(1) Non-transfer arc. The tungsten electrode is connected to the negative electrode of the power supply, and the nozzle is connected to the positive electrode of the power supply. The plasma arc is generated between the tungsten electrode and the inner surface of the nozzle (see Figure 11-2a). The workpiece itself is not energized, but is indirectly heated and melted. The effective utilization of heat It is not high, so it is not suitable for welding and cutting of thicker materials.
(2) Transfer arc. The tungsten electrode is connected to the negative pole of the power supply, and the welding piece is connected to the positive pole of the power supply. First, after a small arc is ignited between the tungsten pole and the nozzle, the circuit between the tungsten pole and the welding piece is immediately connected, and then the circuit between the nozzle and the tungsten pole is cut off. At the same time, the arc between the tungsten electrode and the nozzle is extinguished, and the arc is transferred to the tungsten electrode and the weldment to burn directly. This type of arc is called a transfer arc (see Figure 11-2b). This plasma arc can directly heat the workpiece and improve the effective utilization of heat, so it can be used for welding and cutting workpieces with medium thickness and above.
3) Combined arc. A plasma arc in which a transition arc and a non-transition arc coexist is called a combined arc (see Figure 11-2c). The two arcs of the combined arc are powered by two power sources respectively. The main power source is applied between the tungsten electrode and the weldment to generate a plasma arc, which is the main welding heat source. Another power source is applied between the tungsten electrode and the nozzle to generate a small arc, called a sustaining arc. Combined arc is mainly used for micro-arc plasma welding and spray welding of powder materials.
（1) Small hole type plasma arc welding. Using the characteristics of high plasma arc energy density and good arc stiffness, the welding part of the weldment is completely penetrated, and a small hole through the weldment is produced. Under the action of surface tension, the molten metal does not drip from the small holes (the small hole effect). As the torch is moved forward, the small hole locks behind the arc, resulting in a fully penetrated weld. The welding current range of small hole plasma arc welding is 100~300A, which is suitable for welding 2~8mm thick alloy steel plates.
(2) Penetration plasma arc welding. When the plasma gas flow is small and the compression degree of the arc column is weak, the plasma arc only penetrates the weldment during the welding process, but the fusion welding process without the pinhole effect is called penetration type plasma arc welding, which is mainly used for thin plates. Single-sided welding, double-sided forming and multi-layer welding of thick plates.
(3) Micro-beam plasma arc welding. Penetration plasma arc welding with welding current below 30A is called microbeam plasma arc welding. When the welding current is less than 10A, the arc is unstable, so the combined arc is often used. Even when the welding current is as small as 0.05~10A, the arc still has good stability. Generally used for welding filaments and foils.
Plasma arc welding is mainly used for welding stainless steel, heat-resistant steel, titanium alloy, and refractory and special metal materials such as tungsten and molybdenum.
Plasma arc welding can also use a specially constructed welding torch to generate a plasma arc for surfacing, spray welding and spraying on the surface of the weldment.
The plasma arc temperature is as high as tens of thousands of degrees Celsius, and the concentration of ozone, nitrogen oxides and other toxic gases and metal dust generated by high temperature and strong arc radiation is much higher than that of argon-induced welding. The relative intensity of ultraviolet radiation with a wavelength of 2600~2900A is 1.0 for argon arc welding and 2.2 for plasma arc welding. Toxic gases and ultraviolet radiation are the main harmful factors.
The velocity of the plasma arc flame is very high, and when it is ejected from the nozzle at a high velocity of 10,000 m/min, noise is generated. In addition, during the operation of plasma arc welding, there are harmful factors such as high-frequency electromagnetic field, thermal radiation, radioactivity and electric shock.