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Manufacturing Processes > Joining processes > Tungsten Inert-Gas Welding(TIG)

 

Tungsten Inert-Gas Welding(TIG)

 

Process description

An electric arc is automatically generated between the workpiece and a non-consumable tungsten electrode at the joint line. The parent metal is melted and the weld created with or without the addition of a filler rod. Temperatures at the arc can reach 12 000°C. The weld area is shielded with a stable stream of inert gas, usually argon, to prevent oxidation and contamination.

Materials

Most non-ferrous metals (except zinc), commonly, aluminum, nickel, magnesium and titanium alloys, copper and stainless steel. Carbon steels, low alloy steels, precious metals and refractory alloys can also be welded. Dissimilar metals are difficult to weld.

TIG

Process variations

  • Portable manual or automated a.c. or d.c. systems. a.c commonly used for welding aluminum and magnesium alloys.
  • Pure helium or more commonly, a helium/argon mix is used as the shielding gas for metals with high thermal conductivity, for example copper, or material thickness greater than 6mm giving increased weld rates and penetration.
  • Pulsed TIG: excellent for thin sheet or parts with dissimilar thickness (low heat input).
  • TIG spot welding: used on lap joints in thin sheets.

Economic considerations

  • Weld rates vary from 0.2 m/min for manual welding to 1.5 m/min for automated systems.
  • Automation is suited to long lengths of continuous weld in the same plane.
  • Automation is relatively inexpensive if no filler is required, i.e. use of close fitting parts.
  • Process is suited to sheet thickness less than 4 mm, heavier gauges become more expensive due to argon cost and decreased production rate. Helium/argon gas is expensive but may be viable due to increased production rate.
  • It is economical for low production runs. Can be used for one-offs.
  • Tooling costs are low to moderate.
  • Equipment costs are moderate.
  • Direct labor costs are moderate to high. Highly skilled labor required for manual welding. Setup costs can be high for fabrications using automated welding.
  • Finishing costs are low generally. There is no slag produced at the weld area, however, some grinding back of the weld may be required.

Typical applications

  • Chemical plant pipe work
  • Nuclear plant fabrications
  • Aerospace structures
  • Sheet-metal fabrication

Design aspects

  • Design complexity is high.
  • Typical joint designs possible using TIG are: butt, lap, fillet and edge (see Appendix B – Weld Joint Configurations).
  • Design joints using minimum amount of weld, i.e. intermittent runs and simple or straight contours, although TIG is suited to automated contour following.
  • Design parts to give access to the joint area, for vision, electrodes, filler rods, cleaning, etc.
  • Wherever possible horizontal welding should be designed for, however, TIG welding is suited to most welding positions.
  • Sufficient edge distances should be designed for. Avoid welds meeting at end of runs.
  • Balance the welds around the fabrication’s neutral axis where possible.
  • 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.
  • Provision for the escape of gases and vapors in the design is important.
  • Minimum sheet thickness =0.2 mm.
  • Maximum thickness, commonly:
    • Copper and refractory alloys =3mm
    • Carbon, low alloy and stainless steels; magnesium and nickel alloys =6mm
    • Aluminum and titanium alloys =15 mm.
  • Multiple weld runs required on sheet thickness ≥5 mm.
  • Unequal thicknesses are difficult.

Quality issues

  • Clean, high quality welds with low distortion can be produced.
  • Access for weld inspection important, e.g. Non-Destructive Testing (NDT).
  • 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.
  • Control of arc length important for uniform weld properties and penetration.
  • Need for jigs and fixtures to keep joints rigid during welding and subsequent cooling to reduce distortion on large fabrications.
  • Backing strips can be used for avoiding excess penetration, but at added cost and increased setup times.
  • Selection of correct filler rod important (where required).
  • Care needed to keep filler rod within the shielding gas to prevent oxidation.
  • Workpiece and filler rod must be away from the tungsten electrode to prevent contamination which can cause an unstable arc.
  • Shielding gas must be kept on for a second or two to allow tungsten electrode to cool and prevent oxidation.
  • Tungsten inclusions can contaminate finished welds.
  • Welding variables should be preset and controlled during production.
  • Automation reduces the ability to weld mating parts with inherent size and shape variations; reduced by automation however, it does reduce distortion, improve reproduction and produces fewer welding defects.
  • ‘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 typically ±0.5 mm.

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