Device and method for joining thermoplastic parts

Abstract

A joining device (1) for a first welded part (A1) to a second welded part (A2) comprising at least one electrically conductive resistive element (2) including a central connecting portion (21), configured to be mounted along the common weld zone (W) between the inner surfaces (SI1, SI2) of the two welded parts (A1, A2) for welding them, and two lateral electrical connection portions (22), configured to be mounted outside the common weld zone (W), the lateral portions (22) being configured to be electrically connected to a current source so as to allow the temperature of the central portion (21) to rise to an operating temperature, the central connecting portion (21) comprising a plurality of independent resistive conductive strands. Abstract figure: Figure 2

Description

Title of the invention: Device and method for joining thermoplastic parts. Technical field

[0001] The present invention relates to the field of thermoplastic parts assembly and more specifically to the joining by resistive welding of high-performance thermoplastic parts, in particular, in the aeronautical field.

[0002] It is known to use parts made from a high-performance thermoplastic material, such as polyaryletherketone known as PAEK, or polyetherketoneketone known as PEKK, which exhibits good mechanical properties. Reinforcing fibers, commonly carbon fibers, are generally impregnated into the thermoplastic matrix to strengthen the part, giving it both strength and lightness.

[0003] A method for joining two composite parts to be welded is known from documents EP3488999A1 and WO2022122881A1, in which a graphene film is deposited at the interface between the two parts along the entire bonding area. Graphene, having significant thermal diffusion properties, is connected to a resistive heating system. Upon heating, the graphene uniformly melts the thermoplastic matrix at the interface between the two parts to be welded along the entire bonding area, which is advantageous. The two parts are then welded together by the fusion of their thermoplastic matrices.

[0004] Furthermore, patent application EP4043188A2 discloses a bonding method in which a conductive element, comprising a mesh or fabric covered with graphene, is positioned at the interface between the two thermoplastic parts to be welded.

[0005] In practice, the conductive element forms a barrier between the thermoplastic parts to be welded, which limits the mixing of the thermoplastic matrices. Indeed, the resistive conductive element remains integrated within the welded area. The mechanical strength of the bond between the thermoplastic parts is therefore affected.

[0006] The invention thus aims to eliminate at least some of these drawbacks by proposing a new device and a new method for joining two thermoplastic parts. PRESENTATION OF THE INVENTION

[0007] The invention relates to a device for joining a first part to be welded to a second part to be welded along a common welding zone, configured to form a welded zone once the parts to be welded are joined, each a weldable part being made of a composite material comprising reinforcing fibers impregnated in a thermoplastic matrix, each weldable part having an internal surface to be welded, the internal surfaces of the weldable parts being positioned opposite each other, the joining device being configured to be at least partially integrated into the welded area, the joining device comprising: • at least one electrically conductive resistive element comprising: • a central connecting portion, configured to be mounted according to the common welding zone between the inner surfaces of the two parts to be welded, and • two lateral electrical connection portions, configured to be mounted outside the common welding area, the lateral portions being configured to be electrically connected to a current source in such a way as to allow the temperature of the central portion to rise to an operating temperature.

[0008] The invention is remarkable in that the central connecting portion comprises a plurality of resistive conducting strands designated "strands", the strands being independent.

[0009] The resistive conductive element mounted between the two parts to be welded melts the thermoplastic matrix of the parts to enable them to be welded. Heating the interface directly and rapidly limits heat diffusion through the thickness of the part. Only the interface is melted, thus ensuring the integrity of the welded parts.

[0010] Indeed, the resistive conductive element allows for uniform, rapid, and homogeneous heating of the internal surfaces of the parts to be welded at the central joint area, limiting the risk of localized overheating. Such a resistive conductive element also allows for control of the heating temperature by controlling the applied voltage, which enables precise management of the temperature rise. This allows, in particular, the maintenance of a temperature plateau ensuring the melting and mixing of the thermoplastic matrices, while also controlling the degree of crystallinity of each thermoplastic matrix. Optimal mechanical performance is thus guaranteed in the parts to be welded, even at their interface in the common weld zone.

[0011] The joining device allows two parts to be joined by welding, thereby reducing the mass of the assembly and the aircraft by eliminating the need for additional assembly parts, such as rivets. Welding with a resistive conductive element also allows for efficient, rapid welding without the risk of short circuits or overheating.

[0012] The use of independent resistive conductive strands makes it possible to limit the volume of the conductive element in the area to be welded while avoiding any bonding between volumes that could lead to weaknesses. In the weld zone, the strands have no contact with each other and are therefore independent. Advantageously, the strands allow for the formation of discreet heating zones so as not to "contaminate" the thermoplastic matrix and not to form a barrier between the two parts to be welded. Advantageously, during heating, the thermoplastic matrix spreads between the strands, which ensures a robust mechanical bond. Thus, there is no weakness as with a film or fabric.

[0013] According to one aspect, the central bonding portion is made up of a plurality of strands. Thus, there are only parallel and independent strands in the welded zone. Such strands occupy a small volume in the welded zone, which remains homogeneous, without any weak points.

[0014] Preferably, the strands are parallel to each other. This makes it easier to manufacture the fastening device and to control the heating.

[0015] In one aspect, the strands cover between 10% and 50% of the total surface area of ​​the central bonding portion, preferably around 20%. Such an overlap ensures optimal circulation of the thermoplastic matrix in the welded zone while allowing efficient heating to melt the thermoplastic matrix. The overlap corresponds to the surface area of ​​the strands relative to the total surface area of ​​the central bonding portion.

[0016] Preferably, the first strands are spaced apart from each other by a first spacing step, and the second strands are spaced apart from each other by a second spacing step, different from the first spacing step. This advantageously increases the heating density in the bonding device. This is particularly advantageous when a part to be welded has an excess thickness.

[0017] In one aspect, each strand has a diameter between 0.03 mm and 0.5 mm. This spacing ensures optimal circulation of the thermoplastic matrix in the welded area. Preferably, the first strands have a first diameter and the second strands have a second diameter, different from the first diameter. This advantageously increases the heating density in the bonding device.

[0018] Preferably, the first strands are spaced apart from each other by a first spacing step, and the second strands are spaced apart from each other by a second spacing step, different from the first spacing step. This advantageously increases the heating density in the bonding device.

[0019] According to one aspect, each strand comprises a body covered with a conductive coating. According to one aspect, the body is an insulating material, preferably a glass fiber. This limits the amount of conductive material in the welded area, thereby improving mechanical strength.

[0020] According to one aspect, each strand has thickened ends. This improves current transfer between the central connecting portion and the two lateral electrical connection portions. Advantageously, this prevents heating at the ends, i.e., outside the central connecting portion.

[0021] In one aspect, each strand contains graphene. Graphene is a material with high heat resistance and significant electrical conductivity, enabling it to perform its function effectively. Furthermore, the behavior of graphene is well-controlled.

[0022] According to one aspect, each strand is connected to another strand only at its ends. This makes it possible to limit the volume of resistive conductive element in the area to be welded.

[0023] According to one aspect, the solidarity mechanism includes: • at least one first electrical insulation element, positioned in contact with a first face of the resistive conductive element opposite the central connecting portion and configured to be positioned opposite the inner surface of the first part to be welded, and • at least one second electrical insulation element, positioned in contact with a second face of the resistive conductive element opposite the central connecting portion and configured to be positioned opposite the inner surface of the second part to be welded, • the two electrical insulation elements being configured to allow heat transfer and prohibit current flow.

[0024] The electrical insulation elements of the bonding device advantageously limit the contact of the resistive conductive element with the reinforcing fibers of the composite parts to be welded, which limits the transfer of electrical current in the parts to be welded and thus eliminates the risk of short circuit.

[0025] In a preferred embodiment, the securing device comprises at least two electrical connection members, positioned respectively in contact with the two lateral portions of the resistive conducting element so as to allow the electrical connection of the resistive conducting element to the current source.

[0026] Preferably, each electrical connection element is in the form of a copper strip, allowing the use of a highly conductive element, which limits the risk of electrical energy dissipation. Alternatively, each electrical connection element is in the form of a conductive ink, making it possible to avoid adding extra elements and simplifying the storage of the different components.

[0027] Preferably, the electrical connection members are mounted on the same face of the resistive conducting element, which makes it easier to mount the securing device, preferably on an easily accessible upper face.

[0028] Also presented is an assembly of a first weldable part, a second weldable part and at least one joining device as previously presented, each weldable part being made of a composite material comprising reinforcing fibers impregnated in a thermoplastic matrix, the joining device being configured to allow the joining of the first weldable part and the second weldable part according to a common welding area, each weldable part comprising an inner surface, the inner surface of the first weldable part being mounted opposite the inner surface of the second weldable part, the joining device being mounted between the inner surface of the first weldable part and the inner surface of the second weldable part according to the common welding area.

[0029] A method for joining a first weldable part and a second weldable part using the joining device as described above is also presented, each weldable part being made of a composite material comprising reinforcing fibers impregnated in a thermoplastic matrix, the method comprising: • a step of positioning the strands of the resistive conductive element of the joining device between the inner surfaces of the two parts to be welded according to the common welding zone, • a heating step of the resistive conductive element to an operating temperature higher than the melting temperature of the thermoplastic matrix of the parts to be welded, by supplying current to the resistive conductive element, so as to assemble the parts together.

[0030] Preferably, the process comprises, after the first heating step, a second heating step of the central portion to a consolidation temperature, lower than the operating temperature, so as to strengthen the welded area between the two parts to be welded.

[0031] Preferably, the consolidation temperature is between 200 and 270°C. PRESENTATION OF THE FIGURES

[0032] The invention will be better understood upon reading the following description, given by way of example, and referring to the following figures, given by way of example. non-exhaustive, in which identical references are given to similar objects.

[0033] Fig. 1 is a schematic representation of an exploded view in longitudinal section of an assembly of two parts to be welded and a fastening device according to one embodiment of the invention.

[0034] Figure 2 is a schematic representation of a perspective view of the device of solidarity of the whole of the [Fig.l].

[0035] Figure 3 is a longitudinal cross-sectional view of the fastening device of the [Fig.2],

[0036] Figure 4 is a schematic top view of the device solidarization without its upper electrical insulation component.

[0037] The [Fig.5] is a schematic close-up top view representation of a plurality of conducting strands of the resistive conducting element.

[0038] The [Fig.6] is a close-up schematic representation from above of a plurality of conducting strands of the resistive conducting element with a variable spacing pitch.

[0039] Figure 7 is a close-up schematic representation from above of a plurality of conductive strands of the resistive conductive element with a variable diameter.

[0040] Fig. 8 is a schematic close-up top view of a plurality of conducting strands of the resistive conducting element, the strands having thickened ends.

[0041] Figure 9 is a close-up schematic cross-sectional representation of a conductive strand with a body and a coating.

[0042] Fig. 10 is a close-up schematic representation in longitudinal section of a conductive strand with a body and a coating whose thickness varies according to the length.

[0043] The [Fig. 11] is a schematic representation of two parts to be welded comprising reinforcing fibers and the joining device before joining.

[0044] The [Fig. 12] is a schematic representation of two parts to be welded from the [Fig. 11] after joining.

[0045] Fig. 13 is a schematic representation of the steps of a joining process according to one implementation method of the invention.

[0046] It should be noted that the figures set out the invention in detail to implement the invention, said figures being of course able to serve to better define the invention where appropriate. DETAILED DESCRIPTION OF THE INVENTION

[0047] The invention relates to a device for joining 1 a first weldable part A1 and a second weldable part A2 ([Fig. 1]). The invention is particularly applicable for joining parts intended to be mounted on an aircraft.

[0048] In this example, the Al, A2 weld-on parts are made of a thermoplastic material and include reinforcing fibers impregnated in a thermoplastic matrix. Preferably, the thermoplastic matrix is ​​a high-performance thermoplastic polymer resin, such as polyetherketone (PEKK) or polyaryletherketone (PAEK), for example. Preferably, the reinforcing fibers are carbon fibers, giving the Al, A2 weld-on parts both strength and lightness. However, it is understood that the thermoplastic matrix and the reinforcing fibers could be made of different materials.

[0049] With reference to [Fig. 1], each weldable part Al, A2 has a flat inner weldable surface SU, SI2 and an outer surface SE1, SE2, opposite the inner surface SU, SI2. It is nevertheless understood that the inner surface SU, SI2 could be different and in particular have a relief.

[0050] The parts to be welded Al, A2 are configured to be joined along a common weld area W and positioned so that their respective inner surfaces SU, SI2 are opposite each other, as will be described in more detail later. The common weld area W is intended to form a welded area once the parts to be welded Al, A2 are joined. In this example, the weld area W is straight, but it is understood that it could extend in various ways, in particular, be curved.

[0051] The joining device 1 according to the invention is configured to be mounted between the two parts to be welded Al, A2, so as to join them. Once the parts to be welded Al, A2 are joined, the joining device 1 is at least partially integrated into the welded area, as will be described in more detail later in this document. In other words, the joining device 1 is consumable and is configured to remain trapped at the interface between the two welded parts Al, A2 at the end of the joining operation. Advantageously, it is no longer necessary to store or move significant equipment during the assembly of two thermoplastic parts to be welded.

[0052] In this example, with reference to Figures 2 and 3, the fastening device 1 extends longitudinally along an axis X, laterally along an axis Y, and vertically along an axis Z, so as to form an orthogonal coordinate system (X, Y, Z). The axis X extends along the length of the welding zone W.

[0053] With reference to [Fig. 2], a longitudinal portion of the fastening device 1 is shown for clarity. The length of the fastening device 1 is adapted according to the welding area W.

[0054] In this embodiment, the securing device 1 comprises a resistive conductive element 2 and two electrical insulation members 3A, 3B, positioned along the vertical axis Z on either side of the resistive conductive element 2, as well as two electrical connection members 4 for supplying electrical power to the resistive conductive element 2. The resistive conductive element 2 is electrically conductive and is configured to be supplied by a current source C.

[0055] With reference to [Fig.3], the resistive conductive element 2 according to the invention comprises a central connecting portion 21 and two lateral electrical connection portions 22, extending along the lateral axis Y on either side of the central connecting portion 21. The central connecting portion 21 is configured to remain, at least in part, in the welded area.

[0056] Remarkably, the central bonding portion 21 comprises a plurality of independent resistive strands 20, preferably parallel to each other. Preferably, the central bonding portion 21 consists of a plurality of independent resistive strands 20, preferably parallel to each other. In the weld zone, the strands have no contact with each other.

[0057] Unlike a film or a fabric, a plurality of parallel resistive conductive strands 20, hereinafter referred to as "strands", makes it possible to provide between them a space which facilitates the passage of the thermoplastic matrix, which improves the quality of the bond without forming a barrier which could affect the mechanical resistance.

[0058] Preferably, each strand 20 is connected to another strand 20 only by its ends, in particular, via the lateral electrical connection portions 22.

[0059] The use of parallel resistive strands 20 makes it possible to ensure a compromise by allowing, on the one hand, to heat sufficiently the internal surfaces SU, SI2 of the parts to be welded Al, A2 to melt optimally the thermoplastic matrix while avoiding the formation of a surface limiting the passage of molten thermoplastic matrix.

[0060] The structure of the strands 20 will henceforth be presented with reference to figures 4 to 8.

[0061] The strands 20 are arranged in a row which extends along the length of the area to be welded W so that each strand has a short length and can be electrically supplied.

[0062] As illustrated in [Fig. 5], the strands 20 are spaced apart from each other by a spacing pitch p. With reference to [Fig. 5], the spacing pitch p is constant. Alternatively, with reference to [Fig. 6], the resistive conducting element 2 may have different spacing pitches p1, p2. Preferably, the spacing pitch p may be variable. This advantageously allows the heating density to be increased in a welding area by reducing the spacing pitch p.

[0063] According to one aspect, the strands 20 define an overlap ratio of between 10% and 50%, preferably on the order of 20%. The overlap ratio corresponds to the surface area of ​​the strands 20 divided by the total surface area of ​​the central bonding portion 21. Such an overlap ratio ensures optimal circulation of the thermoplastic matrix in the welded zone while allowing efficient heating to melt the thermoplastic matrix.

[0064] With reference to [Fig. 5], each strand 20 has a diameter d between 0.03 mm and 0.5 mm. Such a diameter d allows for the formation of a heating zone sufficient to melt the thermoplastic matrix of the parts to be welded, Al, A2. With reference to [Fig. 5], the diameter d is constant. Alternatively, with reference to [Fig. 7], the resistive conductive element 2 can have different diameters d1, d2, d3. This advantageously allows for increasing the heating density in a welded zone by increasing the diameter. Preferably, the average diameter is between 0.1 mm and 0.2 mm. According to a preferred aspect, in order to obtain a heating density that varies according to the width defined along the Y-axis, the diameter d of a strand 20 can vary according to the length of a strand 20, as illustrated in [Fig. 8].

[0065] According to one aspect, with reference to [Fig. 8], each resistive conductive strand 20 has thickened ends. This advantageously improves the connection with the lateral electrical connection portions 22 and concentrates heat generation in the central connection portion 21.

[0066] According to one aspect, each strand 20 is made solely of a resistive conductive material, for example, graphene, which has high electrical conductivity and high heat resistance. Alternatively, each strand 20 can be made of a different material, for example, carbon nanotubes.

[0067] According to one aspect, with reference to figures 9 and 10, each strand 20 comprises a body 201 covered with a conductive coating 202. Preferably, each strand 20 consists of a body 201 covered with a conductive coating 202. The thickness of the conductive coating 202 is preferably between Ipm and 50pm.

[0068] In this example, the body 201 is an electrically insulating material. Preferably, the body 201 is a glass fiber, but it is understood that other insulating materials could be suitable. The coating 202 is preferably made of graphene. In one aspect, the coating 202 is deposited onto the body 201, in particular, by spraying.

[0069] As illustrated in [Fig. 1] and [Fig. 3], the resistive conducting element 2 has a first face FA (lower face) and a second face FB (upper face), opposite the first face FA. The resistive conducting element 2 thus has a General shape: flat and thin. The resistive conductive element 2 remains flexible to conform to the internal surfaces SU, SI2 of the parts to be welded Al, A2.

[0070] In this example, the resistive conductive element 2 has a length J extending along the longitudinal axis X (shown in [Fig. 2]), a width L extending along the lateral axis Y, and a thickness Ep2 extending along the vertical axis Z (shown in [Fig. 3]). The thickness Ep2 is preferably between 30 µm and 500 µm. The resistive conductive element 2 is thus thin and light, and has little impact on the mechanical characteristics of the parts to be welded Al, A2 and of the assembly formed by the parts to be welded Al, A2 and the joining device 1.

[0071] The central connecting portion 21 is configured to be mounted along the common weld area W between the inner surfaces SU, SI2 of the two parts to be welded Al, A2 so as to weld them, as shown in [Fig. 1]. Conversely, the two lateral electrical connection portions 22 are configured to be mounted outside the common weld area W.

[0072] The central portion 21 is configured to heat up to an operating temperature Tf, preferably between 330°C and 450°C, which is higher than the melting temperature (which is known) of the thermoplastic matrix of the parts to be welded Al, A2, as will be described in more detail later.

[0073] The two lateral electrical connection portions 22 are configured to be electrically connected, directly or indirectly, to the external current source C (shown in Figures 2 and 3) so as to allow the temperature of the central portion 21 to rise to the operating temperature Tf.

[0074] As shown in Figures 2 and 3, the resistive conductive element 2 mounted between the two Al, A2 parts to be welded is configured to melt the thermoplastic matrix of the Al, A2 composite parts to be welded, thus enabling their welding by mixing the thermoplastic matrices. Heating the interface directly and rapidly limits heat diffusion through the thickness of each Al, A2 part to be welded. Only the internal SU, SI2 surfaces are melted, ensuring the integrity of the Al, A2 welded parts.

[0075] As described previously, with reference to Figures 1 to 3, the fastening device 1 also comprises a first insulating element 3A and a second electrically insulating element 3B, positioned along the thickness of the fastening device 1, i.e., along the vertical axis Z, on either side of the resistive conductive element 2. Such insulating elements 3A, 3B are known from patent application WO2022122881A1. They are porous to allow the passage of thermoplastic matrix and advantageously form an electrical barrier.

[0076] The first electrical insulating member 3A is positioned in contact with the first face FA of the resistive conducting element 2 opposite the central portion 21 and is configured to be positioned opposite the inner surface SU of the first weld-on part Al, as shown in [Fig. 1]. Similarly, the second electrical insulating element 3B is positioned in contact with the second face FB of the resistive conductive element 2 opposite the central portion 21 and is configured to be positioned opposite the inner surface SI2 of the second weld-on part A2. In other words, the resistive conductive element 2 is sandwiched between the two electrical insulating elements 3A, 3B located above and below the resistive conductive element 2. Each electrical insulating element 3A, 3B is configured to limit the risk of contact between the resistive conductive element 2 and the conductive reinforcing fibers of the composite weld-on parts Al, A2, as was the case in the prior art.Preferably, each electrical insulation element 3A, 3B has a surface area greater than that of the common welding area W. Thus, any contact between the resistive conductive element 2 and the parts to be welded Al, A2 is prohibited.

[0077] For this purpose, each electrical insulation element 3A, 3B is preferably made of a composite material comprising a thermoplastic matrix, which has low electrical conductivity. Even more preferably, each electrical insulation element 3A, 3B is made of a high-performance thermoplastic material, such as PEKK or PAEK as described above.

[0078] According to one aspect, as shown in Figures 2 and 3, the fastening device 1 further comprises two electrical connection members 4, positioned respectively in contact with the two lateral portions 22 of the resistive conductive element 2 so as to allow the electrical connection of the resistive conductive element 2 to the current source C. In other words, the current source C is indirectly connected to the resistive conductive element 2 via the electrical connection members 4. This makes it possible to form a robust and practical connection. Preferably, each electrical connection member 4 is configured so as not to be in contact with any of the parts to be welded A1, A2 (as shown in [Fig. 1]). Preferably, each electrical connection member 4 is in the form of a copper strip, electrically connected to the current source C, for example by an electrical cable.Alternatively, each electrical connection element 4 is in the form of a conductive ink deposited directly on one of the faces FA, FB of the resistive conductive element 2.

[0079] Preferably, as shown in figures 2 and 3, the two electrical connection members 4 are mounted on the same face FA, FB of the resistive conductive element 2, so as to facilitate the assembly of the securing device 1.

[0080] Preferably, the two electrical connection members 4 being mounted on the lateral portions 22 of the resistive conducting element 2, i.e. outside the common welding area W, these are configured to be removed after the operation of joining the two parts to be welded Al, A2. It goes without saying that the joining device 1 could include a different number of electrical connection elements 4,

[0081] Preferably, the bonding device 1 can be supplied as individual elements or as a roll that can be cut to the desired length. When supplied as individual elements, the bonding device 1 can comprise strands 20 that are adapted to the desired heating density by varying the spacing p, the diameter d, and / or the thickness of the strands 20. This allows for customized heating, which is particularly advantageous for welding complex parts such as stiffeners to an aircraft skin. Preferably, the bonding device 1 is in the form of a patch.

[0082] Thanks to the fastening device 1, the resistive conductive element 2 according to the invention allows optimal heating of the internal surfaces SU, SI2 of each part to be welded Al, A2 at the central bonding portion 21, limiting the risk of local overheating. The presence of strands 20 allows the thermoplastic matrix to flow between the strands 20, which ensures optimal mechanical bonding.

[0083] The resistive conductive element 2 allows control of the heating temperature of the central portion 21 by controlling the applied voltage via the lateral electrical connection portions 22. Such control allows precise management of the temperature rise of the resistive conductive element 2 and maintains a stable temperature plateau, ensuring homogeneous mixing of the thermoplastic matrices of the parts to be welded Al, A2 and the joining device 1, as well as control of the degree of crystallinity of the thermoplastic matrix. Optimal mechanical performance is thus guaranteed, even at the interface between the two parts to be welded Al, A2 once joined. The presence of electrical insulation elements 3A, 3B eliminates any electrical risk while ensuring optimal heat transfer.The joining device 1 allows two parts to be welded Al, A2 to be joined opposite each other in a robust manner and without degradation of mechanical performance.

[0084] A method for joining two thermoplastic parts to be welded Al, A2 will now be described. The joining method allows the joining of a first part to be welded Al and a second part to be welded A2, as described previously and is carried out by means of the joining device 1 described previously.

[0085] As shown in [Fig. 13], the method according to the invention first comprises a positioning step El of the securing device 1 between the The internal surfaces SU, SI2 of the two parts to be welded Al, A2 are positioned according to the common weld area W as illustrated in [Fig. 11]. To do this, an operator positions the joining device 1 on the internal surface SU of the first part to be welded Al at the common weld area W, the first part to be welded Al thus serving as a support for the joining device 1. The operator then positions the second part to be welded A2 on the joining device 1 at the common weld area W, so that the joining device 1 is sandwiched between the internal surface SU of the first part to be welded Al and the internal surface SI2 of the second part to be welded A2.

[0086] In a second connection step E2, the operator then connects, in this example, the electrical connection members 4 to the current source C and activates the current source C so as to electrically supply the electrical connection members 4. The current is then transmitted to the lateral connection portions 22 of the resistive conductive element 2.

[0087] The process then comprises a first heating step E3 of the central portion 21 of the resistive conductive element 2 by supplying current to the lateral portions 22. The temperature of the strands 20 of the central portion 21 increases until it reaches the operating temperature Tf, for example between 330°C and 450°C, which is higher than the melting temperature of the thermoplastic matrix of each weld-piece Al, A2, at the inner surface SU, SI2, via each electrical insulation member 3A, 3B. The molten thermoplastic matrices of the two weld-pieces Al, A2 mix with the thermoplastic matrices of the electrical insulation members 3A, 3B so as to form a robust bond. Advantageously, with reference to [Fig. 12], the thermoplastic matrices of the Al, A2 weldable parts and the electrical insulation elements 3A, 3B mix optimally due to the strands 20, which ensures a robust mechanical bond.

[0088] Thanks to the bonding device 1 according to the invention, the thermoplastic matrix of the internal surfaces SU, SI2 of the parts to be welded Al, A2 can melt, without the electrical energy which flows in the resistive conductive element 2 being transferred to the reinforcing fibers of each part to be welded Al, A2, thus eliminating any risk of short circuit for example.

[0089] During heating, the operator applies a uniform pressure P by means of a pressure member 9 on the two parts to be welded Al, A2 in order to limit their relative movements and allow a mixing of material.

[0090] Once the thermoplastic matrices of the two parts to be welded Al, A2 have melted, the operator reduces the voltage of the current source C, so as to lower the temperature of the resistive conducting element 2. The process then includes a second heating step E4 of the central portion 21 of the conducting element The resistive element 2 is heated to a consolidation temperature Te lower than the operating temperature Tf, for example between 200 and 270°C, so that the two parts to be welded, Al and A2, are welded together under the pressure P applied by the pressure element 9, which ensures uniform welding. In this step, as the thermoplastic matrices of the two parts to be welded, Al and A2, cool, they solidify, forming the welded area. The temperature of the central portion 21 of the resistive conductive element 2 is managed sequentially by means of the electrical connection elements 4. This second heating stage E4 allows the temperature of the resistive conductive element 2 to be lowered gradually, thus preserving the mechanical properties of each thermoplastic matrix and controlling the crystallization to achieve the desired degree of crystallization.

[0091] After the thermoplastic matrices of the two weld pieces Al, A2 have solidified, the central portion 21 of the resistive conductive element 2 is mixed with the thermoplastic matrices of the weld pieces Al, A2. In other words, the central portion 21 of the resistive conductive element 2 is trapped at the interface between the two weld pieces Al, A2, in the welded area. Advantageously, the resistive conductive element 2 is permeable to the thermoplastic matrix and does not form a barrier.

[0092] When the material comprising the mixture of the two thermoplastic matrices of the two parts to be welded Al, A2 has solidified in the welded area, the process includes a removal step E5 of the lateral portions 22 of the resistive conductive element 2 and the electrical connection members 4, so as to remove all electrical connection means. Since the resistive conductive element 2 is no longer electrically powered, any risk of short circuit is eliminated. Only the central portion 21 of the resistive conductive element 2 and the electrical insulation members 3A, 3B then remain in the welded area. As the central portion 21 consists only of independent strands 20, there is no area of ​​mechanical weakness related to the presence of a large and continuous elementary volume of resistive conductive element 2. Such a removal step E5 can be carried out, for example, by detaching the lateral portions 22 along dotted lines formed on a pre-cut line.Thus, only strands 20 are retained in the welded area, which has only a slight impact on mechanical performance.

Claims

Demands

1. A joining device (1) of a first weldable part (Al) to a second weldable part (A2) along a common weld zone (W), configured to form a welded zone once the weldable parts (Al, A2) are joined, each weldable part (Al, A2) being made of a composite material comprising reinforcing fibers impregnated in a thermoplastic matrix, each weldable part (Al, A2) having an internal weldable surface (SU, SI2), the internal surfaces (SU, SI2) of the weldable parts (Al, A2) being positioned opposite each other, the joining device (1) being configured to be at least partially integrated into the welded zone, the joining device (1) comprising: • at least one electrically conductive resistive conductive element (2) comprising: • a central connecting portion (21),configured to be mounted along the common welding zone (W) between the inner surfaces (SU, SI2) of the two parts to be welded (Al, A2) for welding them, and • two lateral electrical connection portions (22), configured to be mounted outside the common welding zone (W), the lateral portions (22) being configured to be electrically connected to a current source (C) so as to allow the temperature of the central portion (21) to rise to an operating temperature (Tf), • the joining device (1) being characterized in that the central connecting portion (21) comprises a plurality of resistive conductive strands (20) designated "strands" (20), the strands (20) being independent.

2. A fastening device (1) according to claim 1, wherein the central bonding portion (21) is made up of the plurality of strands (20).

3. A fastening device (1) according to any one of claims 1 to 2, wherein the strands (20) cover between 10% and 50% of the surface total central portion of link (21), preferably, of the order of 20%.

4. A fastening device (1) according to any one of claims 1 to 3, wherein each strand (20) has a diameter (d) between 0.03mm and 0.5mm.

5. A fastening device (1) according to any one of claims 1 to 4, in which each strand (20) comprises a body (201) covered with a conductive coating (202).

6. A fastening device (1) according to claim 5, wherein the body (201) is an insulating body, preferably a glass thread.

7. A fastening device (1) according to any one of claims 1 to 6, wherein each strand (20) has thickened ends.

8. A bonding device (1) according to any one of claims 1 to 7, wherein each strand (20) comprises graphene.

9. A joining device (1) according to any one of claims 1 to 8, wherein each strand (20) is connected to another strand (20) only by its ends.

10. A joining device (1) according to any one of claims 1 to 9, wherein the joining device (1) comprises: • at least one first electrical insulation member (3A), positioned in contact with a first face of the resistive conductive element (2) opposite the central connecting portion (21) and configured to be positioned opposite the inner surface (SU) of the first piece to be welded (Al), and • at least one second electrical insulation member (3B), positioned in contact with a second face of the resistive conductive element (2) opposite the central connecting portion (21) and configured to be positioned opposite the inner surface (SI2) of the second piece to be welded (A2), • the two electrical insulation members (3A, 3B) being configured to permit heat transfer and prohibit current flow.

11. Assembly of a first weldable part (Al), a second weldable part (A2) and at least one fastening device (1) according to any one of claims 1 to 10, each weldable part (Al, A2)

12. being made of a composite material comprising reinforcing fibers impregnated in a thermoplastic matrix, the joining device (1) being configured to allow joining of the first weld piece (Al) and the second weld piece (A2) according to a common welding zone (W), each weld piece (Al, A2) comprising an inner surface (SU, SI2), the inner surface (SU) of the first weld piece (Al) being mounted opposite the inner surface (SI2) of the second weld piece (A2), the joining device (1) being mounted between the inner surface (SU) of the first weld piece (Al) and the inner surface (SI2) of the second weld piece (A2) according to the common welding zone (W). Method for joining a first weldable part (Al) and a second weldable part (A2) using the joining device (1) according to any one of claims 1 to 10, each weldable part (Al, A2) being made of a composite material comprising reinforcing fibers impregnated in a thermoplastic matrix, the method comprising: • a positioning step (El) of the strands (20) of the resistive conducting element (2) of the joining device (1) between the inner surfaces (SU, SI2) of the two parts to be welded (Al, A2) according to the common welding zone (W), • a heating step (E3) of the resistive conductive element (2) to an operating temperature (Tf) higher than a melting temperature of the thermoplastic matrix of the parts to be welded (Al, A2), by supplying current to the resistive conductive element (2), so as to assemble the parts together.

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