Copper aluminum connection solution for electric vehicle high voltage wiring harness

In order to improve the driving range of the vehicle and reduce energy consumption, the high-voltage wiring harness needs to be lightweight design. Aluminum and aluminum alloy conductors have the characteristics of good electrical conductivity and low density, and are one of the good conductors for lightweight automotive wire harnesses. This paper mainly talks about the copper-aluminum connection of high voltage wire harnesses. The following is the main text.

Due to its own physical properties, aluminum conductors have a certain gap with copper conductors in conductivity, mechanical properties and creep properties. In particular, due to the large difference in thermal expansion coefficient of copper-aluminum connections, gaps or holes are easily formed at the connection interface during the cold and thermal shock during charging and discharging, resulting in increased resistance and temperature rise. In addition, due to the potential difference between copper and aluminum, it is easy to form electrochemical corrosion. Therefore, if the copper-aluminum connection can be more stable and safe application, there must be a good solution in terms of mechanical properties and electrochemical corrosion.

01 Problems in copper-aluminum connection

The traditional high-voltage wiring harness is mainly composed of copper cables, and the two ends of the copper cable conductor are connected to copper terminals or copper bars, which can be easily obtained and reliable connection without electrochemical corrosion problems. In the context of the development of charging power to high-power charging, the current charging technical standard "GB/ T20234.1-2015" allows the maximum charging current to be 250A, and the standard being developed and updated has raised the charging current to 400A, 600A, and even more than 1000A. In the case of no cooling measures at the end of the vehicle, when the current is increased, according to Joule's law (Q=I2Rt), the conductor resistance must be reduced to prevent the vehicle from thermal failure and other problems. The effective measure to reduce the resistance is to increase the cross-sectional area of the conductor, under normal circumstances, the maximum current carrying capacity of 120mm2 copper cable is 500A, in order to obtain higher current carrying capacity, the cross-sectional area of the cable is larger than 120mm2, such a large size will lead to wiring harness mass overweight, bending radius is too large and other problems. Therefore, lightweight conductors such as aluminum bars or rods will usher in the application of opportunities. Because of its rectangular shape, the aluminum bar has a larger heat dissipation area and better conductivity under the same cross-sectional area. When the wiring harness is arranged, the rectangular shape is conducive to the laying of the plane and occupies less space. At the same time, the structural rigidity of the aluminum bar makes it possible to use less wiring harness to fix the buckle and obtain good wiring harness layout performance. According to the material properties of aluminum conductor itself, there are some inherent problems in mechanical properties, electrical properties and corrosion resistance when connecting with copper terminals

02 Copper-aluminum connection scheme

2.1 Selection of aluminum alloy materials

The physical parameters of copper and aluminum have obvious differences in mechanical and electrical properties.

Aluminum has a lower strength than copper, but its coefficient of thermal expansion is 1.35 times that of copper. The difference in the coefficient of thermal expansion is a challenge to the reliability of copper-aluminum connections. The specific choice of which series of aluminum alloy needs to be selected according to the connection scheme. For example, 1 series aluminum alloy with good weldability can be selected, and 6 series or 8 series aluminum alloy with high strength and good creep resistance can be selected.

Under normal circumstances, aluminum alloys that can be used as conductors are mainly 1 series, 6 series and 8 series, of which the 1 series main grade is 1350/1050, the 6 series aluminum alloy main grade is 6101, and the 8 series aluminum alloy main grade is 8030/8176. The main differences between different aluminum alloys are as follows:

1)1 series aluminum alloy is mainly characterized by aluminum content of more than 99.00%, conductivity of about 61%, good corrosion resistance, good welding performance, its disadvantages are soft texture, low strength, connection strength is not high;

2)6 series aluminum alloy is mainly characterized by magnesium and silicon as the main strengthening phase, has good mechanical properties and electrical conductivity, suitable for bolted connection, generally its electrical conductivity of about 55%IACS;

3) The main feature of 8 series aluminum alloy is that some rare earth or trace elements are added to the alloy system to play a strengthening role, with high mechanical strength, and its creep resistance is basically comparable to that of copper alloy, as shown in Figure 1. The addition of alloy components of aluminum alloy conductors greatly improves their electrical conductivity and connection properties. When the current is overloaded, the alloy components play a continuous connection role, which increases the creep resistance of aluminum alloy conductors.

2.2 Copper-aluminum connection method

The reliability of connection between copper terminal or copper bar and aluminum conductor wire harness needs to consider mechanical properties such as creep resistance and stress relaxation, and also needs to consider the electrical connection problems caused by aluminum surface oxide film. Usually, the main ways of copper and aluminum connection are flash butt welding, brazing, friction stir welding and bolt connection, of which flash butt welding, brazing, friction stir welding and other welding connections can effectively avoid the problem of aluminum surface oxide film, and bolt connection needs to carefully consider the challenges brought by aluminum surface oxide film.

Copper and aluminum creep resistance The following issues need to be considered in the welding of copper and aluminum.

1) The coefficient of thermal expansion of copper and aluminum is not the same as the thermal conductivity, the amount of expansion is not the same when welding, if the slow heating, the time is too long will make the difference between the expansion is greater, therefore, in the welding need to pay attention to control the heat of the welding surface and welding time.

2) The welded joint obtained by composite welding of dissimilar metal materials is brittle, and it is easy to form a brittle phase in the process of melting and welding of copper and aluminum. This is because it is easy to form copper and aluminum intercompounds at the joint, and its main component is copper aluminized, that is, brittle compounds are generated at the copper-aluminum welding joint, which is easy to reduce the strength of the welding head.

The melting point of copper and aluminum has a large difference, the difference is about 400℃, and it is easy to appear that aluminum has melted but copper has not been welded, and it is necessary to debug the appropriate process.

Another common connection method of copper-aluminum connection is bolt connection, and aluminum bar 6101 is selected as the conductor of copper-aluminum connection, and the connection method of bolt connection or welding connection can be selected. When the bolt connection is selected, it is necessary to design an anti-loose bolt connection structure to prevent the thermal expansion and contraction of the copper and aluminum connection due to the cold and thermal shock, resulting in gaps or holes in the connection interface and stress relaxation. For the torque setting of bolt connection, it is necessary to obtain the appropriate contact stress and contact resistance through a series of tests or calculations, and the determination condition can be determined according to the ratio of the change of contact resistance to the change of contact stress <-0.1μΩ/MPa. In this case, the corresponding torque is the appropriate torque, and its calculation formula is shown in equation (1).

Where :mv is the ratio of resistance to stress change, and when it is <-0.1μΩ/MPa, it is considered to be stable contact; ∂Rv is the change value of contact resistance. ∂σm is the change in contact stress.

Several connection methods of copper aluminum connection have advantages and disadvantages, whether they can match the reliable connection of copper terminals and aluminum bars of electric vehicles, it is necessary to test and evaluate the mechanical and electrical properties after connection.

03 Experimental Verification

Welding performance test of copper-aluminum bar

1) Copper and aluminum bars were selected as the research objects. T2 copper and 1350 aluminum were selected as the copper and aluminum bars. Flash butt welding, silver brazing, friction stir welding and ultrasonic welding were used for welding and connection, among which flash butt welding and friction stir welding were used for butt joint. Silver brazing and ultrasonic welding adopt lap welding, and the welding form is shown in the figure. The size of the copper is 4.5mm×45mm×500mm, and the size of the aluminum bar is 4.5mm×45mm×500mm.

The mechanical and electrical properties of the welded sample are tested in the cop-aluminum connection mode. The mechanical properties test mainly tests the pull out force of the joint in the direction of 180°, and the electrical properties mainly test the connection resistance and temperature rise under the condition of 380A. The stripping force of the 90° line of the two kinds of welded joints is tested in the lap welding mode.

2) By comparing the pull out force in the direction of 180°, it can be seen that the combination strength of flash butt welding and friction stir welding is better, and the connection resistance is lower. At the rated current of 380A, after 2h load, the temperature rise of the joint position is relatively close, which is 36K and 34K respectively. The 180° pulling force of silver brazing and ultrasonic welding is slightly small, and the 90° peeling force is small, which does not meet the requirements of use, and the contact resistance and temperature rise of silver brazing are high, and the current carrying capacity is weak.

3) Comparison of welding performance of 4.5mm×45mm copper-aluminum bar

04 Conclusion

Through the connection analysis of aluminum alloy and copper conductor, the following conclusions are drawn in the selection of aluminum bar material, connection mode and surface treatment.

1) By comparing the mechanical and electrical properties of different welding methods, friction stir welding is recommended as a reliable connection method.

2) When the copper aluminum connection is bolted, if the copper aluminum connection area cannot be guaranteed to be in a dry environment, it is recommended to do silver plating on the surface of the aluminum bar, and it is necessary to design an anti-loose structure to prevent the stress relaxation behavior caused by the difference in the coefficient of thermal expansion of copper aluminum, so as to obtain a more stable electrical connection.

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