3. Arc plasma

New words and word combinations.

1. a plasma jet – струя плазмы

2. a transfered arc – перемещенная дуга / дуга прямого действия

3. a nozzle opening – отверстие наконечника

4. reinforced plastic – армированный пластик

5. graphite [‘grӕfait] – графит

6. a mandrel - оправка

7. to chuck in the lathe – зажимать в патроне токарного станка

8. to spin – вращение

Plasma is the condition of the stars, including the Sun, and thus is very common in the Universe, although unknown in a natural form on earth. Man can now make a plasma jet, which in this sense can be defined as a stream of gas heated to such a high temperature that at least part of the gas is ionized. Under these circumstances, controlled temperatures up to 20.000 - 25.000°F may be generated. In arc plasma equipment, this temperature is reached by passing th,e gas at very high velocities through an electric arc. Gases usually used are hydrogen, nitrogen,argon, or air.

Basically, there are two types of arc plasma devices: transferred and nontransferred arc. In transferred arc equipment, the arc traverses from to the cathode to the work which is the anode. In nоntransferred equipment the arc is struck between tungsten electrode and a water-cooled electrode.

This equipment is purely a plasma generating device and in this form the gun can be used for welding and cutting anymaterial known. Other substances can, howevеr, be introduced in the plasma stream and this leads to one of the most valuable industrial uses of plasma the spraying of materials for surface coating. Metal in the form of wire or powder is fed into the nozzle of the gun, melted in the plasma jet, and sprayed through the nozzle opening. The inert gas carrying the melted particles often travels at speed up to 10.000 fps. The spraying process is used for coating materials with refractory substances. Coatings have excellent bond to most of any base material including some reinforced plastics, grapfhites and carbons.

Virtually any material that may be melted and solidified again may by sprayed with an arc plasma torch. Some materials which are of industrial importance are: refractory metals such as tungsten, tantalum, molibdenum, rhenium, and borides of these metals; refractory carbides of niobium, hafnium, titanium and other metals; refractory oxides of thorium, magnesium, zirconium and aluminium.

Spraying to produce parts of irreqular shape has proved to be one of the most successful and useful applications of the plasma arc torch. A mandrel is preparated to the precise internal diameter of the part to be made. Mandrels may be brass,aluminium or other suitable materials . The mandrel is chucked in a lathe and rotated. The selected material is sprayed by moving the torch back and forth across the spinning mandrel. When desired thickness has been reached the torch application is stopped and the mandrel dissolved out.

As a source of extreme temperatures plasma has many other potential uses. Metal cutting is already an established application. Intensive work is being carried out in the field of welding. It is evident, with continuing research and development, that the range of industrial applications of plasma will steadily increase.

Questions to the text.

1. Where plasma in natural state very common?

2. What is a plasma jet?

3. Is it possible to produce arc plasma experimentally?

4. What gases are usually used in producing the arcs plasma?

5. How is the temperature reached?

6. Describe briefly the spray coating process.

7. What is the difference between the transferred and non-transferred arc equipment? What material are mandrels made of?

8. What is the most useful application of the arc plasma?

9. In what form is the spraying metal fed into the nozzle?

10. What future does this process have?