Unit 3. Power Supplies

To supply the electrical energy

necessary for arc welding processes, a

number of different power supplies can be

used. The most common classification is

constant current power supplies and

constant voltage power supplies. In arc

welding, the voltage is directly related to

the length of the arc, and the current is

related to the amount of heat input.

Constant current power supplies are most

often used for manual welding processes

such as gas tungsten arc welding and shielded metal arc welding, because

they maintain a relatively constant current even as the voltage varies. This is

important because in manual welding, it can be difficult to hold the electrode

perfectly steady, and as a result, the arc length and thus voltage tend to

fluctuate. Constant voltage power supplies hold the voltage constant and vary

the current, and as a result, are most often used for automated welding

processes such as gas metal arc welding, flux cored arc welding, and

submerged arc welding. In these processes, arc length is kept constant, since

any fluctuation in the distance between the wire and the base material is

quickly rectified by a large change in current. For example, if the wire and

the base material get too close, the current will rapidly increase, which in turn

causes the heat to increase and the tip of the wire to melt, returning it to its

original separation distance.

The type of current used in arc welding also plays an important role in

welding. Consumable electrode processes such as shielded metal arc welding

and gas metal arc welding generally use direct current, but the electrode can

be charged either positively or negatively. In welding, the positively charged

anode will have a greater heat concentration, and as a result, changing the

polarity of the electrode has an impact on weld properties. If the electrode is

positively charged, it will melt more quickly, increasing weld penetration and

welding speed. Alternatively, a negatively charged electrode results in more

shallow welds. Non-consumable electrode processes, such as gas tungsten arc

welding, can use either type of direct current, as well as alternating current.

However, with direct current, because the electrode only creates the arc and

does not provide filler material, a positively charged electrode causes shallow

welds, while a negatively charged electrode makes deeper welds. Alternating

current rapidly moves between these two, resulting in medium-penetration

welds. One disadvantage of AC, the fact that the arc must be re-ignited after

every zero crossing, has been addressed with the invention of special power

units that produce a square wave pattern instead of the normal sine wave,

making rapid zero crossings possible and minimizing the effects of the

problem.

2 Consumable Electrode welding is shielded metal arc welding

(SMAW), which is also known as

manual metal arc welding (MMA) or

stick welding. Electric current is used to

strike an arc between the base material

and consumable electrode rod, which is

made of steel and is covered with a flux that protects the weld area from

oxidation and contamination by producing CO2 gas during the welding

process. The electrode core itself acts as filler material, making a separate

filler unnecessary. The process is very versatile, requiring little operator

training and inexpensive equipment. However, weld times are rather slow,

since the consumable electrodes must be frequently replaced and because

slag, the residue from the flux, must be chipped away after welding.

Furthermore, the process is generally limited to welding ferrous materials,

though special electrodes have made possible the welding of cast iron, nickel,

aluminum, copper, and other metals. The versatility of the method makes it

popular in a number of applications, including repair work and construction.

Gas metal arc welding (GMAW), also known as metal inert gas (MIG)

welding, is a semi-automatic or automatic welding process that uses a

continuous wire feed as an electrode and an inert or semi-inert gas mixture to

protect the weld from contamination. Since the electrode is continuous,

welding speeds are greater for GMAW than for SMAW. However, because

of the additional equipment, the process is less portable and versatile, but still

useful for industrial applications. The process can be applied to a wide

variety of metals, both ferrous and non-ferrous. A related process, flux-cored

arc welding (FCAW), uses similar equipment but uses wire consisting of a

steel electrode surrounding a powder fill material. This cored wire is more

expensive than the standard solid wire and can generate fumes and/or slag,

but it permits higher welding speed and greater metal penetration.

Submerged arc welding (SAW) is a high-productivity welding method

in which the arc is struck beneath a covering layer of flux. This increases arc

quality, since contaminants in the atmosphere are blocked by the flux. The

slag that forms on the weld generally comes off by itself, and combined with

the use of a continuous wire feed, the weld deposition rate is high. Working

conditions are much improved over other arc welding processes, since the

flux hides the arc and no smoke is produced. The process is commonly used

in industry, especially for large products.

3 Non-Consumable Electrode Methods

Gas tungsten arc welding (GTAW),

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