Chapter 8 Gas Metal and Flux Cored Arc Welding 219 Copyright Goodheart-Willcox Co., Inc. Metal surfaces can be cleaned mechanically or chemically. Abrasive cloth or wire brushing can be used on lightly oxidized metals. Any abrasive from grinding should be wiped away prior to welding. Any chemicals used should be properly removed per manufacturer’s requirements prior to welding. Highly oxidized, pitted, or dirty base metals may need to be fl ame or arc cut, machined or ground to remove the affected material. If the metal being welded is expected to remain oxidized prior to welding, select a welding wire with more deoxidizers. An ER70S-6 GMAW electrode wire has more silicon than the other carbon steel electrodes and is a good choice for welding on oxidized base metal. Discuss the application with a welding wire supplier to obtain the best welding wire for the job. Joint designs for gas metal arc and fl ux cored arc welding are similar to those used for shielded metal arc welding. The groove angle used for GMAW or FCAW may be smaller than the angle used for SMAW. See Figure 8-42. It is possible to use this narrower angle for two reasons. The wire diameters are smaller, and GMAW and FCAW penetrate better than SMAW. A 45° groove angle takes less fi ller metal to fi ll than a 75° groove angle. Welding time is also less. Therefore, there are savings in fi ller metal and weld time. A key to consistently producing high-quality welds is proper preparation of the weld joint. Set the base metal to form the desired or specifi ed weld joint, such as a T-joint or butt joint. Tack weld the base metal. Depending on the size and thickness of the metal, weld a 1/2″ to 3/4″ (13 mm–19 mm) tack about every 3″–4″ (75 mm–100 mm). Welders often feather the tack welds. Feathering is the technique of smoothing the top of the tack weld with a grinder and grinding the start and end of the tack weld. Feathering makes it easier to join the weld bead with the tack welds. Tack welds must melt and get proper pen- etration. They become part of the fi nished weld joint. 8.6 Electrode Extension Electrode extension, sometimes referred to as stickout, is the distance that the end of the welding wire sticks out beyond the end of the contact tip. See Figure 8-43. A B SMAW GMAW/FCAW 60°–75° 30°–45° Goodheart-Willcox Publisher Figure 8-42. Arc welding beads compared. A—Typical groove angle and weld bead for SMAW. B—Typical groove angle and weld bead for GMAW and FCAW. Less filler metal and welding time are required to fill the groove. Nozzle Contact tip Electrode extension distance Contact tip- to-work distance Welding wire Nozzle- to-work distance Arc length Base metal Goodheart-Willcox Publisher Figure 8-43. The electrode extension distance and other distances important in GMAW and FCAW are shown.