Laser welding stack foil

Abstract

A method for laser deep penetration welding of a stack of metal foils (22) to a metal sheet (24) is disclosed. The method independently adjusts the power of a focused center beam and the power of a focused annular beam to form a weld (34) through all of the foils (22) and the sheet (24). The annular beam provides sufficient power to heat the metal to about the melting temperature, to widen the opening of a keyhole, and to stabilize the molten pool. The center beam provides sufficient additional power to form the keyhole. The power of the annular beam is on for a longer time than the power of the center beam. Multiple such welds are formed to provide mechanical strength and electrical conductivity.

Description

[0001] priority

[0002] This application claims priority to U.S. patent application serial number 16 / 828,194, filed on March 24, 2020, the disclosure of which is incorporated herein by reference in its entirety. Technical Field

[0003] The present invention generally relates to welding using a focused beam of laser radiation. In particular, the present invention relates to welding stacks of metal foils using a focused central beam and a focused annular beam. Background Art

[0004] Laser radiation beams are increasingly used to cut, drill, weld, mark and scribing workpieces made of a variety of materials, including metals and metal alloys. Conventional machining produces unwanted defects, such as microcracks that can propagate when the workpiece is stressed, thereby degrading and weakening the workpiece. Laser machining minimizes such unwanted defects, is generally cleaner, and produces 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 locates each weld point or weld seam while minimizing parallel heating. It is useful to distinguish between two main laser welding regimes. Conductive welding occurs at lower laser powers and lower power densities. The absorbed laser power heats the radiated material, melting the material in each part to be joined, which flows, mixes, and then solidifies. Keyhole welding occurs at higher laser powers and higher power densities, sufficient to vaporize some of the radiated material. The pressure of the vaporized material on the surrounding molten material opens a channel through the molten material with a characteristic narrow and deep profile, which allows deep penetration of the laser beam. The finished deep penetration weld is generally narrower, deeper, and stronger than conductive welding. However, maintaining a stable keyhole in the hot and dynamic pool of molten material can be difficult.

[0006] Lithium-ion batteries are a key enabling technology for portable electronic devices, electric vehicles, and most other contemporary rechargeable electrical devices. Each cell of the battery consists of two stacks of thin metal foils immersed in an electrolyte. The metal is usually aluminum or copper, and the typical thickness of the foil is about 10 micrometers (μm). There are typically 20 to 40 individual foils in each foil stack. The foil stack can be rolled into a cylinder or laid flat. The electrolyte is a lithium salt dissolved in a solvent. Each foil stack is electrically connected to a metal tab protruding from the cell for electrical connection. Depending on the voltage and current requirements of the electrical device, multiple cells are electrically connected in series and / or in parallel to form a battery. Multiple batteries can be electrically connected in series and / or in parallel to form a battery pack.

[0007] The mechanical and electrical connection of each foil in the stack to the corresponding sheet is critical to the integrity, reliability, and performance of the battery. However, joining multiple thin metal foils to a much thicker metal sheet is a challenge, especially when the foil stack and sheet are made of different materials. The resulting joint must be strong, durable, and have low electrical resistance. Precision resistance welding is used, but for these highly conductive metals, it relies on interfacial resistance, and the high thermal conductivity of these metals means that large amounts of current must be applied. Ultrasonic welding is used, but requires mechanical compression of the parts to be joined, which must be performed before any assembly. Aluminum has a durable oxide layer that must be broken in this non-laser process. For these reasons, laser welding is preferred, providing precise power delivery to minimize overall heat buildup. Deep-penetration laser welding can form strong welds across the entire thickness of the foil stack and sheet. Some battery designs include additional foil-to-sheet joints for connecting battery cells within the battery, which also benefit from deep-penetration laser welding.

[0008] One problem with deep penetration laser welding is that molten material is ejected from the weld pool due to vapor pressure, shear stress caused by the vapor flow, recoil caused by evaporation, and rapid circulation of the material around the weld pool (which overcomes the surface tension of the molten material). Droplets of this ejected material, called "spatter," re-agglomerate on the metal surface, causing contamination. This ejection causes material to be lost from the top and bottom surfaces of the completed weld, thereby weakening the completed weld. Another problem with deep penetration laser welding is inconsistent penetration depth due to the complex dynamics involved in forming and maintaining the keyhole.

[0009] A fast, simple, and reliable process is needed to laser weld metal foil stacks to metal sheets, producing clean, consistent welds. The process must produce strong joints with low electrical resistance across the entire thickness of each foil stack and sheet. Preferably, the process should not increase the cost of each weld and be compatible with high-volume automated manufacturing. SUMMARY OF THE INVENTION

[0011] In one aspect, a method for laser welding a plurality of copper foils to a copper sheet according to the present invention includes assembling the foils into a stack and compressing the foil stack and the sheet to form a workpiece. A focused beam of laser radiation is directed onto the surface of the workpiece at a location to be welded. The focused beam includes a central beam and concentric annular beams. The power of the annular beam is ramped to a first power and maintained for a first time sufficient to heat the surface of the workpiece at the location to be welded to at least 1100 Kelvin (K). The power of the central beam is ramped to a second power, and after the surface of the workpiece at the location to be welded reaches 1100 Kelvin, the central beam reaches a second power. The power of the central beam is maintained for a second time sufficient to melt through all of the foils and the sheet, thereby forming a weld connecting all of the foils and the sheet.

[0012] In another aspect, a method for laser welding a plurality of aluminum foils to an aluminum sheet according to the present invention includes assembling the foils into a stack and compressing the foil stack and the sheet to form a workpiece. A focused beam of laser radiation is directed onto the surface of the workpiece at a location to be welded. The focused beam includes a central beam and concentric annular beams. The power of the annular beam is ramped up to a first power while the power of the central beam is ramped up to a second power. The power of the annular beam is maintained for a first time. The power of the central beam is maintained for a second time, and then the power of the central beam is ramped down. The first time is longer than the second time. The second time is sufficient to melt through all of the foils and the sheet, thereby forming a weld connecting all of the foils and the sheet. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The accompanying drawings, which are incorporated in and constitute a part of this specification, 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.

[0015] Figure 1A is a side view of a partial cross section, schematically showing a preferred embodiment of a laser welding device for implementing the laser welding method of the present invention, the device comprising a laser source generating at least two laser radiation beams, an optical fiber and a focusing lens.

[0016] Figure 1B It is schematically shown Figure 1A Cross-sectional view of a detail of an optical fiber, which has a central core for guiding the central beam and an annular core for guiding the annular beam.

[0017] Figure 2 is a graph of beam power versus time, schematically illustrating the laser welding method according to the present invention, using Figure 1A and 1B The equipment solders the copper foil stack to the copper sheet.

[0018] Figure 3A is a plan view, and Figure 3B is used Figure 2 An enlarged plan view of a weld in a copper workpiece produced by the method of the present invention, the photograph showing the surface of the foil stack which is the front surface during welding.

[0019] Figure 4 is a graph of beam power versus time, schematically illustrating the laser welding method according to the present invention, using Figure 1A and 1B The equipment welds the foil stack to the aluminum sheet.

[0020] Figure 5A is used Figure 4 Plan view of a weld in an aluminum workpiece produced by the method of the present invention, the photograph showing the surface of the foil stack which is the front surface during welding.

[0021] Figure 5B yes Figure 5A A plan view of the workpiece during welding. The photo shows the surface of the piece, which is the back surface during welding.

[0022] Figure 6 is used Figure 4 A plan view of a weld in an aluminum workpiece produced by the method of the present invention, the photograph showing the surface of the sheet, which is the front surface during welding. Detailed Description of the Invention

[0024] Referring now to the drawings, wherein like parts are represented by like numerals, Figure 1A and 1B The apparatus 10 used in the laser welding method of the present invention is schematically shown. A laser source 12 delivers at least two beams of laser radiation to a focusing lens 16 via an optical fiber 14. The optical fiber 14 includes a central core 40 for guiding a central beam of laser radiation. The central core 40 has a concentric low-refractive index cladding 42, and the optical fiber 14 also includes an annular core 44 for guiding an annular beam of laser radiation. The annular core 44 is concentrically located between the low-refractive index cladding 42 and the low-refractive index cladding 46. The laser source 12 is configured to deliver the central beam to the central core 40 and to deliver the annular beam to the annular core 44. Laser systems integrating such a laser source with such an optical fiber are commercially available. For example, Highlight Laser Systems is available from Coherent Inc. of Santa Clara, California. TM FL-ARM laser. A special feature of this laser system is that the power of the central beam and the annular beam can be selected and adjusted independently.

[0025] Focusing lens 16 forms a focused beam 18, comprising a focused central beam depicted as converging solid lines and concentrically focused annular beams depicted as converging dashed lines. The focused beams converge to a focal point 20, where the focused central beam has a much smaller diameter than the concentrically focused annular beams. Focused beam 18 is directed onto a workpiece comprising a stack 22 of metal foils and a metal sheet 24 to be welded together. Stack 22 and sheet 24 can be made of copper, nickel, nickel-plated copper, aluminum, a steel alloy, or any other metal or metal alloy selected for use in manufacturing cells or batteries. The stack and sheet can be made of the same metal or different metals. Stack 22 and sheet 24 are compressed by a fixture 26 to form the workpiece and eliminate gaps between individual foils in the stack and between the stack and sheets. Any remaining gaps are preferably less than 20% of the thickness of a single foil, and most preferably less than 10% of the thickness of a single foil. In the figure, the workpiece and fixture are shown in cross-section.

[0026] The workpiece is supported by a translation stage 28 and moved as needed. The focal point 20 is located near the front surface of the workpiece. The focal point is moved laterally across the workpiece by a beam scanner 30, which may include an acousto-optic deflector or a galvanometer-driven mirror to achieve rapid movement of the focal point 20 during laser welding. The beam scanner 30 and the focusing lens 16 can be conveniently integrated into a laser processing head 32. For example, the focusing lens 16 can be a flat-field objective lens to maintain a consistent depth of focus across the entire surface of the workpiece accessible to the laser processing head 30. The processing head 32 may also include an optional beam expander (not shown here) located between the optical fiber 14 and the focusing lens 16 to expand and collimate the beam emitted from the optical fiber before focusing. When exposed to the focused beam 18, a weld 34 is formed through the workpiece.

[0027] Figure 2 The power in the central core and the power in the annular core versus time in a preferred embodiment of a laser welding method 50 according to the present invention is schematically illustrated for welding multiple foils to a sheet. The foils and sheet are made of copper, nickel-plated copper, or a copper alloy. The foils are initially assembled into a stack, and the foil stack is pressed onto the sheet using a fixture. The front surface of the workpiece is exposed to a focused beam 18, which is directed onto the location on the front surface to be welded. The power of the annular beam is ramped to a first power P1 and maintained at this power for a first time T1 sufficient to heat the workpiece surface at the location to be welded to at least 1100K, more preferably above 1300K. During this first time T1, the annular beam preheats the workpiece surface. It is known that for copper at approximately 1300K, thermal conductivity decreases abruptly and optical absorption increases abruptly for radiation with a wavelength of approximately 1μm. Heating copper to this temperature decreases thermal conductivity by approximately a factor of two and increases near-infrared light absorption by approximately a factor of three.

[0028] At the end of the first time T1, the power of the center beam is ramped up to a second power P2 and maintained at the second power P2 for a second time T2, sufficient to melt through all foil stacks 22 and sheets 24. Melting by the center beam forms a weld 34 that connects all foil stacks 22 and sheets 24. The focused center beam preferably forms a keyhole in the molten material, penetrating all foils and sheets. The power of the annular beam is ramped down to a third power P3, while the power of the center beam is ramped up to the second power P2. The third power P3 is selected to be sufficient to keep the opening of the keyhole open, thereby allowing metal vapor to escape from the keyhole and minimizing the formation of spatter. During the second time T2, the power of the center beam is preferably limited to the minimum power required to melt through all foils and sheets, while the power of the annular beam is preferably limited to the minimum power required to prevent spatter. Minimizing the total power applied to the workpiece reduces the heat-affected zone around the completed weld and prevents the formation of any unwanted defects in the welded workpiece.

[0029] Figure 3A is used Figure 2 A plan view photograph of an exemplary workpiece welded by method 50. Figure 3B This is a higher magnification plan view of the same welded workpiece. The workpiece is a stack of 25 copper foils pressed onto a nickel-plated copper sheet. Each foil is 6 μm thick, and the sheet is 300 μm thick.

[0030] refer to Figure 1A and 1B , laser source 12 is Highlight TM The FL8000-ARM laser delivers up to 4 kW of power at a wavelength of 1070 nanometers through each core of the optical fiber 14 independently. The diameter of the central core 40 is 100 μm, and the outer diameter of the annular core 44 is 290 μm. The laser processing head 32 is a SCANLAB intelliSCAN 30 scan head, available from SCANLAB GmbH of Puchheim, Germany. The focusing lens 16 provides a magnification of 1.4 times, resulting in a focused central beam with a diameter of approximately 140 μm and an outer diameter of the focused annular beam of approximately 405 μm. During the welding process, nitrogen shielding gas is directed at the location to be welded on the workpiece. As shown, the focused beam 18 is applied to the surface of the stack at an approximately normal angle of incidence (90°). An angle of incidence between 80° and 90° has been found to be beneficial. The fixture 26 is machined from a single piece of metal and pressed against the workpiece around the location to be welded. The focused beam 18 is directed onto the workpiece through a cutout in the fixture.

[0031] Figure 3A and 3BThe surface of the stack is shown, which is the front surface of the workpiece exposed to the focused beam and is therefore the entry surface during deep penetration welding. The workpieces are welded at 42 positions along two staggered rows. In the example described, the rows are welded sequentially by directing the focused beam to successive positions along a row. At each position, as shown in FIG. Figure 2 As shown, power is applied through the ring and the center core to form a weld. Here, the first power P1 = 1500 watts (W), the second power P2 = 1450 watts, the third power P3 = 1000 watts, the first time T1 = 1 millisecond (ms) and the second time T2 = 1 ms.

[0032] Here, "off-power" means that during prolonged exposure to the focused beam, the power is too low to melt the workpiece surface and not damage the workpiece. The off-power in the example shown is 0W. The power of the annular beam is ramped up from the off-power to the first power P1 in about 3ms. The power of the central beam is ramped down from the off-power to the second power P2 in about 1ms, while the power of the annular beam is simultaneously ramped down from the first power P1 to the third power P3. The power of the central beam is ramped down from the second power P2 to the off-power, and the power of the annular beam is simultaneously ramped down from the third power P2 to the off-power in about 1ms. Power is applied at each position for a total time of about 7ms, and the total time required to complete all 42 welds is about 0.54 seconds. This total time includes the time spent translating the focused beam between positions.

[0033] The preheat provided by the focused annular beam during the first time T1 reduces the near-infrared power required for deep melting and welding provided by the focused central beam. Preheating thereby reduces the laser energy applied to the workpiece and reduces the heat-affected zone. Preheating enables the focused central beam to penetrate deeper into the workpiece, further improving weld quality.

[0034] exist Figure 3A and 3B In the laser welding method 50 employed in the example of FIG, the power of the center beam is ramped up to a second power P2 while the power of the annular beam is simultaneously ramped down to a third power P3. However, this simultaneous ramping is not necessary for a successful weld. It is sufficient for the center beam to reach the second power P2 after the stack surface reaches 1100K, and for the two focused beams to maintain at least that temperature together until the center beam reaches the first power P2. Similarly, it is not necessary for both beams to be ramped down to the cutoff power simultaneously for a successful weld. For certain workpieces, it may be advantageous to maintain the power of the annular beam until the center beam is ramped down to a power at which the keyhole collapses.

[0035] The laser welding method 50 will have a typical first power P1 between about 1350 W and about 1650 W, a typical second power P2 between about 1300 W and about 1600 W, and a typical third power P3 between about 700 W and about 1300 W. The first time T1 is typically between about 0.2 ms and about 10 ms, and the second time T2 is typically between about 0.1 ms and about 5 ms in duration.

[0036] Figure 4 The diagram schematically illustrates the relationship between the power in the center core and the energy in the annular core versus time in another preferred embodiment of a laser welding method 60 according to the present invention, used for welding multiple foils to a sheet. Here, both the foils and the sheet are made of aluminum or an aluminum alloy. Again, the foils are initially assembled into a stack, and the foil stack is pressed onto the sheet using a fixture. The front surface of the workpiece is exposed to a focused beam 18, which is directed onto the location on the front surface of the workpiece to be welded. The power of the annular beam is ramped up to a first power P1, while the power of the center beam is ramped down to a second power P2. The power of the annular beam is maintained for a first time T1 and then ramped down. The power of the center beam is maintained for a second time T2, which is shorter than the first time T1, and then ramped down. The second time T2 is selected to be long enough to melt through all foil stacks 22 and sheet 24. Melting by the center beam forms welds 34, which connect all foil stacks 22 and sheet 24. The first time T1 is selected to be long enough to allow for controlled collapse of the keyhole and controlled shrinkage of the molten pool. This controlled collapse and shrinkage allows metal vapor to escape from the weld pool, thereby preventing voids in the welded workpieces and providing annealing to minimize the formation of any cracks.

[0037] Figure 5A and 5B is used Figure 4 A plan view photograph of an exemplary workpiece welded by method 60. The workpiece is a stack of 24 aluminum foils pressed onto an aluminum sheet. The thickness of each foil is 12 μm, and the thickness of the sheet is 400 μm. The laser source, optical fiber, laser processing head, focusing lens, shielding gas, and fixture are Figure 3A and 3B Same as the example. Figure 5A The surface of the stack is shown, which is the front surface of the workpiece exposed to the focused beam and is therefore the entry surface during deep penetration welding. Figure 5B The back surface of the sheet is shown. The workpiece is welded at 42 positions along two staggered rows, with the rows welded sequentially. At each position, as Figure 4 As shown, power is applied through the ring and the center core to form a weld. Here, the first power P1 = 1500W, the second power P2 = 1600W, the first time T1 = 5ms, and the second time T2 = 1ms.

[0038] In the example described, the cutoff power was 0 W. The power of the annular beam was ramped up from the cutoff power to a first power P1 in approximately 3 ms, while the power of the center beam was simultaneously ramped up from the cutoff power to a second power P2. The power of the center beam was ramped down from the second power P1 to the cutoff power in approximately 1 ms. The power of the annular beam was ramped down from the first power P1 to the cutoff power in approximately 1 ms. The power was applied at each position for a total of approximately 9 ms, and the total time required to complete all 42 welds was approximately 2.06 seconds. In this example, it was found that the simultaneous ramping of the annular beam to the first power P1 and the center beam to the second power P2 was beneficial in preventing spatter.

[0039] Figure 6 is used Figure 4 A plan view photograph of another exemplary workpiece welded using method 60. The workpiece here is an aluminum sheet pressed onto a stack of 24 aluminum foils. Each foil has a thickness of 12 μm, and the sheet has a thickness of 400 μm. The laser source, optical fiber, shielding gas, and fixture are the same as those in the above example. The laser processing head 32 is an RLSK 3D Remote Laser Welding Head, available from II-VI Inc. of Kleinmachnow, Germany. The focusing lens 16 provides a magnification of 3 times, so that the diameter of the focused central beam is approximately 300 μm, and the outer diameter of the focused annular beam is approximately 870 μm. The figure shows the surface of the sheet, which is the workpiece surface exposed to the focused beam and, therefore, the entry surface during deep penetration welding. The workpiece was welded at 40 locations along two staggered rows, with the rows welded sequentially. Here, the first power P1 = 1500 W, the second power P2 = 1600 W, the first time T1 = 6 ms, and the second time T2 = 1 ms.

[0040] In the example described, the cutoff power was 0 W. The power of the annular beam was ramped up from the cutoff power to a first power, P1, in approximately 8 ms, while the power of the center beam was simultaneously ramped up from the cutoff power to a second power, P2. The power of the center beam was ramped down from the second power, P1, to the cutoff power in approximately 1 ms. The power of the annular beam was ramped down from the first power, P1, to the cutoff power in approximately 2 ms. The total time for applying power at each position was approximately 16 ms, and the total time required to complete all 40 welds was approximately 1.07 seconds.

[0041] A typical workpiece for laser welding methods 50 and 60 will have 20 to 100 individual foils welded to a sheet. The typical thickness of each foil is between about 5 μm and about 15 μm. Typical sheet thicknesses are between about 100 μm and about 500 μm. Typical focused central beam diameters are between about 100 μm and about 400 μm, and typical focused annular beam diameters at the workpiece surface are between about 300 mm and about 1200 mm. Laser welding method 60 has a typical first power P1 between about 1350 W and about 1650 W, and a typical second power P2 between about 1450 W and about 1750 W. The first time T1 will typically be between about 0.5 ms and about 20 ms, and the second time T2 will typically have a duration between about 0.1 ms and about 5 ms.

[0042] Figure 5A 、 5B Together with 6, it is demonstrated that the welding method of the present invention works with the focused beam 18 incident on the outer surface of the stack 22 or the opposing outer surface of the sheet 24. It is generally necessary to make multiple welds to securely connect a stack of foils to the sheet. A greater number and higher density of welds will provide a stronger joint and higher conductivity. However, those skilled in the art will recognize that there is a tradeoff between the total number of welds and the total time to complete all of them, so the number and density of welds will be specific to the particular application. By varying the number of individual welds, the total area of ​​the welded workpiece can be easily scaled. The method of the present invention can be used to connect the foil stack to the anode or cathode sheet within a battery cell. The method of the present invention can also be applied to connect the foils of battery cells within a battery pack that are electrically connected.

[0043] A beam scanner 30 laterally translates the focused beam 18 between a plurality of locations on the workpiece to be welded. Welds are made at each of these locations by passing a pulse of laser radiation through each core of the optical fiber, the focusing lens, and the workpiece. Figure 2 and Figure 4 The power profiles of the pulse pairs used to make each weld for the laser welding methods 50 and 60 are depicted, respectively. Although linear power ramping is shown and discussed herein, for simplicity of illustration and description, the welding methods of the present invention can be optimized by applying other types of power ramping to the center beam and / or annular beam, such as exponential power ramping.

[0044] Although the annular beam and the central beam are maintained at constant powers (first power P1 and second power P2), respectively, in the examples herein, the welding method of the present invention can be further optimized by modulating the powers of these beams during the first time T1 and the second time T2. Figure 4In the laser welding method 60 , the power of the annular beam is rapidly cycled between a first power P1 and a much lower power to expel metal vapor. Alternatively, the power is ramped down from the first power P1 to a lower power after the keyhole is closed to anneal the metal and prevent crack formation during the weld. The key requirement is to provide sufficient power in the annular beam to widen the keyhole opening during deep penetration welding, stabilize the molten pool, and then allow for controlled collapse of the keyhole and controlled shrinkage of the molten pool.

[0045] In summary, an inventive method is described above for welding a stack of metal foils to a sheet of metal using a focused laser beam comprising a central beam and an annular beam. The focused annular beam has sufficient power to heat the locations to be welded to approximately the melting temperature of the metal. The focused central beam has sufficient additional power and lasts for a sufficient time to form a deep penetration weld connecting all of the foils and the tab. The annular beam lasts for a longer time, which provides preheating for high thermal conductivity metals (such as copper) and post-heating for metals that are prone to vapor trapping (such as aluminum). The method is repeated to form welds at a sufficient number and density of locations to provide the mechanical strength and conductivity required for a particular application.

[0046] 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. Instead, the present invention is limited only by the appended claims.

Claims

1. A method for laser welding a plurality of copper foils to a copper sheet, comprising the following steps: assembling the copper foils into a foil stack, and compressing the foil stack and the copper sheet to form a workpiece; directing a focused beam of laser radiation onto the surface of the workpiece at a location to be welded, the focused beam comprising a central beam and concentric annular beams; ramping the power of the annular light beam to a first power; maintaining the power of the annular beam for a first time sufficient to heat the surface of the workpiece at the location to be welded to at least 1100 Kelvin; ramping the power of the central beam to a second power, the central beam reaching the second power after the surface of the workpiece at the location to be welded reaches 1100 Kelvin; and The power of the central beam is maintained for a second time sufficient to melt through all of the foils and sheets, thereby forming a weld connecting all of the foils and sheets. 2 . The method of claim 1 , wherein during the step of maintaining the power of the annular light beam, the power of the annular light beam is maintained at a first power.

3. The method of claim 1, wherein during the step of maintaining the power of the ring-shaped light beam, the power of the ring-shaped light beam is modulated. The method of claim 1 , wherein during the step of maintaining the power of the central light beam, the power of the central light beam is maintained at a second power. The method of claim 1 , wherein during the step of maintaining the power of the central light beam, the power of the central light beam is modulated.

6. The method of claim 1, wherein during the step of ramping up the power of the center beam, the power of the annular beam is ramped down to a third power. The method of claim 6 , wherein the power of the annular light beam is maintained at the third power during a second time. The method of claim 6 , wherein the power of the annular light beam is modulated during the second time period.

9. The method of claim 6, wherein the third power is between 700 watts and 1300 watts.

10. The method of claim 1, wherein the first power is between 1350 watts and 1650 watts.

11. The method of claim 1, wherein the second power is between 1300 watts and 1600 watts.

12. The method of claim 1, wherein the duration of the first time is between 0.2 milliseconds and 10 milliseconds.

13. The method of claim 1, wherein the duration of the second time is between 0.1 milliseconds and 5 milliseconds.

14. The method of claim 1, wherein the focused beam is directed onto the workpiece surface at an angle of incidence between 80 degrees and 90 degrees.

15. The method of claim 1, wherein 20 to 100 foils are welded to the copper sheet.

16. The method of claim 1, wherein each foil has a thickness between 5 microns and 15 microns.

17. The method of claim 1, wherein the steps of directing the focused beam, ramping up the power of the annular beam, maintaining the power of the annular beam, ramping up the power of the center beam, and maintaining the power of the center beam are repeated multiple times to form multiple welds at multiple locations.

18. The method of claim 1, wherein the central beam has a diameter on the workpiece surface of 100 microns to 400 microns, and the annular beam has a diameter on the workpiece surface of 300 microns to 1200 microns.

19. A method for laser welding a plurality of aluminum foils to an aluminum sheet, comprising the following steps: assembling the aluminum foil into a foil stack, and compressing the foil stack and the aluminum sheet to form a workpiece; Directing a focused beam of laser radiation to the surface of a workpiece at a position to be welded, wherein the focused beam includes a central beam and a concentric annular beam; ramping the power of the annular beam to a first power while ramping the power of the central beam to a second power; maintaining the power of the annular light beam for a first time; and maintaining the power of the central light beam for a second time and then ramping down the power of the central light beam, the first time being longer than the second time; The second time is sufficient to melt through all the foils and sheets, thereby forming a weld connecting all the foils and sheets.

20. The method of claim 19, wherein during the step of maintaining the power of the annular beam, the power of the annular beam is maintained at a first power.

21. The method of claim 19, wherein during the step of maintaining the power of the ring-shaped light beam, the power of the ring-shaped light beam is modulated.

22. The method of claim 19, wherein during the step of maintaining the power of the central light beam, the power of the central light beam is maintained at a second power.

23. The method of claim 19, wherein during the step of maintaining the power of the central light beam, the power of the central light beam is modulated.

24. The method of claim 19, wherein the first power is between 1350 watts and 1650 watts.

25. The method of claim 19, wherein the second power is between 1450 watts and 1750 watts.

26. The method of claim 19, wherein the duration of the first time is between 0.5 milliseconds and 20 milliseconds.

27. The method of claim 19, wherein the second time has a duration between 0.1 milliseconds and 5 milliseconds.

28. The method of claim 19, wherein the focused beam is directed onto the workpiece surface at an angle of incidence between 80 degrees and 90 degrees.

29. The method of claim 19, wherein 20 to 100 foils are welded to the aluminum sheet.

30. The method of claim 19, wherein each foil has a thickness between 5 microns and 15 microns.

31. The method of claim 19, wherein the steps of directing the focused beam, ramping up the power of the annular beam and the center beam, maintaining the power of the annular beam, and maintaining the power of the center beam are repeated multiple times to form multiple welds at multiple locations.

32. The method of claim 19, wherein the central beam has a diameter on the workpiece surface of 100 microns to 400 microns, and the annular beam has a diameter on the workpiece surface of 300 microns to 1200 microns.

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