【Abstract】 With the application of aluminum wire in automotive wiring harness gradually widely, this paper analyzes the connection technology of aluminum power harness, analyzes and compares the performance of different connection methods, so as to select the connection mode of aluminum power harness in the later stage.

01 Overview

With the promotion of the application of aluminum wire harness in automobiles, the application of aluminum wire instead of traditional copper wire is gradually increasing. However, in the application process of aluminum wire replacing copper wire, electrochemical corrosion, high temperature creep and conductor oxidation are the problems that must be faced and solved in the application process. At the same time, the application of aluminum wire replacing copper wire should meet the electrical and mechanical properties of the original copper wire to avoid performance degradation.

In order to solve the problems of electrochemical corrosion, high temperature creep and conductor oxidation in the application process of aluminum wire, there are four mainstream connection methods in the industry at present, which are: friction welding pressure connection, friction welding, ultrasonic welding and plasma welding.

The connection principle and structure of these four kinds of connections are analyzed and their performance is compared below.

02 Friction welding pressure connection

Friction welding pressure joint, first using copper rod and aluminum rod friction welding, and then stamping the copper rod to form the electrical connection end. The aluminum rod is machined to form the aluminum crimping end, and the copper-aluminum terminal is made. Then the aluminum wire is inserted into the copper aluminum terminal. The aluminum crimping end is hydraulically crimped through the traditional wire harness crimping device to complete the connection between the aluminum wire and the copper aluminum terminal, as shown in Figure 1.

Figure 1

Compared with other forms of connection, friction welding pressure joint, through the friction welding of copper rod and aluminum rod, the copper aluminum alloy transition zone is formed, and the welding surface is more uniform and dense, which effectively avoids the thermal creep problem caused by the different thermal expansion coefficient of copper and aluminum, and the formation of alloy transition zone also effectively avoids the electrochemical corrosion caused by the different metal activity between copper and aluminum. Subsequently, the salt spray water vapor is isolated through the heat shrink tube seal, which also effectively avoids the generation of electrochemical corrosion. Through the hydraulic crimping of the aluminum wire and the aluminum crimp end of the copper-aluminum terminal, the aluminum wire monofilm structure and the oxide layer of the inner wall of the aluminum crimp end are destroyed and stripped, and then the cold welding is completed between the monofilm and the aluminum conductor and the inner wall of the crimp end, which improves the electrical performance of the connection and provides the most reliable mechanical properties.

03 Friction Welding

In friction welding, aluminum tubes are used to crimp and shape aluminum wires. After cutting the end face, friction welding is performed with copper terminals. The welding connection between conductor and copper terminal is completed through friction welding, as shown in Figure 2.

Figure 2

First, the aluminum tube is installed on the conductor of the aluminum wire through crimping connection, and the monofilm structure of the conductor is plastic through crimping to form a tight circular section, and then the welding section is smoothed through turning to complete the preparation of the welding surface. One end of the copper terminal is the electrical end connection structure, and the other end is the welding connection surface of the copper terminal. The welding surface of the copper terminal and the welding surface of the aluminum wire are welded by friction welding, and then the welding flash is cut and trimmed to complete the connection processing of the friction welding aluminum wire.

Compared with other connection forms, friction welding forms a transition connection between copper and aluminum through the friction welding connection between copper terminals and aluminum wires, effectively reducing the electrochemical corrosion of copper and aluminum. The copper aluminum friction welding transition zone is sealed by a heat shrink tube with adhesive in the later stage, and the welding area will not come into contact with air and moisture, further reducing corrosion. In addition, the welding area is a direct connection between the aluminum conductor and the copper terminal through welding, effectively increasing the drawing force of the joint, and the processing process is simple.

However, the disadvantages also exist when aluminum wires are connected to copper and aluminum terminals as shown in Figure 1. The application of friction welding in wire harness manufacturers requires the addition of special friction welding equipment, which has poor versatility and increases the fixed asset investment of wire harness manufacturers. Secondly, in the process of friction welding, the mono-wire structure of the wire and copper terminals are directly friction welded, resulting in holes in the friction welding connection area. The presence of impurities such as dust will affect the final welding quality, resulting in the mechanical and electrical properties of the welded connection are unstable.

04 Ultrasonic welding

Ultrasonic welding of aluminum wires is to use ultrasonic welding equipment to connect aluminum wires and copper terminals through the high-frequency oscillation of the welding head of the ultrasonic welding equipment to connect aluminum wires and aluminum wires and copper terminals together to complete the connection between aluminum wires and copper terminals, as shown in Figure 3.

Figure 3

Ultrasonic welding connection is the aluminum wire and copper terminal in the case of ultrasonic high-frequency vibration, the vibration friction between copper and aluminum to complete the connection between copper and aluminum, because copper and aluminum are face-centered cubic metal crystal structure, in the high-frequency oscillation environment, to complete the atomic replacement in the metal crystal structure, forming an alloy transition layer, effectively avoiding the occurrence of electrochemical corrosion. At the same time, during the ultrasonic welding process, the oxide layer on the surface of the aluminum conductor monofilaments is peeled off, and then the welding combination between the monofilaments is completed, which improves the electrical and mechanical properties of the joint.

Compared with other connection forms, ultrasonic welding equipment is a common processing equipment for wire harness manufacturers, without the need for new fixed asset investment, and the terminal uses copper stamping terminals, the terminal cost is lower, so it has the best cost advantage. However, the disadvantages also exist, compared with other connection forms, the mechanical properties of ultrasonic welding are weak, and the vibration resistance is poor, so the use of ultrasonic welding connections is not recommended in the high-frequency vibration area.

05 Plasma Welding

In plasma welding, copper terminal and aluminum wire are crimped and connected. Then, by adding solder, plasma arc is used to heat the welding area, melt the solder, fill the welding area, and complete the aluminum wire connection, as shown in Figure 4.

Figure 4

Plasma welding aluminum wire first adopts plasma welding copper terminal, and completes the crimping and tightening of aluminum wire through crimping. The plasma welding terminal forms a cylindrical structure after crimping. Then, the welding area of the terminal is filled with zinc-containing solder, and the zinc-containing solder is added to the crimping end. Then through the capillary action into the crimp area of the wire gap, to complete the copper terminal and aluminum wire connection processing.

Plasma welding aluminum wire through crimping to complete the tight connection of aluminum wire and copper terminals, providing reliable mechanical properties. At the same time, in the crimping process, through 70% to 80% compression ratio, complete the damage and stripping of the conductor oxide layer, effectively improve the electrical performance, reduce the contact resistance of the connection point, prevent the heat of the connection point. Then, zinc-containing solder is added to the end of the crimping zone, and the welding zone is heated by equal ion beam. The zinc-containing solder is heated and melted, and the gap filling of the solder in the crimping zone is completed through capillary action, so that the salt spray and water vapor in the crimping zone can be isolated and electrochemical corrosion can be avoided. At the same time, because the solder is isolated and buffered, a transition zone is formed, which effectively avoids the occurrence of thermal creep and reduces the risk of increasing the connection resistance under hot and cold shock. Through the plasma welding of the connection area, the electrical properties of the connection area are effectively improved, and the mechanical properties of the connection area are further improved.

Compared with other forms of connection, plasma welding separates the copper terminal from the aluminum wire through the transition welding layer and the strengthening welding layer, effectively reducing the electrochemical corrosion of copper and aluminum. The reinforced welding layer wraps the end face of the aluminum wire, and the copper terminal and the guide core of the wire will not come into contact with air and water, further reducing corrosion. In addition, the transition welding layer and the reinforced welding layer tightly fix the copper terminal and the aluminum wire joint, effectively increasing the drawing force of the joint, and the processing is simple. However, the disadvantages also exist at the same time, the application of plasma welding in the harness manufacturer needs to increase the special equipment for plasma welding, which is poor in versatility and increases the fixed asset investment of the harness manufacturer. Secondly, in the plasma welding process, the solder completes the gap filling of the crimp area through the capillary action, which is uncontrollable. The final welding quality of the plasma welding joint area is not stable, resulting in large deviations in electrical and mechanical properties.

06 Summary

To sum up, aluminum wire connection methods commonly used in the four industries have their own advantages and disadvantages. No matter what kind of connection method can meet the problems of electrochemical corrosion, high temperature creep, conductor oxidation and so on during the connection between aluminum wire and copper terminal, ensuring the connection performance between aluminum wire and copper terminal, but there are differences in stability, cost, performance and other aspects. The most suitable connection can be selected according to comprehensive evaluation of cost requirements, loading environment and other factors.

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