Introduction to welding processes

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
Introduction to 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

6
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

7
Solid phase welding
 Hot processes
 Forge welding
 Friction welding
 Diffusion bonding
 Cold processes
 Ultrasonic welding
 Explosive welding

8
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

9
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

10
Arc energy
Q = 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

11
• 103 Watts/cm2 melts most metals
• 106 -107 Watts/cm2 vaporizes most metals
• 103 to 106 Watts/cm2 typical for fusion welding

12
Manual Metal Arc Welding
MMAW,
SMAW,
Stick electrode welding
Manual welding

13
 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

14

15
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

16
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

17
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)

18
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 positions
1=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

20
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

21
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

22
Submerged arc welding
SAW,
Sub-arc

23

24

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)

26
Submerged arc welding - Equipment
Power source
Welding head and
control box
Welding head travel
Flux recovery system
(optional)
Positioners and
Fixtures

27
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

28
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

29
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

30
Submerged arc welding – Tandem arc

31
Gas shielded arc process
Tungsten Inert Gas welding (TIG)
Gas tungsten arc welding (GTAW)

32
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

33
Gas Tungsten arc welding

34
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)

35
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

37
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

38
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

39
Gas Tungsten Arc Welding - Automation

40
Gas Tungsten Arc Welding – A TIG

41
GMAW and FCAW
Gas metal arc welding
(MIG, MAG, CO2 welding)
Flux cored arc welding

42
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)

43

44
 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

45

46
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

49
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

50
Gas metal arc welding – Wire size

51
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

52
Gas metal arc welding - Developments

53

54

55

56
Plasma Cutting, Welding & Surfacing

58
Neutral Flame
Oxidising Flame
Carburising Flame
Oxy-Acetylene Welding

59
Thermit welding

60
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

61
Laser welding

62
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

63
Electron Beam Welding

64
Size of weld beads in
(a) electron-beam or laser-beam welding
(b) conventional arc welding.

65
Solid-State Welding
 Heat
 Pressure
 Time
 NO Melting
 NO Filler Material
 Intimate Contact
Usually Requires Deformation
 Works with Dissimilar Metals

66
Resistance Welding

67
Resistance spot welding Robots

68
Flash Butt Welding

69
Friction Welding

70
Friction Stir Welding

71
Friction Stir Welding

72
Explosive Welding

Introduction to welding processes

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Introduction to welding processes