Field of the Disclosure

[0001] The present disclosure is related to laser welding of metal sheets, and more particularly to a high speed laser welding method for fuel cell

separators.

Background of the Disclosure

[0002] Laser welding methods for welding metal sheets together are known. US 2017/0028515 A1 discloses an example of device and method for welding a plurality of flat metal sheets together, said metal sheets being clamped and laser welded.

[0003] However, metal sheets used as fuel cell separators are thin. In such a case, the thermal expansion of the metal caused by the laser beam can distort the flat shape of the sheets, and create a gap between them. A known method for compensating the gap created between the metal sheets to be welded is to use full penetration welding.

[0004] According to such a method, both the top sheet and the bottom sheets are melted around the welding surface, which is the surface reached by the laser beam. This allows the liquid melted metal to bridge the gap between the top and the bottom metal sheets, and therefore produce a continuous weld between said metal sheets.

[0005] However, in order to achieve such a full penetration method, a wish amount of energy per unit of length is required. Too high amounts of energy present the risk of cutting the sheets, rather than welding them. In addition, in order to achieve the full penetration welding, the displacement of the laser beam is slowed down, in order to let both the top and the bottom sheets time to melt. Nevertheless, by doing that, the process speed is limited and the risk of cutting the sheets remain.

Summary of the Disclosure

[0006] In the configurations described in the above prior art, the process of welding metal sheets together is slowed down due to the necessity of filling the gap created between the metal sheets during full penetration welding by the laser beam. Therefore, it remains desirable to provide a more efficient method for laser welding metal sheets.

[0007] According to the present disclosure, a method of laser welding a first and a second metal sheet is provided. The method comprises:

Clamping the first and the second metal sheets together with a clamping device, the shape of the first and second metal sheets being such that, when the first and second metal sheets are clamped together, a force is applied by the first metal sheet on the second metal sheet at a welding surface in a stacking direction;

Laser welding the first metal sheet with the second metal sheet at the welding surface.

[0008] In the present disclosure, a stacking direction is a direction in which, before the clamping step, the first and the second metal sheets are stacked one on the other. Usually, the stacking direction is the vertical direction. For example, the first sheet may be positioned above the second metal sheet in the stacking direction.

[0009] According to this method, at least one of the metal sheets, for example the first metal sheet, does not have a flat shape. In most cases, the first metal sheet is shaped such that, when the first sheet is stacked on the second metal sheet before the step of clamping, the contact surface between the first sheet and the second sheet is discontinuous. The welding surface, which is a surface at which the first and second metal sheets have to be welded, is a surface at which the first and second metal sheets are in contact with each other when the first sheet is stacked on the second sheet, and when said first and second metal sheets are clamped together.

[0010] According to this method, at the clamping step, clamped portions of the first and second metal sheets are clamped by the clamping device. In other words, the clamping device sandwiches said portions so as to hold the first and the second metal sheets together. At this step, the first metal sheet is shaped so that a force applied by the clamping device on the first metal sheet is transmitted to the welding surface, so that a force is applied on said welding surface in the stacking direction. This force allows holding together the first and second metal sheets at the welding surface in a tight manner.

[0011] Thus, at the laser welding step, this force applied at the welding surface is opposed to the thermal expansion of the metal caused by the laser beam at said welding surface. Preferably, the force exerted at the welding surface is greater than or equal to a deformation force of the preformed metal sheet caused by the heat of the laser beam at the laser welding step. In such a way, the gap created between the first and the second metal sheet is reduced, or even eliminated, so that it is no more necessary to create a large amount of molten metal to bridge said gap between the sheets. Therefore, partial penetration welding becomes applicable, instead of full penetration welding. In partial penetration welding, a lower volume of molten metal is generated, since only a small portion of the second sheet is melted. This can be achieved with a lower energy input, and therefore the laser welding can proceed at a faster speed, so that the process efficiency is improved.

[0012] According to embodiments of the present disclosure, the clamping device applies a pressure on the first and second metal sheets on both sides of the welding surface, as seen along the stacking direction.

[0013] By doing that, the force can be uniformly applied at the welding surface. In addition, no liberty degree exists between the first and the second metal sheets when said first and second metal sheets are clamped together, so that the force is more effectively concentrated at the welding surface.

[0014] According to embodiments of the present disclosure, the method comprises, before clamping the first and the second metal sheets together, a step of preforming at least one of the first and the second metal sheets so that, when the first and second metal sheets are clamped together, a force is applied by the first metal sheet on the second metal sheet at the welding surface in a stacking direction.

[0015] At least one of the first and the second metal sheets may be shaped, for example, by stamping, before the clamping step, that is, before the first and the second metal sheets are stacked. Such a preforming step, for example by stamping, is easy to perform and allows reaching the above mentioned effect that a force is applied on the welding surface at the clamping step.

[0016] According to embodiments of the present disclosure, at least one of the first and the second metal sheets comprises a groove, the groove having a U-shape in a cross section perpendicular to a longitudinal direction of the groove.

[0017] The groove may extend along a longitudinal direction of the first metal sheet. When the first metal sheet, for example, comprising the groove, is stacked on the second metal sheet, a bottom portion of the U-shaped groove is in contact with the second metal sheet. Thus, when the first and second metal sheets are clamped together, the clamping device applies a pressure on the first metal sheet, and a force is thus applied on the bottom portion of the U- shaped groove in the stacking direction.

[0018] According to embodiments of the present disclosure, a width of the U-shaped groove is between 0,1 and 5 mm, preferably between 0,3 and 3 mm, most preferably between 0,5 and 1 mm, and a depth of the U-shaped groove is between 0,05 and 3 mm, preferably between 0.1 and 1 ,5 mm, most preferably between 0,2 and 0,5 mm.

[0019] The width of the U-shape groove is considered in a direction perpendicular to the stacking direction, and the depth of the U-shape groove is considered in the direction of the stacking direction. These dimensions allow having a substantially flat sheet, although said sheet is preformed, while reaching the effect of applying a force on the welding surface.

[0020] According to embodiments of the present disclosure, the welding surface includes a stripe extending along a bottom portion of the groove.

[0021] In other words, the welding surface may be a continuous line extending along the bottom portion of the groove. The welding surface may also be a discontinuous line extending along the bottom portion of the groove. Thus, the laser welding step may be performed along said bottom portion of said groove. Since a force is applied at said bottom portion thanks to the U-shaped groove, a gap created between said bottom portion and the second metal sheet is reduced.

[0022] According to embodiments of the present disclosure, when the first and the second metal sheets are clamped together, on each side of the welding surface, a clamped portion of the first metal sheet and a clamped portion of the second metal sheets are maintained in contact with each other by the clamping device, and a non-contact portion of the first metal sheet, which is a portion not in contact with the second metal sheet, is arranged between the clamped surface of the first metal sheet and the welding surface.

[0023] The clamped portion of the first or the second metal sheet is a portion where a force is applied by the clamping device to said first or second metal sheet. The clamped portions of the first and second metal sheets are portions where the first and second metal sheets are sandwiched together by the clamping device. According to this configuration, on each side of the welding surface, a part of the force applied by the clamping device at the clamped portion is transmitted to the non-contact portion, a force being then transmitted from the non-contact portion to the welding surface in the stacking direction.

[0024] According to embodiments of the present disclosure, when the first and the second metal sheets are clamped together, a non-contact portion of the first metal sheet is arranged only on one side of the welding surface, and the clamping device applies a pressure on at least part of the non-contact portion.

[0025] In this configuration, a non-contact portion is arranged only on one side of the welding surface. The other side of the welding surface may only be a contact portion in which both metal sheets continuously contact each other, without any non-contact portion. Thus, the force applied by the clamping device on the non-contact portion on the one side of the welding surface is transmitted to the welding surface in the stacking direction, and the force applied by the welding surface on the other side of the welding surface allows maintaining the first and the second metal sheets in contact with each other, so that no gap is formed between said metal sheets.

[0026] According to embodiments of the present disclosure, when the first and the second metal sheets are clamped together, a non-contact portion of the first metal sheet is arranged on both sides of the welding surface, and the clamping device applies a pressure on at least part of each non-contact portion.

[0027] In this configuration, the force applied by the clamping device on the non-contact portion on each side of the welding surface is transmitted to the welding surface in the stacking direction. Thus, a force is applied at the welding surface from both sides of the welding surface by the clamping device. In this case, a groove may be arranged between the non-contact portion on one side of the welding surface and the non-contact portion on the other side of the welding surface. Thus, when the clamping device applies a force on said non- contact portions, this force is transmitted toward the bottom portion of the groove.

[0028] According to embodiments of the present disclosure, on each side of the welding surface, the clamping device clamps the first and the second metal sheets together, at a distance lower than 5 mm, preferably lower than 2 mm, more preferably lower than 1 mm from said welding surface. [0029] This distance may be determined according to the shape of the metal sheets, for example the dimensions of the groove formed on one of these sheets. This distance can be set so as to increase or decrease the force applied at the welding surface, another parameter being the shapes of the first and the second metal sheets.

[0030] According to embodiments of the present disclosure, the second metal sheet is flat.

[0031] Preferably, the second metal sheet is flat at least at the welding portion. This configuration allows a reliable contact between the first and the second metal sheets, thus increasing the efficiency of the method.

[0032] According to embodiments of the present disclosure, the clamping device comprises a first clamping jig exerting a pressure on one side of the welding surface, and a second clamping jig exerting a pressure on the other side of the welding surface.

[0033] According to embodiments of the present disclosure, the first and second metal sheets are fuel cell separators.

[0034] This method is advantageous when applied to fuel cell separators, since said fuel cell separators already need to be preformed by a stamping process.

[0035] It is intended that the present disclosure encompasses all

combinations of the above-described elements and those within the

specification, except if they are not technically compatible.

[0036] It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the disclosure, as claimed.

[0037] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the disclosure and together with the description, serve to explain the principles thereof.

Brief description of the drawings [0038] Figure 1 shows a cross section of an exemplary apparatus for welding two metal sheets according to a laser welding method of the prior art ;

[0039] Figure 2 is a flowchart highlighting a method of laser welding two metal sheets according to embodiments of the present disclosure; [0040] Figure 3 shows a cross section of a first example of a preformed metal sheet and the laser welding method according to the present disclosure;

[0041] Figure 4 shows a cross section of a second example of a preformed metal sheet and the laser welding method according to the present disclosure;

[0042] Figure 5 shows a cross section of a third example of a preformed metal sheet and the laser welding method according to the present disclosure.

Description of the embodiments

[0043] Reference will now be made in detail to exemplary embodiments of the disclosure, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts.

[0044] Figure 1 shows a cross section of an exemplary apparatus for welding two metal sheets according to a laser welding method of the prior art. The apparatus comprises a clamping device 10, comprising a support 13, a first clamping jig 11 and a second clamping jig 12. A first metal sheet 20 is stacked on a second metal sheet 30, said second metal sheet 30 being disposed on the support 13. A stacking direction is a direction in which, before the clamping step, the first and the second metal sheets 20, 30 are stacked one on the other. In this case, the stacking direction is the vertical direction, that is, the up/down direction on figure 1. The first metal sheet 20 may be referenced as the upper metal sheet, positioned above the second metal sheet 30, that is, the lower metal sheet, in the stacking direction. In addition, the first and the second metal sheets 20, 30 may be stainless steel sheets, and may have a thickness between 25 pm and 200 pm.

[0045] During the clamping step, the first clamping jig 11 and the second clamping jig 12 are moved downward in the stacking direction, so as to come into contact with the first metal sheet 20, and to apply a pressure on the first metal sheet 20. The first metal sheet 20 and the second metal sheet 30 are thus clamped between the support 13, the first and the second clamping jigs 20 and 30, as shown by the arrows on figure 1.

[0046] At this stage, the first and the second clamping jigs 11 and 12 apply a pressure on the first metal sheet 20 on both sides of a welding surface 21 , which is a surface at which the first and the second metal sheets have to be welded together. A width S of the welding surface in the cross section of the metal sheets is between 0.5 and 8 mm. The width S also corresponds to a portion of the metal sheets which is not clamped by the clamping device 10. A groove 15 is provided in the support 13 below the welding surface 21 , so as to prevent the support 13 from being melted at the laser welding step. The groove 15 may also be used for applying a shielding gas, for example argon, to prevent oxidation of the metal sheets during welding.

[0047] The apparatus also comprises a laser device (not shown). At the laser welding step, the laser device generates a laser beam 40 which is concentrated at the welding surface 21. The laser beam 40 is moved along a welding line extending between the first and the second jig 11 , 12, in a direction perpendicular to the cross section illustrated in figure 1. The laser beam 40 melts the metal of the first and the second metal sheets 20, 30, so that said first and second metal sheets 20, 30 are welded together along this line.

[0048] However, the heat generated by the laser beam 40 generates a thermal expansion F of the first metal sheet 20, in a direction opposite to the stacking direction. A deformation of the first metal sheet 20 is thus generated. The deformed portion of the first metal sheet 20 is the portion having the width S which is not clamped by the clamping device 10. With such a deformation, a gap g is created between the first and the second metal sheet 20, 30. Such a gap g may be greater than 10 pm, or even greater than 20 pm according to the distance S. In order to fill this gap and to ensure a reliable welding between the first and the second metal sheets 20, 30, the laser beam has to move more slowly along the welding line, in order to melt a sufficient amount of metal of both the first and the second metal sheets 20, 30, and to ensure a reliable welding of the two metal sheets.

[0049] Figure 2 is a flowchart highlighting an exemplary method of laser welding two metal sheets according to an embodiment of the present disclosure.

[0050] One of the first and the second metal sheets is firstly preformed (step S1 ). According to this preforming step, in this embodiment, the first metal sheet is deformed so as to obtain a non-flat sheet having a groove. Then, the preformed metal sheet and the other metal sheet are stacked one on the other and placed in the clamping device so as to be clamped together (step S2). Thereafter, the first and second metal sheets are laser welded together

(step S3). [0051] Figure 3 shows a cross section of a first example of a preformed metal sheet and the laser welding method according to the present disclosure. According to this example, before the clamping step S2, the first metal sheet 20 is preformed at the preforming step S1 so as to form a groove 22, for example by stamping said first metal sheet 20, the groove 22 extending along a longitudinal direction of the first metal sheet 20, which is a direction

perpendicular to the cross section illustrated in figure 3. In this cross section, the groove 22 has a U-shape, said U-shape having a width w between 0,5 and 1 mm, and a depth d between 0,2 and 0,5 mm. The U-shaped groove 22 comprises a bottom portion 23 which comes into contact with the second metal sheet 30 when the first and the second metal sheets 20, 30 are stacked.

[0052] At this time, before the clamping step S2 (corresponding to the left part of figure 3), the first metal sheet 20 comprises a first raised portion 20a on one side of the groove 22 (the left side in figure 3), and a second raised portion 20b on the other side of the groove 22 (the right side in figure 3). The first and the second raised portions 20a, 20b are portions of the first metal sheet 20 which are not in contact with the second metal sheet 30, before the clamping step S2, even when the bottom portion 23 of the groove 22 contacts the second metal sheet 30.

[0053] Then, at the clamping step S2 (corresponding to the right part of figure 3), the first metal sheet 20 and the second metal sheet 30 are pressed between the support 13 and the first and second clamping jig 11 , 12. More specifically, the first jig 11 applies a force on a portion of the first raised portion 20a, so that a part of said first raised portion 20a comes into contact with the second metal sheet 30. Thus, when the first and second metal sheets 20, 30 are clamped together, the first raised portion 20a is divided into a clamped portion 20a1 , which is in contact with a clamped portion 30a1 of the second metal sheet 30, said clamped portions 20a1 and 30a1 being clamped between the support 13 and the first jig 11 , and a non-contact portion 20a2, which is not in contact with the second metal sheet 30. The non-contact portion 20a2 extends between the clamped portion 20a1 and the top end of a first branch 22a of the U-shaped groove 22. In addition, a distance L between a limit between the clamped portion 20a1 and the non-contact portion 20a2, and the bottom portion 23 of the groove 22, that is, the welding surface 21 , is between 1 and 5 mm. [0054] Symmetrically, the second jig 12 applies a force on a portion of the second raised portion 20b, so that said second raised portion 20b comes into contact with the second metal sheet 30. Thus, when the first and second metal sheets 20, 30 are clamped together, the second raised portion 20b is divided into a clamped portion 20b1 , which is in contact with a clamped portion 30b1 of the second metal sheet 30, said clamped portions 20b1 and 30b1 being clamped between the support 13 and the second jig 12, and a non-contact portion 20b2, which is not in contact with the second metal sheet 30. The non- contact portion 20b2 extends between the clamped portion 20b1 and the top of a second branch 22b of the U-shaped groove 22.

[0055] In this configuration, the first metal sheet 20 thus deformed by the clamping device 10 acts as a spring against the second metal sheet 30. In other words, with the deformations created by the first and the second jigs 11 , 12, the non-contact portions 20a2 and 20b2 act as levers, and apply a force F on the first and second branches 22a, 22b of the U-shaped groove 22, said force being directed toward the bottom portion 23 of said U-shaped groove 22, that is, the welding surface 21.

[0056] In such a manner, at the laser welding step S3, the deformation of the first metal sheet 20 caused by the heat of the laser beam 40, is

counteracted by the force F applied at the welding surface 21 in the stacking direction. Thus, the gap g created between the first and the second metal sheets 20, 30 is maintained lower than 10 pm, preferably lower than 5 pm, more preferably lower than 2 pm. Therefore, a lower amount of metal needs to be melted compared to the prior art, so that the laser welding process speed can be increased. For example, the laser welding speed, which is the displacement speed of the laser beam 40, can be increased from 750 mm/s, in the case of two flat metal sheets, to 1350 mm/s or more, in the present disclosure using a preformed metal sheet.

[0057] Figure 4 shows a cross section of a second example of a preformed metal sheet and the laser welding method according to the present disclosure. This second example differs from the first example in that each of the raised portions 20a, 20b comprises a leg portion 20a3, 20b3, which bears on the second metal sheet 30. Thus, the non-contact portion 20a2 extends between the leg portion 20a3 and the first branch 22a of the U-shaped groove 22, and the non-contact portion 20b2 extends between the leg portion 20b3 and the second branch 22b of the U-shaped groove 22.

[0058] In this embodiment, the first clamping jig 11 comes into contact with the non-contact portion 20a2. Contrary to the first example, at the clamping step S2, the raised portion 20a is not further deformed to come into contact with the second metal sheet 30, since the deformation of said raised portion 20a is prevented by the leg portion 20a3. In the same manner, the second clamping jig 12 comes into contact with the non-contact portion 20b2. Contrary to the first example, at the clamping step, the raised portion 20b does not come into contact with the second metal sheet 30, since the deformation of said raised portion 20b is prevented by the leg portion 20b3.

[0059] In such a manner, the transfer mechanism of the force applied by the clamping device 10 differs from the first example in that a part of the force applied by the clamping jigs 11 , 12, is directly transmitted from the non-contact portions 20a2, 20b2 toward the bottom portion 23 of the U-shaped groove 22.

[0060] Figure 5 shows a cross section of a third example of a preformed metal sheet and the laser welding method according to the present disclosure.

In this example, a raised portion 20c is formed only on one side of the welding surface 21. According to this example, no groove 22 is provided. On the other side of the welding portion 21 , both the first and the second metals sheets 20, 30 are flat. Thus, the first clamping jig 11 clamped the first and the second metal sheets 20, 30 which are in contact to each other, and the second clamping jig 12 applies a force on a non-contact portion 20c2. In the same manner as in the second example, the non-contact portion 20c2 does not come into contact with the second metal sheet 30, due to the presence of the leg portion 20c3. Thus, the force F is applied by the second jig 12 from the non- contact portion 20c2 to the welding surface 21. At the same time, the first jig 11 allows maintaining firmly the first and the second metal sheets 20, 30 together, so as to prevent any movement of the first metal sheet 20 relative to the second metal sheet 30 at the laser welding step S3.

[0061] Although the present disclosure herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present disclosure. [0062] It is intended that the specification and examples be considered as exemplary only, with a true scope of the disclosure being indicated by the following claims.