Introduction to welding processes r1 1

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Introduction to welding

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Overview of joining methods

Mechanical methods Screwed fasteners, rivets,

Adhesive bonding Brazing and Soldering

Base metal does not fuse. Molten filler drawn into close-fit joints by capillary

action (surface tension forces). Brazing filler melts >450 C, solder <450 C

Welding


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Weld

A joint produced by heat or pressure or both

So there is continuity of material.

Filler (if used) has a melting temperature

close to the base material


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Welding processes

Fusion welding Welding in the liquid state with no pressure Union is by molten metal bridging

Solid phase welding Carried out below the melting point without filler

additions Pressure often used Union is often by plastic flow


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Basic Requirements of Welding Process

Source of Heat Chemical ReactionElectrical - Arc, Resistance, InductionMechanical

Protection from AtmosphereGas ShieldingFluxMechanical ExpulsionVacuum

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Fusion welding heat sources

Power beams

LaserElectron beam

Spot, seam and projection welding

Electroslag

Electric arcChemical reactionElectric resistance

Oxyfuel gas welding

Thermit welding

MMAWGMAWGTAWFCAWSAW


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Solid phase welding

Hot processes Forge welding Friction welding Diffusion bonding

Cold processes Ultrasonic welding Explosive welding


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Some arc welding processes

MMAW - manual metal arc welding SAW - submerged arc welding GTAW - gas tungsten arc welding (TIG) GMAW - gas-metal arc welding (MIG, MAG) FCAW - flux cored arc welding


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The electric arc Electric discharge between 2

electrodes through ionised gas

10 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 on workpiece

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- Cathode drop zone

Anode drop zone

Peak temperatures

18,000 K


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Arc energyQ = arc energy in kJ/mmE = current in ampsI = arc voltageV = travel speed in mm/min

Low arc energy• Small weld pool size• Incomplete fusion• High cooling rate• Unwanted phase transformations• 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)


Q = E x I

V

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• 103 Watts/cm2 melts most metals

• 106 -107 Watts/cm2 vaporizes most metals

• 103 to 106 Watts/cm2 typical for fusion welding

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Manual Metal Arc Welding

MMAW,

SMAW,

Stick electrode welding

Manual welding


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Heat source - arc between metal and a flux coated electrode (1.6- 8 mm diameter)

Current 30-400A (depends on electrode size)

AC or DC operation

Power 1 to 12 kW


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Manual Metal Arc Welding Minimum equipment

Power source (ac or dc, engine driven or mains transformer)

Electrode holder and leads May carry up to 300 amps

Head shield with lens protects face & eyes Chipping hammer to remove slag Welding gloves protect hands from arc

radiation, hot material and electric shock


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Manual Metal Arc Welding Process features

Simple portable equipment Widely practiced skills Applicable to wide range of materials, joints,

positions About 1kg weld deposited per arc-hour Portable and versatile Properties can be excellent Benchmark process


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Manual Metal Arc Welding Covered electrodes

Core wire Solid or tubular 2mm to 8mm diameter,

250 to 450mm long

Coating Extruded as paste, dried

to strengthen Dipped into slurry and

dried (rare) Wound with paper or

chord (obsolete)


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Manual Metal Arc Welding Functions of coating

Slag protects weld pool from oxidation Gas shielding also protects weld pool Surface tension (fluxing) Arc stabilising (ionising) Alloying and deoxidation Some ingredients aid manufacture

(binder and extrusion aids)


Manual Metal Arc Welding AWS A5.1 classification

E XXXX - H

Useable positions1=all positions2=flat + horizontal4=vertical down

Tensile Strengthin KPSI

Flux type 20 = Acidic (iron oxide) 10, 11 = Cellulosic12, 13 = Rutile24 = Rutile + iron powder27 = Acidic + iron powder16 = basic18, 28 = basic + iron powder

Hydrogen level (HmR)H = 5 ml / 100g of WMR = low moisture pick-up


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Manual Metal Arc Welding Applications

Wide range of welded products: light structure & Heavy steel structures Workshop and site High integrity (nuclear reactors, pressure

equipment)

Ideal where access is difficult - construction site, inside vessels, underwater

Joins a wide range of materials


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Manual Metal Arc Welding Limitations

Low productivity Low power Low duty cycle (frequent electrode

changes)

Hydrogen from flux coatings Electrode live all the time

Arc strike, stray current and electric shock risks


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Submerged arc welding

SAW,

Sub-arc


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Submerged arc welding - Features

High productivity 2 to 10 kg/hour Up to 2m/min

Bulky, expensive and heavy equipment

Flat and horizontal positions only

Thicker sections (3mm and above)

Mostly ferrous materials (also Ni alloys)


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Submerged arc welding - Equipment

Power source Welding head and

control box Welding head travel Flux recovery system

(optional) Positioners and

Fixtures


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Submerged arc welding - Consumables

Solid or cored wires Granular fluxes

Agglomerated, fused or sintered Alloying activity

• Contribution to weld metal chemistry from flux Basicity

• Acid fluxes made from manganese oxide, silica, rutile are easy to use

• Basic fluxes (MgO, CaO, CaF2, Al2O3) provide excellent toughness welds


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Submerged arc welding - Applications

Long straight welds in heavier material Vessel longitudinal and circumferential

welds Flange to web joints of I beams

Flat or horizontal position Flux has to be supported

Access has to be good


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Submerged arc welding Process variations

Surfacing and hardfacing Wire and strip electrodes

Semi-automatic Multiple electrodes

2 (and more) electrode wires From one or more power sources

Iron powder additions to groove


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Submerged arc welding – Tandem arc

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Gas shielded arc process

Tungsten Inert Gas welding (TIG)

Gas tungsten arc welding (GTAW)


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Gas Tungsten Arc Welding

Alternative names - GTAW,TIG (Tungsten Inert Gas), Argonarc

Heat source is an electric arc between a non-consumable electrode and the workpiece

Filler metal is not added or is added independently


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Gas Tungsten arc welding

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Gas Tungsten Arc Welding

Heat source - arc between a tungsten tip and the parent metal

30-400A, AC or DC

10-20V

0.3-8kW

Inert gas shielding

Consumable filler rod can be used (1 to 4mm diameter)


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Gas Tungsten Arc Welding - Process features

Excellent control Stable arc at low power (80A at 11V) Independently added filler Ideal for intricate welds eg root runs in pipe or thin sheet Low productivity 0.5kg/h manual

High quality Clean process, no slag Low oxygen and nitrogen weld metal Defect free, excellent profile even for single sided welds


Gas Tungsten Arc Welding - Equipment

Welding power source with constant current characteristic

DC for most metals, AC for Al Arc starting by high frequency (5000V, 0.05A) Sequence timers for arc starting, arc finishing &

gas control

Water- or gas-cooled torch with tungsten electrode

Electrode may contain thoria or zirconia, etc


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Gas Tungsten Arc Welding - Shielding gases

Torch is fed with an inert or reducing gas Pure argon - widespread applications Argon-helium - Higher arc voltage, inert Argon-2% hydrogen - Cu alloys & austenitic steel Torch gas must not contain oxygen or CO2

Backing (or purge) gas Used for all single-sided welds except in carbon steel Argon, nitrogen, formier gas (N2 + H2)

Supplementary shielding Reactive metals: Ti, etc Gas filled chambers or additional gas supply devices


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Gas Tungsten Arc Welding - Filler metals

Autogenous welding (no filler) Filler wire or rod of matching composition

C-Mn & low alloy steel Stainless Steel Al, Mg, Ti Cu & Ni

Consumable inserts - filler preplaced in joint


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Gas Tungsten Arc Welding - Automation


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Gas Tungsten Arc Welding – A TIG

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GMAW and FCAW

Gas metal arc welding

(MIG, MAG, CO2 welding)

Flux cored arc welding


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A continuous solid wire, small diameter

GMAW uses solid wire, no flux FCAW uses flux-filled wire

Fed through the gun to the arc by wire feeder.

The weld pool may be protected from oxidation by shielding gas.

High productivity 3 kg/h or more Direct current (DCEP mostly)


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MIG Welding

Heat source - arc between parent metal and consumable electrode wire (0.6 to 1.6mm diameter)

60-500A, DC only

16-40V

1 to 20kW



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Gas metal arc welding - Equipment

Welding power source Wire feeder mechanism

May be in power source cabinet

Gun with gas supply & trigger switch

Manual (semiautomatic) guns Automatic torches available Can be fitted to robot etc


Gas metal arc welding – Metal transfer

Spray Higher current & voltage, argon-rich gas

Short circuiting (dip) Low current and voltage, CO2

Globular Intermediate current

Pulsed current power sources Adjustable frequency One droplet per current pulse.


48Current

Voltage

No arc (birds-nesting)

Burn-back and unstable arc Spray

Globular

Short circuiting


Gas metal arc welding – Metal transfer

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Gas metal arc welding - Consumables

Solid Wires (GMAW) A wide variety of alloys are available

Flux cored arc welding (FCAW) Gas shielded flux cored wires Self-shielded flux cored wires

• Used outdoors Metal cored wires

• Light flux cover


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Gas metal arc welding – Wire size

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Gas metal arc welding - Gas mixtures

Inert gases (MIG) Argon or helium or mixtures of these Active base metals, Al, Mg, Ti

Active gases (MAG and FCAW) Carbon dioxide Argon plus oxygen and/or carbon dioxide Nitrogen, hydrogen


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Gas metal arc welding - Developments

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Plasma Cutting, Welding & Surfacing

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Neutral Flame

Oxidising Flame

Carburising Flame

Oxy-Acetylene Welding

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Thermit welding

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Laser Welding

• Photons transmit energy and heat

• Energy intensity up to 109 Watts/cm2

• Depth to width of hole up to 50x

• Automatic controllers needed

• 90% efficiency

• Reflectors don’t weld easily

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Laser welding

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Electron Beam Welding

• Electrons strike surface and generate heat

• Best performed in a vacuum

• Workpiece must be a conductor

• Magnetic fields affect beam

• Current to 1/2 A

• Power to 100 kW

• X-rays produced

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Electron Beam Welding

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Size of weld beads in

(a) electron-beam or laser-beam welding

(b) conventional arc welding.

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Solid-State Welding

Heat Pressure Time NO Melting NO Filler Material Intimate Contact

Usually Requires Deformation Works with Dissimilar Metals

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Resistance Welding

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Resistance spot welding Robots

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Flash Butt Welding

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Friction Welding

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Friction Stir Welding

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Friction Stir Welding

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Explosive Welding

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ANY QUESTIONS

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Introduction to welding processes r1 1

Technology

Transcript of Introduction to welding processes r1 1