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Manufacturing Processes > Joining processes > Plasma Arc Welding (PAW)

 

Plasma Arc Welding (PAW)

 

Process description

A plasma column is created by constricting an ionized gas through a water-cooled nozzle reaching temperatures of around 20 000 °C. The plasma column flows around a non-consumable tungsten electrode, which provides the electrical current for the arc. The plasma provides the energy for melting and fusion of the base materials and filler rod (when used).

Materials

  • Most electrically conductive materials.
  • Commonly and stainless steels, aluminum, copper and nickel alloys, refractory and precious metals.
  • Not cast iron, magnesium, lead or zinc alloys.
PAW

Process variations

  • Portable manual or automated a.c. or d.c. systems: d.c. system most common.
  • Two modes of operation used for welding:
    • Melt-in fusion for reduced distortion uses low currents
    • Key hole fusion at higher currents for full penetration on thick materials.
  • Choice of gas and their proportions important for two modes of operation:
    • Plasma gas: argon or argon–hydrogen mix
    • Shielding gas: argon or argon–hydrogen mix. Also helium or helium–argon mix used.
  • Plasma arc cutting: for cutting, slotting and profiling materials up to about 40mm thickness using the key-holing mode of operation.
  • Plasma arc spraying: melting of solid feedstock (e.g. powder, wire or rod) and propelling the molten material onto a substrate to alter its surface properties, such as wear resistance or oxidation protection.
  • Filler rod sizes between Ø1.6 and Ø3.2mm typically.

Economic considerations

  • Weld rates vary from 0.4 m/min for manual welding to 3 m/min for automated systems.
  • Alternative to TIG for high automation potential using key hole mode.
  • Welding circuit and system more complex than TIG. Additional controls needed for plasma arc and filters and deionizers for cooling water mean more frequent maintenance and additional costs.
  • Economical for low production runs. Can be used for one-offs.
  • Tooling costs low to moderate.
  • Equipment costs generally high.
  • Direct labor costs moderate.
  • Finishing costs low.

Typical applications

  • Engine components
  • Sheet-metal fabrication
  • Domestic appliances
  • Instrumentation devices
  • Pipes

Design aspects

  • Design complexity high.
  • Typical joint designs possible using PAW: butt, lap, fillet and edge .
  • Design joints using minimum amount of weld, i.e. intermittent runs and simple or straight contours wherever possible.
  • Balance the welds around the fabrication’s neutral axis.
  • Distortion can be reduced by designing symmetry in parts to be welded along weld lines.
  • The fabrication sequence should be examined with respect to the above.
  • Design parts to give access to the joint area, for vision, filler rods, cleaning, etc.
  • Sufficient edge distances should be designed for. Avoid welds meeting at end of runs.
  • Mostly for horizontal welding, but can also perform vertical welding using higher shielding gas flow rates.
  • Filler can be added to the leading edge of the weld pool using a rod, but not necessary for thin sections.
  • Minimum sheet thickness =0.05 mm.
  • Maximum thickness, commonly:
    • Aluminum =3mm
    • Copper and refractory metals =6mm
    • Steels =10mm
    • Titanium alloys =13mm
    • Nickel =15 mm.
  • Multiple weld runs required on sheet thickness ≥10 mm.
  • Unequal thicknesses difficult.

Quality issues

  • High quality welds possible with little or no distortion.
  • Provides good penetration control and arc stability.
  • Access for weld inspection important, e.g. NDT.
  • Tungsten inclusions from electrode not present in welds, unlike TIG.
  • Joint edge and surface preparation important. Contaminates must be removed from the weld area to avoid porosity and inclusions.
  • A heat affected zone always present. Some stress relieving may be required for restoration of materials original physical properties.
  • Not recommended for site work in wind where the shielding gas may be gusted.
  • Need for jigs and fixtures to keep joints rigid during welding and subsequent cooling to reduce distortion on large fabrications.
  • Care needed to keep filler rod within the shielding gas to prevent oxidation.
  • Tungsten inclusions can contaminate finished welds.
  • Nozzle used to increase the temperature gradient in the arc, concentrating the heat and making the arc less sensitive to arc length changes in manual welding.
  • Plasma arc very delicate and orifice alignment with tungsten electrode crucial for correct operation.
  • Important process variables for consistency in manual welding: welding speed, plasma gas flow rate, current and torch angle.
  • ‘Weldability’ of the material important and combines many of the basic properties that govern the ease with which a material can be welded and the quality of the finished weld, i.e. porosity and cracking. Material composition (alloying elements, grain structure and impurities) and physical properties (thermal conductivity, specific heat and thermal expansion) are some important attributes which determine weldability.
  • Surface finish of weld excellent.
  • Fabrication tolerances a function of the accuracy of the component parts and the assembly/jigging method, but typically ±0.25 mm.

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