212 Modern Welding Copyright Goodheart-Willcox Co., Inc. Because helium does not conduct electricity as well as argon, higher arc voltages are required, which creates more spatter. Helium allows fi ller metal to be deposited at a faster rate than is possible with argon. Wider weld beads are produced with helium because of the higher voltages used. Helium is lighter than argon and requires two to three times greater fl ow to provide an equivalent level of protection. In addition to requiring a greater fl ow rate, which results in more gas being used, helium is more expensive than argon. Even though the cost of helium may be greater than that for argon, the ben- efi ts of helium for the right welding application make helium an excellent choice. Carbon Dioxide Carbon dioxide (CO 2 ) gas has a higher thermal (heat) conductivity than argon and requires a higher voltage than argon when used in GMAW and FCAW–G. Since carbon dioxide (CO2) is heavy, it covers the weld well. Therefore, less gas is needed. CO2 costs about 80% less than argon. This price difference varies from location to location. Beads made with CO2 have a very good contour. The beads are wide and have deep penetration and no under- cutting. The arc in a CO2 atmosphere causes a great deal of spattering in GMAW. This is reduced by keep- ing the arc length short. Deoxidizers like aluminum, manganese, or silicon are often added to fi ller metal intended for use with CO2. The deoxidizers remove the oxygen from the weld metal. For GMAW, carbon dioxide is used only with short circuiting or globular transfer. It is also the most common shielding gas for FCAW–G. Warning Good ventilation is required when using pure CO2. About 7%–12% of the CO2 becomes dangerous CO (carbon monoxide) in the arc. The amount of CO increases with the arc length. Argon-Helium Mixtures of argon and helium help to produce a bal- ance between deep penetration and a stable arc. A mixture of 25% argon and 75% helium gives deeper penetration than 100% argon gas, but with equivalent arc stability. Spatter is almost zero when a 75% helium mixture is used. Argon-helium mixtures are used on thick nonferrous sections. Argon-Carbon Dioxide Welding ferrous alloys with spray transfer and an arc shielded by pure argon can be erratic and will often have undercutting at the toe of the weld. Mix- ing CO 2 in argon helps stabilize the arc and makes the molten metal in the weld pool more fl uid. This helps to eliminate undercutting when carbon steels are welded with the GMAW process using spray transfer. Adding 8%–10% CO 2 eliminates undercutting, elimi- nates the erratic arc, produces a fl uid weld pool, and allows for spray transfer. A mixture of 75% argon and 25% carbon dioxide or 80% argon and 20% carbon dioxide also produces a more stable arc and fl uid weld pool. These are used with short circuiting and globu- lar metal transfer. FCAW–G with a mixed gas typically uses a 75% argon–25% CO 2 gas mixture. This mixture pro- duces a better and more controllable arc than straight CO 2 and allows for a spray-like metal transfer. The resulting weld has better strength than a weld made with straight CO 2 . Argon-Oxygen Argon-oxygen gas mixtures are used on low-alloy, carbon, and stainless steels. A 1%–2% oxygen mix- ture produces beads with penetration that is wider and less fi nger-shaped. The addition of oxygen also improves the weld contour, makes the weld pool more fl uid, and eliminates undercutting. Also, oxy- gen stabilizes the arc and reduces spatter, but causes the metal surface to oxidize slightly. This oxidization generally does not reduce the strength or appear- ance of the weld to an unacceptable level. If more than 2% oxygen is used with low-alloy steel, a more expensive welding wire with additional deoxidizers must be used. Helium-Argon-Carbon Dioxide A helium-argon-carbon dioxide shielding gas mixture is used to weld austenitic stainless steel with the short circuiting transfer method. A mixture of 90% He, 7 1/2% Ar, and 2 1/2% CO2 is often used and produces a low bead.