Cables are components with a relatively high cost in the high-voltage connector harness of electric vehicles. The conductor materials commonly used are mainly copper and copper alloys. Copper has good electrical and mechanical properties and is an ideal material for electrical conduction. what is the copper-aluminum connection of high-voltage wiring harness for electric vehicles?
As the charging current of electric vehicles develops towards 400A or higher, if copper continues to be used as a cable conductor, cables with a specification of 95mm2 or larger must be used, which will increase the mass of the harness and the entire vehicle, which is not conducive to the development strategy of reducing energy consumption and increasing the range.
In addition, due to the potential difference between copper and aluminum, electrochemical corrosion is likely to occur. Therefore, in order to ensure the stable and safe application of copper-aluminum connection, good solutions are needed in terms of mechanical properties and electrochemical corrosion.
Problems with copper-aluminum connection
Traditional high-voltage wiring harnesses are mainly composed of copper cables. The two ends of the copper cable conductor are connected to copper terminals or copper bars, which can easily obtain reliable connections without the problem of electrochemical corrosion.
In the context of the development of charging power towards high-power charging, the maximum charging current allowed by the current charging technology standard “GB/T20234.1-2015″ is 250A, and the standards being formulated and updated have increased the charging current to 400A, 600A, and even more than 1000A.
In the case of no additional cooling measures on the vehicle end, when the current increases, according to Joule’s law (Q=I2Rt), the conductor resistance needs to be reduced to prevent the vehicle from thermal failure and other problems.
An effective measure to reduce the negative charge is to increase the cross-sectional area of the conductor. Generally, the maximum current carrying capacity of a 120mm2 copper cable is 500A. To obtain a higher current carrying capacity, the cross-sectional area of the cable must be larger than 120mm2. Such a large size will lead to problems such as overweight of the harness and excessive bending radius. Therefore, lightweight conductors such as aluminum bars or aluminum rods will have an opportunity to be applied.
Due to its rectangular shape, aluminum bars have a larger heat dissipation area and better conductivity under the same cross-sectional area. When arranging the harness, the rectangular shape is conducive to flat laying and occupies less space. At the same time, the structural rigidity of the aluminum bar itself allows it to use fewer harness fixing buckles during installation to obtain good harness layout performance.
According to the material properties of the aluminum conductor itself, when connected to the copper terminal, there are some inherent problems in terms of mechanical properties, electrical properties, corrosion resistance, etc., as described below. In order to solve or reduce the performance degradation caused by the difference in the characteristics of copper and aluminum materials, it is necessary to study the effects of aluminum alloy materials, connection methods and surface treatment on the mechanical properties and electrical properties of copper-aluminum connections.
Copper-aluminum connection scheme
Selection of aluminum alloy materials
The physical parameters of copper and aluminum have obvious differences in mechanical properties and electrical properties. Table 1 shows the physical parameters of copper and aluminum. It can be seen from the table that the strength of aluminum is lower than that of copper, but its thermal expansion coefficient is 1.35 times that of copper. The difference in thermal expansion coefficient is a challenge to whether the copper-aluminum connection is reliable. The specific series of aluminum alloys to be selected needs to be selected according to the connection scheme. For example, if better weldability is required, 1 series aluminum alloy can be selected, and if higher strength and good creep resistance are required, 6 series or 8 series aluminum alloy can be selected.
Under normal circumstances, the aluminum alloys that can be used as conductors are mainly 1 series, 6 series and 8 series series, among which the main grades of 1 series are 1350/1050, the main grades of 6 series aluminum alloy are 6101, and the main grades of 8 series aluminum alloy are 8030/8176. The main differences between different aluminum alloys are as follows:
The main characteristics of 1 series aluminum alloys are that they contain more than 99.00% aluminum, have a conductivity of about 61%, good corrosion resistance, and good welding performance. Their disadvantages are that they are soft, have low strength, and have low connection strength;
The main characteristics of 6 series aluminum alloys are that they are mainly strengthened by magnesium and silicon, have good mechanical properties and conductivity, and are suitable for bolt connection. Generally, their conductivity is about 55% IACS;
The main characteristics of 8 series aluminum alloys are that some rare earth or trace elements will be added to the alloy system to play a strengthening role. They have high mechanical strength and their creep resistance is basically comparable to that of copper alloys, as shown in Figure 1. The addition of alloy components to aluminum alloy conductors greatly improves their conductivity and connection performance. When the current is overloaded, the alloy components play a continuous connection role, which increases the creep resistance of aluminum alloy conductors.
Copper-aluminum connection method
The connection reliability of copper terminals or copper bars and aluminum conductor harnesses needs to consider mechanical properties such as creep resistance and stress relaxation, and also needs to consider the electrical connection problems caused by the oxide film on the aluminum surface. Generally, the main ways to connect copper and aluminum are flash butt welding, brazing, stir friction welding and bolt connection. Among them, flash butt welding, brazing, stir friction welding and other welding connections can effectively avoid the problem of aluminum surface oxide film, while bolt connection needs to carefully consider the challenges brought by aluminum surface oxide film.
The following issues need to be considered when welding copper and aluminum.
The thermal expansion coefficient and thermal conductivity of copper and aluminum are different. The expansion amount when heated during welding is different. If the heating is slow, the expansion difference will be greater if the time is too long. Therefore, it is necessary to pay attention to controlling the heating condition of the welding surface and the welding time during welding.
The welded joint obtained by composite welding of dissimilar metal materials is relatively brittle. According to the copper-aluminum binary phase diagram in Figure 2, it can be seen that brittle phases are easily generated during the melting and welding of copper and aluminum. This is because copper-aluminum compounds are easily generated at the joint, and its main component is copper aluminide, that is, brittle compounds are generated at the copper-aluminum weld, which can easily lead to a decrease in the strength of the welded joint.
The melting points of copper and aluminum differ greatly, about 400℃. When welding, aluminum is easily melted but copper is not fully welded, so an appropriate process needs to be debugged.
Another common connection method for copper-aluminum connection is bolt connection. If 6101 is used as the conductor for copper-aluminum connection, bolt connection or welding connection can be selected. When bolt connection is selected, a bolt connection structure that prevents loosening should be designed to prevent the thermal expansion and contraction caused by dry and hot shocks in the copper-aluminum connection, which may cause gaps or holes in the connection interface and stress relaxation. The torque setting for bolt connection requires a series of tests or calculations to obtain the appropriate contact stress and contact resistance. The judgment condition can be determined according to the ratio of contact resistance change to contact stress change <-0.1uQ/MPa. The corresponding torque at this time is the appropriate torque, and its calculation formula is shown in the figure below.
Where: mv is the ratio of resistance to stress change, and it is considered to be stable contact when it is <-0.1uQ/MPa; aRv is the contact resistance change value; dom is the contact stress change value.
The load curve when tin and copper, tin and tin are connected. The dotted line is an asymptote with a slope of -0.1. When the dotted line is tangent to the solid line, it means that the stress and resistance at this time are relatively appropriate, and the stress must not be less than this value. Therefore, the torque of the bolt connection needs to be verified by experiment before it can be set as a reliable value. For example, the M6 bolt has different materials, gaskets, and contact areas, and needs to be set to different torques to obtain stable connection performance. Generally, the national standard M6 bolt torque can be set to 8.5N·m.
The several connection methods of copper-aluminum connection have their own advantages and disadvantages. Whether they can match the reliable connection of copper terminals and aluminum bars of electric vehicles requires testing and evaluation of the mechanical properties and electrical properties after connection.
Experimental verification
Welding performance test of copper and aluminum bars
Test materials and methods for copper and aluminum bar welding connection joints
Copper and aluminum bars were selected as research objects. The copper bar material was T2 copper and the aluminum bar material was 1350 aluminum alloy. Flash butt welding, silver brazing, stir friction welding and ultrasonic welding were used for welding connection. Flash butt welding and stir friction welding were used for butt connection, and silver brazing and ultrasonic welding were used for lap welding. The welding form is shown in Figure 4. The size of copper is 4.5mmx45mmx500mm, and the size of aluminum bar is 4.5mmx45mmx500mm.
The mechanical and electrical properties of the welded samples were tested. The mechanical property test mainly tested the pull-off force of the joint in the 180°direction, and the electrical performance test mainly tested the connection resistance and temperature rise under 380A conditions. The two welding connections of lap welding need to test the peeling force of the 90° line of defense additionally.
Experimental results of copper and aluminum bar welding connection joints
The test results of the four copper and aluminum bar welding joints are shown in Table 2. By comparing the pull-off force in the 180° direction, it can be seen that the flash butt welding and stir friction welding have better bonding strength and lower connection resistance. At the rated current of 380A, after 2h of loading, the temperature rise at the joint position is relatively close, 36K and 34K respectively. The pull-off force in the 180° direction of silver brazing and ultrasonic welding is slightly smaller, and the peeling force in the 90° direction is small, which does not meet the use requirements. The contact resistance and temperature rise of silver soldering are high, and the current carrying capacity is poor.
Performance test of copper-aluminum bolt connection joint
The material of the aluminum busbar is 6101 aluminum alloy, the material of the copper busbar is T2 copper, the size of the aluminum busbar material is 4.5mmx45mm, and the size of the copper busbar material is 3.5mmx35mm. The samples are connected with a torque of 8.5N·m using M6 bolts, and the electrical performance tests are carried out before and after salt spray. Sample type 1 is an aluminum busbar without electroplating, sample type 2 is an aluminum busbar with silver electroplating, sample type 3 is a copper busbar without electroplating, and sample type 4 is a copper busbar with silver plating on the surface. The salt spray condition is 96h neutral salt spray, the temperature rise test current is 380A, and the temperature rise test results.
Experimental analysis
Copper-aluminum welding joint test analysis
Through the test results of the four welding methods, it can be seen that flash butt welding forms a relatively reliable connection joint by chemically fusing aluminum and copper bars, and forms a Cu/AI mixture at the joint interface. It has good mechanical and electrical properties, and the tensile strength is the highest among the four welding methods. The welding resistance is low, and the temperature rise is relatively small, which is suitable for the welding connection of copper and aluminum.
High-voltage wiring harnesses have high requirements for technical cleanliness and are usually produced in an environment that meets CG2 or above in VDA19. Since flash butt welding is not environmentally friendly during the production process and the size control is difficult, this connection method is not recommended unless necessary.
Since silver brazing is a brazing material with silver-copper alloy added to the copper-aluminum connection interface, silver melts first during welding because its melting point is lower than that of copper. Under the condition of applying pressure, copper and aluminum are welded together, but the melting point of silver is higher than that of aluminum, which makes the deformation of copper and aluminum unbalanced when heated and pressed, resulting in insufficient strength of the joint when stretched and peeled. From the test results, it can be seen that the peeling force and connection voltage of silver brazing are the worst among the three connection methods. This connection method is not recommended for copper-aluminum connection in high-voltage wiring harnesses.
Friction stir welding is a solid phase connection method. During the friction stir welding process, a cylindrical stirring head with a special shoulder and a pin convex is rotated and inserted into the workpiece to be welded. The friction between the stirring head and the welded material generates friction heat, which makes the material thermoplastic.
When the stirring tool moves forward along the interface to be welded, the thermoplasticized material is transferred from the front to the rear of the stirring head, and the solid phase connection between the workpieces is achieved under the action of mechanical forging of the stirring tool.
The test results show that the tensile properties and connection resistance of stir friction welding are good. It is one of the relatively reliable methods among the four connection methods. The manufacturing process is simple and can be applied to the welding of 1 series, 6 series and 8 series aluminum alloys. It has the characteristics of high efficiency and stable production. It is a good copper-aluminum connection method.
Ultrasonic welding combines the welded parts together through the high-frequency vibration of the amplitude and the pressure of the welding head. This connection method is more maturely used in the connection between wires and terminals, and can obtain lower resistance and higher bonding force. The test results show that the connection resistance and temperature rise are relatively low, but the connection strength is low. The main reason is that in the connection between copper terminals and aluminum bars, when the thickness of the copper-aluminum bar is relatively small, the equipment welding energy is sufficient, and ultrasonic welding is reliable. However, for the larger specifications of the aluminum bar, the welding is not thorough due to the influence of the welding equipment energy, and the connection strength is low.
In summary, among the four welding connection methods, the method suitable for high-voltage wiring harnesses is mainly stir friction welding. This welding method has a stable and reliable process and is relatively environmentally friendly.
Test analysis of copper-aluminum bolted joints
Due to the dense oxide layer on the surface of aluminum alloy conductors, the conductivity of the oxide film is relatively poor, and a corresponding structure is required to puncture it to improve the conductivity of the termination. If bolt connection is used, special structures and appropriate torques need to be designed to ensure the reliability of the connection. Based on previous experience data, M6 bolts need to use a torque of 8.5N.m to obtain better mechanical and electrical connections.
According to the data in Table 3, when the aluminum surface is not electroplated, its connection with copper and aluminum in any state is in an unreliable state, and the temperature rise after salt spray increases sharply, making the product unsafe. After the aluminum surface is silver-plated, the temperature rise before and after salt spray is qualified when connected with bare copper and silver-plated copper.
When the unplated aluminum busbar is connected to the unplated copper busbar, the temperature difference before and after salt spray is >200℃℃. The main reason is that there is a potential difference between aluminum and copper. When there is a salt solution medium, electrochemical corrosion occurs at the copper-aluminum interface, forming defects such as voids, which accelerates the increase of the connection resistance. As the current carrying time increases, the temperature continues to increase.
When the unplated aluminum is connected to the plated copper busbar, although the copper busbar has an electroplated silver layer, the temperature rise difference is still close to 200℃ after 96h salt spray, indicating that serious electrochemical corrosion still occurs between copper and aluminum.
Most storage and transportation environments are not airtight, and there are a lot of water vapor and medium in the environment. In the presence of electrolytes, the copper-aluminum connection interface is very likely to form a galvanic reaction and electrochemical corrosion, which leads to a sharp increase in contact resistance and temperature rise, causing the product to fail.
Therefore, for high-voltage wiring harness products, when the copper-aluminum connection may be in a non-dry environment, it is recommended to silver-plate the copper and aluminum surfaces to prevent electrochemical corrosion and obtain more reliable connection performance.
Conclusion
Through the connection analysis of aluminum alloy and copper conductor, the following conclusions are drawn in terms of aluminum busbar material selection, connection method, surface treatment, etc.
By comparing the mechanical properties and electrical properties of different welding methods, stir friction welding is recommended as a reliable connection method.
When bolt connection is selected for copper-aluminum connection, if the copper-aluminum connection area cannot be guaranteed to be in a dry environment, it is recommended to silver-plate the surface of the aluminum busbar. At the same time, an anti-loosening structure should be designed to prevent stress relaxation caused by the difference in thermal expansion coefficients of copper and aluminum, so as to obtain a more stable electrical connection.
Post time: Oct-28-2024
