Method for fastening a wire to a connection element

Abstract

The invention relates to a method for fastening a wire (10) to a connection element (12), in which method the strain-hardened wire (10) produced from a copper alloy containing at least 70% copper, in particular at least 75% copper, is integrally joined (V2) to the connection element (12) by applying heat, wherein an elongation at break of the wire (10) after strain hardening and before joining is set to a value (V1) which is greater than or equal to a predefined minimum elongation at break.

Description

[0001] Method for attaching a wire to a connecting element

[0002] The present invention relates to a method for attaching a wire to a connecting element.

[0003] DE 10 2007 021 321 A1 discloses a stator of an electrical machine with multiple coils interconnected on one side of the stator. The stator comprises at least one busbar, to which two of the multiple coils are interconnected by heat-crimping or welding the ends of the busbar to one end of each of the coils.

[0004] Furthermore, DE 10 2017 205 078 A1 discloses a spring clip for securing at least one electrical line to a connection element. The at least one electrical line and the connection element are form-fitting and electrically connectable to each other.

[0005] The object of the present invention is to provide a solution which enables a particularly durable connection of a wire to a connecting element.

[0006] This object is achieved according to the invention by the subject matter of the independent claim. Further possible embodiments of the invention are disclosed in the subclaims, the description, and the figures.

[0007] The invention relates to a method for attaching a wire to a connecting element. The wire is, in particular, a thin, long, flexible component made of metal. The wire has, in particular, a circular cross-section. Alternative cross-sectional shapes are also possible. The wire is, in particular, electrically conductive. The connection between the wire and the connecting element establishes an electrical connection between the wire and the connecting element. An electrical connection can thus be provided via the connection between the wire and the connecting element. For example, the wire can be a conductor of an electrical machine, in particular an electric traction machine for a motor vehicle, the conductor being connected to a voltage source or a load by being connected to the connecting element.

[0008] The method provides for a cold-worked wire made of a copper alloy with at least 70% copper, in particular with at least 75% copper, to be joined to the connecting element in a material-to-material bond using heat. The wire in particular has a copper content of no more than 99.9%. The material from which the wire is made is therefore a low-alloy copper compound and not pure copper. It has been shown that a particularly high strength of the connection between the wire and the connecting element can be achieved through material-to-material joining if the elongation at break of the wire after cold-worked processing and before joining is set to a value that is greater than or equal to a predetermined minimum elongation at break. This minimum elongation at break can, for example, be predetermined depending on the material of the wire.If the elongation at break of the wire after work hardening and before joining is below the minimum elongation at break specified for the wire material, this may result in the joined wire having only a particularly low strength and, as a result, the connection between the wire and the connecting element having a particularly low strength.

[0009] The principle of work hardening is based on the introduction of additional dislocations into a material during plastic deformation. With every plastic deformation process, new dislocations are always introduced into the material. The dislocations prevent each other from moving, which increases the strength. Due to work hardening, plastic deformation of the material only occurs at higher stress values. This means that the work-hardened material has an increased yield strength. By setting the elongation at break of the wire before joining to a value that is greater than or equal to the minimum elongation at break, the particularly high strength of the wire can be achieved by joining the wire and, as a result, the particularly high fatigue strength of the connection between the wire and the connecting element can be achieved.

[0010] It is possible for a winding wire of an active part of an electrical machine, in particular a rotor, to be attached to the connection element as the wire. Alternatively, the active part can be a stator of the electrical machine. In particular, the winding wire is part of a coil component of the electrical machine. In particular, the connection element can be held on a switching ring of the rotor. The described method thus enables the winding wire to be connected to the switching ring of the rotor in a particularly secure and electrically conductive manner. The wire is therefore the electrical conductor of the rotor, which is wound into a coil. The particularly durable connection of the winding wire to the connection element and, via the connection element, to the switching ring of the rotor enables a particularly long service life of the rotor to be achieved.

[0011] In a possible development of the invention, the elongation at break of the wire is set to at least 5% before joining. Elongation at break is a material science parameter that indicates the remaining extension of a tensile specimen after break, relative to an initial gauge length. Elongation at break characterizes the deformability or ductility of a material. A value of 5% can therefore be specified as the minimum elongation at break for any wire material. It has been found that, provided the remaining elongation at break of the wire after work hardening and before joining is at least 5%, a particularly high strength of the wire can be achieved after joining. If the work hardening in the wire is too high before the joining process and thus the elongation at break of the wire before joining is less than 5%, the strength of the joined wire drops disproportionately due to the heat input during joining.It is therefore provided that the work hardening of the wire is terminated at the latest when the elongation at break has reached a value of 5%, in order to prevent the elongation at break from falling below this value. Alternatively, the work hardening can be at least partially reversed by heat treatment of the work-hardened wire in order to increase the elongation at break of the wire to at least 5%. In a further possible embodiment of the invention, it is provided that a copper-zirconium wire, for example CuZrO.1, or a copper-chromium wire, for example CuCr, or a copper-chromium-zirconium wire CuCrZr is used as the wire. In this case, the respective wire has a conductivity of 75 to 98% according to IACS. In particular, the CuZrO.1 wire is alloyed with only 0.1% Zr.Due to their properties, particularly with regard to electrical conductivity, these wires are particularly well-suited for use as conductors in the active parts of electric traction machines. Using copper-zirconium wire, copper-chromium wire, or copper-chromium-zirconium wire, a particularly good electrical conductivity and simultaneously high-strength connection between the wire and the connecting element can be achieved.

[0012] In a further possible embodiment of the invention, it is provided that the wire is cold-formed for work hardening and, during this time, is subjected to a heat treatment to adjust the elongation at break, or alternatively, the already work-hardened wire is subjected to a heat treatment to adjust the elongation at break. In other words, work hardening can be part of the method according to the invention and the wire can be cold-formed for work hardening. To ensure that the elongation at break of the wire after work hardening and before joining does not fall below the predetermined minimum elongation at break, the wire can be subjected to a heat treatment during work hardening to adjust the elongation at break. For example, the wire can be alternately cold-formed and subsequently heat-treated for work hardening until a predetermined wire shape is achieved.For example, it is possible that the wire is drawn out elongated for work hardening, whereby the diameter of the wire is reduced through cold forming. The wire can then be subjected to heat treatment. The wire can then be drawn out elongated again as part of cold forming in order to further reduce the diameter of the wire. It is possible that the wire is then subjected to further heat treatment. In particular, the wire is drawn out elongated for as long or as often as necessary until the diameter of the wire corresponds to a predetermined diameter for the wire. The respective heat treatment can partially reverse the hardening of the wire through cold forming and thus increase the elongation at break of the wire. If the wire is cold formed for work hardening, the wire is plastically deformed below its recrystallization temperature.The resulting hardening increases the material's strength. The recrystallization temperature is approximately 40 to 50% of the absolute melting temperature and depends on the material and the degree of deformation applied. Cold forming can increase strength and reduce ductility, and thus elongation at break.

[0013] Alternatively, it is possible that work hardening is not part of the method according to the invention and that the work-hardened wire is subjected to a heat treatment to adjust the elongation at break. In other words, the work-hardened wire is used and subjected to heat treatment in order to increase the elongation at break of the wire to at least the specified minimum elongation at break, in particular to at least an elongation at break of 5%. For example, the work-hardened wire can be delivered from a supplier to a producer, who manufactures the connection between the wire and the connecting element. The producer can then heat-treat the work-hardened wire to increase the elongation at break of the wire to at least the specified minimum elongation at break.In particular, the heat treatment for adjusting the elongation at break and the heat input for joining are two separate process steps, allowing each of the process steps to be precisely adapted to the respective function of the process. In other words, the heat input during the heat treatment can be optimized such that the elongation at break of the wire before joining is set particularly precisely to a specified value, in particular to a value above the specified minimum elongation at break. The heat input for joining can be selected in such a way that a particularly reliable, material-to-material connection between the wire and the connecting element can be achieved.

[0014] In an alternative possible embodiment of the invention, it is provided that the wire is work hardened as part of the process and that the work hardening is terminated as soon as the wire reaches a predetermined value for the elongation at break. This predetermined value for the elongation at break is in particular above the predetermined minimum elongation at break, in particular above 5%. In this case, heat treatment to adjust the elongation at break can be omitted. For example, if the wire is work hardened by cold forming, the cold forming is terminated as soon as the wire reaches the predetermined value for the elongation at break. This makes it particularly easy and inexpensive to ensure that the elongation at break of the wire before joining is greater than or equal to the predetermined minimum elongation at break.Furthermore, this allows the adjustment of the elongation at break and the work hardening to be carried out particularly quickly, since the work hardening is simply stopped when the specified value for the elongation at break is reached.

[0015] In a further possible embodiment of the invention, it is provided that the wire is hot-crimped and / or soldered and / or welded to the connecting element. Soldering is a thermal process for the material-to-material joining of materials, whereby a liquid phase is created by melting a solder or by diffusion at the interfaces. In particular, the wire can be soft-soldered or hard-soldered to the connecting element. Welding is the joining or fusion of workpieces using heat and / or pressure so that the workpieces form a single unit. In particular, the wire can be laser-welded or electron beam-welded to the connecting element. Crimping is a joining process in which two components are joined together by plastic deformation, for example by flanging, squeezing, crimping, or folding.A crimp connection is only partially removable and can only be replaced with suitable tools during repairs. Crimping enables the creation of a homogeneous, difficult-to-remove electrical connection between a conductor, especially a wire, and a connecting element, especially a terminal. This connection ensures a high level of electrical and mechanical safety.

[0016] Hot or thermal crimping is a technique from the field of resistance welding. A combination of pressure and the introduction of heat creates a permanent connection. The heat energy can be introduced by introducing an electric current into the wire. Alternatively, the process can be carried out without a current flow and only with a heat flow, for example by heating the crimping pliers using induction or a furnace. This means that hot crimping has the side effect that the generated heat burns off any insulating varnish layer that may be present on the wire. This eliminates the need for chemical or mechanical removal of the insulating varnish from the wire before making the connection. Hot crimping therefore enables the wire to be connected to the connecting element particularly quickly, especially when the wire is coated with an insulating varnish.The most important parameters when creating a connection using a hot crimp technique are the pressure with which the crimp connection is pressed, the amount of heat used (e.g., the heat generated when the electrical current is applied due to the electrical or thermal resistance of the material being welded), and the time for which the generated heat acts on the material. The resulting connection is characterized by very high electrical conductivity and low electrical contact resistance.

[0017] In this context, it is particularly intended that the wire is heated during hot crimping for 300 to 1500 milliseconds, in particular for 500 to 1000 milliseconds, to a temperature of 500 °C to 1000 °C, in particular from 650 °C to 900 °C. The stated hot crimp parameters for hot crimping the wire to the connecting element can achieve a particularly high strength of the wire and the connection of the wire to the connecting element after joining.

[0018] In a further possible embodiment of the invention, a wire with a yield strength of at least 420 megapascals is used as the wire. The wire can have a yield strength of at least 420 megapascals, particularly before work hardening and / or after work hardening and before joining. The yield strength of the wire used is at least 420 megapascals, regardless of the elongation at break. This allows for a particularly high fatigue strength of the connection between the wire and the connecting element.

[0019] Further features of the invention may emerge from the claims, the figures, and the description of the figures. The features and combinations of features mentioned above in the description, as well as the features and combinations of features shown below in the description of the figures and / or in the figures alone, can be used not only in the respective combinations specified, but also in other combinations or on their own, without departing from the scope of the invention.

[0020] The drawing shows:

[0021] Fig. 1 is a process diagram for a method for attaching a wire to a connecting element, and Fig. 2 is a schematic side view of a wire inserted in a recess of a connecting element during joining of the wire to the connecting element.

[0022] In the figures, identical and functionally identical elements are provided with the same reference numerals.

[0023] The drawing shows in Fig. 1 a process diagram for a method for attaching a wire 10 shown in Fig. 2 to a connecting element 12. The wire 10 can in particular be a conductor of a coil component of an electric drive motor of a motor vehicle. The connecting element 12 can be arranged on a switching ring of the electric drive motor. For an electrical connection of the conductor to the switching ring, the wire 10 must therefore be connected, in particular joined, to the connecting element 12. Fig. 2 shows how the wire 10 is inserted into a recess 14 of the connecting element 12. Crimping electrodes 18 of a crimping tool are applied on the outside to respective legs 16 of the connecting element 12 that border the recess 14. By moving the crimping electrodes 18 towards one another, the wire 10 and the connecting element 12 are hot-crimped by means of the crimping tool for a materially bonded connection and thus for joining.

[0024] The wire 10 is made of a copper alloy with at least 70% copper, in particular with at least 75% copper. In the present case, the wire 10 is a copper-zirconium wire or a copper-chromium wire or a copper-chromium-zirconium wire. It is provided that the wire 10 in this case has a yield strength of at least 420 megapascals. In order to ensure that the wire 10 and the connection of the wire 10 to the connecting element 12 have particularly high strength after joining, the method provides that in a first method step V1, an elongation at break of the wire 10 after work hardening of the wire 10 is set to a predetermined value, which is greater than or equal to a predetermined minimum elongation at break. In the present case, 5% is specified as the minimum elongation at break. Subsequently, the wire 10 is joined to the connecting element 12 in the second process step V2, in this case hot-crimped.Alternatively, the wire 10 can be soldered and / or welded to the connecting element 12. During hot crimping, the wire 10 is heated to a temperature of 500°C to 1000°C, in particular 650°C to 900°C, for 300 to 1500 milliseconds, in particular 500 to 1000 milliseconds.

[0025] The method thus provides for the work-hardened wire 10, whose elongation at break is set to a value greater than or equal to the predetermined minimum elongation at break, to be joined to the connecting element 12 in a material-to-material bond. It is possible for the wire 10 to be work-hardened before joining and for the work-hardening to be terminated as soon as the wire 10 reaches a predetermined value for the elongation at break, wherein the predetermined value is greater than or equal to the predetermined minimum elongation at break. Alternatively, it can be provided that the wire 10 is cold-formed for work-hardening and, during this time, is subjected to a heat treatment to set the elongation at break. It is further alternatively possible for the work-hardened wire 10 to be subjected to a heat treatment to set the elongation at break.In other words, either the work hardening of the wire 10 can be stopped as soon as the wire 10 has reached the predetermined value for the elongation at break, or the wire 10 can be subjected to at least one heat treatment during or after the work hardening in order to set the elongation at break of the wire 10 before joining to the predetermined value, which is greater than or equal to the predetermined minimum elongation at break.

[0026] When joining high-strength, alloyed copper wires, such as soldering, crimping, or welding, an energy input can lead to a disproportionate reduction in strength depending on the degree of work hardening of the respective wire 10 and thus to a reduced product service life. Within the scope of the method, an optimum between the degree of work hardening of the alloyed wire 10 and the energy input during joining is utilized to achieve maximum strength of the joint between the wire 10 and the connecting element 12.

[0027] In this case, a specific level of work hardening in the alloyed wire 10 can be set through prior heat treatment. The level of work hardening can be adjusted to the parameters of the joining process, particularly with regard to temperature and time, as well as the content of alloying elements in the copper wire. For hot crimping, an elongation of the wire 10 of at least 5% may be necessary in order to achieve maximum strength of the connection after the joining process. If the work hardening in the wire 10 is too high before the joining process and the elongation at break of the wire 10 is therefore below the specified minimum elongation at break, for example because the elongation at break is below 5%, then the strength of the joined wire 10 drops disproportionately due to the heat input during joining. During work hardening, the wire 10 can be drawn elongated, particularly at room temperature, whereby a diameter is reduced from 5 mm to 2 mm.Subsequently, the wire 10 can be subjected to heat treatment to adjust the elongation at break, during which the wire 10 is heated in particular to a temperature of 400 to 450 °C.

[0028] Overall, the invention shows how a cyclically loaded, high-strength and current-carrying copper connection can be manufactured.

[0029] List of reference symbols

[0030] 10 wire

[0031] 12 connecting element

[0032] 14 Recess

[0033] 16 legs

[0034] 18 Crimp electrode

[0035] V1 to V2 respective process steps

Claims

Patent claims 1. Method for fastening a wire (10) to a connecting element (12), in which the cold-hardened wire (10) made of a copper alloy with at least 70% copper, in particular with at least 75% copper, is joined to the connecting element (12) in a material-to-material manner with the introduction of heat (V2), wherein an elongation at break of the wire (10) after cold-hardening and before joining is set to a value (V1) which is greater than or equal to a predetermined minimum elongation at break.

2. Method according to claim 1, characterized in that the elongation at break of the wire (10) is set to the value which is greater than or equal to the predetermined minimum elongation at break of 5%.

3. Method according to claim 1 or 2, characterized in that a copper-zirconium wire or a copper-chromium wire or a copper-chromium-zirconium wire is used as the wire (10).

4. Method according to one of the preceding claims, characterized in that the wire (10) is cold-formed for the work hardening and is subjected to a heat treatment during this process to adjust the elongation at break, or the work-hardened wire (10) is subjected to a heat treatment to adjust the elongation at break.

5. Method according to one of claims 1 to 3, characterized in that the wire (10) is work hardened and the work hardening is terminated as soon as the wire (10) reaches a predetermined value for the elongation at break.

6. Method according to one of the preceding claims, characterized in that the wire (10) is hot-crimped and / or soldered and / or welded to the connecting element (12).

7. Method according to claim 6, characterized in that the wire (10) is heated during hot crimping for 300 to 1500 milliseconds, in particular for 500 to 1000 milliseconds, to a temperature of 500 °C to 1000 °C, in particular from 650 °C to 900 °C.

8. Method according to one of the preceding claims, characterized in that a wire with a yield strength of at least 420 megapascals is used as the wire (10).

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