DESC:TECHNICAL FIELD OF THE INVENTION
[0001] This disclosure relates to a method of manufacturing aluminium sheets for bus bars for use in an electric vehicle battery assembly, and aluminium sheets thereof.

DESCRIPTION OF THE BACKGROUND ART
[0002] Electrified vehicles are being developed to reduce our dependence on internal combustion engines which consume fossil fuels. In general, electrified vehicles differ from conventional internal combustion by their powertrains. The powertrains of electric vehicle are equipped with a battery module having a plurality of battery cells for storing and supplying power to the vehicle. Conventional motor vehicles, by contrast, rely exclusively on the internal combustion engine to power and propel the vehicle. Conventional rechargeable batteries such as lithium-ion or nickel metal hydride batteries are arranged in series or parallel to obtain any desired voltage or current for the vehicle.
[0003] The battery cells of electric vehicle are connected to one another to form a battery array in order to achieve the necessary voltage and power levels. Bus bars and other devices are commonly used to sufficiently connect the battery cells. A bus bar is a metal strip or bar that conducts electricity and is used for electrical power distribution.
[0004] The battery cells have a first terminal, a second terminal and the bus bar is coupled to the first and second terminals respectively of the battery cells to place the battery cells in a series connection and a parallel connection. In order to achieve the desired voltage and power, the battery cells must be connected securely to one another.
[0005] One technique for coupling the bus bar to terminals of the battery cells is through wire bonding, where a wire connects the bus bars to the battery cell terminals to create connections. When connecting cells in series, bus bars and high current interconnects link the positive terminal of one cell, or a parallel group of cells, to the negative terminal of the next cell or the next parallel group of cells. The wire bonding allows for the expected current to pass through the wire bonds without significant heating of the wires due to the current, while allowing the wires bonds to break in an overcurrent condition, such as would be expected during a short circuit.
[0006] Bus bars made of copper are prevalently used due to the high electrical conductivity, but at a higher weight and cost. Aluminium and its alloys, copper and occasionally steels are used as materials of construction for such bus bars due to their high electrical conductivity, availability and/or cost considerations. Poor electrical conductivity would lead to resistance heating of the bus bars and potential safety issues during charging or discharging of the cells.
[0007] Lightweight aluminium alloys are suited for this product as they lead to weight reduction of the vehicle and highest current carrying capacity per unit weight. Copper bus bars are also more expensive compared to aluminium while steel-based bus bars have lower electrical conductivity and current carrying capacity.
[0008] To enhance the overall corrosion resistance, electroless nickel coating is applied to the aluminium bus bars. Such a coating approach poses practical problems such as delamination, cracking, and improper and non-uniform coating on the surface of the bus bars which adversely affects the corrosion resistance of the bus bars. Hence, it is desirable to obtain the required corrosion resistance by eliminating the need for electroless nickel coating.
[0009] It has been observed that there exists a problem in conventional bus bars that the joint between the bus bar and the wire has poor pull strength and has a tendency of disconnection during handling, packing, unpacking, and unpacking operations involving the electric powertrain.
[0010] Hence, it is critical to ensure that the joint between the wire and the bus bar is strong and does not fail during service. Pull testing of wire bonds is carried out to measure the bond strength and higher bond strength is desirable. Furthermore, it is required that the bus bars have sufficient strength to withstand handling during packing, unpacking and assembly processes.

SUMMARY OF THE INVENTION
[0011] An object of the invention is to provide aluminium sheets for bus bars that addresses one or more limitations of conventional bus bars and the process for manufacturing the same.
[0012] An object of the present invention is to provide aluminium sheets for bus bars exhibiting high wire bonding strength.
[0013] Another object of the invention is to provide a process for manufacturing the aluminium sheets for bus bars exhibiting wire bonding strength of greater than 1 kgf.
[0014] In an aspect, the present invention provides aluminium sheets for bus bars for an electric traction system of a vehicle, the bus bars comprising: an electrically conductive body configured for coupling to an electrical device obtained by a process comprising of degreasing and thermal treatment of a cold rolled aluminium sheets having predetermined dimensions.
[0015] In an aspect, thermal treatment is carried out at a temperature in the range of 120- 300oC for duration of 3-5 Hrs.
[0016] In an aspect, the method of manufacturing aluminium sheets for bus bars comprises: casting a feedstock having at least 99.5% w/w aluminium, at most 0.05% w/w manganese, at most 0.05% w/w titanium, at most 0.05% w/w chromium, and at most 0.05% w/w vanadium, into ingots in a casting machine; hot rolling the ingots to form sheets of a first thickness; cold rolling the hot rolled sheets in a cold rolling machine to form sheets of a second thickness; heating the cold rolled sheets to a temperature of 120-300°C for 3-5 hours for forming a uniform aluminium oxide layer of a thickness greater than 5 nm on the surface of the sheets, for enhancing corrosion resistance of the sheets; chemically treating the sheets for stabilizing the aluminium oxide layer and degreasing the sheets, for enhancing wire-bondability of the sheets.
[0017] According to an embodiment, the aluminium oxide layer formed on the cold rolled sheets has a thickness less than 50 nm.
[0018] According to an embodiment, the chemical treatment includes treating the sheets with an acid having a pH ranging from 4 to 7and then washing the sheets by hot water for stabilizing the aluminium oxide layer and degreasing the sheets, for enhancing wire-bondability of the sheets.
[0019] According to an embodiment, the feedstock comprises at most 0.10% w/w of a combination of manganese, titanium, chromium, and vanadium.
[0020] According to the embodiment, the feedstock comprises at most 0.01% w/w manganese, at most 0.02% w/w titanium, at most 0.01% w/w chromium and at most 0.01% w/w vanadium, and a combined composition of manganese, titanium, chromium and vanadium is lesser than 0.05% w/w.
[0021] According to the embodiment, the feedstock comprises at least 99.5% w/w of aluminium, at most 0.04% of manganese w/w, at most 0.02% w/w of titanium, at most 0.002% w/w of chromium, at most 0.006% w/w of magnesium, at most 0.02% w/w of copper, at most 0.005% w/w of zinc, at most 0.15% w/w of silicon and at most 0.3% w/w of iron.
[0022] According to an embodiment, the feedstock has a conductivity of at least 59% IACS.
[0023] According to an embodiment, after casting and prior to cold rolling, the cast ingots are homogenized by heating to a temperature of 500-620°C; the homogenized cast ingots are hot rolled to form sheets of the first thickness of 2-15 mm; and the hot rolled sheets are annealed for improving a ductility thereof.
[0024] According to the embodiment, after hot rolling and prior to the chemical treatment, the sheets are cold rolled to reduce a thickness thereof by at least 50%. After hot rolling and prior to the chemical treatment, the sheets are cold rolled to the second thickness of 0.5 to 6 mm
[0025] According to an embodiment, the casting machine is a continuous casting machine or a direct chill casting machine.
[0026] According to another aspect of the invention, a sheet for manufacturing aluminium bus bars comprises 99.5% w/w aluminium, at most 0.05% w/w manganese, at most 0.05% w/w titanium, at most 0.05% w/w chromium, and at most 0.05% w/w vanadium, and a uniform aluminium oxide layer having a thickness of 5-50 nm on a surface thereof for enhancing corrosion resistance.
[0027] According to the present invention, the sheets can be flat rectangular or coiled.

BRIEF DESCRIPTION OF DRAWINGS
[0028] The above and other aspects, features, and advantages of certain exemplary embodiments of the present invention will be more apparent from the following description taken in conjunction with the accompanying drawings in which:
[0029] Figure 1 through Figures 1A-1E depicts rust formation on the bus bars subjected to said salt spray testing according to an exemplary embodiment of the present invention.
[0030] Figure 2 shows performance analysis of an exemplary bus bar of 4 mm thickness fabricated by the method of the present invention, with the performance parameters being temperature, time, and current.
[0031] Persons skilled in the art will appreciate that elements in the figures are illustrated for simplicity and clarity and to help in improving an understanding of certain exemplary embodiments of the present disclosure and that the drawings are not intended to limit the scope of disclosure.

DETAILED DESCRIPTION OF THE INVENTION
[0032] In describing and claiming the invention, the following terminology will be used in accordance with the definitions set forth below. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, the preferred methods and materials are described herein. As used herein, each of the following terms has the meaning associated with it in this section. Specific and preferred values listed below for individual process parameters, substituents, and ranges are for illustration only; they do not exclude other defined values or other values falling within the preferred defined ranges.
[0033] As used herein, the singular forms "a," "an," and "the" include plural reference unless the context clearly dictates otherwise.
[0034] The terms “preferred” and “preferably” refer to embodiments of the invention that may afford certain benefits, under certain circumstances. However, other embodiments may also be preferred, under the same or other circumstances.
[0035] Furthermore, the recitation of one or more preferred embodiments does not imply that other embodiments are not useful and is not intended to exclude other embodiments from the scope of the invention.
[0036] As used herein, the terms “comprising” “including,” “having,” “containing,” “involving,” and the like are to be understood to be open-ended, i.e., to mean including but not limited to.
[0037] As used herein, the term “bus bar” refers to metal strip or bar that conducts electricity and is used for electrical power distribution. The term “w/w” refers to a weight ratio of the weight of a referred component to the total weight of the bus bar.
[0038] As used herein, the term “thermal treatment” refers to a heat treatment process that changes the physical and sometimes also the chemical properties of a material to increase ductility and reduce the hardness to make it more workable.
[0039] As used herein, the term “degreasing” refers to the process utilized for reduction of level of carry over rolling oil on the sheet surface. The process involves washing of the strip surfaces by pressurized hot water.
[0040] As used herein, the term “wire bonding” refers to an ultrasonic, metal-metal friction welding process that is used to connects battery cells to form battery pack.
[0041] While the present invention is susceptible of embodiment in various forms, there is hereinafter described a presently preferred embodiment with the understanding that the present disclosure is to be considered an exemplification of the invention and is not intended to limit the invention to the specific embodiment illustrated.
[0042] The present invention provides a bus bar for an electric traction system of a vehicle, comprising: an electrically conductive body configured for coupling to an electrical device obtained by a process comprising of degreasing and thermal treatment of a cold rolled aluminium sheet having predetermined dimensions.
[0043] In an aspect, heating is carried out at a temperature in the range of 120-300oC for duration of 3-5 Hrs.
[0044] The feedstock is a highly conductive aluminium alloy, preferably having conductivity greater than 59% IACS. This is achieved by controlling the composition of the alloy. Preferably minimum aluminium content of the alloy should be 99.5% w/w. Presence of small amounts of manganese, titanium, chromium and vanadium leads to drastic reduction in conductivity and should be restricted to 0.05% w/w each and 0.10% w/w cumulative. Preferably, manganese, chromium and vanadium are restricted to 0.01% w/w each and titanium is restricted to 0.02% w/w, and the combined composition of manganese, chromium, vanadium and titanium is 0.05% w/w cumulative. Trace amounts of iron and silicon up to 0.4% w/w may be present in the alloy.
[0045] The alloy of the above composition is then either direct chill cast or continuously cast. The cast ingots are optionally homogenized by heating to temperature is the range of 500- 620oC and then hot rolled to sheets of a first thickness in the range of 2-15 mm. Hot rolled sheets are then cold rolled to a second thickness or a desired thickness of the bus bar, generally in the range of 0.5 to 6 mm. The sheets may optionally be inter- annealed to improve its ductility, i.e., rollability and/or control the strength of the end product. Thicknesses employed in electric vehicle battery casing is preferably in the range of 0.5 to 6 mm. The amount of cold rolling imparted to the sheets is dependent on the final desired strength of the bus bar. Preferably minimum 50% cold rolling reduction is imparted to the sheets for achieving high strength needed for the end product.
[0046] Surprisingly, wire bondability of the bus bars is enhanced significantly when the cold rolled aluminium sheets to be used as bus bars are subjected to thermal treatment and chemical treatment. The thermal treatment involves heating the cold rolled sheets to a temperature of 120-300°C for 3-5 hours for forming a uniform aluminium oxide layer of a thickness greater than 5 nm on the surface of the sheets, for enhancing corrosion resistance of the sheets. This oxide layer assists in improving the corrosion resistance without impairing the wire bonding operation resulting in high bond strength.
[0047] The thermally treated sheets are then subjected to a chemical treatment process for improving the wire-bondability of the sheets. According to an embodiment, said chemical treatment process involves degreasing the sheets with an acid having a pH ranging from 4 to 7 and washing the sheets by a continuous line of hot water for eliminating any left-over oil or grease that may be present on the sheets from the cold rolling operation and improves the wire-bondability of the sheets.
[0048] The present invention provides the aluminium sheets for bus bars of electric vehicles that do not require nickel coating. Thereby, the present invention reduces the cost as well as one of the steps of manufacturing without compromising corrosion resistance properties. Further, the aluminium bus bars manufactured by the method of the present invention provide better connectivity with the wire of the battery assembly, i.e., said aluminium bus bars exhibit wire bonding strength greater than 1 kgf.
[0049] Example:
The feedstock composition was controlled as follows: aluminium ranging from 99.56% w/w to 99.63% w/w, manganese ranging from 0.002% w/w to 0.037% w/w, titanium ranging from 0.012% w/w to 0.02% w/w, chromium ranging from 0.001% w/w to 0.002% w/w, magnesium ranging from 0.001% w/w to 0.006% w/w, copper ranging from 0.001% w/w to 0.016% w/w, zinc ranging from 0.001% w/w to 0.003%. w/w, silicon ranging from 0.074% w/w to 0.126% w/w and iron ranging from 0.21% w/w to 0.295% w/w and sheets were manufactured according to the principles described above, and then aluminium bus bars were fabricated from the sheets.
[0050] The aluminium bus bars fabricated from the sheets manufactured by the method of the present invention were found to exhibit high tensile strength greater than 120 MPa. The conductivity of the bus bars was found to be greater than 59% IACS.
[0051] The aluminium bus bars fabricated from the sheets manufactured by the method of the present invention were subjected to salt spray testing for up to 700 hours, in accordance with the standard ASTM B117. Figure 1 through 1A-1E depicts rust formation on the bus bars subjected to said salt spray testing. Post exposure to salt spray, white rust was observed to be concentrated around pierced holes of the bus bars and no significant corrosion was observed in other regions of the bus bars. There was no formation of red rust at the galvanic joints.
[0052] The aluminium bars fabricated from the sheets manufactured by the method of the present invention were also subjected to 72 hours of CASS (Copper Accelerated Salt Spray) testing. It was noted that the corrosion resistance exhibited by the bus bars of the present invention is comparable to the corrosion resistance offered by 1100 aluminium alloy bus bar with Ni electroless coating used conventionally for manufacturing bus bars.
[0053] Significantly and advantageously, the bus bars when wire bonded without additional cleaning was found to have a bond strength greater than 1 kgf. Typically, 1100 aluminium alloy bus bar with Ni electroless coating exhibits bond strength of only 0.4-0.6 kgf.
[0054] Further, bus bars with a thickness of 4mm were prepared from the aluminium sheets manufactured in accordance with the method of the present invention. Figure 2 shows performance analysis of an aluminium bus bar obtained from the sheet manufactured by the method of the present invention with the performance parameters being temperature, time, and current.
[0055] The results obtained from the performance analysis are summarized as follows in Table 1:
Table 1
4 mm bus bar
1C 36
1.7C 54
3C 82° C

[0056] While particular embodiments of the invention have been described, it is not intended that the invention be limited thereto, as it is intended that the invention be as broad in scope as the art will allow and that the specification be read likewise. It will be appreciated that the embodiments are not limited to those described herein and may vary accordingly as well.
[0057] Many modifications may be made without departing from the basic spirit of the present invention. Accordingly, it will be appreciated by those skilled in the art that the invention may be practiced other than has been specifically described herein.

,CLAIMS:
1. A method of manufacturing aluminium sheets for bus bars, the method comprising:
casting a feedstock having at least 99.5% w/w aluminium, at most 0.05% w/w manganese, at most 0.05% w/w titanium, at most 0.05% w/w chromium, and at most 0.05% w/w vanadium, into ingots in a casting machine;
hot rolling the ingots to form sheets of a first thickness;
cold rolling the hot rolled sheets in a cold rolling machine to form sheets of a second thickness;
heating the cold rolled sheets to a temperature of 120-300°C for 3-5 hours for forming a uniform aluminium oxide layer of a thickness greater than 5 nm on the surface of the sheets, for enhancing corrosion resistance of the sheets;
chemically treating the sheets for stabilizing the aluminium oxide layer and degreasing the sheets, for enhancing wire-bondability of the sheets.

2. The method of manufacturing aluminium sheets for bus bars as claimed in claim 1, wherein the aluminium oxide layer formed on the cold rolled sheets has a thickness less than 50 nm.

3. The method of manufacturing aluminium sheets for bus bars as claimed in claim 1 or claim 2, wherein the chemical treatment includes degreasing the sheets with an acid having a pH ranging from 4 to 7 and washing the sheets by a continuous line of hot water.

4. The method of manufacturing aluminium sheets for bus bars as claimed in any one of claims 1 to 3, wherein the feedstock comprises at most 0.10% w/w of a combination of manganese, titanium, chromium, and vanadium.

5. The method of manufacturing aluminium sheets for bus bars as claimed in any one of claims 1 to 4, wherein the feedstock comprises at most 0.01% w/w manganese, at most 0.02% w/w titanium, at most 0.01% w/w chromium and at most 0.01% w/w vanadium, and a combined composition of manganese, titanium, chromium and vanadium is lesser than 0.05% w/w.

6. The method of manufacturing aluminium sheets for bus bars as claimed in any one of claims 1 to 5, wherein the feedstock contains at least 99.5% w/w of aluminium, at most 0.04% w/w of manganese, at most 0.02% w/w of titanium, at most 0.002% w/w of chromium, at most 0.006% w/w of magnesium, at most 0.02% w/w of copper, at most 0.005% w/w, at most 0.15% w/w and at most 0.3% w/w of iron.

7. The method of manufacturing aluminium sheets for bus bars as claimed in any one of claims 1-6, wherein the feedstock has a conductivity of at least 59% IACS.

8. The method of manufacturing aluminium sheets for bus bars as claimed in any one of claims 1 to 7, wherein after casting and prior to cold rolling,
the cast ingots are homogenized by heating to a temperature of 500-620°C;
the homogenized cast ingots are hot rolled to form sheets of the first thickness of 2-15 mm; and
the hot rolled sheets are annealed for improving a ductility thereof.

9. The method of manufacturing aluminium sheets for bus bars as claimed in any one of claims 1-8, wherein after the hot rolling and prior to the chemical treatment, the sheets are cold rolled to reduce a thickness thereof by at least 50%.

10. The method of manufacturing aluminium sheets for bus bars as claimed in any one of claims 1 to 9, wherein after the hot rolling and prior to the chemical treatment, the sheets are cold rolled to the second thickness of 0.5 to 6 mm.

11. The method of manufacturing aluminium sheets for bus bars as claimed in any one of claims 1-10, wherein the casting machine is a continuous casting machine or a direct chill casting machine.

12. A sheet for manufacturing aluminium bus bars comprising:
99.5% w/w aluminium, at most 0.05% w/w manganese, at most 0.05% w/w titanium, at most 0.05% w/w chromium, and at most 0.05% w/w vanadium, and a uniform aluminium oxide layer having a thickness of 5-50 nm on a surface thereof for enhancing corrosion resistance.