Plasma arc welding

Plasma arc welding (PAW) is an arc welding process similar to gas tungsten arc welding (GTAW). The electric arc is formed between an electrode (which is usually but not always made of sintered tungsten) and the workpiece. 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 is 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.

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PAW advantages

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; reducing distortion and allowing square-butt joints in material up to ½” (12 mm) thick.
• Greater arc stability allows a much longer arc length (stand-off), and much greater tolerance to arc length changes.

PAW limitations

• 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.

Gases

At least two separate (and possibly three) flows of gas are used in PAW:

• Plasma gas – flows through the orifice and becomes ionized
• Shielding gas – flows through the outer nozzle and shields the molten weld from the atmosphere
• Back-purge and trailing gas – required for certain materials and applications.

These gases can all be same, or of differing composition.

Key process variables

• Current Type and Polarity
• DCEN from a CC source is non standard
• AC square-wave is common on aluminum and magnesium
• Welding current and pulsing - Current can vary from 0.5 A to 1200 A; Current can be constant or pulsed at frequencies up to 20 kHz
• Gas flow rate (This critical variable must be carefully controlled based upon the current, orifice diameter and shape, gas mixture, and the base material and thickness.)

Other plasma arc processes

Depending upon the design of the torch (e.g., orifice diameter), electrode design, gas type and velocities, and the current levels, several variations of the plasma process are achievable, including:

• Plasma Arc Welding (PAW)
• Plasma Arc Cutting (PAC)
• Plasma Arc Gouging
• Plasma Arc Surfacing
• Plasma Arc Spraying

Plasma arc cutting (PAC)

When used for cutting, the plasma gas flow is increased so that the deeply penetrating plasma jet cuts through the material and molten material is removed as cutting dross. PAC differs from oxy-fuel cutting in that the plasma process operates by using the arc to melt the metal whereas in the oxy-fuel process, the oxygen oxidizes the metal and the heat from the exothermic reaction melts the metal. Unlike oxy-fuel cutting, the PAC process can be applied to cutting metals which form refractory oxides such as stainless steel, cast iron, aluminum, and other non-ferrous alloys.

See plasma cutter.

Suggested additional reading

American Welding Society, Welding Handbook, Volume 2 (8th Ed.)