Chapter 8 Gas Metal and Flux Cored Arc Welding 209 Copyright Goodheart-Willcox Co., Inc. Gas metal arc welding of carbon steel can be done with 100% argon, 100% carbon dioxide, or various mixtures of these two gases and other shielding gases. There are advantages and disadvantages to be consid- ered when selecting a shielding gas. Shielding gases are commonly delivered in cylin- ders. Shielding gas cylinders can be ordered with a single shielding gas or a shielding gas mixture. Each cylinder is labeled by the supplier with its contents. If a gas mixture is used, it can be mixed on-site by combining multiple pure gases in a double- or triple- unit gas mixer. See Figure 8-30. Such units allow the secondary gas to be mixed by percentage with the primary gas. As an example, consider a 75% argon, 25% CO 2 mixture. Argon is the primary gas and the carbon dioxide is the secondary gas. The gas mixer unit allows the secondary gas to be added as a percentage to the primary gas, in this case 25%. A knob is used to adjust the percentage of the secondary gas. The per- centage of the secondary gas cannot exceed 50%. Shielding Gas Selection for GMAW Metals Gases % Uses and results Aluminum Ar 50%Ar-50%He 25%Ar-75%He He Good transfer, stable arc, little spatter. Removes oxides. Hot arc — 3/8″ to 3/4″ (10 mm to 19 mm) thickness. Removes oxides. Hot arc, less porosity, removes oxides — 1/2″ to 1″ (13 mm to 25 mm). Hotter, more gas 1/2″ (13 mm) and up. Removes oxides. Magnesium Ar 75%He-25%Ar Good cleaning. Hotter, less porosity, removes oxides. Copper (deox.) 75%He-25%Ar Ar Preferred. Good wetting, hot. For thinner materials. Carbon steel CO2 Ar-2%O2 Ar-5%O2 75%Ar-25%CO2 50%Ar-50%CO2 Short circuiting arc: high quality, low current, out-of-position, medium spatter. Globular arc: fast, cheap, spattery, deep penetration. Spray transfer: fast, stable, good bead shape, little undercut, fluid weld pool. Short circuiting arc: fast, no melt-through, little distortion and spatter. Short circuiting arc: deep penetration, low spatter. Low-alloy steel Ar-2%O2 Removes oxides, eliminates undercut, good properties. High-strength steels 60%He-35%Ar- 5%CO2 75%Ar-25%CO2 Short circuiting arc: stable arc, good wetting and bead contour, little spatter. Good impacts. Short circuiting arc: same except low impact. Stainless steel Ar-1%O2 Ar-5%O2 90%He-7.5%Ar- 2.5%CO2 Spray transfer: no undercutting, stable arc, fluid weld, good shape. Spray transfer: more stable arc. Short circuiting arc: small heat-affected zone, no undercut, little warping. Nickel, monel Ar Good wetting — decreases fluidity. Inconel Ar-He Stable arc on thinner material. Goodheart-Willcox Publisher Figure 8-29. Shielding gas selections for GMAW of various metals. Thermco Instrument Corp. Figure 8-30. Two gas mixers. The one on the left mixes up to 50% CO2 with argon. The one on the right mixes up to 10% oxygen with argon.