An Introduction to the Plasma Arc welding process and technical details
Plasma arc welding sometimes offers greater welding speed than gas tungsten arc welding at lower cost than laser beam welding.
Plasma arc welding (PAW) often is overlooked when a fusion welding process must be selected for high-integrity applications such as those found in the medical, electronics, aerospace, and automotive industries.
This process has been overlooked because it is more complex and requires more expensive equipment than other arc processes and because welders want increased welding speeds such as those found with laser beam welding (LBW). However, automotive manufacturers have turned to PAW for a number of applications, including body panels and exhaust system components.
Gas tungsten arc welding (GTAW), also known as tungsten inert gas (TIG) welding, is commonly used for high-quality welds at slower speeds, while LBW is often selected for higher-speed welding.
PAW sometimes offers greater welding speed than GTAW at lower cost than LBW, and it may be the most effective process for many applications. These include welding stainless steel expandable bellows, where PAW is more tolerant to joint misalignment than LBW and gives better penetration than GTAW; welding coated steels like those used in automotive exhaust systems; and welding in keyhole mode to make full-penetration welds in relatively thick material in a single pass.
Plasma arc welding (PAW) is an arc welding process similar to gas tungsten arc welding (GTAW). The electric arc is formed between an and the work piece. The key difference from GTAW is that in PAW, by positioning the electrode within the body of the torch, the plasma arc can be separated from the shielding gas envelope. The plasma then forced through a fine-bore copper nozzle which constricts the arc and the plasma exits the orifice at high velocities (approaching the speed of sound) and a temperature approaching 20,000 °C. Plasma arc welding is an advancement over the GTAW process. This process uses a non-consumable tungsten electrode and an arc constricted through a fine-bore copper nozzle. PAW can be used to join all metals that are weldable with GTAW (i.e., most commercial metals and alloys). Several basic PAW process variations are possible by varying the current, plasma gas flow rate, and the orifice diameter, including:
- Micro-plasma (< 15 Amperes)
- Melt-in mode (15–400 Amperes)
- Keyhole mode (>100 Amperes)
- Plasma arc welding has a greater energy concentration as compared to GTAW.
- A deep, narrow penetration is achievable, with a maximum depth of 12 to 18 mm (0.47 to 0.71 in) depending on the material
- Greater arc stability allows a much longer arc length (stand-off), and much greater tolerance to arc length changes.
- PAW requires relatively expensive and complex equipment as compared to GTAW; proper torch maintenance is critical
- Welding procedures tend to be more complex and less tolerant to variations in fit-up, etc.
- Operator skill required is slightly greater than for GTAW.
- Orifice replacement is necessary.
For the plasma process is tungsten 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. Large bore diameter should be carefully used for operating current level. A too large a bore diameter, may give problems with arc stability and maintaining a keyhole
Plasma and shielding gases
The normal combination of gases is argon for the plasma gas, with argon plus 2 to 5% hydrogen for the shielding gas only for austenitic stainless steels. 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.
Micro plasma welding:
Micro plasma 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.
The advantages on the normal plasma welding are:
1-Deeper penetration (from higher plasma gas flow)
2-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.
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. For thicknesses up to 15mm, a vee joint preparation is used with a 6mm root face. 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.
When pipe welding, the slope-out of current and plasma gas flow must be carefully controlled to close the keyhole without leaving a hole
Equipment Required List
- Power Supply
- Plasma Console (sometimes external, sometimes built in)
- Water re-circulator (sometimes external, sometimes built in)
- Plasma Welding Torch
- Torch Accessory Kit (Tips, ceramics, collets, electrodes set-up gauges)
The full list of reasons for using the plasma welding process is lengthy but can be summarized into three main features where customers desire the advantages of at least one feature.
- Precision: The plasma process is generally more precise than conventional Tig (remember that enhanced power supplies can create an arc that is different to a conventional Tig arc) Plasma offers the following advantages over conventional Tig:
- Stable, concentrated arc
- Forgiveness in arc length variations (Tig +/- 5%, Plasma +/- 15%)
- Small Part Welding:
- Low amperage capability (many plasma power supplies go down to .1 amps)
- Stable at low amps
- Gentle arc transfer (arc start) with no high frequency noise.
- Short weld times possible (for spot welds - guidewires, tubes etc.)
- High Production Welding:
- Long electrode life offers many more hours of welding than Tig before electrode contamination occurs.
In many applications, many of the unique advantages of plasma combine to benefit the overall welding process.
A Plasma Welder can be added to various semi-automatic and automatic systems. In the video above you can see a demonstration of a SAF longitudinal seam welder recently supplied to a UK company who required the plasma welding application to be fitted to the system.
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