GB2606574A

GB2606574A - Joining process and system

Info

Publication number: GB2606574A
Application number: GB2106912.5A
Authority: GB
Inventors: Paul Johnathan Hogg Professor; Faye Catherine Smith Dr
Original assignee: Avalon Consultancy Services Ltd
Current assignee: Avalon Consultancy Services Ltd
Priority date: 2021-05-14
Filing date: 2021-05-14
Publication date: 2022-11-16
Also published as: GB202106912D0

Abstract

A process and system for joining metal i and thermoplastic composite ii parts comprises heating protrusions iii on the metal part to a predetermined temperature, pressing the heated protrusions into the thermoplastic composite part to a predetermined depth which creates local melted areas iv in the thermoplastic part; cooling of the local areas to provide a substantially robust joint between the metal part and the thermoplastic composite part. Heating may be provided by convection, conduction, induction or radiant heating. The protrusions may be pin-like in shape and formed by additive, subtractive or high energy beam means. The parts are preferably aligned and may be ultrasonically vibrated during insertion. The application also includes a process and a physical system for disjoining metal and thermoplastic composite parts which were previously joined using the invention’s joining process or physical system and which facilitates recycling, re-use and remanufacturing of the parts at the end of their service life.

Description

Joining process and system The present invention relates to a process and a system for joining a metal part to a thermoplastic composite part, with the subsequent possibility of efficiently disjoining them if/when required.

The problem that needs to be solved is to create a strong joint between two parts manufactured separately to specified dimensional tolerances, one made from a metal and the other from a fibre-reinforced thermoplastic matrix composite where the joining process retains the original shape and dimensions of the two parts. The bond needs to be capable of withstanding peel and shear loads in service but also would ideally be reversible so that the parts can be separated when the structure comes to the end of its service life allowing the constituent parts to be recycled, re-used or remanufactured.

In engineering structures the metals may be, but are not limited to, steel, aluminium, titanium, magnesium that are required to be joined to fibre-reinforced thermoplastic polymer matrix composites where the fibres are high performance fibres with a tensile modulus > 15 GPa including, but not limited to, carbon fibres, glass fibres, high performance polymeric fibres, natural fibres and hybrids with multiple fibre types The matrices of the composites are thermoplastic polymers and may contain other additives including chemicals, particles and nanomaterials to provide additional durability, strength or functionality.

The joining of metals and thermoplastic fibre composite components can be achieved using mechanical fastening using bolts, screws, or rivets or via adhesive bonding. Mechanical fastening can be expensive, in some cases it offers poor structural efficiency and inevitably introduces a weight penalty into the structure. Adhesive bonding using a separate adhesive is not usually a good solution as the bonds between the adhesive, typically a thermosetting polymer, and the thermoplastic matrix of the composite may not be very strong and can be susceptible to impact, fatigue, and environmental degradation Some operation to raise the surface energy of the thermoplastic composite part, using for example plasma treatment, can result in better bond strengths but this is not always feasible or cost effective and the bond strength is still quite poor.

Thermoplastic polymers can however act as hot melt adhesives and in some circumstances bond well to metals if introduced to the metal in a molten state Accordingly, a solution could be envisaged whereby the surface layer of the thermoplastic composite part is melted prior to bonding to the metal Heating the metal part to a suitable temperature and bringing the two parts to be joined together could achieve this without losing the shape of the composite part and result in an adequate bond on cooling. Such an approach is cited in JP 6749687B2 (1) granted to the Adashi Light Industry company of Japan in 2020. However there would be concerns about the long-term longevity of that bond, its ability to withstand peel and shock loads, fatigue, and environmental interactions (moisture, solvents etc). Published work by Tan et al, Composites part B, 70 (2015) 35-43, reports that when a metal surface is laser heated in contact with a thermoplastic carbon fibre composite, porosity may occur in the polymer near the interface with the metal due to gaseous decomposition products and shrinkage which would undermine the bond strength and durability. Some other techniques to achieve thermoplastic composite-metal bonding have been proposed with limited success include resistance and induction welding techniques and a development of solid-state welding called friction spot joining for localised j oining. This technique is reported by S.T.Amancio-Filho et al, Materials Science and Engineering A, 528 (2011) 3841-3848, to have had some success in joining thin magnesium overlaps to PPS matrix carbon fibre composites.

A further well-known technique to enhance the strength of a joint between two parts, which might be of similar or different materials is to utilise a mechanical interlocking technique exploiting features created on the surface of the parts. Conceptually this has evolved from simple ideas such as the use of dove tail joints and dowels in woodwork. The use of microscopic pin-like protrusions on metal surfaces to join with polymer matrix composites has been described by TWI Ltd (UK) in 2005 which used a metal surface modified to contain surface protrusions that were introduced by the rastering of an electron beam over the surface of the metal in a controlled fashion. The surface modification technique was patented as WO 2004/028731 Al and the technique was called Surfisculpt". The TWI work, in common with all other studies on similar forms envisaged the modified metal part being joined to a thermosetting composite where the protrusions were pushed into an uncured prepreg stack of composite, or into a fibre preform that was subsequently infused with a liquid thermosetting resin. The final joint was formed when the thermosetting resin was cured and solidified. GB patent GB2507128A from BAE Systems, 2012, expands on this basic idea by ensuring that the metal protrusions extend beyond the surface of the thermosetting composite. After the composite is cured the tips of the protrusions are deformed by deflecting the tip, riveting, spreading, or splitting to modify the cross section, thereby increasing the constraint against peeling. The various techniques examined for use with thermosetting composites have shown promising results with genuine increases in peel strength and impact resistance.

The present invention relates to a process and a system for joining a metal part to a thermoplastic composite part which can be part of a structure, with the subsequent possibility of efficiently disjoining them if/when required, and which have significant novel and inventive advantages over prior art.

Figure 1 is a schematic flow chart for one embodiment of the present invention, illustrating process of joining a metal part to a thermoplastic composite part (A), comprising: heating to a predetermined temperature, a metal part that includes on its surface deliberately created protrusions (B); followed by the pressed insertion of the heated protrusions into the thermoplastic composite part (C), which melts the thermoplastic matrix of the composite locally allowing the protrusions to enter the body of the composite to a predetermined depth, which ultimately allows the metal and thermoplastic composite surfaces to come into contact; followed by cooling (D) which occurs while the metal and thermoplastic composite remain in contact until the melted thermoplastic polymer solidifies, thus resulting in a substantially robust joint between the metal and the composite part.

The protrusions on the surface of the metal part are heated to a temperature that is sufficient to soften and melt the polymer matrix when the protrusions are pushed with a suitable force into contact with the composite. If the polymer matrix is a semi -crystalline polymer this will be above the crystalline melting temperature, Tm If the polymer is amorphous and has no specific melting temperature, then the temperature will need to be sufficient to cause the polymer to flow and be displaced easily by the protrusions.

When the heated protrusions are pushed into the composite the polymer melts locally ahead of the protrusion allowing the protrusion to penetrate the composite by displacing the polymer and pushing aside fibres at the tip of the protrusion The applied force continues to push the metal surface closer to the composite surface and the protrusions becoming embedded in the composite until such time that the main surface of the metal, at the base of the protrusions, contacts the surface of the composite. At this point the hot metal surface causes the polymer at the surface of the composite to melt and wet the metal The two surfaces are retained in this position while the polymer is allowed to cool and solidify.

The local situation where a protrusion has been inserted into the thermoplastic composite and locally the surfaces of the metal and composite parts have come into contact is illustrated in Figure 2, which is a schematic enlarged section illustrating: the metal part (i); the mating surface of the metal part (ia); the thermoplastic composite part (ii); the mating surface of the thermoplastic composite part (Ha); the metal protrusion (iii) and the local melted polymer area (iv). Here the local surface (ia) of the metal part (i) is in contact with the surface (iia) of the composite part (ii). The heated protrusion (iii) and the heated metal surface (ia) have melted the polymer matrix in the composite (H) to form a layer of polymer melt (iv) at the interfaces between the two parts. The solidification of the polymer in this melt layer (iv) results in the protrusions becoming anchored in the composite and a robust joint is formed between the metal and composite parts.

In the next paragraphs, a number of variations will be presented only as examples. The heating of the surface of the metal part containing the protrusions can be achieved by any method that is suitable for the parts being joined. These could include different manifestations of radiant heating, such as infrared or high energy lasers; conductive heating using heater mats or other heat sources; convection heating for example using ovens and induction heating.

Radiant or induction heating may be preferred as these methods could be configured to only heat the surface of the metal part and the protrusions, whereas convection heating and conduction heating are likely to heat the entire part. In some circumstances, conduction heating or induction may be preferred as heat can continue to be supplied during the penetration stage.

The protrusions on the metal surface, which are deliberately created to provide a mechanism for forming a robust bond, may be formed using a variety of processes, which can be included in the process of the present invention. These could include the use of focused high energy sources such as electron beams or lasers to melt and move the surface metal to form the protrusions. Subtractive manufacturing methods for example machining of the surface or selected etching could also be used. A preferred approach would be to use additive manufacturing methods whereby the protrusions are created from material added to the surface. This could be achieved by processes including, but not limited to, 3D printing, laser sintering of powders, and the welding of wire forms onto the surface using a cold metal transfer welding techniques. Additive manufacturing is a preferred approach as it also offers the possibility of creating the protrusions from materials that may differ in composition from the bulk metal of the part which could be advantageous to the strength, durability and functionality of the joint. Additive manufacturing methods may also allow control of the specific shape of the protrusions along their length, for example allowing the protrusions to become progressively thinner or thicker at specific locations to improve the properties of the joint.

After the protrusions have been created on the surface of the metal part and before the joining process is undertaken with the thermoplastic composite part, the metal part may be subjected to conventional processes known in the art to assist in bonding operations. These may include but are not limited to, cleaning, etching and anodising the metal and will be dependent on what metal is used for the part. Similarly the thermoplastic composite may be subjected to surface treatments, known in the art to assist in bonding of composites, which may include but are not limited to, cleaning, etching and plasma treatment.

In order for the heated metal surface to be joined correctly to the thermoplastic composite part, the surfaces that will form the joint will preferably have to be aligned, either before or during the process of bringing the two parts together. Correct alignment of the parts will ensure that the final jointed assembly of the two parts has the required orientation and geometry and the two surfaces mate in the desired manner. Preferably the alignment will be achieved by one or more of the two parts being fixed in a jig or structure and the two parts being brought together in a controlled motion along a linear trajectory.

The penetration of the heated protrusions into the solid thermoplastic composite, may be aided by ultrasonically vibrating the metal or the solid composite or both during the penetration process to allow the protrusions to pass between tows or bundles of fibres in the composite where the matrix has melted In many cases the surfaces of the two parts that are to be joined will both be substantially flat. In other situations the two mating surfaces may possess curvature. When the surfaces have curvature the orientation of the pin-like protrusions may be varied relative to the surface to facilitate penetration of the protrusions into the thermoplastic composite. In all circumstances, but especially when the surfaces possess curvature, it may not be desirable to uniformly cover the surfaces that are to be joined with protrusions at every location.

The thermoplastic composite materials may be comprised of many different fibre architectures. For example, the fibres may be arranged in multiple layers or in a single layer. The layers may have preferred orientations and these layers may consist of substantially straight fibres in bundles. In other cases the fibres may be present in woven fabrics or other textile forms. In some cases the fibres may be present in randomly orientated bundles.

Figure 3 is a schematic section view of yet another embodiment of the invention, which is provided only as an example: system for joining metal and thermoplastic composite parts (1), comprising: metal part (2), which contains protrusions (3); thermoplastic composite part (4); heating means (5); and pressed insertion means (6). This schematic section view is of the system prior to activation. Figure 4 is a schematic section view of the same embodiment as in Figure 1, but showing the system post-activation, with the heated protrusions having passed through the thermoplastic composite, and having caused local melted areas (7), The system works as follows. Heating means (5) heats up protrusions (3) to a predetermined temperature, pressed insertion means (6) inserts the heated protrusions into the thermoplastic composite part to a predetermined depth, which creates local melted areas (7) in the thermoplastic composite part (4), thus after cooling and solidification of the thermoplastic polymer results in a substantially robust joint.

The following are variations of the system. The heating may be by either convection, conduction, induction or radiant heating means. The metal protrusions can be substantially pin-like in shape. The system can also include means for the creation of the metal protrusions, which can be focused high energy sources such as electron beams or lasers. Alternatively, the metal protrusions could be created using additive manufacturing, or subtractive manufacturing. Preferably, the system would also include means for aligning the metal part and the thermoplastic part to predetermined axes which would be carried out before or during the pressed insertion.

Yet another variation is for the system to also include means for ultrasonically vibrating the insertion area such that the vibrations occur contemporaneously with the pressed insertion. The surfaces where the metal part and the thermoplastic part join could be substantially flat, or they could have substantially free curvatures.

Another embodiment of the present invention is a process for disjoining a metal part and a thermoplastic composite part which were previously joined using the process for j oining metal and thermoplastic composite parts or the system for joining metal and thermoplastic composite parts which were presented earlier in the description.

Yet another embodiment is a system for disjoining a metal part and a thermoplastic composite part which were previously joined using the process for joining metal and thermoplastic composite parts or the system for joining metal and thermoplastic composite parts which were presented earlier in the description.

So, when the jointed assembly has reached the end of its service life, for whatever reason, it can be dismantled by a further heating of the metal part to a temperature above the melting and softening point of the polymer. At this point the thermoplastic polymer matrix melts at all points where it interfaces with the metal, namely the main surface and along the length of the protrusions, the bond is destroyed, and the two parts can be separated. Induction heating might be preferred for heating the joint for the dismantling of the assembly as this would heat the metal parts preferentially allowing the thermoplastic polymer to melt around the protrusions and at the metal surface without requiring the bulk of the thermoplastic composite to be heated to the melt temperature. Under these circumstances it could be possible to re-use the composite part in another assembly. Alternatively the metal part of the jointed assembly could be attached to a conductive heating source, such as heater mats, or a radiant heat source applied to the reverse metal surface of the joint to preferentially heat the metal part without directly heating the composite part.

Dismantling of the assembly could also be achieved by heating the entire assembly in an oven to a suitable temperature. In this manifestation of the process the thermoplastic composite would not be retrieved in its original shape but the materials may still be suitable for re-manufacture or recycling. The two parts will have to be physically pulled apart, which can occur manually, mechanically or simply by holding one part up and allowing gravity to pull the other down. In all cases the ability to disjoin the metal and thermoplastic composite part will be important for environmental considerations and facilitate recycling, re-use or remanufacturing.

Claims

Claims: 1 A process for joining metal and thermoplastic composite parts (A), comprising: the heating of deliberately created protrusions on the metal part to a predetermined temperature (B), followed by the pressed insertion of the heated protrusions into the thermoplastic composite part to a predetermined depth which creates local melted areas in the thermoplastic composite part (C); then the cooling and solidifying of the local areas in the thermoplastic composite part (D), thus resulting in a substantially robust joint between the metal part and the thermoplastic composite part.
2 A process for joining metal and thermoplastic composite parts as claimed in Claim Lin which the heating is convection, conduction, induction or radiant heating
3 A process for joining metal and thermoplastic composite parts as claimed in Claim 1, in which the metal protrusions are substantially pin-like in shape
4. A process for joining metal and thermoplastic composite parts as claimed in Claim 1, which also includes the creation of the metal protrusions
5. A process for joining metal and thermoplastic composite parts as claimed in Claim 4, in which the metal protrusions are created using high energy beam means.
6 A process for joining metal and thermoplastic composite parts as claimed in Claim 4, in which the metal protrusions are created using additive manufacturing means.
7. A process for joining metal and thermoplastic composite parts as claimed in Claim 4, in which the metal protrusions are created using subtractive manufacturing means.
8 A process for joining metal and thermoplastic composite parts as claimed in Claim 1, which also includes aligning the metal part and the thermoplastic composite part to predetermined axes which must be carried out before or during the pressed insertion.
9 A process for joining metal and thermoplastic composite parts as claimed in Claim 1, which also includes ultrasonically vibrating the insertion area that is carried out contemporaneously with the pressed insertion
10. A process for joining metal and thermoplastic composite parts as claimed in Claim 1, in which the surfaces where the metal part and the thermoplastic composite part join are substantially flat.
11. A process for joining metal and thermoplastic composite parts as claimed in Claim 1, in which the surfaces where the metal part and the thermoplastic composite part join have substantially free curvatures.
12. A process for joining metal and thermoplastic composite parts as claimed in Claim 1, in which the thermoplastic composite part is multi-layered.
13 A system for joining metal and thermoplastic composite parts (1), comprising: metal part (2), which contains protrusions (3); thermoplastic composite part (4); heating means (5); and pressed insertion means (6), which are collectively arranged such that, heating means (5) heats protrusions (3) to a predetermined temperature, pressed insertion means (6) inserts the heated protrusions into the thermoplastic composite part to a predetermined depth, which creates local melted areas (7) in the thermoplastic composite part (4), thus after cooling results in a substantially robust joint.
14. A system for joining metal and thermoplastic composite parts as claimed in Claim 13 in which the heating means is convection, conduction, induction or radiant heating means.
15. A system for joining metal and thermoplastic composite parts as claimed in Claim 13, in which the metal protrusions are substantially pin-like in shape.
16. A system for joining metal and thermoplastic composite parts as claimed in Claim 13, which also includes means for the creation of the metal protrusions.
17, A system for joining metal and thermoplastic composite parts as claimed in Claim 16, in which the metal protrusions are created using high energy beam means.
18 A system for joining metal and thermoplastic composite parts as claimed in Claim 16, in which the metal protrusions are created using additive manufacturing means.
19. A system for joining metal and thermoplastic composite parts as claimed in Claim 16, in which the metal protrusions are created using subtractive manufacturing means.
20. A system for joining metal and thermoplastic composite parts as claimed in Claim 13, which also includes means for aligning the metal part and the thermoplastic composite part to predetermined axes which is to be carried out before or during the pressed insertion.
21. A system for joining metal and thermoplastic composite parts as claimed in Claim 13, which also includes means for ultrasonically vibrating the insertion area such that the vibrations occur contemporaneously with the pressed insertion.
22. A system for joining metal and thermoplastic composite parts as claimed in Claim 13, in which the surfaces where the metal part and the thermoplastic composite part join are substantially flat.
23. A system for joining metal and thermoplastic composite parts as claimed in Claim.13, in which the surfaces where the metal part and the thermoplastic composite part join have substantially free curvatures.
24. A system for joining metal and thermoplastic composite parts as claimed in Claim 13, in which the thermoplastic part is multi-layered.
A process for disjoining metal and thermoplastic composite parts which were previously joined using the process for joining metal and thermoplastic composite parts as claimed in Claim 1 or the system for joining metal and thermoplastic composite parts as claimed in Claim 13, comprising: the heating of the thermoplastic composite areas that have surface contact with the metal part to a predetermined temperature, followed by physically pulling away the two parts until the metal protrusions and other metal contact areas clear the thermoplastic composite part.
26. A system for disjoining metal and thermoplastic composite parts which were previously joined using either the process for joining metal and thermoplastic composite parts as claimed in Claim 1, or the system for joining metal and thermoplastic composite parts as claimed in Claim 13, comprising: heating means for heating the thermoplastic composite areas that have surface contact with the metal part to a predetermined temperature, means for physically pulling away the two parts until the metal protrusions and other metal contact areas clear the thermoplastic composite part.