Laser welding method

Abstract

A method for laser keyhole welding is disclosed to weld two pieces made of a metal alloy together. The method independently adjusts the power of a focused central beam and the power of a concentrically focused annular beam. At the end of the weld, the power of the annular beam (PA) is decreased, the motion of the focused beam is stopped, the power of the central beam (Pc) is increased, and the power of both beams is initially ramped down quickly and then ramped down slowly. Increasing the power of the central beam equalizes the temperature of the two pieces before solidification and cooling at the termination of the weld. Additional power pulses can be applied to prevent the formation of defects or to eliminate any defects.

Description

[0001] priority

[0002] This application claims priority to U.S. Patent Application Serial No. 16 / 881886, filed May 22, 2020, the disclosure of which is incorporated herein by reference. Technical Field

[0003] This invention generally relates to welding using focused laser radiation beams. More particularly, it relates to welding metallic alloys using focused central beams and focused annular beams. Background Technology

[0004] Laser beams are increasingly used for cutting, drilling, welding, marking, and scribing workpieces made of a variety of materials, including metals and metal alloys. Traditional machining produces unwanted defects, such as microcracks that can propagate when the workpiece is under stress, thus degrading and weakening it. Laser machining minimizes these unwanted defects, is generally cleaner, and results in a smaller heat-affected zone. Laser machining uses a focused laser beam to produce precise cuts and holes with high-quality edges and walls while minimizing the formation of unwanted defects.

[0005] In laser welding, a focused laser beam precisely positions each weld point or line while minimizing incidental heating. It is useful to distinguish between two main states of laser welding. Conductive welding is performed at lower laser power and lower power density. The absorbed laser power heats the irradiated material, melting the material in each part to be joined; this material flows, mixes, and then solidifies. Keyhole welding is performed at higher laser power and higher power density, sufficient to vaporize some of the irradiated material. The pressure of the vaporized material on the surrounding molten material opens a channel through the molten material, typically narrow and deep, allowing for deep penetration of the laser beam. Finished keyhole welds are generally narrower, deeper, and stronger than conductive welds. However, maintaining a stable keyhole in a hot, dynamic pool of molten material is challenging.

[0006] When laser welding certain metals and metal alloys, one problem is the formation of defects, particularly cracks, at the ends of the weld. Some defects are caused by stresses generated during workpiece cooling. These initial defects can weaken the welded workpiece and may propagate further if thermal or mechanical stresses are applied when the finished workpiece is put into use. Unreliable welds in a structure can lead to catastrophic failures. One known solution to mitigate these defects is to rapidly reduce the laser power at the end of the weld, rather than digitally shutting it off. Another known solution is to rapidly increase the focused beam at the end of the weld, thus irradiating a gradually increasing area on the workpiece with a beam of gradually decreasing intensity.

[0007] While these solutions have been successful on many materials, they have proven insufficient for modern high-strength metallic alloys with relatively high thermal conductivity. These materials remain highly susceptible to cracking at the start of laser welding, and especially at the end. U.S. Patent Application No. 16 / 786623, filed February 10, 2020, describes a method for welding such materials, assigned to the assignee of this invention, the full disclosure of which is incorporated herein by reference. The method uses a composite laser beam having a central beam and a larger annular beam, which is focused onto the workpiece being welded. As the focused laser beam approaches the end of the line weld, the power of the annular beam slopes down, while the power of the central beam slopes up. The power of the central beam then slopes down again. While this method effectively prevents defects, during these power changes, the focused laser beam moves across the workpiece surface, creating a gradual taper across the weld width. This taper is typically a few millimeters long, which is unacceptable in some applications requiring a uniform cross-section along the entire length of the weld. The end of the weld may also taper gradually in depth.

[0008] There is a need for a simple and reliable method to produce welds with uniform cross-sections in metals and metal alloys that are prone to cracking at the weld termination. Specifically, it is necessary to produce welds that maintain a minimum width along their entire length. Preferably, this process should not compromise any of the advantages of laser welding, such as welding speed, precision, weld quality, and cost per weld. Invention Overview

[0010] A method for laser welding a workpiece along a weld line according to the present invention includes delivering a focused laser radiation beam to the workpiece. The focused beam has a focused central beam and concentrically focused annular beams. The focused central beam is smaller than the focused annular beam exposed on the workpiece surface of the focused beam. The focused beam moves laterally relative to the workpiece along the weld line. The central beam has a central processing power, and the annular beam has an annular processing power. The power of the annular beam decreases from the annular processing power. When the focused beam reaches an end position on the weld line, the lateral movement of the focused beam relative to the workpiece stops. The power of the central beam increases from the central processing power. The power of the annular beam decreases at a first annular tilt rate, while the power of the central beam decreases at a first central tilt rate. The power of the annular beam decreases at a second annular tilt rate, while the power of the central beam decreases at a second central tilt rate. The second annular tilt rate is less than the first annular tilt rate. The second central tilt rate is less than the first central tilt rate. Brief description of the attached diagram

[0012] The accompanying drawings, which are included in and form part of this specification, schematically illustrate preferred embodiments of the invention and, together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention.

[0013] Figure 1A This is a partial cross-sectional side view, schematically illustrating a preferred embodiment of an apparatus for implementing the method for laser welding of a workpiece according to the invention, the apparatus comprising a laser source for generating at least two laser radiation beams, an optical fiber, a focusing lens, and components for focusing the laser radiation beams relative to the workpiece.

[0014] Figure 1B It is shown schematically. Figure 1A A detailed cross-sectional view of an optical fiber having a central core for guiding a central beam and an annular core for guiding a ring beam.

[0015] Figure 2A This is a graph showing the power versus time for the ring beam and the central beam. Figure 2B This is a graph showing the relationship between the translational speed of the focused beam and time. Figure 2A and 2B Together they formed a use Figure 1A and 1B Timing diagram of an embodiment of the laser welding method for the equipment.

[0016] Figure 3A This is a graph showing the power versus time for the ring beam and the central beam. Figure 3B This is a graph showing the relationship between the translational speed of the focused beam and time. Figure 3A and 3B Together to form a use Figure 1A and 1B A timing diagram of another embodiment of the laser welding method for the device.

[0017] Figure 4A This is a graph showing the power versus time for the ring beam and the central beam. Figure 4B This is a graph showing the relationship between the translational speed of the focused beam and time. Figure 4A and 4B Together they formed a use Figure 1A and 1B A timing diagram of another embodiment of the laser welding method for the equipment.

[0018] Figure 5A and 5B This is an enlarged plan view of a lap weld in a workpiece made of high-strength steel, showing the top and bottom surfaces of the workpiece welded along the welding line according to the invention between the start and end positions.

[0019] Figure 6A and6B yes Figure 5A and 5B A further enlarged plan view of the lap weld, showing in particular the surface around the end position.

[0020] Figure 7A yes Figure 5A and 5B Side view of the lap weld.

[0021] Figure 7B yes Figure 5A and 5B The end view cross-section of the lap weld is approximately midway between the start and end positions.

[0022] Figure 7C yes Figure 5A and 5B Cross-sectional view of the end position of the lap weld. Invention Details

[0024] Referring now to the accompanying drawings, where the same parts are indicated by the same numbers. Figure 1A and 1B The apparatus 10 used in prior art laser processing methods and the laser welding method of the present invention is illustrated schematically. A laser source 12 transmits at least two laser radiation beams to a focusing lens 16 via an optical fiber 14. The optical fiber 14 includes a central core 32 for guiding a central beam of laser radiation. The central core 32 has a low-refractive-index cladding 34. The optical fiber 14 also includes an annular core 36 for guiding an annular laser beam. The annular core 36 is concentrically located between the low-refractive-index cladding 34 and another low-refractive-index cladding 38. The laser source 12 is configured to transmit the central beam to the central core 32 and the annular beam to the annular core 36. Laser systems integrating such laser sources with such optical fibers are commercially available. For example, the Highlight™ FL-ARM laser from Coherent Corporation in Santa Clara, California. A feature of this particular laser system is that the power of the central beam and the annular beam can be independently and with high precision adjusted.

[0025] The focusing lens 16 forms a focused beam 18, which includes a focused central beam depicted as a converging solid line and a concentrically focused annular beam depicted as a converging dashed line. The focused beam converges to a focal point 20, wherein the focused central beam has a smaller diameter than the concentrically focused annular beam. The device 10 may also include an optional beam expander (not shown here) located between the fiber 14 and the focusing lens 16. The focusing lens 16 is shown herein as a fiber-coupled lens assembly, which is typically arranged to allow the beam exiting the fiber to expand internally before focusing.

[0026] A focused beam 18 is directed onto workpiece 22, which initially comprises two parts to be welded together, referred to herein as the "top part" and the "bottom part" for ease of description. The terms "top" and "bottom" do not imply a specific spatial orientation of the workpieces. The two workpieces 22 may be coated or uncoated. The two workpieces 22 may be in direct contact or may be separated by a small gap. For example, galvanized steel is typically welded with gaps up to several hundred micrometers to allow high-pressure zinc vapor to escape. In the figures, the two parts are depicted in cross-section during lap welding. Workpiece 22 is supported and moved by a translation stage 24. The focal point 20 is located near the top surface of workpiece 22. The focal point may be located above, above, or below the top surface. For lap welding, the focal point is preferably located at a depth between approximately 1 mm above the surface and approximately 2 mm below the surface.

[0027] During welding, the translation stage 24 is moved laterally, as indicated by vector M. The weld seam is depicted as a shadow on the workpiece 22 and forms along the desired weld line 26 from the starting position 28 to the ending position 30. The laterally moved workpiece 22 moves the focused beam 18 relative to the workpiece 22 along the weld line 26. The device 10 can also be configured to move the focusing lens 16, thereby moving the focused beam 18 relative to the workpiece 22. The focusing lens 16 may be a component comprising a galvanometer-driven mirror and a planar objective lens for rapidly moving the focused beam 18 laterally relative to the workpiece 22. It should be noted that although a flat workpiece is shown, workpieces with other shapes can be welded using the method of the present invention. For example, molded sheet metal parts for automobile bodies often have complex three-dimensional shapes. A robot with multiple degrees of freedom can weld molded sheet metal parts together along a curved weld line.

[0028] Figure 2A and 2B Together, they form a timing diagram, schematically illustrating a preferred embodiment of the laser welding method 40 according to the present invention using device 10. Figure 2A It is a graph showing the power of the ring beam and the power of the central beam as a function of time. Figure 2B This is a graph showing the lateral translational velocity of the focused beam relative to the workpiece as a function of time. The timing graph spans a time range including the end of linear welding and the termination of welding. At time 0 on the graph, the focused beam moves at a constant speed V along weld 26 between the starting position 28 and the ending position 30. For example, the constant speed V is between 15 mm and 135 mm / s. At time T... S The focused beam reaches the end position and stops.

[0029] Along most of the welding line, there exists a “linear welding” pattern, where the central beam remains at the central processing power P. C And the ring beam remains at the ring processing power PA These powers are selected to form strong welds with a uniform width across the entire workpiece thickness. The optimal ratio of center processing power to ring processing power depends on the material composition of the workpiece. However, for a specific metal alloy, even if the composition is proprietary and therefore unknown, this ratio can be optimized empirically. Typically, for laser welding of high-strength steel alloys, the inventors have determined the optimal ratio P. C :P A The preferred ratio is less than 1:3, and the most preferred ratio is less than 1:8.

[0030] During time T1, as the focused beam approaches its end position, the power of the ring beam decreases from the ring processing power P. A The power is gradually reduced to a lower value P1. This gradual reduction eliminates the transverse crack, which would otherwise propagate to the underside of the weld and become visible on the bottom surface of the welded workpiece. Increasing the rate of power reduction of the annular beam moves the crack toward its termination position, where it will be consumed in subsequent steps of the method of the invention. However, reducing the annular power also alters the cross-sectional shape of the weld, as described below. Therefore, it is preferable to reduce the power at a minimum rate sufficient to reliably eliminate unwanted transverse cracks. At time T... S The power of the ring beam is P1, and the power is maintained for a time T2.

[0031] During time T2, as the focused beam 18 is now stationary at the end position 30, the power of the center beam is reduced from the center processing power P. C The power P2 is increased, thus delivering more laser power to the bottom side of the workpiece through the keyhole. During T2, the weld on the top side of the workpiece widens due to the irradiation of the fixed-focused annular beam. This widening is balanced by increasing the power of the central beam to widen the weld on the bottom side of the workpiece. Therefore, the increase in power prevents the formation of welds with asymmetrical cross-sections and undesirable asymmetrical mechanical stresses that would weaken the weld.

[0032] During time T3, the power of the annular beam decreases at a first annular tilt rate, and the power of the center beam decreases at a first center tilt rate. This decrease during T3 provides controlled collapse of the keyhole and essentially a transition of conductive welding conditions to the end position. Cooling and solidification of the molten material begins during time T3. During the longer time T4, the power of the annular beam decreases at a second annular tilt rate, while the power of the center beam decreases at a second center tilt rate. The power of these beams decreases more slowly during time T4 than during time T3 to provide slower and more controlled solidification of the remaining molten material. The second annular tilt rate is less than the first annular tilt rate, and the second center tilt rate is less than the first center tilt rate.

[0033] At the end of time T4, the power density in the focused annular beam and the focused center beam preferably converges to provide uniform heating of the workpiece surface. By the end of time T4, the annular beam and the center beam have dropped to the "off power" PO, meaning the power is too low to melt or damage the exposed areas of the workpiece. For example, the off power could be 0 watts (W).

[0034] The inventors have discovered that solidification leads to significant cracking if the focused laser beam is simply turned off or even linearly reduced in power upon reaching the end position. Without laser power through the keyhole, solidification begins at the bottom of the molten pool. Rapid grain growth occurs inwards and towards the center of the weld, concentrating stress along the weld line. Cooling is accompanied by material shrinkage, which peels the weld along the weld line. In some cases, the resulting cracks propagate along the entire weld. High-strength steels are particularly prone to cracking along the weld line center, and additional cracks may form around the end position.

[0035] The method of the present invention prevents these defects by providing additional energy to the bottom component through the keyhole during time T2, thus bringing the temperatures of the top and bottom components closer together to minimize interfacial crack propagation to the bottom surface. The welded workpiece has a characteristic "spherical" shape around the end position 30 (in Figure 5A and 5B (See image). When the focused beam is stationary, the sphere begins to form during time T2. By controlling solidification and cooling by ramping down the power during times T3 and T4, slower local grain growth is produced, thereby minimizing crack formation at the termination location. The material at the termination location solidifies and cools radially inward, which concentrates stress in a small volume at the center of the sphere, rather than at the center of the weld line.

[0036] Figure 3A and 3B Together, they form a timing diagram schematically illustrating another laser welding method 50 according to the present invention. Figure 3A It is a graph showing the power in the ring beam and the power in the central beam as a function of time. Figure 3B This is a graph showing the lateral translational velocity of the focused beam relative to the workpiece over time. Welding method 50 is similar. Figure 2A and 2B The welding method 40 also includes the dwell time T between times T2 and T3. 2' During this period, the power of the ring beam remains at approximately P1, and the power of the central beam remains at approximately P2. During the dwell time T... 2'During this process, the focused central beam transmits additional energy through the keyhole, providing extra heating to the bottom component to balance the temperatures of the bottom and top components. The inventors found that this additional heating is advantageous when welding certain materials. Power P1, power P2, and dwell time T can be selected. 2' The weld is formed at a symmetrical ending position in the cross-section. Specifically, the ball is symmetrical about the interface between the top and bottom components, as discussed further below. The dwell time T is tens of milliseconds (ms). 2' That's usually enough.

[0037] exist Figure 3A and 3B In welding method 50, within the total time T1 and T2, the power of the ring beam changes from the ring processing power P. A Reduce to a lower power P1. Figure 2A and 2B In welding method 40, the power of the annular beam decreases to a lower power P1 only within time T1. This difference between welding methods is not critical because the power reduction of the annular beam is gradual. For example, the power decreases by 10% to 20% in the last few millimeters of the weld. Time T1 is typically much longer than time T2.

[0038] Figure 4A and 4B Together, they form a timing diagram schematically illustrating another laser welding method 60 according to the present invention. Figure 4A It is a graph showing the power of the ring beam and the power of the central beam as a function of time. Figure 4B This is a graph showing the lateral translational velocity of the focused beam relative to the workpiece over time. Welding method 60 is similar. Figure 2A and 2B Welding method 40, but includes additional laser power pulses applied near the end of solidification of the molten pool.

[0039] After times T3 and T4, the power P remains off for both the ring beam and the center beam. O In some cases, any remaining molten material at the center of the ball will solidify and cool. In some workpieces, residual voids may form near the top surface of the ball's center. In some workpieces, residual cracks may form within the ball. The additional laser power pulse in welding method 60 prevents these defects by further slowing and controlling the solidification of the remaining molten pool. Alternatively, for a freshly solidified and cooled workpiece, the additional pulse remelts the hot material on the top surface, eliminating any residual voids, residual cracks, or other defects.

[0040] In welding method 60, during time T5 after times T3 and T4, the annular beam and the central beam are maintained at off power while the molten pool solidifies and its volume decreases. Then, a laser power pulse is applied during time T6. The annular processing power P... A In comparison, the power of the annular beam P3 during time T6 is lower, and it is also lower than the central processing power P. C In comparison, the power of the central beam P4 during time T5 is also lower. The energy applied during time T6 is at most sufficient for surface remelting, and the power density is preferably uniform. Therefore, the power P4 of the smaller focused central beam is generally less than the power P3 of the focused ring beam. Although a rectangular pulse is depicted in the figure, other pulse shapes would be effective if the power and energy are sufficient to slow down the solidification or remelting of the surface. For some workpieces, applying multiple pulses to slow down the solidification and remelting of the surface may be advantageous.

[0041] Figure 5A and 5B It shows the use Figure 4A and 4B An enlarged plan view of lap welds in high-strength steel workpieces manufactured using welding method 60, with details such as... Figure 3A and 3B The dwell time T in method 50 2' The workpiece consists of two 1.4mm thick galvanized Gen3 steel pieces with a 0.3mm gap between them. The laser system includes Highlight... TM An FL10000-ARM laser and a scanner mounted on a robotic arm are used to translate the focused beam. Before time TS, the focused beam is translated along the welding line at a speed of 46 mm / s, with annular processing power P. A It is 3800W, and the central processing power P C The power rating is 1000W. Other welding parameters are: P1 = 3500W, P2 = 1600W, P3 = 200W, P4 = 50W, T1 = 100ms, T2 = 5ms, T... 2' =50ms, T3=60ms, T4=160ms, T5=70ms, and T6=30ms.

[0042] Figure 5A The top surface exposed to the focused laser beam is shown, and Figure 5B The bottom surface is shown. The focused beam scans along weld line 26 from right to left, from the starting position 28 to the ending position 30. The weld has a generally uniform surface width along most of the weld line, which is approximately the diameter of the focused annular beam. As mentioned above, the weld has a spherical portion around the ending position. This weld surface is free of cracks. The weld line is indicated by a dashed line in the drawing so as not to obscure any cracks in the center of the weld.

[0043] Figure 6A and 6B yes Figure 5A and 5B An enlarged photograph of the workpiece surrounding the sphere. Figure 6A The top surface is shown, and Figure 6B The bottom surface is shown. There are no obvious cracks along the center line or inside the sphere. There are no obvious cavities or other defects near the center of the sphere.

[0044] Figures 7A to 7C yes Figure 5A and 5B A magnified photograph of the cross-section of the workpiece. Figure 7A The entire weld from starting position 28 to ending position 30 is shown. The weld has a generally uniform height along most of the weld line and there are no obvious discontinuities, cracks, or voids. The weld tapers slightly around ending position 30, as described below.

[0045] Figure 7B The cross-section of the weld is shown at approximately the midpoint between the start and end positions. This location is exposed to the focused beam before time T1, while the central beam is at the central processing power P. C And the ring beam is at the ring processing power P A The weld has a roughly uniform width that extends through the workpiece, or (equivalently) the weld has relatively flat walls, as required. This means the weld is symmetrical about the interface between the two parts. The weld has menisci on its top and bottom surfaces, which is typical of gap-crossing lap welds when no filler material is used.

[0046] Figure 7C The cross-section of the weld at the termination position is shown. The weld extends through the entire thickness of the workpiece. Generally, the weld is symmetrical between the top and bottom, with approximately the same amount of molten material in each component. This symmetry creates a "butterfly" or "hourglass" shape in the cross-section at the termination position. The waist located at the interface between the two parts has approximately the same width as the rest of the weld (~1.8 mm) (~1.6 mm, by...). Figure 7B The cross-section (represented in the figure) indicates that the weld termination is not a weak point in the welded workpiece. There are no obvious cracks or voids at the termination location. The diameter of the sphere measured on the top surface of this cross-section is approximately 3.1 mm.

[0047] The welding method of this invention can be applied to various metal alloys. For example, high-strength steel alloys "Gen3" and "XGen3" are known third-generation steels in the art. This method can also be applied to duplex steels, such as "DP600" and "DP980". This method is also suitable for… and Branded steel, which can be obtained from ArcelorMittal SA of Luxembourg.

[0048] Although lap welding was used above as an example, those skilled in the art will recognize that the method of the present invention can be applied to other configurations, such as fillet welding or butt welding. For applications where welding is not required across the entire thickness of the workpiece, the method still controls heating, and then controls the solidification and cooling at the weld bottom, corresponding to the deepest penetration of the keyhole into the workpiece. The method will still mitigate crack formation and form a characteristic sphere at the weld end. This is achieved by using an optimal ratio P of center processing power to ring processing power. C :P A This will form a weld with a roughly uniform width along most of the weld line between the exposed surface and the bottom of the weld.

[0049] Although linear power tilt has been shown and discussed herein, for the sake of simplicity, the welding method of the present invention can be further optimized by using other types of power tilt, such as a central beam and / or annular beam. For example, exponential power tilt.

[0050] The present invention has been described above with reference to preferred embodiments and other embodiments. However, the present invention is not limited to the embodiments described and depicted herein. Rather, the present invention is limited only by the appended claims.

Claims

1. A method for laser welding workpieces along a welding line, comprising the following steps: A focused laser beam is delivered to the workpiece, the focused laser beam having a focused central beam and concentric focused annular beams, the diameter of the focused central beam being smaller than the diameter of the focused annular beams exposed on the workpiece surface by the focused laser beam. The focused laser beam is moved laterally relative to the workpiece along the welding line, the central beam having a central processing power and the annular beam having annular processing power; Reduce the power of the ring beam from the ring processing power; When the focused laser beam reaches the end position of the welding line, the lateral movement of the focused laser beam relative to the workpiece is stopped. Increase the power of the central beam by processing power from the center; The power of the annular beam is reduced at a first annular tilt rate, while the power of the central beam is reduced at a first center tilt rate; and The power of the annular beam is reduced at a second annular tilt rate, and the power of the central beam is reduced at a second center tilt rate, wherein the second annular tilt rate is less than the first annular tilt rate, and the second center tilt rate is less than the first center tilt rate.

2. The method for laser welding a workpiece along a welding line according to claim 1, wherein the laser radiation is transmitted from a laser source to a focusing lens via an optical fiber, and the focusing lens forms the focused laser radiation beam.

3. The method for laser welding a workpiece along a welding line according to claim 2, wherein the optical fiber includes a central core for guiding the central beam and an annular core for guiding the annular beam.

4. The method for laser welding a workpiece along a welding line according to claim 1, wherein the workpiece comprises two pieces to be overlapped and welded together.

5. The method for laser welding workpieces along a welding line according to claim 4, wherein the two pieces are separated by a gap.

6. The method for laser welding workpieces along a welding line according to claim 4, wherein the ratio of the center processing power to the ring processing power is selected to form a weld seam with a uniform width passing through the two pieces.

7. The method for laser welding workpieces along a welding line according to claim 1, wherein the ratio of the center processing power to the ring processing power is less than 1:

3.

8. The method for laser welding a workpiece along a welding line according to claim 1, wherein the focus is located at a focusing depth relative to the exposed surface of the workpiece, the focusing depth being in the range of 1 mm above the exposed surface and 2 mm below the exposed surface.

9. The method for laser welding a workpiece along a welding line according to claim 1, wherein the steps of sloping down the power of the annular beam at a second annular rate and sloping down the power of the central beam at a second center rate, wherein the energy of the annular beam and the central beam is sloping down to a shut-off power.

10. The method for laser welding a workpiece along a welding line according to claim 9, wherein the shut-off power is 0 watts.

11. The method for laser welding a workpiece along a welding line according to claim 9, wherein the shut-off power is less than the power that melts the surface of the workpiece.

12. The method for laser welding a workpiece along a welding line according to claim 1, wherein the step of reducing the power of the annular beam is from the annular processing power to a lower power, and the step of increasing the power of the center beam is from the center processing power to a higher power, and then the lower power and the higher power are maintained for a period of time before the steps of reducing the power of the annular beam and reducing the power of the center beam.

13. The method for laser welding a workpiece along a welding line according to claim 12, wherein the workpiece comprises two pieces to be welded together, and the lower power, the higher power, and the dwell time are selected such that the two pieces have the same temperature at the end position.

14. The method for laser welding a workpiece along a welding line according to claim 12, wherein the lower power, the higher power, and the dwell time are selected such that the temperature on the exposed surface is equal to the temperature at which the keyhole penetrates the workpiece to its deepest point.

15. The method for laser welding a workpiece along a welding line according to claim 12, wherein the lower power, the higher power, and the dwell time are selected to form a weld with a cross-sectionally symmetrical shape at the end position.

16. The method for laser welding a workpiece along a welding line according to claim 12, wherein the power of the annular beam is reduced from the annular processing power to the lower power between 10% and 20%.

17. The method for laser welding a workpiece along a welding line according to claim 1, further comprising the step of applying a laser power pulse after the steps of the sloping power of the annular beam and the sloping power of the central beam.

18. The method for laser welding a workpiece along a welding line according to claim 17, wherein the laser power provided by the pulse is sufficient to slow the solidification of the molten material at the end location.

19. The method for laser welding a workpiece along a welding line according to claim 17, wherein the laser power provided by the pulse is sufficient to remelt the exposed surface.

20. The method for laser welding a workpiece along a welding line according to claim 1, wherein the workpiece is made of a metal alloy selected from Gen3 steel, XGen3 steel, DP600 steel and DP980 steel.

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