Plasma is the fourth state of matter (other three being: solid, liquid and gas). It is hot ionized arc vapour. In arc welding this arc plasma is blown away by moving gas streams, but in a plasma torch it is contained and used effectively giving rise to the following processes:

• Plasma arc welding

• Micro-plasma arc welding

• Plasma spraying

Plasma Welding

• Plasma welding is an extension of TIG welding. The main difference is the water cooled nozzle in between the electrode and the work. This causes constriction of the arc column, resulting in very high arc temperature between 16,600-3300oC. Fig. 2.22 shows two main types of torhes in common use: Transferred Arc and Non­transferred Arc. In the first type the tip of the tungsten electrode (d. c. negative) is located within the torch nozzle. The torch consists of an electrode, a watercooled nozzle, for arc constriction and a passage each for supply of water and gas. A power supply unit provides d. c. The welding area is blanketed by shielding, gas supplied through an outer gas cup. Transferred arc transfers heat directly from electrode in the torch to the workpiece.

• When the gas (argon) is fed through the arc it becomes heated to the plasma tempera­ture range (16,600 - 33.000OC). The arrangement is such that the arc first strikes to the nozzle. The plasma so formed is swept out through the nozzle and the main cur­rent path is then formed between the electrode and the work piece. The transferred (constricted) arc may be used for cutting metals that are not so readily cut by oxy - acetylene torch (non-ferrous metals and stainless steel). For best cutting action ar­gon/hydrogen or nitrogen hydrogen mixtures are used. This requires high output voltage welding machines. A non-transferred arc is established between the electrode and torch nozzle indpendent of the workpiece. The heat is carried by the hot gases (plasma) coming out from the torch. The transferred arc delivers heat more effec­tively to the workpiece as the heat is generated by the anode spot on the workpiece as well as the plasma jet heat. Thus it is most commonly used.

Fig. 2.22 Plasma arc welding

• Plasma welding makes use of the key-hole technique. When the plasma jet strikes metal it cuts or keyholes entirely through the workpiece making a small hole and

molten metal in front of the arc flows around the arc column, and is drawn behind the hole by surface tension. Thus butt welds on 12.5 mm or larger thicknesses could be made in a single pass with full penetration. It is good for welding plates accessible from one side only.

• Plasma arc welding can weld carbon steels, stainless steels, copper, brass, aluminium, titanium, monel and inconel including hastalloys, molybdenum and tantalum etc. Micro-Plasma Arc Welding is a modified process using currents between 0.1-10 A. It is capable of welding extremely thin sheets and foils between 0.05-1.6 mm thickness. The precise control of heat is achieved through ‘‘Pulsed mode’’ operation.

Plasma Spraying: In non-transferred arc torch the arc is struck between electrode and nozzle. The rate of gas flow through this torch is moderately high and a jet of plasma issues from the nozzle. For spraying, powder or wire is injected inta the plasma stream which is hot enough to melt any solid that does not decompose or sublime. Thus ceramics may be

sprayed on to a metal surface. When metal is sprayed, high density caating is obtained. Shield­

ing gases could be either argon or nitrogen or 5-25% hydrogen mixed with nitrogen or argon. The non-transferred torch is also known as a plasma device. Plasma heat could also be used to melt metal for certain applications.