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The tungsten electrode is an electrode of argon arc welding, usually the negative pole of the power supply. The quality of the tungsten electrode material directly affects the welding quality, therefore, the tungsten electrode must meet strict requirements.
The role and requirements of tungsten electrode
The role of tungsten
Tungsten is a refractory metal material that can withstand high temperatures. Its melting point is 3653~3873K and its boiling point is 6173K. It has good electrical conductivity and high strength. During argon arc welding, the tungsten electrode is used as an electrode to conduct current, ignite the arc and maintain the normal combustion of the arc.
Requirements for tungsten electrodes
In addition to high temperature resistance, good electrical conductivity, and high strength, the tungsten electrode should also have a strong ability to emit electrons (easy arc ignition, stable arc), large current carrying capacity, long life and good pollution resistance.
Tungsten electrodes must be cleaned, polished or ground. Cleaning and polishing refers to the removal of surface impurities by chemical cleaning after drawing or forging processing. See Table Types and chemical composition requirements of tungsten electrodes for the chemical composition requirements of tungsten electrodes.
Types, grades and specifications of tungsten electrodes
Tungsten electrodes are classified according to their chemical composition: pure tungsten electrodes (brands are W1, W2), thoriated tungsten electrodes (brands are WTh-7, WTh-10, WTh-15), cerium tungsten electrodes (brands are WCe-20), zirconium tungsten There are five kinds of poles (the brand is WZr-15) and lanthanum tungsten poles. The length range is 76~610mm, and the available diameter range is generally 0.5~6.3mm.
Characteristics of various tungsten electrodes
Pure tungsten electrode
Pure tungsten electrode w (W) is more than 99.85%, the price of pure tungsten electrode is not very high, and it is generally used in situations where the requirements are not strict. When using alternating current, the current carrying capacity of pure tungsten electrode is low, and the anti-pollution ability is poor, which requires a high no-load voltage for the welding machine, so it is rarely used at present.
Thorium tungsten electrode
Thorium tungsten electrode This is a tungsten electrode added with thorium oxide with a mass fraction of 1% to 2%. It has a high electron emission rate, a good current carrying capacity, a long service life, and good anti-pollution performance. When using this kind of tungsten electrode, the arc ignition is easier and the arc is more stable. Its disadvantages are high cost and trace radioactivity.
Cerium Tungsten
A cerium tungsten electrode is made by adding 2% cerium oxide to pure tungsten. Compared with thoriated tungsten electrode, it has the following advantages: when welding with low DC current, it is easy to establish an arc, the arc ignition voltage is 50% lower than that of thoriated tungsten electrode, and the arc burns stably; the compression degree of the arc column is better. , the arc beam is longer, the heat is concentrated, the burning rate is 5%~50% lower than that of thoriated tungsten, the number of grinding ends is less, and the service life is extremely longer than that of thoriated tungsten; the maximum allowable current density is 5% higher than that of thoriated tungsten %~8%; radioactivity is extremely low. It is the tungsten electrode suggested by our country to use as much as possible.
Zirconium Tungsten
The performance of zirconium tungsten is between pure tungsten and thoriated tungsten. When used for AC welding, it has the ideal stability characteristics of pure tungsten electrode and the comprehensive performance of thoriated tungsten electrode’s current carrying capacity and arc ignition characteristics.
Lanthanum Tungsten
It is still under development, and China has designated it as the fifth category.
Grade definition
It is more popular to determine the grade according to its chemical element symbol and the average content of the chemical composition. The grade is shown in Figure Examples of tungsten grades as an example. The grades and chemical compositions of various tungsten electrodes are shown in Table Types and chemical composition requirements of tungsten electrodes.
Specification of tungsten electrode
Manufacturers supply tungsten electrodes with a length ranging from 76 to 610 mm. The diameters of commonly used tungsten electrodes are:
0.5mm、1.0mm、1.6mm、2.0mm、2.5mm、3.2mm、4.0mm、5.0mm、6.3mm、8.0mm and 10mm.
Carrying capacity of tungsten electrode
The carrying capacity of the tungsten electrode is also called the allowable current of the tungsten electrode. The current carrying capacity of the tungsten electrode is mainly determined by the diameter, current type and polarity. If the welding current exceeds the allowable current of the tungsten electrode, the tungsten electrode will be strongly heated, melted and evaporated, which will cause arc instability, affect the welding quality, and cause defects such as pores and tungsten inclusions in the weld; at the same time, the shape of the weld is rough and irregular . Table Recommended allowable current range according to tungsten electrode diameter lists the recommended allowable current range according to the electrode diameter. During the welding process, the welding current shall not exceed the allowable current upper limit stipulated by the tungsten electrode.
Tungsten tip geometry
The shape of the tip of the tungsten pole has a great influence on the combustion stability of the welding arc and the shape of the weld seam.
When using alternating current, the tip of the tungsten pole should be ground into a hemispherical shape; when using direct current, the tip of the tungsten pole is conical or frusto-conical, which is easy to ignite the arc at high frequency, and the arc is relatively stable. The taper at the end of the tungsten pole also affects the penetration depth of the weld. Reducing the taper angle can reduce the width of the weld bead and increase the penetration depth of the weld. Commonly used tungsten electrode tip geometry is shown in Figure Common Tungsten Electrode Tip Geometry.
Grinding the tungsten electrode should use a special hard abrasive fine grinding wheel, and the uniformity of the geometric shape after grinding the tungsten electrode should be maintained. When grinding tungsten electrodes, a sealed or ventilated grinder should be used, a mask and protective glasses should be worn during grinding, and hands and face should be cleaned after grinding.