Method for mechanically bonding first sheet made of metal and second sheet made of composite material comprising fibres, and device for carrying out said method

Abstract

Method for mechanically bonding a first sheet (L1) made of metal and a second sheet (L2) made of a composite material comprising fibres, in which a support (S) provided with at least one hole (H) and a punch (P) having a flat base (B) and coaxial with said hole (H) are used, said hole (H) having a perimeter and said punch having a perimeter inscribed in the hole perimeter, the method comprises the steps of placing the first sheet (L1) and the second sheet (L2) such that the sheets are located between the punch and the support (S) and such that the first sheet (L1) is closer to the side of the support (S) and the second sheet (L2) is closer to the side of the punch (P), and then punching with the punch (P) such that the first sheet (L1) and the second sheet (L2) are positioned between the punch and the support (S) and such that the first sheet (L1) is closer to the side of the support (S) and the second sheet (L2) is closer to the side of the punch (P). The first sheet (L1) is punched until a portion (C) of the punched first sheet (L1) is cut off. The invention also relates to a device for carrying out the method.

Description

Technical Field

[0001] The present invention belongs to the specific field of achieving mechanical interlocking joints (joints, connections) between metals and carbon fiber reinforced polymers through stamping. Background Art

[0002] Multi-material design (MMD) of structural components is an attractive strategy for producing lightweight vehicle components that meet safety requirements at low cost and high production volumes. MMD also promotes the application of the most suitable materials in each produced part, customizing component properties to balance maximum weight reduction, processability, performance, durability, and cost. Therefore, the interest in MMD in the automotive industry is on the rise, especially for combinations of light metals, such as combinations of aluminum and carbon fiber reinforced polymer (CFRP).

[0003] Taking this into account, the joining between these different materials is very important because it is usually the weakest point of the structure and determines its structural efficiency. Traditional joining strategies for metal-FRP multi-material structures include mechanical fastening, adhesive bonding, hybrid mechanical adhesive bonding, and welding.

[0004] Mechanical fastening strategies create mechanical interlocks between the two materials, and their main advantages are their low cost, simple processing, easy maintenance, and insensitivity to the working environment.

[0005] Conventional mechanical joining strategies between metals and CFRP include bolt joining, self-piercing riveting (SPR), and mechanical clinching. However, these conventional methods either increase the weight of the structure by including fasteners or create the need to drill holes in the composite material, which may introduce damages such as delamination or fiber breakage, reducing the load-bearing capacity of the structure.

[0006] Alternative mechanical joining processes have also been proposed to overcome the previous drawbacks, such as pin joining, ring joining, and other joining methods. However, all of these alternative methods involve highly complex processing, which hinders their industrial application in high-volume markets such as the automotive market.

[0007] Modifications to conventional mechanical joining have been developed, such as including bonded metal inserts in composite materials to reduce the stress concentration factor at holes in the composite material, or placing dry fibers onto a tacky prepreg fabric to direct the fibers to match the load path and improve the stiffness and strength of bolted joints. Conventional crimping has also been adjusted to extend its suitability for materials with low ductility, such as CFRP. Some examples are using spring dies, friction-assisted crimping, or softening the polymer matrix in thermoplastic composites to increase the toughness and formability of the composite. However, all of these modifications imply additional process steps or complexity. SUMMARY OF THE INVENTION

[0008] To overcome the limitations of the prior art, the present invention provides a method for mechanically joining a first sheet made of metal and a second sheet made of a composite material including fibers, wherein a support member provided with at least one hole and a punch having a flat base and coaxial with the hole are used, the hole having a perimeter and the punch having a perimeter inscribed in the perimeter of the hole, the method comprising the steps of: a. Placing the first sheet and the second sheet such that the sheets are located between the punch and the support member, and such that the first sheet is closer to the support member side and the second sheet is closer to the punch side; b. Punching with the punch until a portion of the first sheet being punched is cut off.

[0009] The expression "the hole has a perimeter and the punch has a perimeter inscribed in the perimeter of the hole" means that in a plant view, i.e., from above along the punching axis, the perimeters will have substantially the same shape, but they will be separated by a gap that will allow the second sheet to be continuous.

[0010] The present invention provides a cost-effective and weight-penalty-free mechanical joining technique between metals and composite materials including fibers based on a single-step stamping process. Such a method can be easily implemented in an automotive production line where stamping operations are commonly used. It does not increase the weight of the part by adding fasteners; instead, it removes a portion of the metal material, making the part lighter. Additionally, holes are formed in the metal material, which, due to its higher plasticity, alleviates stress concentration more efficiently than composite materials. Thus, damage to the composite material is minimized without increasing the structural weight or the complexity of the joining process.

[0011] Furthermore, the method allows achieving joining strength within the same range as other mechanical joining techniques while minimizing damage to the composite material without increasing the structural weight or the complexity of the joining process.

[0012] It must be noted that the developed joining process forms a tight surface contact between the metal sheet and the composite material. If both materials are conductive and there is a large difference in their electrochemical potentials, as occurs with aluminum and carbon fiber, this arrangement is vulnerable to galvanic corrosion in the presence of an electrolyte. To this end, a potential mitigation concept would be to add an insulating layer between the two materials, such as a fiberglass layer, a resin-rich layer, or an adhesive layer.

[0013] In some embodiments, step b) is performed until the flat base of the punch reaches the plane defined by the contact plane between the first layer and the support, and the portion of the first sheet being punched is cut off.

[0014] In some embodiments, the hole is a circular hole, the punch is cylindrical, the circular hole has a diameter D H and the punch has a diameter D P , such that D H > D P .

[0015] In some embodiments, the following ratio is defined: t is the total thickness of the first sheet, the second sheet, and the third anti-adhesive sheet, if the third anti-adhesive sheet is present; c is included in the range of [3%; 75%].

[0016] Preferably, c is included in the range of [13%; 42%].

[0017] In some embodiments, an anti-adhesive layer is placed on the second sheet before stamping, and the anti-adhesive layer is an anti-adhesive sheet or oil.

[0018] In some embodiments, the first sheet is made of aluminum and the second sheet is made of carbon fiber reinforced polymer.

[0019] In some embodiments, the first sheet is a carbon fiber reinforced polymer, a glass fiber reinforced polymer, an aramid, or a natural fiber reinforced polymer.

[0020] In some embodiments, the fibers are long fibers woven in a twill 2x2 or unidirectional pattern.

[0021] In some embodiments, the polymer is: - an epoxy or phenolic thermosetting matrix; or - a thermoplastic.

[0022] In some embodiments, a third sheet is present between the first sheet and the second sheet. The third sheet may be an electrical insulator to avoid current coupling between the first sheet and the second sheet.

[0023] Advantageously, the method includes another step: c. Curing the junction by locally applying pressure and temperature.

[0024] Post-curing methods for joining CFRP and aluminum alloy sheets are already known to reduce the damage generated in the composite material. In the context of the present invention, it allows a significant improvement in the joint.

[0025] Preferably, step c is carried out by placing the joined sheets in a press provided with a lower die and an upper die. As an alternative, step c is performed by placing the joined sheets in a bag, forming a vacuum in the bag, and applying temperature to the bag.

[0026] The present invention also relates to a device for performing the method according to any of the above variants, the device comprising a support provided with at least one hole and a punch having a flat base and coaxial with the hole, the hole having a perimeter and the punch having a perimeter inscribed within the perimeter of the hole. Preferably, the hole is a circular hole and the punch is cylindrical, the circular hole having a diameter D H and the punch having a diameter D P , such that D H > D P .

[0027] The present invention also relates to a vehicle comprising a component made of a first sheet and a second sheet, the first sheet being made of metal and the second sheet being made of a composite material comprising fibers, wherein the first sheet and the second sheet are joined by a method according to any of the variants of the innovative method.

[0028] The present invention also relates to a vehicle component, such as a hybrid metal composite B-pillar, a battery box, an aircraft component, other aviation vehicle components, such as an unmanned aerial vehicle, a water vehicle component, or other vehicle components, comprising a component made of a first sheet and a second sheet, the first sheet being made of metal and the second sheet being made of a composite material comprising fibers, wherein the first sheet and the second sheet are joined by a method according to any of the variants of the innovative method. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] To make the description complete and to provide a better understanding of the present invention, a set of drawings is provided. The drawings form part of the description and illustrate an embodiment of the present invention, which should not be construed as limiting the scope of the present invention, but should only be construed as an example of how the present invention can be implemented. The drawings include the following figures: Figure 1 is a top perspective photograph of a product obtained by the method according to the present invention. The top layer shown is the anti-adhesion foil or the third sheet.

[0030] Figure 2 shows a hole in the lower plate with a cut-out obtained after the joining process.

[0031] Figure 3 is a view seen from one side of the obtained joined metal layer.

[0032] Figure 4 is a micrograph of a cross-section of the obtained joined part after applying the pressing and curing step.

[0033] Figure 5 is the same as Figure 4 but a micrograph of the same cross-section examined by epifluorescence microscopy.

[0034] Figure 6 is a schematic cross-section of the device according to the present invention.

[0035] Figure 7 schematically shows various configurations obtained during the entire stamping process.

[0036] Figure 8 shows the structures before and after the curing process when pressure and temperature are applied using a flat press.

[0037] Figures 9 to 12 shows the configurations of different steps of the curing process using a pressure and temperature press.

[0038] Figure 13 shows the cured joint obtained by a vacuum curing process with temperature application.

[0039] Figure 14 a of shows the mechanical interlocking joint between AA5754 H111 and twill 2x2 prepreg after stamping and before curing.

[0040] Figure 14 b of shows the mechanical interlocking joint between AA5754 H111 and twill 2x2 prepreg after curing from the Al side.

[0041] Figure 14c shows the mechanical interlock joint between AA5754 H111 and twill 2x2 prepreg after curing from the CFRP side.

[0042] Figure 14 d shows the mechanical interlock joint between AA5754 H111 and UD prepreg after curing from the Al side.

[0043] Figure 14 e shows the mechanical interlock joint between AA5754 H111 and UD prepreg after curing from the CFRP side.

[0044] Figure 15 is the load - stroke curve recorded during the stamping process.

[0045] Figure 16 and Figure 17 show a component with a complex geometry obtained by the method according to the invention. Detailed Description

[0046] In Figure 1 is disclosed a device according to one aspect of the invention for performing the method according to the invention. The device includes a support S provided with at least one circular hole H and a punch P having a flat base B and coaxial with the hole H, the circular hole H having a diameter D H , and the punch having a diameter D P , such that D H > D P .

[0047] This device allows the execution of a method for mechanically bonding a first sheet L1 made of metal and a second sheet L2 made of a composite material including fibers.

[0048] As Figure 7 shown, the method includes the following steps: a. Placing the first sheet L1 and the second sheet L2 such that the sheets are located between the punch and the support S, and such that the first sheet L1 is closer to the support S side, while the second sheet L2 is closer to the punch (P) side, as Figure 7 shown; b. Punching with the cylindrical punch P, as Figure 7 shown in b of Figure 7 and Figure 7 shown in d of

[0049] until the portion C of the first sheet L1 being punched is cut off, as Figure 7In the embodiment shown, the punch does not reach the plane defined by the contact plane between the first layer L1 and the support S, but the thickness of the composite sheet L2 allows the circular portion of the metal sheet L1 to be cut off.

[0050] In other embodiments, step b) can be carried out until the flat base B of the punch P reaches the plane defined by the contact plane between the first layer L1 and the support S, and the portion C of the first sheet L1 to be punched is cut off.

[0051] As Figure 1 shown, in some embodiments, before stamping, an anti-adhesion layer L3 is placed on the second sheet L2, and the anti-adhesion layer L3 is an anti-adhesion sheet or oil.

[0052] Example: A rolled sheet of AA5754 H111 aluminum alloy with a thickness of 1.5 mm and having from SHD Composites 650 g / m of the toughened epoxy resin system of MTC275 2 2x2 twill prepreg and having an M77 fast-curing ring from Hexcel 300 g / m of oxygen resin 2 UD prepreg.

[0053] In a practical embodiment, as the first sheet L1, a rolled sheet of AA5754 H111 aluminum alloy with a thickness of 1.5 mm is used. Due to the high formability and good corrosion resistance of such an alloy, such an alloy is the most common non-heat-treatable aluminum alloy in the automotive industry for producing medium-strength parts. The aluminum sheet is water-jet cut perpendicular to the rolling direction. Such an aluminum sheet is joined to a CFRP sheet with a thickness of 0.60 mm to 0.65 mm.

[0054] As the second sheet L2, two types of CFRP prepregs with a thermosetting epoxy matrix are used to manufacture them: a twill 2x2 prepreg with a toughened epoxy resin system MTC275 from SHD Composites (MTC275 C650 HS 12K 36% RW twill 2x2 650 g / m 2 ), and a unidirectional (UD) prepreg with a fast-curing epoxy resin system M77 from Hexcel (M77 38% RW UD 300 g / m 2 ). For the twill 2x2 specimens, one sheet of MTC275 twill 2x2 650 g / m 2 is used as the CFRP substrate, while for the UD specimens, two sheets of M77UD 300 g / m 2 are used in a 0 / 90° layup to obtain a thickness of 0.60 mm. 2 For the UD specimens, two sheets of M77UD 300 g / m 2 are used in a 0 / 90° layup to obtain a thickness of 0.60 mm.

[0055] The mechanical joining process for bonding aluminum to CFRP involves laying an uncured prepreg layer L2 on top of an aluminum sheet L1 and stamping the entire system, where the CFRP faces the punch side of the set-up. By adjusting the cutting clearance and the punch stroke, the aluminum sheet L1 is fully perforated while the carbon fibers are not perforated ( Figure 14 of a). Instead, these carbon fibers are pressed against the walls of the holes in the aluminum, creating a mechanical interlock between the two materials. When the joining occurs in a state of polymer viscous flow, delamination and tearing of the CFRP are reduced.

[0056] The specimens are then cured by thermoforming ( Figure 14 from b to Figure 14 of e). Co-curing of the composite epoxy resin on the aluminum occurs, so in addition to the mechanical interlock, an adhesive bond exists between the two substrates L1 and L2.

[0057] Stamping equipment and parameters The mechanical interlock joining is achieved by a stamping tool mounted on a ZwickRoell universal testing machine with a 50 kN load cell as shown in Figure 6 . The aluminum surface is thoroughly cleaned with acetone using a wiping cloth to remove any surface contaminants.

[0058] In Figure 6 the geometric characteristics of the punches P and S are shown. The cutting clearance c, Equation 1 (where D H is the die diameter and D P is the punch diameter) is defined as the percentage that relates the gap between the punch and the die to the thickness t of the material being stamped: In these examples, cutting clearances ranging from 13% to 42% are used to allow the carbon fibers to slide between the punch and the die. Another relevant stamping parameter is the punch stroke, which is the penetration of the punch into the substrate being stamped.

[0059] In this example, the joining is performed at a constant stamping speed of 10 mm / min by placing the CFRP on the punch side and the aluminum on the die side. The punch diameter is set to 10 mm as it is a typical hole diameter found in automotive body-in-white parts and the fillet radius is 0 mm. Different combinations of the test process parameters (die diameter and punch stroke) are tested to determine their effect on the mechanical properties of the joint and to establish a process window. The die diameters tested are 12.0 mm, 11.7 mm, 11.4 mm, 11.3 mm, 11.2 mm, 11.1 mm, 10.9 mm, 10.8 mm, 10.7 mm, and 10.6 mm, which correspond to gap values ranging between 13% and 42%.

[0060] During the stamping process as shown in Figure 15 the load - stroke curve is recorded. Four different stamping regimes are observed. The first (I) corresponds to the deformation and strain hardening of aluminum as the depth of the cutting edge flip is formed. The second (II) corresponds to the cutting of aluminum, generating fracture and a smooth shear edge, and the third (III) corresponds to the complete stamping of the aluminum sheet. As the punch stroke increases during regime III, the fibers gradually break until maximum fiber breakage occurs at the start of regime IV, in which the stamping is completed.

[0061] However, when stamping an aluminum and CFRP prepreg system, four stamping regimes ( Figure 17 ) are found. The first two regimes correspond to the same phenomena as in aluminum stamping, but regime III starts with the aluminum being completely stamped without fiber breakage. As the punch stroke increases during regime III, the fibers gradually break until maximum fiber breakage occurs at the start of regime IV, in which the stamping is completed.

[0062] Therefore, the process parameters that allow the generation of a mechanical interlock joint are defined as those falling within stamping regime III ( Figure 15 ), in which the aluminum has been completely stamped, but the CFRP has not been completely stamped. Thus, according to each die diameter, a first stamping with a long punch stroke is performed to obtain a complete load - stroke and to define the different stamping regimes. Then, several stampings with different punch strokes falling within stamping regime III are performed.

[0063] Curing step In the illustrated embodiment, the curing process is carried out by compression molding using a flat steel die. In this case, a flat die is used because the test tube is flat, but more complex geometries can also be envisioned.

[0064] Silicone paper L4 is added to prevent the epoxy resin from adhering to the die, and a silicone frame FS is added to prevent the epoxy resin and carbon fiber from flowing out of the test tube, as shown in Figure 9 .

[0065] Then, the sheets L1, L2 having aluminum and carbon fiber already bonded in the previous step are placed in the die, as shown in Figure 10 , and then, as shown in Figure 11 , the top cover TC of the flat die is added.

[0066] Figure 8 a schematically shows the joined sheets before curing, while Figure 8b shows the joint after the system is placed in a hot plate press, where a pressure of 27 bar (2.7 MPa) and 150 °C are applied for 35 minutes to obtain Figure 12 the product shown in

[0067] Using a stereomicroscope as shown in Figure 4 and an epi-fluorescence inspection as shown in Figure 5 generated cross-sections of the joint. The twill 2x2 weave from the CFRP can be clearly observed in both images. In Figure 5 the epoxy fluorescence generates a clearer contrast between the fibers appearing in black and the epoxy appearing in yellow. This indicates that the carbon fibers are pressed against the flipped depth, while the area of the holes in contact with the fracture depth is filled with resin. The carbon fibers have filled the remaining part of the aluminum holes.

[0068] The curing process can also be completed by generating a vacuum with a vacuum bag and heating the system to 150 °C for 35 minutes. In this case, the joint section is as shown in Figure 13 because the vacuum body presses the fibers and the resin, making the final shape exhibit a concave surface.

[0069] The present invention can be carried out on a flat substrate or in a substrate having a complex geometry (such as an Ω-shaped profile), as shown in Figure 16 and Figure 17 The process of generating the joint can be carried out before or after continuously extruding (conform) a metal sheet, which allows joining parts with complex shapes.

[0070] An intermediate layer can also be added between the first sheet and the second sheet. For example, a glass fiber composite layer can be added between the aluminum sheet and the carbon fiber composite layer to avoid galvanic coupling between the two materials.

[0071] In this document, the term "comprising" and its derivatives (such as "including", etc.) should not be understood in an exclusive sense, that is, these terms should not be construed as excluding the possibility that the described and defined content may include other elements, steps, etc.

[0072] The present invention is clearly not limited to the specific embodiments described herein, but also covers any variations that any person skilled in the art can contemplate within the general scope of the present invention as defined in the claims (for example, regarding the selection of materials, dimensions, components, structures, etc.).

Claims

1. A method for mechanically joining a first sheet (L1) made of metal and a second sheet (L2) made of a composite material comprising fibers, wherein, Using a support (S) provided with at least one hole (H) and a punch (P) having a flat base (B) and coaxial with the hole (H), the hole (H) having a perimeter and the punch having a perimeter inscribed in the hole perimeter, the method comprises the following steps: a. Placing the first sheet (L1) and the second sheet (L2) such that the sheets are located between the punch and the support (S), and such that the first sheet (L1) is closer to the support (S) side while the second sheet (L2) is closer to the punch (P) side; b. Punching with the punch (P) until the portion (C) of the first sheet (L1) being punched is cut off.

2. The method according to claim 1, wherein, Performing step b) until the flat base (B) of the punch (P) reaches the plane defined by the contact plane between the first layer (L1) and the support (S), and such that the portion (C) of the first sheet (L1) being punched is cut off.

3. The method according to claim 1, wherein The hole (H) is a circular hole (H), the punch (P) is cylindrical, and the circular hole (H) has a diameter D H And the punch has a diameter D P , so that D H >D P .

4. The method according to claim 3, wherein, Define the following ratios: t is the total thickness of the first sheet (L1), the second sheet (L2), and the third anti-adhesive sheet (L3) if the third anti-adhesive sheet exists; c is included in the range of [3%; 75%].

5. The method according to claim 4, wherein, c is included in the range of [13%; 42%].

6. The method according to any one of the preceding claims, wherein, Placing an anti-adhesive layer (L3) on the second sheet (L2) before punching, the anti-adhesive layer (L3) being an anti-adhesive sheet or oil.

7. The method according to any one of the preceding claims, wherein, The first sheet (L1) is made of aluminum and the second sheet (L2) is made of carbon fiber reinforced polymer.

8. The method according to any one of the preceding claims, wherein, The first sheet (L2) is carbon fiber reinforced polymer, glass fiber reinforced polymer, aramid, or natural fiber reinforced polymer.

9. The method according to claim 8, wherein, The fibers are long fibers woven as twill 2x2 or unidirectional.

10. The method according to claim 8, wherein, The polymer is: - Epoxy or phenolic thermosetting matrix; or - Thermoplastic.

11. The method according to any one of the preceding claims, comprising a third sheet disposed between the first sheet and the second sheet, the third sheet preferably being an electrical insulator.

12. The method according to any one of the preceding claims, further comprising the step of: c. Curing the joint by locally applying pressure and temperature.

13. The method according to claim 12, wherein, Performing step c) by placing the joined sheets (L1, L2) in a press provided with a lower die and an upper die.

14. The method according to claim 12, wherein, Performing step c) by placing the joined sheets in a bag, forming a vacuum in the bag, and applying temperature to the bag.

15. The method according to any one of the preceding claims, wherein, The assembly made of the first sheet (L1) and the second sheet (L2) is flat or has a complex geometry, preferably having an Ω-shaped cross-section.

16. An apparatus for performing the method according to any one of claims 1 to 11, the apparatus comprising a support (S) provided with at least one hole (H) and a punch (P) having a flat base (B) and coaxial with the hole (H), the hole (H) having a perimeter, and the punch having a perimeter inscribed in the hole perimeter.

17. The device according to claim 16, wherein, The hole (H) is a circular hole (H) and the punch (P) is cylindrical, and the circular hole (H) has a diameter D H and the punch has a diameter D P , such that D H > D P .

18. A vehicle, comprising a component made of a first sheet (L1) and a second sheet (L2), the first sheet (L1) being made of metal and the second sheet (L2) being made of a composite material including fibers, wherein, The first sheet (L1) and the second sheet (L2) are joined by the method according to any one of claims 1 to 15.

19. A vehicle component, such as a hybrid metal composite B-pillar, a battery box, an aircraft component, other aviation vehicle components, such as an unmanned aerial vehicle, a water vehicle component or other vehicle components, comprising a component made of a first sheet (L1) and a second sheet (L2), the first sheet (L1) being made of metal and the second sheet (L2) being made of a composite material comprising fibers, wherein, The first sheet (L1) and the second sheet (L2) are joined by the method according to any one of claims 1 to 15.

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