Gas metal arc welding

There are four primary methods of metal transfer in GMAW, called globular, short-circuiting, spray, and pulsed-spray, each of which has distinct properties and corresponding advantages and limitations.

[citation needed] The spray-arc transfer variation was developed in the early 1960s, when experimenters added small amounts of oxygen to inert gases.

The second most common type of electrode holder is semiautomatic water-cooled, where the only difference is that water takes the place of air.

To achieve a similar effect, sometimes a constant current power source is used in combination with an arc voltage-controlled wire feed unit.

This grants the operator additional control over the heat input into the weld, but requires significant skill to perform successfully.

All commercially available electrodes contain deoxidizing metals such as silicon, manganese, titanium and aluminum in small percentages to help prevent oxygen porosity.

[21] The choice of a shielding gas depends on several factors, most importantly the type of material being welded and the process variation being used.

lts low cost makes it an attractive choice, but because of the reactivity of the arc plasma, spatter is unavoidable and welding thin materials is difficult.

The desirable rate of shielding-gas flow depends primarily on weld geometry, speed, current, the type of gas, and the metal transfer mode.

Perhaps most importantly, the four primary variations of GMAW have differing shielding gas flow requirements—for the small weld pools of the short circuiting and pulsed spray modes, about 10 L/min (20 ft3/h) is generally suitable, whereas for globular transfer, around 15 L/min (30 ft3/h) is preferred.

The spray transfer variation normally requires more shielding-gas flow because of its higher heat input and thus larger weld pool.

As is the case with many other manual skills, experience and practice will lead to a weldor (operator) developing a high level of proficiency.

During training, apprentice weldors are advised to watch the trailing edge of the weld puddle, not the arc, to ascertain they are progressing down the joint at an optimum rate.

[32]  Most guns are designed so that when the grip (handle) is parallel to the work surface, a suitable lead angle will result.

With pure inert gases, e.g., straight argon, the bottom of the torch is often slightly in front of the upper section, while the opposite is true when the welding atmosphere is carbon dioxide.

Excessive stick-out may also cause the shielding gas to not adequately blanket the fusion zone, leading to atmospheric contamination and a porous and unsound weld.

Development of position-welding skill takes experience, but is usually mastered by most welding apprentices before reaching journeyman status.

However, there is an increased tendency for weld drip, leading to the aforementioned cratering and undercutting, avoidable with a proper weaving technique.

As well as possessing good gun-handling skills, the weldor must know how to correctly configure the welder (machine) to suit the characteristics of the weldment, the wire type and shielding gas(es) being used, and in some cases, the orientation of the joint to be welded.

Such configuration involves setting voltage, wire-feed speed and gas-flow rate, as well as using the correct gun nozzle to achieve proper shielding gas dispersal.

Because of its higher thermal conductivity, aluminum welds are especially susceptible to greater cooling rates and thus additional porosity.

To reduce it, the workpiece and electrode should be clean, the welding speed diminished and the current set high enough to provide sufficient heat input and stable metal transfer but low enough that the arc remains steady.

Transparent welding curtains, made of a polyvinyl chloride plastic film, are often used to shield nearby workers and bystanders from exposure to the arc.

The method was originally developed as a cost efficient way to weld steel using GMAW, because this variation uses carbon dioxide, a less expensive shielding gas than argon.

When the droplet finally detaches either by gravity or short circuiting, it falls to the workpiece, leaving an uneven surface and often causing spatter.

This causes a short circuit and extinguishes the arc, but it is quickly reignited after the surface tension of the weld pool pulls the molten metal bead off the electrode tip.

[20][48][49] For thin materials, cold metal transfer (CMT) is used by reducing the current when a short circuit is registered, producing many drops per second.

In comparison with short arc GMAW, this method has a somewhat slower maximum speed (85 mm/s or 200 in/min) and the process also requires that the shielding gas be primarily argon with a low carbon dioxide concentration.

However, the method has gained popularity, since it requires lower heat input and can be used to weld thin workpieces, as well as nonferrous materials.

[57] DCEP, or DC Electrode Positive, makes the welding wire into the positively-charged anode, which is the hotter side of the arc.