must travel in the transverse direction (commonly referred to as y-direction). As fully UD laminates do not generate significant heat, it can be concluded that little current can travel in the y direction of individual plies and it is the z direction in which current flows between multiaxial plies enabling heating.

Research gaps

While the electrical properties and induction heating behaviour of traditional composites has been experimentally investigated and modelled, there is comparatively little work in this area relating to additive manufacturing. Additively manufactured continuous carbon fibre (CCF) grids can generate sufficient induction heating to facilitate improved consolidation⁴⁰ and the concept of using additively manufactured CCF to manufacture a susceptor material for induction welding has been patented.⁴¹ In the current paper, the ability of tow steering, made possible by additive manufacturing, to increase the inductance of CFRP laminates is demonstrated. Tow steering, in the context of the current paper, refers to automated placement of individual pre-impregnated tows of CCF along predefined paths using fused filament deposition (FFD). The CCF is impregnated with nylon to form a filament with a FVF of 34%.⁴² CCF filament used in the current work has been shown to have a fibre direction conductivity of 8000 S/m, a transverse conductivity of 11 S/m and an out of plane conductivity of 3 S/m⁴³. To the best of the authors knowledge there are no studies comparing the influence of tow steering on induction heating and no studies that demonstrate induction welding of such materials. The current work aims to address these gaps.

Aim and objectives

The overarching aim of this study is to evaluate if tow steering, enabled by additive manufacturing, can produce composites with electrical properties that can enable processes such as induction welding. The steps carried out in the current study are a shown in Figure 2. Firstly, samples are manufactured in several formats using the Markforged Mark Two printer. To assess whether the novel mesostructures, enabled by tow steering, result in heating rates sufficient for induction welding, the induction heating rates of the samples are measured and compared to the heating rate of a carbon fibre (CF) polyetheretherkeytone (PEEK) sample. Based on the heating trials, a sample format is selected for induction welding and lap shear samples are induction welded and tested.

Figure 2.

Overview of methodology for current study.

Experimentation

Heating trials

Heating trials were carried out on all the materials and layups. The CF PEEK sample was measured as a baseline to compare other materials to. The quasi isotropic CCF and concentric CCF were investigated to see if the tow steering options would change the inductive properties enough to result in significantly increased heating. Furthermore, based on the heating results a material could be selected for induction welding. To investigate the heating behaviour, all samples were exposed to an alternating current and the temperature on the surface of the laminate was measured.

Sample manufacturing

The additively manufactured samples were manufactured using a Markforged Mark Two printer. This printer can print several composite materials including short carbon fibre reinforced nylon (referred to by Markforged as Onyx), CCF nylon filament, continuous glass reinforced nylon and continuous Kevlar reinforced nylon. A schematic of the printer is given in Figure 3 showing the single print head with two separate nozzles and feeds for the Onyx and CCF.

Figure 3.

Schematic of Markforged Mark Two printer setup.

The Mark Two printer uses the Eiger slicer software to prepare models for printing. This software offers less customisability than slicers such as Cura and the user cannot edit the G code directly.⁴⁴ This means there are limited toolpaths available and nozzle temperature and roof/floor wall count are fixed by the software. In the current work the roof/floor layers on all CCF heating trial samples, which are manufactured from onyx, were removed prior to testing. The nozzle temperature for thermoplastic materials such as Nylon (and Onyx) in the printer is 275°C, while the temperature for the fibre nozzle is 255°C.⁴⁴ Two additively manufactured carbon fibre composite materials are investigated in the current study. The first composite material is short carbon fibres embedded in nylon, referred to by its brand name Onyx. Onyx material has a FVF of circa 11%⁴² and it is supplied as a filament with a diameter of 1.75 mm. The second composite material is CCF filament, with a diameter of 0.4 mm.⁴⁴ This material is referred to by Markforged as continuous carbon fibre; however, it is a continuous carbon fibre embedded in nylon with a FVF of 34%.⁴² A press formed sample consisting of a continuous carbon fibre weave, embedded in a PEEK matrix is also used for comparison. All additively manufactured samples were manufactured as part of the project. However, the CF PEEK sample was supplied by Eirecomposites (Galway, Ireland). The additively manufactured samples were Onyx, CCF with a concentric layup and CCF with quasi-isotropic layup of dimensions 40 mm by 40 mm by 2 mm. Two samples were manufactured from each material type. Table 1 gives sample details of the samples tested as part of the heating trials. Further details on the manufacturing are given in the following sections.

Table 1.

Details of samples tested as part of the heating trials.

Sample	Material	Mesostructure
CF PEEK	CF PEEK	Woven fabric
Quasi isotropic CCF	CCF nylon	Quasi isotropic
Concentric CCF	CCF nylon	Concentric
Onyx	Short fibre nylon	Short fibre

CF PEEK sample

The CF PEEK material was based on a 5 harness satin weave and was 2 mm thick with a FVF of 52%. Two samples 40 mm by 40 mm were extracted from the material for heating trials. The CF PEEK material was chosen as research has shown that this material can be sufficiently heated by an induction field to enable induction welding²¹ and the material supplier (Eirecomposites) had experimentally verified this.

Quasi-Isotropic CCF samples

The quasi-isotropic samples consisted of 14 layers of CCF material resulting in a total thickness of 2 mm. Figure 4 gives the layup of the CCF in the quasi-isotropic samples. The outer perimeter is Onyx material shown in white and the remainder of the layer is a continuous tow of CCF shown in blue. The ends of this tow are highlighted by a red dot for emphasis. The laminate layup is [ 0\ −45\ +90\ +45\ 0\ −45\ +90\ +45\ 0\ −45\ +90\ +45\ 0\ −45]. The white areas in the sample are areas which are too small to be filled by CCF and are instead printed in Onyx. Based on the FVF of 34% for the CCF filament and the fact that the outer walls are made from onyx the FVF is slightly less than 34%.

Figure 4.

Layup details of CCF quasi-isotropic sample showing 0°, -45°,90° and 45°. The red arrows denote the fibre direction, the red dots denote the ends of the fibre tow. The blue lines represent the individual fibre paths. The white lines represent areas of Onyx.

Concentric CCF samples

The concentric samples consisted of 14 layers of CCF material resulting in a sample 2 mm thick. Figure 5 gives the layup of the CCF in the concentric samples taken from the Eiger software, as well as an image of the layup taken during the layup process. The printer prints the outer perimeter in Onyx material shown in white. The printer then prints CCF shown in blue. The ends of this tow are highlighted by a red dot for emphasis. The white areas in the sample are areas which are too small to be filled by CCF and are instead printed in Onyx. Based on the FVF of 34% for the CCF filament and the fact that the outer walls are made from onyx the FVF is slightly less than 34%.

Figure 5.

Layup details for CCF concentric sample showing typical example of concentric layup. A is taken from the Markforged Eiger software and B is a photo of a sample taken during the layup process. The red arrows denotes the fibre direction, the red dots denote the end of the fibre tow, the blue lines represent the individual CCF filament path and the white areas are areas of Onyx.

Onyx samples

The Onyx samples consists of 20 layers of Onyx material resulting in as sample 2 mm thick. The samples were layed down with alternating +45° and −45° toolpaths throughout the sample.

Heating trial experimental setup

The apparatus used for testing is shown in Figure 6. The circular coil, manufactured from copper tubing of diameter 3 mm with an outer diameter of 17 mm was used for the testing. A Ceia Powercube 45/900 generator with a power rating of 45kVAr (average at the inductor) and an input current rating of 16A max was used in conjunction with a Master Controller v3 + control unit for the testing. The circular copper coil was cooled internally with a TAE Evotech water chiller. A Testo 883 thermal camera was aimed at the underside of the samples during testing with the emissivity set to .94. The coil was placed 3 mm above each sample. All samples were supported using an additively manufactured sample holder. The generator was set to 40% power, a frequency of 1046 Khz, and 40 V, and it was set to run for 1 second. These settings were chosen after trials, as they induced significant heating in the CF PEEK sample without damaging the sample due to overheating. A foot pedal was used to start the test. A timer was activated manually at the start of each test and after 3 seconds the thermal image of the underside of the sample was recorded. Two samples of each material were used, and 3 measurements were taken per sample. Each sample was allowed to cool for a minimum of 4 minutes between tests. Figure 6 shows an image of a sample placed on the sample holder prior to measurement.

Figure 6.

Test setup for induction heating trails showing the induction head, stand, thermal camera, coil, sample and sample holder.

Heating results

A summary of the results of the heating trials for all samples is given in Table 2. The figures given are based on the maximum value of the temperature measured after 3 seconds. The table gives the sample description, the result of each measurement, the average temperature for each sample type and the average temperature increase for each sample type. The average temperature increase is calculated as the average maximum temperature, minus the laboratory ambient temperature of 18°C. The table shows that the quasi-isotropic samples show an average increase of 18°C while the concentric samples show an average increase of 66°C. This represents an increase of over 260% and as such demonstrates the influence of tow steering on electrical behavior. The temperature increase for the CF PEEK sample is lower than that of the concentric sample at 51°C. No temperature increase was measurable for the Onyx material, hence these results are not presented here. The results show that the concentric tow pattern results in a higher temperature than both the quasi isotropic sample and the CF PEEK sample. This is despite the concentric CCF sample having a substantially lower fiber volume than the CF PEEK sample (circa 34 for the CCF v 59 for the CF PEEK). The results showed that the concentric CCF sample generated significant heat in the induction field. Based on this result trial welding was carried out as a proof of concept for induction welding of CCF samples. The following sections give further details of the heating trial results as well as details of induction welding trials carried out.

Table 2.

Heating trials for all samples showing the temperature after 3 seconds, the average temperature change for each sample type and the average temperature increase, ΔT (All results are in °C).

Sample description	Sample ID	Trial 1	Trial 2	Trial 3	Average	Δ T
CF PEEK	C1	69	69	69	69	51
CF PEEK	C2	68	68	68	69	51
Quasi isotropic CCF	Q1	37	35	35	36	18
Quasi isotropic CCF	Q2	36	36	38	36	18
Concentric	CF1	82	82	81	84	66
Concentric	CF2	86	84	87	84	66

Figure 7 shows a representative thermal image recorded for a CF PEEK sample, all samples showed a very similar pattern, so only a single result is presented here for clarity. The image shows that the heating pattern is a mirror image of the induction coil, this agrees with the results from literature.^21,45,46 Induction welding of fabric CF PEEK has been successfully demonstrated by several researchers, eg.^1,21,34

Figure 7.

Thermal images of CF PEEK sample.

Figure 8 shows a representative thermal image recorded for a quasi-isotropic sample, all samples showed a very similar pattern, so only a single result is presented here for clarity. The average maximum temperature reached across all samples is 36°C. To successfully induction weld nylon the temperature will have to be close to the nozzle temperature of 275°C. To achieve this temperature would not be practical, as it would require a more powerful generator and increased heating times. The maximum temperature value shows good repeatability as does the heating pattern. The heating pattern for all tests is not a mirror image of the coil, it is a symmetric pattern with the maximum temperature occurring at the sample edge.

Figure 8.

Thermal images of quasi-isotropic CCF sample.

Figure 9 shows a representative thermal image recorded for a the concentric CCF samples. The images show that the heating forms a square pattern concentrated under the coil location. The average maximum temperature across all samples is 84°C. While this temperature is well under the likely weld temperature of the material, given that the induction duration was only one second, it is likely that this material can be induction welded. Based on this result induction welding trials were carried out with this layup type. It is clear from the images that the heating pattern generated follows the concentric layup presented in Figure 5. The fiber architecture of the part is similar to the quasi-isotropic sample in terms of fiber volumes and the continuous nature of each fiber layer. The only major difference is the concentric layup of the CCF. This result clearly demonstrates that tow steering, enabled by additive manufacturing, can be used to vary the inductive properties, and consequently heating rates, of a laminate.

Figure 9.

Thermal images for heating trials carried out on the concentric CCF sample.

Induction welding trials

Based on the results or the heating trials, induction welding trials were carried out on subcomponents with a susceptor consisting of a concentric CCF layup. 12 subcomponents were manufactured and spot welded together to form 6 lap shear test samples. The samples were then tested to failure. The testing was carried out to demonstrate a practical application of altering electrical properties via tow steering.

Subcomponent manufacture

Two separate subcomponents, shown in Figure 10, were printed for each lap shear trial. The first subcomponent, which will be referred to as the susceptor layup had one region printed to give the sample strength and the other to facilitate induction. The part had a geometry 75 by 25 by 3 mm and contained the concentric CCF layup for electrical properties and, UD CCF for strength and Onyx on the bottom layer. To avoid Onyx being printed on the roof layer of the susceptor layup the print was removed from the printer when the final concentric layer was printed. The other subcomponent, which will be referred to as susceptorless layup, had a geometry 75 by 25 by 2 mm and contained only UD CCF and Onxy. The sample area used of 25 by 25 mm is much smaller than the sample size chosen for the heating trials (40 by 40). Figure 10 is an image of the two printed subcomponents prior to induction welding and a schematic of a side view of the layup of the samples on the bottom. In the schematic, the concentric layup is shown in blue, the Onyx in grey and the UD CCF in black. A total of 6 susceptor layups and 6 susceptorless layups were manufactured. The UD material was added to improve the strength of the subcomponents and avoid the samples breaking away from the weld after initial trials failed in this manner. It should be noted that spot welding individual samples is more challenging than continuous welding as the entire weld is close to the edge and end sections cannot be removed as carried out in previous research.

Figure 10.

Top: Lap shear sub components prior to joining. Bottom: Side schematic of layup of subcomponents with the concentric CCF shown in blue, the UD CCF shown in black and the Onyx shown in grey.

Induction welding

The apparatus used for induction welding is shown in Figure 11. The apparatus consisted of a stand, glass fiber clamps and the induction head. For this experimentation the induction head was connected to a robot for the purpose of further experimentation not reported here. The pressure was applied using two glass fiber blocks, shown in Figure 12, which were bolted together to apply the clamping force. Glass fiber was used as it is electrically transparent. The blocks were 30 mm thick and a 20 mm diameter, 29 mm deep, blind hole was drilled in the top clamp to allow the coil be positioned 1 mm from the sample when the clamps were assembled. The subcomponents were aligned in the glass clamps with 625 mm² overlap and held in place using high temperature adhesive tape. The clamp was bolted closed and placed below the induction coil as shown in Figure 11. With this setup the distance between the induction coil and the sample was 1 mm.

Figure 11.

Induction welding setup used for experimental trials.

Figure 12.

Left: Glass fibre clamps, sample and bolts and unwelded lap shear specimen held in place with high temperature adhesive tape, Right: Glass fibre clamps assembled prior to welding showing the assembly bolts and hole enabling the coil to be placed close to the sample.

The sample was placed in the clamps with the susceptor layup on the bottom and the susceptorless layup on the top as shown in Figure 13. Induction heating is most intense near the coil so setting the samples up like this ensured that the highest heat was generated at the weld surface. Figure 13 also shows that while the distance from the coil to the sample was 1 mm the distance from the coil to the susceptor (commonly referred to as the coupling distance) was 3 mm.

Figure 13.

Schematic of subcomponent clamped in place prior to welding. The concentric layup is shown in blue.

The welding trial was carried out with 90% power and a duration of 8 seconds and there was no relative movement between the coil and the samples during welding. This trial resulted in a successful induction welding of all samples. This is, to the best of the authors knowledge, the first demonstration of induction welding of a composite part manufactured using FFD available in the literature and the first example of a structure with one area designed for mechanical properties and another area designed for improved inductive performance. The welded samples are shown in Figure 14. All samples had compressed in the weld area and had a small circular indentation in line at the location of the induction coil hole. This occurred because the 1 mm thick glass was not as stiff as the surrounding clamp area. Furthermore, all samples had areas where the polymer had melted and pushed out over the edge of the weld surface as highlighted in Figure 14.

Figure 14.

Induction welded samples. Note: the sample surface has been marked by the grips at either end and the surface and some samples have a texture that gives them a similar appearance to fabric, however all samples are manufactured from CCF and Onyx as outlined above.

Weld strength testing and results

All specimens were tested in tension to verify the strength of the welds. An Instron 34TM-30 tensile test machine was used for testing. The sample was clamped in the grips and the test was set to run at a rate of 2 mm/min. The resulting lap shear strengths, calculated by dividing the force in newtons by the overlap area of the samples (625 mm²), are given in Figure 15. The average value was 14.2 N/mm² with a standard deviation of 1.8 N/mm².

Figure 15.

Lap shear test results of all specimens.

For all samples, the failure occurred via delamination in the composite adherend, parallel to face of the weld surface, rather than on the face of the weld surface itself. Figure 16 shows the failure surface of samples 1 and 3. For samples 1 and 4, the UD plies were delaminated in the UD section, whereas the remaining samples failed via delamination of the concentric pattern. Figure 16 shows that the samples failed via delamination in the composite and it also shows more clearly evidence of melted polymer pushing out during welding. Inspection of the failure surface, and the weldline indicates that some weld edges and the weld centre were exposed to less heat than the rest of the weld surface and the weld quality was not uniform.

Figure 16.

Image of the failure surface of samples 1 and 3 showing that samples failed via delamination of the adherend.

Discussion

The current work presents the results of heating trials of several additively manufactured materials as well as a proof-of-concept induction welding of samples manufactured using FFD without the inclusion of an additional susceptor.

Influence of concentric tow steering on sample heating

The induction heating results of the concentric CCF samples show that the concentric CCF sample generates significant heat in an induction field. This heating rate exceeds the heating rate of the CF PEEK fabric material despite having a considerably lower fiber volume. Comparing the heating of the concentric sample to the quasi-isotropic sample, illustrates that the concentric tow steering is the reason for the increased heating, as this is the only difference between the samples. Given that the fiber direction conductivity of the fabric material is likely to be close to 13,900 S/m,²¹ compared to 8000 S/m for the concentric sample and that the out of plane and transverse conductivities are low,⁴³ the tow steering must be responsible for the increased inductance. For inductive heating to occur, global current loops must form in the sample. An image with the heating pattern manually overlayed on the layup pattern of a concentric sample is shown in Figure 17. It can clearly be seen that the heating pattern matches the direction of the individual tows and as such the current is flowing predominantly along the fiber direction where the conductivity is highest. Furthermore, the fact that the heating pattern matches the layup, shows that the current is flowing in individual tows, where conductivity is highest, rather than between tows. The concentric pattern aligns the conductive paths in the fibres with the geometry of the coil and hence with the path in which the global current loops are formed. It is clear from the results that the concentric pattern facilitates the flow of global current loops in the sample.

Figure 17.

Thermal image overlayed on an image of the concentric layup with an outline of the coil shown in black.

Influence of quasi-isotropic tow steering on sample heating

The results of the quasi-isotropic CCF samples show that while some heating occurs the maximum temperature is low in comparison to the concentric CCF samples. The CCF quasi-isotropic fiber architecture differs from traditional layups in several ways. Each CCF layer is printed from one continuous piece of CCF. This means that at the edges of the sample the CCF loops back on itself, resulting in a continuous electrically conductive path between adjacent fiber filaments at the edges of the part. While this undoubtedly results in conductive paths at the edge of the sample, it is evident that this did not enable sufficient current loops to flow and generate significant induction heating. This is likely because the x and y conductivities are so low and this prevents current effectively flowing between individual layers. The heating pattern measured was not a mirror image of the coil. While induction heating of fabric material typically results in a heating pattern that is a mirror image of the coil, quasi-isotropic materials have been shown to demonstrate significantly different heating patterns.^10,37

Influence of Onyx short fiber material on sample heating

No temperature increase was measurable with the Onyx Material samples. This shows that that the Onyx material (short carbon fibre embedded in a nylon matrix) generates very little heat when subject to an induction field. The Onyx material is comprised of nylon embedded with randomly oriented short carbon fibres with a FVF of circa 11% and the fibres have a length of 168+/-37 µm.⁴⁴ While the fibres are conductive the matrix is not. The conductivity of such a system can be represented by a percolation-based model.⁴⁷ The percolation model states that for a material comprising of a randomly oriented conductive phase, within and non-conductive phase, there exists a percolation threshold. Below the percolation threshold the material will be an insulator and above the threshold the material will be a conductor with its conductance largely dependent on volume of conductor in the material. The results suggest that the 11% FVF is below the percolation threshold. The percolation threshold is dependent on a wide range of variables including the anisotropy and geometry of the conductive phase and hence this result should not be extrapolated to similar systems with higher fiber volumes or different fiber geometries.

Tow steered concentric susceptor induction welding results

The induction welding presented here is proof-of-concept which was carried out to demonstrate that the added design flexibility offered by FFD could enable processes such as induction welding. The susceptor layup is the first example of a composite component where part of the layup is designed to improve mechanical properties (UD layers required for strength) and part of the component designed to enable electrical based processes. Furthermore, the induction welding trial is the first successful demonstration of induction welding of additively manufactured CCF samples with intrinsic susceptors in literature. During welding, melted polymer was pushed out from the weld indicating overheating occurred and there was some evidence of edge effects. The concentric pattern was not optimised for temperature uniformity and there was no temperature measurement carried out at the weld surface. Based on this, it is possible that the area of the spot weld was less than the area used to calculate the lap shear strength, and the actual lap shear strengths may be higher than that reported here. Process parameter optimization, looking at applied pressure, coil geometry, sample size, coil distance, coil current and the inclusion of temperature measurement and a control loop could eliminate these defects and improve overall weld quality, but this would require additional experimentation, tooling and hardware. For all samples tested the failure occurred away from the weld surface. This, in addition to the fact that the welding setup was not optimised, indicates that the lap shear value 14 MPa represents a lower bound of the lap shear value of the induction welded joint. Recent work looking at adhesive bonding of nylon CCF showed that samples bonded with 3M, DP6301 NS resulted in a lap shear strength of 13Mpa.⁴⁸ The lap shear results in the current study of 14 MPa exceed what would be expected from commercially available adhesive used with the mark forged system. The current study was limited to low fiber volume nylon and limited fiber architectures due to the limitations of the Markforged materials and the Eiger software. Other additive manufacturing systems that do not have these constraints could be employed to further enhance electrical properties.

Broader implications and potential applications

To date, defects caused by overheating could only be addressed by tight control of the process parameters. Several researchers addressed this by removing the ends of welded assemblies^1,23 to remove the defects while others resorted to tooling arrangements that altered induction behavior of the specimen edges.²⁵ The ability to tailor the electrical properties of the laminate via tow steering offers a potential solution to these issues as it has many potential advantages for induction welding including (i) enabling local induction heating in the susceptor only to reduce cooling requirements (ii) enabling the susceptor area to be designed for electrical properties while the remaining laminate could be designed for structural properties, (iii) automating the process of adding the susceptor, (iv) enabling susceptor design with altered properties in key areas to facilitated uniform efficient heating and reduced edge effects as demonstrated previously with metallic susceptors.^28,31 Some of these advantages could also be achieved with the use of a local fabric susceptor or tight control of the process and design parameters. However, the current work shows that tow steering is a novel, highly customizable, process parameter that can be used to optimize induction welded structures.

This work has focused on induction heating and induction welding, but locally tailoring electrical properties, using tow steering, could be useful in any process that relies on the electrical properties of composites. These applications could include induction based contactless sensors, heating during manufacture, curing adhesives, de-icing, thermal control of variable topology morphing composites, lightning strike protection of composites or resistance welding. Unfortunately, there are very limited materials that can be manufactured using tow steering as defined in this paper, the process is relatively slow, and, as there is no consolidation pressure during manufacture, the laminate quality is poor in comparison to traditional composite materials.

The tow steering, made possible through additive manufacturing, is suitable mainly for thermoplastic based continuous composites. This design flexibility showcases a potential advantage of thermoplastic composites over their thermoset rivals.

Conclusions

The aim of this study is to evaluate if the design flexibility of additive manufacturing can result in electrical properties that could enable processes such as induction welding. Several CFRP samples were additively manufactured from short carbon fibre nylon filament and CCF. The heating rates of the samples were measured and compared to a weldable CF PEEK fabric material. Based on these results the material with the highest heating rate was selected, and proof-of-concept induction welds were carried out. The results provide a valuable insight into the heating rates of additively manufactured CFRP and demonstrate that tow steering can enable new processes that are dependent on electrical properties. The work presented here may enable more efficient processes from an energy and cost perspective. In addition, it indicates that it may be possible to replace metallic susceptors in CFRP structures with additively manufactured CFRP, improving the recyclability of these structures. Furthermore, by integrating these functionalities into the structure, at a local level, it is likely that the overall weight and hence the environmental cost could be reduced. Future work will focus on modelling the induction heating process to facilitate the design of tow paths optimised for electrical functionalities.

The following conclusions can be drawn from the work:

(1) The concentric layup pattern, enabled by tow steering, increased the maximum temperature reached by over 260%, compared to equivalent additively manufactured laminates with a traditional quasi-isotropic layup pattern.

(2) The increased temperature measurement indicates that the concentric samples increased inductance and hence increased global electrical conductivity.

(3) Induction welding can successfully be achieved using CCF additively manufactured using tow steering to increase inductance.

(4) FFD enables the production of carbon fibre parts which are tailored for mechanical properties in one location and electrical properties in other locations.

Footnotes

Acknowledgements

This research is supported by the Department of Business, Enterprise, and Innovation and administered by Enterprise Ireland under the Disruptive Technologies Innovation Fund, Mi-Drone Project. The authors would also like to acknowledge the support from Science Foundation Ireland through the MaREI Research Centre for Climate, Energy and Marine, I-Form Centre for Advanced Manufacturing, and Enterprise Ireland through Construct Innovate.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article

Funding

This research is funded by the Department of Business, Enterprise, and Innovation and administered by Enterprise Ireland under the Disruptive Technologies Innovation Fund, Mi-Drone Project (Ref: DT 2020 0221). The authors would also like to acknowledge the financial support from Science Foundation Ireland through the MaREI Research Centre for Climate, Energy and Marine (Grant no. 12/RC/2302_2), I-Form Centre for Advanced Manufacturing (16/RC/3872 and 21/RC/10295_P2), and Enterprise Ireland through Construct Innovate (TC-2022-0033).

Use of AI

During the preparation of this work the authors used Microsoft Copilot and Microsoft word Editor to improve language and readability. In addition, Google Gemini was used during online searches. After using these tool/service, the authors reviewed and edited the content and take full responsibility for the content of the publication.

ORCID iD

Michael Flanagan

Data Availability Statement

Data will be made available on request.*

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Enabling electrical functionality of carbon fibre composites via tow steering during additive manufacturing

Abstract

Keywords

Introduction

Background of induction welding of composites

Theory of induction heating of carbon fibre reinforced polymer composites

Challenges of induction Welding

Research gaps

Aim and objectives

Experimentation

Heating trials

Sample manufacturing

CF PEEK sample

Quasi-Isotropic CCF samples

Concentric CCF samples

Onyx samples

Heating trial experimental setup

Heating results

Induction welding trials

Subcomponent manufacture

Induction welding

Weld strength testing and results

Discussion

Influence of concentric tow steering on sample heating

Influence of quasi-isotropic tow steering on sample heating

Influence of Onyx short fiber material on sample heating

Tow steered concentric susceptor induction welding results

Broader implications and potential applications

Conclusions

Footnotes

Acknowledgements

Declaration of conflicting interests

Funding

Use of AI

ORCID iD

Data Availability Statement

References