Chapter 19 GMAW–Equipment Gas Metal Arc Welding • GMAW Current Selection • GMAW Welding Power Sources • GMAW Equipment • GMAW Welding Wire

Constant-current (CC) power sources have a steep volt-ampere (V-A) curve, while constant-voltage (CV) power sources have a slightly sloping V-A curve. GMAW uses a direct current (DC) welding power source capable of producing CV. CV welding power sources are also referred to as constant-potential (CP) machines because the terms voltage and potential mean the same thing. Unlike constant-current (CC) power sources that have a steep volt-ampere (V-A) curve, CV power sources have a slightly sloping V-A curve. See Figure 19-1.

CV power sources are self-correcting with respect to arc length, which means that arc length remains constant with changes in electrode extension. An increase in electrode extension results in a slight increase in voltage. The increase in the electrode extension also increases the resistance in the wire, which results in a decrease in the welding current. Similarly, a decrease in electrode extension decreases voltage slightly. The decrease in electrode extension reduces the resistance in the wire and results in an increase in the current. In either case, the arc length and voltage remain constant with changes in the electrode extension. See Figure 19-2.

GMAW equipment includes a welding gun cable and gun assembly, a wire feeder, shielding gas, and a workpiece lead with a workpiece connection. In addition to a DC/CV welding power source, GMAW equipment includes a welding gun cable and gun assembly, a wire feeder, shielding gas, and a workpiece lead with a workpiece connection. See Figure 19-3. Additional equipment may be added to automate the system.

Welding gun components include a handle with a conductor tube and trigger, a contact tip, a gas nozzle, a gas diffuser, and an insulator. Welding gun components include a handle with a conductor tube and trigger, a contact tip, a gas nozzle, a gas diffuser, and an insulator. See Figure 19-4. The handle and conductor tube allow easy positioning of the gun by the operator.

Welding guns are available with curved or straight conductor tubes. Welding guns are available with curved or straight conductor tubes. Curved conductor tubes are typically used for semiautomatic applications. Straight conductor tubes are used for mechanized and automatic applications. See Figure 19-5.

The wire feeder may be portable, mounted on the welding machine, or mounted elsewhere to facilitate welding in a large area. A wire feeder automatically advances the welding wire from the wire spool, through the welding gun cable liner and welding gun, to the arc. The wire feeder must be selected to match the power source used for the GMAW application. Constant-speed wire feeders are typically used with CV welding machines. The wire feeder may be portable, mounted on the welding machine, or mounted elsewhere to facilitate welding in a large area. See Figure 19-6. Wire feeders are designed for use with a wide range of solid and metal-cored welding wire from 0.023² to 1/16². The wire feed speed control on the wire feeder can be adjusted to vary the wire feed speed.

Drive roll grooves can be U-shaped, V-shaped, or knurled depending on the type of welding wire. Drive rolls for GMAW are grooved, and the tension on the drive rolls can be adjusted to grip the welding wire without slipping. Grooves can be U-shaped, V-shaped, or knurled depending on the type of welding wire. U-shaped grooves are used with soft wires, V-shaped grooves are used with steel wires, and knurled grooves are used with metal-cored and flux-cored wires. See Figure 19-7.

The wire outlet guide must be aligned closely with the groove in the drive rolls, without touching it. The drive rolls and the welding gun liner must be properly sized to match the diameter of the welding wire. The wire outlet guide must be aligned closely with the groove in the drive rolls without touching it. See Figure 19-8. Misalignment of the liner and the drive rolls can impede the welding wire, causing problems with burnback and bird nesting.

Inert gases do not react readily with other materials, making them useful as shielding gases for arc welding. Atmospheric gases can be excluded by using an inert gas for shielding. Inert gases do not react readily with other materials, making them useful as shielding gases for arc welding. Argon and helium are inert gases that are commonly used for shielding. See Figure 19-9.

Different shielding gases produce different weld bead contour and penetration characteristics. Although it is not inert, carbon dioxide (CO2) can also be used for shielding the weld area if compensation is made for its oxidizing tendencies. CO2, argon, or helium can be used alone or mixed for different applications. All shielding gases should be welding grade. Welding-grade shielding gases are over 99% pure. They provide the best protection and produce the best results. Different shielding gases produce different weld bead contour and penetration characteristics. See Figure 19-10.

The welding area should be protected with windbreaks to prevent gas drifts. Gas drift may occur with high travel speeds or in drafty conditions around the weld area. Gas drift commonly results in inadequate gas shielding. The welding area should be protected with windbreaks to eliminate breezes. It may also be necessary to increase the gas flow to provide better coverage. Some welding guns also have adjustable nozzles that can be positioned to improve gas coverage. See Figure 19-11.

The American Welding Society (AWS) classification system for carbon steel electrodes consists of a series of letters and numbers. Welding wire for GMAW should be similar in composition to the base metal. The copper coating on GMAW electrode wire aids in arc starting, improves wire feeding, and prolongs contact tip life. Welding wire for GMAW is classified by a letter and number system. See Figure 19-12. The E identifies the device as an electrode. The R stands for rod and indicates that it can be used as a non-current-carrying filler rod, as in GTAW. The number specifies the tensile strength of the deposited weld in thousands of pounds per square inch, for example, 70 = 70,000 psi. The S indicates a solid wire. The letter C in this position would indicate composite (metal-cored) wire. The digit following the dash indicates the chemical composition of the welding wire. Using this system enables a welder to choose the correct steel welding wire base on AWS specifications.

The American Welding Society (AWS) has developed similar classification systems for metals other than carbon steel. The American Welding Society (AWS) has developed similar classification systems for metals other than carbon steel. See Figure 19-13.

Welding position and the type of metal to be welded are factors that must be considered when selecting welding wire. Basic welding wire diameters include 0.023², 0.030², 0.035², 0.045², 0.052², 1/16², and 1/8². Generally, welding wire of 0.023², 0.030², or 0.035² is best for welding thin metal, although it can be used to weld low- and medium-carbon steel and medium-thickness, high-strength/low-alloy (HSLA) steel. Medium-thickness metal normally requires 0.045² or 1/16² diameter welding wire. See Figure 19-14.