arc welding, shielded metal arc welding, metal inert gas welding

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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