METHOD FOR PRODUCING A DOUBLE-LAYER HEAT EXCHANGER WALL

Description

Technical field

[0001] The present invention relates to a method of manufacturing a double-layer heat exchange wall, this double-layer wall being intended in particular to equip devices of the heat exchanger type.

[0002] Such heat exchangers find application in the chemical industry or in the energy sector. State of the prior art

[0003] Heat exchangers are devices for transferring thermal energy from a first fluid to a second fluid, without mixing them.

[0004] Different types of two-fluid heat exchangers exist, including those equipped with a so-called "double-layer" heat exchange wall which comprises two layers of different or identical thicknesses and each intended to come into contact with one of the heat transfer fluids.

[0005] The particular structure of the double-layer wall has the advantage of providing increased safety to the heat exchanger equipped with it, since each layer performs, in a redundant manner, a double function, namely the sealing function to prevent the two heat transfer fluids from coming into contact with each other and the function of resisting the pressure of the heat transfer fluids.

[0006] This dual function is particularly important, for example, in cases where the heat exchanger is used in installations such as chemical reactors, in which it is necessary to ensure, in a safe and efficient manner, the heat exchange between a first fluid such as reactive molten metals or metal salts, for example based on sodium, lithium or potassium, and a second fluid such as water. Indeed, any failure of the heat exchange wall in such installations would have serious consequences for operation due to the chemical reactivity of the heat transfer fluid with the medium to be cooled. In this specific case, the stresses in service, in addition to the static pressure of the fluids and the corrosion of the surfaces, arise from the thermal stresses and the fatigue stresses imposed by the operating cycles or the vibrations induced by the thermo-hydraulics.

[0007] Most double-layer heat exchange walls consider leaving a mechanical clearance, or gap, between the two layers.

[0008] By connecting this gap to a detection system, it then becomes possible to detect the presence of fluid coming from the piercing of one of the two layers before the integrity of the second is altered.

[0009] However, the presence of this gap is particularly penalizing with regard to the thermal conductivity properties of the heat exchange wall and, moreover, does not allow for an effective response to a hypothetical simultaneous piercing of the first layer and the second layer constituting it.

[0010] To improve the thermal conductivity of double-layer heat exchange walls, it has been proposed to at least partially fill the gap between the first layer and the second layer.

[0011] Thus, document US 2013 / 0205861, referenced [1] in the remainder of this description, proposes a method for manufacturing a heat exchange wall formed by a double-layer tube in which, after a step of polishing the internal surface of the external tube as well as the external surface of the internal tube, braided wires are interposed in the gap formed by the external and internal tubes and then drawing and heat treatment are carried out.

[0012] Document CN 203928838, referenced [2],proposes a method for manufacturing a heat exchange wall also formed by a double-layer tube which comprises a step of filling the gap between the inner and outer tubes with a metal powder. By ensuring a radial distribution of the temperature, this metal powder gives good thermal conductivity to the double-layer tube and also makes it possible to mitigate the risks of degradation linked to sudden thermal transitions.

[0013] Document US 2013 / 070889, referenced [3], proposes a method for manufacturing a double-layer tube implementing a cold machining step of an assembly formed by an internal tube inserted inside an external tube in which: either the outer tube, which is made of ferritic steel with at least 2% by mass of chromium, comprises, at its inner surface, a layer of calamine oxide containing chromium with a thickness of between 10 µm and 30 µm and the inner tube, which is made of steel or an alloy with at least 2% by mass of chromium, comprises, at its outer surface which is machined and / or ground to a thickness of at least 0.1 mm, a layer of calamine, or the inner tube, which is made of ferritic steel with at least 2% by mass of chromium, comprises, at its outer surface, a layer of calamine oxide containing chromium with a thickness of between 10 µm and 30 µm and the outer tube, which is made of steel or an alloy with at least 2% by mass of chromium, comprises, at its inner surface which is machined and / or ground to a thickness of at least 0.1 mm, a layer of calamine.

[0014] The cold machining step implemented in the process of the document[3] is carried out in such a way as to achieve a reduction rate of 5% to 30% of the external thickness.

[0015] Document GB 2 241 339, referenced [4], also proposes a method for manufacturing a heat exchange wall comprising three concentric metal tubes which are in intimate contact, at their interfaces, so as to ensure good thermal contact between the three tubes. This intimate contact can be obtained by brazing or welding. In the document [4], the intermediate tube may be made of an iron or steel-based material and the inner and outer tubes of copper or a copper alloy.

[0016] While they improve the thermal conductivity properties of double-layer tubes, the manufacturing processes described in the documents [1] has [4] have thermal conductivities that are too penalizing for the intended operation.

[0017] The aim of the present invention is, consequently, to propose a method for manufacturing a double-layer heat exchange wall which has the dual function of sealing and resistance to the pressure of heat transfer fluids, in particular by resisting the propagation of fatigue defects, this double-layer exchange wall then offering maximum guarantees of integrity in service for a minimum penalty in thermal conductivity. Statement of the invention

[0018] The above-stated and other objects are achieved by a method of manufacturing a double-layer heat exchange wall comprising a first layer and a second layer, the first and second layers being metallic.

[0019] According to the invention and in accordance with claim 1, the manufacturing method comprises the following successive steps (i) to (v), and optionally (vi): (i) the supply of: a first sheet intended to form the first layer and having a thickness e 1 , a second sheet intended to form the second layer and having a thickness e 2 , and an iron strip Fe 0 < having a thickness e 3 of between 10 µm and 100 µm and, advantageously, of between 50 µm and 100 µm; (ii) the assembly of the first and second sheets and the iron strip Fe 0 < , the strip being interposed between the first and second sheets; (iii) the mechanical pressing of the assembly obtained at the end of the previous step under a minimum pressure of 1 MPa; (iv) the peripheral welding of the pressed assembly obtained at the end of step (iii); (v) the heat treatment of the welded assembly obtained at the end of step (iv), this heat treatment being carried out by hot isostatic compression conducted at a temperature between 800°C and 1200°C, at a pressure between 10 8< Pa and 2.10 8< Pa, for a period of between 1 h and 3 h; and, where appropriate, (vi) additional treatment such as bending, folding, quenching, normalized tempering or annealing.

[0020] The method according to the invention makes it possible to form, between the first and second layers, a metallic interphase made of pure iron. This interphase, which is ductile and dense and which fills the entire initial volume of the gap existing between the first and second sheets, thus makes it possible to ensure a uniform mechanical junction between the first and second layers. The absence of play between these first and second layers makes it possible to avoid the movement of these two layers relative to each other: these two layers now form only a single wall, they then deform in the same way as a monolithic wall during mechanical stress or subsequent shaping. Thanks to the manufacturing method according to the invention, a double-layer wall is obtained having a geometric quality that does not require calibration after the heat treatment step by hot isostatic pressing.

[0021] The pure iron metallic interphase also makes it possible to give the double-layer wall excellent thermal conductivity properties by ensuring very good heat transfer between the first and second layers, which has the effect of increasing the efficiency of a heat exchanger equipped with such a double-layer wall. Unexpectedly and surprisingly, the Inventors found that this ductile and dense iron Fe 0 < interphase ensures a thermal conductivity which is greater than or equal to 80% of the thermal conductivity of an equivalent solid wall, or even greater than 95% in the particular case of the use of the iron strip Fe 0 < .

[0022] This ductility of the iron metal interphase also makes it possible to deflect and / or stop the propagation of fatigue cracks, particularly high-cycle fatigue cracks, which would have formed and propagated within one of the two layers, thus preserving the integrity of the other layer. In particular, when fatigue cracks are deflected, they propagate in the ductile metal interphase, thus providing a 30% longer life at room temperature.

[0023] This iron interphase is also characterized by a seal allowing it to prevent the capillary propagation of fluid coming from such a crack between the two layers.

[0024] The method according to the invention makes it possible to manufacture a double-layer wall having a thickness e tot .

[0025] The method according to the invention comprises steps (i) to (v), and optionally (vi), mentioned above and detailed below.

[0026] During step (i) of the manufacturing method according to the invention, two sheets are provided, a first sheet which is intended to form the first layer of the double-layer wall, and a second sheet which is intended to form the second layer of this same double-layer wall.

[0027] The first sheet has a thickness, noted e 1 , while the second sheet has a thickness, noted e 2 .

[0028] According to one embodiment, the thickness e 1 of the first sheet and the thickness e 2 of the second sheet are each between 1 mm and 30 mm, advantageously between 1 mm and 5 mm and, preferably, between 1 mm and 2 mm. However, although included in intervals of identical values, the thicknesses e 1 and e 2 of the first and second sheets are not necessarily identical.

[0029] It is specified that the expression "between ... and ..." which is used above but also in the rest of this description to define an interval, must be understood as defining not only the values of the interval, but also the values of the limits of this interval.

[0030] In step (i) of the manufacturing process according to the invention, a strip is also provided which is made of iron with oxidation state 0, i.e. pure iron Fe 0< .

[0031] This iron strip Fe 0< has a thickness, noted e 3 , which is between 10 µm and 100 µm and, advantageously, between 50 µm and 100 µm.

[0032] The manufacturing method according to the invention comprises, after step (i), a step (ii) of assembling the first and second sheets and the iron strip Fe 0 < , the strip being interposed between the first and second sheets.

[0033] In other words, at the end of step (ii), an assembly is obtained in which the iron strip Fe 0< is sandwiched between the first and second sheets.

[0034] In an alternative embodiment of the manufacturing method according to the invention and in accordance with claim 2, steps (i) and (ii) which have just been described can be respectively replaced by the following steps (i') and (ii'): (i') the supply of: a first sheet intended to form the first layer and having a thickness e 1 , and a second sheet intended to form the second layer and having a thickness e 2 , this second sheet comprising, on one of its surfaces, an iron coating Fe 0<; and (ii') the assembly of the first sheet and the second coated sheet, the iron coating Fe 0< being placed between the first and second sheets,

[0035] During step (i') of this variant of the manufacturing method according to the invention, two sheets are provided, a first sheet which is intended to form the first layer of the double-layer wall, and a second sheet which is intended to form the second layer of this same double-layer wall.

[0036] As before, the first sheet has a thickness, noted e 1 , while the second sheet has a thickness, noted e 2 .

[0037] According to one embodiment, the thickness e 1 of the first sheet and the thickness e 2 of the second sheet are each between 1 mm and 30 mm, advantageously between 1 mm and 5 mm and, preferably, between 1 mm and 2 mm. As already indicated previously, although included in intervals of identical values, the thicknesses e 1 and e 2 of the first and second sheets are not necessarily identical.

[0038] In this variant, the second sheet comprises, on one of its surfaces, a coating of iron with oxidation state 0, i.e. pure iron Fe 0< .

[0039] This iron coating Fe 0< has a thickness, noted e 3 , between 10 µm and 100 µm and, advantageously, between 50 µm and 100 µm.

[0040] In other words, the second coated sheet has a thickness, noted e 2' , such that e 2' = e 2 + e 3 .

[0041] According to an embodiment of this variant of the manufacturing method according to the invention, the iron coating Fe 0< is obtained by cold spraying of iron powder Fe 0< onto one of the surfaces of the second sheet.

[0042] In this variant, the manufacturing method according to the invention comprises, after step (i'), a step (ii') of assembling the first sheet and the second coated sheet, the iron coating Fe 0< being placed between the first and second sheets.

[0043] In other words, at the end of step (ii'), an assembly is obtained in which the iron coating Fe 0< is positioned between the first and second sheets.

[0044] The choice of using a Fe 0< iron strip with a thickness of 10 µm to 100 µm, as well as the choice of using a 10 µm to 100 µm thick Fe 0< iron coating by cold spraying, makes it possible to create, between the first and second layers, a ductile Fe 0< iron interphase. This ductile Fe 0< iron interphase makes it possible to significantly slow down and deflect the propagation speed of fatigue cracks that would have formed within one of these two layers and, consequently, to increase the fatigue life of the other (undamaged) layer by up to 30% and, therefore, of the double-layer wall manufactured by the method according to the invention.

[0045] As will be seen in the examples below, the choice will advantageously fall on the production of the iron interphase from the intercalation of an iron strip Fe 0 < which makes it possible to obtain a double-layer wall having mechanical and thermal conductivity properties which are superior to those of a double-layer wall in which the iron interphase is obtained by cold projection of an iron powder Fe 0 < .

[0046] In an advantageous variant, the method according to the invention further comprises a step of cleaning the surfaces of the first sheet and the surfaces of the second sheet or of the second coated sheet, this cleaning step being carried out prior to step (ii) or (ii') of assembly.

[0047] In another advantageous variant, the method according to the invention further comprises a step of grinding the first sheet and the second sheet, optionally coated, this grinding step being carried out prior to the assembly step (ii) or (ii').

[0048] The manufacturing method according to the invention comprises, after step (ii) or (ii') of assembly, a step (iii) of mechanical pressing of the assembly obtained at the end of step (ii) or (ii'), this mechanical pressing being carried out under a minimum pressure of 1 MPa.

[0049] The choice of such a mechanical pressing step (iii) has the advantage of further reducing the volume of the gap existing between the first sheet and the second sheet, possibly coated. In doing so, the quantity of gas contained between these two sheets becomes negligible.

[0050] The manufacturing method according to the invention comprises, after step (iii) of mechanical pressing, a step (iv) of peripheral welding of the assembly as obtained at the end of step (iii). In doing so, a weld of the periphery of the first and second sheets is obtained.

[0051] This welding step (iv), which can be carried out using any welding technique allowing a gas-tight weld to be obtained, makes it possible to isolate the iron interphase Fe 0 < interposed between the first and second sheets.

[0052] The welding step (iv) may in particular be carried out by laser, by rod, by electron beam or even, and advantageously, by an arc welding process with a non-consumable electrode, where appropriate in the presence of a filler metal. This latter type of welding may in particular be carried out by TIG welding (TIG being the acronym for the English expression "Tungsten Inert Gas") without filler metal.

[0053] The presence of the iron Fe 0 < interphase makes it possible not to affect the composition of the welded joints nor the composition of the first and second sheets which therefore retain their mechanical properties. This choice of iron Fe 0 < interphase therefore makes it possible to ensure assembly, by means of a reliable weld, of the double-layer walls manufactured by the method according to the invention with the structures of a device which they are intended to equip, for example with those of a heat exchanger. Indeed, since the iron Fe 0 < of the interphase only marginally modifies the composition of the peripheral weld, the interphase therefore does not alter the mechanical properties of this peripheral weld thus produced.

[0054] The manufacturing method according to the invention comprises, at the end of the welding step (iv), a step (v) of heat treatment of the welded assembly as obtained at the end of step (iv).

[0055] This heat treatment step (v) is carried out by hot isostatic pressing (HIP) conducted at a temperature between 800°C and 1200°C, at a pressure between 10 8< Pa and 2.10 8< Pa, for a duration between 1 h and 3 h.

[0056] This hot isostatic pressing step (v) makes it possible to weld, by diffusion, the first sheet and the second sheet, possibly coated, and in doing so, to obtain a uniform mechanical junction between these first and second sheets. At the end of the hot isostatic pressing step (v), this uniform mechanical junction is formed by the iron interphase Fe.

[0057] In an advantageous variant, the method according to the invention further comprises at least one step of rectification of the assembly, this or these rectification steps being implemented prior to the welding step (iv) and / or prior to the heat treatment step (v).

[0058] In a particularly advantageous variant, the method according to the invention does not comprise a degassing step between the mechanical pressing step (iii) and the welding step (iv) and / or does not comprise a degassing step between the welding step (iv) and the heat treatment step (v).

[0059] The elimination of one or two degassing steps, which are particularly complex to implement on an industrial level in the context of the manufacture of double-layer walls, contributes to a significant reduction in cycle times, costs and production lead times.

[0060] The method according to the invention makes it possible to manufacture double-layer walls of variable lengths and geometries.

[0061] The materials of the first tube and the second tube (excluding the coating) may be the same or different. These materials may include iron.

[0062] In an advantageous variant, the first and second sheets are made of a material comprising iron, this material being advantageously chosen from iron Fe 0 < and a steel, for example martensitic steel such as Eurofer-97,

[0063] In a preferred embodiment, the materials of the first and second sheets are identical,

[0064] In a variant, the method according to the invention further comprises at least one step (vi) of additional treatment. Such additional treatment (vi) is intended in particular to confer a specific shape and / or particular mechanical properties to the double-layer wall manufactured by the method according to the invention.

[0065] This additional treatment (vi) may, for example, be chosen from bending, folding, quenching, normalized tempering and annealing, or even be a combination of additional treatments (quenching followed by normalized tempering).

[0066] By means of such additional treatment (vi), the double-layer wall can, for example, be shaped into a tube with a circular, square or rectangular section.

[0067] The invention will be better understood in light of the additional description which follows and which refers to the Figures 1 and 2 annexed.

[0068] It is specified that this additional description is given only as an illustration of the subject of the invention and that it must in no case be interpreted as a limitation of this subject which is defined by claims 1 to 11. Brief description of the drawings

[0069] There figure 1 illustrates the evolution of the progression of a fatigue crack (noted a and expressed in mm), at room temperature, as a function of the number of cycles (noted N) of two double-layer walls obtained by a process according to the invention (noted RS and CS) and of a similar massive reference wall (noted Massive). The figure 2illustrates the evolution of the equivalent thermal conductivity (noted λ and expressed in W / mK) as a function of the temperature (noted T and expressed in K) of two double-layer walls obtained by a process according to the invention (noted RS and CS) and of a similar massive reference wall (noted Massive). Detailed description of specific embodiments 1. Manufacture of walls double layer by the process according to the invention and a reference wall 1.1. A first double-layer wall, denoted RS, was manufactured by the method according to the invention,

[0070] Two Eurofer-97 sheets were rolled to give them a thickness of 6.00 mm (+0.0 / -0.1 mm) for the first sheet and a thickness of 4 mm (+0.0 / -0.1 mm) for the second sheet.

[0071] A sheet of Fe 0 < metal iron was then rolled until a strip with a thickness of 100 µm was obtained.

[0072] After grinding and cleaning the surfaces of the two sheets, the two sheets and the strip were assembled in a sandwich by interposing this metallic iron strip between the first and second sheets.

[0073] The assembly was then mechanically pressed using a press to exert a pressure of 1 MPa, followed by peripheral TIG welding around the entire perimeter of the assembly.

[0074] The welded assembly was then subjected to heat treatment by carrying out a hot isostatic compression (HIC) cycle conducted at a temperature of 1100°C, at a pressure of 1200 bar (1.2.10 8< Pa), for 1 h, then to additional heat treatments of quenching after holding at a temperature of 980°C for 30 min and tempering at 760°C for 90 min.

[0075] At the end of these heat treatments, we obtain the RS double-layer wall. 1.2. A second double-layer wall, denoted CS, was manufactured by the method according to the invention,

[0076] Two Eurofer-97 sheets were rolled to give them a thickness of 6.00 mm (+0.0 / -0.1 mm) for the first sheet and a thickness of 4 mm (+0.0 / -0.1 mm) for the second sheet.

[0077] A cold spray deposition of metallic iron powder was then carried out on one of the surfaces of the second sheet so as to form an iron coating with a thickness of more than 100 µm.

[0078] After rectification of the deposit to a thickness of 100 µm and then cleaning of the surfaces of the two sheets, the first sheet and the second coated sheet were assembled, the metallic iron coating being positioned between the first and second sheets.

[0079] The assembly thus obtained was then subjected to the stages of mechanical pressing, welding, heat treatment by CIC and additional heat treatments described in chapter 1.1. above.

[0080] At the end of these heat treatments, we obtain the double-layer CS wall. 1.3. A reference wall, noted Massive, was also created.

[0081] A Eurofer-97 sheet was rolled to give it a thickness of 10.00 mm (+0.0 / -0.1 mm).

[0082] The sheet was then subjected to the CIC heat treatment and additional heat treatment steps described in chapter 1.1. above.

[0083] At the end of these heat treatments, we obtain the Massive reference wall. 2. Characterization of the RS and CS double-layer walls and the Massive reference wall

[0084] 2.1. 10 x 10 x 55 mm bending specimens were taken from each of the RS, CS and Massive walls. A 2 mm notch, marked by point A on the Figure 1 , was carried out in each of the specimens, respectively noted RS, CS and Massive, in order to allow the initiation of a fatigue crack during fatigue loading.

[0085] The propagation of this notch was then examined by subjecting the specimens to 3-point bending stress cycles at a constant maximum stress of 60% of the yield strength of the sheet material. A minimum force of 150 N is maintained so as not to detach the tool from the specimen. The path of the crack is followed in situ by image correlation.

[0086] With reference to the area marked B on the Figure 1, it is observed that the RS and CS specimens in accordance with the invention are characterized by delayed crack propagation compared to that of the Massive reference specimens.

[0087] With reference to the area marked C on the Figure 1 , it is also noted that the RS and CS specimens in accordance with the invention show a stagnation of the crack length at the interphase (propagation distance of 6 mm). This leads to an increase in the lifetime of the RS and CS double-layer walls compared to that of the Massive reference wall.

[0088] These results clearly demonstrate that the choice of a ductile Fe 0< interphase makes it possible to significantly modify the crack propagation speed and significantly increase the fatigue life at room temperature of the undamaged layer.

[0089] 2.2. The thermal conductivity of each of the RS, CS and Massive specimens was measured.

[0090] Referring to the results illustrated on the figure 2 , it is observed that the RS and CS specimens manufactured by the process according to the invention have a thermal conductivity λ very close to the Massive reference specimen and that this thermal conductivity coefficient represents at least 80% of the thermal conductivity of the Massive reference wall for the CS double-layer wall and nearly 95% for the RS double-layer wall. Bibliography

[0091] [1] US 2013 / 0205861 A1 [2] CN 203928838 U [3] US 2013 / 070889 A1 [4] GB 2 241 339 A

Claims

1. Method for manufacturing a double-layer heat-exchange wall comprising a first layer and a second layer, the first and second layers being metallic, this manufacturing method being characterised in that it comprises the following successive steps (i) to (v), and optionally (vi): (i) the provision: . of a first metal sheet intended to form the first layer and having a thickness e1, . of a second metal sheet intended to form the second layer and having a thickness e2, and . of a leaf of iron Fe0 having a thickness e3 of between 10 µm and 100 µm and, advantageously, between 50 µm and 100 µm; (ii) the assembly of the first and second metal sheets and the leaf of iron Fe0, the leaf being interposed between the first and second metal sheets, (iii) the mechanical pressing of the assembly obtained at the end of the previous step at a minimum pressure of 1 MPa; (iv) the peripheral welding of the pressed assembly obtained at the end of step (iii); (v) the heat treatment of the welded assembly obtained at the end of step (iv), this heat treatment being implemented by hot isostatic pressing conducted at a temperature of between 800°C and 1200°C, at a pressure of between 108 Pa and 2.108 Pa, for a period of between 1 hour and 3 hours; and, optionally, (vi) a supplementary treatment such as curving, bending, quenching, normalised ageing or annealing.

2. Method according to claim 1, wherein steps (i) and (ii) are respectively replaced by the following steps (i') and (ii'): (i') the provision: . of a first metal sheet intended to form the first layer and having a thickness e1, and . of a second metal sheet intended to form the second layer and having a thickness e2, this second metal sheet comprising, on one of its surfaces, a coating of iron Fe0, this coating having a thickness e3 of between 10 µm and 100 µm and, advantageously, between 50 µm and 100 µm, the coated second metal sheet having a thickness e2' such that e2' = e2 + e3; and (ii') the assembly of the first metal sheet and of the coated second metal sheet, the coating of iron Fe0 being placed between the first and second metal sheets.

3. Method according to claim 2, wherein the coating of iron Fe0 is obtained by cold spraying of iron powder Fe0 on one of the surfaces of the second metal sheet.

4. Method according to any one of claims 1 to 3, wherein the welding step (iv) is implemented by an arc welding method with a non-meltable electrode, where appropriate in the presence of a filler metal.

5. Method according to any one of claims 1 to 4, further comprising a step of cleaning the surfaces of the first metal sheet and the surfaces of the coated second metal sheet, this cleaning step being implemented prior to the assembly step (ii) or (ii').

6. Method according to any one of claims 1 to 5, further comprising a step of grinding the first and second metal sheets, this grinding step being implemented prior to the assembly step (ii) or (ii').

7. Method according to any one of claims 1 to 6, further comprising at least one step of grinding the assembly, this or these grinding steps being implemented prior to the welding step (iv) and / or prior to the heat treatment step (v).

8. Method according to any one of claims 1 to 7, which does not comprise a degassing step between the mechanical pressing step (iii) and the welding step (iv) and / or between the welding step (iv) and the heat treatment step (v).

9. Method according to any one of claims 1 to 8, wherein the first and second metal sheets are produced from a material comprising iron, this material advantageously being selected from iron Fe0 and a steel.

10. Method according to any one of claims 1 to 9, wherein the materials of the first and second metal sheets are identical.

11. Method according to any one of claims 1 to 10, wherein the thicknesses e1 and e2 of the first and second metal sheets are between 1 mm and 30 mm, advantageously between 1 mm and 5 mm, and preferentially between 1 mm and 2 mm.