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Plasma arc welding
Plasma Arc Welding (PAW) is an extension of the Gas Tungsten Arc Welding (GTAW)
process. The arc is formed between an electrode (which is usually but not always
made of a 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 oC.
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:
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Micro-plasma (< 15A)
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Melt-in Mode (15 – 400A)
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Keyhole Mode (>100A)
PAW (Plasma arc welding) – Advantages
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Plasma Arc Welding has a greater energy concentration as compared to GTAW.
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A deep, narrow penetration is achievable; reducing distortion and allowing square-butt joints in material up to ½” (12mm) thick.
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Greater arc stability allows a much longer arc length (stand-off), and much greater tolerance to arc length changes.
PAW – Limitations
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PAW requires relatively expensive and complex equipment as compared to GTAW; proper torch maintenance is critical
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Welding procedures tend to be more complex and less tolerant to variations in fit-up, etc.
PAW Gases
At least two separate (and possibly three) flows of gas are used in PAW:
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Plasma Gas – flows through the orifice and becomes ionized
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Shielding Gas – flows through the outer nozzle and shields the molten weld from the atmosphere
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Back-Purge and Trailing Gas – required for certain materials and applications.
These gases can all be same, or of differing composition.
PAW – Key Process Variables
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Current Type and Polarity
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DCEN from a CC source is standard
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AC square-wave is common on aluminum and magnesium
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Welding Current & Pulsing - Current can vary from 0.5A to 1200A; Current can be constant or pulsed at frequencies up to 20 kHz
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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:
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Plasma Arc Welding (PAW)
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Plasma Arc Cutting (PAC)
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Plasma Arc Gouging
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Plasma Arc Surfacing
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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.
Suggested Additional Reading
American Welding Society, Welding Handbook, Volume 2 (8th Ed.)
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