Plasma Arc Welding or PAW is an arc gas welding process similar to Gas Tungsten Arc Welding (GTAW). More candidly, PAW is a gas-shielded process using a constricted arc between the metal piece and the non-consumable tungsten electrode. The difference between GTAW and PAW is in the way of setting electrodes in the nose of the torch. Know various aspects of the plasma arc welding process.
- Plasma Arc Welding Principle
- PAW Welding Types
- Variables of Plasma Welding
- PAW Operating Modes
- Plasma Welding Consumables
- PAW Shielding Gas
- Plasma Welding Machines
- Advantages of Plasma Arc Welding
- Disadvantages of PAW Welding
- Plasma Welding Applications
- PAW Welding Safety Precautions
Plasma Arc Welding Principle
Basically, PAW welding is an extension of TIG welding which employs non-consumable tungsten electrodes. It is a process of an arc welding which produces coalescence because of heating. It happens due to the constricted arc setup between the tungsten electrode and the water-cooled nozzle. The plasma welding uses two inert gases. Further, one shields the arc plasma and other forms the arc plasma. Welders may add or may not add filler metal.
Plasma Arc Welding Types
Generally, plasma arc welding has two types. They are as follows.
Transferred Arc Process
In this process, an arc is formed between the electrode and a workpiece. The arc has a high energy density and plasma jet velocity aiming to melt and cut of metals. Welders can apply plasma welding on non-ferrous metals and stainless steel for cutting them. It can be performed at high arc travel speeds for welding as well.
Non-transferred Arc Process
In this type, the arc is formed between the water-cooled constricted nozzle and the electrode. And the arc plasma comes out from the nozzle in the form of flame. Welders can handle the arc flame in a better way or move it from one place to another. The welding process can be used for producing high-density metal coatings. For initiating a non-transferred arc, it needs a high-frequency unit in the circuit.
Variables of Plasma Welding
PAW uses possibly two or three gases which may be the same or combination of different ones. They are as follows:
- Plasma Gas
- Shielding Gas
- Back-purge and Trailing Gas
Plasma Arc Welding Variables
- Current Type and Polarity
- AC Square-Wave
- Welding Current and Pulsing
- Gas Flow Rate
The process normally operates with DC. The modes can be achieved by plasma gas flow rate and varying bore diameter. There are three operating modes.
Micro-plasma (0.1 to 15A) – Welders can operate arc at the mode by keeping very low welding currents. The columnar arc remains stable despite the arc length is varied up to 20mm.
Medium Current (15 to 200A) – Normally, at higher currents of 15 to 200A, the features of plasma arc welding look similar to TIG welding. Yet, as the plasma is constricted, the arc remains stiffer. However, there is a possibility of improving weld pool penetration just by increasing the plasma gas flow rate. But it causes a risk of air and shielding gas entrainment.
Keyhole Plasma (Over 100A) – Welders can create a very powerful plasma beam by increasing the flow of plasma gas and welding current. As a result, it provides full penetration in a material. The process is suitable for thicker materials up to 10mm of stainless steel.
Plasma Welding Consumables
Every welding process uses consumables for performing welds. Plasma welding employs a copper nozzle and the electrode is tungsten 2% thoria. The diameter of the electrode tip is not as critical as for GTAW. It should be maintained around 30 to 60 degrees. The large bore diameter may cause instability in the arc.
Argon is the normal combination of gases for plasma gas. And as a shielding gas, argon is mixed with 2% to 5% hydrogen. Helium can also be used as plasma gas but being hotter results in a reduction of nozzle current rating. Besides this, the lower mass of helium causes difficulty in keyhole mode.
Basically, PAW welding is a modification of TIG welding. As plasma is an intense source of heat and used in this process for fusing metals. Normally, there are two types of plasma arc welding machines. They are-
1. Plasma Welding Machine – Capable of welding 100 microns to 1 mm thin metal sheets
2. Micro Plasma Welding Machine – Capable of welding very thin sheets (100 microns to 5 mm)
Advantages of Plasma Arc Welding
Every arc welding process consists of both advantages and disadvantages side by side. The plus point of PAW welding is in controlling and quality production.
- The design of torch allows better control of the arc along with higher tolerance for torch standoff distance.
- It produces typically cleaner and smoother welds.
- In PAW, the plasma jet and higher heat concentration allow faster travel speeds.
- There are smaller heat-affected zones in plasma welding.
Disadvantages of Plasma Arc Welding
Despite having advantages, the limitations of plasma arc welding are as follows.
- The startup costs of the plasma welding process are relatively high. Further, the equipment is very expensive.
- As being more specialized welding type, it requires more intense training and expertise.
- It causes the emission of ultraviolet and infrared radiation.
- It produces higher noise of around 100dB.
Plasma Arc Welding Applications
The use of the PAW welding process in the welding industry is as follows.
- Welders employ plasma arc type to weld pipes and tubes of titanium or stainless steel.
- Aerospace and marine industries employ plasma arc welding in their operations on a wide scale.
- For welding or coating on a turbine blade, PAW is used.
- Other uses of plasma welding are in the electronic and repair industries.
Arc Welding Safety Precautions
As we all know, welding is a joining process that uses heat to fuse two or more metals together. This is like welders are consciously playing with fire using certain welding codes and parameters. Therefore, it becomes necessary to keep in mind the safety of self and the things around.
- An authorized person should inspect the area and identify precautions.
- Make sure all combustibles should be kept 35 feet away from the welding place and properly protected.
- Keep fire extinguishers near and ready for immediate use.
- Welders must wear personal protective equipment such as helmets, leather gloves, protective clothing, and wear ear muffs or plugs.
- There should be proper ventilation, exhaust, or respiratory system installed.
And, follow other safety measures as per the requirements of the welding place.
Thus, the above-mentioned information on various aspects of plasma arc welding or PAW is beneficial for students, welders, and other professionals in the welding industry.
Related: Other Welding Types