Gas Tungsten Arc Welding (TIG) and Allied Processes
What this domain tests
Polarity and electrode selection by material, shielding practice, parameters, and awareness of allied processes.
Core rules
- Polarity. DCEN for carbon steel, stainless, and most metals - it concentrates heat in the work and spares the tungsten. AC for aluminium and magnesium: the positive half cycle breaks up the refractory oxide (cleaning action) and the negative half cycle limits tungsten melting, producing a balled tip.
- Electrodes. 2% thoriated (red) for DCEN steel; pure (green) or zirconiated (brown) for AC aluminium because a balled tip is stable; ceriated and lanthanated are general-purpose alternatives.
- Shielding gas. Argon for most work at roughly 8–14 l/min; helium or argon-helium for thick sections and high-conductivity metals because it gives a hotter, wider arc. A gas lens gives better coverage; back-purge stainless to prevent sugaring.
- Current guide. About 30–40 A per millimetre of thickness on steel with DCEN; aluminium needs more current because heat conducts away from the puddle.
- Allied processes. Plasma arc welding (constricted arc, keyholing), A-TIG (activating flux), orbital and hot-wire TIG.
Worked example
Choose parameters for a 3 mm aluminium butt versus a 3 mm stainless butt. Stainless: DCEN, argon at about 10 l/min, 2% thoriated tungsten, roughly 80–110 A, tight 60–75° included angle, back purge. Aluminium: AC, argon at about 12 l/min, pure or zirconiated tungsten, roughly 120–150 A, balanced cleaning, oxide wire-brushed and degreased first. Reasoning: aluminium conducts heat away about five times faster than steel and carries an oxide film melting near 2050 °C, so it needs both the extra current and the AC cleaning half-cycle; stainless needs DCEN to keep the arc concentrated and the tungsten intact.
Traps
Using DCEP for TIG (the tungsten melts); using a thoriated electrode on AC aluminium; forgetting to remove the oxide film before welding; welding stainless without a purge and getting oxidised 'sugar'.