Plasma Welding

When electric arc welding is chosen to weld sheets or tubes up to 8 mm thick in a single pass, it is advantageous to use a process derived from TIG welding: plasma welding. The plasma arc welding process ensures high productivity and higher penetrations.

In some applications, the use of TIG welding and Plasma welding are necessary and complementary. The root pass is performed by one method and the filling by the other.

The principle of plasma welding

principle scheme of plasma welding with external filler wire electrode and plasma gas
Diagram showing the principle of plasma welding with filler metal. The tungsten electrode is installed inside the water cooled nozzle and in which the plasma gas is diffused. A nozzle is mounted around the cooled nozzle to diffuse the annular shielding gas (Argon, most of the time). The assembled parts are butt welded.

Definition

PLASMA ARC WELDING is basically an extension of the gas tungsten arc welding (GTAW or TIG) process. However, it has a much higher arc energy density and higher plasma gas velocity by virtue of the plasma being forced through a constricting nozzle.

The plasma arc welding process is generally used for welding and cladding. It can weld the most common metals up to a thickness of approx.10 mm. Using suitable torches, it can even be used on aluminium, up to a plate thickness of approximately 5 mm.

The main applications for plasma welding also called PAW are in the construction of containers and the manufacture of pipes where welding automation is very popular and effective. Micro-plasma welding is used in technology for household appliances, electronics, aviation and space technology, medical technology and apparatus and instrumentation engineering.

plasma welding paw automated welding lathe automated machine
Example of a welding lathe equipped with a plasma arc welding machine. The plasma welding torch moves along or around the part. This lathe has been designed to perform welds in a single pass on 316L stainless steel 4, 8 or 10mm thick.

Advantages of plasma welding

  • Mechanical constriction of the arc by a water-cooled nozzle enables welding of greater thickness in square butt preparation (3mm to10mm) with a lower heat affected zone (keyhole weld).
  • Increased welding speed from 50mm/min to 500mm/min, depending on materials and the material thickness.
  • Reduction in the number of weld layers on greater thickness when combined with the hot wire process.
  • High electrode service-life due to the use of a pilot arc.
  • Excellent weld quality.
  • Minimal distortion.
  • Single sided welding with good root penetration.
  • Welding in all positions with synchronisation of pulsed current and plasma gas.
longitudinal plasma arc welding Polycar horizontal with external filler material
Polycar equipped with a plasma welding torch with wire spool. This plasma automated welding machine is mounted on I-beam fixed rail. This plasma arc welding equipment has been designed to perform longitudinal or circular welds in horizontal or vertical position

Performance characteristics of the plasma welding process using filler wire

  • Butt welding of materials with thickness from 3mm to 8 mm using a square edge preparation.
  • Possibility of using the hot wire process for the filling layer(s).
  • Synchronisation of Pulse current and plasma gas for out of position welding.

Weld preparation for plasma welding

With material thickness from 5mm to 7mm: v- preparation (30°, 70° or 90°, according to the process combination and the number of filler layers). By maintaining the correct distance between the torch nozzle and workpiece, this will ensure that plasma welding arc ignition will be trouble free.

Characteristics of the weld pool in plasma arc welding

In plasma keyhole welding, the cross-section of the weld pool has a wineglass shape. At the root the weld pool is very narrow, whilst it widens disproportionately at the top. The rapid cooling of the root and the slower solidification of the upper area, balance the weld pool.

principle of plasma welding and keyhole welding
Polycar equipped with a plasma welding torch with wire spool. This plasma automated welding machine is mounted on I-beam fixed rail. This plasma arc welding equipment has been designed to perform longitudinal or circular welds in horizontal or vertical position

Behaviour of the weld pool in plasma welding

An important fact in examining the influencing factors, is the time difference between the solidification of the weld pool in the area of the root, and in the region of the surface. The greater the difference the better the control of the weld pool.

The surface tension and viscosity of the weld pool are basic factors governing its equilibrium.

Factors maintaining the stability of the weld pool

These factors can influence the equilibrium of the weld pool created by the plasma welding machine:

  • Size of the weld pool
  • Physical properties of the weld pool
  • Weld preparation
  • Speed of welding

Size of the weld pool in plasma welding

It is not possible to influence the viscosity of the molten metal. It is, however, possible to control the volume of the weld metal and its solidification, by specifically modifying the welding parameters, during the welding process.

Physical properties of the plasma weld pool

It is easier to control the weld pool if it meets the following specifications:

  • A square edge preparation of 90 degrees.
  • Diameter of keyhole should be kept as small as possible (between 1 and 3 mm).

Weld preparation

In order not to affect the equilibrium of the weld pool, the plasma welding parameters must be controlled and the seam preparation consistently good. This consistency has a direct influence on the regularity of the weld. The plasma nozzles are specially designed for specific maximum currents; on approaching this limit; stray arcs may form, affecting the plasma arc and the flow of the plasma gas.

Welding speed

If plasma keyhole welding is carried out at a welding speed, which is too slow, this will produce an irregular weld or, in extreme cases, may cause the weld to fall away. In practice, welding current and gas volume should be set so that the plasma jet is just strong enough to fully penetrate the workpiece. To obtain a perfect weld, the welding speed must be adapted to these parameters.

Criteria for using plasma keyhole welding

Longitudinal welding presents no particular problems in any position for every plasma arc welding machines. The synchronisation of the impulse current and plasma gas needs an appropriate control system.

Two difficulties, however, are associated with orbital plasma welding:

  1. Closing of the keyhole
  2. Risk of concave penetration from 6 to 9 o’clock. All other positions are easy to master.

The flow rate of the plasma gas must be set so as to prevent the material from being “blown out” and to eliminate bonding errors or unintentional inclusions.

With a special nozzle, plasma torches can also be used for TIG welding. The centre gas and plasma gas are controlled separately. The following variations are available when using a power source and a plasma/TIG torch:

  • normal TIG method
  • TIG double gas method 
  • Plasma /plasma keyhole welding
plasma welding comparison diagram with tig gtaw productivity arc voltage vs current intensity
In the diagram, the characteristic curves U = f(I) compare the arc voltage values recorded as a function of the intensity of the welding current between the plasma and TIG processes. It turns out that, for the same current intensity, the voltage values in plasma welding are twice as high as in TIG. This shows the influence of the electrode/workpiece distance (which is longer in the plasma arc welding process) and that of the second gas (plasma gas).

Each of these methods can also be used to increase productivity (improved deposition rate) in combination with the Plasma/TIG hot-wire process.

Plasma welding with pulsed plasma gas

The method of plasma keyhole welding using pulsed gas was developed in the POLYSOUDE application technology division. This development allows plasma welding in all positions. The flow speed of the plasma gas is used to generate the kinetic energy required to fully control the keyhole.

The synchronisation of pulsed welding current and plasma gas reduces linear power input. The volume and therefore the control of the plasma weld pool can be determined by this “sewing machine effect”.

Sewing machine effect

  • high current ð fusion of base metal
  • low current ð cooling
  • heightening of this effect by reduction of plasma gas flow in low current phase ð precision control of the keyhole

Who we are

Polysoude specialises in the design, development and manufacture of innovative arc welding solutions. The brand is synonymous with world-leading expertise in 3 core sectors: automatic orbital TIG welding systems, automated TIG and plasma welding solutions and TIGer™ weld overlay.
Situated in Nantes since 1961, the company manufactures a variety of power sources and orbital welding equipment, with a speciality in orbital plasma welding and plasma automated welding as well as Narrow Gap Welding and weld overlay cladding.

Nowadays, as a smart factory oriented company, Polysoude products are all designed to be ready for Industry 4.0.

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