All Welding

7/27/2019 All Welding

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Classification of welding processes

Welding is a process of joining two metal pieces as a result of significant diffusion ofthe atoms of the welded pieces into the joint (weld) region. Welding is carried out byheating the joined pieces to melting point and fusing them together (with or withoutfiller material) or by applying pressure to the pieces in cold or heated state.

Advantages of welding:

Strong and tight joining;

Cost effectiveness;

Simplicity of welded structures design;

Welding processes may be mechanized and automated.

Disadvantages of welding:

Internal stresses, distortions and changes of micro-structure in the weld region;

Harmful effects: light, ultra violate radiation, fumes, high temperature.

Applications of welding:

Buildings and bridges structures;

Automotive, ship and aircraft constructions;

Pipe lines;

Tanks and vessels;

Rairoads;

Machinery elements.

Welding processes Arc welding

Carbon Arc Welding;

Shielded Metal Arc Welding (SMAW);

Submerged Arc Welding (SAW);

Metal Inert Gas Welding (MIG, GMAW);

Tungsten Inert Gas Arc Welding (TIG, GTAW);

Electroslag Welding (ESW);

Plasma Arc Welding (PAW);

Resistance Welding (RW);

Spot Welding (RSW);

Flash Welding (FW);

Resistance Butt Welding (UW) ;

Seam Welding (RSEW);

Gas Welding (GW);

Oxyacetylene Welding (OAW);

Oxyhydrogen Welding (OHW);

Pressure Gas Welding (PGW);

Solid State Welding (SSW);

Forge Welding (FOW);

Cold Welding (CW);

Friction Welding (FRW);

Explosive Welding (EXW);

Diffusion Welding (DFW);

Ultrasonic Welding (USW);

Thermit Welding (TW); Electron Beam Welding (EBW);

Laser Welding (LW).

Principles of arc welding

Arc welding is a welding process, in which heat is generated by an electric arcstruck between an electrode and the work piece.Electric arc is luminous electrical discharge between two electrodes through ionizedgas.

Any arc welding method is based on an electric circuit consisting of the followingparts:

Power supply (AC or DC);

Welding electrode;

Work piece;

Welding leads (electric cables) connecting the electrode and work piece to the

power supply.


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Work piece is not a part of welding electric circuit in Twin Carbon Electrode ArcWelding, therefore the welding torch may be moved from one work piece to otherwithout extinguishing the arc.

Shielded Metal Arc Welding (SMAW)

Shielded metal arc welding (Stick welding, Manual metal arc welding) uses a

metallic consumable electrode of a proper composition for generating arc betweenitself and the parent work piece. The molten electrode metal fills the weld gap and

joins the work pieces.

This is the most popular welding process capable to produce a great variety ofwelds.

The electrodes are coated with a shielding flux of a suitable composition. The fluxmelts together with the electrode metallic core, forming a gas and a slag, shieldingthe arc and the weld pool. The flux cleans the metal surface, suppliessome alloying elements to the weld, protects the molten metal from oxidation and

stabilizes the arc.The slag is removed after Solidification.

Advantages of Shielded Metal Arc Welding (SMAW):

Simple, portable and inexpensive equipment;

Wide variety of metals, welding positions and electrodes are applicable;

Suitable for outdoor applications.Disadvantages of Shielded Metal Arc Welding (SMAW):

The process is discontinuous due to limited length of the electrodes;

Weld may contain slag inclusions;

Fumes make difficult the process control.

Submerged Arc Welding (SAW)

Submerged Arc Welding is a welding process, which utilizes a bare consumablemetallic electrode producing an arc between itself and the work piece within agranular shielding flux applied around the weld.

The arc heats and melts both the work pieces edges and the electrode wire. Themolten electrode material is supplied to the surfaces of the welded pieces, fills theweld pool and joins the work pieces.

Since the electrode is submerged into the flux, the arc is invisible. The flux is partiallymelts and forms a slag protecting the weld pool from oxidation and otheratmospheric contaminations.

Advantages of Submerged Arc Welding (SAW):

Very high welding rate;

The process is suitable for automation; High quality weld structure.

Disadvantages of Submerged Arc Welding (SAW):

Weld may contain slag inclusions;


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Limited applications of the process - mostly for welding horizontally located

plates.

Metal Inert Gas Welding (MIG, GMAW)

Metal Inert Gas Welding (Gas Metal Arc Welding) is a arc welding process, in

which the weld is shielded by an external gas (Argon, helium, CO2, argon+ Oxygen or other gas mixtures).

Consumable electrode wire, having chemical composition similar to that of the parentmaterial, is continuously fed from a spool to the arc zone. The arc heats and meltsboth the work pieces edges and the electrode wire. The fused electrode material issupplied to the surfaces of the work pieces, fills the weld pool and forms joint.

Due to automatic feeding of the filling wire (electrode) the process is referred to as asemi-automatic. The operator controls only the torch positioning and speed.

Advantages of Metal Inert Gas Welding (MIG, GMAW):

Continuous weld may be produced (no interruptions);

High level of operators skill is not required; Slag removal is not required (no slag);

Disadvantages of Metal Inert Gas Welding (MIG, GMAW):

Expensive and non-portable equipment is required;

Outdoor application are limited because of effect of wind, dispersing the shielding

gas.

Tungsten Inert Gas Arc Welding (TIG, GTAW)

Tungsten Inert Gas Arc Welding (Gas Tungsten Arc Welding) is a weldingprocess, in which heat is generated by an electric arcstruck between a tungsten non-consumable electrode and the work piece.

The weld pool is shielded by an inert gas (Argon, helium, Nitrogen) protecting themolten metal from atmospheric contamination.The heat produced by the arc melts the work pieces edges and joins them. Fillerrod may be used, if required.

Tungsten Inert Gas Arc Welding produces a high quality weld of most of metals. Flux

is not used in the process.

Advantages of Tungsten Inert Gas Arc Welding (TIG, GTAW):

Weld composition is close to that of the parent metal;

High quality weld structure

Slag removal is not required (no slag);


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High welding rate;

High penetrating capability (keyhole effect);

Disadvantages of Plasma Arc Welding (PAW):

Expensive equipment;

High distortions and wide welds as a result of high heat input.

Resistance Welding (RW)

Resistance Welding is a welding process, in which work pieces are welded due to acombination of a pressure applied to them and a localized heat generated by a highelectric current flowing through the contact area of the weld.

Heat produced by the current is sufficient for local melting of the work piece at thecontact point and formation of small weld pool (nugget). The molten metal isthen solidifies under a pressure and joins the pieces. Time of the process and values

of the pressure and flowing current, required for formation of reliable joint, aredetermined by dimensions of the electrodes and the work piece metal type.

AC electric current (up to 100 000 A) is supplied through copper electrodesconnected to the secondary coil of a welding transformer.

The following metals may be welded by Resistance Welding:

Low carbon steels - the widest application of Resistance Welding

Aluminum alloys

Medium carbon steels, high carbon steels and Alloy steels (may be welded, but

the weld is brittle)

Advantages of Resistance Welding:

High welding rates;

Low fumes;

Cost effectiveness;

Easy automation;

No filler materials are required;

Low distortions.Disadvantages of Resistance Welding:

High equipment cost;

Low strength of discontinuous welds;

Thickness of welded sheets is limited - up to 1/4 (6 mm);

Resistance Welding (RW) is used for joining vehicle body parts, fuel tanks, domesticradiators, pipes of gas oil and water pipelines, wire ends, turbine blades, railwaytracks.

The most popular methods of Resistance Welding are:

Spot Welding (RSW)

Flash Welding (FW)

Resistance Butt Welding (UW)

Seam Welding (RSEW)

Spot Welding (RSW)Spot Welding is a Resistance Welding (RW) process, in which two or moreoverlapped metal sheets are joined by spot welds.

The method uses pointed copper electrodes providing passage of electric current.The electrodes also transmitt pressure required for formation of strong weld.Diameter of the weld spot is in the range 1/8 - 1/2 (3 - 12 mm).

Spot welding is widely used in automotive industry for joining vehicle body parts.


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Flash Welding (FW)Flash Welding is a Resistance Welding (RW) process, in which ends of rods (tubes,sheets) are heated and fused by an arc struck between them andthen forged (brought into a contact under a pressure) producing a weld.

The welded parts are held in electrode clamps, one of which is stationary and the

second is movable.

Flash Welding method permitts fast (about 1 min.) joining of large and complexparts.Welded part are often annealed for improvement of Toughnesstoughness of theweld.

Steels, Aluminum alloys, Copper alloys, Magnesium alloys, Copper alloys and Nickelalloys may be welded by Flash Welding.

Thick pipes, ends of band saws, frames, aircraft landing gears are produced byFlash Welding.

Resistance Butt Welding (UW)Resistance Butt Welding is a Resistance Welding (RW) process, in which ends ofwires or rods are held under a pressure and heated by an electric current passingthrough the contact area and producing a weld.

The process is similar to Flash Welding, however in Butt Welding pressure andelectric current are applied simultaneously in contrast to Flash Welding where

electric current is followed by forging pressure application.

Butt welding is used for welding small parts. The process is highly productive andclean. In contrast to Flash Welding, Butt Welding provides joining with no loss of thewelded materials.


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A great Information Site about Plasma Welding !

Plasma Welding, Arc Plasma Welding

Plasma arc welding (PAW) is a process in which coalescence, or the joining ofmetals, is produced by heating with a constricted arc between an electrode and theworkpiece (transfer arc) or the electrode and the constricting nozzle (nontransfer

arc).

Shielding is obtained from the hot ionized gas issuing from the orifice, which may besupplemented by an auxiliary source of shielding gas.

Shielding gas may be an inert gas or a mixture of gases. Pressure may or may notbe used, and filler metal may or may not be supplied. The PAW process is shown infigure 10-35.


b. Plasma Welding, Arc Plasma Welding Equipment.

(1) Plasma Welding, Arc Plasma Welding Power source.

A constant current drooping characteristic power source supplying the dcwelding current is recommended; however, ac/dc type power source can beused.

It should have an open circuit voltage of 80 volts and have a duty cycle of 60percent.

It is desirable for the power source to have a built-in contactor and provisionsfor remote control current adjustment. For welding very thin metals, it shouldhave a minimum amperage of 2 amps.

A maximum of 300 is adequate for most plasma welding applications.

(2) Plasma Welding, Arc Plasma Welding Welding torch.

The welding torch for plasma arc welding is similar in appearance to a gastungsten arc torch, but more complex.

(a) All plasma torches are water cooled, even the lowest-current range torch.

This is because the arc is contained inside a chamber in the torch where itgenerates considerable heat.

If water flow is interrupted briefly, the nozzle may melt. A cross section of aplasma arc torch head is shown by figure 10-36.

During the nontransferred period, the arc will be struck between the nozzle ortip with the orifice and the tungsten electrode.

Manual plasma arc torches are made in various sizes starting with 100 ampsthrough 300 amperes. Automatic torches for machine operation are alsoavailable.



(b) The torch utilizes the 2 percent thoriated tungsten electrode similar to thatused for gas tungsten welding. Since the tungsten electrode is located insidethe torch, it is almost impossible to contaminate it with base metal.


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(2) The process variables for plasma arc welding are shown by figure 10-41.Most of the variables shown for plasma arc are similar to the other arc weldingprocesses. There are two exceptions: the plasma gas flow and the orificediameter in the nozzle. The major variables exert considerable control in theprocess. The minor variables are generally fixed at optimum conditions for thegiven application. All variables should appear in the welding procedure.Variables such as the angle and setback of the electrode and electrode typeare considered fixed for the application. The plasma arc process does responddifferently to these variables than does the gas tungsten arc process. Thestandoff, or torch-to-work distance, is less sensitive with plasma but the torchangle when welding parts of unequal thicknesses is more important than withgas tungsten arc.

g. Variations of the Process.

(1) The welding current may be pulsed to gain the same advantages pulsingprovides for gas tungsten arc welding. A high current pulse is used formaximum penetration but is not on full time to allow for metal solidification.This gives a more easily controlled puddle for out-of-position work. Pulsingcan be accomplished by the same apparatus as is used for gas tungsten arcwelding.

(2) Programmed welding can also be employed for plasma arc welding in thesame manner as it is used for gas tungsten arc welding. The same powersource with programming abilities is used and offers advantages for certaintypes of work. The complexity of the programming depends on the needs ofthe specific application. In addition to programming the welding current, it isoften necessary to program the plasma gas flow. This is particularly importantwhen closing a keyhole which is required to make the root pass of a weldjoining two pieces of pipe.

(3) The method of feeding the filler wire with plasma is essentially the same as

for gas tungsten arc welding. The "hot wire" concept can be used. This meansthat low-voltage current is applied to the filler wire to preheat it prior to goinginto the weld puddle.

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