The GMAW process offers significant productivity gains over GTAW and is particularly suited to manual and automated welding. Although the level of control is reduced compared to GTAW, WM’s save speed and usability is much higher.
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The types of metal transfer possible with GMAW are (1) short, (2) spherical, (3) spray, and (4) impulse spray. Selecting the WM transmission mode requires information about the weld design/thickness, location of the weld, desired deposition rate, and welder skill level.
Short circuit transfer mode is used in all welding positions, provides good weld puddle control and is considered a low heat welding process. However, since this transmission mode operates in the lower current range, it is more susceptible to incomplete splicing errors. Spherical mode is mainly used for surface welds like cladding. Sputter transfer occurs at the highest current and voltage levels and is therefore characterized as a medium to high heat input and relatively high deposition rate welding process. Spray application is suitable for welding thick flat sections due to good deposition, high productivity and low spatter rate.Although transfer sputtering is less prone to incomplete fusion failures than shorts, its relatively high heat input can induce secondary phase precipitation in the HCS of corrosion-resistant nickel-base alloys and reduce their post-weld corrosion resistance. Pulse spray mode is a variant of spray transmission where the welding power alternates from low to high. While spray transfer is still achieved at higher amperages, the lower average power allows
Pulsed Spray Welding to be used on thinner base metals and in all welding positions. The main advantage is the lower average current, which reduces the total welding heat input and the resulting benefits. A specially designed current source is required to produce a pulsed output.The electrical polarity in the GMAW is the Direct Current Positive Electrode (DCEP). Typically, shielding gas flow rates are between 15 and 20 l/min. For optimal shielding, it is recommended to make the gas tank of the welding gun as large as possible. Pure Ar and mixtures of Ar + He, Ar + He + CO2 and He + Ar + CO2 can be used as protective gasses. Gasses containing CO2 produce a very stable arc, excellent out-of-position welding characteristics and excellent Ni-base welding characteristics with
carbon steel.However, since carbon dioxide is present, the surface of the WM is severely oxidized. This oxidized condition can increase the likelihood of incomplete casting defects. Therefore, it is strongly recommended that multipass welds made with gasses containing CO2 be lightly ground between passes to remove the oxidized surface. When using Ar and He mixtures, the weld surface should be bright and shiny with minimal oxidation. In multi-pass welding, grinding between passes is not mandatory.When 100% AR is used, there may be some oxidation on the weld surface. It is recommended to use a coarse wire brush and/or light sanding (80 grit) between passes. As with the GTAW, backflushing is required to ensure the root side of the weld joint is not severely oxidized. Alternatively, fabricators can weld without backflushing if they grind the root side of the weld to remove oxidation.
