Introduction
Gas tungsten arc welding is ideally suited for welding aluminum alloys. All thicknesses of aluminum can be welded with this process, with or without filler metal.
Aluminum alloys form refractory surface oxides, which make joining more difficult. For this reason, most welding of aluminum is performed with alternating current (using high-frequency arc stabilization) because ac provides the surface cleaning action of DCEP in addition to the deeper penetration characteristics of DCEN. Sometimes DCEP is used for welding thin aluminum sections. DCEN with helium as the shielding gas is used for the high-current automatic welding of sections over 6.35 mm (1/4 in.) thick. Since DCEN produces no cleaning action, the aluminium workpieces must be thoroughly cleaned immediately prior to welding.
For welding with ac, electrodes of pure tungsten, ceriated tungsten, and zirconiated tungsten are recommended. Only thoriated tungsten electrodes are used on aluminum with dc.
Argon shielding is generally used for welding aluminium with ac because it provides better arc starting, better cleaning action, and superior weld quality than helium. When DCEN is used, helium shielding allows faster travel speeds and achieves deeper penetration. However, the poor surface cleaning action of this combination may result in porosity.
Aluminum and magnesium alloys have high thermal and electrical conductivities. Therefore, high welding current and short welding time are needed.

Electrode Selection
Pure tungsten electrodes (EWP) contain a minimum of 99.5% tungsten, with no intentional alloying elements. The current carrying capacity of pure tungsten electrodes is lower than that of the alloyed electrodes. Pure tungsten providesgood arc stability when used with alternating current, either balanced-wave or continuous high frequency. The tip of the EWP electrode maintains a clean, balled end that promotes good arc stability. EWP electrodes may also be used with dc, but they do not provide the arc initiation and arc stability characteristics offered by thoriated, ceriated, or lanthanated electrodes. Pure tungsten electrodes are generally considered low-cost electrodes and are normally used for the welding of aluminum and magnesium alloys.

Current Selection
When the tungsten electrode is connected to the positive terminal (DCEP), a cathodic cleaning action is created at the surface of the workpiece. This action occurs when welding most metals, but it is most important when welding aluminum and magnesium because the cleaning action removes the refractory oxide surface that inhibits the wetting of the base metal surfaces of the weldment by the weld metal.


Inert Gas
Argon (Ar) is an inert monatomic gas with a molecular weight of 40. It is obtained from the atmosphere by the separation of liquefied air. Welding-grade argon is refined to a minimum purity of 99.99%. This is acceptable for the gas tungsten arc welding of most metals except the reactive and refractory metals, for which a minimum purity of 99.997% is required. For this reason, weldments in reactive and refractory metals are often fabricated in purge chambers from which all traces of air have been removed prior to initiating the welding operation. Argon is used more extensively than helium for shielding because argon offers the following advantages:
1. Smoother, quieter arc action;
2. Reduced penetration;
3. Cleaning action when welding materials such as aluminum and magnesium;
4. Lower cost and greater availability;
5. Lower flow rates for good shielding;
6. Better cross-draft resistance; and
7. Easier arc starting.

WELDING PROBLEMS AND REMEDIES

Filler Metal
Refer AWS A5.10 for filler metal for aluminium.

Safety
All personnel within the immediate vicinity of a welding operation should have adequate protection from the radiation produced by the welding arc. Generally, the highest ultraviolet radiant energy intensities are produced when argon shielding gas is used and when aluminum or stainless steel is welded.

Source: AWS Welding Handbook