Introduction to welding processes

1

Introduction to welding

2

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


3

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


4

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


5


Basic Requirements of Welding Process

Source of Heat

Chemical Reaction

Electrical - Arc, Resistance, Induction

Vacuum

Mechanical

Protection from Atmosphere

Gas Shielding

Flux

Mechanical Expulsion

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Fusion welding heat sources

Power beams

Laser

Electron beam

Spot, seam and

projection welding

Electroslag

Electric arcChemical reactionElectric resistance

Oxyfuel gas

welding

Thermit welding

MMAW

GMAW

GTAW

FCAW

SAW


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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

+

-Cathode

drop zone

Anode

drop zone

Peak

temperatures

18,000 K


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Arc energyQ = arc energy in kJ/mm

E = current in amps

I = arc voltage

V = 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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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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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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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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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)



AWS A5.1 classification

E XXXX - H

Useable positions1=all positions

2=flat + horizontal

4=vertical down

Tensile Strength

in KPSI

Flux type 20 = Acidic (iron oxide)

10, 11 = Cellulosic

12, 13 = Rutile

24 = Rutile + iron powder

27 = Acidic + iron powder

16 = basic

18, 28 = basic + iron powder

Hydrogen level (HmR)H = 5 ml / 100g of WM

R = low moisture pick-up


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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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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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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 arcSpray

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

Technology

Transcript of Introduction to welding processes