Plasma torch is a device that produces plasma arc and conducts welding during plasma arc welding. Plasma arc welding gun is mainly composed of three parts: upper gun body, lower gun body and nozzle. The function of the upper gun body is to fix the electrode, cool the electrode, conduct electricity, adjust the length of the tungsten pole, etc. The function of the lower gun body is to fix the nozzle and protective cover, cool the lower gun body and nozzle, transport ion gas and protective gas, so that the nozzle conducts electricity, etc. Reliable insulation, good air tightness and high coaxiality are required between upper and lower gun bodies.
1 insulation cap 2 ionic gas inlet 3 cooling water outlet 4 non-transfer arc and transfer superconducting wire (connected to the negative pole of power supply) 5 non-transfer lone Line (connected to the positive pole of power supply) 6 cooling water inlet 7 protective gas inlet 8 tungsten electrode 9 protective gas cover 10 pressure nozzle
1 protective cover; 2 nozzle gland; 3 tungsten electrode; 4 the nozzle; 6 Sealing gasket; 7 gas screen; 8 shots; 9 Insulation column; 10 sealing gasket; 11 insulation sleeve; 12 Mount the gun body; 13 tungsten electrode clamp; 14 the sleeve; 15 Pressing nut; 16 Insulation cap; 17 Adjusting screw; 18 Insulation cover; 19 Sealing gasket; 20 brass washer; 21 water and electricity joint; 22 Insulating handle
1 nozzle; 2 protective cover; 3 pairs of central 4 air screen; 5 shots; 6 insulation sleeve; 7 tungsten clamp; Tungsten electrode; 9. Mount the gun body; 10 Adjusting nut; 11, 14 sealing gasket; 12 Insulation cover; 13 compression spring; 15 tungsten sleeve; 16 Insulation cap; 17 Welding gun handle; 18 insulation column
Nozzle is a key part of plasma arc welding gun. Its basic structure is shown in Figure Nozzle infrastructure. The structure and size of the nozzle play a decisive role in the performance of the plasma arc. Its main size is the nozzle aperture d, the length of the passage l, and the compression Angle a
Figure Nozzle infrastructure The first two nozzles are cylindrical compression channels, which are widely used in plasma arc welding. FIG.Nozzle infrastructure c shows the convergent diffusion single-pass nozzle, which weakens the compression effect on the plasma arc, but this nozzle is suitable for welding thick plates with high current. The nozzle shown in Figure Nozzle infrastructure b is cylindrical with three holes. Three hole nozzle in addition to the center of the main hole, its left and right each have a small hole, mutual symmetry. The plasma jet from the two holes can change the circular temperature field produced by the plasma arc into an ellipse. When the long axis of the elliptic temperature field is parallel to the welding direction, the welding speed can be increased and the width of the weld heat affected zone can be reduced. For example, cylindrical three-pass nozzles can weld 30-50% faster than single-pass nozzles.
(a) cylindrical single pass (b) cylindrical triple pass (c) Convergent extended single pass type
Aperture d determines the diameter and energy density of the plasma arc. The size of d is determined by the current and the flow rate of ionic gas. Table 《The relationship between plasma arc current and nozzle aperture》 lists the relationship between plasma arc current and nozzle aperture. For a certain current value and ionic gas flow rate, the larger the aperture, the smaller the compression effect. If the aperture is too large, the compression effect is lost; If the aperture is too small, it will cause double arc phenomenon and destroy the stability of plasma arc.
The relationship between plasma arc current and nozzle aperture
| orifice diameter d(mm) | 0.8 | 1.6 | 2.1 | 2.5 | 3.2 | 4.8 |
| Plasma arc current (A) | 1~25 | 20~75 | 40~100 | 100~20 | 150~300 | 200~500 |
| Ion airflow (Ar) (L/min) | 0.24 | 0.47 | 0.94 | 1.89 | 2.36 | 2.83 |
When the aperture d is constant and the length l increases, the compression effect on the plasma arc is also enhanced. Usually, ld is used to represent the compression characteristics of nozzle passage, which is called the passage ratio. See Table 《Nozzle pass ratio》 for the commonly used pore channel ratio. When the channel ratio exceeds a certain value, double arcs will also occur.
Nozzle pass ratio
| nozzle diameter d(mm) | orifice throat ratio l/d | compression angle α | Plasma arc type |
| 0.6~1.2 | 2.0~6.0 | 25~45 | Associative arc |
| 1.6~3.5 | 1.0~1.2 | 60~90 | transferred arc |
The compression Angle has little effect on the compression of plasma arc. Considering the match with the extreme shape of tungsten, a is usually selected as 60°~90°, among which 60° is the most widely used.
Generally, pure copper is used as nozzle material. Direct water cooling must be used for high-power nozzles. In order to improve the cooling effect, the wall thickness of nozzles should not be greater than 2 ~ 2.5mm.
Plasma arc welding gun uses needle tungsten or cerium tungsten electrode. Tungsten electrodes with 0.15%~0.40% mass fraction are also used abroad. Table《Thorium tungsten electrode diameter and permissible current range》lists the permissible current ranges for tungsten electrodes.
| The electrode diameter d(mm) | 0.25 | 0.5 | 1.0 | 1.6 | 2.4 | 3.2 | 4.0 | 5.0~9.0 |
| current range(A) | <=15 | 5~20 | 15~80 | 70~150 | 150~250 | 250~400 | 400~500 | 500~1000 |
Thorium tungsten electrode diameter and permissible current range
Figure 《Electric extreme shape》 shows the common electrical extremes. In order to facilitate the arc initiation and ensure the stability of the plasma arc, the electric extreme part is generally ground into a sharp cone Angle of 30°-60°, and the top is slightly ground flat. When the tungsten electrode diameter is large and the current is large, the electrode can also be ground into other shapes to slow down the burning loss.
(a) tapered; (b) Round platform; (c) Tapered round platform; (d) conical sphericity; (e) spherical
FIG. 《The shrinkage length and coaxiality of the electrode》a shows the shrinkage length of the electrode, which has great influence on the compression and stability of the plasma arc. Generally, lg=(1±0.2)mm is selected to increase, then the compression degree is improved; Lg is too large, easy to produce double arc phenomenon.
Coaxiality has an important effect on the stability of plasma arc and weld forming. The eccentric electrode will cause plasma arc deflection, bad weld forming and easy to form double arc. The coaxiality of the electrode can be measured according to the distribution of high frequency sparks between the electrode and the nozzle (Figure 《The shrinkage length and coaxiality of the electrode》b). In welding, it is generally required that high frequency sparks cover 75% to 80% of the circumference.
(a) Shrinkage of the electrode; (b) Distribution of electrode coaxiality and high frequency spark
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