Method for bonding aluminum electrical lines to copper tubing

Abstract

It would be desirable to improve the method of connecting aluminum electrical lines to copper connections in such a way that a long-term stable electrical connection between the line and the connection can be produced quickly and inexpensively with as few quality variations as possible. For this purpose, it is proposed to hard solder the line to the connection in a first step and to soften the aluminum-Litz wire bundle in a second step to effect a diffusion and / or fusion bond with the hard solder layer.

Description

[Technical field]

[0001] The invention relates to a method for connecting an electrical line made of aluminum or an aluminum alloy as a Litz wire to an electrical connection made of copper or a copper alloy, which electrical connection comprises, in addition to a connecting element, a tube for receiving the end of the electrical line, which may be a tube that is open on one or both sides. [Background technology]

[0002] The use of copper or copper alloys as conductors for electric current has been known for a long time. In motor vehicles, especially in e-mobility, it is desired to replace these electrical conductors made of copper or copper alloys with conductors made of aluminum or aluminum alloys for weight reasons. From EP 2 362 491 A or EP 2 621 022 A, it is already known to use aluminum lines which are first crimped and then welded to a connecting element made of another material.

[0003] Such joints, for example when used with aluminum tracks and copper connection elements, present significant problems with regard to the duration of mechanical stability and electrical conductivity of the joint. If moisture penetrates into such joints, contact corrosion occurs, which increases the contact resistance and significantly reduces the mechanical durability of the joint.

[0004] However, the main problem when joining electrical aluminum tracks with connecting elements made of dissimilar materials is that aluminum has a strong affinity for oxygen and is therefore covered within a very short time with an electrically insulating, dense, very hard and very stable oxide layer, also called corundum, which has a melting point of about 2050°C, i.e. significantly higher than the melting point of aluminum, which is about 660°C, or that of copper, which is about 1080°C.

[0005] Due to the shape, usually circular, of the aluminum wires of Litz wire, voids form between the wires, with or without a non-conductive coating. Moisture can penetrate into these voids, leading to local corrosion of the aluminum wires, which leads to mechanical weakening and increased electrical resistance. However, voids also exist in formed wires, with or without a coating.

[0006] Furthermore, during melting of both the aluminum and copper materials to be connected, brittle and relatively highly resistive intermetallic phases are formed, so that in this region a large heat generation occurs during the subsequent current flow. Due to the increased temperatures, the intermetallic layer becomes thicker over time. Due to the brittleness of the joint, it easily breaks even under small mechanical loads. According to EP 2 621 022 A1, an attempt is made to avoid the formation of intermetallic phases by using cost-intensive CUPAL sleeves.

[0007] Another problem is that the melting temperatures of the two connection partners are very far apart, so that it is possible that the aluminum has already gone into the melt, while a dissimilar material such as copper has not yet reached its melting temperature. This leads to poorly welded joints that do not reach the required strength. Since poorly welded joints of this kind are usually not visible from the outside, it is possible that electrical conductors that are poorly connected to the connection element in this way will be used.

[0008] The time lag between the crimping performed in the first stage, where the oxide layer is destroyed, and the spatially separated second stage, where the elements are welded together, as in the prior art, also leads to new oxidation occurring, which leads to strong untraceable variations in the quality of the joint, which in turn depends heavily on the quality of the aluminium litz wire. Summary of the Invention [Problem to be solved by the invention]

[0009] The object of the present invention is to produce a long-term stable electrical connection between an electrical conductor made of aluminum or an aluminum alloy and an electrical connection made of copper or a copper alloy, with quality fluctuations that are minimized as much as possible, while avoiding the above-mentioned disadvantages, and further to make it possible to produce this connection quickly and inexpensively. [Means for solving the problem]

[0010] To solve the problem, the following method steps are proposed: a) coating a brazing material on the inside of a tube or using a tube having a brazing material coated on the inside of the tube; b) after insertion of the end of the electrical line into the tube, plastically deforming the tube in the region of the end of the tube facing the connection element, in which the non-conductive oxide layer of the litz wire is destroyed by indentation and the individual wires of the litz wire are deformed in such a way that the individual wires are pressed together without play; c) applying a pre-determinable pressure and a pre-determinable current for a pre-determinable time via the electrodes, depending on the geometry and material properties of the tube and the litz wire, to induce Joule heating of the aluminum-litz wire bundle, the Joule heating melting the brazing material and creating a brazing material bond between the inner surface of the tube and the outer area of ​​the aluminum-litz wire bundle that is in contact with the inner surface; d) softening the aluminum-litz wire bundle by applying a different pre-settable pressure and a different pre-settable current for a different pre-settable time, also depending on the geometry and material properties of the tube and the litz wire, so that a diffusion of the aluminum-litz wire bundle and / or at least a partial melting of the aluminum-litz wire bundle is performed; is proposed.

[0011] Here, a hard solder is understood to mean a metal which, together with aluminum or an aluminum alloy, has a eutectic melting temperature below the melting temperature of aluminum or an aluminum alloy. Preferably, a hard solder is selected which acts as a diffusion barrier to prevent the formation of intermetallic phases between aluminum and copper. For less expensive, durable and stable joints, a coating with a soft solder can also be used. The pressing of the tube during the plastic deformation not only destroys the oxide layer on the individual wires of the litz wire, but also presses the wires tightly against one another so that no gaps remain between the individual wires any more into which oxygen or gases can penetrate or remain, so that reoxidation can no longer take place in the pressed-in areas. The tight joining of the litz wires with each other and thus the associated increase in the contact surface of the litz wires reduces the overall contact resistance of the wires. The two-stage nature of this joining, i.e. the attachment of the outer aluminum wire or alloy of the aluminum wire to the coating by hard brazing, then prevents the formation of a new oxide layer on the surface of the litz wire, and the contact resistance, in particular between the copper tube and the outer surface of the litz wire, is minimized. A subsequent diffusion and / or partial melting process for the core of the litz wire minimizes the contact resistance between the individual wires of the litz wire. In this case, for the deformation process, a separate tool with a different size of working surface can be used, for example, relative to the working surface of the electrode for the brazing process. For each of these two stages, at least one set of process parameters is usually required that differ from each other. The stages can also be divided into several intervals or sections with different pressure or force / current / time settings. This allows the energy supply to be more finely metered, for example, to prevent excessive melting of the wire.

[0012] However, it is also exemplary to carry out method features b) and c) simultaneously or directly following one another.

[0013] In this case, the electrodes are used both to form the plastic deformation and to carry out the brazing process, which allows a shorter time between the deformation and brazing processes to be achieved, so that re-oxidation of the litz wire surface can be sufficiently prevented.

[0014] During the direct successive or simultaneous plastic deformation by the electrodes, the electrodes additionally cause deformation of the tube and compaction of the litz wire wire. The required force can be significantly reduced by a constant or modulated preheating current flowing through the electrodes. The compaction, brazing, diffusion or melting and cooling phases can be defined by different pressing forces and / or energy inputs. However, the compaction phase can also be partially integrated into the brazing and / or diffusion phases. The latter describes a process in which a separate continuous compaction takes place during the brazing and / or diffusion phases. The force application period during the ramp-like temperature increase to the soft brazing temperature, as well as to the brazing temperature, can also be used for compaction. Due to the spatially different temperature distributions in the composite of the tube and the litz wire, the brazing, diffusion or melting processes can overlap in time.

[0015] The diffusion process is also controlled via the pressing force, temperature and time, where the temperature in the bonding zone must be between the lowest melting temperature and the highest recrystallization temperature of the bonding partners.

[0016] It has proven advantageous to plastically deform the area of ​​the end of the tube facing the electrical line, so that the tube abuts against the electrical line in the form of a sealing cuff.

[0017] This ensures that the litz wire is held more securely in the tube for a subsequent or simultaneous hard brazing process. The transition area of ​​the litz wire between the plastic deformation on the connection side and the plastic deformation on the line side should be loaded as little as possible in order to avoid damage to the litz wire wire by overstretching. Furthermore, the contact resistance of the line side sealing cuff should be significantly higher than that of the plastic deformation on the connection side, so that shunting for the currents required for hard brazing, diffusion or melting is avoided.

[0018] If the individual wires of the litz wire are not fixedly bonded to one another merely by diffusion, but rather the core of the litz wire is partially or completely melted, it is advantageous to mold a melt reservoir between the deformation on the connection element side and the deformation on the line end side.

[0019] This achieves that the melt reaches the melt reservoir in an emergency but cannot flow outwards, the size of which can be determined before the deformation process, since the melt can also reach between the completely uncompacted areas of the litz wire and does not have to flow outwards from there.

[0020] The plastic deformation may again be carried out prior to or simultaneously with the brazing process.

[0021] Advantageously, during the deformation process and / or the brazing process and / or the diffusion or melting process, the pressing tool or the electrode can be heated or consists of a material which is heated when electric current is passed through it.

[0022] Basically, the heat required for brazing, diffusion or melting should be generated by the current flowing through the elements to be joined on the basis of Joule self-heating. In the deformation process used for compression, the materials to be joined become more flexible due to the heated tool or electrode, so that less force is required for the pressing process. The reduction in the pressing force leads to a gentle deformation, elongation and bending of the tube and the Litz wire wire, which results in a smaller pressure load on the electrode. During brazing, less heat required for brazing will flow through the electrode, as long as the electrode is heated. If the electrode is made of, for example, a tungsten material, it will heat up during the current flow, which results in an additional heat source for the brazing process and / or the diffusion process and / or the melting process.

[0023] Exemplary, the tools of the press tool pair and / or the electrodes of the electrode pair are moved simultaneously.

[0024] This reduces the stretching of the wires on the upper or lower outer sides of the litz wire next to the compacting region of the litz wire.

[0025] During hard brazing, it is advantageous to use an electrode with a small working surface relative to the working surface of the deformation tool in the compression step, so that the non-conductive lateral restrictions of the tube give it a substantially rectangular or square shape in the deformation and current-carrying areas.

[0026] Due to the different size of the pressing surface of the forming tool relative to the electrode, lateral fastening zones are generated in the tube, which serve to prevent the electrode from coming into lateral contact with the plastically deformed tube. The tube can be softened by the compression of the forming tool and the heat generation during the brazing. However, the limiting parts, which contact all over on the top and bottom sides as well as on the sides, prevent expansion, which may occur, for example, in the case of a hexagonal profile. The release of the compressed connection is prevented by the surrounding pinching. That is to say, the additional side pinching prevents the tube or the compacted Litz wire from expanding. The coating of the lateral limiting components is preferably neither electrically nor thermally conductive. Here, it is desirable that as little heat dissipation or electrical shunting occurs as possible. Preferably, ceramic-coated steel or directly ceramic is used. The contour of the tube is formed during the plastic deformation. During the subsequent brazing, diffusion or melting, the lateral limiting parts prevent the crimped connection from loosening.

[0027] The invention will be explained in more detail below on the basis of the drawings which illustrate the invention. [Brief description of the drawings]

[0028] [Figure 1] FIG. 2 shows an example according to the invention with pre-deformation for hard brazing and then diffusion of a Litz wire. [Diagram 2] FIG. 13 shows another example with pre-deformation, sealing cuffs and melt reservoir. [Diagram 3] FIG. 13 shows a third example with pre-deformation, sealing cuffs and a small melt reservoir. [Figure 4] FIG. 1 shows an example according to the invention with a plastic deformation process carried out by an electrode. [Diagram 5] FIG. 5 shows the embodiment according to the invention shown in FIG. 4 with additional sealing cuffs. [Figure 6] FIG. 13 shows examples for the structure of electrodes. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029] 1 shows in cross section an electrical connection 1 consisting of a copper tube 2 and a connection element 3, into which the end of an aluminum Litz wire 4 is pressed. The inside of the tube 2 is coated with a hard solder material 5.

[0030] The tube 2 has already been subjected to a plastic deformation 6 on the connection side, whereby the effective area of ​​the deformation tool (not shown) is greater than the area of ​​the electrode 7, so that a fastening zone 8 is formed in the tube 2 next to the centrally placed electrode 7, which fastening zone 8 ensures that the brazing and / or diffusion or melting current flows mainly via the Litz wire 4, as indicated by the current line 9.

[0031] Under the plastic deformation 6 on the connection side, the litz wire 4 is strongly compacted, which on the one hand destroys the oxide layers of the individual wires of the litz wire 4 and presses the individual wires of the litz wire 4 against each other without play. The application of current to the electrodes first leads to Joule self-heating in the area of ​​higher contact resistance between the tube 2, the brazing solder 5 and the litz wire 4, which leads to a bonding of the outer layer of the litz wire 4 to the tube 2 by melting the brazing solder, thereby reducing the contact resistance in this area. If current is applied further with different process parameters, which are usually used for brazing, Joule heating occurs in the area that provides a higher contact resistance between the individual wires of the litz wire, which leads to the individual wires being bonded to each other by diffusion or melting processes, thereby further reducing the contact resistance between the individual wires. Here, the heat introduction can be assisted by heating or self-heating of the electrodes.

[0032] 2 shows that during the formation of the connection-side plastic deformation 6, a further deformation is formed on the track side as a sealing cuff 10. The sealing cuff 10 is pressed smaller into the pipe 2 than the connection-side plastic deformation 6. However, the sealing cuff 10 ensures that the melt produced in the melting zone 11 can only reach the melt reservoir 12 and cannot reach the outside.

[0033] 3, in addition to the connection-side plastic deformation 6, a line-side deformation is also provided for forming the sealing cuff 10. In this example, however, the melt reservoir 12 is very small or does not exist at all. Any melt that may occur can certainly reach the spaces between the individual wires of the litz wire 4 in the region of the drawing zone 13, but it cannot escape likewise due to the sealing cuff 10.

[0034] Figure 4 shows an electrical connection 1 which has been deformed by an electrode 7 and which then undergoes a brazing process in a first step and a diffusion or welding process in a second step. Since the electrode 7 is also partially bonded to the side wall of the plastic deformation 6 on the connection side, a relatively large area of ​​molten brazing material 14 can be seen in Figure 4.

[0035] Figure 5 shows a variation on figure 4, where additionally a sealing cuff 10 has been pressed in, and figure 5 likewise shows a melt reservoir 12 between the sealing cuff and the plastic deformation 6 on the connection side.

[0036] Fig. 6 shows the electrodes 7, 7'. In particular, Fig. 6 shows that these two electrodes 7 can be brought closer to each other, so that the outer wires of the litz wire 4 are stretched almost equally during the plastic deformation process. This does not stretch the litz wire as much as the electrode pair 7, 7', in which only one electrode 7 is moved.

[0037] Furthermore, it can be seen that the tube 2 is strongly crushed together with the litz wire 4 inside the tube 2, so that the tube 2 takes on a substantially rectangular shape. In order to prevent the tube 2 from being able to escape laterally between the electrodes 7, 7', a lateral restriction 15 is provided, which additionally ensures that the tube 2 and the litz wire 4 maintain their plastic deformation without any possible expansion, so that gas cannot reach the litz wire 4 in the subsequent joining step.

[0038] It is therefore important that after the compaction process the tube is brazed to the wire on the surface of the litz wire in a first step and in a second step the inner areas of the litz wire are fixedly bonded to each other by diffusion and / or melting.

[0039] It is known that in Al-Cu melts, highly resistive and brittle Al-Cu intermetallic phases (IMPs) grow. This is a known practical problem. The high resistance leads to local overheating in the crimped joint, and the brittleness leads to cracks in the joint when external forces or temperature fluctuations occur, with a strongly fluctuating contact resistance and local overheating. In the cracks, contact corrosion can also form between these electrochemically very different materials. Proposals have also arisen to apply an antioxidant, usually a tin layer, to the aluminum wire and / or copper tube and to braze them together. In this case, CuSn intermetallic phases form, which are likewise highly resistive and brittle. This also leads to practical failures.

[0040] In order to avoid this practical risk, a diffusion barrier is additionally proposed which forms a eutectic with aluminum. The temperature of the eutectic must be lower than the melting temperature of the two bonding partners Al and Cu. The properties of the diffusion barrier prevent the occurrence of critical Al-Cu intermetallic phases. Besides silver materials, suitable coating materials for this are in particular nickel materials and their alloys. No strength loss with temperature occurs with silver or nickel brazing materials, as in the case of tin or zinc brazing materials. Nickel is successfully used as a diffusion barrier during wire bonding (avoidance of Kirkendall voids), soft soldering (avoidance of epsilon-eta layers) and spot welding (avoidance of tin whiskers). Thus, during the first step, a hard solder joint is created between the coating applied to the copper and the not yet melted Al wire, which has been pressed to remove the oxide layer. The coating can likewise be oxidized, but the oxide layer is also destroyed by the preceding deformation process.

[0041] After the formation of the hard solder joint between nickel and aluminum, the Al composite continues to heat up in a second step until the Al wire passes into a locally limited melting or diffusion joint between the electrode zones. This is not a resistance welding of the aluminum-litz wire, but a partial melting or diffusion that occurs completely or only at the contact surfaces that come into contact with each other. The heating process required for this must be controlled in a closed / open loop so that the hard solder joint is made before melting. This sequence is achieved by dividing the joining process into at least two different sections, which usually have different adjustment parameters. In the first section, starting with a preheating phase (stepwise and / or ramp-like current increase), the well-metered energy required for hard soldering is introduced. In the second section, the energy required for the melting or diffusion of the Al wire is supplied. Here, other adjustment parameters are usually required. The important parameters that have to be adjusted are the current, the time and the pressing force. If a plastic deformation process with additional heating of the forming tool is used, the correspondingly used energy can be spent on the total energy of the hard soldering-diffusion-melting process. Depending on the mechanical dimensions, electrode size, electrode material, deformation zone formation, etc., different sections with different adjustments may be required.

[0042] The brazing filler material may be inserted as a molded part, but is preferably applied onto the tube and possibly the Litz wire by electrochemical or physical (e.g. spraying, sputtering) methods. The coating provides additional oxidation protection for the substrate.

[0043] The current flow during bonding takes place via opposing electrodes in contact with the upper and lower surfaces. The electrical conductivity of the electrodes can be selected in such a way that they generate additional heat for the bonding process. The aim is to generate the required heat in the materials to be bonded by Joule heating, but the additional electrode heat can accelerate the heating process and reduce the heat dissipation via the electrodes. The heat required for bonding is thus not mainly supplied via the self-heating of the electrodes, but is generated by the current flow in the copper tube-coating-aluminum Litz wire bonding system. Here too, nickel is advantageous as a material with low electrical conductivity, since the Joule heat is generated in the contact zone additionally directly to the aluminum part.

[0044] A small amount of additional heating by the electrode material can be beneficial as it reduces heat flow through the electrodes.

[0045] The effect of the electrode current can also be influenced by the contour of the contact surface and the contact side, where the electrodes can be designed as plane-parallel or partially or completely convex or concave. [Explanation of symbols]

[0046] 1 Electrical Connections 2 tubes 3 Connection elements 4 Litz wire 5. Hard solder 6 Plastically deformed part on the connection side 7 electrodes 8 Fastening Zone 9 Current Lines 10 Seal Cuffs 11 Melting Zone 12 Melt storage section 13 Extension Zone 14 Molten brazing material 15 Lateral Restrictions

Claims

1. A method for connecting an electrical line formed as a litz wire (4) made of aluminum or an aluminum alloy to an electrical connection (1) made of copper or a copper alloy, the electrical connection (1) having, in addition to a connection element (3), a tube (2) for receiving an end of the electrical line, comprising the following method steps: a) coating the inner surface of the tube (2) with a brazing material (5) or using a tube (2) with an inner surface coated with a brazing material (5); b) after insertion of the end of the electrical line into the tube (2), plastically deforming the tube (2) in the region of its end facing the connection element, in which the non-conductive layer of the litz wire (4) is destroyed by indentation and the individual wires of the litz wire (4) are deformed in such a way that the individual wires are pressed together without play; c) applying via electrodes (7, 7') at least a predeterminable pressure and at least a predeterminable current for at least a predeterminable time, depending on the geometry and material properties of the tube (2) and the Litz wire (4), in order to induce Joule heating of the aluminum-Litz wire bundle, the Joule heating causing the brazing material (5) to melt and resulting in a brazing material bond between the inner surface of the tube (2) and the outer area of ​​the aluminum-Litz wire bundle which is in contact with the inner surface; d) softening the aluminum-Litz wire bundle by applying at least another predeterminable pressure and at least another predeterminable current for at least another predeterminable time, also depending on the geometrical shape and the material properties of the tube (2) and the Litz wire (4), so that a diffusion of the aluminum-Litz wire bundle and / or at least a partial melting of the aluminum-Litz wire bundle takes place; The method includes:

2. 2. The method according to claim 1, wherein method features b) and c) are carried out simultaneously or directly following one another.

3. 3. The method according to claim 1, further comprising the step of: e) plastically deforming the tube (2) in the region of its end facing the line, so that after insertion of the end of the electrical line into the tube (2), the tube (2) abuts against the electrical line in the form of a sealing cuff (10).

4. 4. The method according to claim 3, further comprising the step of: forming a melt reservoir (12) between the connection element side deformation and the track end side deformation.

5. 5. The method according to claim 3, wherein process steps e) and f) are carried out simultaneously with process step b) or at the latest before the start of process step d).

6. 6. The method according to claim 1, wherein during method step b) and / or method step c) and / or method step d), the pressing tool or electrode (7, 7') is heated or the pressing tool or electrode (7, 7') consists of a material which is heated when electric current is passed through it.

7. 7. The method according to claim 1, further comprising simultaneously moving the tools of the pressing tool pair and / or the electrodes of the electrode pair.

8. 8. The method according to claim 1, wherein in method step c) an electrode (7, 7') is used which has a smaller working surface relative to the working surface of the deformation tool in method step b), and a non-conductive lateral restriction (15) of the tube (2) provides the desired shape in the deformation and current carrying areas.

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