Method of laser welding

Abstract

Laser welding an aluminium tab to a steel terminal of an electrical cell is performed using an oval wobble for the laser beam path 252. The laser wobble has a swing of amplitude A, which is greater than 0.45 mm. The average velocity of the laser spot is at least 60 mm / s; and the frequency of oscillations is 400 to 600 Hz. The aluminium tab may be part of a busbar, in a vehicle battery pack. The oval wobble may be a circular wobble, whereby the length of a first axis is equal to the length of a second axis. Oval oscillations may be inclined relative to the laser weld path.

Description

TECHNICAL FIELD The present invention relates generally to a method of laser welding an aluminium tab to an electrical cell having a steel terminal. Particularly, but not exclusively, the invention relates to welding aluminium tabs to steel terminals of cells to electrically and mechanically connect a busbar assembly to the cells. The cells and connected busbar assembly may form part of a vehicle battery module. Aspects of the invention relate to a method, to a battery module, to a battery pack, and to a vehicle. INTRODUCTION There has recently been increased interest in providing battery-powered vehicles, which has led to developments in vehicle battery, in particular vehicle traction battery, technology. Vehicle battery modules typically comprise a plurality of cells electrically connected to one another to produce the required voltage, energy and current capacity. The connections are made by a busbar assembly, typically via a plurality of “tabs” that each contact a terminal of one or more cells. It is important to ensure that the connections between the cells and the tags are produced very reliably, as there may be hundreds of connections within an individual battery module and several such battery modules within a vehicle battery pack, and it is not acceptable for even one of the connections to be faulty. It is also desirable to produce the welds as quickly as possible. Copper has typically been used for busbar tabs, as it provides good conductivity. However, copper is relatively expensive and heavy, so it would be desirable to use an alternative material such as aluminium. However, it has hitherto not been possible to achieve adequate welds with materials that would provide a cost and / or weight improvement over copper. It is an object of the present invention to at least partially mitigate one or more of the above problems. SUMMARY OF THE INVENTION According to an aspect of the invention for which protection is sought, there is provided a method of laser welding an aluminium tab to an electrical cell having a steel terminal, the method comprising: placing a first surface of the tab in contact with the terminal of the cell; and welding the tab to the terminal by directing a laser beam generated by the laser welding system onto a second surface of the aluminium tab to produce a weld, the second surface being opposite the first surface, wherein: the weld has a predetermined weld path; the laser welding system is configured to control the beam to produce an oval wobble about the weld path, the oval wobble having an amplitude parallel to the weld path of greater than 0.45mm, preferably 0.5mm or more, more preferably 0.55mm; the average velocity of the laser spot on the tab is at least 60mm / s preferably 65mm / sec, more preferably 70mm / sec or more; and the frequency of the oscillations is 400-600Hz. In some embodiments, the oval wobble may have an amplitude perpendicular to the weld path greater than 0.45mm, preferably 0.5mm or greater, and more preferably 0.55mm. It will be understood that in some embodiments the weld path may not be straight, and in such embodiments the “average velocity” may not correspond to an average velocity calculated in a cartesian coordinate system. Instead, the average velocity is given by the total length of the weld path, divided by the time taken for the laser spot to travel along the weld path to produce the weld. Advantageously, this provides welds with improved mechanical properties as compared to prior art welds joining aluminium to steel. Furthermore, welding in an “aluminium-on-top” configuration allows the method to be used for electrically and mechanically connecting tabs of a busbar component to the terminals of the cells. The present inventors have experimentally shown that welds produced as described above provide an unexpectedly advantageous combination of mechanical properties. In an embodiment, the wobble amplitude parallel and / or perpendicular to the weld path may be 0.6-1 mm. Optionally, the average velocity of the laser spot on the tab may be 75-120mm / s. Furthermore, in some embodiments the average velocity of the laser spot on the tab is at least 80mm / s, preferably at least 90mm / s and / or the oscillations have an amplitude of at least 0.65mm, preferably at least 0.7mm. Advantageously, these welding parameters have been found to provide welds with particularly good mechanical properties, especially pull and peel strength. In some embodiments, the amplitude A (measured in mm) and the velocity v (measured in mm / s) may satisfy one or more of the following equations: A + (v / 107) > 1.4 A + (v / 107) >1.378 A + (v / 107) > 1.3 A + (v / 107) > 1 Any one of the above equations may be satisfied in addition to or instead of the requirement that the velocity is 60mm / sec or more and the amplitude is 0.45mm or more. In an embodiment, the laser power may be 300-500W, preferably around 400W. The laser power density in the spot may be 5.5-7.5 x 107 W / cm2, preferably around 6.5 x 107 W / cm2. In an embodiment a shielding gas may be provided in the vicinity of the tab and the terminal whilst the weld is being produced. The shielding gas may be argon of high purity, for example 99.99% purity. However, it will be understood that other shielding gases may also be useful. In an embodiment, wherein the oval wobble comprises oscillations having a first component in a direction parallel to the weld path and a second component in a direction normal to the weld path, wherein the component in the direction parallel to the weld path defines a length of the oval along a first axis thereof, and wherein the component in the direction normal to the weld path defines a length of the oval along a second axis thereof. One of the first and second axes may be a major axis and the other may be a minor axis. Alternatively, neither of the first and second axes may be the major axis of the oval, and the major axis of the oval may be inclined relative to the weld path. In such embodiments, the ratio length of the major axis to the minor axis may be between 4:1 and 1:1, preferably between 2:1 and 1:1. Optionally the length of the length of the second axis is 25-400% of the length of the first axis, preferably wherein the length of the second axis is 50-200% of the length of the first axis. That is to say, the ratio of the length of the first axis to the length of the second axis may be between 1:4 and 4:1, preferably between 1:2 and 2:1. In an embodiment, the oval wobble is a circular wobble, and the length of the first axis is equal to the length of the second axis. It will be understood that throughout the present specification, the described shape of the “wobble” refers to the shape of the movement of the beam as a result of the oscillations, and excluding the component of the motion that results in an overall displacement along the weld path. Accordingly, given an actual route followed by a beam, the shape of the oscillations can be deduced by determining the average velocity along the weld path, and subtracting the motion due do the movement of the beam along the weld path at the average velocity. In an embodiment, the cell is a cylindrical cell. The present invention may be particularly advantageously applied when joining aluminium busbar tabs to cylindrical cells, as such cells often have a steel casing. Furthermore, welds produced according to the present invention typically do not penetrate through the steel, which helps to ensure the jelly roll within a cylindrical cell is not damaged by the welding process. In an embodiment, the laser is a single mode laser. In an embodiment, the laser is a fiber laser. In an embodiment the laser is a Ytterbium laser. In an embodiment the laser power is ramped down at end of weld. In an embodiment the laser power during welding is 300-500W, preferably around 400W. It will be understood that laser welding systems typically allow the output power to be selected, and the above range refers to the laser power output from the laser, as opposed to the power consumed by the laser. In an embodiment the laser spot diameter on the aluminium tab is 25-35 micrometres. The laser power density in the spot may be 5.5-7.5 x 107 W / cm2, preferably around 6.5 x 107 W / cm2. In an embodiment the steel terminal comprises a nickel plating. In an embodiment the steel terminal has a thickness of 0.2-0.5mm. This may be a typical thickness for commercially available cylindrical cells, especially in sizes that are typically used in automotive traction batteries, such as 21700 cells. In an embodiment the aluminium tab has a thickness of 0.2-0.4mm. This may provide a cost and weight saving as compared to prior art copper tabs, without increasing the electrical resistance to any significant extent. In an embodiment the weld path has a length of 5-20mm. Preferably the weld path has a length of around 15mm. In an embodiment the weld path is circular, optionally wherein the circular weld path has a diameter of 2-4mm. In an embodiment the weld path is discorectangle shaped, optionally wherein the discorectangle shape has a perimeter length of 6-15mm. It will be understood that the term “stadium shaped” is equivalent term to discorectangle. In an embodiment, the cell comprises a first end surface, wherein a first terminal of the cell is located in a central region of the first end surface and at least part of a second terminal of the cell is located in a peripheral region of the first end surface, and the step of welding the tab to the terminal comprises welding the tab to the central region of the first end surface. In an embodiment the method further comprises welding a second tab to the peripheral region of the first end surface. Advantageously, this allows a single-sided busbar assembly to be used. In an embodiment the second tab is welded to the peripheral regions of the first end surfaces of two adjacent cells. According to another aspect of the invention, there is provided a method of mechanically and electrically connecting a busbar component to a plurality of cells, wherein the busbar assembly comprises a plurality of aluminium tabs, the method comprising welding each of the aluminium tabs to one or more of the cells, according to a method as described above. According to another aspect of the invention, there is provided a battery module comprising a busbar component mechanically and electrically connected to a plurality of cylindrical cells according to a method as described above. According to another aspect of the invention, there is provided a battery pack comprising a plurality of battery modules as described above. According to another aspect of the invention, there is provided a vehicle comprising a battery module or a battery pack as described above. It will be understood that the wobble amplitude and frequency defines the wobble speed. Accordingly, the present invention may alternatively be defined in terms of wobble speed. The wobble speed may be at least 900mm / s, preferably at least 1000mm / s; the wobble speed may be less than 1300mm / s. Within the scope of this application, it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and / or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and / or features of any embodiment can be combined in any way and / or combination, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and / or incorporate any feature of any other claim although not originally claimed in that manner. BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of the invention will now be described by way of example only, with reference to the accompanying figures, in which: Figures 1A-C show different views of a cylindrical cell that may be used in a vehicle battery module (PRIOR ART); Figure 2A shows a laser welding system (PRIOR ART); Figure 2B shows a schematic illustration of the motion of a laser spot with circular wobble; Figure 3A shows tabs welded to respective cell terminals in a first configuration; Figure 3B shows tabs welded to cell terminals in another configuration Figures 4A and 4B show schematic representations of the welding arrangement; Figure 5A is an illustration of the test welds used in the pull strength and fatigue tests; Figure 5B is an illustration of the test welds used in the T-peel tests; Figure 6 shows representative images of the microstructure observed for each weld condition; Figures 7A-C shows a schematic representation of the expected weld cross section for various welding conditions; Figure 8 shows the results of the pull and T-peel strength tests; Figure 9 shows a contour plot illustrating the relationship between laser spot velocity, oscillation frequency and peel strength; Figures 10A and 10B show SEM images of weld cross section from different experimental conditions; Figures 11A and B show SEM-EDS analyses of two different locations on a single weld condition; Figure 12 shows a longitudinal section of an end region of a weld produced according to a weld condition; Figure 13 shows a longitudinal section of an centre region of a weld produced according to the same condition as figure 12; Figures 14A and C show microhardness profiles of welds produced according to different experimental conditions; Figures 14B and D show Vickers hardness indents for welds produced according to different experimental conditions; Figure 15 shows the results of fatigue testing for a particular experimental condition; Figure 16 shows a flow chart illustrating a method in an embodiment of the present invention; Figure 17 shows a vehicle in an embodiment of the present invention. DETAILED DESCRIPTION Figures 1A-C show different views of a conventional cylindrical cell 100. Cylindrical cells 100 are widely available in a variety of different sizes. For example, in traction batteries for vehicles cells having a diameter D of 21mm and a length L of 70mm are often used. Such cells are typically referred to as 21700 cells (the first two numbers referring to the diameter D, in mm, and the last three numbers referring to the length L, in tenths of mm). However, it will be understood that other sizes of cell may also be used in embodiments of the present invention. As will be well understood by the skilled person, the cell 100 comprises a positive terminal 100P, a negative terminal 100N, and vent means 100V. The positive terminal is provided by a steel end cap 106 in a central region of the first end 104 of the cell, and the negative terminal is provided by a steel cylindrical case 108. The steel cylindrical case 108 covers the second end 102, the entire cylindrical surface between the first and second ends, and a peripheral region 100S of the first end surface. The peripheral region of the first end surface may also be referred to as a “shoulder” region 100S of the first end surface 104. In commercially available cells, it is sometimes the case of the end cap that defines the positive terminal 100P on the first end surface 104 and protrudes beyond the shoulder region of the first end surface, although this is not the case in the cell shown in figure 1. This allows a substantially planar connector to be connected to the positive terminal and not the negative terminal. As will be well understood by the skilled person, it is important to avoid direct electrical connections between the positive and negative terminals, as such connections create a short circuit which may damage the cell. As shown in figure 1, the cell 100 comprises three vent means 100V in the first end surface 104, between the steel end cap 106 that defines the positive terminal 100P and the shoulder region 100S of the steel cylindrical case 108. The vent means 100V are gaps that are covered by a material that will rupture to allow hot gases to escape through the gaps between the end cap 106 and steel cylindrical case 108 in the event of excessive pressure occurring inside the cell, thereby mitigate against the risk of the cell exploding. Cells 100 may be grouped together within a housing and electrically connected together by a busbar assembly to create a battery module. Furthermore, as will become apparent from the following description, in some embodiments a plurality of cells 100 may be mechanically joined together to form a group of cells, and a battery module may comprise one or more of such groups of cells. Figure 2A shows a laser welding system (not shown), which produces a laser beam 202. Beam 202 passes through a diverging lens 204, and a converging lens 206, before being deflected towards a target location in the welding plane 212 by first and second mirrors 208, 210. Each of the mirrors 208, 210 is rotatable about a single axis under the control of a respective galvanometer 214, 216. The axes about which the mirrors 208, 210 are rotated about are mutually perpendicular, so that the first mirror 208 controls the target location of the beam in the X direction and the second mirror 210 controls the target location of the beam in the Y direction. The lens 204 is movable along an axis parallel to the initial direction of the beam 202, which allows the position of the focal point of the laser, and therefore the welding plane, to be adjusted in the Z direction. It will be understood that the laser may be focused to provide a “spot” in the welding plane of a predetermined size. Furthermore, it will be understood that in some laser welding systems the lens 204 may be fixed, so that the focus position of the welding laser is not adjustable in the Z direction. Such systems are referred to as two-dimensional, as they are only operable to direct the beam in the X and Y directions. A controller (not shown) is operable to control the power of the laser and to selectively turn the laser on or off. The controller is also operable to adjust the focal position by moving the lens 204, and to adjust the target position of the laser by controlling the angular positions of the mirrors first and second mirrors 208, 210, via the respective galvanometers 214, 216. Accordingly, an operator may program the controller to make a predetermined set of welds by actuating the laser only when it is directed at selected target regions. As will be well understood by the skilled person, laser welding systems such as the one illustrated in figure 2A are able to produce welds very rapidly and with fine control over the weld power and shape. They are therefore particularly useful in situations in which several components in close proximity must be rapidly welded together. It will be understood that the lenses 204, 206 and the mirrors 208, 210 and associated galvanometers 214, 216 may all be contained within a common housing, which may have a suitable openings to receive the incoming beam and allow the focussed and directed beam to leave the housing. The arrangement of lenses and galvanometer-controlled mirrors shown in figure 2A may be referred to as a three-dimensional scanning head, because it is operable to move the focus position of the beam in three dimensions. The present invention particularly relates to the application of laser welding systems to welding aluminium to steel, in particular in situations in which the aluminium is positioned on top of the steel (i.e. the aluminium is closer to the scanning head than the steel). Such welding operations are of significant interest, as they have the potential to reduce weight and cost as compared to situations in which copper is welded to steel. However, prior art methods have not been able to produce adequate strength for many applications. The present inventors have developed a method of laser welding aluminium to steel that is able to reliably produce welds of sufficient strength for use in critical applications such as welding busbar tabs to cell terminals. A significant feature of the method of the present invention is the use oscillation of the laser spot about a weld path and specific ranges of other parameters. Figure 2B shows a schematic illustration of the motion of the laser spot along the surface of the workpiece. Figure 2B shows a route 250 that the beam spot follows. The route 250 proceeds along the weld path 252 in the direction illustrated by arrow 252A, but with a superimposed circular “wobble” along direction Wo. The wobble has an amplitude A, which is equal to the diameter of the circles. The laser welding system is programmed to control the beam spot to move with a velocity v along the weld path 252, and with superimposed oscillations having an amplitude A, and a frequency f. It will be understood that the speed of the beam due to the wobble depends on the amplitude and frequency of the oscillations. The overall velocity of the beam at any given time is given by superimposing the velocity due to the wobble and the velocity v along the weld path 252. Depending on the velocity along the weld path 252, the amplitude A of the wobble and the frequency f of the wobble there will be a degree of overlap between successive circular wobbles as indicated by O in Figure 2B. Figure 3A shows two aluminium tabs 302 that are each welded to the positive terminals 100P on the first ends 104 of respective cells 100, via welds 304. The tabs 302 may be electrically connected to a component of a busbar assembly. Furthermore, other tabs that are connected to a different component of the busbar assembly may be welded to the negative terminal 100N of the cell 100. The tabs that are connected to the negative terminal may be located at any suitable location on the negative terminal, although they would typically be located either on the second end surface 102 or the shoulder region 100S of the cell. It will be understood that, although the welds in the experiments discussed below are performed on test specimens rather than actual cells, the properties of the steel are very similar to those that would be used for the casings of typical electrical cells, and the aluminium is of a composition that could readily be used for producing tabs for connection to a busbar assembly as shown in figure 3. Indeed, the present inventors have used the weld parameters discussed below to join aluminium tabs to commercial 21700 cells. The electrical resistance of the welds was found to be within acceptable limits and the electrical performance of the cells was not degraded by the welds. Accordingly, the welds 304, as well as any other welds connecting the negative terminals to a busbar component, may be produced using weld parameters corresponding to any of the experimental conditions described herein. Although the welds 304 are straight lines in the illustrated embodiment, it will be understood that any suitable shape may be used in embodiments of the present invention. Figure 3B shows another arrangement of aluminium tabs 308, 310 that can be used in embodiments of the present invention to electrically connect a plurality of cylindrical electrical cells 100 to a single-sided busbar assembly. The welds to the negative terminal are located in the shoulder region 100S, and the welds to the positive tab 100P are in a central region of the first end. The welds 312 connecting the negative tab 310 to the shoulder regions 100S of the cells each have the shape of a single, continuous, curved line. As can be seen in figure 3B, a single negative tab 310 is connected to the shoulder region of two adjacent cells. In the embodiment shown in figure 3B, the welds 314 connecting the positive tabs 308 to the positive terminals are discorectangle shaped and as such have two straight regions. Advantageously, the discorectangle shaped welds may provide improved mechanical strength, and welding a single tab 310 to the terminals of two adjacent cells reduces the overall number of tabs required to connect a given group of cells to a busbar component. Figures 4A and 4B show schematic representations of the welding arrangement used in the pull strength and fatigue strength experiments described herein. Figure 4A shows a side view of the configuration of the samples and the weld, and figure 4B shows the samples and weld from above. As can be seen in figures 4A and 4B, a sheet of aluminium 402 is placed on top of a sheet of steel 404 in an overlapping configuration. Each of the aluminium and steel specimens are sheets having a length of 75mm, a width of 25mm and a thickness of 0.3mm. The aluminium and steel specimens are placed in contact with one another with a 25mm long section of the aluminium in contact with the steel. The sections that of the specimens that contact each other are in contact along the entire width of the 25mm long contact portion. Each of the specimens has a 0.3mm thick shim 408 connected to the surface of the specimen that will be placed in contact with the other specimen. As best seen in figure 4A, the shims cover a 25mm by 25mm area at the opposite end of the surface to the area that will be placed in contact with the other specimen. These shims allow the specimens to be gripped by a testing system whilst maintaining the orientation of the specimen parallel to the direction of the force applied by the testing system. In each of the experiments, a 15mm long straight weld 406 is made at the centre of the overlap region. The weld is made in a direction perpendicular to the long axes of the specimens. As discussed above, the aluminium is placed on top of the steel, so that the beam makes contact with the aluminium before the steel. The aluminium is alloy 1070. The chemical compositions of the aluminium samples is shown in table 1, below: Element Fe Si V Zn Cu Ti Mg Mn Al Minimum concentration (wt %) 0.25 0.2 0.05 0.04 0.04 0.03 0.03 0.03 >99.7 Table 1: Chemical composition of aluminium alloy 1070 samples Although the experiments described herein have been performed using aluminium alloy 1070, it will be understood that other aluminium alloys, and especially other 1000 series aluminium alloys, would also be suitable as thermo-physical properties are very similar between different 1000 series aluminium alloys. For example, aluminium alloy 1050 is commonly used for tabs to connect cells to busbar components, and the weld parameters discussed herein would be equally suitable for use in joining tabs made from 1050 aluminium to the terminals of steel cells. The steel is a commercially available formulation with the trade name Hilumium, which is available from Tata Steel. Hilumium is an electrolytically nickel-plated, diffusion-annealed cold rolled steel strip of 0.3mm thickness, interstitial free with very low carbon. This steel has high formability to allow the cell case to be deep drawn. The actual chemical composition of the Hilumium is not publicly available but the composition of a typical Interstitial-free steel, DC 06, is given in table 2, below: Element C P S Mn Ti or Ni Fe Minimum concentration (wt %) 0.08 0.02 0.02 0.25 0.3 balance Table 2: Chemical composition of steel DC 06 The above-described aluminium samples are considered to be representative of a desirable material for use as busbar tabs for connecting cell terminals to busbar components, as busbar tabs made from this material would be significantly lighter and cheaper than equivalent copper components. The steel Hilumium was selected because it is used for the casings of many commercially-available electrical cells, and is it considered to be representative of the steels that are typically used in cell casings. EXPERIMENTAL RESULTS The present inventors have performed experiments to determine the pull strength, fatigue strength and peel strength of welds produced on samples illustrated in figures 4A and 4B, with various different laser welding settings. Figure 5A shows the loading configuration used in the pull strength and fatigue strength tests. Although the shims 408 are not shown in figure 5A, it will be understood that they are used to load the samples into the testing machine, and axial forces are applied to the samples in the directions indicated by the arrows. In the case of the pull strength tests, the maximum force applied before failure was recorded. For the fatigue tests a cyclic load varying between a maximum and minimum load was applied, with the ratio between the maximum and minimum loads set at 10:1. The number of cycles to failure was recorded. Figure 5B shows the configuration used for the peel strength tests. The samples 502, 504 used in the peel strength tests are similar to those used in the fatigue and pull strength tests, except that they have a 90 degree bend 25mm from the end, and the welds 506 are provided halfway between the bend and the nearest end, as shown in figure 5B. The present inventors have tested both configurations extensively, using various different welding parameters, as set out in table 3, below: Welding velocity (mm / s) Wobble amplitude (mm) Wobble velocity (mm / s) A 50 0.4 628 B 50 0.6 942 C 50 0.8 1256 D 75 0.4 628 E 75 0.6 942 F 75 0.8 1256 G 100 0.4 628 H 100 0.6 942 1 100 0.8 1256 Table 3: Welding parameters for the experimental conditions For all conditions, the laser power was maintained ay 390W, the focal position was on the upper surface of the aluminium specimen, the wobble frequency was 500Hz, and the laser power density on the upper surface of the aluminium specimen was 6.37x107 W / cm2. 99.99% pure argon was used as a shielding gas in all conditions, and was supplied coaxially at a flow rate of 10 litres / min. In all conditions, good contact welds were achieved without excessive spattering. The welded specimens were cross-sectioned for Metallographic analysis. Since the crosssection of wobble welds show considerable variation along the weld length, at least three cross-sections were analysed for every parameter setting. Also, welds were sectioned in the longitudinal direction along the weld line. The samples were fine-polished to 1 pm. The weld samples were analysed under the optical microscope for defects, geometry, and microstructures. Sigma FE-SEM from Zeiss was used for microstructural investigation. SEM-EDS (Scanning electron microscopy - Energy dispersive spectral analysis) was performed on selective samples to examine IMC (intermetallic compound) phases in welds. Micro-hardness testing was conducted to examine the hardness of different phases formed during weld solidification. Micro-hardness profiles were taken on the cross-sections with a load of 50gf (gram force) in uniform intervals. The indentation time was maintained as 10 seconds. Figure 6 shows representative optical microscope images of the weld cross-sections corresponding to the parameter settings A to I. The weld cross-section shows the section across the circular wobble-region where the laser beam has interacted multiple times based on v and wobble parameters, A and wobble frequency. The weld pool is formed by repetitive melting and re-solidification as the circular wobble covers the region. The balance between this complex laser melting and wobble parameter dictates the geometry of the weld. Although power density is constant for all the welds, the variations in wobble amplitude, A, and welding velocity, v, have a significant influence on weld geometry. As A is varied from 0.4mm to 0.8mm, the weld depth is significantly decreased. As A is amplified, the wobble controller increases the wobble velocity correspondingly as shown in table 3. Hence the laser beam interactiontime with the metal surface is reduced significantly. In micro-welding of thin sheets, this variation is sufficient to cause a considerable difference in depth of penetration. As v increases, both the weld depth and we Id-interface size are reduced. At high speed, the interaction of laser photons with the material surface is considerably reduced resulting in lower energy absorption. At low v, the laser beam interacts on the material surface for longer and the overall laser energy absorption is improved, resulting in large size welds. As seen in the cross-sectional images, the variation in weld shape is observed with a change in parameters. In a circular wobbling pattern, the weld cross-section shows the overlapped weld segments as shown in the schematic in figures 7A-C. In the ideal condition, two distinct weld segments can be seen, as shown in figure 7A. However, in reality, the weld segments get mixed either in the top sheet, as shown in figure 7B, or completely as shown in figure 7C. For dissimilar metal welding, the weld shape shown in figure 7B is preferred to that shown in figure 7C, as less mixing is favourable to avoid the formation of harmful intermetallic compounds (IMC). Returning to the images shown in figure 6, welds made with parameters F to I (high v and wider A) show less mixing of Fe (iron) and Al (aluminium). Welds of parameters A to E show complete mixing of Fe and Al corresponds to the pattern shown in figure 7C. These welds also show cracks. The crack severity is not consistent along the weld seam and cracks are observed in various locations along the weld line. The crack severity reduces as A and v increase. Also, crack severity is directly related to the mixing of Fe and Al. Welds of parameters A to E show cracks, but the rest of the welds are fairly clean, free from cracks. The formation of cracks is due to the thermal stress during weld-solidification. The cracks are mostly aligned in the solidification direction. Since the level of mixing and cracking are related, it can be inferred that cracking severity is related to the volume of IMC. Weld microstructure with a high volume of IMC phases undergoes severe thermal stresses due to huge variation in coefficient of thermal expansion. Welds with low IMC phases can endure the thermal stress during solidification and form a crack-free weld microstructure. Moreover, partial penetration of welds is favoured in cell interconnect joints due to the heat sensitivity of Lithium active materials in the cell which can be thermally abused or damaged affecting the electrochemical performance of the cell. Considering all these factors, welds of parameter settings F, H, and I are considered the most suitable aluminium tab to steel cell laser welding. The mechanical strength of welds tested in terms of Tensile-shear and T-Peel loading conditions are given in figure 8. The mean pull strength ranges from 150 N to 350 N. Parameters C, E, F, H, and I show noticeably higher pull strength than other parameters. From the pull strength data, it can be observed that the pull strength increases with the increase in wobble amplitude, A, and traverse velocity, v. As A increases, the weld interface area increases but as v increases weld depth decreases. The wider weld interface area enhances the load-carrying capacity of the weld. However, this trend is not consistent and the data show huge scatter which may be attributed to the complex nature of laser-material interaction and consequent weld microstructure. T-Peel results show much lower mechanical strength and exhibit significant variation. From the T-peel strength data, the parameters can be grouped into 2 categories; Group I relates to the parameters A to E, and group II corresponds to ‘F’ to T. The Group II welds show considerably higher Peel strength than those of group I. The average peel strength of group I welds is around 25 N compares to 100 N or above for group II welds. Figure 8 shows the trend lines for both pull and peel test data. Peel strength increases linearly with parameter settings from ‘A’ to T and the R2 value of the linear trend line is 0.706 which implies the direct relationship with process parameters. Group II welds show a substantial increase in peel strength which indicates the influence of other factors in addition to weld geometry. Some of the welds in Group I (Parameters B, C, and E) have wider weldinterface area but demonstrate low Peel strength. Weld defects such as cracks and porosity are observed in group I welds which can diminish weld strength significantly. However, the Pull test data have not been severely influenced by these weld defects. Therefore, the T-peel strength can be attributed to weld microstructure, the formation of new phases during weld solidification and weld defects. The contour plot in Figure 9 shows the relationship between wobble amplitude A (shown on the vertical axis) and welding velocity v (shown on the horizontal axis) with peel strength. The region 902 shows a peel strength of less than 40N, the region 904 shows a peel strength of 40-80N, and the region 906 shows a peel strength of more than 80N. From weld geometry and mechanical strength viewpoints as discussed above, the parameter settings which produce welds of partial penetration and low mixing of Fe and Al are applicable. Weld microstructures of full and partial penetration welds were investigated with SEM. Figures 10A and B show SEM- backscattered electron images of weld cross-sections. Figure 10A shows a weld made according to parameter B which represents the full depth of penetration. Figure 10B shows a weld made according to parameter H, which illustrates partial penetration. The images clearly show the weld boundaries and mixing of Al and Fe without chemicaletching. The flow lines indicate the complex molten metal flow inside the weld pool due to the repeated heating and solidification. From the geometry of welds-B and H, It is evident that the formation of welds occurs at a high solidification rate. Fe and Al have significant differences in thermophysical properties, such as melting point and coefficient of thermal expansion. Joining dissimilar metals by fusion welding may lead to differential heating and asymmetric weld pool development. However, in this study, welds are of uniform shape despite the large difference in melting point between Al and Fe due to the exceptional, rapid heating and cooling rate. However, the major difference in the welds B and H is the mixing region; weld H has a relatively minor mixing of Fe in the Al side. On the other side, weld B shows almost complete mixing of Fe on Al and vice versa. In SEM-BSE images shown in figures 10A and B, Fe and Al can be distinguished with image contrast; Al is dark, and Fe is bright. Similarly, the phases present in the weld pool can also be distinguished- Fe-rich phases are much brighter than that of Al-rich phases. Yet this demarcation does not differentiate the IMG phases precisely and needs to be investigated with SEM-EDX for chemical composition. Figure 11 shows the chemical composition analysis of the weld produced with parameter H. Figure 11A shows the Fe and Al mix zone and 11B illustrates the weld root. Fe-AI mix zone exhibits Al-rich phases Fe2Als, Fe4Ali3 and the presence of small amounts of Iron and Iron-rich phase FeAI. The major portion is covered by continuous Fe2Als and needle-like Fe4Ali3. The presence of Fe particle on the Al side shows that the Fe atoms vaporised in the process of keyhole dynamics and solidified in the surrounding of Al. Due to the narrow homogeneity range and close compositional difference FeAh phase is not clearly distinguished from Fe2Als. At the root side, Fe-rich phases FeAl and Fe3AI are seen as shown in Figure 11B. Al-rich composition is also observed in the root of the weld along the flow lines which indicate the solidification patterns. Fe-rich phases are preferred microstructures to Al-rich due to the less complex crystal structure and brittleness. The welds undergo the highest cooling rate at the root side. This can also facilitate the formation of favourable Fe-rich IMC phases at the weld root. Weld B shows similar characteristics to weld H in terms of Fe migration towards the Al side, although the migration is greater in weld B. However, in weld B the weld pool is completely mixed and the IMC phases are observed in the entire weld pool. In weld B, Al-rich phases are found in the Al side and most of the Fe side. Unlike weld H, the weld root is with very little Fe-rich IMC and high Al-rich IMC (figure 11B). This observation is in line with previous research works on laser welding of Steel and Aluminium alloys. Most of the studies in steel on top configuration observed significant Al-rich microstructure but little Fe-rich phase. The formation of Al-rich phases is favoured to Fe-rich thermodynamically as the AG of Al-rich phases is much lower than that of Fe-rich phases as shown in Table 4 (below). However, the present inventors have discovered that the mixing of Fe and Al and the formation of IMC phases seems to be heavily influenced by the spatial distribution of laser energy using beam wobbling. In welds of high process velocity and larger wobble amplitude (parameters F to I), low mixing of Fe and Al is observed as laser energy is distributed in proper intervals leading to optimum heat and molten flow conditions. Whereas welds of low velocity and small wobble amplitude (parameters A to E) show severe mixing of Fe and Al and consequent IMC formation throughout the weld pool. In the latter case, the laser energy is concentrated in a narrow region which leads to deeper weld penetration and complete mixing. Phases Al (At %) Hardness (HV) Crystal Structure AG (KJ / mol) Fe2AI5 70-73 1000-1100 Orthorhombic -19.64 Fe4Ali3 / FeAh 74.5-76.6 820-980 BC monoclinic -22.87 FeAI2 66-66.9 1000-1050 Triclinic -17.0 FeAl 23-55 400-520 Simple cubic (B2 type) -11.09 Fe3AI 23-34 250-350 FCC -4.83 Table 4: Fe-AI IMC properties Longitudinal sections of welds were analysed to understand the influence of the spatial distribution of laser energy. Figure 12 shows the longitudinal section of weld H, taken at the weld-end region. At the end of the weld which is shown on the right side of the image, the laser energy is absorbed for a longer time due to the time lagging for the worktable to come to a complete stop. This transient condition leads to deep weld penetration, high concentration of IMC phases, and cracking at the weld end region. This can be rectified by laser power ramping-down at the weld end. However here it is shown to demonstrate the shift in cracking and formation of IMC phases for a minor change in laser energy distribution. The left side of the image which represents the steady heat transfer conditions shows low IMC phases and crack-free microstructure. Figure 13 shows the centre region in the longitudinal section representing the characteristic of welds of parameters F to I. The weld interface shows intermittent Fe and Al mix zone, and a major area is free from IMC. These welds are able to conduct electrical current without overheating and energy loss. The Inter-wobble region where the laser is not directly interacted on the material also shows fairly good contact as a brazelike structure with a thin IMC layer which can facilities a good electrical contact. The microstructure of various intermetallic phases can be attributed to the Micro-hardness profiles. Figure 14A and B shows the micro-hardness profile and the macrograph of the weld cross-sections after Micro-hardness tests with the hardness indents for weld B, and figures 14C and D show the same results for weld condition H. The data confirm the findings from weld microstructures. Weld B shows predominantly high hardness regions correspond to Al-rich phases (Figure 14 A, and B). Pure aluminium shows the hardness of about 40 HV0.025 whereas the Al-rich phase shows the highest hardness, close to 1000 HV0.025 corresponding to Fe2Als and 600 to 850HV related to Fe4Ali3. As shown in Table 4, the FeAh hardness is close to Fe2Als, so this phase cannot be distinguished by this approach. At the root side and near HAZ, some Fe-rich phases are observed which shows hardness 250 to 500 HV0.025. In the weld H, (Figure 14- C, and D) a small IMC-high hardness area and a large pure Al region are observed. The root side of the weld shows hardness values of 200-300 HV0.025 resembles FesAI. The hardness profiles appear similar to the solidification pattern lines seen in the SEM images in Figures 10-13 which implies that weld microstructures are influenced by the weld pool flow behaviour. The optimum welds are achieved with parameters F, H, and I as these welds are of partial penetration depth and offer high mechanical strength despite the presence of IMC phases. However, condition E also appears to produce acceptable results. The Li-ion cell interconnect joints undergo mechanical vibrations in vehicle operation and need to be tested for their dynamic mechanical resistance. Fatigue tests were performed on the welds of parameter, H and the result is shown in figure 15. The maximum load was chosen from the Pull test data shown in figure 8. The maximum load is usually selected to be lower than that of the yield point, but here loads much higher than the yield point were chosen. The welds show good performance even at high loading conditions. At the maximum load of 175N, the welds can withstand 1 million cycles without failure. In summary, the present inventors have developed a method for joining aluminium to steel in an “aluminium on top” configuration that could be used in electric battery assembly. The laser energy was controlled precisely by parameters-wobble amplitude and traverse velocity. Welds of high traverse velocity and wider wobble amplitude offer partial penetration welds with low intermetallic phases. Full penetration welds show high intermetallic formation, cracking, and low strength. Laser beam wobbling is found to be controlling the spatial distribution of laser energy along the weld line and thus form optimum thermal and molten flow conditions. The fatigue strength of the optimum parameter welds was found to be over 1 million load cycles at 175N, cyclic load. This joining solution of Aluminium tabs with Ni-coated steel can be useful to replace the costly copper tab welding in EV battery applications. Very good quality welds were observed with 0.6-0.8mm wobble amplitude and 75-100 mm / sec traverse velocity for the selected laser parametric conditions. However, based on the experimental results it is expected that at least acceptable welds could be formed with an average velocity of 60mm / sec or more, preferably 65mm / sec, more preferably 70mm / sec, and an amplitude of 0.45mm or more, preferably 0.5mm or more, more preferably 0.55mm or more. The oscillation frequency may be between 400-600Hz. Although circular oscillations are preferred, it will be understood that a circular wobble is merely a special case of an oval wobble in which the minor and major axes have equal lengths, and any oval wobbles would be expected to produce similar results to circular wobbles. Should a non-circular oval wobble be used, the amplitude of the wobble may be considered to be the length measured parallel to the direction of the weld. Furthermore, the length perpendicular to the weld direction may be 25-400%, preferably 50-200%, more preferably 75-133% of the length parallel to the weld direction. It will be understood that other ranges of the above parameters may also be useful. For example, in addition to the velocity being 60mm / sec or more and the amplitude being 0.45mm or more, one or both of the following criteria may be met: the velocity is at least 80mm / s, preferably at least 85mm / s: or the oscillation amplitude is at least 0.65mm, preferably at least 0.7mm. In some embodiments, the amplitude A (measured in mm) and the velocity v (measured in mm / s) may satisfy one or more of the following equations: A + (v / 107) > 1.4 A + (v / 107) >1.378 A + (v / 107) > 1.3 A + (v / 107) > 1 Any one of the above equations may be satisfied in addition to or instead of the requirement that the velocity is 60mm / sec or more and the amplitude is 0.45mm or more. Figure 16 shows a flow chart illustrating a method 1600 of laser welding an aluminium tab to an electrical cell having a steel terminal in an embodiment of the present invention. The method begins at step 1602, in which a first surface of the tab is placed in contact with the terminal of the cell. The method then proceeds to step 1604, in which the tab is welded to the terminal by directing a laser beam generated by the laser welding system onto a second surface of the aluminium tab to produce a weld, the second surface being opposite the first surface. In the method shown in figure 16, the weld has a predetermined weld path, the laser welding system is configured to control the beam to produce an oval wobble about the weld path, the oval wobble having an amplitude parallel to the weld path of greater than 0.45mm, the average velocity of the laser spot on the tab is at least 60mm / s, and the frequency of the oscillations is 400-600Hz. Figure 17 shows a vehicle 1700 incorporating a battery module 1720 according to an embodiment of the present invention or a battery pack 1710 comprising two or more battery modules 1720 according to one or more embodiments of the present invention. Within the present specification, the terms “beam oscillation” and “beam wobble” have been used interchangeably. Furthermore, although the experiments described herein used oval wobble, it is expected that similar weld parameters may also be useful with other wobble shapes, for example a linear wobble across the weld path, a figure of eight (8) wobble or an infinity-shaped («) wobble. Each feature disclosed in this specification (including any accompanying claims, abstract and drawings), may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features. The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed. The claims should not be construed to cover merely the foregoing embodiments, but also any embodiments which fall within the scope of the claims.

Claims

1. A method of laser welding an aluminium tab to an electrical cell having a steel terminal, using a laser welding system the method comprising:placing a first surface of the tab in contact with the terminal of the cell; andwelding the tab to the terminal by directing a laser beam generated by the laser welding system onto a second surface of the aluminium tab to produce a weld, the second surface being opposite the first surface,wherein:the weld has a predetermined weld path ;the laser welding system is configured to control the beam to produce an oval wobble about the weld path, the oval wobble having an amplitude (A) parallel to the weld path of greater than 0.45mm;the average velocity (V) of the laser spot on the tab is at least 60mm / s; andthe frequency of the oscillations is 400-600Hz.

2. A method as claimed in claim 1, wherein the average velocity (V) of the laser spot on the tab is at least 80mm / s and / or the oscillations have an amplitude (A) of at least 0.65mm.

3. A method as claimed in claim 1 or claim 2, wherein the oval wobble comprises oscillations having a first component in a direction parallel to the weld path and a second component in a direction normal to the weld path, wherein the component in the direction parallel to the weld path defines a length of the oval along a first axis thereof, and wherein the component in the direction normal to the weld path defines a length of the oval along a second axis thereof.

4. A method as claimed in claim 3, wherein the length of the length of the second axis is 25-400% of the length of the first axis, preferably wherein the length of the second axis is 50-200% of the length of the first axis.

5. A method as claimed in claim 3 or claim 4, wherein the oval wobble is a circular wobble and the length of the first axis is equal to the length of the second axis.

6. A method as claimed in any preceding claim, wherein the cell is a cylindrical cell.

7. A method as claimed in any preceding claim, wherein the laser is a single modelaser.

8. A method as claimed in any preceding claim, wherein the laser is a fibre laser.

9. A method as claimed in any preceding claim, wherein the laser is a Ytterbium laser.

10. A method as claimed in any preceding claim, wherein the laser power is ramped down at end of weld.

11. A method as claimed in any preceding claim, wherein the laser power during welding is 300-500W.

12. A method as claimed in any preceding claim, wherein the laser spot diameter on the aluminium tab is 25-35 micrometres.

13. A method as claimed in any preceding claim, wherein the steel terminal comprises a nickel plating.

14. A method as claimed in any preceding claim, wherein the steel terminal has a thickness of 0.2-0.5mm.

15. A method as claimed in any preceding claim, wherein the aluminium tab has a thickness of 0.2-0.4mm.

16. A method as claimed in any preceding claim, wherein the weld path has a length of 5-20mm.

17. A method as claimed in any preceding claim, wherein the weld path is circular, optionally wherein the circular weld path has a diameter of 2 to 4mm.

18. A method as claimed in any one of claims 1-16, wherein the weld path is discorectangle shaped, optionally wherein the discorectangle shape has a perimeter length of 6-15mm.

19. A method as claimed in any preceding claim, wherein the cell comprises a first end surface , wherein a first terminal of the cell is located in a central region of the first end surface and at least part of a second terminal of the cell is located in a peripheral region of the first end surface, and the step of welding the tab to the terminal comprises welding the tab to the central region of the first end surface.

20. A method as claimed in claim 19, wherein the method further comprises welding a second tab to the peripheral region of the first end surface.

21. A method as claimed in claim 20, wherein the second tab is welded to the peripheral regions of the first end surfaces of two adjacent cells.

22. A method of mechanically and electrically connecting a busbar component to a plurality of cells, wherein the busbar assembly comprises a plurality of aluminium tabs, the method comprising welding each of the aluminium tabs to one or more of the cells, according to a method as claimed in any preceding claim.

23. A battery module comprising a busbar component mechanically and electrically connected to a plurality of cylindrical cells according to a method as claimed in claim 22.

24. A battery pack comprising a plurality of battery modules as claimed in claim 23.

25. A vehicle comprising a battery module as claimed in claim 23 or a battery pack as claimed in claim 24.

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