Class_5_Baics of Arc Welding

8/21/2019 Class_5_Baics of Arc Welding

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Manufacturing processes

Joining Process : Welding

Brazing

Soldering

Casting

Metal Forming


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Welding?

Weldingis the process of joining two metal pieces as

a result of significant diffusion of the atoms of the

welded pieces into the joint (weld) region.

Welding is carried out by heating the joined pieces to

melting point and fusing them together (with or

without filler material) or by applying pressure to the

pieces in cold or heated state
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Advantages of welding

Strong and tight joining;

Cost effectiveness;

Simplicity of welded structures design;

Welding processes may be mechanized and

automated


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Disadvantages of welding

Internal stresses, distortions and changes of micro-

structurein the weld region;

Harmful effects: light, ultra violate radiation, fumes,

high temperature.
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Applications of welding

Buildings and bridges structures;

Automotive, ship and aircraft constructions;

Pipe lines;

Tanks and vessels;

Railroads;

Machinery elements


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ARC

Arc is an electric discharge between twoelectrodes which takes place through anelectrically conducting hot ionized gas

known as plasma. It is a high current low voltage electric

discharge operating in range of 10 to 2000amperes and at 10 to 50 volts.

Bell shaped


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Arc division

Divided into 5 parts.

Cathode spot

Cathode drop zone

Arc column

Anode drop zone

Anode spot


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Arc striking

Touch start

Scratch start

Imposing High frequency unit


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Physics of ARC

Thermionic emission of electrons

Liberation of electrons from heated electrodes.

Kinetic energy of free electrons increases to a

point where they can escape from surface. -ve ions (electrons emitted from cathode)

+ve ions (Emitted from anode)


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Physics of ARC

Electrons are attracted towards, anode andpositive ions towards cathode.

During their journey, they interact with

each other and gas molecules. They further dissociate into ve and +ve

ions.

This creates an avalanche motion. Thus plasma channel is formed.

66% heat at anode.


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Basics of Arc Welding

Electrode:Consumable? Or Non Consumable

Shielding: Gas (Inert or Active or Mixture)? FluxPolarity :Straight ? Or Reverse?

Power source characteristics : CC? or CV


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Electrode

Electrode:

Consumable : Melt during welding

Non Consumable: Does not melt during welding


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Shielding

Shielding:Gas (Inert or Active or Mixture)? Flux

Molten metal in the weld pool is chemically active

and it reacts with the surrounding atmosphere.As a result weld may be contaminated by oxideand nitride inclusions deteriorating its mechanicalproperties.

Neutral shielding gases(argon, helium) and/orshielding fluxes are used for protection of theweld pool from atmospheric contamination.Shields are supplied to the weld zone in form of aflux coating of the electrode or in other forms.


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Polarity

Polarity :Straight ? Or Reverse?

It should be noted that this pertains to DC welding only. AC

welding switches polarity 60 times per second and can be

thought of as both positive and negative

DCEN = DCSP=DC straight polarity= heat to work piece

DCEP = DCRP=DC reverse polarity = heat to electrode


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Power source characteristics

Power source characteristics :CC? or CV


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Volts

Amps

OCV

Constant Current/Amperage Volt Amp Characteristic

Large change in voltage

=

Smaller change in amperage

Welding Voltage

Large

arc gap

Small

arc gap


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Volts

Amps

OCV

Constant Voltage Volt/Amp Characteristic

Small change in voltage

=

large change in amperageChanges in

arc length

The self

adjusting arc.


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Arc welding

is the weldingprocess, in which heat is generated by an electric arc struck

between an electrode and the work pieceAny arc welding method is based on an electric circuit consisting of the following

parts:

Power supply (AC or DC);

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

power supply.

Electric arc between the electrode and work piece closes the electric

circuit. The arc temperature may reach 10000F (5500C), which issufficient for fusion the work piece edges and joining them.

When a long join is required the arc is moved along the joint line.The front edge of the weld pool melts the welded surfaces when therear edge of the weld pool solidifiesforming the joint.
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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 arcbetween itself and the parent

work piece. The molten electrode metal fills the weld gap and

joins the work pieces

The electrodes are coated with a shielding fluxof a suitable

composition. The flux melts together with the electrode

metallic core, forming a gas and a slag, shielding the 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.
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Advantages of Shielded Metal Arc Welding

(SMAW):

Simple, portable and inexpensiveequipment;

Wide variety of metals, weldingpositions and electrodes areapplicable;

Suitable for outdoor applications.

Disadvantages of Shielded Metal Arc

Welding (SMAW):

The process is discontinuous due tolimited length of the electrodes;

Weld may contain slag inclusions; Fumes make difficult the process

control.


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Submerged Arc Welding (SAW )

Submerged Arc Weldingutilizes a bare consumable metallicelectrode producing an arcbetween itself and the work piecewithin a granular shielding fluxapplied around the weld.

The arc heats and melts both the work pieces edges and theelectrode wire. The molten electrode material is supplied to thesurfaces of the welded pieces, fills the weld pool and joins thework pieces.

Since the electrode is submerged into the flux, the arc isinvisible. The flux is partially melts and forms a slagprotecting the weld pool from oxidation and other atmosphericcontaminations
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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;

Limited applications of the process- mostly for welding horizontally

located plates.
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SAW at Anup Engineering


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Gas Metal Arc Welding (GMAW)

Gas Metal Arc Weldingis the arc welding process, inwhich the weld is shieldedby an external gas (argon,helium, CO2, argon + oxygen or other gas mixtures).

Consumable electrode wire, having chemical composition

similar to that of the parent material, is continuously fedfrom a spool to the arczone. The arc heats and melts boththe work pieces edges and the electrode wire. The fusedelectrode material is supplied to the surfaces of the workpieces, fills the weld pool and forms joint.

Due to automatic feeding of the filler wire (electrode) the

process is referred to as a semi-automatic. The operatorcontrols only the torch positioning and speed.
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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 ofeffect of wind, dispersing the shielding gas.


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Tungsten Inert Gas Arc Welding (TIG, GTAW)

Tungsten InertGas Arc Welding (Gas Tungsten ArcWelding)is the welding process, in which heat isgenerated by an electric arcstruck between a tungstennon-consumable electrode and the work piece.

The weld pool is shieldedby an inert gas (argon, helium,

nitrogen) protecting the molten metal from atmosphericcontamination.

The heat produced by thearc melts the work pieces edgesand joins them. Filler rodmay be used, if required.

Tungsten Inert Gas Arc Welding produces a high qualityweld of most of metals. Flux is not used in the process.
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Advantages of Tungsten Inert Gas Arc

Welding (TIG, GTAW):

Weld composition is close to that ofthe parent metal;

High quality weld structure

Slag removal is not required (noslag);

Thermal distortions of work piecesare minimal due to concentrationof heat in small zone.

Disadvantages of Tungsten Inert Gas

Arc Welding (TIG, GTAW):

Low welding rate;

Relatively expensive;

Requires high level of operatorsskill.
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Plasma Arc Welding (PAW)

Plasma Arc Weldingis a welding processutilizing heatgenerated by a constricted arcstruck between a tungsten non-consumable electrode and either the work piece (transferredarc process) or water cooled constricting nozzle (non-transferred arc process).

Plasmais a gaseous mixture of positive ions, electrons andneutral gas molecules.
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Advantages of Plasma Arc

Welding (PAW)

Requires less operator skill dueto good tolerance of arc tomisalignments;

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.


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Edge preparations


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Comparison


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THE EFFECT OF WELDING PARAMETERS


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Process/C

haracteris

tics

GMAW GTAW SMAW SAW PAW

ELECTRODE

SHIELDING

POLARITY

POWER

SOURCE


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Process/

Charact

eristics

GMAW GTAW SMAW SAW PAW

ELECTRODE C NC C C NC

SHIELDING G G F F G

POLARITY DCEP DCEN DCEP/DCEN DCEP DCEN/

DCEP(Al)

POWER

SOURCE

CV CC CC(Dual

Control)

CV CC

Class_5_Baics of Arc Welding

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