Gas tungsten-arc (GTA) welding is an arc welding process that produces coalescence of metals by heating them with an electric arc between a nonconsumable tungsten electrode and the base metal. The weld pod, arc, electrode, and the heated section of the work pieces are protected from atmospheric contamination by a gaseous shield; otherwise, atmospheric oxygen and nitrogen will combine with the molten weld metal and result in a weak, porous weld. The shielding gas is usually an inert gas, such as helium, argon, or a mixture of gases.

The electrode used in GTA welding is generally tungsten or a tungsten alloy because other refractory metals would erode too rapidly at the high arc temperatures involved.

GTA welds are stronger, more ductile, and more corrosion-resistant than other types of arc welds. The weld zone has 100-percent protection from the atmosphere; therefore, no flux is required. Since no flux is required, it eliminates flux or slag inclusions in the weld, and there are no sparks, fumes, or spatter. With GTA welding, the welding heat, amount of penetration, and bead shape can be very accurately controlled, and the bead surface is smooth and uniform.

Any standard dc or ac welding machine can be used to supply the current for gas tungsten-arc welding. However, it is important that the generator or transformer have good current control in the low range. This is necessary to maintain a stable arc, especially when welding thin gauge materials. Specially designed machines with all of the necessary controls are available for gas tungsten-arc welding. Many of the power supply units are made to produce both ac and dc current. The choice of an ac or dc machine depends on what weld characteristics may be required. Some metals are

joined more easily with ac current, while others get better results when dc current is used.

Alternating current, high-frequency (ACHF) welding is a combination of dc straight polarity and dc reverse polarity. One half of the complete ac cycle is DCSP and the other half is DCRP. Unfortunately, oxides, scale, and moisture on the work piece often tend to prevent the full flow of current in the reverse polarity direction. If no current whatsoever flowed in the reverse polarity direction during a welding operation, the partial or complete stoppage of current flow would cause the arc to be unstable and sometimes go out. To prevent this, ac welding machines incorporate a high-frequency current flow unit. The high-frequency current is able to jump the gap between the electrode and the work piece, piercing the oxide film and forming a path for the welding current to flow.