GB2539300A

GB2539300A - Component joining method and component joining structure

Info

Publication number: GB2539300A
Application number: GB1602441.6A
Authority: GB
Inventors: Lewis John; Brown Mark
Original assignee: Tata Motors European Technical Centre PLC; Tata Motors Ltd
Current assignee: Tata Motors European Technical Centre PLC; Tata Motors Ltd
Priority date: 2015-02-12
Filing date: 2016-02-11
Publication date: 2016-12-14
Also published as: GB201505480D0; GB201602441D0; WO2016128920A1

Abstract

Aspects of the present disclosure relate to a component joining method comprising; interposing an intermediate member 32 between at least one first component 4 and a second component 2, joining the intermediate member, the at least one first component and the second component to one another by electrical welding through the intermediate member, wherein the electrical welding is arranged to bring the at least one first component and the second component in mechanical and electrical communication with one another at least in part through the intermediate member. The intermediate member 2 is formed from a rivet or insert that is inserted through a perforation in the first component. Preferred materials for the components include aluminium and copper. A component joining structure so formed finds particular application in attaching battery cell tabs to battery busbars.

Description

COMPONENT JOINING METHOD AND COMPONENT JOINING STRUCTURE

TECHNICAL FIELD

The present disclosure relates to a component joining method and a component joining structure, and more particularly to a component joining method and a component joining structure using resistive welding. More particularly, though not exclusively, the disclosure relates to a method of joining metallic components of dissimilar thicknesses.

BACKGROUND OF THE INVENTION

The process of resistance welding is an established technique in which electrodes are brought into face contact on respective sides of components being welded and a high current is passed between the electrodes and thus through the components. The resistive heating effect at the point of contact between the components results in localised fusion and joining of the parts. By appropriate selection of electrode materials, electrode geometries and weld process parameters (current, time and pressure) it is generally possible to provide the correct heat balance whereby the heating primarily occurs at the point of contact where the weld is required.

However, where the components being welded are of dissimilar physical thickness it can be difficult to provide the correct heat balance. In particular, large disparity in the thickness of the components results in difficulty in achieving correct heat balance within the structure and consequently the area of fusion may extend to the surface of the thinner component. When this occurs it is likely to result in a secondary weld occurring between the thinner component and the welding electrode. This problem is particularly prevalent when welding thin metallic foils to thicker components and is undesirable because the welding process may have to be disrupted in order to release the electrode, in which case it is necessary to clean the electrode prior to recommencing welding.

It is therefore an object of embodiments of the present invention to circumvent or at least partially solve or ameliorate the above problem and limitations or shortcomings of joining components of disparate physical thickness according to known techniques.

SUMMARY OF THE INVENTION

Aspects and embodiments of the present invention relate to a component joining method, a component joining structure, a battery cell module, and a vehicle as claimed in the appended claims.

According to an aspect of the present invention there is provided a component joining method comprising; interposing an intermediate member between at least one first component and a second 5 component, joining the intermediate member, the at least one first component and the second component to one another by electrical welding through the intermediate member, wherein the electrical welding is arranged to bring the at least one first component and the second component in mechanical and electrical communication with one another at least in part through the intermediate member.

In the interests of clarity, the join between the intermediate member, the at least one first component and the second component may consist of a weld there-between providing mechanical and electrical connection between said components.

This aspect of the invention is beneficial in that it enables components having different physical thicknesses to be successfully welded without the weld nugget extending to the face of the thinner material.

In an embodiment, the at least one first component comprises a plurality of first components and the electrical welding is arranged to bring the plurality of first components and the second component in mechanical and electrical communication with one another at least in part through the intermediate member.

In an embodiment, the method comprises perforating the or each first component to provide an aperture adapted to receive the intermediate member.

Optionally, the method comprises inserting the intermediate member through the aperture in the or each first component so as to extend there-beyond.

The or each first component may be perforated prior to insertion of the intermediate member through the aperture so formed. Alternatively, the aperture may be provided concurrently or simultaneously with inserting the intermediate member by using a self-piercing intermediate 35 member.

In one embodiment, the intermediate member comprises at least one of an insert, a fastener and a rivet.

In these embodiments of the invention the intermediate member operates as a shunt resistance connected across the various series resistances associated with the or each first component and the second component layers. This is advantageous in that it facilitates control of the welding process by making the shunt resistance associated with the intermediate element the predominant resistance.

Perforating the first component is beneficial in that a periphery of the aperture so formed may be substantially free from corrosion, oxidation and / or any other contaminants which might otherwise be present on the surface of the first component and which could otherwise compromise the quality of the weld. The action of perforating may therefore obviate further preparation of the surface of the first component, e.g. by chemical, electrochemical or mechanical cleaning or ablating the surface of the first component. Accordingly, the foregoing embodiments are beneficial in that they enable an electrical fusion weld to be formed between the first and second components at least in part at an interface between the periphery of the intermediate member and the periphery of the aperture in the first component.

In one embodiment, perforating comprises physically removing a portion of material from the first component, for example using a punch and die set.

Optionally, the second component is physically thicker than the or each first component by a factor of at least five. Further optionally, the second component is physically thicker than the or each first component by a factor of at least ten.

The intermediate member may comprise substantially the same material as the or each first component or the second component. In an embodiment, the intermediate member comprises an alloy containing substantially the same material as the or each first component or the second component.

In one embodiment the intermediate member, the or each first component and the second component all comprise copper. Alternatively, the intermediate member, the or each first component and the second component may all comprise aluminium.

In another embodiment, the intermediate member comprises a dissimilar material to the or each first component or the second component. The intermediate member may comprise a material which alloys with the material of the or each first component and the second component, in particular without forming brittle inter-metallic materials, compounds or residues.

In an embodiment, the intermediate member comprises a single material.

In an alternative embodiment, the intermediate member may comprise a composite material.

For example, a composite form of the intermediate member may comprise a stem and a head consisting of different materials. Specifically, a composite intermediate material may comprise copper, silver, tin or nickel in order to modify the physical and electrical characteristics of the member and / or the welding process.

Optionally, at least one of the intermediate member, the or each first component and the second component may comprise a surface including tin (Sn). Optionally, the tin may comprise a surface layer of about 5 pm thickness. This is beneficial where the tin has a high resistivity and low melting point relative to that of the material comprising the intermediate member, the or each first component and the second component because the tin (Sn) promotes and localises the weld interface within the structure. Subsequently, the tin melts and alloys with material of the intermediate member, the or each first component and the second component (typically copper in certain embodiments).

In embodiments of the present method comprising perforating the or each first component to provide an aperture there-in, the intermediate member may be received in the aperture in the or each first component so as to form an interference fit with the or each first component. The interference fit may be provided between the periphery of the intermediate member and that of the aperture of the or each first component.

The method may comprise mechanically forming the intermediate member so as to retain the same within the aperture in the or each first component. Without limitation: the process of mechanically forming the intermediate member may include clinching, swaging or riveting the intermediate member.

In one embodiment, the method comprises mechanically forming the intermediate member prior to joining the intermediate member, the or each first component and the second component to one another by electrical welding. Where the or each first component comprises an aperture, the intermediate member may be introduced through the aperture so as to extend beyond the or each first component prior to mechanically forming the intermediate member. This is beneficial where a plurality of first components are to be joined to a second component because the mechanical forming process retains the plurality of first components together until welded. The or each first component and the second component are consequently joined mechanically and electrically by the weld thereafter irrespective of whether the intermediate member was or was not formed prior to welding.

According to another aspect of the present invention there is provided a component joining structure comprising at least one thinner first component and a thicker second component welded in mechanical and electrical communication with one another at least in part through an intermediate member interposed there-between.

In this structure the join between the intermediate member, the at least one first component and the second component may consist of a weld there-between providing mechanical and electrical connection between said components.

Optionally, the second component is thicker than the at least one first component by a factor of at least five, further optionally by a factor of at least ten. The present component joining structure according to the present aspect is advantageous in that it enables thin materials such as metallic foils to be successfully welded to thicker metallic materials such as metallic bars. Accordingly, components of widely disparate thicknesses can be joined to one another mechanically and electrically without the weld extending to the surface of the thinner component.

In one embodiment of the present component joining structure, the intermediate member, the at least one first component and the second component all comprise copper. In an alternative embodiment of the present component joining structure, the intermediate member, the at least one first component and the second component all comprise aluminium. The component joining structure provides the benefit of reliably joining metals of low resistance (relative to say iron or steel) by electrical resistance welding.

In another embodiment, the component joining structure comprises a plurality of first components.

According to another aspect of the present invention there is provided a battery cell module comprising a component joining structure as described in the previous aspect of the invention, formed using the method of the foregoing aspect of the invention.

In embodiments of this aspect of the invention the or each first components may comprise a battery cell connection such as a battery cell tab and the second component may comprise a battery busbar.

According to another aspect of the present invention there is provided a battery cell module comprising one or more cells, the or each cell having one or more relatively thin cell tabs welded in mechanical and electrical communication with a relatively thicker bus-bar at least in part through an intermediate member interposed there-between. Optionally, the intermediate member comprises a rivet passing through the cell tab.

According to yet another aspect of the present invention there is provided a vehicle comprising a battery cell module according to a foregoing aspects of the invention.

According to one aspect of the present invention there is provided a component joining method, a component joining structure a battery cell module, or a vehicle substantially as hereinbefore described.

Within the scope of this application it is expressly envisaged 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

One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 shows a schematic, cross sectional representation of a conventional resistance welding arrangement known in the art for joining two metallic components of dissimilar physical thickness; Figure 2 shows a schematic, cross sectional representation of a conventional resistant welding arrangement known in the art for joining a plurality of metallic components of dissimilar physical thickness; Figure 3 shows a schematic, cross sectional representation of an improved joining configuration according to an embodiment of the present invention; Figure 4 shows a schematic, cross sectional representation of an intermediate joining member according to an embodiment of the present invention; Figure 5 shows a schematic, cross sectional representation of an alternative intermediate joining member according to another embodiment of the invention; Figure 6 shows a schematic, cross sectional representation of an improved joining configuration according to another embodiment of the present invention; Figure 7 shows an image of a plurality of battery cell tabs, each cell tab welded to a copper busbar using an embodiment of the present method; Figure 8 shows an optical micrograph of a weld cross section made using an embodiment of the present method; Figure 9 shows a schematic representation of an electric vehicle comprising a traction battery having a plurality of battery cell modules according to an embodiment of the present invention.

DETAILED DESCRIPTION

Referring to Figure 1, in conventional resistance spot welding techniques, a first component 4 having a small physical thickness is typically joined to a second component 2 having a larger physical thickness by applying a physical force in combination with an electric current via welding electrodes 6a, 6b. The resistance heating effect of the electric current causes the material(s) of the first and second components to melt and fuse together to form a weld nugget 8.

A localised projection 10 is optionally formed in the surface of one of the components being joined (usually the thicker component) to create a higher electrical resistance path thus facilitating fusion of the components at the point of contact.

However, as described above, where the components being welded are of dissimilar physical thickness it can be difficult to provide the correct heat balance during the welding process. In particular, large disparity in the thickness of the components (for example components having a ratio of thickness around 10:1) results in difficulty in achieving correct heat balance within the structure and consequently the area of fusion may extend to the surface of the thinner component.

One practical example of this is where resistance welding is used to form interconnections between cell terminals and busbars in lithium ion traction battery modules for electric vehicles. It is a key requirement in such traction battery modules that the welded interconnections between the cell terminals and busbars are of high mechanical strength and integrity, but also provide a low resistance electrical connection between the cell terminals and the busbars. A low resistance electrical connection is imperative in reducing unwanted 12R heating losses in the interconnections caused by the large currents which flow there-through when the battery module is in use. This in turn improves the safety and reliability of the battery module.

A further complication is that lithium ion cell manufacturers commonly use nickel plating as means of corrosion protection of copper cell terminals. Because the melting point of nickel (1453°C) is significantly higher than that of copper (1084°C) and because the electrical conductivity of nickel is only 20% that of copper significantly more weld energy (higher current and / or longer time) is required to create fusion at the interface and to form a weld. The joint thus becomes hotter which in turn results in further extension of the fusion area to the surface of the thinner component.

It is recognised in conventional processing techniques that there is beneficial effect in selectively removing the nickel plating from the cell terminal in the areas where welds are to be made. Various processes can be used -mechanical, chemical, electrochemical or laser ablation -however all of these techniques represent an additional process overhead.

Referring to Figure 2, the situation is exacerbated where multiple components 4a, 4b of thin physical thickness are to be joined to another component 2 of greater thickness. One practical example of this is where resistance welding is used to form interconnections between a plurality of cell terminals to a busbar in a traction battery module. This occurs where cells within the battery module are connected in parallel. In this case the complex resistance profile of multiple weld interfaces makes the process difficult to control.

Referring to Figure 3, a method according to a first embodiment of the invention comprises interposing an intermediate member 32 between at least one first (thinner) component 4, 4a, 4b and a second (thicker) component 2 prior to joining the components by resistance welding. In this embodiment of the invention, an aperture 42 is made in the at least one first (thinner) component 4, 4a, 4b to receive the intermediate member 32. The intermediate member 32 may take the form of an insert 44 which is introduced in to the aperture 42 prior to resistance welding.

Applicant has determined that the use of such a suitably shaped intermediate member 32 or insert 44 between the components to be joined 4, 4a, 4b, 2, provides a thermal mass which is larger than that of the thinner component(s) and which is locatable spatially between the thinner component(s) 4, 4a, 4b and the thicker component 2. This in turn enables the heat distribution within the weld to be controlled and precludes the weld nugget extending to the surface of the thinner component 4, 4a, 4b.

More specifically, referring to Figure 4, the method according to this embodiment comprises perforating the at least one first (thinner) component 4, 4a, 4b, for example by drilling or piercing, to provide said aperture 42 to receive the intermediate member 32 or insert 44. The perforation process is arranged to provide a clean-sided aperture 42 through the at least one first (thinner) component 4, 4a, 4b, substantially free from fraying and swarfe.

Applicant has determined that the process of perforating the at least one first (thinner) component 4, 4a, 4b, in particular by piercing using a punch / die, is beneficial in that it exposes a clean weld interface at the periphery of the aperture 42 so formed. This is particularly beneficial where the material comprising the at least one first (thinner) component 4, 4a, 4b has a surface coating applied, for example an anticorrosion surface treatment. Referring to the example described above, wherein nickel plating may be applied as means of corrosion protection of a copper component, the present embodiment obviates the requirement to remove the nickel plating prior to welding.

After having perforated the at least one first (thinner) component 4, 4a, 4b, the intermediate member 32 or insert 44 is inserted into the aperture 42 in the at least one first (thinner) component 4, 4a, 4b, and retained therein by suitable means which may be merely by gravity or some other means, e.g. adhesive, mechanical retention etc. Alternative to punching or drilling the at least one first (thinner) component 4, 4a, 4b, a self-piercing intermediate member 32 or insert 44 is used to perforate the at least one first (thinner) component 4, 4a, 4b to provide said aperture 42. In this variation of the embodiment the process of perforating the at least one first (thinner) component 4, 4a, 4b occurs concurrently and simultaneously with the process of inserting the intermediate member 32 or insert 44 into the aperture 42 formed thereby.

The relative dimensions of the intermediate member 32 and the aperture 42 are arranged to provide a close physical interrelationship once assembled. Optionally, the outer periphery of the insert 44 shall be arranged to be a close physical fit, optionally an interference fit, with the inner periphery of the aperture 42. The insert 44 may comprises a substantially circular cross section cylinder and the aperture 42 may have a substantially circular cross section; in which case the outer circumference of the insert 44 may be an interference fit with the circumference of the aperture 42.

Optionally, the insert 44 is shaped at one end to modify the weld characteristics, for example the end which is intended to abut the second component 2 during the welding process is optionally profiled to a point. In one configuration, the abutting end of the insert 44 is arranged to have a substantially conical form. Alternatively, the abutting end of the insert 44 is arranged to have one of a substantially frustoconical form, a hemispherical form, a semi-spherical from, a semi-ellipsoidal form, and a domed form.

In a further embodiment illustrated in figure 5 having a plurality of first (thinner) components 4a, 4b, the insert 44 comprises a rivet 52 and the method includes the step of clinching the rivet 52 after insertion through the aperture 42. Without limitation, the clinching process comprises pressing the rivet 52 against a mandrel to form the abutting end for the subsequent weld. Optionally, the clinching process comprises rotary or orbital riveting using a peen tool held at a fixed angle to create a sweeping line of pressure around the periphery of the shank of the rivet 52, progressively forming the abutting end for the weld. The clinching process has the benefit of securing the rivet 52 in the aperture 42.

Optionally, the clinching process also profiles the abutment end of the rivet ready for welding.

The profiled end of the intermediate member 32, insert 44 or rivet 52 performs the same function as the conventional projection on the second (thicker) component 2 to be joined but obviates the same.

The joining process is completed by resistive welding the at least first (thinner) component 4, 4a, 4b to the second (thicker) component 2 via the intermediate member 32, in this case the rivet 52. Specifically, electrodes 6a and 6b are applied to the intermediate member 32 and the second component respectively. A mechanical force and electric current are applied to the intermediate member, which melts, flows and fuses to the first and second components 4, 4a, 4b, 2. The components are thus brought in to electrical and mechanical communication with each other via and through the intermediate member 32.

The skilled person will appreciate that the at least one first (thinner) component 4, 4a, 4b and the second (thicker) component 2 can be considered in terms of an electrical equivalent circuit for the purposes of resistive welding. Specifically, the interfaces between the layers may be considered as electrical resistances connected in series. It is the series configuration of the various resistances associated with the numerous layers which makes electrical (resistive / inductive) welding of the components difficult to control hitherto. The intermediate member 32 of the embodiments of the present invention described herein can be considered in electrical terms as a shunt resistance connected across the various series resistances associated with the component layers. Accordingly, the invention facilitates control of the welding process by making the shunt resistance associated with the intermediate element 32 the predominant resistance.

Furthermore, it is believed that the manner in which the intermediate member 32 melts and flows during the welding process may have an additional beneficial effect in fusing with the first and second components 4, 4a, 4b, 2. By way of a more detailed description, during the initial stages of the welding process the intermediate member 32 melts and expands outwardly within the aperture 42 in the at least one first (thinner) component 4, 4a, 4b. In embodiments where the intermediate member 32 comprises a rivet 52, the shank of the rivet 52 expands radially outward within the aperture 42. At this initial stage the electrical current from the electrodes 6a and 6b passes predominantly through the intermediate member causing the same to heat up and melt.

As the welding process proceeds, the intermediate member 32 continues to expand outwardly until it has filled the aperture 42 in the at least one first (thinner) component 4, 4a, 4b. The molten intermediate member 32 consequently comes in to physical, thermal and electrical communication with the material of the at least one first (thinner) component 4, 4a, 4b at the periphery of the aperture. The material of the at least one first (thinner) component 4, 4a, 4b in this region begins to heat up and melt due to conduction of thermal and electrical energy from the intermediate member 32. In other words, the at least one first (thinner) component 4, 4a, 4b acts as a parasitic electrical resistance connected electrically in parallel with the resistance of the intermediate member 32. Accordingly, some of the welding current from the electrodes 6a, 6b passes through the at least one first (thinner) component 4, 4a, 4b causing resistive (12FI) heating and melting thereof. The heating effect is therefore twofold. Consequently, the molten material of the at least one first (thinner) component 4, 4a, 4b fuses and coalesces readily with that of the molten intermediate member 32 to form a weld of high mechanical strength and integrity, as well as low electrical resistance.

During the welding process, the profiled end of the intermediate member 32, insert 44 or rivet 52 also acts as a weld projection in contact with the second (thicker) component 2.

Hence, an additional weld nugget is formed at the interface between the profiled end of the intermediate member 32, insert 44 or rivet 52 and the second (thicker) component 2. Consequently, the molten material at the end of the intermediate member 32 fuses effectively with that of the second (thicker) to form a weld there-between of high mechanical strength and integrity, as well as low electrical resistance.

Thus, the welding process of the present invention enables the at least one first (thinner) component 4, 4a, 4b, and the second (thicker) component 2 to be joined together both mechanically and electrically via the intermediate member 32.

Referring now to Figure 7, the inventors have established that the method of the embodiment of Figure 3 is further improved when joining copper components 2, 4, 4a, 4b, by forming a layer of tin (Sn) 9 on at least one of the contacting surfaces of the intermediate member 32 and the second (thicker) component 2 prior to joining the components by resistance welding. The layer of tin 9 is typically 5pm thick and is formed by electroplating.

Other methods of forming the tin layer may be also be useful.

Specifically, the inventors have discovered that the layer of tin 9 melts preferentially during the welding process to promote and localise welding at the tin layer 9. The tin 9 produces a metallic flux which alloys readily with the copper of the second (thicker) component 2. This effect is caused by the relatively high electrical resistivity of the tin compared with that of copper (causing 12R heating in the tin layer), in combination with the relatively low melting point of the tin compared with that of copper. By way of comparison, see Table 1 below.

Material Resistivity (ohms.m @ 20°C) Melting point (PC) Copper (Cu) 1.68 x 10-' 1084 Nickel (Ni) 6.99 x 10-8 1453 Tin (Sn) 1.09 x 10-7 232

Table 1

The method of the embodiment of Figure 7 is beneficial in that it enables lower welding currents to be employed while maintaining a weld of high mechanical strength and integrity, and low electrical resistance. The lower welding current reduces the likelihood of the weld nugget extending to the surface of the at least one first (thinner) component 4, 4a, 4b.

Figure 7 shows an image of a battery module 60 according to an embodiment of the invention having a plurality of battery cell tabs 4a, each cell tab 4a welded to a copper busbar 2 using an embodiment of the present method. In this embodiment, each cell tab 4a is welded to a respective busbar 2 using a plurality of intermediate members 32. The battery cell tabs 4a typically comprise copper foil having a thickness of 200pm (0.2mm). Without limitation, the busbars 2 comprise the same material (copper) as the cell tabs 4a, however the busbars 2 typically have a thickness of 2000 pm (2mm). This represents an order of magnitude difference (a factor of about 10) in the thickness of the cell tabs 4a compared with that of the busbars 2. The intermediate members 32 also comprise copper. In alternative arrangements both the cell tabs 4a and the busbars may comprise aluminium.

Although the ratio of the thickness of the busbars 2 to the thickness of the cell tabs 4a is described above as 10:1, other ratio's may also be applicable. For example, the present invention finds useful application where the ratio of the thickness of the busbars 2 to the thickness of the cell tabs 4a is 5:1.

Referring to Figure 8, a micrograph of a weld cross section made using an embodiment of the present method shows fusion welding of a plurality of first (thinner) components 4a, 4b to the intermediate member 32 at the interface between the periphery of the intermediate member 32 and the periphery of the aperture 42.

Similarly, the micrograph illustrates the abutment end of the intermediate member 32 fused to the surface of the second (thicker) component 2.

Figure 9 shows a schematic representation of an electric vehicle 100 comprising a traction battery having a plurality of battery cell modules 60 according to an embodiment of the present invention.

It will be appreciated that various changes and modifications can be made to the present invention without departing from the scope of the present application.