METHOD FOR FACE-SIDE LASER WELDING

Description

[0001] The invention relates to a method having the features of the preamble of claim 1.

[0002] There are numerous applications where joining flanges of metal components, held together with their flat sides against each other, are welded together at their joints – the parallel end faces of the joining flanges. In many cases, laser welding is used for this purpose. The joining flanges of the components being joined can be adjacent end regions following the longitudinal extent of the joints, for example, of sheet metal workpieces, or sections of such a workpiece that have been specifically bent or flanged to form the joining flanges. To enable the end-face welding, the joining flanges are fixed together in a suitable manner, for example, using a suitable clamping device, until the welding is completed.

[0003] From DE 10 2013 107 228 B3, an end-face laser welding of joining flanges of two joining partners is known, where the joining partners are sheet steel parts. The laser welding process disclosed in this document is designed to supply a central area of ​​the weld with a lower energy input and the adjacent edge areas located within the joints with a higher energy input. In order to perform the welding across the desired joint width, the welding laser beam is moved back and forth in an oscillating motion transverse to the feed direction.In this previously known method, this welding laser beam movement is also used to introduce the required varying energy input across the weld cross-section, either by extending the dwell time and / or increasing the laser beam energy in the edge regions of the weld, where welding with higher energy input is required. This allows the weld zone to penetrate relatively deep into the joints in these areas.

[0004] A problem with welding sheet steel components, especially in vehicle body construction, is that the components are coated with a metal layer. This coating can be a corrosion-inhibiting layer, such as a zinc coating. Typically, such a zinc coating is applied as hot-dip galvanizing or electrolytic galvanizing. The difficulty in joining these sheet metal components lies in the fact that the melting point and vaporization temperature of the metallic coating are significantly lower than the melting point of the steel.This has the consequence that when welding two workpieces where at least one joining partner has a metal coating with an evaporation temperature below the melting point of the steel, as is the case, for example, with a zinc coating, the coating material spontaneously and explosively vaporizes due to the heat input required for welding. In the case of a zinc coating, this is also referred to as zinc degassing. The result is increased spatter formation, which also carries away parts of the molten weld pool and causes increased porosity in the weld seam. This porosity can be explained by the fact that the vaporized zinc cannot degas freely. Furthermore, the welding process induces residual stresses in the weld seam.In combination with the existing diffusion possibilities, the remaining liquid zinc can embrittle the weld through a process known as "Liquid Metal Embrittlement" (LME). Therefore, such a weld generally does not meet the strength requirements otherwise placed on the welded component, especially under cyclic or dynamic loads, as the material embrittlement increases the risk of cracking.

[0005] In the welding process disclosed in DE 10 2013 107 228 B3, a gap must be maintained between the joining flanges of the joining partners when welding galvanized sheets. This gap must be neither too large nor too small. To maintain the gap along the length of the joints, correspondingly high demands are placed on the clamping device. Furthermore, it is unavoidable in this process that some of the laser energy is lost, specifically the portion of the laser beam that passes through the gap between the joining partners.

[0006] WO 2016 / 169791 A1 discloses a method by which two such joining partners can be welded together in the area of ​​their joining flanges by end-face laser welding with reduced zinc degassing problems. In this previously known method, the weld pool is kept liquid for a longer period by means of laser beam oscillation transverse to the feed direction at a frequency between 80 Hz and 1000 Hz. In addition, the laser beam oscillation introduces a certain dynamic into the weld pool. Both measures support a controlled degassing process through the weld pool. However, the required welding depth also increases with increasing sheet thickness. This makes it increasingly difficult, especially when two joining partners are zinc-coated, to ensure that the degassing process is complete or controlled, as the zinc has to travel a longer distance to reach the surface.Even though this process allows for end-to-end laser welding of joining partners, at least one of which has a coating with an evaporation temperature below the melting point of the steel material, the feed rate is limited, in particular, by the necessary laser oscillation perpendicular to the feed direction. Consequently, more time must be allocated for the laser welding process in series production.

[0007] A laser welding process for coated steel sheets is known from EP 0 771 605 A2. According to this process, the sheets are positioned at an acute angle to each other in the weld area before welding, such that the weld can be formed as a fillet weld. This provides an additional degassing path between the two joining partners. A corresponding process is also known from DE 10kl 2008 033 113 A1.

[0008] DE 10 2012 104 362 A1 discloses a method for step-welding a face-to-face flange connection. In order to control degassing that occurs when joining coated steel sheets, the weld is executed only as a step-weld according to this previously known method.

[0009] In JP 2005 324235 A, a welding process for laser welding two coated steel sheets, primarily used for manufacturing fuel tanks for vehicles, is described. This prior art method aims to prevent porosity resulting from the coating's evaporation temperature, which is already below the melting point of the steel. This previously known laser welding process is based on the understanding that, to prevent such porosity, the joining flanges of the joining partners are kept at a distance from each other to provide a degassing gap. A disadvantage noted is the use of spacers for arranging the joining partners, which sometimes have to be removed after welding. Furthermore, the required width of the joining flange in previously known methods is criticized.To improve such a laser welding process, this prior art proposes arranging the joining partners in such a way that a degassing gap is provided, opening away from the laser-exposed joint surface. It is provided that the joint edges of adjacent joints are spaced apart, and thus the degassing gap widens from this gap width located in the region of the joint edges in the direction of heat input. In this prior art, it is essential that the distances in the region of the opposing joint edges and in the region of the end of the joining flanges, as well as the width of the joining flanges, are in a very specific geometric ratio to one another.To bridge the gap between the two opposing butt edges, the welding process described in this prior art is carried out using two lasers, each with its laser beam directed at one of the butt edges, so that the molten metal fills the gap between the two butt edges. Although satisfactory welding results for coated sheet steel parts can be achieved with this prior art method, maintaining the required distance between the two opposing butt edges for the laser welding process remains a challenge, especially when producing longer welds. Furthermore, particular attention must be paid to ensuring that the distance between the joining flanges is increased in order to achieve the desired welding result.Especially when thinner sheet steel parts are added, this requires higher standards for the dimensional accuracy of the forming of the joining flanges.

[0010] Based on the prior art discussed at the outset, the invention therefore aims to further develop a method for end-face laser welding of the joints of two joined flanges of two joining partners made of a steel material in such a way that the welds can be carried out with a higher feed rate and more effective coating degassing, avoiding porosity and LME, and thus faster than with the method known from WO 2016 / 169791 A1. Furthermore, the disadvantages identified in JP 2005 324235 A are to be avoided.

[0011] According to the invention, this problem is solved by a generic method mentioned at the outset, having the features of claim 1.

[0012] The term "joining flange" used in this context refers to the edge section of one joining partner by which it is connected to the joining flange of the other joining partner. Thus, the joining flange of one joining partner is defined as that section which, starting from the adjacent joint, corresponds to the penetration depth of the material melted during welding.

[0013] In this process, unlike the prior art discussed earlier, the joining partners are not held parallel with their opposing sides, but rather at an angle to each other. This angle opens away from the laser-exposed joint surface. The laser welding is performed in this position of the joining flanges relative to each other. Due to the angular positioning of the joining flanges, a degassing gap is created, extending away from the point of current melting in the direction of the weld depth and increasing in width. This gap allows vaporization products of the coating material, generated by the heat input, to escape.This does not mean that no evaporation products are carried away from the coating material towards the laser-exposed side, but rather that the majority of the evaporation products are carried away in a controlled manner via the degassing gap. The geometry of the degassing gap described above creates a pathway for the removal of these products, offering them less resistance than degassing through the weld pool. Therefore, the evaporation products are carried away, at least for the most part, through the degassing gap, away from the joining zone. In the inventive method, the entire laser energy can be introduced into the material to be melted.

[0014] In the aforementioned angular arrangement of the mutually facing joining flanges for forming the degassing gap, it is provided, in contrast to the teaching of JP 2005 324235 A, that these flanges, when held together for the joining process, only have line contact or quasi-line contact. This allows evaporation products of the metal coating to be carried away via the degassing gap as soon as welding begins. At the same time, the line contact and the weld pool formed along the line contact create an effective barrier for the coating's evaporation products.The laser beam, directed at the adjoining joints, preheats the coated surface in the direction of the developing weld seam due to the inclined arrangement of the opposing sides of the joining flanges. This preheating is achieved by the laser beam and its reflections off the opposing sides of the joining partners, as well as, if necessary, by the weld pool itself, which penetrates the weld pool with its preceding heat front. This heats the joining flanges, and in particular the metallic coating, bringing them to the vaporization temperature of the metallic coating before the steel material melts this area.

[0015] The degassing gap formed by holding the joining flanges together as described above provides the pathway through which the degassing products are carried away, which occurs automatically due to the pressure generated during evaporation. In the process according to the invention, the molten metal remains largely free of evaporation products, so that the weld seam is extremely low in porosity, if not completely pore-free. Consequently, the quality of the resulting weld seam is excellent and meets the highest static requirements, even under oscillating and dynamic loads. Since, in this process, the degassing of the metal coating does not occur through the weld pool, the pool does not need to be kept liquid for longer than is necessary to melt the steel material in order to achieve a controlled degassing process.As a result, the laser beam can be moved at a correspondingly high feed rate in this process. Studies have shown that this method can be carried out at feed rates of more than 10 m / min.

[0016] The angled arrangement of the opposing sides of the joining flanges, required to form the degassing gap, can be achieved by the corresponding arrangement of the joining partners when the joining flanges are straight. It is also possible for at least one of the two joining partners to be angled or curved in the area of ​​its joining flange, with the end of the curve being the joint that is held against the joint of the other joining partner during the laser welding process. A further advantage of such an arrangement of the joining flanges is that they can be clamped in the angled position using simple means. Unlike the prior art, they do not need to be clamped via their parallel, adjacent sides. Instead, it is possible to fix one of the two joining flanges and press the other against it to create the desired line contact and compensate for any tolerances.

[0017] The advantages of this process include not only the exceptional weld quality achieved even when welding metal-coated components and the potential for high welding speeds, but also the fact that the joining flanges only need to be as wide as the weld depth. Consequently, they can be made particularly narrow. This reduces the weight of the component formed from the joined parts, for example, a structural component for a vehicle. At the same time, the installation space required by such a component is reduced, which is a significant advantage, especially in the area of ​​compact vehicles. Due to the inclined arrangement of the joining flanges relative to each other, which forms the degassing gap, these gaps can be created by curved component extensions, further reducing the installation space required by the component.

[0018] The aforementioned advantages of degassing are already realized when the angle enclosed by the joining flanges is very small, for example, only 1°, preferably between 6° and 8°. The opening angle of the degassing gap should not be chosen too large to ensure a weld with a sufficiently deep root. Therefore, the joining flanges for forming the degassing gap are typically not arranged at an angle to each other exceeding 20°, as the strength is correspondingly reduced at larger angles.

[0019] The laser welding process does not require a specific material thickness for the components being joined. Instead, this is determined by the other requirements placed on the component being manufactured.

[0020] In the method according to the invention, the laser beam is further provided that it is directed at an angle perpendicular to the feed direction onto the joining zone of the buttresses of the two joining flanges, thereby promoting the degassing process. This is attributed to the increased reflections occurring at the opposing surfaces of the joining flanges in the area of ​​the degassing gap, resulting in less energy loss and correspondingly more intensive preheating before the actual melting of the metal-coated surface areas. In addition, it has been observed that with an angled laser, the energy can be introduced deeper into the material than is the case without an angled laser.It is assumed that the angularly arranged joining flanges, in combination with the angled laser, lead to improved reflection behavior, making it possible to introduce energy deeper into the material compared to conventional methods with the same laser power. This allows the weld to self-seed into the joining plane. In other words, a homogeneous weld essentially forms between the two angularly arranged joining partners, following the contours of the components. The angle at which the laser beam is inclined perpendicular to the feed direction should not exceed 25°. At a steeper inclination, the weld may no longer be formed to the necessary depth while maintaining a high feed rate.

[0021] Because the vaporization products of the metallic coating are carried away from the laser-treated surface, the laser optics and other adjacent equipment are also protected from weld spatter or similar contaminants. Nevertheless, it can be advantageous to provide an airflow across the joining zone, thus directing the welding beam across the area. This airflow blows away any vaporization products that may be escaping from this side, thereby protecting the laser and adjacent equipment from contamination.

[0022] Additionally, the laser beam can be moved back and forth in an oscillating motion transverse to the feed direction following the longitudinal extent of the welds, as described in WO 2016 / 169791 A1. The longer fluidity of the weld pool resulting from this measure, and the weld pool dynamics introduced within it, have a positive effect on preventing porosity within the weld. If the laser beam is guided in an oscillating motion transverse to the feed direction, even metal-coated steel components can be joined with a result that meets the requirements, where the metallic coating has a melting point higher than that of the steel.The oscillation of the laser beam and the resulting weld pool dynamics introduce into the weld pool first disrupt the cohesive phases that form from such a metallic coating during the welding process and then distribute them uniformly, i.e., finely dispersed, within the weld seam. Typically, the path amplitude of such an oscillating laser beam movement corresponds to approximately two-thirds of the contact area width formed by the impacts.

[0023] In addition to a non-penetrating, i.e., perpendicular, laser alignment in the feed direction, the joining zone can also be preheated in the feed direction by directing the laser beam at an angle to the feed direction, deviating from the perpendicular. An angle of inclination between 5° and 45° relative to the perpendicular to the joining zone is considered suitable in this context. Due to the angled position, the laser beam is directed at the joining zone in the feed direction with a penetrating angle. The laser beam then pushes a larger heat front ahead of it, which is particularly advantageous when joining metal-coated components whose coating has a vaporization temperature below the melting point of the steel. In this case, the metallic coating on the joint surface vaporizes almost completely before the steel melts.Overall, this reduces seam collapse and improves the quality of the weld.

[0024] The invention is described below with reference to exemplary embodiments and the accompanying figures. These show: Fig. 1a - 1c: The arrangement of the joining flanges of two joining partners for end-face laser welding ( Figure 1a ), at the beginning of the welding process in the initial stage of the formation of a weld pool at a point in the joining zone ( Figure 1b ) and at an advanced stage of welding ( Figure 1c ), Fig. 2a - 2c: in an enlarged cross-sectional view of the in Figure 1 Welding process shown before the start of material melting ( Figure 2a ), in the initial stage of the formation of the sweat bath ( Figure 2b ) and after completion of the welding process ( Figure 2c ), Fig. 3a - 3c:a representation of a weld corresponding to that of the sequence of figures 1a - 1c with geometrically differently designed joining partners and Fig. 4: a representation of a macro section of two joining partners, corresponding to the representation of the weld according to Figure 3a - 3c have been added.

[0025] Figure 1a Figure 1 shows an edge section of two joining partners 1, 1.1, which are not shown in detail. The joining partners 1, 1.1 are sheet steel parts, such as those used for the manufacture of structural components of vehicles. The joining partners 1, 1.1 are hot-dip galvanized and therefore have a metallic coating, namely the zinc coating. The joining partners 1, 1.1 are in the Figure 1a with their joints 2, 2.1 following the longitudinal extent of the component, held adjacent to one another. In the Figure 1In the position shown of the two joining partners relative to each other, the opposing butt edges 2.2, 2.3 are adjacent. Thus, both joining partners 1, 1.1 are in line contact with each other. The in Figure 1a The upper section of the depicted parts of the joining partners 1.1 forms the joining flanges. The extent of the joining flanges 3, 3.1 is in Figure 1a Indicated by curly brackets. The two joining partners 1, 1.1 are arranged at an angle of approximately 10° to each other to enable the previously described line contact at the opposing butt edges. This arrangement results in the opposing surfaces 4, 4.1 of the joining partners 1, 1.1 being at an angle to each other and enclosing a gap 5. The gap width increases from the abutting edges 2.2, 2.3 in the direction away from the joints 2, 2.1.

[0026] In the Figure 1aIn the arrangement shown, the two joining partners 1, 1.1 are held in a clamping device (not shown in the figures) for the purpose of creating an end-face weld to join the two joining partners 1, 1.1. Preferably, the joining partners 1, 1.1 are held in such a way that the abutting edges 2.2, 2.3 are under a certain preload.

[0027] To join the two joining partners 1, 1.1 together, a laser beam 6 is directed at the joints 2, 2.1 (see figure). Figure 1b The laser beam 6 is defocused. However, it is also conceivable that the laser is focused at this point. The diameter of the laser spot is shown in the diagram. Figure 1bThis is evident. Due to the heat input to the two joints 2, 2.1, the zinc coating on the joints 2, 2.1 evaporates before the steel material begins to melt. The same applies if a zinc coating is only present on the facing sides 4, 4.1 of the joining partners 1, 1.1, and is no longer present on the end faces of the joints 2, 2.1, for example, due to a previous processing step such as trimming the joining partners. The weld pool is identified by reference numeral 7. Due to the laser beam and the resulting continuous heat input, the weld pool 7 self-penetrates from the joints 2, 2.1 into the joining partners 1, 1.1, i.e., it finds its own path. A heat front forms ahead of the expansion of the weld pool 7 into the depth, which heats the zinc coating on the mutually facing sides of the joining partners 1, 1.1.Since the evaporation temperature of the zinc coating is below the melting temperature of the steel, the zinc coating has begun to evaporate, or this process is already completely or partially completed, before the steel melts. Due to the gap 5, which is also referred to as the degassing gap in this context, the evaporation products of the zinc coating are carried away by following the path of least resistance. This is the path through the degassing gap 5 away from the weld pool 7, as shown in [reference]. Figure 1b This is indicated by a block arrow. Due to the removal of the vaporization products in the same direction in which the weld zone spreads into the depth of the weld seam, it essentially cuts its way down automatically. With this process, the formation of the weld seam into the depth does not have to work against the counterflow of vaporization products from the metal coating.

[0028] Figure 1c The figure shows the joining zone after welding is largely complete. The weld pool 7 has clearly penetrated sufficiently deep into the joining partners 1, 1.1. The zinc coating located on the facing sides 4, 4.1 of the joining partners 1, 1.1 continues to evaporate away from the weld pool 7 through the degassing gap 5.

[0029] Figures 2a to 2c illustrate the previously described procedure again using an enlarged section view of the area of ​​the adjacent butt edges 2.2, 2.3 of the two joining partners 1, 1.1. In contrast to the Figure 1a to 1c The described joining method is carried out according to the invention in the exemplary embodiment of the Figure 2a to 2cThe laser beam 6.1 is directed at the two joints 2, 2.1 at an angle perpendicular to its feed direction. Although the two joining partners 1, 1.1 are in contact with each other in the area of ​​the opposing joint edges 2.2, 2.3, as described above, small gaps actually exist between the two joining partners 1, 1.1 due to unavoidable material irregularities. This means that through these openings, as in Figure 2a As shown, laser beam components pass through and are reflected at the facing surfaces 4, 4.1 within the degassing gap 5. This penetration of laser beams at the surfaces 4, 4.1, which is facilitated by the inclined laser beam 6.1, heats the zinc coating 8, 8.1 at the facing surfaces of the joining partners 1, 1.1. The passage of laser beams through the unavoidable remaining gaps ends when, as shown in Figure 2bAs shown, in the area of ​​the adjoining butt edges 2.2, 2.3, the material begins to melt and a weld pool 7 forms. Due to preheating and the heat front extending around the weld pool 7, the zinc coating 8, 8.1 in the immediate vicinity of the weld pool 7 is already melted and, as a rule, either completely or at least largely evaporated before the weld pool melts into the steel material in these areas. This is especially true if the zinc coating 8, 8.1 does not extend, or only partially extends, in the area of ​​the opposing butt edges 2.2, 2.3 of the joining partners 1, 1.1, and thus mainly on the opposing surfaces 4, 4.1 of the joining partners 1, 1.1. This can result, for example, from prior trimming operations on the end faces of the respective joining partners 1, 1.1. Figure 2bThe dashed line shows an isotherm I, which represents the evaporation temperature of the zinc coating 8, 8.1. The temperature of the weld pool 7 is above the temperature of the isotherm shown. I, which precedes the penetration of the weld pool 7 into the depth. Accordingly, the zinc coating 8, 8.1 is at least partially vaporized before these parts of the steel material melt, preceding the spread of the molten pool 7. The vaporization products are carried away from the weld pool 7 by the pathway created by the degassing gap 5, as schematically shown in the Figure 2b shown.

[0030] Figure 2c shows the completed weld. The weld seam is located at the joining partners 1, 1.1 in Figure 2c The depth of the weld shown also represents the width of the joining flanges 3, 3.1 as part of the joining partners 1, 1.1.

[0031] Figures 3a to 3c schematically illustrate the joining of two further components 1.2 and 1.3. While in the embodiment shown in Figures 1a to 1c, the components 1 and 1.1 were both planar and arranged at an angle to each other, in components 1.2 and 1.3, component 1.3 is curved in the area of ​​its joining flange 3.2. In this embodiment, the degassing gap is provided by the curved design of the joining flange 3.2 of component 1.3, with a line contact between the components at the end of its curvature. However, it is also conceivable that both components are curved and are held together opposite each other at their respective outer radius or curvature ends by means of line contact. The welding process is carried out in the same way as described in the sequence of figures 1a to 1c or 2a to 2c.

[0032] Figure 4Figure 1 shows a macro section of a weld seam produced according to the welding process described in the preceding figures. The illustration shows that the molten material has an ideal shape and is free of pores.

[0033] To investigate the load-bearing capacity of such welds, a bumper was manufactured from two zinc-coated sheet steel components welded end-to-end. One component is a hat-shaped profile in cross-section, the open end of which is closed by a steel plate. Both components were welded end-to-end as described in the preceding figures. The thickness of the sheet steel components used was 2 mm. The weld depth was between 2.5 mm and 3 mm. Drop tests carried out with this component, involving drop heights of 3 m and drop masses exceeding 500 kg, did not result in any failure of the welds. In all tests performed, the base material failed at some points, but the weld itself did not fail.This result was surprising, since otherwise, in such components, especially under dynamic load, the weld seam usually represents the respective weak zone.

[0034] The invention has been described using exemplary embodiments. Without departing from the scope of the applicable claims, a person skilled in the art can see numerous other possibilities for implementing the invention. Reference symbol list

[0035] 1, 1.1, 1.2, 1.3 Joining partner 2, 2.1 Butt joint 2.2, 2.3 Butt edge 3, 3.1, 3.2 Joining flange 4, 4.1 Surface 5 Gap / Degassing gap 6, 6.1 Laser beam 7 Weld pool 8, 8.1 Zinc coating II inotherm

Claims

1. A method for laser welding end faces of the joints (2, 2.1) of two joining flanges (3, 3.1, 3.2) of two joining partners (1, 1.1, 1.2, 1.3) made of a steel material and held together, at least one of which is provided with a metallic coating having an evaporation temperature below the melting temperature of the steel material, wherein the joining flanges (3, 3.1, 3.2) of the two joining partners (1, 1.1, 1.2, 1.3) are held together in line contact and enclosing an angle opening towards the laser-subjected side for the laser welding process in the region of their mutually facing edges (2.2, 2.3), whereby a degassing gap (5) between the joining flanges (3, 3.1, 3.2) which increases in the direction of heat input, is provided, by which evaporation products of the coating material (8, 8.1) resulting from the heat input are removed, characterized in that the laser beam (6.1) is directed inclined transversely to the feed direction towards the joining zone to be melted of the joining flanges (3, 3.1, 3.2) held together, so that preheating occurs before the actual melting of the metal-coated surface regions by reflections of the laser beam (6.1) on the mutually facing sides of the joining partners (1, 1.1, 1.2, 1.3).

2. The method according to claim 1, characterized in that the joining flanges (3, 3.1, 3.2) are held inclined to each other at an angle between 1° and 20°.

3. The method according to any one of claims 1 or 2, characterized in that the laser spot hitting the joining flanges (3, 3.1, 3.2) is set with a diameter that is larger than the maximum gap width between the joining flanges (3, 3.1, 3.2) held together in the region of the joints (2, 2.1), from which maximum gap width the degassing gap (5) increases with respect to its width.

4. The method according to any one of claims 1 to 3, characterized in that the laser beam (6.1) is directed at the joining zone of the joints (2, 2.1) at an angle of inclination of no more than 25 degrees transverse to the feed direction.

5. The method according to any one of claims 1 to 4, characterized in that an airflow flowing transversely to the laser beam direction and guided through the laser beam is provided on the joint side subjected to the laser beam.

6. The method according to any one of claims 1 to 5, characterized in that the laser beam is moved back and forth in an oscillating manner transverse to the feed direction following the longitudinal extension of the joints (2, 2.1).

7. The method according to claim 6, characterized in that the oscillation frequency of the laser beam does not exceed 1000 Hz.

8. The method according to claim 6 or 7, characterized in that the excursion amplitude of the oscillation movement of the laser beam does not exceed two thirds of the joint surface width formed by the joints (2, 2.1) held together.

9. The method according to any one of claims 1 to 8, characterized in that the joining zone of the joints (2, 2.1) intended for melting is gradually preheated in the feed direction before melting.

10. The method according to claim 9, characterized in that the laser beam is directed at the joining zone at an angle between 5° and 45° relative to the vertical.

11. The method according to any one of claims 1 to 10, characterized in that the metal coating of the at least one joining partner is a zinc coating (8, 8.1).

12. The method according to any one of claims 1 to 11, characterized in that the width of the joining zone to be melted is smaller than the surface width formed by the joints (2, 2.1) held together and the laser beam (6.1) is guided in such a way that an outer edge region is not melted at both joints (2, 2.1) to form a weld pool support.