Plasma-welding-tips:

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Powerful Plasma Welding of Thick Joints

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Plasma-welding-tips are useful reminders.

The main characteristics of Plasma Arc Welding (PAW) are stressed in this page.

That should be useful for the correct exploitation of its specific advantages.

The first of the Plasma-welding-tips is a correct definition of PAW.

It should help for understanding the differences that make the process unique.


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A jet stream of ionized gas is called Plasma.

While neutral gas is an insulator, ionized gas conducts electricity.

Plasma Arc Welding is defined as an extension of gas shielded tungsten arc welding (GTAW).

A collimated plasma is generated by an electric arc striking a column of shielding gas passing through a constricting nozzle.

The copper alloy nozzle is called constricting because it presents a limited diameter orifice.

The shielding gas jet becomes highly ionized during its transit through the arc.

The concentrated and collimated jet stream of ionized gas is composed of nearly equal number of electrons and ions of gas atoms and molecules.

Negative electrons and positive ions (atoms stripped of one or more electrons) run in opposite directions.

The plasma column exits from the constricting nozzle at a very high temperature, about 16,700 °C or 30,000 °F.

The flow pattern generated by the constricting nozzle gives very good directional control.

It provides also a favorable depth-to-width cross sectional weld shape.

Plasma Arc Welding can be performed with or without additional filler metal.

In manual applications, the filler metal is added as needed by hand.

In the mechanized version, the filler metal is added from the side by a wire feeder.

Plasma-welding-tips concerns understanding the specific features of the process, detailed below.

Then it is all about using its capabilities for obtaining successful welding results.

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Arc Modes Plasma-welding-tips

The arc is produced between a nonconsumable tungsten electrode and either the workpiece (transferred arc mode) or the constricting nozzle (nontransferred arc mode).

In the transferred arc mode, the electric current is discharged directly from the torch to the workpiece through the conducting plasma.

The workpiece is included in the electric circuit but the nozzle is not.

Heat is obtained by a combination of:

  • anodic heating, where electrons from the negative tungsten electrode impinge on the workpiece, and
  • from the plasma jet radiating its heat to the surroundings.

Plasma-welding-tips

In certain cases, where the current is too high for a given orifice size, a secondary arc may be produced in the torch between the nozzle and the electrode, a case called double arcing.

This condition is a faulty one and must be avoided as it causes damage to the nozzle while impairing weld quality.

It can be corrected by reducing the current or by selecting a larger orifice nozzle.

In the non transferred arc mode, the workpiece is excluded from the electric circuit.

This mode is useful for cutting and joining non conductive materials or where low energy concentration is needed.

The arc is struck and maintained between the electrode and the constricting nozzle. Heat comes only from the plasma jet.

In practice a non transferred low power pilot arc is always ignited at start for arc initiation (even in transferred mode applications).

The power supply providing the superposed high frequency is then disconnected after the main arc is established.

Shielding gas Plasma-welding-tips

The plasma jet must be limited in flow to avoid turbulence.

Therefore for most operations, to protect the weld pool from air contamination, an additional stream of the same or of a different shielding gas is provided through an outer gas nozzle.

Orifice gas is usually argon.

Shielding gas can be argon or a mixture of argon and 2-5% of H2.

For welding high conductivity metals, like aluminum or copper, argon, helium or mixtures of 75%He-25%Ar may be used.

Operating modes Plasma-welding-tips

1 - Melt-in mode

In this mode a small pool of molten material is obtained in front of the torch.

This is similar to GTAW except for the higher stiffness of the plasma jet.

It is a remarkable fact that here, at variance from GTAW, arc length is not critical.

This mode is further characterized according to current level as follows.

Microplasma includes processes performed with current from 0.1 to 15 A.

It is most useful for delicate and precision joints (visual control performed under a microscope) to be welded with minimum heat input.

Medium current plasma uses current levels from 15 to about 100 A.

Advantages of Melt-in mode:

  • Improved arc stability at low current levels
  • Narrow beads (from higher ratio depth-to-width) mean less distortion
  • Greater directional stabiliy
  • Less fixturing needed
  • If filler metal is required, feeding is easier because of greater torch standoff
  • Less electrode grinding as it is more protected
  • Less workpiece contamination by electrode
  • Welding out of position is easier as standoff is not critical.

Limitations:

  • Less tolerance for joint misalignment
  • Heavier torches hard to manipulate
  • Stricter maintenance of nozzle condition.

2 - Keyhole mode Plasma-welding-tips

Over 100 A the process is applied to penetrate the full thickness of the joint, producing a hole.

Generally performed in flat position from 1.6 mm to 9.5 mm (1/16 to 3/8 in.)

In certain cases it is reported possible up to 19 mm (3/4 in.) in any position.

Molten metal flows around the plasma jet and solidifies behind the keyhole as the torch progresses along the joint.

This mode, with higher currents, permits welding of relatively thick material, without beveling of the edges, without filler metal, in one single pass, at higher welding speed than other processes would allow.

There is some danger of keyhole closure porosity if the termination sequence is not perfectly correct.

Advantages of Keyhole mode:

  • Less danger of trapped gas
  • Less transverse distortion
  • Deeper penetration permits less passes, reducing weld time and distortions
  • Easier preparation, square butt joints, no filler metal.

Limitations:

  • More process variables require stricter control
  • Mostly for flat position
  • Stricter maintenance of nozzle condition.

Welding Current Plasma-welding-tips

Direct Current with Electrode Negative (DCEN) is the most used, except for aluminum and magnesium.

For maximum heat input control, the direct current can be pulsed between two levels, a low current level permitting partial solidification and a higher level for melting and welding.

For aluminum and magnesium it is advantageous to use the cleaning feature provided by cathodic etching, obtained by reverse polarity with Electrode Positive.

However a constant current (DCEP) would heat excessively the electrode.

Therefore either an alternating current is used or a pulsed square wave current with alternating pulses of opposing polarity:

  • a long pulse of DCEN (to maximize heat input to the workpiece)
    followed by
  • a very short pulse of DCEP (for oxide removal).

The square wave alternating current with individually adjustable current polarity, current levels and duration times is also called Variable Polarity Plasma Arc (VPPA).

Power sources are of inverter type or of Silicon Controlled Rectifiers (SCR).

The power source is generally of constant current type.

Minimum open circuit of 80V is required to permit easy arc initiation.

Besides control of welding current and voltage it should also allow controlled adjustment of upslope for the welding start and of downslope for weld termination.

Torch Plasma-welding-tips

Torch size is designed as a function of the nominal power rating.

Concentricity of electrode and nozzle must be assured for consistent results.

Cables are needed for power supply, while hoses convey the gases (inner plasma or orifice gas and outer or shielding gas) and liquid coolant.

Lighter torches are available for manual welding.

Sturdy construction permits continued use at elevate current levels for mechanized welding.

Summary of Plasma-welding-tips: Advantages

In the plasma arc torch the nonconsumable tungsten electrode is inside, at a certain distance, called electrode set-back, from the constricting nozzle face.

Therefore no touch starting is possible, there is no danger of electrode contamination of the workpiece, a major advantage of PAW and a considerable source of downtime in GMAW.

Arc Length

As the plasma column is almost cylindrical, any minor change in torch standoff distance (between torch and workpiece) has no consequence on the weld.

This is a major advantage vs. GTAW where arc voltage and arc spread are affected by torch distance (arc length).

Another advantage of PAW as compared to GTAW, is the directional stability (stiffness) of the plasma column.

While arc blow may be a serious hindrance in GTAW, the plasma jet is much stiffer and therefore much less affected by magnetic fields.

It is remarked that this characteristic requires less skill on the part of the welder for producing acceptable welds.

High current density and high energy concentration characterize PAW, permitting lower heat input for plasma welding.

Low speed plasma jets are obtained by large orifices, permitting low gas flow rate and low currents.

Note: For plasma cutting, on the contrary, high jet speed, high current, high orifice gas flow rate would be required.

But the most important advantage of PAW at high current levels is the capability of operating in keyhole mode, mostly without filler metal.

This unique feature is present only in more complex processes like Electron Beam Welding (EBW) and Laser Beam Welding (LBW).

As already indicated the keyhole mode permits reduced amount of joint preparation, very deep penetration for a single pass without filler metal, high welding speed and economic performance.

Plasma-welding-tips: Limitations

Cost of equipment is higher than that of GMAW. The PAW process has less tolerance for poorly fitted joints. Higher complexity of equipment requires more attention in scheduled maintenance of the torch.

More parameters to control may also confuse less prepared welders and operators.

Concluding Plasma-welding-tips

Applications requiring either precise low heat input with controllable low current (microplasma) or deep penetration in thick joints with minimum preparation (keyhole mode) and high quality economic performance are those that best exploit the unique advantages of PAW.

An Article on Double-Stage PAW Process was published (7) in Issue 113 of Practical Welding Letter for January2013.
Click on PWL#113 to see it.

An Article on Cross-Arc Welding was published (7), in Issue 158 of Practical Welding Letter for October 2016.
Click on PWL#158.

A note on Plasma Arc Gouging was published (8) in Issue 161 of Practical Welding Letter for January 2017.
Click on PWL#161.

To get at no cost each issue of PWL as it is published, please subscribe.

AWS C5.1-73
Recommended Practices for Plasma-ARC Welding
American Welding Society
01-Jan-1973, 68 pages

AWS A5.12/A5.12M-98
Specification for Tungsten and Tungsten Alloy Electrodes for Arc Welding and Cutting
American Welding Society
23-Mar-1998, 16 pages

Watch the following Video on

OERLIKON PLASMA WELDING

http://www.youtube.com/watch?v=jo376zPns8I

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For basic information on the physics of electric arc for welding, see our new page on Arc Welding.




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Plasma-welding-tips are useful reminders of typical and advantageous applications of PAW and of the main differences that provide to the PAW process its unique and most sought for characteristics.