Chapter 8 Gas Metal and Flux Cored Arc Welding 197 Copyright Goodheart-Willcox Co., Inc. The various methods of transfer deposit metal at dif- ferent rates, Figure 8-2. Methods of transferring metal across the arc include the following: • Globular transfer. • Spray transfer. • Pulsed spray transfer. Short circuiting, globular transfer, spray transfer and pulsed spray transfer will each be discussed in the next four subheadings. 8.2.1 Short Circuit GMAW (GMAW–S) Short circuit gas metal arc welding (GMAW–S) is used with relatively low welding currents. Welding current, which is controlled by the wire feed speed, is lower for short circuiting transfer than any other transfer method. Thus, short circuiting transfer is particularly suitable for thin metal sections. Short circuiting transfer is also useful for fi lling large root gaps or gaps between poor-fi tting parts. Short circuiting transfer can be used in all posi- tions. All position welds are made easily because there is no metal transfer across the arc. The weld pool cools and solidifi es rapidly using the short circuiting arc. Short circuiting transfer results in low heat input into the base metal. Since GMAW–S has a small weld pool and low heat input, it is used to weld in the overhead or vertical welding position. In short circuiting transfer, the arc is terminated or goes out when the electrode touches the molten weld pool. Surface tension from the weld pool pulls the molten metal from the end of the electrode into the pool. A pinch force is an electrical force around the electrode that squeezes the molten end of the electrode. The combined effects of surface tension and the pinch force separate the molten metal from the electrode. This separated portion of the elec- trode fl ows into the weld pool and fl attens out. See Figure 8-3. Once the molten metal from the electrode is sepa- rated from the electrode, the current jumps the gap between the new end of the electrode and the weld pool, reestablishing the arc. The arc melts the base metal and the end of the electrode. The continuously fed electrode again touches the molten pool and the process repeats. The process of shorting the electrode to the work repeats about 20 to 200 times per second. The pinch force, which acts to separate the end of the electrode, is created by current fl owing through the electrode. Arc voltage, the slope of the power source, and the circuit resistance determine the strength of the pinch force. These factors—voltage, slope, and resistance—also affect the welding current. If a 150A current is set on the arc welding machine, the amperage may rise rapidly to the maximum out- put of the machine when the electrode short circuits that could be 500A or more. An inductance circuit is built into the arc welding machine to control and slow down any rapid rises in current. GMAW Deposition Rates GMAW method Metal deposited lb/hr kg/hr Short circuiting Globular Spray Pulsed spray 2–6 4–7 6–12 2–6 0.9–2.7 1.8–3.2 2.7–5.4 0.9–2.7 Goodheart-Willcox Publisher Figure 8-2. The approximate rate at which carbon steel filler metal is deposited with various GMAW methods. Arc reignites Wire nears another short circuit Metal short-circuits to weld pool Pinch force squeezing off droplet Pinch force Shielding gas envelope A B C D Goodheart-Willcox Publisher Figure 8-3. The sequence of metal transfer during the short circuiting transfer method. A—The welding wire short-circuits to the weld pool. There is no arc at this point. B—A magnetic pinch force squeezes off a droplet of molten electrode metal. C—The welding arc reignites. D—The electrode nears another short circuit condition and the process repeats itself.