Welding process
Gas tungsten arc welding (GTAW)
An arc burns between a non-consumable tungsten electrode and the work under inert gas shielding, with filler added separately by hand, giving the cleanest and most precisely controlled arc weld.
How it works
A tungsten electrode carries the arc without melting, because tungsten has the highest melting point of any metal. Inert shielding gas, usually argon or an argon-helium blend, flows through the torch cup and blankets both the electrode and the pool. Filler metal is dipped in separately, so heat input and filler addition are controlled independently.
Polarity governs where the heat goes and whether the arc cleans the surface. Electrode negative puts most of the heat into the work and keeps the tungsten cool, which suits steel and stainless. Alternating current alternates between heating the work and stripping the refractory oxide film off aluminium and magnesium, which is why aluminium is welded on AC.
Because there is no flux and no transferring filler in the arc, the process produces no slag and very little spatter, and the weld chemistry stays close to the filler and base metal. The price is speed: the welder must feed filler by hand, deposition rates are low, and the joint has to be genuinely clean before an arc is struck.
Strengths
- Highest weld quality and cleanliness of the common arc processes, with no slag and minimal spatter
- Precise control of heat input and filler addition, ideal for thin material and root passes
- Welds almost any metal, including aluminium, titanium, magnesium, and exotic alloys
- Can be run autogenously with no filler at all where the joint design allows it
Limitations
- Low deposition rate and slow travel speed, making it costly on heavy sections
- Demands high welder skill, since both hands work independently at the same time
- Very sensitive to contamination; oil, oxide, or moisture on the joint shows up immediately
- Shielding gas is easily disturbed, so wind screening and adequate purge are essential
Key variables
- Polarity and current type
- Electrode negative for steel and stainless, alternating current for aluminium so the arc's cleaning half-cycle strips the oxide film.
- Tungsten type and taper
- Alloy content and included grind angle set arc starting behaviour, arc stability, and how concentrated the arc cone is.
- Shielding gas and flow
- Argon gives a soft controllable arc; helium additions raise heat and travel speed. Too high a flow causes turbulence and draws in air.
- Arc length
- Held entirely by the welder. A long arc widens the bead, reduces penetration, and lets the shield break down at the edges of the pool.
- Filler wire addition
- Rate and timing of dips control bead profile and dilution, and the wire must stay under gas coverage to avoid oxidised deposits.
What an inspector watches for
- Check the purge on stainless and alloy root passes, since heat tint or sugaring on the inside surface signals an inadequate purge
- Look for tungsten inclusions and a contaminated or balled electrode tip, which point at dipping or excess current
- Verify tungsten type and grind geometry against the procedure, because both alter arc shape and penetration
- Confirm filler wire identification and that the wire's hot end stayed inside the gas shield rather than being pulled out to oxidise