Patent Application
20170136694
This invention relates to the additive manufacture (AM) of composite materials reinforced with structural fibres or particulates.
AM of composites is currently only suitable for those composites limited in performance by way of usage, temperature, strength or stiffness, such as thermoplastic polymers like nylons and those with a low volume fraction of discontinuous/particulate reinforcements. This limits their usage to low performance structures and non-safety critical applications. It is, however, possible to make metal AM parts that meet such structural and safety critical part requirements
Manufacture of conventional polymer composite laminates using high performance fibres involves laying up long reinforcing fibres in a plastics matrix material. Whereas short fibre or particulate reinforced AM materials are available, such materials are not available with any preferential alignment of the fibres or particulates, which would allow increased directional performance. The incorporation of long fibres, for example, carbon or glass fibres, in the AM process is problematic.
It is an object of the invention to address at least some of these shortfalls and to enable net-shape manufacture of higher performance composites.
According to one aspect of the present invention there is provided a method of forming a composite component by additive layer manufacturing, the method including the steps of providing an elongate tape of carbon nanotubes (CNTs), applying stretching force to the tape whereby to align the carbon nanotubes to the tape and form an aligned tape, impregnating the tape with matrix material, forming the aligned tape into portions of required size and transferring the portions as required to a component build location whereby to form the component, layer by layer.
The concept builds on the recent developments enabling alignment of carbon nanotube arrays in fine unidirectional 2D planes. These arrays can be laminated or consolidated into high strength or stiffness composite laminates with properties exceeding those of conventional aerospace grade composites, for example, 8552/IM7-Hexcel.
The present invention aims to attain AM by constructing a conveyor process delivering fine tapes of aligned CNT (typically 1-10 mm wide and of varying lengths) which may be assembled and consolidated in-situ to build a tool-less composite component. The width of the tape may be selected according to need. For example, a component with no narrow features, like thin walled sections, or small, intricate features such as apertures with narrow border areas may allow wider tapes to be used. Whereas a component with such features or sections may require a tape in the narrower range in order to obtain the necessary degree of manufacturing precision.
AM processes are those depositing material as and where it is required, whether in powder, liquid or solid form. An example of this is Fused Deposition Modelling (FDM). Here the material is heated prior to deposition and it adheres to and consolidates with the substrate or previous layer of material when it cools. Alternatively, energy (e.g. thermal or ultraviolet) may be provided, once the material is deposited, and this causes the deposited material to adhere to and consolidate with the substrate. Another type of AM process is the selective consolidation process, in this case a layer of material is deposited, and then a selective region of the material is consolidated. Consolidation can be either by applying radiation (e.g. thermal or ultraviolet), or by an adhesive. Powder bed or stereo lithography are examples of AM processes using selective consolidation.
The CNT source material can be provided by different methods, with varying degrees of scalability. Suitable CNT source materials may include vapour catalysed veils, spun forest arrays and solution-produced bucky-paper, comprising an array of CNTs, forming a 2D paper-like sheet. The CNT source will be represented by the word “tape” which, as used herein, is to be understood to include the above CNT source materials and any suitable elongate lengths of CNT material, whether matted, woven or otherwise, and also rovings or yarns which may not be flat in cross section.
In order to attain the required level of CNT alignment in the tape, the tape may be progressively stretched in the range of 10-50% strain over the original length. Stretching is achieved through the application of a controlled stretching force in a direction which will generally be longitudinally of the elongate tape. Investigations indicate that optimum alignment can be attained with approximately 40% of even stretch. A challenge which the inventors have had to overcome is to achieve even straining without localised necking in the tape. The best technique found was to use a series of double rollers. Traction may be provided either by pinching the tape between counter-rotating rollers or by friction, from wrapping the tape around one or more rollers.
Differential speed between successive rollers or sets of rollers is used to attain the target strains. The level of stretch between each successive roller or set of rollers may be controlled to result in 5-10% strain. Consequently, obtaining a target stretch of 40%, for example, requires 4 to 8 roller stations. The stretching strain rate is also critical and should be controlled in the range of 1×10−3 to 1×10−2 per second. The rollers may be made from smooth and low surface energy materials such as low coefficient of friction polymers or PTFE. In some arrangements, use of a soft elastomeric roller surface material may be advantageous and the roller size may be increased.
In order for the tape to achieve uniform alignment it may be necessary to employ a secondary medium to lubricate the dry tape. This may be a volatile solvent, for example, acetone or IPA or even water, in which case the continuous alignment process needs to allow for a dry-off step before the resin is applied. Alternatively, the lubrication may be via impregnation with a liquid resin, for example, epoxy or polyester, or solid thermoplastic resin, which may later form a matrix in the additively manufactured composite component. In this case, after the impregnated resin has acted as the lubricant, it may need to be further activated, through heat or a chemical catalyst, to undergo a cure or re-melting for the consolidation process. As a further alternative, the lubricating agent may be an oil or low molecular weight organic liquid that may be washed off the CNT aligned tape before impregnation with composite matrix resin.
Alternatively, the longitudinal strain may be attained through use of a double vacuum diaphragm technique whereby the tape is retained within a first vacuum bag, which in turn forms a sealed bag over a curved tool. Formation of vacuum in a second bag stretches the bagging film over the curved tool, thereby applying tensile strain to the first bag and its contents. Similarly, stretching may be obtained by temporary bonding onto an elastomeric backing strip, whereby uniform strain applied to the backing strip promotes even stretch in the CNT tape.
The CNT tape used with the roller stretching arrangement may be fed from a continuous semi-infinite spool. However it is possible to run the process with considerably shorter lengths of tape, down to perhaps 20 cm in length. Where longer lengths of tape are not available, it is possible for shorter lengths of tape to be joined or spliced to create longer lengths, to enable an efficient continuous process line. In such cases the joint lines may be marked out and may be excluded from use in later stages of the additive manufacturing process.
The width of tape may be typically controlled in the range of 0.5-5 mm. The tape's areal weight may be in the range of 2-30 grams/sq m and more preferably in the range of 3-15 grams/sq m.
If the tape is in the form of a yarn, of nominally circular cross section, the yarn may be in the range of 2-50 Tex (i.e. grams per Km) and more preferably 5-10 Tex.
Upon leaving the alignment module the aligned tape may be surface treated to impart increased covalent functionality or hydrophilic characteristics to enhance CNT/matrix interaction in order to improve the properties of the resulting composite material.
The tape then enters a resin impregnator unit where resin may be added in the form of a liquid, slurry, spray or dry powder. The impregnated tape may then be deposited as a layer on a substrate or upon a prior layer, through a nozzle applicator as part of an AM build process.
The resulting pre-impregnated fibre tape may achieve fibre (CNT) weight fractions in the range of 10-60 wt % CNT, delivering increased reinforcement efficiency from that presently known.
According to a further aspect of the invention which may be suitable for powder bed or other selective consolidation additive manufacturing techniques, following fabrication of the impregnated tape emerging from the impregnator/consolidator/feeder, the tape is segmented by cutting, chopping or otherwise into particulates for further processing. Micromechanical calculations suggest that aligned CNT arrays in the range of 10 to 200 microns will provide substantial and increasing reinforcement effects. The pre-impregnated aligned tape may be chopped into fine aligned particles in or above the above range using a guillotine or milling or contactless process. It is possible that the segmentation process may be carried out in multiple stages, for example, ˜5 mm long guillotined pieces processed to a finer particle size using milling or cryo-milling.
The deposition direction of the tape may be controlled to enable addition of any in-plane direction, e.g. to achieve a cross ply or quasi-isotropic lamination lay-up. For example, fibre alignment in the directions 0°, ±30°, ±45°, ±60°, 90° is often used, in various high-strength structures but any chosen set of directions may be adopted, according to the nature of the invention. Out-of-plane orientations, as required, may also be achieved, using the method of the invention, as explained below.
Thus, the example of a work piece in the shape of a cube may be used. Laying the tapes on top of each other on the X-Y plane would mean that all the load carrying fibres (or CNT filaments) would be in the X-Y plane. In order to get some filaments aligned in the Z direction, the sides of the component may be clad with tape layers during the build. When a core of the component is partially built up with successive X-Y plies, either it or the applicator nozzle may be rotated to allow the sides of the component to be clad with tape. Once sufficient Z-plane tape build-up has been achieved, the component build may then revert back to the X-Y plane. This alternating process may continue for as long as the component design dictates.
As an alternative, a secondary deposition nozzle may also be deployed to provide disposable supporting structure, or “scaffold” structure, for any component overhangs. This way Z-oriented plies may be laid up on a support structure which can be removed at the end of the build. This is a known practice in 3D printing where any major overhangs in the component are printed on top of a strategically printed scaffold. The scaffold is made from a cheaper and easily machinable or soluble material.
The added resin may be a thermoplastic, e.g. nylon, ABS, PC, PPS. More preferably, for high strength use, it will be a thermoset, for example, epoxy, BMI or polyester. In the case of thermoplastics, a melting heater control may be incorporated in the nozzle, similar to that used in fused deposition modelling techniques. In the case of thermosets, an external curing source, for example, an ultraviolet or infrared source or a laser may be used to cure or partially cure the deposition layers.
With the process of the invention it is possible to selectively deposit conductive patterns in the 3D structure of the component to impart additional functionality such as de-icing heater tracks, integrated strain sensors, integrated antennae, frequency selective surfaces or selective electromagnetic/radar absorbing layers. To expand the range of such functionality, the aligned tapes may be additionally metallised, for example, by the application of a metal particulate loaded binder or by an electrochemical process.
Multiple deposition heads may be used, with each head providing fibres with different functionalities. Thus, the surface treatment unit, illustrated, may be replaced by a functionalization unit, which could then additionally be used for surface treatment or application of secondary materials such as metallic coatings for, say, sensing applications.
Examples for such high performance composites produced using net-shape AM include aircraft structural components such as wing box ribs, stringers, brackets or structural sub-assemblies; wing control surfaces, for example, slats, flaps and spoilers; semi-structural components such as access panels or avionics packages. It is noted that the size and scale of such articles may be much smaller and miniaturised for unmanned vehicle applications.
Further applications may include manufacture of space frames or geodesic composite structures when a series of internal struts, fins or cell structures may be optimally designed to provide tailored load paths in a weight efficient architecture. Numerical techniques are often employed to attain topologically optimised load paths for strength, stiffness or damage tolerance.
According to another aspect of the invention, once the aligned tape is produced, it is deposited on the substrate or work piece using laser ablation.
In a first method, a laser ablative process called LIFT (Laser Induced Forward Transfer) is used. LIFT is a known process. For this process, the aligned tape to be deposited is attached to a backing tape to form a supported tape. The LIFT process is enabled by a super-heated vapour being generated by the laser between the aligned tape and the backing tape. In most cases sufficient vapour is generated from contaminants and other coatings that may be between the aligned tape and the backing tape (such as an adhesive layer). However, if necessary, an interface between the backing tape and the aligned tape can be deliberately seeded with materials to improve laser absorption and hence improve the generation of vapour to enable the LIFT process.
The supported tape is placed in close proximity to the substrate, for example in the range of 10 s of microns to 500 microns, with the aligned tape facing the substrate. A laser pulse is then discharged to irradiate a small region of the supported tape. In the irradiated region, the laser pulse causes a portion of the aligned tape to be lifted off the backing tape and projected onto the substrate. The process is then repeated with the laser being directed at a fresh portion of the tape until all usable aligned tape in a given region thereof has been used up and a fresh region is exposed to the laser to be progressively projected onto the substrate.
Relative movement is effected between the laser and the tape and between the tape and the work piece, during the tape deposition process, in order to allow the laser to irradiate different portions of the tape and for different portions of the tape to be positioned over differing portions of the work piece. These relative motions may be organised in the most suitable way for the particular conditions of a particular work piece and particular ALM tooling. It is generally envisaged that 100% of the aligned tape would be deposited, save for particular geometric arrangements of work piece dictating otherwise, like certain corner regions, for example.
A deposition head of the apparatus, for use with aligned tape, may include a feed spool of tape, feeding supported tape past a deposition region to a collection spool for the backing layer and any remaining aligned tape attached to the backing layer. The head may also include a pulse laser and means to move the direction of laser focus with respect to the tape. The work piece may also be movable with respect to the aligned tape, for example, laterally and/or longitudinally of the tape and rotationally with respect to the tape.
In a second method, no backing tape is used. Instead, the aligned tape is placed directly on the substrate (or close to it); it is then irradiated with laser pulses, causing irradiated parts of the tape to be cut and deposited on the substrate. In order to allow for the aligned tape losing much of its material, following deposition of irradiated regions of the tape, the aligned tape may be pushed as well as well as pulled across an irradiation region.
For both methods, once the portions of aligned tape are deposited, their cohesion with the substrate, or preceding layer, by consolidation, can be further enhanced by post processing. Post processing may be in the form of heating with a radiation source, for example, laser diode, infra-red laser, microwave source, ultrasonic source, etc. During such post processing, consolidation may benefit from simultaneous application of pressure.
Once the aligned tape layer is deposited, it is coated with a thermoplastic/resin substrate coating which is the host for the composite material. The whole process is repeated layer by layer to build the 3-dimensional component.
Alternatively, the aligned tape may be pre-impregnated with resin, to form either a prepreg. or partial prepreg., in order to overcome the need for the application of a resin layer.
A suitable laser for the LIFT process would be a Q-switched Nd:YAG laser, producing a laser energy of 100 mJ or more. The precise values depend on the laser pulse length, area of aligned tape to be deposited using lift and support tape thickness. Smaller energies may also be suitable, in certain circumstances. Other types of pulsed lasers with similar laser parameters could also be used. Lasers with shorter pulse lengths, of the order of pico-seconds or femto-seconds could also be used.
According to a further aspect of the invention, following fabrication of the impregnated aligned tape, the material may be cut, chopped or otherwise segmented into particulates for further processing. Micromechanical calculations suggest that aligned CNT tapes having CNTs in the size range of 10 to 200 microns will provide substantial and increasing reinforcement effects. In this aspect the pre-impregnated aligned tape may be chopped into fine aligned particles in the above size range using a guillotine or milling or contactless process. It is possible that this latter process may be carried out in multiple stages, e.g., ˜5 mm long guillotined pieces processed to a finer particle size using milling or cryo-milling.
The resulting particles containing unidirectional arrays of CNT may then be processed as feed stock in a powder bed or other selective consolidation AM process. Prior to selective consolidation, the particles in a layer to be processed may be aligned in a desirable, unified direction with the application of an aligning influence, such as an ultrasonic field or local electric or magnetic field, prior to consolidation. The matrix material may, again, be a thermoplastic or a thermoset. In the case of the latter the heating source, e.g., laser, can be used to promote consolidation as well as cure.
The field may also be imposed to preferentially align and rotate the aligned particles in the X, Y and Z directions to attain tailored fibre reinforcement in desired directions, throughout the composite component.
For intermediate fibre lengths (>200 microns to mms), ferromagnetic (or other) markers may be deposited on the aligned tape at distances equivalent to the lengths of short fibres required. These markers will be used to aid the alignment of the fibres.
After stretching the tape, the CNTs become aligned. The aligned tape is then marked with a ferromagnetic material at intervals of desired fibre ribbon length. Marking could be done with any suitable ink deposition tool, such as an ink jet, extrusion nozzle. Marker dimensions may be off the order of 10-100 microns but may be smaller or larger. Marker sizes are optimised to provide suitable sensitivity to the markers and their movement through the resin binder. Thus their size is subject to the size, position and proximity of the magnetic field and to the viscosity of the resin. Once the aligned tape is marked, the fibre ribbons are sliced to a desired length.
For additive manufacture by the powder bed or other selective consolidation process, stock material may be made either by mixing dry unimpregnated fibre ribbons with resin, prior to deposition, or depositing a layer of resin and then depositing the dry fibre ribbons onto the resin. At this stage the fibre alignment of ribbons with respect to one another is random. A magnet is then passed over the resin/ribbon mix. The magnetic field interacts with the ferromagnetic dots and, as it passes over them, the dots are forced to move towards the magnet. Viscosity and surface tension of the resin prevent the fibre ribbons from leaving the resin and attaching themselves to the magnet (which in the absence of the resin they might do, with a strong enough magnet field). Furthermore the viscous drag from the resin on the fibre ribbons limits the movement of the ribbons to rotational movement, as if the tails of the ribbons were pinned.
The invention will now be described with reference to the accompanying drawings, of which:—
a) to e) show a sequence of steps according to the invention for transforming a UD array of CNT prepreg to a sintered array of aligned CNT composite;
Referring to the drawings,
The tape 2 then enters a tape alignment stretching module 3, as further illustrated in
When the aligned tape 2 leaves the alignment module 3 it is subject to surface treatment by plasma at a plasma treatment station 4. Such surface treatment assists impregnation of the tape 2 with matrix material.
At this stage, the tape 2 can also be treated with a metallic coating 7, if a layer for de-icing, for example or strain sensing or if an integrated antenna or frequency selective surface or selective electromagnetic/radar absorbing layer is required.
The aligned tape 2 next enters an impregnator, consolidator, feeder module 5 where liquid resin is sprayed onto the tape 2 from a resin applicator 6. The sprayed tape 2 is then rolled by one or more rollers, in a consolidator stage within the module 5, to more evenly impregnate the tape with resin. Next, the tape 2 can be either directly fed from a feed nozzle in the module 5 (not separately shown) onto a substrate 11 to commence fabrication of a component 8 layer by layer (see discussion of
In the case of component build on a substrate 11, further heat, pressure and additional resin may each be optionally applied to the CNT tape 2, during its laying onto the substrate. And, once laid, the tape 2 may be trimmed before a succeeding layer or layers are applied. It will be appreciated that if a tape 2, of say 5 mm width, is used and the component is wider (or longer) than this, more than one width of tape will be required to cover a layer of the component. It is also within the scope of the invention to use tape sources of differing width, and indeed to mix types of tape, in order to build components which demand such design flexibility.
Turning now to
The stretching/aligning/feeder rollers 12 and consolidation roller(s) 14 are mounted in a deposition head 15 which also houses the tape cutter 9. Movement of the deposition head 15 relative to the component 8 is in the direction of arrow 16, during layup of the tape 2. It will be appreciated that this relative movement may be achieved by moving the head 15, the component 8 or both. The double arrow 54 represents a heat or other energy source.
In this embodiment, the work piece 8 is mounted on a 5-axis manipulator 17 which will allow layup of plies of tape 2 on any one of the faces 18 of the component and in any desired direction.
In
Referring to
The supported tape 26 is placed in close proximity to the substrate 11, for example in the range of 10 s of microns to 500 microns, with the aligned tape 27 facing the substrate. A laser pulse is then discharged to irradiate a small region 38 of the supported tape 26. In the irradiated region 38, the laser pulse causes a portion 39 of the aligned tape to be lifted off the backing tape 28 and projected onto the substrate 11. The process is then repeated with the laser 36 being directed at a fresh portion of the tape 26 until all usable aligned tape in a given region thereof has been used up and a fresh region is exposed to the laser 36 to be progressively projected onto the substrate 11.
Referring now to
For both methods, once the portions 39 of aligned tape are deposited, see
The whole process is repeated layer by layer to build the 3-dimensional component 8, see
Referring to
The field may be imposed to preferentially align and rotate the aligned particles 43 in the X, Y and Z directions to attain tailored fibre reinforcement in desired directions, throughout the composite component.
Referring to
Once the aligned tape 27 is marked, the tape is spliced into ribbons 43 of desired length, see
Referring to
Referring to
With the ribbons 43 aligned to a desired direction, within the resin layer 50, the layer can be consolidated and post processed, as described earlier, to form a new consolidated layer of the component 8. It will be appreciated that the direction of ribbon orientation can be altered, layer 50 by layer, and even in the midst of a layer, one or more times, if the component design requires it.
| Number | Date | Country | Kind |
|---|---|---|---|
| 14275125.4 | May 2014 | EP | regional |
| 1409040.1 | May 2014 | GB | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/GB15/50297 | 2/4/2015 | WO | 00 |
