Insider’s Guide to Plasma Arc Welding (PAW)

Plasma arc welding (PAW) is an arc welding process wherein metal joining is achieved by the generated heat from the arc created between a tungsten electrode and the welding machine nozzle or workpiece. The arc formed between the tungsten electrode and the machine nozzle is known as a non-transferred arc, while the arc formed between the tungsten electrode and workpiece is known as a transferred arc. The plasma arc welding process utilizes two inert gases; one forms the plasma arc, and the second shields the plasma arc. On plasma arc welding filler metal may or may not be required.

Plasma arc welding is an advancement of the gas tungsten arc welding process GTAW (TIG) and is comparatively a new process to the industry (patented in 1963). Due to its complexity of the process and high investment, it is yet to gain popularity.

Plasma welding process and its types

Plasma arc welding is of two types:

1. Non-transferred plasma arc welding.
2. Transferred plasma arc welding.

In the first one, the arc is between the tungsten electrode and the nozzle, and in the second one, the arc is between the tungsten electrode and the workpiece.

Non-transferred PAW

The non-transferred arc type PAW uses direct polarity (DCEN). The tungsten electrode is connected to the negative terminal of the DC source. The inner nozzle is connected to the positive terminal of the DC source. An arc is struck between the tungsten electrode and the inner nozzle with the help of a high-frequency unit. This arc is forced through the small orifice of the inner nozzle, and hence the arc becomes constricted. An inert gas is allowed to flow through the inner nozzle at a low flow rate, and this inert gas passes through the arc, gets ionized, and becomes plasma. This plasma arc is used for welding. This process is used for welding thin metal sheets. Non-transferred plasma usually is not dependent on the distance between the nozzle and the workpiece.

Transferred PAW

The transferred arc type PAW uses direct polarity (DCEN). The tungsten electrode is connected to the negative terminal of the DC source. The workpiece is connected to the positive terminal of the DC source. A pilot arc is produced between the tungsten electrode and the inner nozzle using the high-frequency unit. The arc is forced through the small orifice of the nozzle and becomes constricted. The inert gas is allowed to flow through the inner nozzle, and it passes through the arc, gets ionized, and comes out of the inner nozzle at high velocity.

The plasma gas strikes the workpiece and conducts the electric current from the tungsten electrode to the workpiece, and an arc is struck between the electrode and the workpiece. Now, the pilot arc is automatically switched off by the system. This process is used for the welding of thick plates. Plasma arc welding uses two inert gasses, one to form the plasma and the other for shielding the weld pool from atmospheric air and contaminations. Filler metal may or may not be required.

Difference between the non-transferred arc PAW and transferred arc PAW process

Plasma arc welding weld positions

The plasma arc welding can be used in all the welding positions viz. horizontal, vertical, overhead, seam welding of pipes, and circumferential butt welding of pipes; however, suitable fixtures may have to be designed wherever necessary.

Type of joints for plasma arc welding

For plasma arc welding of metals up to 25 mm thick, weld joints like a square butt joint, J, or V types of weld joints are used, and welding is done by keyhole as well as non-keyhole methods. The keyhole method is usually preferred for welding thick metals. For the keyhole welding method, the typical weld joint used is a square butt joint, and for root pass welding (specifically for butt welding of thick-walled pipes), a U groove joint is used.

Plasma arc welding gasses

PAW needs two gases and sometimes three. The first is the plasma gas that flows through the inner nozzle orifice and gets ionized, the second is the shielding gas that flows through the outer nozzle (surrounding the inner nozzle), and the third gas is passed on the other side of the welding to avoid purging (like passing an inert gas into the pipe during the seam welding of pipe). The third gas is specific to some applications only. Most of the time, the same gas is used for all the two/three places; however, they can be different also.

Weldability of metals by plasma arc welding

Plasma arc welding can typically be used for welding most of the commercial metals viz. alloy steel, low, medium, and high carbon steels, stainless steel, tool steel, titanium, aluminum, precious metals, etc. Then some metals are difficult but possible to weld by plasma arc welding viz. bronze, magnesium, wrought iron, cast iron, etc. however, they are not popular.

Plasma arc welding can weld all the metals that the GTAW process can weld. However, the PAW process may not be the best and economical in all cases.

Metal thickness

The keyhole method of plasma arc welding can be used for metals with thicknesses from 1.6 mm to 12 mm thickness; however, the actual maximum thickness can vary depending on metal type. The melt-in mode of plasma arc welding can be used for sheets of thickness as less as 0.1 mm to 3.2 mm thickness in a single pass, and the multi-pass melt-in mode can be used for metal thickness 6.4 mm or more.

Plasma arc welding process

A brief step by step process for PAW is explained below:

• Preparation for the joint and cleaning of the workpiece.
• Position and clamp the workpiece in the fixture.
• Set up the welding machine with welding parameters for the workpiece.
• Initiation of the arc unlike in other arc welding processes, the arc in PAW cannot be initiated by touching the tungsten electrode with the workpiece since the tungsten electrode is positioned inside the inner (plasma) nozzle.
• A pilot arc is initiated between the tungsten electrode and the inner nozzle, and this is usually done using a high-frequency unit in the power source.
• The pilot arc helps ionization of the inert gas. Since the ionized gas is electrically conductive, it helps initiate the main arc between the tungsten electrode and the workpiece.
• Once the main arc is established between the tungsten electrode and the workpiece, the pilot arc is taken out. The inert gas flow through the inner (plasma) nozzle is maintained between 0.5 to 5 LPM (liters per minute) since excessive flow rate can cause instability in the weld pool.
• The welding torch is moved in the direction of the welding either manually or automatically as the case may be. The filler wire can be used if needed, and as in GTAW welding, it is always positioned in front of the torch.
• Two types of welding techniques are used in PAW: the keyhole technique, and the non- keyhole (weld-in) technique.
• In the keyhole technique, the high temperature and high energy density constricted plasma arc is utilized for the high penetration (up to 100% workpiece thickness), resulting in complete melting of the workpiece under the constricted plasma arc.
• The inert gas coming out of the inner orifice is not sufficient to shield the weld pool. Hence, an inert gas at a higher flow rate is passed through the outer gas nozzle (that surrounds the inner nozzle) to protect the weld pool from atmospheric air and contaminations. The inert gas passed through the inner and outer nozzles can be the same or different, and the flow rate depends on the inert gas used.
• As the welding torch and arc move forward, the metal is melted and fills the keyhole left behind.
• At the end of the welding, the keyhole is filled up suitably.
• Turn off the power supply and gas.
• Allow the weld to cool, and the weld may need cleaning.

READ: Principle Of Plasma Arc Welding And Differences From GTAW (TIG).

Plasma welding different modes

Plasma welding different modes

Plasma arc welding can be worked in three different modes by varying the current, size of the inner nozzle orifice, and the flow rate of plasma gas, and they are:

1. Micro plasma arc welding.
2. Medium current or melt-in plasma arc welding.
3. Keyhole method of plasma arc welding.

1. Micro-plasma arc welding

Micro-plasma arc welding with currents ranging from 0.1 to 10 or 15 amperes. This can be used for welding very thin sheets (as thin as 0.1 mm) and sections made with thin wires. The narrow arc looks like a needle and minimizes distortion. Filler wire usually is not used in this process.

Advantages of Micro PAW

• The process is fast and leaves a negligible heat-affected zone.
• The process gives a consistent and highly stable arc compared to other welding processes.
• Welding current as small as 0.1 ampere is available for welding delicate workpieces.
• This process can be fully automated.

Disadvantages of Micro PAW

• The cost of the equipment is very high when compared to GTAW.

Application of Micro PAW

• Widely used in nuclear and aerospace industries.
• Used in the automobile industry for the welding of exhaust pipes of racing cars and high-performance motorbikes.
• This process is highly suitable for welding stainless steel, nonferrous metals, silver, titanium, nickel-based alloys, etc.

2. Medium current or melt-in plasma arc welding

Medium current or melt-in plasma arc welding (15 to 200 amperes) is an alternative to the conventional GTAW process. The advantages are higher depth of penetration and higher welding speed.

3. Keyhole method of plasma arc welding

The name keyhole describes the shape of the hole formed in the workpiece during the welding of a tight square edge butt joint. Keyhole welding in PAW is done using a current of more than 100 amperes. At this high current and higher flow of plasma gas, the plasma arc will be very powerful and can penetrate the entire thickness of the workpiece to make a keyhole. As the welding progresses, the keyhole gets filled with the molten metal to form the weld bead. At the end of the weld, the last keyhole is suitably filled with weld metal. The keyhole welding can be used for welding stainless steel plates 10 mm thick in one pass.

The main advantage of the keyhole method of welding is its ability to penetrate into the thick root sections quickly, resulting in a uniform narrow weld bead with a comparatively less heat-affected zone (HAZ). The keyhole method helps deeper penetration, faster welding and gives good quality weld bead. When welding thicker plates by the non-keyhole method and using filler metal, plasma gas flow is reduced. The keyhole welding method can be used for the welding of low alloy steels and stainless steels (2.5 to 10 mm thickness) and aluminum alloy plates of up to 20 mm thickness.

Equipment used in plasma arc welding

The equipment used for plasma arc welding has the following units:

1. Electric power source.
2. High frequency (HF) generator.
3. Welding torch.
4. Non-consumable electrode.
5. Plasma gas.
6. Shielding gas.
7. Filler wire, cables, hoses, wire brush, etc.

1. Electric power source

A direct current (DC) source with drooping V-I (voltage-current) characteristics and a voltage of 60 – 80 volts or more is used for plasma arc welding. Drooping denotes that as the welding current increases, there will be a decrease in the terminal voltage of the welding machine. Both generator or rectifier type DC sources can be used; however, rectifier type is usually preferred. When using helium as the inert gas, the voltage requirement can be more than 70 volts. The higher voltage can be obtained by connecting two power sources in series or initiate the arc initially with argon at the normal voltage and then switch over to the helium gas.

PAW process typically uses a constant current type power source with direct current electrode negative (DCEN) polarity, and the current can range from 50 to 300 amperes. However, in line with the GTAW process, AC (alternating current) and DCEP (direct current electrode positive) may be used for welding aluminum and its alloys with special welding torches and higher size tungsten electrodes.

2. High frequency (HF) generator

The purpose of the HF generator in PAW is similar to the GTAW. However, in PAW, the HF is used to generate the pilot arc (not the main arc), and the second thing is the arc between the tungsten electrode and the plasma nozzle (not between the tungsten electrode and the workpiece as in GTAW). As the welding torch comes closer to the workpiece, the main arc is started, and the pilot arc is put off.

3. Plasma welding torch

The plasma welding torch can be transferred arc type or non-transferred arc type. The welding torch used in PAW may look the same as the TIG welding torch, but it is more complex in construction. The PAW torches are water-cooled since considerable heat is generated inside the torch. The welding torch has different parts viz. non-consumable tungsten electrode, an inner constricting plasma gas nozzle, outer gas nozzle for the shielding gas, the passages for cooling water, electric cables, and gas hoses. The torch has a system for water cooling to take away the excess heat.

The plasma nozzle is made of copper. The bore (orifice) of the plasma gas nozzle is vital, and a fine orifice used with high current and high flow of plasma gas can lead to too much nozzle wear out or even melting. It is essential to use the nozzle of proper bore size, considering the current and flow rate of the plasma gas. A larger bore size can cause problems with arc stability. Even otherwise, the copper nozzle wears out and needs frequent replacement. The pilot arc is struck between the tungsten electrode and the plasma gas nozzle. PAW torches are available for both the manual as well as the automatic type of operation. The Manual type has a current rating of up to 300 amperes.

4. The non-consumable electrode

The non-consumable electrode used in PAW is tungsten with 2% thorium; the addition of thorium helps to start the arc. The tip of the tungsten electrode (tip diameter and angle) in PAW is not as significant as in GTAW and can be maintained between 30 and 60 degrees. The contamination of the tungsten electrode with the workpiece metal is almost impossible since it is positioned safely inside the inner nozzle.

5. Plasma gas

The normal inert gasses used for plasma gas are argon, helium, or a mixture of argon and helium. Plasma arcs can be formed using inert gas in laminar flow (low pressure and low flow) or turbulent flow (high pressure and high flow). Laminar flow is used for plasma arc welding since using turbulent flow will blow off the molten metal from the welding zone.

Argon is the preferred gas used for plasma; however, helium, or a mixture of argon and helium, may also be used depending on the welded metal.

6. Shielding gasses

Typically used shielding gasses in PAW are argon, helium, or a mixture of argon and helium. The gas is required at two places, through the inner nozzle as the plasma gas and through the outer nozzle (surrounding the inner nozzle) as the shielding gas. The flow rate of the inert gas through the inner nozzle is low, and when it comes out of the nozzle as plasma gas, it is unable to protect the weld pool from the atmospheric air and contaminations. The shielding gas at a higher flow rate is passed through the outer nozzle, and this shields the weld pool from atmospheric air and contaminations. The shielding gas passed through the outer nozzle can be the same as the plasma gas, or it can be different. Depending on the metals being welded, either an inert gas, a mixture of inert gasses, or a mixture of inert gas and an active gas can be used to shield gas.

Argon is the most commonly used shielding gas. Helium may be used for non-keyhole welding. A mixture of argon and hydrogen (2 to 5%) gives higher thermal energy (than only argon), and this will be helpful for keyhole welding of nickel-based alloys, copper-based alloys, and stainless steels. Active gas can be used for shielding if it does not affect the weld quality.

7. Filler wire

The filler material used in GTAW can be used in PAW also. Filler wire is manually fed in the manual type of PAW; however, a wire feeder is used for the automatic type of PAW.

Advantages and disadvantages (limitations) of plasma arc welding

Advantages

• Plasma arc welding is a versatile process and gives clean and precise weld beads, and due to its longer arc lengths, it can create deep penetration high-quality welds.
• PAW has better welding speeds, ability to weld thick plates and even hard metal plates than GTAW.
• The distance between the torch and the workpiece is not a critical issue in PAW, giving much freedom to the welder during manual PAW.
• PAW is adaptable for the keyhole method of welding.
• The plasma arc has better stability and is not easily deflectable, and this is important for making root welding and one-side weld joints.
• Adding filler metal is easier in PAW compared to GTAW.
• The energy concentration of the welding arc in PAW is higher than GTAW.
• PAW can narrow and deep penetration (12 to 18 mm or more) depending on the workpiece metal.
• Less time spent for joint preparation since beveling can be eliminated for plates welded up to 10 mm thick.
• Better joint quality due to penetration up to 100%.
• The constricted arc leads to narrow weld beads, reduced heat-affected zone (HAZ), and low distortion.
• Good-looking weld bead and reduced or complete elimination of finishing operations required after welding.

Disadvantages (limitations)

• Plasma arc welding needs two types of inert gasses compared to one type in GTAW. This adds to the cost.
• Water cooling is essential even for torches of a low current rating.
• The equipment used in PAW is expensive compared to GTAW, and the construction of the welding torch is critical and proper maintenance of the torch is essential.
• The equipment used in PAW is bulky and heavy and hence has low portability compared to GTAW.
• The arc voltage control is not easy with PAW, which poses a problem when welding with filler wire.
• The welding torches used in PAW are not as readily available as in GTAW.
• The high-frequency unit is used more often in PAW, and this can interfere in the working of the electrical and electronics computers and instruments placed nearby.
• As with GTAW, PAW is also unsuitable for outdoor use since the moving atmospheric air can disturb the plasma gas and shield gas.
• PAW process gives out harmful infrared and ultraviolet rays that are harmful to human beings. In addition to this, there is high noise of 100 DB, which is harmful to the people around. Hence, special protection is required due to higher radiation and noise during welding.
• The skill level required for a PAW welder is higher than the GTAW welder.

Plasma arc welding process limitations

Some limitations concerning the equipment used in the plasma arc welding process and extra care are necessary. The limitations are:

• The welding torches used in PAW are delicate, complex, and all are water-cooled.
• The shape of the tungsten electrode tip and the inner nozzle orifice is essential, and they should be maintained concentric within close limits.
• The level of current used depends on the tungsten electrode tip; using a very high current may damage the tip.
• The welding torch of PAW has narrow passages for cooling water, passages for the flow of gas, ways for cables, and has a control console. All this makes it complex and requires periodical preventive maintenance.

Applications of plasma arc welding

• The micro PAW process is used for welding thin sheets and wire mesh sections. The arc looks like a needle, and the distortion is less.
• PAW process is used in boiler manufacturing using stainless steel, chemical engineering industries, ship-building industry, and pre-fabrication of gasoline and gas pipelines.
• Plasma arc welding is comparatively a new process and is yet to be introduced in many industries.
• A combination of PAW and GTAW is found useful for circumferential butt welding of large diameter pipes. PAW is positioned in the front to do the penetration and GTAW to fill weld it.
• Welding of stainless steel and titanium pipes and tubes.
• Submarine manufacturing, electronic component manufacturing, and manufacturing of jet engines.
• Aerospace industries and cryogenics.
• Plasma welding is usually used for longitudinal seam welds of rolled pipes (6 to 8 mm wall thickness). Vessels of large diameter used in food and beverage industries can use plasma arc welding for seam welding.

Plasma cutting

When the current and rate of plasma gas flow is sufficiently increased, the molten metal formed during the keyhole formation can be expelled at the bottom of the plate as the torch moves forward, which is how plasma cutting works. Plasma cutting is specifically suitable for cutting nonferrous metals like aluminum, nickel, copper, and alloys of these metals (these metals are difficult to cut by other methods like oxy-acetylene cutting). A mixture of argon and hydrogen or other suitable gas mixtures can be used as plasma gas for plasma cutting of nonferrous metals. Plasma torches can be used for either welding or cutting.

Safety consideration during plasma arc welding

• All arc welding processes, including PAW, can be dangerous if necessary safety precautions are not taken.
• A welder has to deal with electric current, electric arc, hot components, welding fumes, thermal radiations, etc. He/she must take all the safety precautions and must wear safety gear for protection.
• A welder has to wear leather hand gloves, long sleeve jackets, shoes, good-quality welding helmets (with flip-able welding glasses), and a mask (if there is no provision of built-in protection from fumes in the helmet).
• PAW process gives out harmful infrared and ultraviolet rays that are harmful to human beings. In addition to this, there is high noise of 100 DB, which is harmful to the people around. Hence, special protection is required due to higher radiation and noise during welding.
• The open-circuit voltage used in PAW is high, and safety measures are required to overcome electrical hazards.
• The welding enclosure should have good ventilation for the quick exit of the toxic gases formed during welding. Also, the welding enclosure must not contain inflammable/combustible items like fuel, oil, paper, etc.
• Conventional standard welding helmets have dark plates on the front to prevent exposure to thermal radiation. The latest helmet designs have a liquid crystal-type faceplate that automatically darkens when exposed to the welding arc.
• The place of PAW welding should have a suitable fire extinguisher nearby.

Conclusion

Plasma arc welding is a relatively new process compared to other established welding processes, and its versatility is unknown to many people. However, PAW has many advantages over GTAW/TIG, and it will be adapted in more and more industries in the days to come.

References

• Wikipedia Encyclopedia.
• The Welding Institute (TWI).
• EL PRO CUS Website.
• Fronius International Website.
• ScienceDirect Website.