256 Section 3 Gas Metal and Flux Cored Arc Welding Copyright Goodheart-Willcox Co., Inc. melted while welding. A ceramic or copper backing is used when welding mild steel or stainless steel. A stainless steel backing is used when welding aluminum. When welding a butt joint, the electrode should be directly over the centerline of the weld. The welding arc should equally melt both pieces being welded. If one piece is melting more than the other, adjust the location or angle of the gun until both pieces melt equally. A work angle of 0° is used, as shown in Figure 17-15. Use the backhand welding method so the tip of the electrode points opposite the direction of travel. A travel angle of 20°–25° should be used. This gun position can also be described as 65°–70° from the surface of the workpiece. Multiple passes or weld beads are required to fi ll the grooves for thick base metals. The keyhole method is used only on the root pass. After the root pass, each weld pass is similar to laying a weld bead on a plate. Good penetration into the base metal and the previous weld bead is required. Figure 17-16 shows a multiple-pass groove weld. Each weld bead must be thoroughly cleaned before another weld pass is made. When FCAW is used, all slag must be removed after each weld pass is made. Remember, some FCAW electrodes are designed for only single-pass welding. Verify the electrode being used is designed for multiple-pass welding. Figure 17-17 shows how multiple passes are made to fi ll a thick plate groove weld. Exercise 17-4 Welding a Square Groove Butt Joint 1. Obtain two pieces of mild steel measuring 1/8″ × 1 1/2″ × 6″ (3.2 mm × 40 mm × 150 mm). 2. Clean one edge on each piece. 3. Select the correct welding electrode, variables, and shielding gas for short-circuiting transfer. Set up the machine and set the correct flow rate. 4. Position the two pieces so there is a 3/32″ (2.38 mm) gap between them. Tack weld the pieces together in three places. 5. Support the pieces so a full penetration weld can be made. A backing is optional. 6. Align the gun and electrode with the center of the joint before welding. Press the switch on the 0° work angle Vertical Vertical A B 20°–25° travel angle 65°–70° from the surface Goodheart-Willcox Publisher Figure 17-15. These are the suggested gun angles for welding a butt joint. A—The gun is aligned vertically with the weld axis. This is a 0° work angle, as shown in the end view. B—Welding is done using the backhand welding method. A 20°–25° drag travel angle is used.