ARC WELDING PROCESSES
SHIELDED METAL ARC WELDING
Shielded metal arc welding is a manual arc welding process in which the heat for welding is generated by an arc, established between a flux-covered consumable electrode and the work-piece. The electrode tip, weld puddle, arc, and adjacent areas of the work-piece are protected from the atmospheric contamination by a gaseous shield obtained from combustion and decomposition of the flux covering. Additional shielding is provided for the molten metal in the weld puddle by a covering of molten flux (slag). Filler metal is supplied by the core of the consumable electrode and, with certain electrodes, from metal powder mixed with the electrode covering.
Shielded metal arc welding is the most widely used welding process for joining metal parts, mainly because of its versatility. Also equipment is less complex, more portable and less costly than other arc welding processes.
1. Shielded metal arc welding can be done indoors or outdoors.
2. Joints in virtually any position that can be reached with an electrode (for example, joints directly overhead and vertical joints), locations can be welded.
3. Restricted areas, which are inaccessible locations for most welding processes can be welded.
4. Joints in almost any location can be welded, because the power-supply leads can be extended for relatively long distances.
5. Welding equipments are light and portable.
6. Shielded metal arc welding electrode materials are available for matching the properties of most base metals. Thus, the properties of a joint can match those of the metals joined.
1. Deposition rate and efficiency of shielded metal arc welding process are lower than the most of other arc welding processes. Electrodes have fixed lengths and therefore welding must be stopped after each electrode is consumed.
2. Deslagging is required after each pass, to remove the slag covering that forms on the weld. In gas metal arc welding, multiple passes can be made without stopping for slag removal, since no flux is used.
GAS METAL ARC WELDING
Gas metal arc welding (often called as MIG/MAG welding) is an arc welding process in which the heat for welding is generated by an arc between a consumable electrode and the work-piece. The electrode, a bare solid wire that is continuously fed to the weld area, becomes the filler metal as it is consumed. The electrode, weld puddle, arc and adjacent areas of the base metal are protected from atmospheric contamination by a gaseous shield provided by a stream of gas, or mixture of gases, fed through the electrode holder (torch). The gas shield must provide full protection, because even a small amount of entrapped air can contaminate in the weld deposit.
1. Gas metal arc welding process is a greater speed welding process with respect to shielded metal arc welding. Because of ;
* Continuous feed of filler metal, so that the welder donít need to stop to replace the used-up electrode as required in shielded metal arc welding.
** Absence of slag which must be removed after each pass when welding with coated electrodes, and also this provides better weld quality because of less risk of slag entrapment to the weld.
*** Smaller diameter electrode wire than for shielded metal arc welding for a given welding current. Thus, current density is higher and weld-metal deposition rate is greater.
2. Gas metal arc welding process results in weld metal with a low hydrogen content, which can be important, especially in welding hardenable steels.
3. The potential for deep penetration with gas metal arc welding process can sometimes allow using of smaller fillet welds, and producing more consistent root penetration, than with shielded metal arc welding.
4. Gas metal arc welding is also better adapted than shielded metal arc welding for joining of thin sheets, although the gas tungsten arc process is often used for welding thin sheets when no filler metal is required.
5. Applicable in semiautomatic and fully automated welding systems.
1. Equipment for gas metal arc welding is more complex, and consequently is more costly and less portable than shielded metal arc welding equipments.
2. In gas metal arc welding torch must be close to the work-piece and consequently gas metal arc welding is less adaptable than the shielded metal arc process for welding in difficult to reach areas.
3. In hardanable steels, gas metal arc welded joints can be more susceptible to weld metal cracking, because there is no slag cover to reduce the rate of cooling.
4. Gas metal arc welding requires positive protection from strong drafts, which blow the stream of shielding gas away from the weld; for this reason, the gas metal arc welding process may be less practical than the shielded metal arc welding outdoors.
FLUX CORED ARC WELDING
Flux cored arc welding is a process in which the heat for welding is produced by an arc between a tubular consumable electrode wire and the work-piece, with shielding provided by gas evolved during combustion and decomposition of a flux contained within the tubular electrode wire, or by the flux gas plus an auxiliary shielding gas. The method that uses an auxiliary gas shield is similar to gas metal arc welding, which employs a solid consumable electrode and depends on externally applied gas shield for protecting the arc and molten metal. The self shielding method is more closely related to shielded metal arc welding, which also depends on the combustion and decomposition of a solid flux to provide the gaseous shield. In shielded metal arc welding, the flux is outside of the electrode, which limits the form of the electrode to a straight length (a stick electrode). The flux is inside a tubular electrode, which can be coiled and supplied to the arc as a continuous wire.
The method is applicable to semiautomatic work (manually manipulated electrode holder) and to the various machine and automatic welding procedures.
A disadvantage of flux cored arc welding is that the deposit is covered with solid slag, much the same as, although thinner than, the deposit made by shielded metal arc welding. But with certain flux cored electrodes, slag removal is not required or no slag is formed during welding.
GAS TUNGSTEN ARC WELDING
Gas tungsten arc welding (often called TIG welding) is an arc welding process in which the heat is produced between a non consumable electrode (tungsten electrode) and the work-piece. The electrode, the weld puddle, the arc and adjacent heated areas of the work-piece are protected from atmospheric contamination by a gaseous shield. This shield is provided by stream of gas (usually an inert gas), or mixture of gases. The gas shield must provide full protection; even a small amount of entrained air can contaminate the weld metal.
1. Gas tungsten arc welding process is adaptable to both manual and automatic operation, and can be used to produce continuous welds, intermittent welds and spot welds.
2. Because the electrode is non consumable, a weld can be made by fusion of the base metal with or without the addition of filler metal.
3. All position welding process and is especially well adapted to the welding of thin metals.
4. Provides good root pass penetration without weld porosity.
5. Heat input is concentrated in the weld area and decreases deformation in the work-piece.
6. Provides smooth and uniform welds, no need for cleaning of the weld.
1. Weld metal deposition rate of gas tungsten arc welding process is lower than other arc welding processes.
2. Gas tungsten arc welding is not an economical process for thick base metals.
SUBMERGED ARC WELDING
Submerged arc welding is an arc welding process in which the heat for welding is supplied by an arc (or arcs) developed between a consumable electrode (or electrodes) and work-piece. The arc is shielded by a layer of granular flux, which gets molten (slag), and blankets the molten weld and base metal near the joint and protects the molten weld metal from atmospheric contamination. In submerged arc welding, the electric current flows through the arc and weld puddle, which consists of molten flux and molten weld metal. The heat of the arc melts the electrode, flux, and some base metal, forming a weld puddle that fills the joint. In addition, the flux cover, acting as a protective shield, may supply deoxidizers and scavengers that react chemically with the weld metal. Fluxes for submerged arc welding of alloy steels may also contain alloying ingredients that modify the composition of the weld metal.
Submerged arc welding is an automatic operation. In some applications of automatic submerged arc welding, two or more electrodes are fed simultaneously to the same joint. The electrodes can be side-by-side and fed into the same weld puddle (called as twin arc), or they can be spaced just far enough apart to permit two weld puddles to solidify independently (called as tandem arc), producing high welding speed and high deposition rate.
1. The process can be used at high welding speeds and deposit i on rates to weld cylindrical or flat plate, or pipe of any size or thickness, can also be used for hardfacing.
2. Produces sound welds with good mechanical properties.
3. No weld spatter and arc rays during welding, therefore less protection req uired for welder.
4. Requires less groove angle on the work piece with respect to other processes
5. The submerged arc process can be used indoor or outdoors
1. Flux is subjected to contamination that may cause weld porosity.
2. To obtain welds of good quality, the base metal must be homogeneous, and essentially free of scale, rust, oil and other contaminants.
3. Slag must be removed from the weld bead, and this is sometimes a difficult operation. In multi pass welding; slag must be removed after each pass, to avoid entrapment in the weld metal.
4. The process usually is not suitable for metals thinner than 5 mm, because burn-through can occur.
5. Except for special applications, welding is largely restricted to flat positions for groove welds, and to the flat and horizontal positions for fillet welds, to avoid runoff of flux. The process is not adaptable to all metals and alloys.