Figure 8-9. - Tungsten electrode shapes for ac and dc welding.

Tungsten electrodes are usually color-coded at one end. A green color indicates that the rod is pure tungsten; yellow indicates a 1-percent thoriated tungsten rod; red indicates a 2-percent thoriated tungsten rod; and brown indicates that the rod is alloyed with zirconium.

To produce good quality welds with the GTA process, you must shape the electrode properly. The general practice is to use a pointed electrode with dc welding and a spherical end with ac welding (fig. 8-9).

Shielding Gas

Shielding gas for GTA welding can be argon, helium, or a mixture of argon and helium. Argon is by far the most popular. When compared to helium, argon has greater cleaning action and provides a more stable arc. Argon is heavier than air; therefore, it provides a blanket over the weld that protects it from contaminants. Helium, being lighter than air, requires a higher gas flow than argon and is therefore more expensive to use. However, as a shielding gas, helium - allows greater penetration and faster welding speeds because the arc is hotter in the helium atmosphere than in the argon atmosphere. The opposite is true for GMA welding; therefore, a mixture of argon and helium is sometimes used in welding metals that require a higher heat input. Table 8-4 lists a selection of shielding gases recommended for various metals for both the GTA and GMA welding processes. Notice that for most GTA welding operations, you use pure argon.

Regulators

The primary difference between the regulators used for oxyfuel welding and for GTA/GMA welding is that the working pressure on the oxyfuel regulators is shown in pounds per square inch (psi) while the regulators used

Figure 8-10. - Combination regulator and flowmeter.

Figure 8-11. - Cross section of flowmeter.

for GMA/GTA show the flow of shielding gas in cubic feet per hour (cfh) or liters per minute (lpm). Regulators used for GMA/GTA welding have a flowmeter instead of a working pressure gauge along with the cylinder pressure gauge. See figure 8-10.

The flowmeter consists of a plastic or glass tube that contains a loosely fitting ball. As the gas flows up the tube, it passes around the ball and lifts it up. The more