Laser welding of metal foil stacks to metal substrates

Abstract

A method for laser welding a stack of metal foils (120) to a metal substrate includes a step of securing (210) the stack of metal foils (120) between a surface (130S) of a metal substrate and a removable clamp (140) such that a side of the stack formed by an edge of the foils (120) is located on an inner portion of the surface (130S), the clamp (140) being retracted from the side of the stack. A first laser welding step (220) interconnects the foils with an initial laser weld joint by successively tracing (222) a plurality of lateral paths along the foil edge (120E). A second laser welding step (230) connects the interconnected stack of foils (120) to the substrate by tracing a path along the interface between the initial laser weld joint and the substrate surface with a laser beam.

Description

[Technical Field]

[0001] (Priority) This application claims priority to U.S. Provisional Application Serial No. 63 / 152,534, filed February 23, 2021, the entire disclosure of which is incorporated herein by reference.

[0002] The present invention relates generally to laser welding of stacks of metal foils to metal substrates, and in particular as applied to the production of electrochemical batteries such as lithium ion batteries. [Background technology]

[0003] Beams of laser radiation are increasingly being used to cut, drill, weld, mark, and engrave workpieces made from a wide range of materials, including metals and metal alloys. Traditional mechanical processes create undesirable defects, such as microcracks, that propagate when the processed workpiece is subjected to stress, thereby degrading and weakening the processed workpiece. Laser processing, which minimizes such undesirable defects, generally results in fewer contaminants and smaller heat-affected zones. Laser machining uses a focused laser beam to produce precision cuts and holes with high-quality edges and walls, while minimizing the formation of undesirable defects.

[0004] In laser welding, a focused laser beam precisely positions each weld or seam while minimizing incidental heating. It is useful to distinguish between two main laser welding regimes. Conduction welding occurs at lower laser powers and lower power densities. The absorbed laser power heats the irradiated material, melting it at each part to be joined, causing it to flow and mix, then solidify. Keyhole welding occurs at higher laser powers and higher power densities, sufficient to vaporize a portion of the irradiated 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, allowing for deep penetration of the laser beam. The finished keyhole weld is generally narrower, deeper, and stronger than conduction welding.

[0005] Lithium-ion batteries are a key enabling technology for portable electronic devices, electric vehicles, and most other modern rechargeable electrical devices. Each cell of the battery contains two stacks of thin metal foils immersed in or coated with an electrolyte. The metal is most often aluminum or copper, and the foils have a typical thickness of about 10 microns (μm). Typically, there are 20 to 40 individual foils in each foil stack. The foil stacks can be combined into a cylinder or laid flat. The electrolyte contains a lithium salt. Each foil stack is electrically connected to a metal tab that protrudes from the cell for electrical connection. Multiple cells are electrically connected in series and / or parallel to form a battery, depending on the voltage and current requirements of the electrical device. Multiple batteries can be electrically connected in series and / or parallel to form a battery pack.

[0006] The mechanical attachment and electrical connection of each foil in the stack to an individual tab is critical to the battery's integrity, reliability, and performance. However, joining multiple thin metal foils to a much thicker metal tab is challenging. The completed joint must be strong, durable, and have low electrical resistance. Precision resistance welding is used, but it relies on interface resistance for these metals with high electrical conductivity, and the high thermal conductivity of these metals means that a large current must be applied. Ultrasonic welding is used, but it requires mechanical compression of the parts to be joined, which must be joined prior to any assembly. Aluminum has a durable oxide layer that must be destroyed in such non-laser processes. For these reasons, laser welding has emerged as an attractive alternative, offering precise delivery of power and minimizing overall heat buildup. Keyhole laser welding can form a strong weld through the entire thickness of the foil stack and tab. Some battery designs include additional foil-to-tab joints for attaching and connecting the cells within the battery, which also benefit from keyhole laser welding. Summary of the Invention [Means for solving the problem]

[0007] Disclosed herein is a method for laser welding a metal foil stack to a metal substrate. The method is useful in the manufacture of battery cells, e.g., lithium-ion battery cells, where the method can be used to weld an anode metal foil stack or a cathode metal foil stack to a metal tab for electrical connection. The disclosed method is a two-step process that avoids the difficulties of welding materials with significantly different thicknesses together in a single laser welding operation. The disclosed method is specifically tailored to laser welding a metal foil stack to only a single metal substrate to minimize the form factor of the resulting welded structure.

[0008] During laser welding, the metal foil stack is secured between a metal substrate to which the metal foil will be connected and a removable clamp. The metal foil stack is welded only to the metal substrate, not the clamp, so the clamp can be removed from the welded structure. To avoid welding the metal foil stack to the clamp, the clamp is retracted from the edge of the metal foil.

[0009] Laser welding is performed in two steps. First, the metal foils are laser welded together in a relatively weak manner, due to the relatively thin nature of the metal foils, to prevent spatter and excessive bending and / or cracking of the metal foils. This first laser welding step serves both to electrically interconnect the metal foils and to structurally strengthen the metal foil stack. Next, benefiting from the improved strength of the welded metal foil stack, a stronger laser weld is used to form a robust, high-quality weld joint connecting the welded metal foil stack to the relatively thick metal substrate. The first laser welding step may rely solely on conduction welding, while the second laser welding step may utilize keyhole welding. The present specification also provides, for example, the following: (Item 1) 1. A method for laser welding a stack of metal foils to a metal substrate, comprising: clamping the metal foil stack between a surface of the metal substrate and a removable clamp such that a side of the stack formed by an edge of the metal foil is positioned on an inner portion of the surface of the metal substrate, the removable clamp being retracted from the side of the stack; interconnecting the metal foil with an initial laser weld joint, the interconnecting step comprising using a laser beam to continuously trace a plurality of lateral paths along the metal foil edge; connecting the interconnected stack of metal foils to the metal substrate by tracing a path along the interface between the initial laser weld joint and the metal substrate surface with a laser beam; A method comprising: (Item 2) Item 10. The method of claim 1, wherein each of the lateral paths, when traced, is closer to the metal substrate surface than any preceding one of the lateral paths. (Item 3) 3. The method of claim 1, further comprising the step of displacing the removable clamp from the stack of metal foils after the connecting step. (Item 4) the securing step includes clamping the metal substrate and the metal foil stack between the removable clamp and a backing plate; the method further includes, after the connecting step, displacing the removable clamp from the backing plate to facilitate removal of the metal substrate and the metal foil stack to be welded together by the interconnecting and connecting steps. 2. The method according to any of the preceding items. (Item 5) 10. The method of claim 1, wherein in the interconnecting step, each trace of a lateral path produces a weld line, and the weld lines produced by adjacent lateral paths of a pair spatially overlap. (Item 6) 10. The method of claim 9, wherein the connecting step includes melting a portion of the initial laser weld joint closest to the metal substrate surface without melting a portion of the initial laser weld joint furthest from the metal substrate surface. (Item 7) 10. The method of claim 1, wherein in the connecting step, the laser beam scans a repeating two-dimensional scanning pattern along the interface. (Item 8) Item 8. The method of item 7, wherein in the connecting step, the path traced by the laser beam traverses the interface back and forth in a circular or elliptical manner. (Item 9) In the interconnecting step, the laser beam forms the initial laser weld joint by conduction welding; the connecting step includes keyhole welding the interconnected metal foil stack to the metal substrate. 2. The method according to any of the preceding items. (Item 10) 10. The method according to claim 9, wherein in the interconnecting step, the laser beam is an annular laser beam. (Item 11) 10. The method according to claim 9, wherein in the connecting step, the laser beams include a central laser beam and an annular laser beam. (Item 12) the securing step includes offsetting the foil edges from one another so that sides of the stack are inclined in the direction toward the removable clamp; In each of the interconnecting step and the connecting step, the laser beam is incident along a direction at an oblique angle to the surface of the metal substrate. 2. The method according to any of the preceding items. (Item 13) Item 13. The method of item 12, wherein each of the metal foils has a thickness of 5 to 30 microns, and the bonding step produces an average offset between the edges of each pair of adjacent metal foils, averaged across the stack, in the range of 20 to 200 microns. (Item 14) 14. The method according to claim 12, wherein in the interconnecting step, each of the lateral paths has a length of at least 5 millimeters, and the laser beam has a width in the range of 100 to 500 microns. (Item 15) 15. The method of claim 12, wherein the connecting step includes forming a melt pool along the interface, and the width of the melt pool in a dimension transverse to the interface is in the range of 0.1 to 2 millimeters. (Item 16) 3. The method according to any preceding item, wherein the thickness of each of the metal foils is 5 to 30 microns. (Item 17) 17. The method of claim 16, wherein the stack comprises at least 10 metal foils. (Item 18) Item 18. The method according to item 16 or 17, wherein the metal foil is made of aluminum. (Item 19) Item 19. The method according to item 18, wherein the metal substrate is made of aluminum or an aluminum alloy. [Brief explanation of the drawings]

[0010] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate generally preferred embodiments of the present invention and, together with the general description given above and the detailed description of preferred embodiments given below, serve to explain the principles of the invention.

[0011] [Figure 1] FIG. 1 illustrates a battery cell assembly during manufacturing, according to an embodiment. [Figure 2] FIG. 2 is a flowchart of a method for laser welding a stack of metal foils to a metal substrate, according to an embodiment. [Figure 3] 3A-3D illustrate a method for interconnecting a tilted stack of metal foils with an initial laser weld joint, according to an embodiment. [Figure 4] 4A-4C illustrate a method for joining a stack of metal foils already interconnected by an initial laser weld joint to a metal substrate, according to an embodiment. [Figure 5] FIG. 5 illustrates a repeating two-dimensional scanning pattern that can be used in the method of FIGS. 4A-4C. [Figure 6] FIG. 6 illustrates a cross-sectional profile of a composite laser beam useful for performing the laser welding of the method of FIG. 2, according to one embodiment. [Figure 7] FIG. 7 illustrates a method for interconnecting a tilted stack of metal foils with an initial laser weld joint formed by tracing multiple transverse paths with a laser beam, according to an embodiment. [Figure 8] FIG. 8 illustrates a non-tilted configuration of a stack of metal foils, according to an embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0012] Detailed Description of the Invention Referring now to the drawings, wherein like components are designated by like numerals, FIG. 1 illustrates, in a cross-sectional side view, one battery cell assembly 100 during manufacture. The assembly 100 includes multiple metal foils 120 and a material layer 110 disposed thereon. Each material layer 110 contains an electrolyte, e.g., a lithium salt. The metal foils 120 form either the anode set or the cathode set of the battery cell. The metal foils 120 are interleaved with a second set of metal foils, not shown in FIG. 1 , such that one metal foil 120 can be the anode foil and the second set of metal foils can be the cathode foil, or vice versa. The metal foils 120 extend beyond the material layers 110 to reach a metal tab 130. The metal tab 130 is configured to form an electrical contact with the metal foil 120. A portion of the metal foil 120 is stacked on a surface 130S of the metal tab 130 in preparation for laser welding the metal foil 120 to the metal tab 130. The thickness of each metal foil 120 may be in the range of 5 to 30 microns (μm). For comparison, the thickness of the metal tab 130 may exceed 0.5 millimeters (mm). The assembly 100 may include 10 or more metal foils 120, e.g., 20 to 40 metal foils 120, stacked on the metal tab surface 130S. The metal foils 120 may be made of, for example, aluminum, and the metal tab 130 may be made of aluminum or an aluminum alloy. Alternatively, the metal tab 130 may be made of another metal or metal alloy, e.g., copper, a copper alloy, or a steel alloy.

[0013] FIG. 2 is a flowchart of one method 200 for laser welding a stack of metal foils to a metal substrate. Method 200 can be applied to battery assembly 100 to laser weld the stack of metal foils 120 to metal tab 130 so as to electrically connect each metal foil 120 to metal tab 130. However, method 200 is also applicable to other scenarios in which a stack of metal foils needs to be welded to a metal substrate. Assembly 100 represents just one example of a metal foil stack and metal substrate that can be laser welded by method 200. Without departing from the scope of this specification, the stack of metal foils 120 can be a stack of different types of battery cell metal foils or even non-battery-related metal foils. Similarly, metal tab 130 can be a metal substrate of a different type of battery assembly or a non-battery-related metal substrate.

[0014] Method 200 includes steps 210, 220, and 230. Step 210 secures a stack of metal foils between a surface of a metal substrate and a removable clamp. Step 210 arranges the metal foil stack and the removable clamp so that (a) the side of the metal foil stack is positioned within an inner region of the metal substrate surface away from the outer periphery thereof, and (b) the removable clamp is recessed from the side of the stack. In one embodiment of step 210, a stack of distal metal foils 120 is secured between a metal tab surface 130S and a removable clamp 140. An edge 120E of the metal foil 120 forms a side of the stack that is located on an inner portion of the metal tab surface 130S, i.e., a non-zero distance away from the outer periphery 130E of the metal tab surface 130S. The removable clamp 140 is recessed from the side of the stack formed by the edge 120E.

[0015] Step 220 is a first laser welding step that interconnects the metal foils with an initial laser weld joint. In one embodiment of step 220, laser beam 180 welds edges 120E of metal foils 120 together while the stack of metal foils 120 is secured between metal tab 130 and removable clamp 140. Step 220 includes step 222, in which the laser beam is used to sequentially trace multiple lateral paths along the metal foil edges. Each lateral path is generally parallel to the metal foil edges. In one embodiment of step 222, each path traced by laser beam 180 extends parallel to edges 120E, i.e., perpendicular to the plane of FIG. 1 .

[0016] In a top-to-bottom implementation of step 222, the laser beam first traces a path farthest from the metal substrate surface, and each subsequent lateral path traced by the laser beam is closer to the metal substrate surface than the previous lateral path already traced by the laser beam. In one example of this top-to-bottom implementation, the laser beam 180 first traces a path near the top of the stack of metal foils 120, i.e., farthest from the metal tab surface 130S and closest to the removable clamp 140. This trace extends parallel to the edges 120E, i.e., perpendicular to the plane of FIG. 1 . As the laser beam 180 traces this first lateral path, it melts the edges 120E of two or more of the top metal foils 120 to weld them together. Next, the laser beam 180 traces lateral paths closer to the metal tab surface 130S, welding more metal foils 120 to those already welded. This process continues until edges 120E of all metal foils 120 are welded together at the initial weld joint. One embodiment of a top-down implementation of step 222 is discussed in further detail below with reference to Figures 3A-3D.

[0017] Step 220 operates on relatively thin metal foils. Step 220 does not attempt to weld these thin metal foils to a thicker metal substrate. Thus, step 220 relatively lightly welds the metal foils together, which may avoid undesirable results such as excessive bending and / or cracking of the metal foils and significant loss of material caused by spatter. Step 220 relies solely on conduction welding, which may avoid such undesirable results. Once step 220 is completed, the initial laser weld joint not only electrically interconnects the metal foils, but also provides structural strength.

[0018] Furthermore, due to the removable clamps being set back from the sides of the stack formed by the foil edges, the foil is not completely secured in place prior to welding. Conduction welding along a series of lateral paths in step 220 is particularly well suited to preventing excessive edge bending and cracking of the foil in this configuration.

[0019] In an alternative embodiment, step 220 traces a differently arranged set of paths, such as multiple paths that are transverse to the foil edge 120E, or another set of paths that cooperate to span across the entire foil edge 120E. This alternative embodiment may utilize conduction welding.

[0020] Step 230 is a second laser welding step that connects the metal foil stack to the metal substrate. Step 230 benefits from the metal foil stack already interconnected with the initial laser weld joint in step 220. The structural strength provided by the initial laser weld joint has at least two benefits. First, welding through the entire metal foil stack is not required because they are already electrically and structurally interconnected by the initial laser weld joint formed in step 220. Second, the strength provided by the initial laser weld joint allows for a stronger weld in step 230, forming a robust, high-quality electrical connection between the metal foil and the metal substrate.

[0021] Step 230 includes step 232, in which a laser beam is used to trace a path along the interface between the initial laser weld joint formed in step 220 and the metal substrate surface. The laser beam may perform a keyhole weld while tracing this path to optimize the quality of the resulting electrical connection. In one implementation of step 230 that implements step 232, laser beam 190 traces a path along the interface between metal tab surface 130S and the initial laser weld joint that interconnects edge 120E of metal foil 120. The path traced in this implementation of step 230 may generally be along a direction perpendicular to the plane of FIG. 1. One embodiment of step 230 is discussed in further detail below with reference to FIGS. 4A-4C.

[0022] Step 232 may utilize a repetitive two-dimensional (2D) scan pattern to optimize keyhole formation and weld quality. The repetitive 2D scan pattern may move back and forth across the interface in a circular or elliptical manner. Welding using a repetitive 2D scan pattern is discussed in further detail below with reference to FIG. 5.

[0023] Steps 220 and 230 may each utilize a shielding gas, such as argon or another inert gas, in the welding area.

[0024] Method 200 may further include step 240, removing the removable clamp from the metal foil stack after completing the two laser welding steps 220 and 230. Because neither step 220 nor step 230 welds the removable clamp to the metal foil stack, step 240 may simply remove the clamp by displacing the clamp from the metal foil stack. In some embodiments of method 200, step 210 sandwiches the metal substrate and metal foil stack between the removable clamp and a backing plate, such as backing plate 150 as shown in FIG. 1 . In such embodiments, step 240 may displace the removable clamp from the backing plate to facilitate removal of the metal substrate and metal foil stack (as welded together by steps 220 and 230).

[0025] The inventors have found that it is advantageous to arrange the metal foil in step 210 so that the sides of the metal stack formed by the foil edges are inclined in a direction toward the removable clamp. In other words, it is advantageous to offset the foil edges from each other so that the sides of the metal stack to be laser welded face each other somewhat away from the metal substrate surface. This configuration allows the laser beam to be incident along a direction that is at an oblique angle to the metal substrate surface, e.g., approximately normal to the sides of the metal stack, in both steps 220 and 230. The inclined configuration also increases the area accessible for laser welding in step 220, thus enabling the formation of a stronger laser weld joint. Furthermore, laser welding of the foil edges in this configuration handles metal foil shrinkage particularly well.

[0026] 3A-3D illustrate one method 300 for interconnecting such a sloped stack of metal foils with an initial laser weld joint. Method 300 is an embodiment of step 220 and may be applied to a stack of metal foils 120 in one embodiment of assembly 100, where the stack has sloped sides. The example depicted in FIGS. 3A-3D has 20 metal foils 120, listed as 120(1)-120(20), with metal foil 120(1) closest to metal tab surface 130S and metal foil 120(20) closest to removable clamp 140. Without loss of generality, method 300 will be discussed herein in the context of metal foils 120 and metal tabs 130. FIG. 3A illustrates one configuration of the path traced by the laser beam in method 300. FIGS. 3B and 3C depict the state of the assembly at one stage during method 300 in a cross-sectional side view and a perspective view, respectively. Figure 3D depicts the completed initial laser weld joint formed by method 300. Figures 3A-3D are best viewed together in the following discussion.

[0027] Method 300 is applied to an arrangement in which edges 120E are offset from one another; thus, generally, the higher a metal foil 120 is in the stack, the further the corresponding edge 120E is set back relative to the edge 120E of metal foil 120(1). As a result, the edges 120E of the stack of metal foils 120 form sloping sides 320S (see FIG. 3B). Deviations from this offset pattern may exist when the placement of the metal foils 120 is subject to imprecision. However, on average, each edge 120E is set back from the edge 120E of any directly adjacent, underlying metal foil 120 by an offset. This offset, averaged across all metal foils in the stack, may be in the range of 20 to 200 μm. The stack of metal foils 120 may have a height 320H in the range of 1 to 3 mm, and the slope angle 320A of the stack's sides 320S (see FIG. 3B) may be in the range of 10 to 80 degrees.

[0028] Laser beam 180 may be incident on side 320S at near-normal incidence. For example, laser beam angle of incidence 380A may be between −20 degrees and +20 degrees relative to the surface normal of side 380A. Near-normal incidence of laser beam 180 onto stack side 320S may optimize the coupling of energy from laser beam 180 to metal foil 120.

[0029] In the method 300, the laser beam 180 traces multiple side paths 310 (see FIG. 3A ), e.g., 3 to 20 side paths 310. The laser beam 180 first traces the side path 310(1) farthest from the metal tab surface 130S. Each subsequent side path 310 is closer to the metal tab surface 130S than any preceding side path 310. The final side path 310 (the fifth side path 310(5) in the example depicted in FIG. 3A ) is closest to the metal tab surface 130S. Each side path 310 may span the entire length 320L of the edge 120E, or a significant fraction thereof. The laser beam 180 may trace each side path 310 in the same direction, e.g., as indicated by the arrows in FIG. 3A , or the laser beam 180 may trace some side paths 310 in opposite directions. In one embodiment, successive lateral paths 310 are traced in opposite directions to form an overall continuous trace having a serpentine shape. As shown in FIGS. 3B and 3C, each tracing of a lateral path 310 by laser beam 180 forms a weld line 320. Each weld line 320 may have a width 320W in the range of 15 to 1,000 μm. After the formation of the first weld line 320(1), each subsequent weld line 320 contacts or overlaps at least the immediately preceding weld line 320. Thus, upon completion of method 300, all edges 120E are laser welded together to form a common initial laser weld joint 330, as shown in FIG. 3D.

[0030] In method 300, laser beam 180 may be operated at a power level that causes only conduction welding and avoids keyhole formation. However, when heated by laser beam 180, metal foil 120 may undergo some amount of shrinkage. If metal foil 120 were fully secured in place, such shrinkage could lead to mechanical stress and cracking of metal foil 120. However, in method 300, the sides of the stack of metal foils 120 are angled and removable clamp 140 is set back a distance 342 (see FIG. 3A) from edge 120E, so that metal foil 120 is not fully secured in place. Metal foil 120 is therefore free to undergo some shrinkage without resulting in cracking. Distance 342 is, for example, in the range of 0.1 to 5 mm.

[0031] In alternative embodiments, laser beam 180 traces paths 310 in a different order, for example, using a reverse tracing order starting at side path 310(5) on the nearest metal tab surface 130S and ending at side path 310(1), or at least partially out of order. Laser beam 180 may even interleave the tracing of different side paths 310.

[0032] 4A-4C illustrate one method 400 for joining a stack of metal foils already interconnected by an initial laser weld joint to a metal substrate. Method 400 is an embodiment of step 230 of method 200 and may be applied to an assembly in which the stack of metal foils is tilted. For example, method 400 may be applied to an assembly that has already been laser welded according to method 300. Without loss of generality, method 400 will be discussed herein in the context of metal foil 120 and metal tab 130 when metal foil 120 is interconnected by initial laser weld joint 330. FIG. 4A illustrates the path traced by the laser beam in method 400. FIGS. 4B and 4C depict, in cross-sectional side and perspective views, respectively, the state of the assembly upon completion of method 400. FIG. 4B further illustrates the propagation direction of laser beam 190 when performing method 400. FIGS. 4A-4C are best viewed together in the following discussion.

[0033] In method 400, laser beam 190 traces a path 440 along the interface between initial laser weld joint 330 and metal tab surface 130S (see FIG. 4A). Path 440 may span the entire length 320L of edge 120E, or a significant percentage thereof. The direction of incidence of laser beam 190 may be near normal to side 320S of stack of metal foils 120. For example, incidence angle 490A of laser beam 190 (relative to the surface normal of side 320S of stack of metal foils 120) may be between −20 degrees and +20 degrees (see FIG. 4B). Incident angle 490A may be similar to incidence angle 380A shown in FIG. 3B. As laser beam 190 traces path 440, laser beam 190 forms a molten pool that solidifies into laser weld joint 450 (see FIGS. 4B and 4C). The laser weld joint 450 electrically and structurally connects the stack of interconnected metal foils 120 to the metal tab 130 .

[0034] Because the stack of metal foils 120 is already interconnected prior to the laser beam 190 performing method 400, the laser beam 190 does not need to weld through the entire stack of metal foils 120. It is sufficient to melt the portion of the initial laser weld joint 330 closest to the metal tab surface 130S (and possibly the immediately adjacent area of ​​the metal foil 120 that is not melted in method 300). The remaining areas of the initial laser weld joint 330 and the stack of metal foils 120 can be left unmelted by method 400. In one example, the melt pool and resulting laser weld joint 450 formed by the laser beam 190 has a width 450W in the range of 0.1 to 2 mm.

[0035] Benefiting from the strength provided by the initial laser weld joint 330, the laser beam 190 may perform a keyhole weld as it traces the path 440 through the laser weld joint 450 to maximize not only the conductivity between the metal foil 120 and the metal tab 130, but also the robustness of the laser weld joint 450. For optimal keyhole formation and optimal quality of the laser weld joint 450, the laser beam 190 may trace the path 440 using a repetitive 2D scanning pattern.

[0036] 5 illustrates one repeating 2D scan pattern 540 that the laser beam 190 may use in the method 400. The repeating 2D scan pattern 540 moves back and forth across the interface between the initial laser weld joint 330 and the metal tab surface 130S in a circular or elliptical manner. To trace the repeating 2D scan pattern 540, the laser beam 190 is directed to loop around a central location while the central location is translated along a linear direction 542.

[0037] As an alternative to the elliptical or circular scanning pattern shown in FIG. 5, the laser beam 190 may trace a sinusoidal or sawtooth pattern that repeatedly intersects the interface between the initial laser weld joint 330 and the metal tab surface 130S.

[0038] 6 illustrates the cross-sectional profile of one composite laser beam 600 useful for performing the laser welding of method 200. Composite laser beam 600 includes a central beam 610 and an annular beam 620. Central beam 610 has a 1 / e wavelength in the range of 15-100 μm. 2 The annular beam 620 may have an outer 1 / e diameter 610D in the range of 100-500 μm. 2 A local minimum in laser power may exist between the central beam 610 and the annular beam 620. The composite laser beam 600 may be a HighLight TM The composite laser beam 600 may be generated by a fiber laser such as an FL-ARM laser, or as discussed in U.S. Patent No. 10,807,190 (issued October 20, 2020, and incorporated herein by reference in its entirety). Alternatively, a single laser beam generated by a standard fiber laser may be manipulated, for example, using refractive and / or diffractive optics, to form the composite laser beam 600.

[0039] In one scenario, the composite laser beam 600 performs both steps 220 and 230 of method 200, e.g., according to methods 300 and 400. In step 220, i.e., method 300, the laser source may be operated with no or insignificant power in the central beam 610 and conduction welding may be performed using only the annular beam 620. Here, the power of the annular beam 620 may be in the range of 50 to 300 watts (continuous wave). The inventors have found that this power level and a scanning speed along the lateral path 310 of 100 to 500 mm / sec are suitable for laser welding 15 μm thick aluminum foil according to method 300. In step 230, i.e., method 400, the central beam 610 and the annular beam 620 have more equal power; for example, the power in each of the central beam 610 and the annular beam 620 may be in the range of 200 to 1,000 watts (continuous wave). The inventors have found that these power levels, along with a linear scan speed of 50-500 mm / sec along direction 542 and a loop speed of 200-800 Hz (i.e., performing 200-800 loops per second), are suitable for laser welding interconnected 15 μm thick aluminum foil to an aluminum substrate according to method 400.

[0040] Without departing from the scope herein, one or both of steps 220 and 230 may utilize another laser beam profile, such as a Gaussian or top-hat laser beam.

[0041] FIG. 7 illustrates a method 700 for interconnecting a slanted stack of metal foils with an initial laser weld joint formed by tracing multiple transverse paths using a laser beam. Method 700 is an embodiment of step 220 and may be applied to a stack of metal foils 120 in an embodiment of assembly 100 where the stack has slanted sides. Method 700 is a modification of method 300, in which laser beam 180 traces multiple transverse paths 710 instead of lateral paths 310. Each transverse path 710 is oriented approximately perpendicular to edge 120E of metal foil 120. Laser beam 180 may trace all of the transverse paths 710 in the same direction, e.g., starting with the metal foil 120 farthest from metal tab surface 130S and tracing in a direction toward metal tab surface 130S. Alternatively, laser beam 180 may trace some of the transverse paths 710 in opposite directions. For example, laser beam 180 may trace transverse path 710 in a continuous serpentine pattern.

[0042] 7, laser beam 180 traces twelve transverse paths 710. Without departing from the scope of this specification, laser beam 180 may trace a different number of transverse paths 710. The weld line formed along each transverse path 710 may overlap with the weld line formed along an adjacent transverse path 710 to combine to form an initial laser weld joint 330, as shown in FIG. 3D.

[0043] In an alternative embodiment, the transverse path 710 is at an oblique angle to the edge 120E.

[0044] Without departing from the scope of this specification, methods 300, 400, and 700 may each be modified to apply to configurations in which the stack of metal foils 120 has no tilt or only a very small tilt. FIG. 8 illustrates such a non-tilted configuration of the stack of metal foils 120, where there is little or no offset between the edges 120E of the metal foils 120. For example, the offset may be in the range of 0 to 20 μm. As a result, the sides of the stack of metal foils 120 are approximately perpendicular to the metal tab surface 130S, corresponding to a tilt angle 320A of 90 degrees, or 80 to 90 degrees.

[0045] The present invention is described above in terms of preferred and alternative embodiments. The present invention, however, is not limited to the embodiments described and depicted herein. Rather, the present invention is limited only by the claims appended hereto.

Claims

1. 1. A method for laser welding a stack of metal foils to a metal substrate, comprising: clamping the metal foil stack between a surface of the metal substrate and a removable clamp such that a side of the stack formed by an edge of the metal foil is positioned on an inner portion of the surface of the metal substrate, the removable clamp being retracted from the side of the stack; interconnecting the metal foil with an initial laser weld joint, the interconnecting step comprising using a laser beam to continuously trace a plurality of lateral paths along the metal foil edge; connecting the interconnected stack of metal foils to the metal substrate by tracing a path along the interface between the initial laser weld joint and the metal substrate surface with a laser beam; A method comprising:

2. The method of claim 1 , wherein each of the lateral paths, when traced, is closer to the metal substrate surface than any preceding one of the lateral paths.

3. The method of claim 1 or claim 2, further comprising the step of displacing the removable clamp from the stack of metal foils after the connecting step.

4. the clamping step includes clamping the metal substrate and the metal foil stack between the removable clamp and a backing plate; the method further includes, after the connecting step, displacing the removable clamp from the backing plate to facilitate removal of the metal substrate and the metal foil stack to be welded together by the interconnecting and connecting steps. The method according to any one of claims 1 to 3.

5. 5. The method of claim 1, wherein in the interconnecting step, each trace of a lateral path produces a weld line, and the weld lines produced by adjacent lateral paths of a pair spatially overlap.

6. 6. The method of claim 1, wherein the connecting step includes melting a portion of the initial laser weld joint closest to the metal substrate surface without melting a portion of the initial laser weld joint furthest from the metal substrate surface.

7. The method of any one of claims 1 to 6, wherein in the connecting step, the laser beam scans a repeating two-dimensional scanning pattern along the interface.

8. 8. The method of claim 7, wherein in the connecting step, the path traced by the laser beam traverses the interface back and forth in a circular or elliptical manner.

9. In the interconnecting step, the laser beam forms the initial laser weld joint by conduction welding; the connecting step includes keyhole welding the interconnected metal foil stack to the metal substrate; The method according to any one of claims 1 to 8.

10. The method of claim 9 , wherein in the interconnecting step, the laser beam is an annular laser beam.

11. The method of claim 9 , wherein in the connecting step, the laser beams include a central laser beam and an annular laser beam.

12. the securing step includes offsetting the foil edges from one another so that the sides of the stack are inclined in a direction toward the removable clamp; In each of the interconnecting step and the connecting step, the laser beam is incident along a direction at an oblique angle to the surface of the metal substrate. The method according to any one of claims 1 to 11.

13. 13. The method of claim 12, wherein each of the metal foils has a thickness of 5 to 30 microns, and the bonding step produces an average offset between edges of each pair of adjacent metal foils, averaged across the stack, in the range of 20 to 200 microns.

14. 14. The method of claim 12 or 13, wherein in the interconnecting step, each of the lateral paths has a length of at least 5 millimeters and the laser beam has a width in the range of 100 to 500 microns.

15. 15. The method of any one of claims 12 to 14, wherein the connecting step comprises forming a melt pool along the interface, the width of the melt pool in a dimension transverse to the interface being in the range of 0.1 to 2 millimeters.

16. A method according to any preceding claim, wherein the thickness of each of the metal foils is between 5 and 30 microns.

17. The method of claim 16 , wherein the stack comprises at least 10 metal foils.

18. 18. The method of claim 16 or claim 17, wherein the metal foil is made from aluminum.

19. 20. The method of claim 18, wherein the metal substrate is made from aluminum or an aluminum alloy.

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