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 present 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 the connection element, a tube for receiving the end of the electrical line, which tube may be 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 some time. In automobiles, particularly in e-mobility, there is a demand for replacing these electrical conductors made of copper or copper alloys with conductors made of aluminum or aluminum alloys for weight reasons. EP 2 362 491 A1 or EP 2 621 022 A1 already disclose the use of aluminum lines that are first crimped onto and then welded to connecting elements made of another material.

[0003] Such joints, for example when used with aluminum lines and copper connection elements, present significant problems with regard to the 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 aluminum electrical lines to connecting elements made of dissimilar materials is that aluminum has a strong affinity for oxygen, and therefore, after a very short time, it becomes covered with an electrically insulating, dense, extremely hard, and extremely stable oxide layer, also called corundum, whose melting point is approximately 2050°C, i.e., significantly higher than that of aluminum, which is approximately 660°C, or that of copper, which is approximately 1080°C.

[0005] Due to the typically circular shape of the aluminum wires in Litz wire, voids form between the wires, regardless of whether they are coated with a non-conductive coating. Moisture can penetrate these voids, causing localized corrosion of the aluminum wire, which leads to mechanical weakening and increased electrical resistance. However, voids also exist in shaped wires, regardless of whether they are coated.

[0006] Furthermore, when both the aluminum and copper materials to be joined melt, a brittle, relatively highly resistive intermetallic phase forms, resulting in significant heat generation in this region during subsequent current flow. This increased temperature causes the intermetallic layer to thicken over time. The brittleness of the joint means that even small mechanical loads can easily lead to joint failure. EP 2 621 022 attempts to avoid this by using a complex CUPAL sleeve.

[0007] Another problem is the extreme difference in the melting temperatures of the two connecting elements. Therefore, there is a risk that the aluminum has already melted, while a dissimilar material, such as copper, has not yet reached its melting temperature. This can result in an insufficient weld that does not achieve the required strength. Since such an insufficient weld is usually not visible from the outside, there is a risk that an electrical conductor that is insufficiently connected to the connecting element will be used.

[0008] Furthermore, the time lag between the crimping in the first stage, in which the oxide layer is destroyed, and the spatially separated second stage, in which the elements are welded together, as is the case with conventional techniques, also leads to the formation of new oxidation, which in turn leads to strong and 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 avoid the above-mentioned drawbacks and to form 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 variations minimized as much as possible, and further to make this connection quickly and inexpensively. [Means for solving the problem]

[0010] To solve the problem, the following method steps are proposed: a) coating the inner surface of a tube with a brazing filler metal or using a tube with an inner surface coated with a brazing filler metal; b) after the 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, which causes the brazing filler metal to melt and create a brazing filler metal bond between the inner surface of the tube and the outer region of the aluminum-litz wire bundle that is in contact with the inner surface; d) softening the aluminum-litz wire bundle by applying different pre-set pressures and different pre-set currents for different pre-set times, also depending on the geometry and material properties of the tube and the litz wire, so that the aluminum-litz wire bundle is diffused and / or at least partially melted; is proposed.

[0011] Here, a hard solder is understood to mean a metal that, 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 that 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 indentation of the tube during plastic deformation not only destroys the oxide layer on the individual wires of the litz wire, but also presses the wires tightly together so that no gaps remain between the individual wires through which oxygen or gases can penetrate or remain. This prevents reoxidation in the indented area. The tight bonding of the litz wires and the resulting increase in their contact surface reduce the overall contact resistance of the wires. The two-stage nature of this bonding, i.e., the attachment of the outer aluminum wire or aluminum wire alloy to the coating by hard brazing, prevents the formation of a new oxide layer on the surface of the litz wire, thereby minimizing the contact resistance, particularly between the copper tube and the outer surface of the litz wire. A subsequent diffusion and / or partial melting process for the litz wire core minimizes the contact resistance between the individual wires. In this case, a separate tool with a different working surface size for the deformation process can be used, for example, compared to the working surface of the electrode for the brazing process. Each of these two stages typically requires at least one set of process parameters that differ from each other. These stages can also be divided into multiple intervals or sections with different pressure or force / current / time settings. This allows for more precise metering of the energy supply, for example, to prevent excessive melting of the wire.

[0012] However, it is also exemplary to perform method features b) and c) simultaneously or directly subsequent to 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, thereby sufficiently preventing re-oxidation of the litz wire surface.

[0014] During the direct, subsequent, or simultaneous plastic deformation by the electrodes, the electrodes additionally deform the tube and compact the litz wire. The required force can be significantly reduced by a constant or modulated preheating current flowing through the electrodes. The compacting, brazing, diffusion, or melting, and cooling phases can be defined by different pressures and / or energy inputs. However, the compacting phase can also be partially integrated into the brazing and / or diffusion phases. The latter describes a process in which a separate, continuous compaction occurs during the brazing and / or diffusion phases. The force application period during the ramp-up to the brazing temperature, as well as the soft brazing temperature, can also be used for compaction. Due to the spatially different temperature distribution within 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 pressure, 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 brazing process. The transition area of ​​the litz wire between the connection-side deformation and the line-side deformation should be as lightly loaded as possible to avoid damaging the litz wire due to overstretching. Furthermore, the contact resistance of the line-side sealing cuff should be significantly higher than that of the connection-side deformation, thereby avoiding shunting of the current required for brazing, diffusion, or melting.

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

[0019] This ensures that the melt reaches the melt reservoir in an emergency but cannot flow outward. The size of the melt reservoir can be determined before the deformation process, because the melt can reach the uncompacted areas of the litz wire and does not have to flow outward.

[0020] The plastic deformation may again be carried out before 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 that heats up when current is passed through it.

[0022] Essentially, the heat required for brazing, diffusion, or melting is generated by Joule self-heating via the current flowing through the components to be joined. During the compression process, the heated tool or electrode makes the materials to be joined more flexible, requiring less force for the pressing process. This reduced pressing force results in gentle deformation, elongation, and bending of the tube and Litz wire, resulting in less pressure on the electrodes. During brazing, as long as the electrodes are heated, less heat will flow through them. If the electrodes are made of tungsten, for example, they will heat up when current flows through them, creating an additional heat source for the brazing, diffusion, and / or melting processes.

[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 area of ​​the litz wire.

[0025] During 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] The different size of the pressing surface of the deforming tool relative to the electrode creates lateral clamping zones in the tube, which serve to prevent the electrode from coming into lateral contact with the deformed tube. The tube can be softened by the compression of the deformed tube and the heat generated during brazing. However, the full-surface contact on the upper and lower surfaces and on the sides prevents expansion, as would occur with a hexagonal profile, for example. The circumferential clamping prevents the compressed connection from releasing. The additional lateral clamping prevents expansion of the tube or compacted Litz wire. The coating of the lateral clamping elements is preferably neither electrically nor thermally conductive. It is desirable to minimize heat dissipation and electrical shunting. Ceramic-coated steel or direct ceramic is preferably used. The tube profile is formed during the deformed tube. During subsequent brazing, diffusion, or melting, the lateral clamping prevents 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 explanation of the drawings]

[0028] [Figure 1] 1 shows an example according to the invention with pre-deformation for hard-brazing and then diffusing a litz wire; FIG. [Figure 2] FIG. 10 shows another example with pre-deformation, sealing cuffs and melt reservoir. [Figure 3] FIG. 10 shows a third example with pre-deformation, sealing cuffs and a small melt reservoir. [Figure 4] 1 shows an example according to the invention with plastic deformation performed by electrodes; [Figure 5] 5 shows the example according to the invention shown in FIG. 4 with an additional sealing cuff. [Figure 6] 10A and 10B show examples for electrode structures. DETAILED DESCRIPTION OF THE INVENTION

[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, where 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] The litz wire 4 is strongly compacted beneath the plastically deformed portion 6 on the connection side, which 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. When current is applied to the electrodes, Joule self-heating first occurs in the areas of higher contact resistance between the tube 2, the brazing filler metal 5, and the litz wire 4. This self-heating causes the outer layer of the litz wire 4 to bond to the tube 2 by melting the brazing filler metal, thereby reducing the contact resistance in this area. When current is applied further, typically with different process parameters for brazing, Joule heating occurs in areas that provide higher contact resistance between the individual wires of the litz wire. As a result, the individual wires bond to each other through a diffusion or melting process, further reducing the contact resistance between the individual wires. Here, heat transfer can be assisted by heating or self-heating of the electrodes.

[0032] 2 shows that during the formation of the joint-side deformation 6, a further deformation is formed on the track side in the form of a sealing cuff 10. The sealing cuff 10 is pressed into the pipe 2 to a smaller extent than the joint-side 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 joint-side plastic deformation 6, a track-side deformation for forming the sealing cuff 10 is also provided. However, in this example, the melt reservoir 12 is very small or does not exist at all. Any melt that may form 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 then subjected to a brazing process in a first step and a diffusion or welding process in a second step. The electrode 7 is also partially bonded to the side wall of the plastically deformed part 6 on the connection side, so that a relatively large area of ​​molten brazing material 14 can be seen in Figure 4.

[0035] Figure 5 shows a variation of Figure 4, in which a sealing cuff 10 has additionally been pressed in, and Figure 5 likewise shows a melt reservoir 12 between the sealing cuff and the plastically deformed part 6 on the connection side.

[0036] Figure 6 shows electrodes 7, 7'. In particular, Figure 6 shows that these two electrodes 7 can be moved 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 a pair of electrodes 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 it, 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', lateral restrictions 15 are provided, which additionally ensure that the tube 2 and the litz wire 4 maintain their plastic deformation without any possible expansion, thereby preventing gas from reaching the litz wire 4 in the subsequent joining step.

[0038] Therefore, what is important is 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 Al-Cu melts develop highly resistive and brittle Al-Cu intermetallic phases (IMPs). This is a known practical problem. The high resistivity leads to local overheating at the crimped joint, and the brittleness leads to cracks at the joint when external forces or temperature fluctuations occur, with highly fluctuating contact resistance and local overheating. Contact corrosion can also form at these cracks between these electrochemically very different materials. It has also been proposed to apply an antioxidant, usually a tin layer, to aluminum wire and / or copper tubes and then braze them together. In this case, Cu-Sn intermetallic phases form, which are also highly resistive and brittle. This can also lead to practical failures.

[0040] To avoid this practical risk, a diffusion barrier that forms a eutectic with aluminum is proposed. The temperature of the eutectic must be lower than the melting point of the two bonding partners, Al and Cu. The diffusion barrier properties prevent the formation of critical Al-Cu intermetallic phases. Suitable coating materials for this purpose are, in addition to silver, nickel and its alloys. Unlike tin or zinc brazing filler metals, silver or nickel brazing filler metals do not lose strength with temperature. Nickel is successfully used as a diffusion barrier during wire bonding (to avoid Kirkendall voids), soft soldering (to avoid epsilon-eta layers), and spot welding (to avoid tin whiskers). Thus, during the first step, a hard solder joint is formed between the coating applied to the copper and the still-unmelted Al wire, which has been pressed to remove the oxide layer. The coating may also oxidize, but the oxide layer is destroyed by the preceding deformation process.

[0041] After the formation of the brazing bond between the nickel and aluminum, the Al composite continues to heat up in the second step until the Al wire transitions to a locally limited melting or diffusion bond between the electrode zones. This is not a resistance welding of the aluminum and Litz wire, but rather a partial melting or diffusion that occurs either completely or only at the contact surfaces. The heating process required for this must be controlled in a closed or open loop so that the brazing bond occurs before melting. This sequence is achieved by dividing the joining process into at least two distinct sections, usually with different adjustment parameters. The first section begins with a preheating phase (a stepwise and / or ramp-like current increase) and introduces the energy required for the brazing. The second section provides the energy required for the melting or diffusion of the Al wire. This typically requires additional adjustment parameters. The key parameters that must be adjusted are the current, time, and pressure. If a plastic deformation process with additional heating of the forming tool is used, the corresponding energy consumption can be used to cover the entire brazing-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 metal may be inserted as a molded part, but is preferably deposited on the tube and possibly the litz wire by electro-deposition or physical (e.g. spraying, sputtering) methods. The coating provides additional oxidation protection for the substrate.

[0043] The current flow during joining is via opposing electrodes in contact with the upper and lower surfaces. The electrode conductivity can be selected so that the electrodes generate additional heat for the joining process. The goal is to generate the necessary heat via Joule heating in the materials to be joined, but the additional electrode heat can accelerate the heating process and reduce heat dissipation via the electrodes. Thus, the heat required for joining is not primarily supplied via self-heating of the electrodes, but rather is generated by the current flow in the copper tube-coating-aluminum Litz wire joining system. Here again, nickel is advantageous as a low-conductivity material because Joule heat is generated in the contact zone directly in addition to the aluminum part.

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

[0045] The effect of the electrode current can also be influenced by the contour of the contact surface and contact side, where the electrodes can be plane-parallel or can be 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 hard solder 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) comprising, in addition to a connection element (3), a tube (2) for receiving the end of the electrical line, the method comprising the following method steps: a) coating the inner surface of the tube (2) with a brazing filler material (5) or using a tube (2) whose inner surface is coated with a brazing filler 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 broken 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 wires (4), to induce Joule heating of the aluminum-litz wire bundle, the Joule heating melting the brazing filler metal (5) and creating a brazing filler metal bond between the inner surface of the tube (2) and the outer region of the aluminum-litz wire bundle that is in contact with the inner surface; d) softening the aluminum-litz wire bundle by applying at least another pre-determinable pressure and at least another pre-determinable current for at least another pre-determinable time, also depending on the geometry and material properties of the tube (2) and the litz wire (4), so that the aluminum-litz wire bundle is diffused and / or at least partially melted; A method comprising:

2. 2. The method of claim 1, wherein method features b) and c) are performed simultaneously or directly following one another.

3. 3. The method according to claim 1, further comprising the step of: e) plastically deforming the pipe (2) in the region of its end facing the electrical line, after insertion of the end of the electrical line into the pipe (2), so that the pipe (2) abuts 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 connecting element-side deformation and the track end-side deformation.

5. 5. The method according to claim 4, 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. 2. 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 that heats up when current is passed through it.

7. 7. The method according to claim 6, wherein the pressing tool and / or the electrode (7, 7') are moved simultaneously.

8. 2. The method according to claim 1, wherein in method step c) an electrode (7, 7') is used which has an active surface smaller than the active surface of the deforming tool for deforming in method step b), and wherein non-conductive lateral restrictions (15) of the tube (2) give the desired shape in the deforming area and in the current-carrying area.

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