7.3 Shielding Gases Used with GTAW
Inert gases are gases that will not react with metals or other gases. Inert gases are used to create a protective bubble around the arc, molten metals, and the tungsten electrode while welding is being done. Argon, helium, argon-helium, and argon-hydrogen mixtures are used to shield the weld during gas tungsten arc welding. Because this process uses a tungsten electrode and inert gases, it is known in the shop as tungsten inert gas (TIG) welding. TIG is an older term that is still commonly used in informal settings. The AWS-recognized name for the process, however, is gas tungsten arc welding. The abbreviation is GTAW.
Argon (Ar) has an atomic weight of 40 and a density of 1.78 gm/l. It is, therefore, a relatively heavy gas. Argon is usually furnished in heavy steel cylinders similar in construction to those used for oxygen. The usual cylinder size is 300 ft3 (8.5 m3). Smaller cylinder sizes are available. Common sizes include 125 ft3 (3.5 m3) and 80 ft3 (2.3 m3). The 80 ft3 (2.3 m3) size is very portable. Argon, as used for welding, is a minimum 99.995% pure. It is capable of being shipped in the liquid form at –300°F (–184°C). Liquid argon can be shipped more cheaply than gaseous argon. However, equipment must be purchased by the user to vaporize the argon for use in welding.
Argon is used more often than helium for the following reasons:
Helium (He) is the lightest inert gas. It has an atomic weight of 4 and a density of 0.178 gm/l. Argon, therefore, is ten times heavier than helium. Helium used for welding is at least 99.95% pure. It is normally shipped and used in gaseous form. The cylinders are similar to those used for oxygen. Helium is typically purchased in cylinder sizes of 300 ft3 (8.5 m3). Smaller cylinders including 125 ft3 (3.5 m3) and 80 ft3 (2.3 m3) are available from welding gas suppliers. Because helium is so light, about two to three times more helium is required compared to argon to shield a given weld area.
The chief advantage of helium over argon is that helium yields a much higher available heat on the metal than is possible with argon. Helium is, therefore, used to weld thick sections of metal or metals with a high heat conductivity, such as copper or aluminum. Helium also produces greater weld penetration.
To produce the same available heat at the metal, higher currents must be used with argon than with helium. Studies have shown that undercutting will occur at the same current levels with either gas. Helium, therefore, will produce better weld results at higher speeds without undercutting than argon will.
Both helium and argon provide good cleaning action with direct current electrode positive (DCEP). However, since direct current electrode negative (DCEN) produces greater heat and penetration, DCEN is the polarity normally used for GTAW. With ac, which is used on aluminum and magnesium, argon provides a better cleaning action. Argon also provides better arc stability with ac than helium does.
Argon and helium can be mixed to obtain desired results. Mixtures provide a combination of the characteristics of each gas. Good arc starting and a stable arc can be combined with increased penetration and increased welding speeds.
Argon-hydrogen mixtures are used to produce higher welding speeds. The possible welding speed is directly proportional to the amount of hydrogen added to argon gas. The addition of hydrogen to argon permits the mixture to carry higher arc voltages. Too much hydrogen will cause porosity (gas pockets) in the weld metal. Porosity weakens a weld. Mixtures of 65% argon and 35% hydrogen have been used on stainless steel with a 0.010″ to 0.020″ (approximately 0.250mm to 0.500mm) root opening. The most common argon-hydrogen ratio is 85% argon and 15% hydrogen. Argon-hydrogen mixtures are used when welding stainless steels, nickel-copper, and nickel-based alloys. Hydrogen produces negative welding effects with most other metals.
Argon, helium, or argon-helium mixtures can be used for most GTAW welding jobs. Manual welding on thin metals is done best with argon because of the low arc voltages and welding currents required. Whenever there is a cross draft, it is advisable to construct a windbreak to block the draft. This will reduce the possibility of the shielding gas being blown away from the weld area.
Inert gases are gases that will not react with metals or other gases. Inert gases are used to create a protective bubble around the arc, molten metals, and the tungsten electrode while welding is being done. Argon, helium, argon-helium, and argon-hydrogen mixtures are used to shield the weld during gas tungsten arc welding. Because this process uses a tungsten electrode and inert gases, it is known in the shop as tungsten inert gas (TIG) welding. TIG is an older term that is still commonly used in informal settings. The AWS-recognized name for the process, however, is gas tungsten arc welding. The abbreviation is GTAW.
Argon (Ar) has an atomic weight of 40 and a density of 1.78 gm/l. It is, therefore, a relatively heavy gas. Argon is usually furnished in heavy steel cylinders similar in construction to those used for oxygen. The usual cylinder size is 300 ft3 (8.5 m3). Smaller cylinder sizes are available. Common sizes include 125 ft3 (3.5 m3) and 80 ft3 (2.3 m3). The 80 ft3 (2.3 m3) size is very portable. Argon, as used for welding, is a minimum 99.995% pure. It is capable of being shipped in the liquid form at –300°F (–184°C). Liquid argon can be shipped more cheaply than gaseous argon. However, equipment must be purchased by the user to vaporize the argon for use in welding.
Argon is used more often than helium for the following reasons:
- It provides easier arc starting.
- It provides a smoother, quieter arc action.
- It requires a lower arc voltage for a given arc length and current. This allows for better control of the weld pool.
- Its use results in reduced penetration.
- It has a lower overall cost and is more readily available.
- It provides a greater metal cleaning action on aluminum and magnesium when used with alternating current.
- It is heavier than helium, and requires a lower flow rate for good shielding.
- Because it is heavier than helium, it provides better cross-draft shielding.
- Because it requires lower arc voltages to maintain the arc, argon has advantages for use with thin metal and out-of-position welding.
Helium (He) is the lightest inert gas. It has an atomic weight of 4 and a density of 0.178 gm/l. Argon, therefore, is ten times heavier than helium. Helium used for welding is at least 99.95% pure. It is normally shipped and used in gaseous form. The cylinders are similar to those used for oxygen. Helium is typically purchased in cylinder sizes of 300 ft3 (8.5 m3). Smaller cylinders including 125 ft3 (3.5 m3) and 80 ft3 (2.3 m3) are available from welding gas suppliers. Because helium is so light, about two to three times more helium is required compared to argon to shield a given weld area.
The chief advantage of helium over argon is that helium yields a much higher available heat on the metal than is possible with argon. Helium is, therefore, used to weld thick sections of metal or metals with a high heat conductivity, such as copper or aluminum. Helium also produces greater weld penetration.
To produce the same available heat at the metal, higher currents must be used with argon than with helium. Studies have shown that undercutting will occur at the same current levels with either gas. Helium, therefore, will produce better weld results at higher speeds without undercutting than argon will.
Both helium and argon provide good cleaning action with direct current electrode positive (DCEP). However, since direct current electrode negative (DCEN) produces greater heat and penetration, DCEN is the polarity normally used for GTAW. With ac, which is used on aluminum and magnesium, argon provides a better cleaning action. Argon also provides better arc stability with ac than helium does.
Argon and helium can be mixed to obtain desired results. Mixtures provide a combination of the characteristics of each gas. Good arc starting and a stable arc can be combined with increased penetration and increased welding speeds.
Argon-hydrogen mixtures are used to produce higher welding speeds. The possible welding speed is directly proportional to the amount of hydrogen added to argon gas. The addition of hydrogen to argon permits the mixture to carry higher arc voltages. Too much hydrogen will cause porosity (gas pockets) in the weld metal. Porosity weakens a weld. Mixtures of 65% argon and 35% hydrogen have been used on stainless steel with a 0.010″ to 0.020″ (approximately 0.250mm to 0.500mm) root opening. The most common argon-hydrogen ratio is 85% argon and 15% hydrogen. Argon-hydrogen mixtures are used when welding stainless steels, nickel-copper, and nickel-based alloys. Hydrogen produces negative welding effects with most other metals.
Argon, helium, or argon-helium mixtures can be used for most GTAW welding jobs. Manual welding on thin metals is done best with argon because of the low arc voltages and welding currents required. Whenever there is a cross draft, it is advisable to construct a windbreak to block the draft. This will reduce the possibility of the shielding gas being blown away from the weld area.