Chapter 8 Gas Metal and Flux Cored Arc Welding 199 Copyright Goodheart-Willcox Co., Inc. At point A, the lower background current is fl owing. The arc melts the end of the wire and the base metal. Just as the electrode shorts to the base metal at point B, the power source reduces the current to near zero. There is no arc. After the short occurs, the current increases. At point C, the current is increasing to create the pinch force, which necks down the welding wire. A very important part of this process is that just as the molten end of the electrode is about to separate, the current is again reduced to a very low level at point D. The sur- face tension of the molten weld pool pulls the molten droplet off the end of the electrode and into the weld pool. There is very little current fl owing when the mol- ten droplet separates from the electrode. This low cur- rent prevents the end of the electrode from exploding or creating spatter. After the droplet leaves the end of the electrode, the current increases to the peak current amount. The peak current is shown as point E in Figure 8-4. Dur- ing this peak current period, the weld pool is very fl uid. The high current helps the weld pool fl ow out to the toe of the weld. Finally, Point A shows a new drop- let forming on the end of the electrode the process is repeating. One complete short circuit cycle is com- pleted and a new one begins. This process is repeated 20 to 200 times per second. 8.2.2 Globular Transfer Globular metal transfer occurs when the welding cur- rent is set slightly above the range used for the short circuiting metal transfer. In the globular metal trans- fer process, the metal transfers across the arc as large, irregularly shaped drops. See Figure 8-5. The drops are usually larger than the electrode diameter. Drops form on the end of the electrode. Each drop grows so large that it falls from the electrode due to its own weight. If the shielding gas contains a high per- centage of inert gas, the drops fall straight into the weld pool. If the shielding gas contains a high percentage of CO2, the drops travel across the arc in random paths, creating spatter. To minimize spatter, a lower arc volt- age, which produces a shorter arc length, can be used. However, too short of an arc voltage allows large drops to contact the base metal before the drops separate from the electrode. The resulting short circuits cause the drops to explode, creating a lot of spatter. One way to minimize spatter when using CO 2 is to increase the current slightly. This creates a deep weld pool that is lower than the surface of the surround- ing metal. This is referred to as a buried arc. Using a buried arc, most of the spatter is contained within the deep weld pool. With a buried arc, a combination of globular and short circuiting transfer occurs. Deeper penetration occurs with a buried arc. The globular transfer method can be used to cre- ate welds faster than the short circuiting transfer method, Figure 8-2. Welds produced with globular transfer are suffi cient quality for many applications. However, because of the excessive spatter and non- uniform metal deposition, this transfer method is not capable of producing truly high-quality welds. Short circuiting and spray transfer are preferred transfer methods. Because the molten metal falls into the weld pool due to gravity alone, this method of transfer can be used only in the fl at position. Arc Shielding gas envelope Spatter Deep weld pool Irregular large droplet forming Droplet may short-circuit when it falls Droplet may fall erratically and cause spatter Buried arc helps to contain droplet to reduce spatter Goodheart-Willcox Publisher Figure 8-5. GMAW globular metal transfer. Drops fall erratically and cause spatter. Note that the buried arc may help contain the drops to reduce spatter.