DC argon tungsten arc welding is divided into DC positive connection and DC reverse connection.
DC positive connection means that the weldment is connected to the positive electrode, and the tungsten electrode is connected to the negative electrode. It is the most widely used form in argon tungsten arc welding. It does not have the effect of removing the oxide film, so it is generally not suitable for welding active metals, such as aluminum magnesium and its alloys. The welding of other metals generally adopts the DC positive polarity connection method, because there is no problem of producing high melting point metal oxides.
DC reverse connection means that the weldment is connected to the negative electrode, and the tungsten electrode is connected to the positive electrode. It has a function of removing the oxide film (commonly known as “cathode crushing”). However, the heat effect of DC reverse connection is unfavorable to welding, because the heat of the anode is more than that of the cathode during argon tungsten arc welding, and when the polarity is reversed, the electrons bombard the tungsten and release a lot of heat, which is easy to burn the tungsten. Therefore, In argon tungsten arc welding, DC reverse polarity connection is rarely used except for welding thin plates of aluminum, magnesium and their alloys.
AC tungsten argon arc welding is a common method for welding aluminum, magnesium and their alloys. In the negative half-wave (the workpiece is the cathode), the cathode has the cleaning effect of removing the oxide film, making the weld surface bright and ensuring the quality of the weld; and In the positive half wave (the tungsten pole is the cathode), the tungsten pole is cooled, and enough electrons can be emitted at the same time, which is conducive to stabilizing the arc. However, there are two main problems in AC argon tungsten arc welding: DC component and poor arc stability. The generation of DC components will weaken the cleaning effect of the cathode, increase the energy consumption of the power transformer, and even cause the danger of overheating or even burning the equipment. When the alternating current of argon tungsten arc welding crosses the zero point, the arc stability is poor, and measures to stabilize the arc at the zero crossing should be taken. The current argon tungsten arc welding has taken measures to eliminate the DC component and stabilize the arc.
When welding aluminum alloys with AC tungsten arc welding, DC components are obviously generated. This is related to the great difference in the physical properties of the tungsten electrode and the base material, and the difference in the arc conduction characteristics during the positive and negative half waves. As shown in Figure Schematic diagram of DC component of AC tungsten arc welding, when AC power is used, the polarity of aluminum and tungsten is constantly changing. When the tungsten pole is negative, because of its high melting point (3400°C), small section size of the tungsten pole, and small thermal conductivity of tungsten, the cathode spot falling on the tungsten pole is easy to maintain high temperature, so the ability to emit electrons is very strong , at this time the arc current is larger, but the arc voltage is lower; on the contrary, when the aluminum workpiece is negative, because the melting point of aluminum is lower, the thermal conductivity is better, the cross-sectional size is larger, and the heat dissipation capacity is stronger, the electrons of aluminum escape The power is higher, so the ability to emit electrons is weaker, so the arc current is small and the arc voltage is high. In this way, the arc voltage and arc current on the two half-waves of alternating current are not equal, which means that the arc has different conductivities in the two half-waves.
It can be seen from Figure Schematic diagram of DC component of AC tungsten arc welding that the arc current of the negative half-wave of the tungsten pole is greater than the arc current of the negative half-wave of the aluminum workpiece. In this way, it is equivalent to connecting a positive DC power supply in series in addition to the AC power supply in the welding circuit (the tungsten pole is negative and the workpiece is positive), forming a DC component in the welding circuit (the direction of the DC component is from the workpiece to the tungsten. pole), this phenomenon is called the “rectification effect” of the arc. This DC component corresponding to the positive polarity will significantly reduce the cathode crushing effect, hinder the removal of the oxide film on the surface of the molten metal, and make the arc unstable, and the weld is prone to defects such as incomplete penetration and poor forming. At the same time, due to the existence of the DC component, the iron core of the welding transformer produces a corresponding DC magnetic flux, which easily saturates the iron core, increases the excitation current of the primary coil of the welding transformer, increases the loss, and even burns out the transformer. Therefore, in AC tungsten arc welding, try to eliminate this DC component as much as possible.
There are usually four methods for removing the DC component of the AC circuit: connecting batteries in series in the welding circuit, connecting resistors in series, connecting variable resistors in parallel with rectifier components, and connecting large capacitors in series. The four methods are shown in Figure Method to Eliminate DC Component.