T
he gas metal arc welding (GMAW) process was first conceived in the 1920s, but it was not available for commercial use until the late 1940s. The primary application was the welding of aluminum. During the early 1950s, it was discovered that reactive gases, such as carbon dioxide (CO2), and mixtures of inert and reactive gases could be used to join a wider range of materials with increased weld purity and production efficiency. The development of more versatile continuous consumable wire electrodes (welding wires) increased the popularity of GMAW. Today, GMAW is used extensively in the automotive industry and in a variety of other manufacturing and fabrication environments.
GAS METAL ARC WELDING Gas metal arc welding (GMAW) is an arc welding process that uses an arc between a continuous wire electrode and the weld pool. GMAW equipment consists of a welding power source, a welding gun cable and gun assembly, electrode wire (welding wire), a wire feeder, shielding gas, and a workpiece lead with a workpiece connection. The continuous wire electrode (welding wire) is fed through the wire feeder and the welding gun cable and gun assembly at a preset speed (the wire feed speed, or wfs). The molten welding wire transfers across the arc where it fuses with the base metal to form the weld. A shielding gas or a combination of shielding gases supplied from an external source is also fed through the welding gun cable and gun assembly. The shielding gas completely covers and protects the weld pool. Metal transfer is the manner in which molten metal transfers from the end of the electrode across the welding arc to the weld pool. GMAW is capable of producing three modes of metal transfer: short circuiting transfer, globular transfer, and spray transfer, as well as a
OBJECTIVES • • • • • •
Describe gas metal arc welding (GMAW). Describe types of power sources used for GMAW. Describe welding guns and their use in GMAW. Explain how wire feeders work. Identify common types of shielding gas used for GMAW. Identify considerations in selecting GMAW welding wire.
variation of spray transfer called pulsed spray transfer. The type of metal transfer used is determined by wire feed speed (which controls the amperage or current), arc voltage, welding wire diameter and composition, and type of shielding gas. A GMAW weld can be applied by the semiautomatic, mechanized, or automatic methods. When semiautomatic welding is used, the wire feed speed, voltage setting, and gas flow rate are preset, but the welding gun is manually operated. The welder directs the welding gun along the weld joint to complete the weld. A constant-voltage welding machine with direct current electrode positive is most commonly used when welding with GMAW.
In mechanized GMAW, the welding operator sets the welding parameters and monitors the welding operation while a mechanical device controls the welding gun along the joint. In automatic GMAW, the welding parameters and welding gun movements are programmed into a computer, and all aspects of the process are controlled by the equipment, such as in a robotic cell in a manufacturing environment. 217