METHOD FOR CONNECTING AN ALUMINUM ELECTRICAL CONDITIONER TO A COPPER PIPE

Description

[0001] The invention relates to a method for connecting an electrical cable, which is formed as a stranded wire made of aluminum or an aluminum alloy, to an electrical connector made of copper or a copper alloy, which, in addition to the connecting element, has a tube for receiving the end of the electrical cable. These tubes can be open on one or both sides.

[0002] The use of copper or copper alloys as conductors for electrical current has long been known. In motor vehicles, especially electric vehicles, the goal is to replace these copper or copper alloy conductors with conductors made of aluminum or aluminum alloys for weight reasons. EP 2 362 491 A1 and EP 2 621 022 A1 already disclose the use of aluminum cables that are first crimped and then welded to connecting elements made of other materials.

[0003] Such connections pose major problems, for example, when using aluminum cables and copper connection elements, with regard to the duration of the mechanical stability of the connection and its electrical conductivity. If moisture penetrates such a connection, contact corrosion occurs, which increases the contact resistance and significantly reduces the mechanical durability of the connection.

[0004] The main problems with connecting aluminum electrical cables to connectors made of dissimilar materials, however, are that aluminum has a strong affinity for oxygen and therefore quickly becomes coated with a dense, electrically insulating, very hard, and highly durable oxide layer. The melting point of this oxide layer, also known as corundum, is approximately 2,050°C, significantly higher than the melting point of aluminum, which is approximately 660°C, or copper, which is approximately 1,080°C.

[0005] The typically round shape of the aluminum wires in the stranded wire, with or without a non-conductive coating, creates voids between the wires. Moisture can penetrate these voids and cause local corrosion of the aluminum wire. This leads to mechanical weakening and an increase in the conductor resistance. However, voids are also present in formed wire, with or without a coating.

[0006] In addition, the melting of the two aluminum and copper materials to be joined can create intermetallic phases that are both brittle and have a high resistance. This can lead to significant heat generation in this area when current is subsequently passed through. This increased temperature causes the intermetallic layer to become even thicker over time. Due to the brittleness of the connection, even minor mechanical stress can easily cause the connection to break. According to EP 2 621 022 A1, attempts are being made to prevent the formation of intermetallic phases by using complex CUPAL sleeves.

[0007] A further problem is that the melting temperatures of the two connecting components differ considerably. This creates the risk that aluminum will already be melting while a dissimilar material, such as copper, has not yet reached its melting temperature. This results in inadequate welds that do not achieve the required strength. Such inadequate welds are generally not visible from the outside, thus creating the risk of using electrical conductors that are poorly connected to the connecting element.

[0008] The time lag according to the prior art between the first crimping step, in which the oxide layer is broken up, and a spatially separated second step in which the elements are welded together can also cause renewed oxidation, which can result in significant, unexplainable fluctuations in the connection quality. The quality of the connection is also highly dependent on the quality of the aluminum strands. JP2010110790A discloses a method for fusing an aluminum wire and a copper terminal in the commutator of a motor. The connection point between the aluminum wire and the terminal is subjected to pressure and electricity to melt it, with a coating provided in the connection area.EP3451455A1 discloses a method for attaching a copper contact sleeve to an aluminum conductor, wherein the individual wires of the conductor are materially bonded to each other and to the contact sleeve, while the fastening band is materially bonded to the contact sleeve. JP5199987B2 discloses a method for joining two metal parts made of different materials (i.e., copper and aluminum) by using a joining auxiliary material consisting of a third metal and having a lower melting point than the first and second metal parts.

[0009] The invention is based on the object of producing long-term stable electrical connections between electrical conductors made of aluminum or an aluminum alloy and an electrical connection made of copper or a copper alloy, while avoiding the disadvantages described, with qualitative fluctuations being excluded as far as possible, which can also be produced quickly and cost-effectively.

[0010] The following procedural steps are proposed to solve the problem: a) Coating the inner surfaces of the tube with a hard solder or using a tube coated with a hard solder on its inner surface; b) After inserting the end of the electrical cable into the tube, plastically deforming the tube in the region of the connection element-side end of the tube, whereby the individual wires of the stranded wire are deformed by over-embossing in such a way that their non-conductive oxide layers are broken up and the individual wires are pressed together without a gap; c) Applying a predeterminable contact force and a predeterminable current for a predeterminable time via electrodes, depending on the geometry and material properties of the tube and the stranded wire, to generate Joule heating of the aluminum stranded wire bundle, whereby the Joule heating melts the hard solder and creates a hard solder connection between the inner surfaces of the tube and the outer areas of the aluminum stranded wire bundle in contact with them;d) by applying, also depending on the geometry and the material properties of the tube and the strand, a further predeterminable contact force and a further predeterminable current for a further predeterminable time, the softening of the aluminium strand bundle in such a way that a diffusion of the aluminium strand bundle and / or an at least partial melting of the aluminium strand bundle occurs.

[0011] A hard solder is a metal that has a eutectic melting point with the aluminum or aluminum alloy below the melting point of the aluminum or aluminum alloy. A hard solder is preferably selected that acts as a diffusion barrier to prevent the formation of intermetallic phases between the aluminum and copper. For less high-quality, durable and stable connections, a coating of soft solder could also be used. By over-embossing the tube during plastic deformation, not only are the oxide layers on the individual wires of the stranded wire broken up, but the wires are pressed so tightly together that no gap remains between the individual wires into which oxygen or gas could penetrate or remain, so that no further oxidation can occur in this over-embossed area.The closer the stranded wires are placed together and the resulting increase in the contact area between the stranded wires reduces the overall contact resistance of the wires. The subsequent two-stage connection, i.e. the bonding of the outer aluminum wires or their alloys to the coating by brazing, prevents the formation of new oxide layers on the surface of the stranded wire and, above all, minimizes the contact resistance between the copper tube and the outer surface of the stranded wire. The subsequent diffusion and / or partial melting process for the core of the stranded wire minimizes the contact resistance between the individual wires of the stranded wire. Other tools can be used for the deformation process, e.g. those with different sized effective surfaces to the effective surfaces of the electrodes for the brazing process.Each of these two stages typically requires at least one set of different process parameters. The stages can also be divided into several intervals or sections with different pressure or force / current / time specifications. This allows for more precise control of the energy supply, for example, to prevent excessive melting of the line.

[0012] However, it is also worth emulating if the process features b) and c) are carried out simultaneously or directly one after the other.

[0013] Although the electrodes are then used both to produce the plastic deformation and to carry out the brazing process, this allows for shorter times between the deformation process and the brazing process, so that the re-oxidation of the strand surface can be largely prevented.

[0014] During immediate or simultaneous plastic deformation by the electrodes, these also deform the tube and compact the stranded wires. The required forces can be significantly reduced by a constant or modulated preheating current flowing through the electrodes. The compaction phase, the brazing phase, the diffusion phase or melting phase, and the cooling phase can be determined by various contact forces and / or energy inputs. The compaction phase can also be partially integrated into the brazing phase and / or diffusion phase. The latter describes a process in which further continuous compaction takes place during the brazing and / or diffusion phase. The duration of the force application during the ramp-like heating to the brazing temperature, but also the soldering temperature, can also be used for compaction.Due to the spatially different temperature distribution within the composite of pipe and strand, the brazing and diffusion or melting processes can overlap in time.

[0015] The diffusion process is also controlled by contact force, temperature, and time. 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 effective to plastically deform the area of ​​the cable-side end of the pipe in such a way that the pipe fits around the electrical cable in the form of a sealing sleeve.

[0017] This ensures that the stranded wire is held even more securely in the pipe for the subsequent, or simultaneous, brazing process. The transition area of ​​the stranded wire between the connection-side and the line-side plastic deformation should be subjected to as little stress as possible to prevent damage to the stranded wires due to overstretching. Furthermore, the contact resistance of the line-side sealing sleeve must be significantly higher than the connection-side plastic deformation to prevent a shunt for the current required for brazing, diffusion, or melting.

[0018] If the individual wires of the stranded wire are not only to be firmly connected to one another by diffusion, but the stranded wire core is to be partially or completely melted, it is advisable to form a melt depot between the deformation on the connection element side and the deformation on the cable end side.

[0019] This ensures that melt can enter the melt reservoir if necessary, but cannot escape to the outside. The size of the melt reservoir can be determined before the forming process, as melt can also enter the non-compacted areas of the strand and does not necessarily escape to the outside.

[0020] Here too, the plastic deformation can be carried out before the brazing process or at the same time.

[0021] Advantageously, the pressing tools or electrodes can be heated during the forming process and / or during the brazing process and / or the diffusion or melting process, or they can be made of a material that heats up when current flows through it.

[0022] In principle, the heat required for brazing, diffusion or melting should be generated by a current flowing through the elements to be joined due to Joule's inherent heating. During the deformation process used for compaction, heated tools or electrodes can make the materials to be joined more pliable, so that less force needs to be applied for the over-embossing. The reduction in contact force leads to gentler forming, stretching and compressing of the tube and the stranded wires and to less compressive stress on the electrodes. During brazing, if the electrodes are heated, less of the heat required for brazing would flow away via the electrodes. If the electrodes are made of tungsten materials, for example, they would heat up when the current flows through them, creating an additional heat source for the brazing process and / or the diffusion process and / or the melting process.

[0023] It is worth imitating if the tools or electrodes of the pressing tool and / or electrode pairs are moved simultaneously.

[0024] This reduces the elongation of the upper and lower outer wires of the strand next to the compaction area of ​​the strand.

[0025] It is advisable to use electrodes with a smaller effective area than the effective area of ​​the forming tools in the compaction step during brazing, and to use non-conductive, lateral boundaries of the tube in the forming and current flow area to give it an essentially rectangular or square shape.

[0026] Due to the different size of the pressing surface of the forming tools and the electrodes, lateral end zones are created on the tube. These serve to prevent lateral contact of the electrodes with the plastically deformed tube. The heating during deforming, compacting and brazing can soften the tube. However, the limits on the top and bottom as well as across the entire side prevent expansion, as would occur with a hexagonal contour, for example. The all-round clamping prevents the compacted connection from breaking down. The additional lateral clamping therefore prevents the tube or the compacted strand from expanding. The coating of the lateral limiting components should preferably be neither electrically nor thermally conductive. If possible, neither heat dissipation nor electrical shunts should occur here. Ceramic-coated steel or direct ceramic is preferably used.The tube's contour is created during plastic deformation. During subsequent brazing, diffusion, or melting, the lateral boundary prevents the pressed joint from loosening.

[0027] The invention is explained in more detail with reference to the accompanying drawings. These show: Fig. 1 shows an example according to the invention with pre-deformation for brazing and subsequent diffusion of the strand, Fig. 2 shows a further example with pre-deformation, sealing sleeve and melt depot, Fig. 3 shows a third example with pre-deformation, sealing sleeve and small melt depot, Fig. 4 shows an example according to the invention with plastic deformation carried out by electrodes, Fig. 5 shows an example according to the invention according to Figure 4 with additional sealing sleeve, and Fig. 6 an example of the structure of the electrodes

[0028] Fig. 1shows, in section, an electrical connector 1 consisting of a copper tube 2 and a connecting element 3, with the end of an aluminum strand 4 inserted into the tube 2. The tube 2 is coated internally with a brazing solder 5.

[0029] The tube 2 has already been subjected to plastic deformation 6 on the connection side. The deformation tool (not shown) had an effective area larger than the area of ​​the electrode 7, whereby termination zones 8 are formed in the tube 2 next to the centrally placed electrode 7, which ensure that the brazing current and / or diffusion or melting current, indicated by the current flow lines 9, flows predominantly via the stranded wire 4.

[0030] Below the connection-side plastic deformation 6, it is indicated that the stranded wire 4 is highly compacted, which, on the one hand, has broken up the oxide layers of the individual wires of the stranded wire 4, and the individual wires of the stranded wire 4 are pressed together without a gap. By applying current to the electrodes, Joule-induced self-heating initially occurs in the area of ​​the higher-ohm contact resistance between tube 2, brazing alloy 5, and stranded wire 4, which creates a bond between the outer layer of the stranded wire 4 and the tube 2 by melting the brazing alloy, thereby reducing the contact resistance in this area.With further current flow, usually with process parameters different from those used for brazing, the Joule heat is generated in the area between the individual wires of the stranded wire, which offers greater contact resistance. This causes the individual wires to bond together through diffusion or a melting process, which in turn reduces the contact resistance between the individual wires. Heat input can be supported by heating or self-heating of the electrodes.

[0031] Fig. 2shows that during the creation of the connection-side plastic deformation 6, a further deformation was formed on the pipe side as a sealing sleeve 10. The sealing sleeve 10 is less strongly embossed into the pipe 2 than the connection-side plastic deformation 6. However, the sealing sleeve 10 ensures that the melt created in the melt zone 11 can only reach a melt depot 12 and not to the outside.

[0032] In Fig. 3 In addition to the connection-side plastic deformation 6, a line-side deformation has also been created to form a sealing sleeve 10. However, the melt deposit 12 is very small or nonexistent here. Any melt that may form can penetrate between the individual wires of the stranded wire 4 in the area of ​​an expansion zone 13, but it also cannot escape through the sealing sleeve 10.

[0033] Fig. 4now shows an electrical connection 1 that was deformed using the electrode 7, and in which the brazing process was subsequently carried out in the first stage and the diffusion or welding process in the second stage. Since the electrode 7 is also partially connected to the side walls of the connection-side plastic deformation 6, larger areas of the molten brazing material 14 can be seen here.

[0034] Fig. 5 shows a variation of Figure 4 . Here, a sealing sleeve 10 was additionally embossed, which in turn shows a melt depot 12 between the sealing sleeve and the connection-side plastic deformation 6.

[0035] Fig. 6shows electrodes 7 and 7'. In particular, it shows that these two electrodes 7 can be moved toward each other. As a result, the outer wires of the stranded wire 4 are stretched approximately equally during the plastic deformation process, which stretches the stranded wire less than in a pair of electrodes 7, 7', where only one electrode 7 is moved.

[0036] Furthermore, it can be seen that the tube 2 with the stranded wire 4 located therein is tightly squeezed, so that the tube 2 assumes a substantially rectangular shape. To prevent the tube 2 from protruding between the electrodes 7, 7' at the sides, a lateral boundary 15 is provided, which additionally ensures that the tube 2 and stranded wire 4 retain their plastic deformation without any expansion occurring, so that no gas can reach the stranded wire 4 during the subsequent connection steps.

[0037] It is therefore essential that, after a compaction process, the tube is soldered to the surface wires of the strand in a first step and that, in a second step, the inner regions of the strand are firmly connected to one another by diffusion and / or melting.

[0038] It is known that an Al-Cu intermetallic phase (IMP) grows in an Al-Cu melt, which is highly resistive and brittle. This represents a well-known field problem. The high resistance leads to partial overheating in the crimp, and the brittleness leads to cracks in the connection point when exposed to external forces or temperature changes, combined with strongly fluctuating contact resistances and partial overheating. Contact corrosion can also develop in these cracks between these electrochemically very different materials. There is also the suggestion of applying oxidation protection, usually a tin layer, to the aluminum wires and / or the copper tube and soldering them together. In this case, CuSn IMPs are formed, which are also highly resistive and brittle. This can also cause field failures.

[0039] To avoid these field risks, a diffusion barrier is proposed, which additionally forms a eutectic with the aluminum. The temperature of the eutectic must be below the melting temperature of the two bonding partners, Al and Cu. The property of the diffusion barrier prevents the formation of critical Al-Cu IMPs. A suitable coating material for this purpose, in addition to silver, is primarily nickel and its alloys. Temperature-induced strength loss, as with tin or zinc solder, does not occur with silver or nickel solder. Nickel is successfully used as a diffusion barrier in wire bonding (avoiding Kirkendahl voids), soft soldering (avoiding epsilon-eta layers), and spot welding (avoiding tin whiskers).During the first step, a brazing bond is created between the coating applied to the copper and the pressed-on aluminum wires, freed from the oxide layer and not yet melted. The coating may also be oxidized, but the preceding deformation process also breaks down its oxide layer.

[0040] After the brazing joint between nickel and aluminum has formed, the aluminum composite heats up further in a second step until the aluminum wires between the electrode zones form a locally limited melt or diffusion bond. This is not resistance welding of the aluminum stranded wires, but rather a complete melting or partial melting or diffusion that occurs only at the adjacent contact surfaces. The heating process required for this must be regulated / controlled so that the brazing connection occurs before melting. This sequence is achieved by dividing the joining process into at least two different stages, usually with different setting parameters. In the first stage, starting with a preheating stage (stepped and / or ramped current increase), the carefully measured energy required for brazing is introduced.In the second stage, the energy required for melting or diffusing the aluminum wires is supplied. Different settings are generally required here. The key parameters that must be adjusted are the current, the duration, and the contact force. If plastic deformation with additional heating of the forming tools is used, the correspondingly applied energy can be incorporated into the total energy of the brazing diffusion melting process. Depending on the mechanical dimensions, the electrode size, the electrode material, the formation of the deformation zone, etc., additional stages with different settings may be required.

[0041] Brazing alloys can be inserted as preforms, but are preferably applied to the tube and, if necessary, the strand by galvanic or physical means (e.g., spraying, sputtering). The coating provides additional oxidation protection for the base material.

[0042] The current flows during the connection process via the opposing electrodes on the top and bottom. The conductivity of the electrodes can be selected so that the electrodes generate additional heat for the connection process. The goal is for the necessary heating to be generated by Joule heat in the material to be joined; however, the additional electrode heat can accelerate the heating process and reduce heat loss via the electrodes. The heat required for the connection is therefore not supplied primarily via the self-heating of the electrodes, but rather is generated by the current flow in the composite system of copper pipe, coating, and aluminum strand. Here, too, nickel, as a poorly conductive material, would be advantageous because the Joule heat is then generated directly in the contact zone in addition to the aluminum component.

[0043] A slight additional heating by the electrode material can be helpful as this reduces the heat flow through the electrodes.

[0044] The effect of the electrode current can also be influenced by the contact surface and the contact-side contour. The electrodes can be plane-parallel, partially or completely convex, or concave. List of reference symbols

[0045] 1Electrical connection 2Pipe 3Connection element 4Wire 5Brazing alloy 6Connection-side plastic deformation 7Electrode 8Termination zones 9Current flow lines 10Sealing sleeve 11Melt zone 12Melt deposit 13Expansion zone 14Melted brazing alloy 15Lateral boundaries

Claims

1. A method for connecting an electrical conductor, which is configured as a stranded wire (4) made of aluminium or an aluminium alloy, to an electrical connection (1) made of copper or a copper alloy, which in addition to the connection element (3) has a tube (2) for receiving the end of the electrical conductor, having the following method steps a) coating the inner surfaces of the tube (2) with a hard solder (5) or using a tube (2) coated on the inner surface with a hard solder (5) b) after introducing the end of the electrical conductor into the tube (2) plastically deforming the tube (2) in the region of the end of the tube (2) on the connection element side, wherein the individual wires of the stranded wire (4) are deformed by over-stamping, such that the non-conductive layers thereof are broken up and the individual wires are pressed against one another in a gap-free manner, c) applying at least a predeterminable contact force and at least a predeterminable current for at least a predeterminable time via electrodes (7, 7') as a function of the geometry and the material properties of the tube (2) and the stranded wire (4) for generating a Joule heating of the aluminium stranded wire bundle, wherein due to the Joule heating the hard solder (5) melts and a hard solder connection is produced between the inner surfaces of the tube (2) and the outer regions of the aluminium stranded wire bundle in contact therewith, characterized in that d) by applying at least a further predeterminable contact force and at least a further predeterminable current for at least a further predeterminable time, also as a function of the geometry and the material properties of the tube (2) and the stranded wire (4), softening the aluminium stranded wire bundle such that a diffusion of the aluminium stranded wire bundle and / or an at least partial melting of the aluminium stranded wire bundle takes place.

2. The method according to Claim 1, wherein the method features b) and c) are carried out at the same time or immediately one after the other.

3. The method according to Claim 1 or 2, wherein e) after introducing the end of the electrical conductor into the tube (2) it is plastically deformed in the region of its end on the conductor side, such that the tube (2) bears in the form of a sealing sleeve (10) against the electrical conductor.

4. The method according to Claim 3, wherein f) a melt reserve (12) is formed between the deformation on the connection element side and the deformation on the conductor end side.

5. The method according to Claim 3 or 4, wherein the method steps e) and f) are carried out at the same time with the method step b) or at the latest before the start of the method step d).

6. The method according to one of Claims 1 to 5, wherein during the method step b) and / or c) and / or d) the pressing tools or electrodes (7, 7') are heated or consist of a material which is heated when current passes through.

7. The method according to one of Claims 1 to 6, wherein the tools or electrodes (7, 7') of the pressing tool and / or electrode pairs are moved at the same time.

8. The method according to one of Claims 1 to 7, wherein in the method step c) electrodes (7, 7') are used with a smaller effective area than the effective area of the deformation tools in method step b) and wherein non-conductive lateral boundaries (15) of the tube (2) provide a desired shape in the deformed and energized region.