How do TIG welding machines work?

This is a welding process in which the heat source is an electric arc between the workpiece and a non-fusible
(refractory) electrode within a protective atmosphere flowing from the torch nozzle.
The role of the gas is to protect the molten metal, the tungsten electrode (W) and the filler metal
(which is why it is left until the gas fades under the torch).
The nature of the gas (argon or helium) will also influence the energy of the arc,
the shape and depth of penetration (the voltages observed with helium are higher than with argon. This results in better penetration),
the surface appearance of the bead (better with helium).

Hydrogen (H) and nitrogen (N) can be used in a mixture with argon (Ar) or helium (He):
gas action in welding the shielding gas will have an impact not only on the geometry of the bead
(better penetration because higher arc voltage with H) but also on the chemistry and metallurgical structure of the bead
(H being strongly reducing, it reduces traces of oxides on the beads / N favours the formation of austenitic structures).
The settings of the pre-gas and post-gas phases during TIG welding are important for oxidation and shrinkage defects.
The refractory electrode consists of tungsten (W) or tungsten alloy.
The choice is made according to the emissivity required in relation to the arc environment
(an environment that is too hot for an alloyed electrode may cause the electrode to disintegrate).
This electrode must be trimmed when it is too polluted, when it comes into contact with the molten metal
or the filler metal or when the tip is damaged (disintegrated due to the temperature).
This process can be used with or without filler metal (142). This process is rather slow with a
low deposition rate, the polarity is called direct (electrode at the negative pole of the generator).
The intensity is adjusted on a TIG station, the arc voltage being a function of the
torch height and the gas used. The torches can be air or water cooled and the nozzles adapted for the gas flow to generate a protective atmosphere.

What are the defects associated with the
TIG-141 welding process?

Greyish bead (on stainless steel): Poor gas protection (right side / reverse side)

Concave bead: not enough filler metal.

Gutter: chamfer too tight, current too strong - may also be related to lack of filler metal.

Uneven penetration: Uneven feed rates, stability of the torch, uneven chamfer (grinding preparation...)

Lack of penetration: Current too low, root clearance (joint preparation, pinching during welding), energy too low (speed too high, gas atmosphere, electrode size), unmelted spot.

Lack of fusion / bonding (rare): Current too low, root clearance and chamfer (access to bottom of joint), position and type of joint (heat input / heat output)

Tungsten inclusion (W): Poorly sized electrode, poorly chosen electrode composition, electrode in contact with the melt (not detectable by visual inspection)

Oxide inclusion: Filler metal enriched with Si (TIG in automatic), the silicates are found in the skin of the bead and must be ground off. Most often on aluminium in manual TIG (alumina layer)

Shrinkage: Arc fade time too short, S and P impurities, shrinkage on solidification too high (clamping of parts, lack of liquid metal)

Blowout: Poor degreasing of the edges to be welded, poor gas protection (nozzle or torch leakage, end of cylinder), fade time too short, leakage in the cooling water circuit, steel elements that volatilize or steel that is not calmed. Special case: closure of a reinforcing saddle or a fur without vent.