Laser welding method

Abstract

In laser welding two insulated wires 106 together, initially, insulation is removed from the end of each wire 106 (e.g. by stripping) to form exposed portions of wire 508, 512. A clamp (figure 3) holds the exposed portions 508, 512 together. The clamp comprises material with high thermal conductivity (at least 100 W / mK) (e.g. a copper alloy). A laser beam welds the exposed portions of wire 508, 512 together. The clamp can act as a heat sink, to draw thermal energy, from welding, away from the insulated sections 510, 514 of the wires 106; thereby preventing heat damage to the insulated sections 510, 514. Respective stripped ends 508, 512 of the wires 106 may be bent to form angles 504, 506 with the insulated wire sections 510, 514. The clamp may comprise sliding planar surfaces (figure 3). The wires 106 may be I-pins or hairpins in a vehicle electric motor stator.

Description

TECHNICAL FIELD The present disclosure relates to a laser welding method. Aspects of the invention relate to a laser welding method, to a method of manufacturing an electric motor for a vehicle, to a system for implementing a laser welding method and to computer readable instructions which, when read by a computer, implement a laser welding method. BACKGROUND It is known to use enamel coated copper wires as conductors in a stator for an electric motor, for example an electric motor for an electric vehicle. In the manufacture of a stator, a length of insulated copper wire is straightened from a coil. Portions of the insulated copper wire are stripped (to expose the copper wire), which is then cut to desired lengths and either left as an l-pin or bent into a hairpin shape. After the l-pins or hairpins are placed in slots within the stator, their exposed copper portions are twisted together and welded to form an electric connection. Achieving a high quality weld joint without damaging the insulation of the covered parts of the wire is important to prevent unintended contact between wires within the stator which would affect the electric motor’s function. This may be achieved by altering the welding parameters, for example weld time, radiation wavelength and power, or by including an insulating material, e.g. paper, between wires within the stator. It is an aim of the present invention to address one or more of the disadvantages associated with the prior art. SUMMARY OF THE INVENTION Aspects and embodiments of the invention provide a laser welding method, a method of manufacturing an electric motor for a vehicle, a system for implementing a laser welding method and computer readable instructions which, when read by a computer, implement a laser welding method. According to an aspect of the invention there is provided a laser welding method for joining two insulated wires of a coil of an electric motor. The method may include providing a first insulated wire and providing a second insulated wire. The method may include removing a portion of insulation from the first insulated wire, thereby forming an exposed portion on the first insulated wire and / or removing a portion of insulation from the second insulated wire, thereby forming an exposed portion on the second insulated wire. The method may include providing a clamp device. The clamp device may include a material having a thermal conductivity of at least 100 W / mK. The method may include clamping together, using the clamp device, the exposed portion of the first insulated wire and the exposed portion of the second insulated wire. The method may include applying a laser beam to the exposed portions of each of the first insulated wire and the second insulated wire, thereby forming a weld joint between the exposed portion of the first insulated wire and the exposed portion of the second insulated wire. According to another aspect of the invention there is provided, a laser welding method for joining two insulated wires of a coil of an electric motor. The method includes providing a first insulated wire and providing a second insulated wire. The method includes removing a portion of insulation from the first insulated wire, thereby forming an exposed portion on the first insulated wire. The method includes removing a portion of insulation from the second insulated wire, thereby forming an exposed portion on the second insulated wire. The method includes providing a clamp device including a material having a thermal conductivity of at least 100 W / mK. The method includes clamping together, using the clamp device, the exposed portion of the first insulated wire and the exposed portion of the second insulated wire, and applying a laser beam to the exposed portions of each of the first insulated wire and the second insulated wire, thereby forming a weld joint between the exposed portion of the first insulated wire and the exposed portion of the second insulated wire. The clamp device contacts the exposed portions of the first and second insulated wires, creating a preferential path for thermal energy from the exposed portions of each of the first and second insulated wires. The clamp device includes a material having a high thermal conductivity, which enables the clamp device to act as a heat sink and draw heat away from the insulation of each of the first and second insulated wires. In this way, heat damage to the insulation of the first and second wires is minimised or prevented. The clamp device may include a first clamp member and a second clamp member. The method may include urging one of the first clamp member or the second clamp member in a first direction toward the first insulated wire, and urging the other of the first clamp member or the second clamp member in an opposing second direction toward the second insulated wire. The laser welding method may thereby include holding the exposed portion of the first insulated wire and the exposed portion of the second insulated wire between the first clamp member and the second clamp member. The contact between each of the first and second clamp members and the exposed portions of the first and second insulated wires contributes to the thermal path that allows thermal energy from the exposed portions of each of the wires to be transferred to the clamp device in preference to the insulation of each of the wires, thereby minimising or preventing damage to the insulation on either or both of the insulated wires. The laser welding method may include sliding the one of the first clamp member or the second clamp member in the first direction toward the first insulated wire, and sliding the other of the first clamp member or the second clamp member in the opposing second direction toward the second insulated wire. The sliding arrangement of the first and second clamp members provides a compact and efficient clamping device. The first clamp member may have a planar surface. The second clamp member may have a planar surface. The clamp device may be configured such that the planar surface of the first clamp member faces the planar surface of the second clamp member. The method may include sliding the planar surface of the one of the first clamp member or the second clamp member relative to the planar surface of the other of the first clamp member or the second clamp member in the first direction toward the first insulated wire, and sliding the planar surface of the other of the first clamp member or the second clamp member relative to the planar surface of the one of the first clamp member or the second clamp member in the second direction toward the second insulated wire. The planar surfaces of the first and second clamp members provide a large surface area for the transfer of heat to the clamp device in preference to the insulation of the insulated wires. The laser welding method may include urging the first clamp member in a third direction to contact the second clamp member, wherein the third direction is perpendicular to the first direction and / or the second direction. The contact between the first and second clamp members further contributes to the thermal path that allows thermal energy from the exposed portions of each of the wires to be transferred to the clamp device in preference to the insulation of the insulated wires by conduction. Each of the first clamp member and the second clamp member may include a through-hole. The method may include passing each of the first insulated wire and the second insulated wire through each of the through-hole of the first clamp member and the through-hole of the second clamp member. The first clamp member and / or the second clamp member may include the material having a thermal conductivity of at least 100 W / mK. The material of the clamp device may include copper or a copper alloy. The first insulated wire and / or the second insulated wire may be a hairpin or an l-pin for a stator winding coil. The laser welding method may include bending the exposed portion of each of the first and the second insulated wires adjacent an insulated portion, such that all surfaces of the exposed portion, within a projection of a cross-section of the exposed portion at the end of the exposed portion, are exposed. The laser welding method may include bending the exposed portion of the first insulated wire, thereby forming a first angle between a welding portion of the exposed portion for providing the weld joint and a part of the insulated portion adjacent the bend in the first insulated wire, and bending the exposed portion of the second insulated wire, thereby forming a second angle between a welding portion of the exposed portion for providing the weld joint and a part of the insulated portion adjacent the exposed portion of the second insulated wire. The laser welding method may include clamping the first insulated wire relative to the second insulated wire such that the angle between the insulated portions adjacent the exposed portions of the first insulated wire and the second insulated wire is at least 90°. Having an angle of at least 90° between the insulation portions adjacent the exposed portions of the first insulated wire and the second insulated wire has been found to be particularly advantageous for minimising the transfer of heat between the first and second insulated wires by convection. In this way, damage to the insulation portions adjacent the exposed portions is minimised or prevented. According to an aspect of the invention there is provided a method of manufacturing an electric motor for a vehicle. The method may include forming part of a winding for a stator of the electric motor by joining the first insulated wire to the second insulated wire using the laser welding method of any of the preceding aspects of the invention. Advantageously, the method enables an electric motor to be manufactured with greater speed and accuracy. According to an aspect of the invention there is provided a system for implementing the laser welding method of any of the preceding aspects of the invention. The system advantageously enables the laser welding method to be performed more efficiently and / or more accurately. According to an aspect of the invention there is provided computer readable instructions which, when read by a computer, implement the laser welding method according to the laser welding method of any of the preceding aspects of the invention. The computer readable instructions may be stored on a computer readable medium. The computer readable instructions may be non-transitory. The computer readable instructions enable the laser welding method to be performed under the control of a computer. According to an aspect of the present invention there is provided laser welding method for joining two insulated wires of a coil of an electric motor. The method may include providing a first insulated wire having a first exposed portion and a second insulated wire having a second exposed portion. The method may include bending the first exposed portion and the second exposed portion. The first and second exposed portions may be bent adjacent an insulated portion of the respective insulated wire. All surfaces of the exposed portion, within a projection of a cross-section of the exposed portion at the end of the exposed portion may thus be exposed. The first exposed portion and the second exposed portion may be clamped together, for example such that the exposed portions are substantially aligned with one another. The insulated portions may extend away from one another. The laser welding method may include applying a laser beam to the first exposed portion and / or the second exposed portion in order to form a weld joint between the first exposed portion and the second exposed portion. Bending the exposed portion of at least one of the insulated wires ensures that the insulated portions of the first and second insulated wires are spaced apart and out of the ‘line of sight’ of the laser. This hinders the transfer of heat between the first and second wires by convection and thus minimises and / or prevents heat damage to the insulation on the first and second insulated wires. According to another aspect of the invention there is provided a laser welding method for joining two insulated wires of a coil of an electric motor. The method includes providing a first insulated wire and providing a second insulated wire. A portion of insulation is removed from the first insulated wire, thereby forming a first exposed portion on the first insulated wire. A portion of insulation is removed from the second insulated wire, thereby forming a second exposed portion on the second insulated wire. The exposed portion of each of the first insulated wire and the second insulated wires are bent adjacent an insulated portion, such that all surfaces of the exposed portion, within a projection of a cross-section of the exposed portion at the end of the exposed portion, are exposed. The exposed portion of the first insulated wire and the exposed portion of the second insulated wire are clamped together such that the exposed portions are substantially aligned with one another and the insulated portions extend away from one another. A laser beam is applied to the exposed portions of each of the first insulated wire and the second insulated wire, thereby forming a weld joint between the exposed portion of the first insulated wire and the exposed portion of the second insulated wire. Bending the exposed portion of at least one of the insulated wires ensures that the insulated portions of the first and second insulated wires are spaced apart and out of the 'line of sight’ of the laser. This hinders the transfer of heat between the first and second wires by convection and thus minimises and / or prevents heat damage to the insulation on the first and second insulated wires. The exposed portions may be substantially aligned with one another in a first direction. The insulated portions may be separated from one another in a second direction. The second direction may be perpendicular to the first direction. The laser welding method may include bending the exposed portion of the first insulated wire, thereby forming a first angle between a welding portion of the exposed portion for providing the weld joint and a part of the insulated portion adjacent the bend in the first insulated wire, and / or bending the exposed portion of the second insulated wire, thereby forming a second angle between a welding portion of the exposed portion for providing the weld joint and a part of the insulated portion adjacent the exposed portion of the second insulated wire. The laser welding method may, for example, include bending the exposed portion of the first insulated wire and / or the exposed portion of the second insulated wire such that the first angle and / or the second angle is at least 120°, for example at least 129°.The laser welding method may, for example, include bending the exposed portion of the first insulated wire and / or the exposed portion of the second insulated wire such that the first angle and / or the second angle is less than 150°, for example less than 144°. Having a first and / or second angle between approximately 120° and approximately 144° has been found to be particularly beneficially for reducing the transfer of heat between the first and second insulated wires by convection, thereby minimising and / or preventing heat damage to the insulation on the insulated portions of the first and second insulated wires. The first angle may be the same as the second angle. Alternatively, the first angle may be bigger than the second angle. In other examples, the first angle may be smaller than the second angle. The angles between the welding portion and a part of the insulated portion adjacent the bend in the respective insulated wire prevents the transfer of heat between the insulated wires by convection and thus contribute to minimising and / or preventing heat damage to the insulation of the first and second wires. The laser welding method may include providing a clamp device. The clamp device may include a material having a thermal conductivity of at least 100 W / mK. The clamp device may be used to clamp the exposed portion of the first insulated wire and the exposed portion of the second insulated wire together. The clamp device contacts the exposed portions of the first and second insulated wires, creating a preferential path for thermal energy from the exposed portions of each of the first and second insulated wires. The clamp device includes a material having a high thermal conductivity, which enables the clamp device to act as a heat sink and draw heat away from the insulation of each of the first and second insulated wires. In this way, heat damage to the insulation of the first and second wires is minimised or prevented. The clamp device may include a first clamp member and a second clamp member. The laser welding method may include urging one of the first clamp member or the second clamp member in a first direction toward the first insulated wire. Additionally or alternatively, the laser welding method may include urging the other of the first clamp member and the second clamp member in an opposing second direction toward the second insulated wire. Either or both of these steps may result in the clamp device holding the exposed portion of the first insulated wire and the exposed portion of the second insulated wire between the first clamp member and the second clamp member. The contact between each of the first and second clamp members and the exposed portions of the first and second insulated wires contributes to the thermal path that allows thermal energy from the exposed portions of each of the wires to be transferred to the clamp device in preference to the insulation of each of the wires, thereby minimising or preventing damage to the insulation on either or both of the insulated wires. The laser welding method may include sliding the one of the first clamp member or the second clamp member in the first direction toward the first insulated wire, and sliding the other of the first clamp member or the second clamp member in the opposing second direction toward the second insulated wire. The sliding arrangement of the first and second clamp members provides a compact and efficient clamping device. The first clamp member may have a planar surface. The second clamp member may have a planar surface. The clamp device may be configured such that the planar surface of the first clamp member faces the planar surface of the second clamp member. The laser welding method may include sliding the planar surface of the one of the first clamp member or the second clamp member relative to the planar surface of the other of the first clamp member or the second clamp member in the first direction toward the first insulated wire, and sliding the planar surface of the other of the first clamp member or the second clamp member relative to the planar surface of the one of the first clamp member or the second clamp member in the second direction toward the second insulated wire. The planar surfaces of the first and second clamp members provide a large surface area for the transfer of heat to the clamp device in preference to the insulation of the insulated wires. The laser welding method may include urging the first clamp member in a third direction to contact the second clamp member. The third direction may be perpendicular to the first direction and / or the second direction. The contact between the first and second clamp members further contributes to the thermal path that allows thermal energy from the exposed portions of each of the wires to be transferred to the clamp device in preference to the insulation of the insulated wires by conduction. The exposed portion of either or both of the first insulated wire and the second insulated wire may have a length of at least 8 mm. The exposed portion of either or both of the first insulated wire and the second insulated wire may have a length of less than 14 mm. Having an exposed portion between approximately 8 mm and approximately 14 mm in length has been found to be particularly advantageous for ensuring that damage to the insulation portion adjacent to the exposed portion is prevented or at least minimised. The first insulated wire and / or the second insulated wire may be a hairpin or an l-pin for a stator winding coil. According to another aspect of the invention there is provided a method of manufacturing an electric motor for a vehicle. The method may include forming part of a winding for a stator of the electric motor by joining the first insulated wire to the second insulated wire using the laser welding method of any of the preceding aspects of the invention. According to another aspect of the invention there is provided a system for implementing the laser welding method according to any of the preceding aspects of the invention. The system may comprise a clamping member configured for use in performing the laser welding method according to any of the preceding aspects of the invention. The system advantageously enables the laser welding method to be performed more efficiently and / or more accurately. According to a further aspect of the invention there is provided computer readable instructions which, when read by a computer, implement a laser welding method according to any of the preceding aspects of the invention. The computer readable instructions may be for controlling a laser welding system comprising a clamping member, which when read by a computer, cause the laser welding system to implement a laser welding method according to any of the preceding aspects of the invention. The computer readable instructions may be stored on a computer readable medium. The computer readable instructions may be non-transitory. The computer readable instructions enable the laser welding method to be performed under the control of a computer. 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 One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: FIG. 1A shows a stator and a coil for an electric motor in accordance with an embodiment of the invention; FIG. 1B shows an insulated wire for the coil of FIG. 1 A; FIG. 1C shows an alternative insulated wire for the coil of FIG. 1A; FIG. 2 shows a flow chart for a laser welding method for joining two insulated wires of a coil of an electric motor in accordance with an embodiment of the invention; FIG. 3 shows a clamp device for use in the laser welding method of FIG. 2; FIG. 4 shows a flow chart for a laser welding method for joining two insulated wires of a coil of an electric motor in accordance with an embodiment of the invention; FIG. 5 shows a schematic representation of two insulated wires that have been joined using the laser welding method of FIG. 4; and FIG. 6 shows a vehicle having an electric motor in accordance with an embodiment of the invention. DETAILED DESCRIPTION Laser welding methods in accordance with embodiments of the present invention are described herein. The laser welding method is for joining two insulated wires of a coil of an electric motor, for example an electric motor including a stator, as shown in FIG. 1 A. The stator includes a coil having a number of insulated wires, e.g. insulated copper wires, in the form of hairpins, for example a hairpin as shown in FIG. 1B. The laser welding method of the embodiment of FIG. 2 includes a clamp device an example of which is shown in FIG. 3. The laser welding method of the embodiment of FIG. 4 will be described with reference to FIG. 5. The electric motor may be included in a vehicle as shown in FIG. 6. Referring now to FIG. 1A, there is shown a stator 100. The stator 100 has a stator housing 102 and a coil 104. The coil 104 includes a number of insulated wires 106. The insulated wires 106, which will be described in more detail with reference to FIG. 1B, are formed from lengths of copper having a rectangular cross section and which are coated with an insulating enamel or varnish, for example polyamide-imides (PAI), polyether ether ketone (PEEK) orpolyamide-imides with polyimide foil (PAI-FEP). Each insulated wire 106 is stripped to expose a length of wire between 8 and 10 mm (an exposed portion, as will be described in more detail below), cut to length andbent into either a hairpin 108, as shown in FIG. 1B oran l-pin 124, as shown in FIG. 1C. The insulated wires 106 are inserted axially into the stator housing 102 as a hairpin basket. The ends of the insulated wires 106 are twisted in accordance with a winding scheme. To form the winding scheme, the ends of the insulated wires 106 are electrically connected, for example by a laser welding method according to an embodiment of the present invention. After the welding process, the ends of the insulated wires 106 are re-insulated and the entire stator 100 is impregnated, for example with a resin, to protect the stator 100 from thermal, electrical and mechanical events. FIG. 1B shows an example of an insulated wire 106 that has been bent into a hairpin 108. The insulated wire 106 is a continuous length of material and has a first arm 110, a second arm 112 and a V- or U-shaped portion 114, wherein the V- or U-shaped portion 114 is between the first arm 110 and the second arm 112. The continuous length of material from which the hairpin 108 is formed has a first end 116 at an end of the first arm 110 which is distal to the V- or U-shaped portion 114 and a second end 118 at an end of the second arm 112 which is distal to the V- or U-shaped portion 114. A portion of the insulation is removed, e.g. by stripping, from each of the first end 116 and the second end 118 of the hairpin 108, thereby forming exposed portions 120, 122 of the insulated wire 106. In each of the exposed portions 120, 122, the copper wire is exposed. Each of the exposed portions 120, 122 has a length that is defined by the distance between the end of the first arm 110 or the end of the second arm 112 and the edge at which the insulation meets the exposed portion 120,122. FIG. 1C showsan example of an insulated wire 106 that has been cut into an l-pin 124. The insulated wire 106 has an arm 126. A portion of the insulation is removed at one end of the l-pin 124, thereby forming an exposed portion 128 of the insulated wire 106. At the exposed portion 128, the copper wire is exposed. The exposed portion 128 has a length 130 that is defined by the distance between the end of the arm 126 and the edge at which the insulation meets the exposed portion 128. The length 130 of the exposed portion 128 is approximately 10 mm. It will be appreciated that the lengths of the exposed portions 120,122 may also be approximately 10 mm. A laser welding method 200 according to an embodiment of the invention will now be described with reference to FIG. 2. In a first step 202, a continuous length of insulated copper wire is provided. Portions of insulation are removed along the length of the copper wire to form the exposed portions 120,122 (step 204). A first insulated wire 106 is formed in step 206 by cutting a first length of copper wire in the exposed portion and bending it in two dimensions to form a first hairpin 108. Similarly, a second insulated wire 106 is formed by cutting a second length of copper wire in an exposed portion and bending it in two dimensions to form a second hairpin 108. In step 208, each of the first hairpin 108 and the second hairpin 108 is inserted into the slots of a stator 100. In step 210, a clamp device comprising a material having a thermal conductivity of at least 100 W / mK is provided. The clamp device 300 is used to clamp together the exposed portion 120, 122 at one end, e.g. the first end 116 of the insulated wire 106 of the first hairpin 108 and the exposed portion 120,122 at one end, e.g. the second end 118 of the insulated wire 106 of the second hairpin 108. In step 212, a laser beam is applied to the exposed portions 120,122 at the one end of the insulated wire 106 of the first hairpin 108 and the exposed portions 120, 122 at the one end of the insulated wire 106 of the second hairpin 108, thereby forming a weld joint between the exposed portion 120,122 of the first hairpin 108 and the exposed portion 120,122 of the second hairpin 108. An example of such a weld joint 502 is shown in FIG. 5, in which an exposed portion 120 of the first insulated wire 106 is welded to an exposed portion 122 of the second insulated wire 106. The clamp device contacts the exposed portions 120, 122 of the insulated wires 106 of the first hairpin 108 and the second hairpin 108, creating a preferential path for thermal energy from the exposed portions 120,122 of each of the first hairpin 108 and the second hairpin 108. This hinders or prevents heat transfer from the exposed portions 120,122 to the insulation surrounding each of the first hairpin 108 and the second hairpin 108. The inclusion of a material having a high thermal conductivity in the clamp device advantageously enables the clamp device to act as a heat sink and draw heat away from the insulation of each of the first hairpin 108 and the second hairpin 108. In this way, the laser welding method 200 advantageously minimises or prevents heat damage to the insulation of the hairpins 108 within the stator 100. An example of a clamp device for use in the laser welding method 200 will now be described with reference to FIG. 3. The clamp device 300 has a first clamp member 302 and a second clamp member 304. In step 212 of the laser welding method 200 of FIG. 2, the first clamp member 302 is urged in a first direction 306 toward the first insulated wire 106 and the second clamp member 304 is urged in an opposing second direction 308 toward the second insulated wire 106. In this way, the exposed portion 120,122 ofthe first hairpin 108 and the exposed portion 120,122 of the second hairpin 108 are held between the first clamp member 302 and the second clamp member 304. The contact between each of the first clamp member 302 and the second clamp member 304 and the exposed portions 120, 122 of the first hairpin 108 and the second hairpin 108 contributes to the path that allows thermal energy from the exposed portions 120,122 of each of the first hairpin 108 and the second hairpin 108 to be transferred by conduction to the clamp device 300 in preference to the insulation of each of the first hairpin 108 and the second hairpin 108. In an embodiment of the invention, the first clamp member 302 and the second clamp member 304 are generally planar and slide relative to each other in the first direction 306 and the second direction 308, respectively. The first clamp member 302 has a first planar surface 310 and the second clamp member 304 has a second planar surface 312. The first planar surface 310 of the first clamp member 302 faces the second planar surface 312 of the second clamp member 304 such that the first and second planar surfaces 310, 312 slide relative to each other in their respective first and second directions 306, 308. The sliding arrangement of the first clamp member 302 and the second clamp member 304 provides a compact and efficient clamp device 300. The first clamp member 302 may be urged toward the second clamp member 304 in a third direction 314, wherein the third direction 314 is perpendicular to the first direction 306 and the second direction 308. In this way, the first planar surface 310 of the first clamp member 302 is brought into contact with the second planar surface 312 of the second clamp member 304. The contact between the first planar surface 310 of the first clamp member 302 and the second planar surface 312 of the second clamp member 304 further contributes to the thermal path that allows thermal energy from the exposed portions 120,122 of each of the hairpins 108 to be transferred to the clamp device 300 in preference to the insulation of either of the hairpins 108. The first clamp member 302 and the second clamp member 304 each include a through-hole 316. In the laser welding method 200, the exposed portions 120,122 at the ends of the first hairpin 108 and the second hairpin 108 is passed through the through-holes 316 of each of the first clamp member 302 and the second clamp member 304. In this way, the exposed portions 120,122 of the hairpins 108 are spaced apart from, or proud of, the first clamp member 302 and the second clamp member 304. The contact between the walls of the through-holes 316 in the first clamp member 302 and the second clamp member 304 and the exposed portions 120,122 of the first and second hairpins 108 further contributes to the thermal path that allows thermal energy from the exposed portions 120,122 of each of the hairpins 108 to be transferred to the clamp device 300 in preference to the insulation of the first and second hairpins 108. The first clamp member 302 and / or the second clamp member 304 of the clamp device 300 may include a material, e.g. copper or a copper alloy, having a high thermal conductivity, e.g. having a thermal conductivity of at least 100 W / mK. This enables the clamp device 300 to act as a heat sink and draw heat away from the insulation of each of the first and second insulated wires 106. In this way, heat damage to the insulation of the first and second hairpins 108 is minimised or prevented. A laser welding method CO according to another embodiment of the invention will now be described with reference to FIG. 4. In a first step 402, a continuous length of insulated copper wire is provided. Portions of insulation are removed along the length to form the exposed portions 120,122,128 (step 404). A first insulated wire 106 is formed in step 406 by cutting a first length of copper wire in the exposed portion and bending it in two dimensions to form a first hairpin 108. Similarly, a second insulated wire 106 is formed by cutting a second length of copper wire in the exposed portion and bending it in two dimensions to form a second hairpin 108. In step 408, each of the first and second hairpins 108 is bent in a third dimension. In more detail, the exposed portion 120,122 at one end of the first hairpin 108 and the exposed portion 120,122 at one end of the second hairpin 108 are each bent adjacent an insulated portion of the respective hairpin 108, such that all surfaces of the exposed portions 120,122, within a projection of a cross-section of the exposed portion 120,122 at the end of the exposed portion 120,122, are exposed. In step 410, the exposed portion 120, 122 of the insulated wire 106 of the first hairpin 108 and the exposed portion 120,122 of the insulated wire 106 of the second hairpin 108 are clamped together such that the exposed portions 120,122 are substantially aligned with one another and the insulated portions extend away from one another. A laser beam is then applied to the exposed portions 120,122 of each of the insulated wire 106 of the first hairpin 108 and the insulated wire 106 of the second hairpin 108 (step 412). In this way a weld joint, e.g. a weld joint 502 as shown in FIG. 5) is formed between the exposed portion 120,122 of the insulated wire 106 of the first hairpin 108 and the exposed portion 120,122 of the insulated wire 106 of the second hairpin 108. The exposed portions 120,122 of the first hairpin 108 and the second hairpin 108 are thus substantially aligned with one another in a first direction. The insulated portions of the first hairpin 108 and the second hairpin 108 are separated from one another in a second direction, wherein the second direction is perpendicular to the first direction. Bending the exposed portions 120,122 of the first insulated wire 106 and the second insulated wire 106 ensures that the insulation of the two insulated wires 106 is out of the 'line of sight’ of the laser. This further hinders the transfer of heat between the insulated wires 106 by convection and thus further minimises or prevents heat damage to the insulation on the first and second insulated wires 106. Step 408 of the laser welding method 400 will be further described with reference to FIG. 5. The exposed portion 120 of the first insulated wire 106 is bent such that a first angle 504 is formed between a welding portion 508 of the exposed portion 120 for providing the weld joint 502 and a part of the insulated portion 510 adjacent to the bend in the first insulated wire 106. Similarly, the exposed portion 122 of the second insulated wire 106 is bent such that a second angle 506 is formed between a welding portion 512 of the exposed portion 122 for providing the weld joint 502 and a part of the insulated portion 514 adjacent to the bend in the second insulated wire 106. The first angle 504 and the second angle 506 are, in this embodiment, the same and are approximately 140 degrees. Referring now to FIG. 6 there is shown a vehicle 600. The vehicle 600 has an electric motor 602 including a stator 100. The core of the stator 100 is manufactured using a laser welding method 200, 400 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. It will, therefore, be appreciated that the clamp device 300 may be used in the laser welding method 200, which may be modified to include the bending step 410 of the laser welding method 400 to reduce the risk of, or prevent, thermal damage to the insulation of the insulated wires 106 during assembly of the stator 100. In the embodiments described above, the lengths of the exposed portions 120, 122, 128 are approximately 10 mm. It will be appreciated that, in other embodiments of the invention, the lengths of the exposed portions of the insulated wires may be varied. The lengths of the exposed portions may be, for example, between approximately 6 mm and approximately 14 mm. In some examples of the invention, the lengths of the exposed portions may be between approximately 8 mm and approximately 10 mm. In the methods described above, first and second hairpins 108 are formed by cutting a length of insulated copper wire in the exposed portions and bending it in 2 dimensions. It will be appreciated that, in alternative embodiments of the invention, first and second l-pins 124 may be formed by cutting a length of insulated copper wire in the exposed portions. In other words, the 2 dimensional bending step may be omitted if l-pins are required instead of hairpins. The laser welding methods 200,400 are described in relation to the welding of one end of a first hairpin 108 to one end of a second hairpin 108. It will be appreciated that the stator 100 includes a plurality of hairpins 108 and thus methods according to the invention are applied to each of the hairpins 108 in the stator 100 to ensure that the hairpins 108 are electrically connected. Similarly, it will be 5 appreciated that the stator may include a plurality of l-pins and that the l-pins of the stator may be laser welded using the laser welding method 200. Use of the clamp device 300 in a laser welding method for welding the exposed portion of a first hairpin 108 to the exposed portion of a second hairpin 108 is described. It will be appreciated that the clamp device 300 is one example of a clamp device that may be 10 used in laser welding methods according to the present invention. Since the stator 100 includes a plurality of hairpins 108, the clamp device may be configured to clamp any number of hairpins 108 to ensure that the laser welding method can be used to electrically connect each of the hairpins 108 in the stator. Similarly, it will be appreciated that the stator may include a plurality of l-pins and that the clamp device may be configured to clamp a plurality of l-pins so that they can be welded together using a laser welding method according to the present invention. 15

Claims

1. A laser welding method for joining two insulated wires of a coil of an electric motor, the method comprising:providing a first insulated wire;providing a second insulated wire;removing a portion of insulation from the first insulated wire, thereby forming an exposed portion on the first insulated wire;removing a portion of insulation from the second insulated wire, thereby forming an exposed portion on the second insulated wire;providing a clamp device comprising a material having a thermal conductivity of at least 100 W / mK;clamping together, using the clamp device, the exposed portion of the first insulated wire and the exposed portion of the second insulated wire; andapplying a laser beam to the exposed portions of each of the first insulated wire and the second insulated wire, thereby forming a weld joint between the exposed portion of the first insulated wire and the exposed portion of the second insulated wire,2. The laser welding method of claim 1, wherein the clamp device comprises a first clamp member and a second clamp member, and the method comprises:urging one of the first clamp member or the second clamp member in a first direction toward the first insulated wire; andurging the other of the first clamp member or the second clamp member in an opposing second direction toward the second insulated wire;thereby holding the exposed portion of the first insulated wire and the exposed portion of the second insulated wire between the first clamp member and the second clamp member.

3. The laser welding method of claim 2, the method comprising:sliding the one of the first clamp member or the second clamp member in the first direction toward the first insulated wire; andsliding the other of the first clamp member or the second clamp member in the opposing second direction toward the second insulated wire.

4. The laser welding method of claim 3, wherein the first clamp member has a planar surface and the second clamp member has a planar surface, and the clamp device is configured such that the planar surface of the first clamp member faces the planar surface of the second clamp member, and the method comprises:sliding the planar surface of the one of the first clamp member or the second clamp member relative to the planar surface of the other of the first clamp member or the second clamp member in the first direction toward the first insulated wire; andsliding the planar surface of the other of the first clamp member or the second clamp member relative to the planar surface of the one of the first clamp member or the second clamp member in the second direction toward the second insulated wire.5, The laser welding method of any one of claims 2 to 4, the method comprising:urging the first clamp member in a third direction to contact the second clamp member, wherein the third direction is perpendicular to the first direction and / or the second direction.

6. The laser welding method of any one of claims 2 to 5, wherein each of the first clamp member and the second clamp member includes a through-hole, the method comprising:passing each of the first insulated wire and the second insulated wire through each of the through-hole of the first clamp member and the through-hole of the second clamp member.

7. The laser welding method of any one of claims 2 to 6, wherein the first clamp member and / or the second clamp member comprises the material having a thermal conductivity of at least 100 W / mK.

8. The laser welding method of any one of claims 2 to 7, wherein the material of the clamp device comprises copper or a copper alloy.

9. The laser welding method of any one of claims 2 to 8, the method comprising:bending the exposed portion of each of the first insulated wire and the second insulated wire adjacent an insulated portion, such that all surfaces of the exposed portion, within a projection of a cross-section of the exposed portion at the end of the exposed portion, are exposed.

10. The laser welding method of claim 9, the method comprising:bending the exposed portion of the first insulated wire, thereby forming a first angle between a welding portion of the exposed portion for providing the weld joint and a part of the insulated portion adjacent the bend in the first insulated wire; andbending the exposed portion of the second insulated wire, thereby forming a second angle between a welding portion of the exposed portion for providing the weld joint and a part of the insulated portion adjacent the bend in the second insulated wire.

11. The laser welding method of claim 9 or 10, the method comprising:clamping the first insulated wire relative to the second insulated wire such that the angle between the insulated portions adjacent the exposed portions of the first insulated wire and the second insulated wire is at least 90°.

12. The laser welding method of any one of claims 1 to 11, wherein the first insulated wire and / or the second insulated wire is a hairpin or an l-pin for a coil of a stator.

13. A method of manufacturing an electric motor for a vehicle, the method comprising:forming part of a coil for a stator of the electric motor by joining the first insulated wire to the second insulated wire using the laser welding method of any one of claims 1 to 12.

14. A system comprising a clamping member configured for use in performing the laser welding method of any one of claims 1 to 12.

15. Computer readable instructions for controlling a laser welding system comprising a clamping member, which when read by a computer, implement a laser welding method according to any one of claims 1 to 12.5

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