Plasma Welding

Plasma Welding - Welding with a directed flow of the plasma arc. It has much in common with argon welding technology.

Standard notation
PAW - Plasma Arc Welding - Welding Plasma arc

The technology of plasma welding
Plasma is a partially or fully ionized gas consisting of neutral atoms and molecules, as well as electrically charged ions and electrons. In this definition the usual arc can be called a plasma. However, with respect to the usual arc of the term "plasma" is not used nearly as well as the usual arc has a relatively low temperature and has low energy content compared to the traditional concept of the plasma.

Scheme of plasma welding

To raise the temperature of the arc and the power of the ordinary and turning it into a plasma using two processes: the contraction of the arc and forced injection of plasma-forming gas in it. The scheme for obtaining the plasma arc is shown in the figure above. Compressing the arc is carried out by placing it in a special device - a plasma torch, the wall is intensively cooled by water. As a result of compression reduces the cross section of the arc and increases its capacity - the amount of energy per unit area. The temperature in the column of the ordinary arc burning in argon and iron vapor is 5000-7000 ° C. The temperature in the plasma arc is 30 000 ° C.

Simultaneously with the contraction of the zone of the plasma arc plasma-forming gas is injected, which is heated by the arc, ionized, and as a result of thermal expansion increases the volume of 50-100 times. This causes the gas to bleed out of the channel of the plasma torch nozzle at high speed. The kinetic energy of moving particles of ionized plasma gas complements the thermal energy released in an arc as a result of electrical processes taking place. Therefore, the plasma arc is a more powerful source of energy than usual.

The main features that distinguish normal from the plasma arc are:

• a higher temperature;
• smaller diameter of the arc;
• cylindrical shape of the arc (as opposed to the usual conical);
• pressure of the arc on the metal is 6-10 times higher than normal;
• ability to maintain an arc at low currents (0,2-30 A).

These distinctive features make the plasma arc as compared with the usual more universal source of heating of the metal. It provides a deeper penetration of the metal while reducing the amount of its melting. The figure shows the shape of melting for a typical arc plasma. The figure shows that the plasma arc - a concentrated source of heat and makes it groove weld large metal thickness. Because of its cylindrical shape and the possibility to significantly increase the length of an arc welding allows for tight spaces, as well as variations of the distance from the burner nozzle to the product.

The form of penetration for the conventional and plasma arc

There are two possible schemes of the process:

• plasma arc welding when the arc between a nonconsumable electrode and the workpiece,
• and the plasma jet when the arc between a nonconsumable electrode and the nozzle of the plasma torch and the gas flow is blown.

The first scheme is the most common.

As the plasma gas used in welding is usually argon, sometimes with the addition of helium or hydrogen. As an inert gas is used most often as argon. The material of the electrode - tungsten-activated yttrium, lanthanum or thorium, and hafnium and copper.

Varieties
Depending on the strength of the current are three types of plasma welding:

• microplasma (I St = 0.1-25A);
• at medium currents (I St = 50-150A);
• at high currents (I St > 150 A).

Microplasma welding
The most common is the microplasma welding. Due to the relatively high degree of ionization of the gas in the plasma torch and using tungsten electrodes with a diameter of 1-2 mm plasma arc can burn at very low currents, ranging from 0.1 A.

Scheme of microplasma welding

Special maloamperny power supply (see figure above), the DC is to get Simmer, continuously burning between the electrode and water-cooled copper nozzle. In summing up the torch to ignite the main arc of the product, which is fed from the source. Plasma-forming gas is supplied through the nozzle of the plasma torch, which has a diameter of 0.5-1.5 mm.

The protective gas is fed through a ceramic nozzle. The plasma torch is water cooled. To ignite the arc in the welding installation oscillators have the duty and the main arc.

Microplasma welding is a very effective way of fusion products of the small thickness to 1.5 mm. The diameter of the plasma arc is about 2 mm, which allows heat to concentrate on a limited section of the product and heat welding area without damaging surrounding areas. This arc has a cylindrical shape, so the depth of penetration and other parameters depend little on the seam of the arc length, which allows manipulation of a welder torch to avoid burn-through characteristic of the normal TIG welding of thin metal.

The main gas is used as the plasma and shielding is argon. However, depending on the metal to be welded to it can be additive, increasing the efficiency of the welding process. When welding steel to a protective argon suitable additive (8-10%) of hydrogen, thus improving the thermal efficiency of the plasma arc. This is due to the dissociation of hydrogen on the periphery of the arc column and its subsequent recombination with the heat on the surface of the base metal. When welding low-carbon steels can be additive to argon carbon dioxide, the welding of titanium - the additive of helium.

Plants for microplasma welding can weld in different modes: continuous line polarity, pulse polarity (allows you to control heat input), pulses of (for aluminum, ensures the destruction of the oxide film), a continuous reversed polarity. The most common installation is the MPU-4u.

The main process parameters are microplasma welding current, voltage, flow of the plasma and shielding gas, the diameter of the nozzle channel, the depth of immersion of the nozzle electrode, the diameter of the electrode.

Microplasma welding has been used successfully in the production of thin-walled tubes and vessels, membranes and welding of the bellows to the massive detail, combined foil thermocouples, in the manufacture of jewelry.

Plasma welding at medium currents
Plasma welding currents on the I St = 50-150A has a lot to do with the argon-arc welding with tungsten electrode. However, due to the higher arc power and heat a limited area, it is more effective. On the energy characteristics of plasma arc is intermediate between the normal arc and electron or laser beam. It provides a deeper penetration than the usual arc with a smaller width of the seam. In addition to energy performance, it is associated with a high-pressure arc on the weld pool, thus decreasing the thickness of the layer of liquid metal under the arc and improves heat transfer into the base metal. Welding can be done with the use of filler wire or without it.

Plasma welding at high currents
Plasma welding at currents more than I = 150A has an even greater force on the metal (plasma arc currents of 150 A to 300 A is equivalent to arc welding consumable electrode).

Accompanied by full penetration welding to form a through hole in the bath. There is a kind of cut parts followed by welding.

The formation of the seam with a through-penetration welding with a plasma at high currents

The metal on the back side of the joint is held by surface tension forces. The range of modes is very limited, as in welding may burn marks.

Plasma welding at high currents used in the fusion of low-carbon and alloy steels, copper alloys, aluminum alloys, titanium and other materials. In many cases, it can significantly reduce the costs associated with cutting edges, increase productivity, improve the quality of the welds.

Plasma welding requires high production standards, compliance with procurement and assembly technology, careful to ensure the cooling conditions of plasma torches and the rules of their operation. Even minor violations of the cooling of the plasma torch as a result of high temperatures and small-diameter nozzle lead to its destruction.

See also:
Plasma Cutting
Plasma Cutting Machines

Welding technology:

manual arc welding semi-automatic welding argon arc welding machine welding electroslag welding plasma welding laser welding electron beam welding

gas welding resistance welding spot welding seam welding projection welding butt welding welding underwater welding in space