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The Plasma Arc Welding Process

Summary

Plasma Arc Welding (PAW) was invented and patented in 1953, by Robert M. Gage, at the Linde/Union Carbide laboratory in Buffalo NY. About 10 years of development and multiple subsequent patents occurred before the devices were brought to market in 1964.

The plasma welding process was introduced to the welding industry as a method of bringing better control to the arc welding process in lower current ranges. Today, plasma retains the original advantages it brought to industry by providing an advanced level of control and accuracy to produce high quality welds in miniature or precision applications and to provide long electrode life for high production requirements.

 

The plasma process is equally suited to manual and automatic applications. It has been used in a variety of operations ranging from high volume welding of strip metal, to precision welding of surgical instruments, to automatic repair of jet engine blades, to the manual welding of kitchen equipment for the food and dairy industry.

How Plasma Welding Works

A plasma is a gas which is heated to an extremely high temperature and ionized so that it becomes electrically conductive. Similar to GTAW (Tig), the plasma arc welding process uses this plasma to transfer an electric arc to a work piece. The metal to be welded is melted by the intense heat of the arc and fuses together.

In the plasma welding torch a Tungsten electrode is located within a copper nozzle having a small opening at the tip. A pilot arc is initiated between the torch electrode and nozzle tip. This arc is then transferred to the metal to be welded.

By forcing the plasma gas and arc through a constricted orifice, the torch delivers a high concentration of heat to a small area. With high performance welding equipment, the plasma process produces exceptionally high quality welds.

Plasma gases are normally argon. The torch also uses a secondary gas, argon, argon/hydrogen or helium which assists in shielding the molten weld puddle thus minimizing oxidation of the weld.

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 gages)

List of Plasma Welding Features and Benefits

Feature Benefit Protected electrode Protected electrode allows for less electrode contamination. This is especially advantageous in welding materials that out gas when welded and contaminate the unprotected GTAW electrode. Length of arc benefit due to arc shape and even heat distribution Arc stand off distance is not as critical as in GTAW. Gives good weld consistency. No AVC needed in 99% of applications, sometimes even with wirefeed. Arc transfer is gentle and consistent Provides for welding of thin sheet, fine wires, and miniature components where the harsh GTAW arc start would damage the part to be welded. Stable arc in welding Reduces arc wander. Arc welds where it is aimed. Allows and arc starting tooling in close proximity to weld joint for optimum heat sinking. Minimal high frequency noise in welding Minimal high frequency noise once pilot arc started, thus plasma can be used with NC controls. Another benefit lies in welding applications involving hermetic sealing of electronic components where the GTAW arc start would cause electrical disturbances possibly damaging the electronic internals of the component to be welded. Arc energy density reaches 3 times that of Tig Causes less weld distortion and smaller welds. Gives high welding speeds Weld times as short as .005 seconds Extremely short and accurate weld times possible for spot welding of fine wires, accurate weld times combined with precision motion devices provide for repeatable weld start/stop positions. Equipment options offer to 10,000 Hz Offers a wide range of pulsing options for varied. pulsing up applications. Low amperage art welding
(as low as 0.05 amp) Allows welding of miniature components or good control in downsloping to a weld edge. Diameter of arc chosen via nozzle orifice This feature assists in predicting the weld bead size.

Features, Benefits, and Applications

Features

P Protected electrode, offers long times before electrode maintenance (usually one 8 Hr Shift) L Low amperage welding capability (as low as 0.05 amp) A Arc consistency and gentle arc starting produce consistent welds, time after time S Stable arc in arc starting and low amperage welding M Minimal high frequency noise issues, HF only in pilot arc start, not for each weld A Arc energy density reaches 3 times that of GTAW. Higher weld speeds possible     W Weld times as short as 5 msecs (.005 secs) E Energy density reduces heat affected zone, improves weld quality L Length of arc benefit due to arc shape and even heat distribution D Diameter of arc chosen via nozzle orifice

Benefits

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.

Applications

Small Part Welding: The plasma process can gently yet consistently start an arc to the tip of wires or other small components and make repeatable welds with very short weld time periods. This is advantageous when welding components such as needles, wires, light bulb filaments, thermocouples, probes and some surgical instruments.

Sealed Components: Medical and electronic components are often hermetically sealed via welding. The plasma process provides the ability to:

1. Reduce the heat input to the part
2. Weld near delicate insulating seals
3. Start the arc without high frequency electrical noise which could be damaging to the electrical internals

Applications include Pressure and Electrical Sensors, Bellows, Seals, Cans, Enclosures, Microswitches, Valves, Electronic Components, Motors, Batteries, Miniature Tube to Fitting/Flange, Food and Dairy Equipment,

Tool Die & Mold Repair: A whole repair industry has sprung up to assist companies wishing to re-use components with slight nicks and dents from misuse or wear. The ability of modern micro-arc power supplies to gently start a low amperage arc and make repairs has provided users with a unique alternative to conventional repair and heat treatment. Both the Micro-Tig and micro-plasma welding processes are used for tool, die and mold repair. For outside edges the Plasma process offers great arc stability and requires less skill to control the weld puddle. To reach inside corners and crevices the TIG process allows the tungsten welding electrode to be extended in order to improve access.

Strip Metal Welding: The plasma process provides the ability to consistently transfer the arc to the workpiece and weld up to the edges of the weld joint. In automatic applications no Arc Distance Control is necessary for long welds and the process requires less maintenance to the torch components. This is especially advantageous in high volume applications where the material outgases or has surface contaminants.

Tube Mill Welding: Tube mills produce tube and pipe by taking a continuous strip of material and rollforming the edges upwards until the edges of the strip meet together at a weld station. At this point the welding process melts and fuses the edges of the tube together and the material exits the weld station as welded tube.

The output of the tube mill depends on the arc welding speed and total time spent welding. Each time the mill shuts down and starts up again there is a certain amount of scrap produced. Thus the most important issues to the tube mill user are:

1. Maximum tube mill weld speed obtainable.
2. Arc stability for optimum weld quality and consistency.
3. Maximum number of hours of welding electrode tip life.

Some tube mills employ plasma welding in order to get a combination of increased weld speed, improved weld penetration and maximum electrode life.

If you want to learn more, please visit our website Rods Gouging Torch Manufacturers.

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Comparison of GTAW and Plasma Welding Energy Input

The following is from a test made with the GTAW (Tig) and Plasma welding processes on a specific strip of test material in order to establish a comparison of the energy input of poth processes. The test results should be used as a general guideline comparison only as welding engineers can change any of the parameters noted below to achieve a different result.

Test Parameters: Manual welding, no clamping device, Cr/Ni steel, 0.102" thicknes. All values determined with measuring instruments.

GTAW: 125 Amps, 12 Volts, 10.24 I.P.M. (26 cm/min) Plasma: 75 Amps, 18 Volts, 13.38 I.P.M. (34 cm/min) Heat Input:

   V x A x 60
-----------------
 Speed in cm/min

GTAW:

  12 x 125 x 60
-----------------  =  3.46 KJ
    26 cm/min

Heat Input:

  18 x 75 x 60
----------------  =  2.38 KJ
    34 cm/min

In addition to the fact that a higher weld speed is possible, the lower heat input brings the following advantages:

• More consistency
• Less distortion.
• Less stress in welded component.
• Lower risk of damaging any heat sensitive parts adjacent to the weld joint.

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Process characteristics

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:

• Microplasma: 0.1 to 15A.
  The microplasma arc can be operated at very low welding currents. The columnar arc is stable even when arc length is varied up to 20mm.
• Medium current: 15 to 200A.
  At higher currents, from 15 to 200A, the process characteristics of the plasma arc are similar to the TIG arc, but because the plasma is constricted, the arc is stiffer. Although the plasma gas flow rate can be increased to improve weld pool penetration, there is a risk of air and shielding gas entrainment through excessive turbulence in the gas shield.
• Keyhole plasma: over 100A.
  By increasing welding current and plasma gas flow, a very powerful plasma beam is created which can achieve full penetration in a material, as in laser or electron beam welding. During welding, the hole progressively cuts through the metal with the molten weld pool flowing behind to form the weld bead under surface tension forces. This process can be used to weld thicker material (up to 10mm of stainless steel) in a single pass.

Power source

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.

Arc starting

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.

Electrode

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.

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. 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.

Applications

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 1995. It has been updated so the web page no longer reflects exactly the printed version.

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