258 Section 3 Gas Metal and Flux Cored Arc Welding Copyright Goodheart-Willcox Co., Inc. wire feeder to the welding gun. Make sure a Tefl on® liner is used in the cable. A pull gun, which pulls the electrode wire through the cable, may be used. A push-pull system is formed when a pull gun pulls the electrode wire through the cable as the wire feeder pushes the wire. A push-pull system helps to prevent bird’s nests from forming. Aluminum oxidizes very easily, forming aluminum oxide. Aluminum oxide has a higher melting temperature than aluminum. The base metal should be chemically or mechanically cleaned before welding. A stainless steel wire brush is recommended to mechanically clean the joint. The tip of the electrode wire may also oxidize. Use a pair of wire cutters to cut the tip off the electrode wire. This should be done each time before starting a new weld. Place your hand over the area to be cut to prevent the piece of wire from fl ying through the air and injuring someone. Forehand welding is better at removing oxides on unwelded areas than backhand welding. However, penetration may be less with the forehand welding method. Aluminum conducts heat very well. The metal you are welding is already preheated as you move along the weld joint. Therefore, it may be necessary to move the welding gun faster to prevent melt-through. Weld Defects Defects produced in GMAW and FCAW are similar to those found in work done using other welding processes. These defects include incomplete penetration, lack of fusion, slag inclusions, undercut, overlap, and porosity. Defects cause a weld to be weaker than its design requirements. Figure 17-18 shows an unacceptable weld. Two common defects in a fi llet weld are undercut and overlap. It is important to use the correct gun angles and gun motion to avoid these defects. Make sure both pieces of metal are being melted. You may need to move the gun toward the toe of the fi llet weld to melt the base metal. Another common defect is slag in the weld. This often happens on a multiple-pass weld. Be sure to completely remove all slag from one weld pass before starting another pass. A GMAW bead will have only very small silicon deposits on the bead. A FCAW bead will have a complete layer of slag over the completed bead. Also remember that some FCAW electrodes are designed for single-pass welding only. The slag must be removed, but no additional weld beads are to be deposited over the fi rst bead. Most defects can be avoided by taking the following steps: • Selecting the correct electrode and shielding gas. • Setting the correct voltage, wire feed speed, and shielding gas fl ow rate. • Maintaining the proper contact tube-to-work distance (nozzle-to-work distance). • Using the proper gun angles. • Moving the gun at an appropriate and constant travel speed. • Removing slag between weld passes. Crater of initial bead is not filled Goodheart-Willcox Publisher Figure 17-18. This weld made with GMAW metal cored electrode is not acceptable because the restart did not start and fill the crater of the initial bead. The weld is weak in the middle.