Method for attaching a wire to a connecting element

Abstract

The invention relates to a 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.

Description

The present invention relates to a method for fastening a wire to a terminal member.DE 10 2007 021 321 A1 discloses a stator of an electric machine having a plurality of coils which are interconnected with one another on one side of the stator. The stator comprises at least one busbar to which two of the plurality of coils are connected by the ends of the busbar being hot-crimped or welded in each case to one end of one of the coils.Furthermore, DE 10 2017 205 078 A1 discloses a spring clip for fixing at least one electrical line to a connection element. The at least one electrical line and the connection element can be connected to one another in a form-fitting and electrical manner.It is the object of the present invention to provide a solution which enables a particularly durable bonding of a wire to a connection element.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 dependent claims, the description and the figures.The invention relates to a method for fastening a wire to a terminal element. The wire is in particular a thin and long-shaped, flexible component which is produced from a metal. The wire has in particular a circular cross section. Alternative cross-sectional shapes are also possible. The wire is in particular electrically conductive. An electrical connection of the wire to the connection element is established via the connection between the wire and the connection element. An electrical connection can thus be provided via the connection between the wire and the connection element. For example, the wire can be a conductor of an electric machine, in particular an electric traction machine for a motor vehicle, wherein the conductor is connected to a voltage source or a load by connecting to the connection element.In the method, it is provided that a work-hardened wire made of a copper alloy with at least 70% copper, in particular with at least 75% copper, is joined to the connection element in a materially bonded manner with the introduction of heat as the wire. The wire has in particular a copper content of at most 99.9%. Thus, the material from which the wire is made is a low alloy copper compound and thus not pure copper. It has been found that a particularly high strength of the connection of the wire to the connection element can be achieved by the material-bonded joining if an elongation at break of the wire is set after the cold-bonding and before the joining to a value which is greater than or equal to a predefined minimum elongation at break. This minimum breaking extension can be predefined, for example, depending on the material of the wire. If the breaking elongation of the wire after the cold-setting and before the joining is below the minimum breaking elongation predetermined for the material of the wire, then it can result that the joined wire only has a particularly low strength and as a result there is a particularly low strength of the connection of the wire to the connection element.The principle of work hardening is based on the introduction of additional dislocations into a material during plastic deformation. During each plastic deformation process, new dislocations are always also introduced into the material. The dislocations interfere with each other in the migration, which results in a strength-enhancing effect. As a result of the work hardening, a 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 breaking elongation of the wire before joining to the value which is greater than or equal to the minimum breaking elongation, the particularly high strength of the wire can be achieved by the joining of the wire and, as a result, the particularly high fatigue strength of the connection of the wire to the connection element can be achieved.It is possible for a winding wire of an active part of an electric machine, in particular of a rotor, to be fastened to the connection element as the wire. Alternatively, the active part can be a stator of the electric machine. In particular, the winding wire is part of a coil component of the electric machine. In particular, the connection element can be held on a switching ring of the rotor. The method described thus enables the winding wire to be connected particularly securely and in an electrically conductive manner to the switching ring of the rotor. The wire is thus the electrical conductor of the rotor, which is wound into a coil. By the particularly durable connection of the winding wire to the connection element and via the connection element to the switching ring of the rotor, a particularly long service life of the rotor can be achieved.In a possible development of the invention, it is provided that the breaking elongation of the wire is set to at least 5% before the joining. The elongation at break is a material science characteristic value indicating the remaining elongation of a tensile sample after breaking with respect to an initial gauge length. The elongation at break characterizes the deformability or ductility of a material. The value of 5% can thus be predefined for each material of the wire as the minimum breaking extension. It has been found that, if the remaining breaking elongation of the wire after work hardening and before joining is at least 5%, a particularly high strength of the wire after joining can be achieved. If the work hardening in the wire before the joining process is too high and if the breaking elongation of the wire before the joining is less than 5%, then the strength of the joined wire decreases disproportionately strongly due to the heat input during the joining. It is thus provided that the work hardening of the wire is ended at the latest when the breaking elongation has reached the value of 5%, in order to prevent the breaking elongation from dropping below the value of 5%. Alternatively, work hardening may be at least partially reversed by heat treating the work hardening wire to increase the breaking elongation 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 CuZr0.1, 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 according to IACS has a conductivity of 75 to 98%. In particular, the CuZr0.1 wire is alloyed only with 0.1% Zr. Because of their properties, in particular with regard to electrical conductivity, the aforementioned wires are materials which are particularly well suited for use as conductors in active parts of electric traction machines. By means of the copper zirconium wire or the copper chromium wire or the copper chromium zirconium wire, a particularly highly electrically conductive and at the same time high-strength connection between the wire and the connection element can be achieved.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 for adjusting the breaking elongation or alternatively the already work-hardened wire is subjected to a heat treatment for adjusting the breaking elongation. In other words, the work hardening can be part of the method according to the invention and the wire can be work hardened for work hardening. In order to ensure that the breaking elongation of the wire after the cold-setting and before the joining is not below the predetermined minimum breaking elongation, the wire can be subjected to a heat treatment during the cold-setting for adjusting the breaking elongation. For example, the wire for work hardening can be cold formed alternately and subsequently heat treated until a predetermined shape of the wire is reached. For example, it is possible that the wire is pulled in length for work hardening, thereby reducing a diameter of the wire by work forming. Subsequently, the wire may be subjected to the heat treatment. Subsequently, the wire may be drawn again in length, during cold working, to further reduce the diameter of the wire. It is possible that subsequently the wire is again subjected to a further heat treatment. In particular, the wire is pulled lengthwise until the diameter of the wire corresponds to a predetermined diameter for the wire. By the respective heat treatment, the consolidation of the wire by the cold forming can be partially revised and thus the breaking elongation of the wire can be increased. If the wire is cold formed for the work hardening, then the wire is plastically formed below its recrystallization temperature. The material strength increases as a result of the solidification occurring in the process. The recrystallization temperature is about 40 to 50% of the absolute melting temperature and depends on the material and the degree of forming applied. By cold forming, an increase in strength and a reduction in ductility and thus in breaking elongation can be achieved.Alternatively, it is possible that the work hardening is not part of the method according to the invention and for adjusting the breaking elongation the work hardening wire is subjected to a heat treatment. In other words, the work-hardened wire is used and subjected to the heat treatment in order to raise the breaking elongation of the wire at least to the predetermined minimum breaking elongation, in particular at least to a breaking elongation of 5%. For example, the work-hardened wire may be supplied from a supplier to a producer who connects the wire to the terminal element. The producer can then heat treat the work-hardened wire in order to raise the breaking elongation of the wire at least to the predetermined minimum breaking elongation. In particular, the heat treatment for setting the elongation at break and the heat input for the joining are two separate process steps, as a result of which each of the process steps can be precisely adapted to the respective function of the process. In other words, the heat input of the heat treatment can be optimized in such a way that the breaking elongation of the wire before the joining is adjusted particularly precisely to a predefined value, in particular to a value above the predefined minimum breaking elongation. The heat input for the joining can be selected in particular such that a cohesive connection between the wire and the connection element can be achieved with particularly high reliability.In an alternative possible embodiment of the invention, it is provided that the wire is work-hardened within the scope of the method and work-hardening is ended as soon as the wire reaches a predefined value for the breaking elongation. This predefined value for the elongation at break is in particular above the predefined minimum elongation at break, in particular above 5%. In this case, a heat treatment for adjusting the elongation at break can be dispensed with. For example, if the wire is work-hardened by cold working, the work-hardening is ended as soon as the wire reaches the predetermined value for the breaking elongation. In this way, it can be ensured in a particularly simple manner and with particularly little effort that the breaking elongation of the wire before the joining is greater than or equal to the predetermined minimum breaking elongation. Furthermore, the setting of the breaking elongation and the work hardening can thereby be carried out particularly quickly, since work hardening is simply ended when the predefined value for the breaking elongation is reached.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 connection element. Soldering is a thermal method for the material-bonding joining of materials, wherein a liquid phase is formed by melting a solder or by diffusion at the boundary surfaces. In particular, the wire can be soft soldered or hard soldered to the connection element. Welding is understood to mean joining or fusion of workpieces using heat and / or pressure so that the workpieces form a unit. In particular, the wire can be laser welded or electron beam welded to the connection element. Crimping is understood to mean a joining method in which two components are joined to one another by plastic deformations, for example by crimping, squeezing, crimping or folding. A crimped connection can only be released to a limited extent and can only be renewed with a suitable tool during repairs. Crimping makes it possible to produce a homogeneous electrical connection, which is difficult to release, between a conductor, in particular the wire, and a connecting element, in particular the connection element. This connection ensures high electrical and mechanical safety.Hot or thermocrimping is a technique from the field of resistance welding. A combination of pressure and initiation of heat generation produces a durable bond. The thermal energy may be introduced into the wire by introducing electrical current. Alternatively, it is possible to work without a current flow and only with a heat flow, for example by heating the crimping tongs by induction or an oven. That is to say that hot crimping has the side effect that any insulating lacquer layer of the wire which may be present is burnt off by the heat which is produced. Chemical or mechanical removal of the insulating lacquer from the wire before the connection is made can thus be omitted. The hot crimping thus enables a particularly rapid connection of the wire to the connection element, in particular if the wire is a wire coated with an insulating lacquer.The most important parameters in the production of a connection by means of a hot crimping technique are the pressure with which the crimped connection is pressed, the amount of heat that is used, for example the heat that is generated when the electric current is introduced by the electrical or thermal resistance of the material to be welded, and the time during which the heat that is generated acts on the material. The resulting connection is distinguished by a very high electrical conductivity and a low electrical contact resistance.In this context, it is provided in particular 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 of 650° C. to 900° C. The mentioned hot crimping parameters for the hot crimping of the wire to the connection element make it possible to achieve a particularly high strength of the wire and the connection of the wire to the connection element after the joining.In a further possible embodiment of the invention, it is provided that a wire with a yield strength of at least 420 megapascals is used as the wire. In this case, the wire can have the yield strength of at least 420 megapascals, in particular before the cold-bonding and / or after the cold-bonding and before the joining. The yield strength of the wire used is in particular at least 420 megapascals, independently of the breaking elongation. This makes it possible to achieve a particularly high durability of the connection of the wire to the connection element.Further features of the invention can be derived from the claims, the figures and the description of the figures. The features and combinations of features mentioned above in the description and 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 respectively specified combination but also in other combinations or alone without departing from the scope of the invention.The drawing shows in: FIG. 1 shows a method diagram for a method for fastening a wire to a connection element, and FIG. 2 shows a schematic side view of a wire inserted into a recess of a connection element during joining of the wire to the connection element.In the figures, identical and functionally identical elements are provided with the same reference numerals.The drawing shows in FIG. 1 a method diagram for a method for fastening a wire 10 shown in FIG. 2 to a connection element 12. The connection element 12 can be arranged on a switching ring of the electric drive machine. For an electrical connection of the conductor to the switching ring, the wire 10 is thus to be connected, in particular joined, to the connection element 12. FIG. 2 shows how the wire 10 is inserted into a recess 14 of the connection element 12. On the outside, respective legs 16 of the connection element 12 delimiting the recess 14 are placed against respective crimping electrodes 18 of a crimping pliers. By moving the crimping electrodes 18 towards one another, the wire 10 and the connection element 12 are hot-crimped by means of the crimping pliers for a cohesive connection and thus for the joining.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 the present case has a yield strength of at least 420 megapascals. In order to achieve that the wire 10 and the connection of the wire 10 to the connection element 12 have a particularly high strength after the joining, it is provided in the method that in a first method step V 1 an elongation at break of the wire 10 after a cold-hardening of the wire 10 is set to a predefined value which is greater than or equal to a predefined minimum elongation at break. In the present case, 5% is specified as the minimum breaking elongation. Subsequently, the wire 10 is joined to the connection element 12 in the second method step V 2, in the present case hot crimped. Alternatively, the wire 10 can be soldered and / or welded to the connection element 12. During the hot crimping, the wire 10 is heated to a temperature of 500° C. to 1000° C., in particular of 650° C. to 900° C., for 300 to 1500 milliseconds, in particular for 500 to 1000 milliseconds.In the method, it is thus provided that the work-hardened wire 10, whose breaking elongation is set to the value greater than or equal to the predetermined minimum breaking elongation, is joined to the connection element 12 in a materially bonded manner. It is possible that the wire 10 is work-hardened before joining and work-hardening is ended as soon as the wire 10 reaches a predefined value for the breaking elongation, wherein the predefined value is greater than or equal to the predefined minimum breaking elongation. Alternatively, it can be provided that the wire 10 is cold formed for the work hardening and during this is subjected to a heat treatment for adjusting the breaking elongation. It is also alternatively possible for the work-hardened wire 10 to be subjected to a heat treatment for adjusting 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 breaking elongation, or the wire 10 can be subjected to the at least one heat treatment during or after the work hardening in order to set the breaking elongation of the wire 10 to the predetermined value, which is greater than or equal to the predetermined minimum breaking elongation, before the joining.During the joining, such as soldering, crimping or welding, of high-strength, alloyed copper wires, an energy input depending on a degree of work hardening of the respective wire 10 can lead to a disproportionate reduction in the strength and thus to a reduced product life. In the context of the method, an optimum between the degree of work hardening of the alloyed wire 10 and an energy input during the joining is utilized to achieve a maximum strength of the joint connection of the wire 10 to the connection element 12.In this case, a targeted level of work hardening in the alloyed wire 10 can be set by a pre-arranged heat treatment. The level of work hardening can be matched to parameters of the joining process, in particular with regard to temperature and time, and to a content of alloying elements in the copper wire. For the hot crimping, an elongation at break of the wire 10 of at least 5% may be necessary in order to be able to obtain a 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 breaking elongation of the wire 10 is thus below the predetermined minimum breaking elongation, for example because the breaking elongation is below 5%, then the strength of the joined wire 10 decreases disproportionately strongly as a result of the heat input during the joining.In the context of the work hardening, the wire 10 can be drawn for a long time, in particular at room temperature, as a result of which a diameter of 5 mm is reduced to 2 mm. Subsequently, the wire 10 can be subjected to the heat treatment for adjusting the elongation at break, within the scope of which the wire 10 is heated in particular to a temperature of 400 to 450° C.Overall, the invention shows how a cyclically loaded, high strength and current carrying copper joint can be made.List of reference characters10 Wire 12 Connection element 14 Recess 16 Leg 18 Crimping electrode V 1 to V 2 Respective method stepsReferences included in the specificationThis list of documents cited by the applicant has been produced in an automated manner and is only included for the better information of the reader. The list is not part of the German patent application or utility model application. The DPMA does not take any adhesion for any faults or omissions.Patent Literature citedDE 10 2007 021 321 A1

[0002] DE 10 2017 205 078 A1

[0003]

Claims

Method for fastening a wire (10) to a connection element (12), in which the work-hardened wire (10) made of a copper alloy with at least 70% copper, in particular with at least 75% copper, is joined (V2) by material bonding to the connection element (12) with the introduction of heat, wherein an elongation at break of the wire (10) is set (V1), after the work-hardening and before the joining, to a value which is greater than or equal to a predefined minimum elongation at break.Method according to claim 1, characterised in that the breaking elongation of the wire (10) is set to the value which is greater than or equal to the predetermined minimum breaking elongation of 5%.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).Method according to one of the preceding claims, characterized in that - the wire (10) is cold-formed for the work hardening and during this is subjected to a heat treatment for adjusting the breaking elongation, or - the work-hardened wire (10) is subjected to a heat treatment for adjusting the breaking elongation.Method according to one of Claims 1 to 3, characterized in that the wire (10) is work-hardened and work-hardening is ended as soon as the wire (10) reaches a predefined value for the elongation at break.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 connection element (12).Method according to claim 6, characterised in that the wire (10) is heated during the 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 of 650°C to 900°C.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).

Images