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Electric Arc
andArc Welding
Instructor: Prof. G. L. Datta
Lecture # 3
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What is an Electric Arc?
An electric arc is a discharge of electriccurrent across a gap in a circuit .
It is sustained by an ionized column of gas(plasma) through which the current flows
To initiate the arc in AW, electrode isbrought into contact with work and thenquickly separated from it by a shortdistance.
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+
Cathodedrop zone
Anodedrop zone
Peaktemperatures
18,000 K
Electric discharge between
2 electrodes through a gas10 to 2000 amps at
10 to 500 V arc voltage
Column of ionised gas at
high temperature
Forces stiffen the arc column
Transfer of molten metal
from electrode to workpiece
Can have a cleaning action,
breaking up oxides onworkpiece
The electric arc
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Electrical constituents of an arc
Cathode
Cathode drop zone
Arc column Anode drop zone
Anode
V = Vc + Va + Vcl
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V
PS
+
Vc
Va
Vcl
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Arc Welding (AW)
A fusion welding process in whichcoalescence of the metals is achieved bythe heat from an electric arc between an
electrode and the work. Electric energy from the arc produces
temperatures ~ 5500 C, hot enough tomelt any metal.
Most AW processes add filler metal toincrease volume and strength of weld joint.
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A pool of molten metal is formed near
electrode tip, and as electrode is moved along
joint, molten weld pool solidifies in its wake.
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Basic configuration of an arc welding process.
Arc Welding
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Two Basic Types of AW
Electrodes
Consumableconsumed during weldingprocess
Source of filler metal in arc welding
Nonconsumablenot consumed duringwelding process
Filler metal may be added separately.
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Consumable Electrodes
Forms of consumable electrodes
1. Welding rods (sticks): 25 to 45 cm in length and10 mm or less in diameter and must be changed
frequently.2. Weld wire: can be continuously fed from spoolswith long lengths of wire, avoiding frequentinterruptions.
In both rod and wire forms, electrode isconsumed by arc and added to weld joint asfiller metal.
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Non-consumable Electrodes
Made of tungsten which resists melting
Gradually depleted during welding
(vaporization is principal mechanism) Any filler metal needed must be supplied
separately into weld pool.
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Arc Shielding At high temperatures in AW, metals are
chemically reactive to oxygen, nitrogen, and
hydrogen in air
Mechanical properties of joint can be
seriously degraded by these reactions
To protect operation, arc must be shielded
from surrounding air in AW processes
Arc shielding is accomplished by:
Shielding gases, e.g., argon, helium, CO2
Flux
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Arc Length
Longer arc lengths = increased puddle heat,flatter welds, deeper penetration
Shorter arc lengths = less puddle heat, flatter
welds, less penetrationUse arc length to control puddle size,
penetration, and burn through.
Normal arc length is 1.5 mm 3.1 mm
Use a slightly longer arc length during a start
or restart.
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Arc energy
Q 0.06EI
v
Q = arc energy in kJ/mmE = arc voltage, voltsI = current, ampsv = travel speed in mm/min
Low arc energy Small weld pool size Incomplete fusion High cooling rate
Unwanted phasetransformations Hydrogen cracking
High arc energy Large weld pool size Low cooling rate Increased solidification
cracking risk Low ductility and strength Precipitation of unwanted
phases (corrosion and ductility)
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Classifications of Arc Welding
ArcWelding
SMAW/
MMAW
MIG /
GMAW
MAG
FCAW
SAW
TIG /
GTAW
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Shielded Metal Arc Welding (SMAW)also known as
Manual Metal Arc Welding (MMAW)
Instructor: Prof. G. L. Datta
Lecture # 4
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Covered electrodes
Core wire Solid or tubular
2 mm to 8 mm
diameter, 250 to 450
mm long
Coating
Extruded as paste,
dried to strengthen
Dipped into slurry
and dried (rare)
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Shielded metal arc welding process
The oldest and the most versatile process Uses a covered consumable electrode consisting of a core
wire around which a concentric clay like mixture of silicate
binders and powdered materials such as fluoride,
carbonates, oxides of metal alloys and cellulose isextruded.
This covering acts as a source of arc stabilizer, gases to
displace air, alloying elements, and slag to protect and
insulate the deposited metal.
About 75 to 80 of the total welding work is done with
shielded electrodes.
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Shielded Metal Arc Welding
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Basic Characteristics of SMAW
Manual welding
Control of arc length important for quality
Application-specific electrode to be used
10 to 15 length of weld per electrodeinterrupted welding for long runs
Any type of power source ac, dc, rectified dc
can be used Weld metal chemical composition adjustable
Weld covered with slag
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SMAW Plant
Power source
Power cable
Electrode holder
Welding hood
Chipping hammer
Wire brush
Hand gloves
Apron
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Current Requirements in SMAW Process
Gauge # Dia of core wire,
mm
Current, amps
10 3.1 80 - 100
8 4 100 - 130
6 5 130 - 160
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Advantages of SMAW
a) Simplest of all arc welding processes
b) Equipment portable
c) Finds innumerable applications due to
availability of wide variety of electrodesd) A wide range of metals and alloys can be
welded
e) Welding in all positions
f) Cost may be less than other welding
processes.
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Limitations of SMAW
(a) Low productivity - Low power, low duty cycle(frequent electrode changes), electrode of finitelength, and low speed of welding.
(b) Possibility of slag inclusion.(c) Hydrogen from flux coatings making the weld prone
to hydrogen embrittlement cracking.
(d) Quality dependent on manual skill.
(e) Low depth of penetration.(f) Welding fume.
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Gas Metal Arc Welding (GMAW) /
Metal-arc Inert Gas Welding (MIG)
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Gas Metal Arc Welding (GMAW)
Uses a consumable bare metal wire aselectrode and shielding accomplished byflooding arc with a gas.
Wire is fed continuously and automatically
from a spool through the welding gun Shielding gases include inert gases such as
argon and helium for aluminum welding, andactive gases such as CO2 for steel welding
Bare electrode wire plus shielding gaseseliminate slag on weld bead - no need formanual grinding and cleaning of slag.
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(CONTACT TIP)
GMAW Terminology
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(CONTACT TIP)
GMAWGas Metal Arc Welding Gun Solid or Metal CoredElectrode
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Gas Metal Arc Welding
EquipmentDCEP
(DCRP)
(Reverse Polarity)
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Copper Coated MS MIG Wire
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Lincoln Electric GMAW Equip.
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GMAW Advantages over SMAW
Better arc time because of continuous wire
electrode
Sticks must be periodically changed in SMAW
Better use of electrode filler metal than SMAW
End of stick cannot be used in SMAW
Higher deposition rates
Eliminates problem of slag removal Can be readily automated
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Flux Cored Arc Welding
Lecture # 5
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Flux-Cored Arc Welding
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Main Features of FCAW
Two Variations: Gas Shielded
Self Shielded
Gas Shielded Like GMAW, but cored electrode contains flux
Requires shielding gas
Self Shielded Like GMAW, but cored electrode contains flux
Flux produces its own shielding gas
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Self Shielded FCAW Process
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Self Shielded FCAW
Vaporizing flux produces Carbon Dioxide
Molten slag coats droplets.
Limited ability to shield puddle from N2 inair, nitride formers added to flux.
Nitrides in the weld metal increasehardness and decrease toughness.
Less sensitive to drafts than gas shielded.
Used where SMAW used, eg. outdoors.
Can use long stickout, low current.
Useful for sheet metal or bridging gaps
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Gas Shielded FCAW Process
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Gas Shielded FCAW
Shielding gas N2, O2, H2O away
75%Ar-25%CO2 for spray transfer.
100% CO2 for short arc transfer.
Deoxidants in flux to tie up O2 from CO2.
Higher deposition rate.
Slag removal is an extra step.
Metal cored electrodes contain no flux, just
metal powders. Therefore it is a GMAW
process, not a FCAW process.
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Flux Cored Electrodes
Uniform distribution of flux is critical Hermetic sealing is critical
to prevent moisture pickup that can lead tohydrogen cracking
Flux in core
Affects arc characteristics
Provides for chemical interaction with weld to
1) add alloying elements 2) tie up N2 and O2 Forms a slag (glassy covering) to protect the
weld from the air
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Flux Cored Arc Welding
Advantages
Higher production rate than SMAW &GMAW
Some electrodes can be used in anyposition
Limitations
Oxide and flux inclusions in weld metal
Nitride inclusions in self shielded FCAW
weld metal Smoke and ultra violet radiation
Slag must be removed between passes
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Assignment # 2
1. Perform an internet search for GMAW &FCAW wire electrodes: Metal, Wire
diameter, Spool size and names of
Manufacturers.
2. Perform an internet search for Shielding
gases: Sources of gases.
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Review Questions
Distinguish between a spark and an arc.
Can you express arc-voltage as a function ofarc-length?
Why SMAW process uses a lower current thanMIG process?
Why cant you use high current in TIG
process? Why do you get high deposition rate from
FCAW process?
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Review Questions
What polarity is used with Gas Metal Arc? Which shielding gas does give the greatest
penetration? The most spatter?
Compare the productivity of all theprocesses which use flux.
Compare the productivity of all the
processes which dont use any flux. Why is MIG preferred over SMAW?
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Review Questions
The following parameters are used in an
SMAW process: I = 120 amps, E = 26 volts, v =
100 mm/min. determine (a) arc power and (b)
heat input into the work piece.
What is meant by low arc energy and high
arc energy? Give a comparison.
Compare the characteristics of (a) SMAW andFCAW, and (b) SMAW and MIG processes.
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Thank You
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