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.

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, somegrinding 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 thickness¼0.2 mm.
. Maximum thickness, commonly:
. Copper and refractory alloys¼3mm
. Carbon, low alloy and stainless steels; magnesium and nickel alloys¼6mm
. Aluminum and titanium alloys¼15 mm.
. Multiple weld runs required on sheet thickness5 mm.
. Unequal thicknesses are difficult.

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