Plasma Arc Welding
Before we learn what is Plasma Arc Welding? First, let us know what is Plasma.
The term plasma means a gas that has been heated to a sufficiently high temperature so that it is transformed into an ionized condition (i.e. into an equal number of electrons and ions) and is able to conduct an electric current.
Plasma is regarded as the fourth state of matter following the three commonly known states: solid, liquid and gaseous. The electric arc with its intense heat, generates a considerable amount of plasma.
In fact, the electric arc consists of a relatively high current discharge sustained through a column of plasma.
Related reading: What is Plasma Arc Cutting Process?
Features of Plasma Arc
In the normal open arc welding processes including the TIG process, the plasma jet has a low velocity and is diffused over a wide area, with the result that the arc loses much of its power and its zonal temperatures are lowered.
Plasma arc processes (these include plasma arc welding, plasma arc cutting, plasma arc surfacing and plasma arc spraying) use a specially designed plasma arc torch, which is a modified version of the TIG torch, in which the tungsten arc is directed through a nozzle through which an inert gas (argon or argon-helium mixture), called the carrier gas, flows.
The nozzle, located between the tungsten electrode and the base metal, prevents the divergence of the arc and constricts it into a small cross-section.
This results in a substantial increase in the resistive heating of the arc, so that the arc temperatures and the arc voltage are raised. From the constricting nozzle, the arc emerges in the form of a high velocity, intensely hot and collimated plasma jet. This plasma jet is termed as a plasma arc.
Advantages of Plasma Arc Welding
Plasma Arc Welding (PAW) process, as compared to TIG, has the following advantages:
- Greater concentration of energy,
- improved arc stability especially at low currents,
- higher heat content,
- the higher velocity of the plasma,
- less sensitivity to variations in arc length,
- No tungsten contamination,
- less welder skill in manual welding,
- solid backing is avoided by adopting the keyhole technique.
Disadvantages of Plasma Arc Welding
The drawbacks of Plasma Arc Welding or PAW process are:
- Equipment costs more,
- the short life of constricting nozzle,
- a welder needs a deeper understanding of the process,
- Increased consumption of inert gas.
Plasma Arc Welding Operation
An arc is created between a tungsten electrode and the welded part, in the manner of the TIG process, a plasma gas is injected into a cooled chamber and allows the arc to pass through a nozzle, and thus increases the power of the arc which becomes very localized.
An additional gas protects the melt pool from atmospheric influences. The plasma process requires a plasma gas and a shielding gas.
The plasma makes it possible to reach a temperature of 15,000 ° C and it is very strongly accelerated by the passage in the nozzle. The term plasma is used when the arc is no longer composed of atoms and molecules, but of ions and electrons.
The very high temperature and concentration of the arc make the plasma suitable for thermal cutting operations and welding operations by the “keyhole” technique.
To understand the operation of the Plasma Arc Welding process, one must know the terminology of the plasma arc torch as shown in Fig. below, and the significance of each term.
Electrode
The tungsten electrode is recessed inside the constricting nozzle so that it cannot touch the workpiece.
Hence tungsten contamination, which commonly occurs in the TIG process, does not occur in this Plasma Arc Welding process. The electrode has negative polarity. Positive polarity causes excessive deterioration of the electrode.
Electrode setback:
This is the distance between the electrode tip and the outer bottom edge of the constricting nozzle.
Orifice gas:
This gas, referred to earlier as the carrier gas, is an inert gas that is fed through the torch to surround the electrode and to enter the arc through the plenum chamber.
It becomes ionized in the arc to form the plasma, which expands due to the heat of the arc and issues from the orifice or orifices of the constricting nozzle at an accelerated speed as the plasma jet.
The jet is so powerful that it is capable of cutting metal. The higher the orifice gas flow rate, the more penetrating is the arc.
For Plasma Arc Welding, the jet is controlled by holding the orifice gas flow rates within the range of 1.5-15 1/min. The orifice gas alone is not adequate to shield the weld pool from the atmosphere. Additional shielding gas is therefore provided through an outer gas nozzle.
Plenum chamber:
This has been referred to under ‘orifice gas’. It is the space between the inside wall of the constricting nozzle and the electrode.
Shielding gas:
As mentioned under ‘orifice gas’, the shielding gas is provided additionally through the outer gas nozzle, in the same manner as in the TIG process to protect the area of the workpiece, over which the arc plasma impinges.
Its composition is the same as the orifice gas, i.e. either argon or argon-helium mixture.
Constricting nozzle:
The arc-constricting nozzle is a critical component of the torch. Its contour partly determines the thermal energy of the arc. Its main dimensions are ‘orifice diameter’ and ‘throat length’, which are shown in the above figure. The nozzle may have single or multiple ports.
The nozzle is usually made of copper and is water-cooled. The fact that copper with a melting point of 1,083° C is able to withstand the plasma temperatures of 24,000° C and above is explained thus: during Plasma Arc Welding, the arc column inside the nozzle is surrounded by an annular layer of cooler gas with a very steep thermal gradient.
This layer of cooler gas provides thermal and electrical insulation to the inside surface of the nozzle. If the smooth flow of this gas layer is disturbed either by insufficient orifice gas flow or excessive arc current, the nozzle may suffer damage due to double arcing, i.e. between the electrode and the nozzle and between the nozzle and the work.
Orifice diameter:
The nozzle orifice size is a critical dimension as stated earlier. It is rated for a given current at a given gas flow rate. If the gas rate is reduced, the current rating at the orifice must be proportionately decreased.
Torch standoff:
It denotes the distance between the outer edge of the constricting nozzle and the workpiece. The torch standoff distance is not important, because changes in arc length have a negligible effect on the Plasma Arc Welding process results.
Much longer torch standoffs are possible with the plasma arc than with the TIG process.
Plasma Arc Properties
In the conventional TIG process, the arc plasma spreads over a large area of the workpiece and the arc is easily deflected by weak magnetic fields.
Plasma Arc Welding, on the contrary, is stiff and unidirectional and very little affected by magnetic fields. The constriction of the arc in the latter results in higher plasma temperatures and arc power.
For example at 200 amp, electrode negative, and an argon flow rate of 19 1/min, a constricted arc passing through a 4.8 mm diameter orifice shows a 100% increase in arc power (mainly because the arc voltage is doubled) and a 30% increase in temperature as compared to an open TIG arc.
The energy of a plasma arc can vary over a wide range depending on:
- plasma current,
- size and shape of nozzle orifice,
- type of orifice gas, and
- flow rate of orifice gas.
Hence the process can be applied with equal efficiency for cutting, welding, surfacing and metal spraying by suitably controlling the four factors. For cutting, for example, a high arc current, a small-sized orifice and high orifice gas flow rate are used to obtain a highly concentrated energy and a high jet velocity.
For welding, a low plasma jet velocity is obtained by using a large orifice, low gas flow rate and low transferred arc currents.
Applications of Plasma Arc Welding
PAW can be used on all metals which are normally welded by the TIG fabrication jobs PAW has been proved to give better flexibility, consistently high quality, and process.
On many better economy. It is therefore an accepted process in shipbuilding, nuclear, electronic, aerospace and many other engineering industries.
Mechanized high current PAW with keyhole technique has been used to obtain high welding speeds and consistently high-quality joints in the following typical applications in the U.S.A.:
1) Stainless steel and titanium tubing (longitudinal welds)
2) Girth joints in pipe fabrication
3) Missile tankage
4) Turbine engine components
Manual PAW with the melt-in technique is usually employed on very thin metal sections, because of the ease and reliability it provides.
Micro plasma welding is successfully used in the following applications because it provides a stable yet cool arc at very low currents:
1) Thin-wire mesh screen filters
2) Small-wire butt welds
3) Relay case fabrication
4) Bellows assemblies
5) Thermal shields
6) Thin-wall pressure vessels
7) Vacuum tube components
8) Thermocouple junctions.
It is possible for highly skilled welders to use micro plasma keyhole technique to achieve uniform weld penetration in applications such as pipe fabrication.
