Chapter 8 Gas Metal and Flux Cored Arc Welding 213 Copyright Goodheart-Willcox Co., Inc. Effect of Shielding Gases in Metal Transfer Short circuiting transfer on aluminum and other non- ferrous metals and alloys requires an inert shielding gas. Short circuiting transfer on steels is done with CO2 or a mixture of argon and CO2. A common mixture is 75% argon–25% CO2. A mixture of helium, argon, and CO2 is used to weld stainless steel. Globular transfer on steel base metals can occur with pure CO2 or with argon with a high percentage of CO2. With CO2, the globules leave the wire in a ran- dom way, and spatter is high. Spray or pulsed spray transfer on nonferrous base metals requires pure argon or an argon-helium mix- ture. Spray transfer or pulsed spray on ferrous base metals occurs only in an atmosphere that has a high argon percentage. When low-carbon steels are welded with the spray or pulsed spray transfer method, argon with 1%–5% oxygen (O2) and argon with 5%–10% CO2 mixtures are used. Small amounts of oxygen or CO2 lower the transi- tion current and help stabilize the arc. Oxygen or CO2 allows the molten metal droplets to leave the electrode more easily. Oxygen or CO2 makes the weld pool more fl uid, reduces the deep fi ngerlike penetration of a pure argon shielded weld, and reduces undercutting. For fl ux cored arc welding, it is important to use the correct shielding gas with the proper electrode. Elec- trodes are designed for use with a certain shielding gas or gas mixture. Carbon dioxide is a common shielding gas for FCAW. The metal transfer method is globular. Some fl uxes allow for a spray-like transfer. Argon alone is not used for FCAW. A common gas mixture for FCAW is 75% Ar–25% CO2. This gas mixture, with the proper electrode, produces a spray transfer method. Less oxidation of the weld occurs when a gas mixture with high argon is used. 8.4.4 Selecting the Proper Shielding Gas Flow Rate for GMAW and FCAW–G Enough gas must fl ow to create a straight-line (lami- nar) fl ow. If too much gas comes out of the nozzle, the gas may become turbulent. Turbulent fl ow of shield- ing gas can draw in the atmosphere from around the nozzle area, causing contamination of the weld. See Figure 8-34. A gas lens can be installed in the welding gun to create a steady laminar gas fl ow. When too little gas fl ows, the weld area is not prop- erly protected. The weld becomes contaminated, and porosity occurs. Too little gas creates a popping sound. Contact tip Nozzle Laminar (straight-line) gas flow A B Turbulent gas flow Goodheart-Willcox Publisher Figure 8-34. Effects of gas flow rate. A—Laminar gas flow is the result of the proper gas flow rate. B—Turbulence occurs when the flow rate is too high.