2. Introduction to welding
Overview of joining methods
Mechanical methods
q Screwed fasteners, rivets,
Adhesive bonding
Brazing and Soldering
q Base metal does not fuse.
q Molten filler drawn into close-fit joints by capillary
action (surface tension forces).
q Brazing filler melts >450 C, solder <450 C
Welding
2
3. Introduction to welding
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
3
4. Introduction to welding
Welding processes
Fusion welding
q Welding in the liquid state with no pressure
q Union is by molten metal bridging
Solid phase welding
q Carried out below the melting point without filler
additions
q Pressure often used
q Union is often by plastic flow
4
5. Introduction to welding
Basic Requirements of Welding Process
Source of Heat
Chemical Reaction
Electrical - Arc, Resistance, Induction
Mechanical
Protection from Atmosphere
Gas Shielding
Flux
Mechanical Expulsion
Vacuum
5
6. Introduction to welding
Fusion welding heat sources
Electric resistance Chemical reaction Electric arc Power beams
Spot, seam and Oxyfuel gas MMAW Laser
projection welding welding GMAW Electron beam
GTAW
FCAW
Electroslag Thermit welding SAW
6
9. Introduction to welding
The electric arc
Electric discharge between 2
electrodes through ionised
gas
Peak - Cathode q 10 to 2000 amps at 10 to
temperatures
drop zone 500 V arc voltage
18,000 K
Column of ionised gas at high
temperature
Forces stiffen the arc column
q Transfer of molten metal
Anode
from electrode to workpiece
drop zone
Can have a cleaning action,
+ breaking up oxides on
workpiece
9
10. Introduction to welding
Arc energy
Q = arc energy in kJ/mm
Q E x E = current in amps
= I V I = arc voltage
V = travel speed in
mm/min
Low arc energy High arc energy
• Small weld pool size • Large weld pool size
• Incomplete fusion • Low cooling rate
• High cooling rate • Increased solidification cracking risk
• Unwanted phase transformations • Low ductility and strength
• Hydrogen cracking • Precipitation of unwanted phases
(corrosion and ductility)
10
11. Introduction to welding
• 103 Watts/cm2 melts most metals
• 106 -107 Watts/cm2 vaporizes most metals
• 103 to 106 Watts/cm2 typical for fusion welding 11
13. Introduction to welding
Manual Metal Arc Welding
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
13
15. Introduction to welding
Manual Metal Arc Welding
Minimum equipment
Power source (ac or dc, engine driven or
mains transformer)
Electrode holder and leads
q 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
15
16. Introduction to welding
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 16
17. Introduction to welding
Manual Metal Arc Welding
Covered electrodes
Core wire
q Solid or tubular
q 2mm to 8mm diameter,
250 to 450mm long
Coating
q Extruded as paste, dried
to strengthen
q Dipped into slurry and
dried (rare)
q Wound with paper or
chord (obsolete)
17
18. Introduction to welding
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) 18
19. Introduction to welding
Manual Metal Arc Welding
AWS A5.1 classification
E XXXX - H
Tensile Strength Hydrogen level
in KPSI (H mR)
H = 5 ml / 100g of WM
R = low moisture pick-
up
Useable positions Flux type
1=all positions 20 = Acidic (iron oxide)
2=flat + horizontal 10, 11 = Cellulosic
4=vertical down 12, 13 = Rutile
24 = Rutile + iron powder
27 = Acidic + iron powder
16 = basic
18, 28 = basic + iron powder
20. Introduction to welding
Manual Metal Arc Welding
Applications
Wide range of welded products:
q light structure & Heavy steel structures
q Workshop and site
q High integrity (nuclear reactors, pressure
equipment)
Ideal where access is difficult -
construction site, inside vessels,
underwater
Joins a wide range of materials
20
21. Introduction to welding
Manual Metal Arc Welding
Limitations
Low productivity
q Low power
q Low duty cycle (frequent electrode
changes)
Hydrogen from flux coatings
Electrode live all the time
q Arc strike, stray current and electric shock
risks
21
25. Introduction to welding
Submerged arc welding - Features
High productivity
q 2 to 10 kg/hour
q 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)
26. Introduction to welding
Submerged arc welding - Equipment
Power source
Welding head and
control box
Welding head travel
Flux recovery system
(optional)
Positioners and
Fixtures
26
27. Introduction to welding
Submerged arc welding - Consumables
Solid or cored wires
Granular fluxes
q Agglomerated, fused or sintered
q Alloying activity
• Contribution to weld metal chemistry from flux
q Basicity
• Acid fluxes made from manganese oxide, silica, rutile are
easy to use
• Basic fluxes (MgO, CaO, CaF2, Al2O3) provide excellent
toughness welds
27
28. Introduction to welding
Submerged arc welding - Applications
Long straight welds in heavier material
q Vessel longitudinal and circumferential
welds
q Flange to web joints of I beams
Flat or horizontal position
q Flux has to be supported
Access has to be good
28
29. Introduction to welding
Submerged arc welding
Process variations
Surfacing and hardfacing
q Wire and strip electrodes
Semi-automatic
Multiple electrodes
q 2 (and more) electrode wires
q From one or more power sources
Iron powder additions to groove
29
31. Introduction to welding
Gas shielded arc process
Tungsten Inert Gas welding (TIG)
Gas tungsten arc welding (GTAW)
31
32. Introduction to welding
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
32
34. Introduction to welding
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)
34
35. Introduction to welding
Gas Tungsten Arc Welding - Process features
Excellent control
q Stable arc at low power (80A at 11V)
q Independently added filler
q Ideal for intricate welds eg root runs in pipe or thin sheet
q Low productivity 0.5kg/h manual
High quality
q Clean process, no slag
q Low oxygen and nitrogen weld metal
q Defect free, excellent profile even for single sided welds
35
36. Introduction to welding
Gas Tungsten Arc Welding - Equipment
Welding power source with constant
current characteristic
q DC for most metals, AC for Al
q Arc starting by high frequency (5000V, 0.05A)
q Sequence timers for arc starting, arc finishing &
gas control
Water- or gas-cooled torch with tungsten
electrode
q Electrode may contain thoria or zirconia, etc
37. Introduction to welding
Gas Tungsten Arc Welding - Shielding gases
Torch is fed with an inert or reducing gas
q Pure argon - widespread applications
q Argon-helium - Higher arc voltage, inert
q Argon-2% hydrogen - Cu alloys & austenitic steel
q Torch gas must not contain oxygen or CO2
Backing (or purge) gas
q Used for all single-sided welds except in carbon steel
q Argon, nitrogen, formier gas (N2 + H2)
Supplementary shielding
q Reactive metals: Ti, etc
q Gas filled chambers or additional gas supply devices
37
38. Introduction to welding
Gas Tungsten Arc Welding - Filler metals
Autogenous welding (no filler)
Filler wire or rod of matching composition
q C-Mn & low alloy steel
q Stainless Steel
q Al, Mg, Ti
q Cu & Ni
Consumable inserts - filler preplaced in joint
38
41. Introduction to welding
GMAW and FCAW
Gas metal arc welding
(MIG, MAG, CO2 welding)
Flux cored arc welding
41
42. Introduction to welding
Gas metal arc welding
A continuous solid wire, small
diameter
q GMAW uses solid wire, no flux
q 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)
42
44. Introduction to welding
Gas metal arc welding
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
44
46. Introduction to welding
Gas metal arc welding - Equipment
Welding power source
Wire feeder mechanism
q May be in power source cabinet
Gun with gas supply & trigger
switch
q Manual (semiautomatic) guns
q Automatic torches available
q Can be fitted to robot etc
46
47. Introduction to welding
Gas metal arc welding – Metal transfer
Spray
q Higher current & voltage, argon-rich gas
Short circuiting (dip)
q Low current and voltage, CO2
Globular
q Intermediate current
Pulsed current power sources
q Adjustable frequency
q One droplet per current pulse.
48. Introduction to welding
Gas metal arc welding – Metal transfer
Burn-back
and unstable arc
Spray
Voltage
Globular
Short
circuiting
No arc (birds-nesting)
Current 48
49. Introduction to welding
Gas metal arc welding - Consumables
Solid Wires (GMAW)
q A wide variety of alloys are available
Flux cored arc welding (FCAW)
q Gas shielded flux cored wires
q Self-shielded flux cored wires
• Used outdoors
q Metal cored wires
• Light flux cover
49
51. Introduction to welding
Gas metal arc welding - Gas mixtures
Inert gases (MIG)
q Argon or helium or mixtures of these
q Active base metals, Al, Mg, Ti
Active gases (MAG and FCAW)
q Carbon dioxide
q Argon plus oxygen and/or carbon dioxide
q Nitrogen, hydrogen
51
60. Introduction to welding
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
60
62. Introduction to welding
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
62
64. Introduction to welding
Size of weld beads in
(a) electron-beam or laser-beam welding
(b) conventional arc welding. 64
65. Introduction to welding
Solid-State Welding
q Heat
q Pressure
q Time
q NO Melting
q NO Filler Material
q Intimate Contact
Usually Requires Deformation
q Works with Dissimilar Metals
65
85. Introduction to welding
Importance of Welding
Wide use in manufacture
Occurs later stage in manufacturing process
q Large number of practitioners
q Cost is high proportion of manufactured item
q Risk and cost of defective welds is high
Technology is complex
q Process control is key to success
85
86. Introduction to welding
Fusion welding
Intense energy source melts base metal locally
q Energy density 0.001 W/cm2 to 1 MW/cm2
q Energy source may be stationary or move at a constant
speed
Filler metal
q From electrode
q Independently added filler
q No filler (autogenous welding)
86
87. Introduction to welding
Why metals do not weld
Surface irregularities
Surface contamination
q Adsorption - A one atom boundary layer forms in
10-9 sec at 100 kPa and in 10-3 sec at a pressure
of 1Pa
q Chemical combination (oxidation)
Joining can occur where heat or pressure are
present ( ex: seizing)
87
99. Introduction to welding
High dilution procedures
Square edges
Low cost of preparation
Fast travel speeds (acid
Single pass with temporary backing fluxes)
Maximum thickness
q 16 mm in one pass, 20 mm
in two
Location of bead is critical
High dilution leads to low
toughness
High cap height, lower
Two pass weld fatigue life
100. Introduction to welding
Vee butt weld procedures
60
One, two or multipass included
Vee or U preparations
Lower currents
Unlimited thickness
Excellent quality 6mm
1.5mm max
100