196 Modern Welding Copyright Goodheart-Willcox Co., Inc. FCAW can be done with shielding gas. Gas-shielded FCAW is known as FCAW–G. Because FCAW–G uses a shielding gas, it is frequently used inside a building where there are no cross winds to disrupt the shielding gas. FCAW can also be done without shielding gas. This is known as FCAW–S. The FCAW–S fl ux cored electrode does less to protect the weld from the atmosphere. The fl ux in the electrode wire does a better job at removing oxygen and nitrogen from the weld pool. FCAW–S is more commonly used outdoors on construction sites. Any cross wind will not affect the weld quality. 8.1 Gas Metal Arc and Flux Cored Arc Welding Principles Gas metal arc welding is a quick and easy process that produces quality welds. GMAW is usually performed with solid wire electrodes. FCAW uses fl ux cored welding wires. See Figure 8-1. A shielding gas or gas mixture must be used with GMAW. GMAW is nearly always performed with direct current electrode positive (DCEP) current. Direct cur- rent electrode negative (DCEN) is rarely used for gas metal arc welding, but has a limited use for surfacing. A straight AC output current is not produced for GMAW or FCAW. However, a variable polarity waveform is possible with an inverter power source. The variable wave combines the penetration of DCEP with a higher deposition of DCEN. It can be used on thinner steel and aluminum base metal. It differs from normal AC in that the positive and negative portions of the output wave are not balanced. For every pound of solid GMAW welding wire used, 92%–98% becomes deposited weld metal. About 82%–92% of fl ux cored arc welding wire is deposited as weld metal. As a comparison, SMAW deposits 60%–70% of the welding wire as weld metal. Some spatter does occur in the GMAW processes. FCAW–G has some spat- ter. FCAW–S has higher levels of spatter. Very little stub loss occurs when continuously fed wire is used. There are very thin glass-like islands over the weld bead after gas metal arc welding. No heavy slag is created because the weld area is shielded by a gas. FCAW produces a slag covering. Some of the fl ux in the FCAW forms a gas around the weld area. Some of the fl ux forms a slag that covers the weld. A welder can spend more time on the welding task with a continuously fed wire process. This improves the cost effi ciency of GMAW and FCAW. The GMAW process can be adapted to a variety of job requirements by choosing the correct shielding gas, electrode size, and welding parameters. Welding parameters include voltage, travel speed, and wire feed rate. The arc voltage and wire feed rate deter- mines the fi ller metal transfer method. 8.2 Metal Transfer Metal transfer is the transfer of molten fi ller metal from the electrode to the weld pool, either by the short cir- cuiting method or by transferring metal across the arc. Shielding gas Contact tip Wire motion Wire motion Electrode Gas nozzle Gas Metal Arc Welding (GMAW) Shielding gas (if used) Contact tip Gas nozzle (if used) Flux Cored Arc Welding (FCAW) Flux cored electrode Goodheart-Willcox Publisher Figure 8-1. Cutaway views of GMAW and FCAW gas nozzles and electrodes. If shielding gas is not used with FCAW, no nozzle is required.