Plasma welding is very similar to TIG as the arc is formed between a pointed tungsten electrode and the workpiece. However, by positioning the electrode within the body of the torch, the plasma arc can be separated from the shielding gas envelope. Plasma is then forced through a fine-bore copper nozzle which constricts the arc. Three operating modes can be produced by varying bore diameter and plasma gas flow rate:
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The plasma arc is normally operated with a DC, drooping characteristic power source. Because its unique operating features are derived from the special torch arrangement and separate plasma and shielding gas flows, a plasma control console can be added on to a conventional TIG power source. Purpose-built plasma systems are also available. The plasma arc is not readily stabilised with sine wave AC. Arc reignition is difficult when there is a long electrode to workpiece distance and the plasma is constricted, Moreover, excessive heating of the electrode during the positive half-cycle causes balling of the tip which can disturb arc stability.
Special-purpose switched DC power sources are available. By imbalancing the waveform to reduce the duration of electrode positive polarity, the electrode is kept sufficiently cool to maintain a pointed tip and achieve arc stability.
Although the arc is initiated using HF, it is first formed between the electrode and plasma nozzle. This 'pilot' arc is held within the body of the torch until required for welding then it is transferred to the workpiece. The pilot arc system ensures reliable arc starting and, as the pilot arc is maintained between welds, it obviates the need for HF which may cause electrical interference.
The electrode used for the plasma process is tungsten-2%thoria and the plasma nozzle is copper. The electrode tip diameter is not as critical as for TIG and should be maintained at around 30-60 degrees. The plasma nozzle bore diameter is critical and too small a bore diameter for the current level and plasma gas flow rate will lead to excessive nozzle erosion or even melting. It is prudent to use the largest bore diameter for the operating current level.
Note: too large a bore diameter, may give problems with arc stability and maintaining a keyhole.
The normal combination of gases is argon for the plasma gas, with argon plus 2 to 5% hydrogen for the shielding gas. Helium can be used for plasma gas but because it is hotter this reduces the current rating of the nozzle. Helium's lower mass can also make the keyhole mode more difficult.
Microplasma welding
Microplasma was traditionally used for welding thin sheets (down to 0.1 mm thickness), and wire and mesh sections. The needle-like stiff arc minimises arc wander and distortion. Although the equivalent TIG arc is more diffuse, the newer transistorised (TIG) power sources can produce a very stable arc at low current levels.
Medium current welding
When used in the melt mode this is an alternative to conventional TIG. The advantages are deeper penetration (from higher plasma gas flow), and greater tolerance to surface contamination including coatings (the electrode is within the body of the torch). The major disadvantage lies in the bulkiness of the torch, making manual welding more difficult. In mechanised welding, greater attention must be paid to maintenance of the torch to ensure consistent performance.
Keyhole welding
This has several advantages which can be exploited: deep penetration and high welding speeds. Compared with the TIG arc, it can penetrate plate thicknesses up to l0mm, but when welding using a single pass technique, it is more usual to limit the thickness to 6mm. The normal methods is to use the keyhole mode with filler to ensure smooth weld bead profile (with no undercut). For thicknesses up to 15mm, a vee joint preparation is used with a 6mm root face. A two-pass technique is employed and here, the first pass is autogenous with the second pass being made in melt mode with filler wire addition.
As the welding parameters, plasma gas flow rate and filler wire addition (into the keyhole) must be carefully balanced to maintain the keyhole and weld pool stability, this technique is only suitable for mechanised welding. Although it can be used for positional welding, usually with current pulsing, it is normally applied in high speed welding of thicker sheet material (over 3 mm) in the flat position. When pipe welding, the slope-out of current and plasma gas flow must be carefully controlled to close the keyhole without leaving a hole.
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This Job Knowledge article was originally published in Connect, April . It has been updated so the web page no longer reflects exactly the printed version.
Posted on December-12-
Plasma arc welding involves joining metals together by heating with a constricted arc between the workpiece (transfer arc) and an electrode (non-transfer arc) OR the constricting nozzle and the electrode.The constricted plasma that is formed is highly ionised and concentrated. In simple terms, plasma arc welding uses plasma (gas) that has been superheated to become highly conductive.
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Plasma arc welding is very similar to TIG welding. This is due to the arc formation that is created between the electrode and the workpiece. However, with plasma arc welding, the arc can be separated from the shielding gas envelope. Therefore, the plasma can be forced through a nozzle to help constrict the arc.
Plasma welding is now standard use in certain industries. It offers faster, more reliable welds that are more flexible. Examples of where plasma welding is used includes:
Better Control
This is one of the biggest advantages of plasma welding. The quality and control of the welded item is very good and this is due to the design of the torch. It increases the effectiveness of the arc control and a greater tolerance for in-torch stand-off distance. It also allows for both hands to move and the head is available for observation and control.
Low Spatter Level
Compared to other welding techniques, plasma arc welding has less spatter due to its efficient energy consumption. It also does not cause warping in thin materials so there is no need to preheat them.
Smaller Heat Affected Zones
Compared to other welding, such as TIG, the area affected by heat is smaller with PAW (plasma arc welding). This is because PAW does not need a filler material, so there is no energy released at the root of the bead. Not only does this reduce the heat affected zone, but also improves the appearance of the welded zone.
Lower Distortion
The lack of filler material also means that there is little or no distortion to the adjacent materials during the weld. The high temperature and high concentration of PAW allows for the keyhole effect, allowing complete penetration.
Suitable For Precision Welding
PAW compared to other welding methods such as TIG, can maintain a stable arc at lower current levels. This means that it is suitable for small precision welding applications. This is why this method of welding is often used in the medical and electronic industry.
Other Advantages Include:
Equipment
The equipment needed for PAW compared to TIG is more complex and costly. The torch also needs water cooling, so this limits how small the torch can be made. TIG torches can be gas-cooled, so can be made to fit into smaller areas.
High Initial Investment
As we have mentioned above, the cost of PAW equipment is high. The equipment also requires qualified technicians for maintenance and this is vital as torch failure can be extremely dangerous for the operator. This also means that additional safety equipment is needed to protect the operator.
Training
To use PAW equipment, operators will require training to know how to operate a specific plasma welding machine. There are no general-purpose plasma machines.
Noise Levels
With PAW, the noise levels are much higher compared to other welding methods. This is due to the arc plasma and the high speed of travel. This intense noise can be harmful to operators, especially young people.
Other Disadvantages Include:
Here at Arc Welding, we are specialists in welding equipment, including plasma machines. Our welding equipment, supplies and tools are available for hire or purchase. We also offer protective equipment and other relevant accessories. Rest assured that with Arc Welding, we have anything you need for one of jobs or ongoing projects. To get in touch, give us a call on 327 or fill out our online enquiry form.
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