Laser welding process and laser welding system

Abstract

Method for laser welding with a laser welding system 12 for joining at least two components 2a, b, wherein during joining a weld seam 4 is produced along a processing path 6, wherein the laser welding system 12 produces a processing laser beam 8 and a tacking laser beam 20, wherein the weld seam 4 is formed with the processing laser beam 8 in a welding process, wherein the weld seam 4 assumes a welding heat state 16, wherein with the tacking laser beam 20 in a tacking process within a tacking region 22 at least one tacking point 24 for tacking the at least two components 2a, b is formed, wherein the at least one tacking region 22 is arranged on the processing path 6 upstream of the weld seam 4, wherein the tacking process is carried out while the weld seam 4 is in the welding heat state 16.

Description

[0001] The invention relates to a method for laser welding and a laser welding system. State of the art

[0002] Laser welding systems are used for a wide variety of manufacturing processes.

[0003] The document DE 10 2018 209 981 A1 discloses a method for joining two components by means of a laser weld seam, wherein the two components are arranged one above the other in a joining area, wherein the first component is pressed in the direction of the second component by means of a clamping device, wherein a laser beam strikes the first component on the side facing away from the second component and melts material of the two components at least indirectly. Disclosure of the invention

[0004] The subject matter of the invention is a method having the features of claim 1 and a laser welding system having the features of claim 13. Preferred or advantageous embodiments of the invention emerge from the subclaims, the following description and the attached figures.

[0005] The method according to the invention is carried out using a laser welding system. The laser welding system is used for joining, i.e., laser welding, at least two components. When joining the at least two components, a weld seam is created along a processing path. The processing path corresponds to the planned weld seam path. The processing path is arranged upstream of the weld seam. The upstream processing path is converted point by point or continuously into the weld seam. In other words, a processing laser beam is guided along the processing path. The processing laser beam creates a weld spot. The weld spot converts the processing path into the weld seam. The weld spot thus moves in a welding direction along the processing path. During laser welding, a molten pool forms around the weld spot, which forms the weld seam upon cooling.With the weld seam, at least two components are joined in a material-to-material manner in the usual way.

[0006] The laser welding system generates the processing laser beam. The processing laser beam is used to create the welding spot and thus the weld seam. The weld seam permanently joins the two components.

[0007] In this process, the weld seam is formed using a processing laser beam in a welding process. During the formation of the weld seam, the weld assumes a welding heat state. The welding heat state is a state of the weld seam during laser welding. In the welding heat state, part of the material of at least one of the components is in a liquid state. The weld spot changes its position with the movement of the processing laser beam, as explained above. The weld seam left behind by the movement of the weld spot cools over time. The cooled weld seam no longer exhibits a welding heat state. The cooled weld seam does not contain any of the component material in a liquid state.

[0008] In addition, the laser welding system generates a tack laser beam in addition to the processing laser beam. The tack laser beam is used to create a tack between the two components. The tack joins the components over a significantly smaller area than the weld seam. Unlike a weld seam, the tack with its smaller area can be removed by applying force, with little or no damage to the surface of the components. The tack is created using at least one tack point. The term "tack point" is understood here to mean a technically concentrated point, but also to mean a tack line or another structure / shape / contour suitable for tackling the at least two components. Tack points are preferably used, which are created as point-like welds on the surface of the adjacent sides of the two components.

[0009] In the method, at least one stapling point is formed with the stapling laser beam in a stapling process in order to effect the stapling of the at least two components as explained above.

[0010] The tack area is a surface area on one of the components in which the tack points can be or are to be arranged. The tack area is determined by, among other things, the deflectability of the tack laser beam, the position of the weld spot, the thermal deformation around the weld spot, the effective range of the tack points, etc. The tack points are spaced apart from the weld point and therefore do not coincide with it. The tack area does not have to be contiguous and can therefore also have separate sub-areas. The tack area is a flat area in which planned tack points are located and in which the planned tack points are created. Completed tack points can fall outside the tack area.

[0011] In the tack welding process, the two components are connected with at least one tack point, preferably several, e.g. 30, tack points. The tack welding process is used to determine the relative position of the two components to one another in the tack welding area. The tack welding points are designed in the connection between the two components so that they can absorb forces during the welding process. The tack welding points are usually not as mechanically resilient as the weld seam. In particular, the tack welding points are produced in a much shorter time than the weld seam. The mechanically more resilient weld seam differs significantly from a tack welding point in particular in terms of its surface area and mechanical load-bearing capacity. Forces acting as loads on the welded component that can be absorbed by the weld seam without breaking the weld seam can lead to the connection between the tack welding points breaking.

[0012] At least one tacking zone is formed along the machining path—viewed in the direction of the welding—in advance of the weld seam. The tacking zone is dimensioned such that the machining path preferably lies within the tacking zone. Alternatively, two tacking zones can be formed along each side of the machining path. The weld seam always lies outside the tacking zone. Converting the machining path into the weld seam therefore shifts, in particular, the planned tacking zone along the machining path, so that the weld seam no longer falls within the tacking zone. The shift of the (planned) tacking zone is therefore to be understood as being in advance of the weld seam.

[0013] The tack area leading up to the weld seam can be used to determine the relative position of the two components, especially in the area of ​​a section of the weld currently being created. This determination can also take into account thermal deformation around the weld point that is only caused by the creation of the weld seam.

[0014] In this process, the tacking process is performed while the weld seam is at the welding temperature. The tacking process is carried out by placing tacking points in the tacking area. As the weld spot moves, the welding temperature changes on the weld seam and the (planned) tacking area shifts.

[0015] By performing the tack process while the weld is at the welding temperature, thermal deformation due to the weld point can be taken into account when setting the tack points by adjusting the distance between the tack points and the weld point. The accuracy of the weld is increased. Thermal distortion of the components after the weld has cooled is minimized. These advantages are achieved, among other things, by not setting the tack points before the weld begins, as is common practice.

[0016] In one embodiment of the method, the welding heat state of the weld seam is reached when the temperature of the weld seam is equal to or greater than the melting point of the material of the components. In particular, the melting point of the material is greater than 500°C, preferably greater than 660°C and in particular greater than 1400°C. Every laser-weldable material has a melting temperature as its melting point. For example, steel has a melting temperature in the range of 1400 to 1500°C. Copper has a melting temperature of approximately 1084°C. Aluminum has a melting temperature of approximately 660°C. Aluminum alloys can have melting points below 600°C to approximately 500°C. During laser welding, the material of the components is heated with the laser beam to such an extent that the melting temperature is exceeded. Preferably, the melting temperature is significantly exceeded, e.g., by more than 200°C. This causes a molten pool to form at the weld point.The temperature of the weld seam changes over time at a specific position along the weld. The temperature is high at the weld point and then drops over time as it cools. This results in a temperature drop that depends on the distance to the weld point. A temperature drop also occurs with a greater time interval between the energy input to the weld point. Continuous wave (CW) lasers are preferred for laser welding. The weld seam is in the welding heat state as long as the temperature of the weld point has not fallen below the melting temperature of the material. The temperature specification refers to the temperature measured on the surface of the weld seam.

[0017] In one embodiment of the method, the weld is in the welding heat state when a molten pool is formed at the weld.

[0018] The molten pool contains molten material from both components. The material is melted in the weld seam using the processing laser beam. A molten pool is created in particular when so much energy is introduced into the weld spot that a temperature above the melting temperature of the material at the weld spot is reached. The molten pool has an expansion that depends on the above-mentioned temperature drop depending on the distance to the weld spot. Thus, the distance from the weld spot and the temporal energy input determine the expansion of the molten pool. The molten pool cools at the edges. If the energy input is absent for an extended period, the expansion of the molten pool decreases until it no longer exists. If this is not present, the material has cooled down. When the weld spot moves, the molten pool moves in line with the movement of the weld spot.

[0019] The molten pool is the welding heat state of the weld seam; the molten pool forms around the weld point. The molten pool moves with the movement of the weld point. The cooled material of the molten pool forms the weld seam. For a homogeneous weld seam, the molten pool is continuously moved along the machining path without cooling in between. A homogeneous weld seam can absorb higher forces than an inhomogeneous weld seam.

[0020] In one embodiment of the method, at least one, preferably several or all, of the stitching points are arranged outside the processing path, i.e., they do not coincide with it. However, the at least one stitching point is preferably always arranged within the stitching area.

[0021] By arranging at least one of the tack points outside the processing path, a second welding operation at the location of this tack point, which would result in the formation of a weld seam, can be prevented. Such a second welding operation can be referred to as "overwelding" of a tack point. The second welding of such a location in the processing path with the processing laser beam leads to inhomogeneities in the weld seam.

[0022] In a further embodiment of the method, the at least one stitching point is arranged adjacent to the movement path.

[0023] The arrangement of the tack points "adjacent" to the processing path means that the tack points are arranged laterally to the processing path at a specific distance. The distance can vary for each tack point. The distance is less than or equal to the maximum distance of the tack area from the weld point. Preferably, the tack points are arranged at a distance from the weld point in the tack area that ideally has the ambient temperature or a temperature below 250°C throughout the entire welding process. Particularly preferably, the distance between the weld point and the tack point is in the range of 0.5 mm to 50 mm and is in particular less than 50 mm, less than 30 mm, or less than 5 mm.

[0024] In particular, two of the tacking points are set so that the machining path runs perpendicular and in the middle of the connecting line of the two tacking points as a transverse line.

[0025] In a further embodiment of the method, at least one of the tack points is arranged such that the components lie against one another without a gap, with a tolerable gap, and / or are aligned flat-flat. In particular, the components are arranged without a gap if the distance between the facing sides of the components is less than 1 mm, preferably less than 0.5 mm, particularly preferably less than 0.05 mm. The surface roughness of the facing sides of the components forms a tolerance range. The distance can fluctuate by twice the tolerance range. In particular, the distance between the facing sides of the components with the tolerable gap is in a range of 1 mm to 1.5 mm. Preferably, the distance is greater with the tolerable gap than with the gap-free arrangement of the components.

[0026] In particular, "plane-to-plane" refers to the arrangement of the components with at least one side oriented toward each other. In other words, in a plane-to-plane alignment, the two components lie one on top of the other. The distance between the facing sides of the components in a plane-to-plane alignment is preferably in a range of 0.1 to 0.5 mm.

[0027] With the gap-free arrangement or with a tolerable gap, components can be tacked or welded more easily.

[0028] The stapling process can be successfully performed even with a small distance or gap between the components, allowing a stable stapling point to be achieved. The small distance or gap is determined by the parameters of the stapling laser beam. The maximum distance or gap between the two components at which the stapling process can be successfully performed is referred to below as the stapling gap.

[0029] The tack gap dimension can be exceeded due to warping of at least one of the components, especially the component with the weld point. The warping is a result of thermal deformation and thus thermal expansion of the component containing the weld point. The weld point has high temperatures. The high temperatures cause thermal expansion of at least the component containing the weld point.

[0030] Due to the curvature of the component with the weld point caused by thermal deformation, a gap occurs between the components around the weld point that is larger than the tack gap. As the distance from the weld point increases, the gap between the two components decreases until it reaches the tack gap. At gaps smaller than the tack gap, the tack point can be set, thus enabling the tack process. The distance from the weld point to the point on one of the components where the gap is reduced to the tack gap is the minimum distance.

[0031] The tack gap is often located at a point on one of the components where the warpage is present due to thermal deformation. Depending on the profile of the warpage caused by thermal deformation, the tack gap can be located at the boundary between the non-warped and warped material of the thermal deformation. The boundary between the non-warped and warped material of the thermal deformation can also be determined by the presence or absence of the tack gap.

[0032] In a further embodiment of the method, at least one of the tacking points is set in advance with the minimum distance along the processing path, preferably and / or in advance with a maximum distance.

[0033] In particular, the distance between the tack points and the weld point is at least approximately five times the thickness of one of the components. If the components have different thicknesses, five times the thickness of the thinner of the two components is preferably used as the minimum distance between the weld point and the tack point. If both components are made of a sheet metal with a thickness of 100 µm each, the preferred distance between the weld point and the tack point is 1 mm. If the thinner of the two components is made of a sheet metal with a thickness of 150 µm, the preferred distance between the weld point and the tack point is 1.5 mm.

[0034] Depending on the feed rate, energy input, or the joining situation, the distance between the tack points and the weld point can be greater or less than five times the thickness of a component. In particular, the distance between the tack points and the weld point can be ten times the thickness of one of the components.

[0035] The minimum distance allows for thermal deformation caused by the weld spot to be taken into account. The maximum distance is determined by the technical limit of the tack beam's mobility. The maximum distance is also determined by the effective range of the tack points. The effective range of the tack points determines the relative position of the two components. The further the tack points are positioned from the weld spot, the more freedom of movement the component material gains. As the freedom of movement increases, the relative position of the two components is determined less.

[0036] In another version, the laser welding system is operated so that the tacking laser beam and the processing laser beam are switched alternately. With alternating switching, either the processing laser beam or the tacking laser beam is active. With alternating switching, the processing laser beam is deactivated for a short period of time. When deactivated, the laser beam is deflected to the tacking point, the laser beam is switched to the tacking laser, the components are tacked, and the laser beam is guided back to the welding point. The deflection is so fast that no processing of the components takes place while the laser beam is moving. Deactivation interrupts the energy supply to the welding point. With the interrupted energy supply, the molten pool reduces its expansion. With the reduced expansion, the edges of the molten pool cool down.In particular, the deactivation period is designed to be so short that the energy supply to the welding point is resumed before the entire molten pool has cooled down.

[0037] While the processing laser beam is deactivated, the laser beam is activated as a tacking laser beam. The position of the welding spot is used to determine the position and thus the distance and orientation of the tacking point to the welding spot. The tacking laser beam is deflected to the distance and a tacking point is set. Once the tacking point has been set, the tacking laser beam is returned to the welding spot. The processing laser beam is reactivated. This completes one cycle of switching from the processing laser beam to the tacking laser beam. In particular, the laser beam is switched on continuously during the cycle and jumps back and forth between the position for the welding spot and the position for the tacking point, thus being activated at the respective positions. The laser beam itself is constantly on and only distributes the laser power spatially between the two positions.

[0038] In particular, the cycle time is limited so that the molten pool, and thus the welding temperature, is always maintained, even when the processing laser beam is inactive, until the end of the weld is reached. This results in a homogeneous weld, as described above.

[0039] The time between cycles is used to heat the molten pool. In addition, the weld spot is moved along the machining path during the time between cycles.

[0040] Especially for thin components, a tack point can be set in a single cycle. For example, foils have a thin thickness. The tack points on foils with a thickness of less than 150 µm can preferably be set in a single cycle. For thicker components, e.g., with a thickness greater than 0.5 mm, multiple cycles may be necessary to set a tack point. For thicker components, the times within and between cycles, and the spacing of the tack points are adjusted accordingly.

[0041] By alternating the processing laser beam and the tacking laser beam, it is possible to tack and weld two components with just one laser beam.

[0042] In a further embodiment of the method, the laser welding system is operated in such a way that the tacking laser beam and the processing laser beam are switched simultaneously.

[0043] When switched on simultaneously, the stapling laser beam and the processing laser beam are active or switched on together at certain times. Preferably, the processing beam is always active. The stapling laser beam is only activated when a stapling point needs to be set.

[0044] With simultaneous switching, the processing laser beam is not switched off. This allows the weld spot and the weld pool to be guided continuously along the processing path. The tacking laser beam can also be activated and can place tacking points in the tacking area independently of the processing laser beam.

[0045] By simultaneously switching the processing laser beam and the tacking laser beam, components with a thickness greater than 0.5 mm can also be tacked and welded using this process. Unlike the alternating switching above, the tacking laser beam does not need to be directed at the same tacking point several times with interrupted time intervals.

[0046] In a further embodiment of the method, the laser welding system generates a laser beam that forms the tacking laser beam and the processing laser beam. In particular, the one laser beam is split into a processing laser beam and a tacking laser beam by means of an optical system for beam shaping the laser beam.

[0047] In particular, the division can be permanent and enable at least temporary simultaneous switching of the processing laser beam and the stapling laser beam. With permanent division, the beam profile of one laser beam is split into two laser beams, the processing laser beam and the stapling laser beam. The division of the laser beams can vary. For example, the processing laser beam can be allocated a share of the laser beam power of 30 to 70% and the stapling laser beam the remaining share of the laser beam power, i.e. 70% to 30%. These shares can be changed as desired. For components with a thickness greater than 150 µm and less than 0.5 cm, for example, the stapling laser beam can have a share of the laser beam power of 50%, while the processing laser beam has a share of the laser beam power of 50%.For components with a thickness greater than 0.5 cm, for example, the stapling laser beam can account for 60% of the laser beam's power, while the processing laser beam accounts for 40% of the laser beam's power. When the stapling laser beam is not needed, a share of up to 100% of the laser beam's power can be allocated to the processing laser beam. After splitting the laser beam, the processing laser beam is deflected to one position, while the stapling laser beam is deflected to another.

[0048] Preferably, one laser beam is alternately switched between the processing laser beam and the tacking laser beam. In principle, during alternating switching, the proportion of the laser beam power for the processing laser beam and the tacking laser beam is not reduced. In individual cases, the proportion of the laser beam power for the processing laser beam or the tacking laser beam can be reduced. Preferably, the proportion of the laser beam power for the processing laser beam and the tacking laser beam is up to 100%. The laser beam is preferably deflected as a processing laser beam to the position of the weld spot. After a certain period of time, the processing laser beam is switched to the tacking laser beam for a short period of time. This switching deactivates the processing laser beam and activates the tacking laser beam. The tacking laser beam is deflected to the position of the tacking laser beam.

[0049] The duration of the short period of time is conditioned or limited by the reduction in the welding heat state. The temperature around the weld spot and / or the molten pool is also included in the welding heat state. These short periods of time can range from a few ns, e.g. 1 ns, to several tens of µs. The short period of time depends on the laser process, material dimensions, and material properties such as thermal conductivity and melting temperature, etc. Before the welding heat state no longer exists, the laser beam switches back from the tacking laser beam to the processing laser beam. This switchover activates the processing laser beam and deactivates the tacking laser beam. By deflecting the processing laser beam onto the welding spot and activating the processing laser beam, energy is again introduced into the welding spot, maintaining the welding heat state.The switching from the processing laser beam to the tacking laser beam and back again can be referred to as a single cycle. Several cycles, e.g., more than 1,000,000, can be performed to weld two components. The cycles can occur at a frequency of a few Hz, e.g., 1 Hz, to several hundred kHz, e.g., 300 kHz. In addition to the switching, the laser beam must be deflected, which rapidly alternates between the positions of the processing laser beam and the tacking laser beam.

[0050] Various optical systems can be used to deflect and shape the machining laser beam and the tacking laser beam. These optical systems include acousto-optic deflectors (AODs), acousto-optic modulators (AOMs), scanners such as galvos, and / or other optical or optomechanical components that move very quickly and / or have a beam-shaping effect. Suitable optical systems could include spatial light modulators (SLMs) or optical phase modulators (LCOS-SLMs).

[0051] In a further embodiment of the method, the laser welding system shapes the laser beam using an optical phased array (OPA) as the optical system. With an optical phased array, the laser beam is split into several laser beams. The several laser beams formed from one laser beam are preferably arranged in a matrix. Alternatively, several laser sources can be combined into one laser beam, which is referred to as coherent beam combining (CBC). With several adjacent laser beams, the phases of the split laser beams are adjusted so that after the laser beams are combined and superimposed, a deflection occurs. The laser beam is shaped by the deflection. The deflection can be used to switch between the position of the processing laser beam and the position of the tacking laser beam.

[0052] The OPA allows laser beam deflection without moving elements. Deflection is possible within picoseconds to nanoseconds. Cycles of the machining laser beam and the tack laser beam can be performed at frequencies in the MHz range. Other systems such as galvo scanners, LCOS-SLM, etc., currently only allow significantly lower frequencies (Hz range down to several tens of kHz, e.g., 30 kHz).

[0053] In a further embodiment of the method, the laser welding system has two separate laser sources.

[0054] The laser beam generated by one laser source forms the processing laser beam and the laser beam generated by the other laser source forms the stapling laser beam.

[0055] With the two permanently available laser beams, the processing laser beam and the tacking laser beam can be switched on simultaneously for a limited time. This allows for preferential tacking and welding of components with greater thickness, e.g., greater than 0.5 cm. However, it can also be cost-effective for thinner components to use one laser beam source for the welding and the tacking process.

[0056] A further object of the invention is the laser welding system which is designed to carry out the method described above

[0057] In particular, the laser welding system has a control module for carrying out the welding process and the tacking process.

[0058] In particular, a control module determines the distance and orientation of the tack point to the weld point. The control module can use sensors to measure the thermal deformation due to the energy input at the weld point in the tack area as weld reinforcement. Preferably, the sensors can also measure the gap formation in the tack area.

[0059] In particular, the weld reinforcement can be determined, for example, using a measuring system based on optical coherence tomography (OCT). In particular, the sensors can also include an optical distance sensor that records the gap size between the components. The data from the aforementioned sensors can be used to determine the time frame and position within the tacking area where a tacking point is placed.

[0060] In particular, the sensor data is fed to a control system or controller, which then generates a forecast of the optimal tacking points, taking into account the position of the welding spot, the processing path, and the at least one tacking area. Preferably, the parameters for the tacking process with the tacking laser beam are determined based on the data determined by the control module. During or before the next activation of the tacking laser beam, the parameters for the tacking laser beam are adjusted to set the tacking point. In particular, the parameters can include, for example, beam profile, deflection, duration, number of laser beam pulses, temporal distribution of the laser beam pulses, etc.

[0061] The control module can be used to determine the parameters of the tack point in order to set a tack point in the tack area that will produce a good weld result.

[0062] The invention is based on the following findings, observations, and considerations and also includes the following preferred embodiments. These embodiments are sometimes referred to as "the invention" for simplicity. The embodiments may also contain parts or combinations of the above-mentioned embodiments or correspond to them and / or may also include previously unmentioned embodiments.

[0063] One idea behind the invention is the following: to enable fixation close to the weld point (weld point), so-called tack points are commonly used during welding today. Especially with flat components, the problem arises that, in the case of large expansions, the tack points actually have to be placed in the "process state," i.e., in the position shortly before welding, when the components have already bulged due to the release of residual stresses, and not in the "cold state" before welding.

[0064] A possible solution would be to carry out a second laser process at the same time, which would set the tack points prior to the actual welding process.

[0065] Another solution is a process with a tack process prior to the welding process. The weld seam is created with a laser (processing laser beam), which is then displayed via a scanner (deflection) that places the tack points in time with the welding process. This significantly reduces warping and gap formation between the two components due to thermal expansion.

[0066] Another solution is to create these tack points with a laser (tack laser beam) and a very fast beam deflection so that the tack points are set at the same time as the welding process without negatively influencing the welding process.

[0067] Highly dynamic lasers, optics, and beam deflection concepts are available today that allow beam deflections up to the MHz range. For example, lasers from Civan can shape and deflect the beam very quickly using a so-called "OPA = Optical Phased Array." Other systems are also available that can move and / or shape the beam very quickly using an acousto-optic deflector (AOD) or acousto-optic modulator (AOM), scanners, or other optical and optomechanical components. Future systems that are currently too slow but could also become faster in the future are also conceivable, using spatial light modulator (SLM) chips or optical phase modulator (LCOS-SLM).

[0068] These lasers and beam deflection systems are used to briefly direct the laser beam onto the tack point during the welding process and then return it to the welding process. This requires jump times of less than 100 µs.

[0069] Using a rapid beam deflection system (deflection), the laser can briefly exit the weld seam between its position on the weld seam to create tack points at different positions. The tack point can be set with a single laser energy input with a duration of t, or the tack point can be created with several short energy inputs of t. The durations and laser energy, which are determined by parameters such as focus diameter, laser power, and beam profile, can vary for the weld seam and tack point and are determined by the highly dynamic beam deflection and / or beam shaping systems.

[0070] Particularly with thin foils, which typically have thicknesses of <150 µm and especially <100 µm, this method takes advantage of the fact that the creation of the tack points requires little energy, medium power, and a short dwell time, thus minimizing the impact on the welding process. For thick sheets, with sheet thicknesses of several mm, this is less advantageous, as the times t for the tack points become longer due to the greater depth and can no longer be set without negatively impacting the process.

[0071] Fast beam-shaping systems, such as those from Civan, are preferred here. This allows the laser beam to be formed into various beam shapes very quickly. Pattern changeover times can be in the MHz range. Likewise, the beam profiles can be moved and modified very quickly, e.g., in the MHz range.

[0072] A beam configuration with multiple steel profiles can be created with a single laser beam that is shaped into a left spot or a right spot using the beam profile. It is also conceivable that more than two spots could be created, or that the spot positions, the beam profile of the spots, or the respective power distributions could change over the course of the process.

[0073] For example, a single-spot beam pattern can be used as a single beam for welding (processing laser beam) and tacking (tacking laser beam). Another beam pattern with two spots can be used for simultaneous welding and tacking point creation in the pre-welding phase with the welding beam (processing laser beam) and the laser beam for the tacking process (tacking laser beam).

[0074] The process uses one beam shape that performs the actual welding process and then quickly switches to another beam shape as needed, e.g., to create a tack spot. Advantageously, in this variant, the laser beam used to create the weld seam remains in the molten pool. Only through beam shaping is a further laser spot (the tack laser beam) generated in the vicinity of the welding laser spot (the welding spot of the processing laser beam) used to create the tack spot.

[0075] The variant with fast beam deflection can also be carried out with a laser or beam shaping setup if different patterns or beam shapes are used for the positions (welding point, tack point).

[0076] This variant has the advantage that larger tack spots can also be created, as it constantly switches between single spots and multi-spots. Different beam profiles, such as beam diameter, can be used for the tack and weld spots. This variant therefore offers even more freedom in process design.

[0077] A control module can measure the weld reinforcement or gap formation in the pre-processing stage, for example, using an OCT measuring system, an optical distance sensor, etc., and, based on the gap size, predict when and where a tack point needs to be placed to still achieve a good process result. For this purpose, the sensor data is fed into a control system, which then, taking into account the path movements (processing path), i.e., the seam path (processing path) of the weld seam across the component to be created, generates a predictive calculation of the optimal tack points and positions the beam accordingly on the component (via the fast beam deflection system or a fast beam shaping system).

[0078] The tack points that tack both joining partners, i.e. a connection is created between the upper and lower sheets through the tack point, which reduces the formation of gaps caused by thermal expansion or residual stresses, etc. For this purpose, the tack points must be designed in such a way that they can absorb the forces that occur, e.g., from thermal expansion, residual stresses, etc., in order to reduce the formation of gaps.

[0079] Tack points are preferably placed in advance of the weld seam. The tack points can be placed next to the future seam (processing path) or on the future seam (processing path). On the seam (processing path) means that the tack points are welded over by the weld seam. To avoid disrupting the welding process, a tack point in the immediate vicinity of the weld seam is preferable to a tack point on the seam (processing path), where the tack point is welded over.

[0080] The position of the tack point relative to the weld (processing path) can change depending on the geometric characteristics of the components. For example, the tack points usually need to be positioned differently on a straight weld than on curves or in component areas with more or less residual stresses within the component, or in areas where the clamping device can clamp the components more or less effectively. However, the method presented here allows for very flexible positioning of the tack points, thus always creating the best conditions for the welding process prior to welding, especially immediately before the welding process.

[0081] The tack points can be created directly in the weld seam precursor. This can be achieved using multiple separate laser systems, by rapid beam deflection of at least one laser system, by rapid beam shaping of at least one laser system, or by a combination of these options.

[0082] Further features, advantages, and effects of the invention will become apparent from the following description of preferred embodiments and the accompanying figures. These show: Fig. 1 schematic representation of two components during laser welding along a processing path with tack points in a perspective view; Fig. 2 a schematic cross-section through the arrangement of Fig. 1 along line II-II

[0083] The Fig. Figure 1 shows a highly schematic representation of two superimposed components 2a, b. Components 2a, b are formed from sheet metal. The two components 2a, b are to be joined using laser welding, i.e., bonded together. For this purpose, the two components 2a, b are welded using a weld seam 4.

[0084] The weld seam 4 is to be created along a processing path 6. The processing path 6 corresponds to the course of the planned weld seam 4. The weld seam 4 has not yet been created on the processing path 6. The processing path 6 lies in advance of the weld seam 4. The weld seam 4 is created using a processing laser beam 8. To create the weld seam 4, the processing laser beam 8 is guided in a welding direction 10 along the processing path 6. The weld seam 4 develops in the welding direction 10 along the processing path 6. In other words, the processing path 6 is converted into the weld seam 4 by the advancement of the processing laser beam 8 in the welding direction 10.

[0085] The processing laser beam 8 is generated by a laser welding system 12. The processing laser beam 8 creates a welding point 14 on the component 2a, b. At the welding point 14, energy is introduced into the components 2a, b. The energy introduction causes a temperature increase. With the temperature increase, part of the material of the two components 2a, b melts at the welding point 14. The weld seam 4 assumes a welding heat state 16 at the welding point 14, which in Fig. 1 is indicated by an arrow. In the welding heat state 16, a molten pool 18 is present. In other words, the molten pool 18 is molten material of the components 2a, b in the liquid state.

[0086] As the processing laser beam 8 moves in the welding direction 10 along the processing path 6, the position of the welding spot 14 changes. This changes the position of the energy input. As the distance from the welding spot 14 increases, the molten material cools and forms the weld seam 4. The molten material also cools if the energy input of the processing laser beam 8 into the welding spot 14 is interrupted. The cooling of the molten material begins at the edges of the molten material. If the energy input at the welding spot 14 is briefly interrupted, the edges of the molten material cool down, but part of the molten material remains in the liquid state. The molten material is present inside the welding spot 14. The molten material has a temperature that is greater than or equal to the melting temperature of the material.Every laser-weldable material has a melting temperature as its melting point. For example, steel has a melting temperature in the range of 1400 to 1500°C. Copper has a melting temperature of approximately 1084°C. Aluminum has a melting temperature of approximately 660°C. Aluminum alloys can have melting temperatures below 600°C to approximately 500°C. The temperature is measured at the weld point 14 using a pyrometer (not shown). The temperature is measured on the surface of one of the components 2a, b, preferably of component 2a where the weld seam 4 is visible from the outside. When measuring the temperature at the weld point 14, the temperature of the weld point 14 is approximately equal to the temperature inside the components 2a, b. As long as molten material is present, the welding heat state 16 exists. Only when all of the molten material has cooled down does the welding heat state 16 no longer exist.

[0087] A homogeneous weld seam 4 is achieved when the welding heat state 16 is maintained continuously and without interruption over the length of the processing path 6. If the welding heat state 16 is no longer present over the length of the processing path 6, an interruption in the weld seam 4 occurs and an inhomogeneous weld seam 4 is created. Welding over the interruption also leads to an inhomogeneous weld seam 4. Overwelding refers to the renewed melting of a cooled weld point. The cooled weld point can be a previous weld seam 4 or a tack point 24. The homogeneous weld seam 4 absorbs higher forces than the inhomogeneous weld seam 4 and is therefore preferred. Depending on the requirements of the component, an inhomogeneous seam may be required.

[0088] The laser welding system 12 generates a tacking laser beam 20 in addition to the processing laser beam 8. The tacking laser beam 20 sets individual tacking points 24 in a tacking area 22.

[0089] The tack area 22 is a surface on one of the components 2a, b, here component 2a. The tack area 22 is formed in advance of the weld seam 4. The tack area 22 therefore does not have a weld seam 4. The tack area 22 begins at the transition between the weld seam 4 and the processing path 6. Part of the tack area 22 extends along the processing path 6 as a rectangular strip, with the strip having a width of approximately 1 cm to 10 cm. Further tack areas 22 (not shown) can also be formed. When two tack areas 22 are formed, for example, one tack area 22 is on one side of the processing path 6 and the second tack area 22 is on the other side of the processing path 6. The maximum distance 26 of the tack area 22 from the weld point 14 is predetermined by the maximum deflection of the tack laser beam 20.

[0090] Due to the energy input and the associated higher temperature of the weld point 14, the material around the weld point 14 bulges. This bulging is referred to as thermal deformation. The bulging occurs in the component on which the weld point 14 is created, in this case, component 2a. The bulging creates a gap between the two components 2a, b. The thermal deformation occurs around the weld point 14 and has a limited, areal extent.

[0091] The stapling process can be successfully performed even with a small distance or gap 32 between the components 2a, b, so that a stable stapling point 24 can be set. The small distance or gap 32 is determined by the parameters of the stapling laser beam 20. The maximum distance or gap 32 between the two components 2a, b at which the stapling process can be successfully performed is hereinafter referred to as the stapling gap 34.

[0092] Due to the curvature of component 2a with weld point 14 due to thermal deformation, a gap 32 occurs around weld point 14 between components 2a, b that is greater than the tack gap 34. As the distance from weld point 14 increases, the gap 32 between the two components 2a, b decreases to the tack gap 34. For gaps 32 that are smaller than the tack gap 34, a tack point 24 can be set, thus enabling a tack process. The distance from weld point 14 to the point on one of components 2a, b at which the gap 32 is reduced to the tack gap 34 is the minimum distance 28.

[0093] The tack gap 34 is often located at a point on one of the components 2a, b where the warping is present due to thermal deformation. Depending on the profile of the warping caused by thermal deformation, the tack gap 34 can be located at the boundary between the non-warping and warping material of the thermal deformation. The boundary between the non-warping and warping material of the thermal deformation can also be determined by the presence or absence of the tack gap 34. The size of the area of ​​thermal deformation varies depending on the material of the component 2a, b and the energy introduced into the weld point 14.

[0094] The tack points 24 are preferably set outside the area with thermal deformation. Tack points 24 are preferably set in the area between the minimum distance 28 and the maximum distance 26. The area between the minimum distance 28 and the maximum distance 26 is an area of ​​the tack area 22. In principle, it is possible for a tack point 24 to be set in the area of ​​thermal deformation, which is associated with increased thermal expenditure (to destabilize the curvature) and increased time expenditure for setting the tack point 24. The tack points 24 serve to determine the relative position of the two components 2a, b to one another. The planned tack points 24 are arranged in the tack area 22. The planned tack points 24 are then created in the tack area 22. The tack area 22 changes its position as the weld seam 4 progresses. The finished tack points 24 fall out of the tack area 22 as the tack area 22 is shifted.

[0095] In particular, the distance between the tack points 24 and the weld point 14 is at least approximately five times the thickness of one of the components 2a, b. If the components 2a, b have different thicknesses, five times the thickness of the thinner component 2a, b of the two components 2a, b is preferably used as the minimum distance between the weld point 14 and the tack point 24. If both components 2a, b consist of a sheet metal with a thickness of 100 µm each, the preferred distance between the weld point 14 and the tack point 24 is 1 mm. If the thinner component 2a, b of the two components 2a, b consists of a sheet metal with a thickness of 150 µm, the preferred distance between the weld point 14 and the tack point 24 is 1.5 mm.

[0096] The tack points 24 have a lower mechanical load-bearing capacity than the weld seam 4. The lower mechanical load-bearing capacity is adjusted by specifying parameters for the tack laser beam 20. The parameters include, for example, beam profile, deflection, duration, number of laser beam pulses, temporal distribution of the laser beam pulses, etc. The tack points 24 are designed to absorb the forces that arise when creating the weld seam 4. Forces that can be absorbed by the weld seam 4 in the welded components 2a, b and do not lead to a break in the weld seam 4 can cause the tack points 24 to break. The weld seam 4 is designed for higher mechanical loads than the tack points 24. The tack points 24 can have different shapes. The different shapes include point-shaped, linear, rectangular, cross-shaped, etc., all of which are represented symbolically here only by a single point.

[0097] The tack points 24 are preferably arranged outside the processing path 6. This allows for the creation of a homogeneous weld seam 4. Arranging the tack point 24 within the processing path 6 (not shown) would result in overwelding of the tack point 24. Overwelding of the tack point 24 creates an inhomogeneous seam. Depending on the requirements of the component, an inhomogeneous seam may be required, e.g., for surfaces that are visible from the outside in the final product.

[0098] The tacking points 24 are arranged laterally to the processing path 6. The respective distance of the tacking points 24 from the processing path 6 can vary. One tacking point 24 is placed on each side of the processing path 6. Two lateral tacking points 24 are placed such that, when the tacking points 24 are connected, the processing path 6 is positioned at the center of the connecting line between both tacking points 24. The connecting line of the tacking points 24 is arranged perpendicular to the processing path 6 at the intersection point with the processing path 6.

[0099] There are several configurations of how the laser welding system 12 generates the processing laser beam 8 and the tacking laser beam 20.

[0100] In a first embodiment, a laser beam is alternately switched to the processing laser beam 8 and the tacking laser beam 20. During the alternating switching, the laser beam is activated and positioned at the position of the welding point 14 for a specific period of time with up to 100% of the laser beam power as the processing laser beam 8. After the period of time has elapsed, the processing laser beam 8 is deactivated at the position of the welding point 14 by positioning the laser beam on the tacking point 24. The laser beam is thus activated as the tacking laser beam 20 with up to 100% of the laser beam power and positioned on the tacking point 24 for a short period of time. After the short period of time has elapsed, the tacking laser beam 20 is deactivated at the tacking point 24 by positioning it again on the welding point 14. The laser beam is then reactivated as the processing laser beam 8.Preferably, the time of the time duration is longer than the time of the time span. The time duration is the time in which the laser beam is switched as processing laser beam 8. The time duration is in the range from 1 microsecond to more than 10 seconds. The short time span is the time in which the laser beam is switched as tacking laser beam 20. The short time span is limited to 1 nanosecond to 5000 microseconds. Preferably, the short time span is in the range from 100 µs to 500 µs. Tacking points 24 can be set in multiple cycles with short time spans. When setting the tacking points 24 with multiple cycles, the short time span can also be less than 100 µs. The laser beam is switched in the processing laser beam 8 state for longer than in the tacking laser beam 20 state.

[0101] The short time period depends on the cooling rate of the liquid material in the melt pool 18. The short time period for setting the tack point 24 is calculated so that the molten heat state 16 is maintained. Thus, the tack laser beam 20 is switched back into the processing laser beam 8 before all of the liquid material has cooled and the molten heat state is no longer present.

[0102] The switching from the processing laser beam 8 to the tacking laser beam 20 and back to the processing laser beam 8 is referred to as a cycle. This cycle is executed very quickly. One cycle lasts approximately 1 µs to 10 µs, which corresponds to a frequency of 0.1 MHz to 1 MHz. Accordingly, more than 1,000,000 cycles can be performed to weld two components 2a, b, for example.

[0103] The switchover makes it possible to perform tacking as a tacking process and welding as a welding process with a single laser beam. By performing the tacking process prior to the welding process and alternating the tacking process with the welding process, reduced thermal distortion of the two components 2a, b can be achieved. Furthermore, the dimensional accuracy of the welded components relative to each other is increased.

[0104] In a second embodiment, the processing laser beam 8 and the tacking laser beam 20 are generated simultaneously by the laser welding system 12. The simultaneous generation of the tacking laser beam 20 and the processing laser beam 8 is preferably only briefly used to set one of the tacking points 24.

[0105] The simultaneous presence of the processing laser beam 8 and the stapling laser beam 20 is optionally achieved by two laser sources. Thus, one laser source generates the processing laser beam 8, and a second laser source generates the stapling laser beam 20.

[0106] As an alternative to the two laser sources, the simultaneous presence of the processing laser beam 8 and the stapling laser beam 20 is possible by splitting a laser beam, i.e., a single laser beam. When splitting the single laser beam, a first portion of the power of the single laser beam is assigned to the processing laser beam 8 and a different portion of the power of the single laser beam is assigned to the stapling laser beam 20. For example, a portion of the power of the single laser beam in the range of 30% to 70% is assigned to the processing laser beam 8. The remaining portion of the power of the single laser beam, thus 70% to 30%, is assigned to the stapling laser beam 20. If the stapling laser beam 20 is not required, it is possible for 100% of the portion of the power of the single laser beam to be assigned to the processing laser beam 8.When the stapling laser beam 20 is needed again, the proportion of the processing laser beam 8 in the single laser beam is reduced and the proportion of the stapling laser beam 20 in the single laser beam is increased.

[0107] By dividing the individual laser beam into a processing laser beam 8 and a tacking laser beam 20, the energy input into the welding point 14 is continuously maintained.

[0108] As a step following the simultaneous or alternating generation of the processing laser beam 8 and the tacking laser beam 20, the deflection is carried out via optical systems.

[0109] For the alternating generation of the processing laser beam 8 and the tacking laser beam 20, an optical phased array (OPA) is used as an optical system for deflection and laser beam shaping. With the optical phased array (OPA), a laser beam is deflected without moving any mechanical components. This enables very fast laser beam deflections. Jumping back and forth between the position of the welding point 14 for the processing laser beam 8 and the position of the tacking point 24 for the tacking laser beam 20 is possible with an OPA at frequencies up to 300 MHz.

[0110] When generating the machining laser beam 8 and the tacking laser beam 20 simultaneously, or when generating with two laser sources, the following optical systems can be used instead of the OPA. The optical systems include an acousto-optic deflector (AOD), an acousto-optic modulator (AOM), and / or other optical or optomechanical components that shape the beam.

[0111] When the processing beam 8 and the tacking laser beam 20 are generated simultaneously, the welding state 16 is always maintained. This maintenance eliminates the need for rapid position changes. When the processing beam 8 and the tacking laser beam 20 are generated simultaneously, scanners such as galvos, spatial light modulators (SLM), or optical phase modulators (LCOS-SLM) can also be used.

[0112] The laser system has a control module 30. The control module 30 sets the parameters for the processing laser beam 8 and the tacking laser beam 20. In particular, the control module 30 determines the distance and orientation of the tacking point 24 to the welding point 14. The control module 30 uses sensors to measure the thermal deformation due to the energy input at the welding point 14 in the tacking area 22 as weld reinforcement. Optionally, the sensors can additionally measure the progression of the gap formation in the tacking area 22. Another option for determining the weld reinforcement is, for example, a measuring system based on an optical coherence tomography (OCT) measuring system. In particular, the sensors can also include, among other things, an optical distance sensor that records the progression of the gap size between the components 2a, b.The data from the above-mentioned sensors are used to determine in which time frame and at which position in the stapling area 22 a stapling point 24 is set.

[0113] In particular, the sensor data is fed to a control system or controller (not shown), which then generates a precalculation of the optimal tacking points 24, taking into account the position of the welding point 14, the processing path 6, and the at least one tacking area 22. Preferably, the parameters of the tacking laser beam 20 for the tacking process with the tacking laser beam 20 are determined based on the data determined by the control module 30. During one of the next activations of the tacking laser beam 20, the parameters for the tacking laser beam 20 are adjusted in order to set the tacking point 24. In particular, the parameters can include, for example, beam profile, deflection, duration, number of laser beam pulses, temporal distribution of the laser beam pulses, etc.

[0114] In the Fig. 2 is a schematic cross-section through the arrangement of Fig. 1 along the line II-II.

[0115] In the Fig. 2 shows the two components 2a, b, the laser welding system 12 with the processing laser beam 8, the tacking laser beam 20 and the control module 30, the welding point 14, the weld seam 4, the welding heat state 16, the molten pool 18, the tacking area 22, the tacking points 24, the maximum distance 26, the minimum distance 28 during the welding process and the tacking process.

[0116] The following points out differences in Fig. 2 to Fig. 1 and additions are described.

[0117] The processing laser beam 8 applies energy to the welding point 14. This energy input leads to the formation of the welding heat state 16 and the molten pool 18 containing a portion of liquid material from the components 2a, b. During welding, the portion of the material of the component 2a, b directed toward the processing laser beam 8 is completely melted. Furthermore, a portion of the material of the component 2a, b further away from the processing laser beam 8 is melted.

[0118] The energy input of the processing laser beam 8 leads to an increase in temperature in the component closer to the processing laser beam 8, here component 2a. The area in front of the weld seam 4 bulges. This bulging is referred to as thermal deformation. During thermal deformation, the gap between the two components 2a, b increases. The distance between the two components 2a, b from one another is referred to as the gap dimension 32. The bulging increases the gap dimension 32 and thus the gap. The increased gap makes it more difficult to set the tack points 24. The tack points 24 are preferably set in an area of ​​the components 2a, b in which the components 2a, b are aligned to one another without a gap, with a tolerable gap dimension 32 and / or flat-to-flat.

[0119] The thermal deformation moves as the weld point 14 changes. The thermal deformation is in advance of the weld point 14. The thermal deformation is in the tack area 22.

[0120] The tack points 24 are placed outside the thermal deformation area. The distance from the occurrence of the thermal deformation to the welding point 14 is the minimum distance 28 for placing the tack points 24 in the tack area 22. The tack points 24 are placed at a distance from the welding point 14, whereby the distance is greater than the minimum distance 28. The tack points 24 are placed laterally to the processing path 6. The tack point 24 shown in the curvature is arranged behind the curvature at the minimum distance 28 from the welding point 14. Tack points 24 can be placed outside the thermal deformation area. For the tack points 24, only a small portion of the material of the component 2a, b that is distant from the processing laser beam 8 is melted, in contrast to the weld seam 4. This means that the tack points 24 can be placed in a shorter time. QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] DE 10 2018 209 981 A1

[0003]

Claims

[1] Method for laser welding with a laser welding system (12) for joining at least two components (2a, b), wherein during joining a weld seam (4) is produced along a processing path (6), wherein the laser welding system (12) generates a processing laser beam (8) and a tacking laser beam (20), wherein the weld seam (4) is formed with the processing laser beam (8) in a welding process, wherein the weld seam (4) assumes a welding heat state (16), wherein at least one tacking point (24) for tacking the at least two components (2a, b) is formed with the tacking laser beam (20) in a tacking process within a tacking area (22), wherein the at least one tacking area (22) is arranged on the processing path (6) in front of the weld seam (4), wherein the tacking process is carried out while the weld seam (4) is in the welding heat state (16). [2] Method according to claim 1,characterized by that the weld seam (4) is in the welding heat state (16) when the temperature of the weld seam (4) is equal to or greater than the melting temperature of the material of the components. [3] Method according to claim 1 or 2, characterized by that the weld seam (4) is in the welding heat state (16) when a molten pool (18) is formed on the weld seam (4). [4] Method according to one of the preceding claims, characterized by that at least one of the tacking points (24) is arranged outside the processing path (6). [5] Method according to one of the preceding claims, characterized by that the at least one tacking point (24) is arranged adjacent to the processing path (6). [6] Method according to one of the preceding claims, characterized bythat the at least one tacking point (24) is arranged such that the components (2a, b) lie against one another without a gap or with a tolerable gap (32) and / or are aligned flat-flat. [7] Method according to one of the preceding claims, characterized by that the at least one tacking point (24) is set in advance with a minimum distance (28) along the processing path (6) preferably and / or in advance with a maximum distance (26). [8] Method according to one of the preceding claims, characterized by that the laser welding system (12) is operated in such a way that the tacking laser beam (20) and the processing laser beam (8) are switched alternately. [9] Method according to one of the preceding claims 1 to 7, characterized by that the laser welding system (12) is operated in such a way that the tacking laser beam (20) and the processing laser beam (8) are switched simultaneously. [10] Method according to one of the preceding claims, characterized by that the laser welding system (12) generates a laser beam which forms the tacking laser beam (20) and the processing laser beam (8). [11] Method according to claim 10, characterized by that the laser welding system (12) shapes the laser beam with an OPA. [12] Method according to one of the preceding claims, characterized by that the laser welding system (12) has two separate laser sources, one laser source forming the processing laser beam (8) and the other laser source forming the tacking laser beam (20). [13] Laser welding system (12), wherein the laser welding system (12) is configured to carry out the method according to one of the preceding claims.

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