Metal Arc Welding

Metal Arc Welding 

A technique in which metal is welded by heat generated by an electric arc struck between two electrodes or between one electrode and the metal work piece. It is a manual arc welding process that uses a consumable electrode coated in flux. An electric current, either alternating current or direct current, forms electric arc between the electrode and the metals to be joined. As the weld is laid, the flux coating of the electrode disintegrates, providing a shielding gas and a layer of slag, both of which protect the weld from contamination until it cools. | Because of the versatility of the process and the simplicity of its equipment and operation, shielded metal arc welding is the most commonly used welding process. SMAW also is known as manual metal arc welding (MMA or MMAW), flux shielded arc welding, and stick welding.

Types and Processes

Arc Welding -Arc welding is a process utilizing the concentrated heat of an electric

arc to join metal by fusion of the parent metal and the addition of metal to joint usually

provided by a consumable electrode. Either direct or alternating current may be used for

the arc, depending upon the material to be welded and the electrode used.

Electroslag Welding -Electroslag Welding (ESW) deposits the weld metal into the

weld cavity between the two plates to be joined. This space is enclosed by water cooled

copper dams or shoes to prevent molten slag from running off. The weld metal is

produced from a filler wire that forms an initial arc with the workpiece until a sufficient

pool of liquid metal is formed to use the electrical resistance of the molten slag.

This process requires special equipment used primarily for horizontal welds of very large

plates up to 36 inches or more by welding them in one pass as in large machinery and

nuclear reactor vessels.

There are also variations of ESW where shielding is provided by an appropriate gas and a

continuous arc is used to provide weld metal. These are termed Electrogas Welding or

EGW machines.

Fluxed-Core Arc-Welding -Fluxed-Core Arc-Welding (FCAW) uses a tubular

electrode filled with flux that is much less brittle than the coatings on SMAW electrodes

while preserving most of its potential alloying benefits.

The emissive fluxes used shield the weld arc from surrounding air, or shielding gases are

used and nonemissive fluxes are employed. The higher weld-metal deposition rate of

FCAW over GMAW (Gas Metal Arc Welding) has led to its popularity in joining

relatively heavy sections of 1" or thicker.

Another major advantage of FCAW is the ease with which specific weld-metal alloy

chemistries can be developed. The process is also easily automated, especially with the

new robotic systems.

Gas Metal-Arc Welding -Gas Metal-Arc Welding (GMAW), also called Metal Inert

Gas (MIG) welding, shields the weld zone with an external gas such as argon, helium,

carbon dioxide, or gas mixtures. Deoxidizers present in the electrode can completely

prevent oxidation in the weld puddle, making multiple weld layers possible at the joint.

GMAW is a relatively simple, versatile, and economical welding apparatus to use. This is

due to the factor of 2 welding productivity over SMAW processes. In addition, the

temperatures involved in GMAW are relatively low and are therefore suitable for thin

sheet and sections less than ¼ inch.

GMAW may be easily automated, and lends itself readily to robotic methods. It has

virtually replaced SMAW in present-day welding operations in manufacturing plants.

Gas Tungsten-Arc Welding -Gas Tungsten-Arc Welding (GTAW), also known as

Tungsten Inert Gas or TIG welding, uses tungsten electrodes as one pole of the arc to

generate the heat required. The gas is usually argon, helium, or a mixture of the two. A

filler wire provides the molten material if necessary.

The GTAW process is especially suited to thin materials producing welds of excellent

quality and surface finish. Filler wire is usually selected to be similar in composition to

the materials being welded.

Atomic Hydrogen Welding (AHW) is similar and uses an arc between two tungsten or

carbon electrodes in a shielding atmosphere of hydrogen. Therefore, the work piece is not

part of the electrical circuit.

Plasma Arc Welding - Plasma Arc Welding (PAW) uses electrodes and ionized gases

to generate an extremely hot plasma jet aimed at the weld area. The higher energy

concentration is useful for deeper and narrower welds and increased welding speed.

Shielded-Metal Arc Welding - Shielded-Metal Arc Welding (SMAW) is one of the

oldest, simplest, and most versatile arc welding processes. The arc is generated by

touching the tip of a coated electrode to the workpiece and withdrawing it quickly to an

appropriate distance to maintain the arc. The heat generated melts a portion of the

electrode tip, its coating, and the base metal in the immediate area. The weld forms out of

the alloy of these materials as they solidify in the weld area. Slag formed to protect the

weld against forming oxides, nitrides, and inclusions must be removed after each pass to

ensure a good weld.

The SMAW process has the advantage of being relatively simple, only requiring a power

supply, power cables, and electrode holder. It is commonly used in construction,

shipbuilding, and pipeline work, especially in remote locations.

Submerged Arc Welding -Submerged Arc Welding (SAW) shields the weld arc using

a granular flux fed into the weld zone forming a thick layer that completely covers the

molten zone and prevents spatter and sparks. It also acts as a thermal insulator, permitting

deeper heat penetration.

The process is obviously limited to welding in a horizontal position and is widely used

for relatively high speed sheet or plate steel welding in either automatic or semiautomatic

configurations. The flux can be recovered, treated, and reused.

Submerged Arc Welding provides very high welding productivity....4-10 times as much

as the Shielded Metal Arc Welding process.

MIG Welding -MIG (Metal Inert Gas) or as it even is called GMAW (Gas Metal Arc

Welding) uses an aluminium alloy wire as a combined electrode and filler material. The

filler metal is added continuously and welding without filler-material is therefore not

possible. Since all welding parameters are controlled by the welding machine, the process

is also called semi-automatic welding.

The MIG-process uses a direct current power source, with the electrode positive (DC,

EP). By using a positive electrode, the oxide layer is efficiently removed from the

aluminium surface, which is essential for avoiding lack of fusion and oxide inclusions.

The metal is transferred from the filler wire to the weld bead by magnetic forces as small

droplets spray transfer. This gives a deep penetration capability to the process and makes

it possible to weld in all positions. It is important for the quality of the weld that the spray

transfer is obtained.

There are two different MIG-welding processes, conventional MIG and pulsed MIG:

1. Conventional MIG uses a constant voltage DC power source. Since the spray

transfer is limited to a certain range of arc current, the conventional MIG process

has a lower limit of arc current (or heat input). This also limits the application of

conventional MIG to weld material thicknesses above 4 mm. Below 6 mm it is

recommended that backing is used to control the weld bead.

2. Pulsed MIG uses a DC power source with superimposed periodic pulses of high

current. During the low current level the arc is maintained without metal transfer.

During the high current pulses the metal is transferred in the spray mode. In this

way pulsed MIG is possible to operate with lower average current and heat input

compared to conventional MIG. This makes it possible to weld thinner sections

and weld much more easily in difficult welding positions.

TIG Welding -TIG-welding (Tungsten Inert Gas) or GTAW-welding (Gas Tungsten

Arc Welding) uses a permanent non-melting electrode made of tungsten. Filler metal is

added separately, which makes the process very flexible. It is also possible to weld

without filler material.

The most used power source for TIG-welding generates alternating current (AC). Direct

current can be used, but due to high heat generation on the tungsten electrode when DCEP

(electrode positive) welding, that particular polarity is not feasible. In some cases DCEN

(electrode negative) is used, however, this requires special attention before welding,

due to the arc's poor oxide cleaning action.

AC TIG-welding usually uses argon as a shielding gas. The process is a multi purpose

process, which offers the user great flexibility. By changing the diameter of the tungsten

electrode, welding may be performed with a wide range of heat input at different

thicknesses. AC TIG-welding is possible with thicknesses down to about 0,5 mm. For

larger thicknesses, > 5 mm, AC TIG-welding is less economical compared to MIGwelding

due to lower welding speed.

DC TIG-welding with electrode negative is used for welding thicknesses above 4 mm.

The negative electrode gives a poor oxide cleaning compared to AC-TIG and MIG, and

special cleaning of joint surfaces is necessary. The process usually uses helium shielding

gas. This gives a better penetration in thicker sections. DC TIG-welding is applicable for

welding thicknesses in the range 0,3 - 12 mm. More and more popular is also pulsed DC

TIG-welding, which makes it possible to weld uniform welds with deeper penetration at

the same heat input. Pulse frequency is usually in the range 1 - 10 Hz.