The PTA process is a special form of arc welding. This process is characterised by a constricted transfer arc and thus by a higher energy density in the beam. A characteristic feature is the separation between the fusion of the substrate and the fusion of the filler metal with the use of two separate sources of heat. The powder is melted in a nontransferring plasma arc (pilot arc) and then applied to the base metal by a transferring plasma arc (main arc).

A special plasma torch is employed for achieving a constricted, high-energy arc. This form of plasma-powder torch is represented diagrammatically in figure 1 and illustrated with an example in figure 2.

Plasma gas: Generation of the plasma and constriction of the arc by the flowing gas in combination with the plasma nozzle

Conveying gas: Transport of the welding filler
Inert gas: Shielding of the melt against oxidation

The plasma and conveying gases are directly involved in the energy transport into the substrate and into the filler metal, respectively. By adjusting the gas flow rates and the welding current, the energy input can be controlled independently of the filler metal quantity. This feature distinguishes the process from other welding methods. Dilution with the base material can thus be limited to values less than 10 per cent. Consequently, large jumps in properties with respect to the substrate can be achieved even in the first pass. Single-pass operation is therefore a typical feature of this process.

A process version with two arcs is usually applied; for this purpose, a pilot arc situated in the interior of the torch is employed for ignition of the plasma arc. In principle, a process version using only a single main arc also yields comparable cladding properties; in this case, however, the power rating of the high-frequency starting device must be higher. All torch components require intensive water cooling for dissipating the heat losses.

The grain size fraction of the most widely used industrial coating processes can be found in the diagram of Image 3.

The process is characterised by a non-fusing, negatively polarised tungsten electrode behind a plasma nozzle and three gas flows:

In correspondence with the field of application, the deposition efficiency ranges between 0.2 and 1.0 kg/h in the case of micro-PTA (MPTA) for very small workpieces, up to 6 kg/h for conventional normal-PTA (NPTA), and up to 30 kg/h for high-performance weld surfacing (HPTA).

The system-specific advantages of the plasma-powder process, such as low dilution of the cladding material with the substrate material and the good open- and closed-loop control characteristics of the plasma heat source, also persist upon increasing the deposition efficiency. In correspondence with the type of torch and the particular application, the dilution values can vary between 5 und 25 per cent. These dilution values result from the design and circuitry of the PTA torch technology. For this purpose, a nontransferring arc is employed for fusion of the filler metal. Consequently, the transferring arc, whose function is the fusion of the substrate, can operate at lower power. During recent years, the economic application of the method has been extended to the field of high-performance cladding.

The powder is supplied by means of a feed system, which comprises a conical worm conveyor, a small wheel conveyor, a conveyor drum, or a rotary-table feeder as metering device. At present, feed systems equipped with a small wheel conveyor provide the best control characteristics.

Modern inverter-type power-supply units are employed as current sources. Nowadays, these units also operate with alternating polarity and can thus be employed for cladding of light metals.