Electromagnetic induction welding device for joining composite materials and relative joining method

CROSS-REFERENCE TO RELATED APPLICATIONS

0001This patent application claims priority from <patcit id="pcit0001" dnum="IT102018000020524"><text>Italian patent application no. 102018000020524 filed on December 20, 2018</text></patcit>.

TECHNICAL FIELD

0002The present invention relates to a portable electromagnetic induction welding device for joining composite materials, in particular for aeronautic applications, to which the ensuing description will make explicit reference, without any loss of generality.

0003The present invention also relates to a method for joining composite materials by means of the aforementioned electromagnetic induction welding device.

PRIOR ART

0004As is known, composite materials are used in various industrial sectors, including the aviation industry. In particular, fibre-reinforced composite materials, commonly referred to as "preimpregnated" or "prepreg", are known, which are generally constituted by a semi-finished product comprising a resin matrix and reinforcing fibres immersed in the matrix. The fibres can be arranged in different configurations, for example, in a single direction, in two or more mutually different directions, or can be arranged to form a fabric. The matrix is used to fix the fibres to each other and possibly to other components during production.

0005Prepregs are generally prepared in the form of tapes and wound in rolls; in order to achieve the desired mechanical properties, prepregs must be subjected to a stabilization process through heat and often also under pressure.

0006The prepregs mainly used in the aviation industry can have a matrix of a thermosetting material or of a thermoplastic material.

0007In the first case (thermosetting materials), the matrix is constituted by polymers that, in opportune temperature conditions and/or in the presence of certain substances, transform into rigid, insoluble and infusible materials. This transformation occurs following cross-linkage reactions (a process known as curing, through which polymer chains undergo a reaction that creates bonds between different chains at a reactive functional group level), which take place between the polymer chains with formation of strong (covalent or ionic) bonds.

0008Before polymerization, thermosetting materials have characteristics of stickiness. These materials can therefore be used to create stratifications, placing different layers one on top of the other, with an opportune sequence or orienting of the different layers. The stratifications are then subjected to a temperature and pressure cycle (in a vacuum bag and in autoclave, using ovens, moulding presses, etc.) which polymerizes the material, raising the molecular weight and inducing the creation of bonds between the macro-molecules (cross-linking), thus transforming it into a material with structural characteristics and mechanical properties suitable for its intended application.

0009Some thermosetting polymers are cross-linked only by heat or through a combination of pressure and heat, while others can be cross-linked through chemical reactions at room temperature (cold cross-linking).

0010In the second case (thermoplastic materials), the matrix resin has a high molecular weight and therefore, on one hand, it does not need to undergo a polymerization cycle, and on the other, does not have characteristics of stickiness.

0011In a first approximation, a thermoplastic-matrix prepreg can be considered a manufactured product in its final state formed by a single lamina. To be able to form a laminate, it is necessary to heat it so as to cause the fusion of at least the contact surfaces of its constituent laminae or layers of thermoplastic prepreg, compress it and then cool it. The temperature to be reached for fusion is the glass transition temperature T_g for amorphous thermoplastics, and the melting point T_f for semi-crystalline thermoplastics.

0012In these cases, the apparatus for producing a laminate based on thermoplastic prepregs must also provide the heat for reaching a temperature (which, depending on the materials, might be excessively high) such as to melt the resin and thus obtain adhesion between the various layers that will constitute the laminate; in addition, for semi-crystalline thermoplastics, cooling that is too rapid might cause amorphisation of the part, with consequent loss of performance characteristics.

0013As previously explained, the processes of stabilization of the prepregs and of joining the various prepreg layers that form the final component usually take place in an autoclave, in ovens or moulding presses. In the case of very large components, such as, for example, structural components in the aeronautics sector, naval sector, etc., the known stabilization and joining processes are excessively expensive and can create numerous undesired constraints.

0014The need is thus felt to develop techniques that allow achieving in-situ stabilization and joining of parts in a composite material, especially when these are very large in size.

0015<patcit id="pcit0002" dnum="US9527237B2"><text>US 9527237 B2</text></patcit> discloses a welding device as defined in the preamble of claim 1 and a method of joining composite materials as defined in the preamble of claim 10.

DESCRIPTION OF INVENTION

0016One object of the present invention is to produce a welding device for joining composite materials that enables meeting the aforementioned need and, at the same time, enables achieving high-quality welds.

0017According to the present invention, a portable welding device is provided as claimed in claim 1 and in its dependent claims.

0018The present invention also relates to a method of joining composite materials as claimed in claim 10.

BRIEF DESCRIPTION OF THE DRAWINGS

0019The present invention will now be described with reference to the accompanying drawings, which show a non-limiting embodiment thereof, in which: <ul id="ul0001" list-style="dash" compact="compact"><li><figref idref="f0001">Figure 1</figref> is a side elevation view, with parts removed for the sake of clarity, of a portable electromagnetic induction welding device made according to the present invention for joining tapes of adherends in electrically conductive composite materials with a polymeric matrix;</li><li><figref idref="f0002">Figure 2</figref> is perspective view on an enlarged scale of a detail of <figref idref="f0001">Figure 1</figref>, with parts removed for the sake of clarity;</li><li><figref idref="f0003">Figure 3</figref> is perspective view, on a further enlarged scale, of the detail of <figref idref="f0002">Figure 2</figref>, with parts removed for the sake of clarity; and</li><li><figref idref="f0003">Figure 4</figref> is a perspective view on an enlarged scale of a detail of <figref idref="f0003">Figure 3</figref>.</li></ul>

PREFERRED EMBODIMENT OF THE INVENTION

0020<figref idref="f0001">Figure 1</figref> shows a portable electromagnetic induction welding device, indicated as a whole by 1, for joining together tapes 2 of adherends in electrically conductive composite materials with a polymeric matrix.

0021In particular, the tapes 2 used generally comprise a resin-based matrix in which reinforcing fibres, suitable for giving special mechanical properties to the composite material, and electrically conductive fibres, in which mainly parasitic electric currents can be induced by the welding device 1, are dispersed.

0022The matrix can be thermoplastic, semi-crystalline or amorphous resin based, or thermosetting resin based.

0023In the first case, the semi-crystalline thermoplastic resin can, for example, be polyether ether ketone, or PEEK, which has a melting point T_f of approximately 340°C. In alternative, this semi-crystalline thermoplastic resin can, for example, be polyether ketone ketone, or PEKK, which has a melting point T_f of approximately 370°C. An example of amorphous thermoplastic resin is represented, for example, by polyetherimide, or PEI, which has a glass transition temperature T_g of approximately 215°C.

0024In the case where a thermosetting resin is used, the latter can, for example, be an epoxy, BMI (bismaleimide) or phenolic resin.

0025The reinforcing fibres can be arranged in one or more unidirectional layers, in several layers having different orientations from each other, or like a fabric.

0026The reinforcing fibres are preferably in carbon; in alternative, other types of reinforcing fibres known in the aeronautics sector can be used, such as, for example, glass fibres or a combination of glass and carbon fibres.

0027The electrically conductive fibres are preferably in carbon and are dispersed in the matrix in at least two different directions, preferably in a random manner in all directions; in alternative, the electrically conductive fibres can also be made in another conductive material, for example in a metal material.

0028Referring to the <figref idref="f0001 f0002 f0003">Figures 1 to 3</figref>, the welding device 1 comprises: <ul id="ul0002" list-style="dash" compact="compact"><li>a fixed base 3, facing a work surface 4, which in turn is designed to receive the tapes 2 to be joined or is defined by at least one tape 2 already positioned to receive one or more other tapes 2;</li><li>an operating head 5, receiving one tape 2 at a time and movable with respect to the base 3 along at least a first movement line L1 parallel to the work surface 4 for stretching the received tape 2 on the work surface 4;</li><li>a motorized arm 6 connecting the operating head 5 to the base 3 and selectively activatable to impart movements to the operating head 5 parallel to the movement line L1 and also parallel to at least a further movement line L2 transversal to the movement line L1 and parallel to the work surface 4; and</li><li>a feeding means 8 selectively activatable to feed one tape 2 at a time to the operating head 5 and connected to the operating head 5.</li></ul>

0029In particular, the movements of the operating head 5 along the movement line L1 are preferably used for stretching the respective tape 2, fed to the operating head 5, along the work surface 4. The movements of the operating head 5 along the movement line L2 are preferably used for moving the operating head 5 in an area of the work surface 4 adjacent and parallel to that of the tape 2 just applied.

0030In detail, the operating head 5 comprises at least one positioning roller 9, receiving one tape 2 at a time from the feeding means 8 and adapted to deviate this tape 2 on the work surface 4, at least one pressure roller 10, spaced from and aligned with the positioning roller 9 along the movement line L1, and at least one inductor 11, interposed between the positioning roller 9 and the pressure roller 10 referring to the movement line L1 and selectively energized, in use, to generate an electromagnetic field suitable for inducing parasitic electric currents in the tapes 2 overlapping each other so as to produce, by the Joule effect, the local fusion of polymer matrices in contact with each other of the tapes 2.

0031The positioning roller 9 is arranged downstream of the pressure roller 10 along the feeding direction of the operating head 5 parallel to the movement line L1.

0032According to a possible alternative that is not shown, the operating head 5 could also comprise two or more positioning rollers 9 and two or more pressure rollers 10.

0033In greater detail, the operating head 5 comprises a carriage 12 provided with positioning and pressure rollers 9, 10 and sliding on the work surface 4.

0034In the case shown (<figref idref="f0003">Figure 4</figref>), the inductor 11 is constituted by a winding of conductive material, normally copper wire, covered by a thin insulation film. Alternatively, the inductor 11 can also be defined by a coil.

0035The inductor 11 is preferably housed in a seat 13 passing through the carriage 12 and protrudes therefrom to interact with the tapes 2 to be joined during the sliding of carriage 12 on the work surface 4.

0036As can be seen in particular in the <figref idref="f0001 f0002 f0003">Figures 1 to 3</figref>, the operating head 5 also comprises a heat sensor 14, preferably a heat camera, mounted in a cantilever fashion on the carriage 12, and configured and positioned on the carriage 12 so as to frame the work area of the inductor 11 and remotely detect the temperature reached on the matrices of the tapes 2 to be joined.

0037Referring to the <figref idref="f0001">Figures 1</figref> and <figref idref="f0002">2</figref>, the motorized arm 6 comprises: <ul id="ul0003" list-style="dash" compact="compact"><li>a slider 15 coupled movably to the base 3 parallel to a horizontal direction X, in turn parallel to the work surface 4 and, in the case shown, transversal to the movement line L1; and</li><li>an articulation element 16, having a first end portion 17 hinged to the slider 15 about a first horizontal axis A parallel to the direction X, and a second end portion 18 hinged to the operating head 5 about a second axis B parallel to the axis A and to the direction X.</li></ul>

0038More specifically, the slider 15 comprises a supporting portion 20, slidably coupled along the direction X to a guide element 21 of the base 3, and a movable portion 22 coupled to the supporting portion 20 in a rotatable manner about a vertical axis C orthogonal to the axes A and B and to the direction X.

0039The articulation element 16 comprises a first connection member 23, defining the end portion 17, and a second connection member 24, defining the end portion 18; the connection members 23, 24 are then hinged to one another around an axis D, parallel to the axes A and B, at their respective concurrent end portions 25, 26, respectively opposite to the end portions 17, 18.

0040In greater detail, the connection member 23 is defined by a single lever hinged on one end to the movable portion 22 of the slider 15 and on the other end to the connection member 24.

0041As can be seen in <figref idref="f0001">Figures 1</figref> and <figref idref="f0002">2</figref>, the connection member 24 comprises a first lever 27, hinged to the connection member 23 about the axis D, and a second lever 28, hinged to the operating head 5 about the axis B; the two levers 27, 28 are then hinged to one another around an axis E orthogonal to the axes A, B and D.

0042Always referring to <figref idref="f0001">Figures 1</figref> and <figref idref="f0002">2</figref>, it can be noted how the operating head 5 is also coupled to the end portion 18 of the articulation element 16, and more specifically to the lever 28, about an axis F, transversal, and more specifically orthogonal, to the work surface 4, as well as orthogonal to the axes A, B and D and passing through the operating head 5.

0043The feeding means 8 basically comprise a roll (in itself known and not shown) of the tape 2, unrolling means (in themselves also known and not shown) for unrolling the tape 2 from the roll, and a flexible conduit 30, extending around the motorized arm 6 and internally carrying a plurality of guide rollers 31 of the tape 2 to be fed to the operating head 5; a downstream end 32 of the flexible conduit 30 is laterally fixed to the operating head 5 in a position adjacent to the positioning roller 9.

0044In use, due to the translation of the slider 15 along the guide element 21, the motorized arm 6, and with it the operating head 5, can move along the horizontal direction X.

0045Whereas, due to the rotations of the first connection member 23, the second connection member 24 and the operating head 5 about the respective axes A, B and D, it is possible to obtain the movement of the operating head 5 along two other directions Y, Z orthogonal to the direction X, thereby creating complex movements resulting from the combination of movements along the directions X, Y, Z; direction Y extends vertically and parallel to the axis C, while the direction Z extends horizontally.

0046The rotation about the axis F enables the operating head 5 to proceed in stretching the respective tape 2 on the work surface 4 along the movement lines L1 arranged according to any orientation with respect to the work surface 4.

0047The rotation about the axis C enables the operating head 5 to work on any work surface 4, flat or curved, arranged in any position around the axis C.

0048The welding device 1 is suitable for being used not only for welding tapes 2 of composite materials to each other, but also for progressively stretching these tapes 2 on a predetermined work surface 4, i.e. on a surface intended to receive these tapes 2 or defined by one or more already positioned tapes 2.

0049Once the device 1 has been positioned near the work surface 4 on which it must operate, a tape 2 is fed from the respective roll inside the conduit 30, to then come out from the latter at the downstream end 32 and be received by the positioning roller 9 of the operating head 5. In the meanwhile, the operating head 5 is fed by the motorized arm 6 along the movement line L1.

0050During this operation, in order to join the tape 2 just stretched on the underlying tape 2, the inductor 11 is fed with an electric current so as to generate a variable electromagnetic field E suitable for inducing the parasitic currents largely in the electrically conductive fibres in the matrices of the tapes 2 to be joined.

0051Since the electrically conductive fibres are dispersed in the respective matrices in at least two different directions, actual "electrical circuits" are created inside the matrices that heat them by the Joule effect.

0052The localized heating is very efficient and enables reaching and locally exceeding, i.e. in the contact areas between the matrices of the two tapes 2, the melting point T_f or the glass transition temperature T_g.

0053Due to the arrangement of the positioning and pressure rollers 9, 10 on the carriage 12, the passage of the pressure roller 10 takes place after the induction of parasitic currents in the overlaid tapes 2.

0054In this way, the pressure is progressively exerted on the areas of the tapes 2 to be joined, after the softening obtained at the interface between the tapes 2 following electromagnet induction and during the cooling of these areas.

0055Preferably, at least the pressure roller 10 is kept cold so as to progressively remove residual heat from the tapes 2.

0056From an examination of the characteristics of the welding device 1 and of the joining method realized according to the dictates of the present invention, the advantages that can achieved therewith are evident.

0057In particular, the welding device 1 is suitable for in-situ operation, both for stabilizing composite materials and for joining several layers or laminae of composite materials.

0058Due to the versatility of movement of the motorized arm 6 and the particular configuration of the operating head 5, the welding device 1 is suitable for processing surfaces or parts of any shape, flat or curved or a combination thereof, in an efficient and particularly rapid manner.

0059Finally, it is clear that modifications and variants can be made to the welding device 1 and the joining method set forth herein without departing from the scope defined in the claims.