Plasma arc welding
Encyclopedia
Plasma arc welding is an arc welding
Welding
Welding is a fabrication or sculptural process that joins materials, usually metals or thermoplastics, by causing coalescence. This is often done by melting the workpieces and adding a filler material to form a pool of molten material that cools to become a strong joint, with pressure sometimes...

 process similar to gas tungsten arc welding
Gas tungsten arc welding
Gas tungsten arc welding , also known as tungsten inert gas welding, is an arc welding process that uses a nonconsumable tungsten electrode to produce the weld...

 (GTAW). The electric arc
Electric arc
An electric arc is an electrical breakdown of a gas which produces an ongoing plasma discharge, resulting from a current flowing through normally nonconductive media such as air. A synonym is arc discharge. An arc discharge is characterized by a lower voltage than a glow discharge, and relies on...

 is formed between an electrode
Electrode
An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit...

 (which is usually but not always made of sintered tungsten
Tungsten
Tungsten , also known as wolfram , is a chemical element with the chemical symbol W and atomic number 74.A hard, rare metal under standard conditions when uncombined, tungsten is found naturally on Earth only in chemical compounds. It was identified as a new element in 1781, and first isolated as...

) 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
Shielding gas
Shielding gases are inert or semi-inert gases that are commonly used in several welding processes, most notably gas metal arc welding and gas tungsten arc welding . Their purpose is to protect the weld area from atmospheric gases, such as oxygen, nitrogen, carbon dioxide, and water vapour...

 envelope. The plasma
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...

 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.
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 millimetre 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.

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 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 cutting (PAC)
  • Plasma arc gouging
  • Plasma arc surfacing
  • Plasma arc spraying

Plasma arc cutting

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
Oxy-fuel welding and cutting
Oxy-fuel welding and oxy-fuel cutting are processes that use fuel gases and oxygen to weld and cut metals, respectively. French engineers Edmond Fouché and Charles Picard became the first to develop oxygen-acetylene welding in 1903...

 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. Since PAC was introduced by Praxair Inc. at the American Welding Society
American Welding Society
The American Welding Society is a nonprofit organization dedicated to advancing the science, technology, and application of welding and allied joining and cutting processes, including brazing, soldering, and thermal spraying...

show in 1954, many process refinements, gas developments, and equipment improvements have occurred.

External links

Microplasma welding
  • http://www.youtube.com/watch?v=T8g1lULZryk
  • http://www.youtube.com/user/multiplazslovenia#p/u/6/SWbUJh4XuMQ


Arc spray welding.
  • http://www.youtube.com/watch?v=BtsywbmjKIE&NR=1
  • http://www.youtube.com/watch?v=ibPPbQC5LeE&feature=related
The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
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