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
Click to view larger JPEG. 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
Click to view larger JPEG. 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.
Plasma Arc Welding
Click to view larger JPEG. 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
Click to view larger JPEG. 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
Click to view larger JPEG. 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:
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.
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 DC-EP (electrode positive) welding, that particular polarity is not feasible. In some cases DC-EN (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 MIG-welding 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.
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