- Heating rate and peak temperature - Cooling rate - Composition of filler metal - Pre/post heating of base metal - Stress relieving heat treatment Controls the microstructure and properties of the weld. 41 Heat transfer in welding process - Heat input to the weld is from the energy source - Heat is conducted away from the weld zone into the base metal - Heat flow depends on thermal properties of base metal and weld metal - Heat affected zone experiences thermal cycle due to heat input - Rapid heating and cooling rates in welding leads to non-equilibrium microstructures - Control of heat input and cooling rate is important to control weld metallurgy

1. 9/4/2012
Casting,
Casting Forming & Welding
(ME31007)
Jinu Paul
Dept. of Mechanical Engineering
Course details: Welding
Topic Hours
1. Introduction to welding science & technology 2-3
2 Welding Processes 4
3 Welding Energy sources & characteristics 1-2
5 Welding fluxes and coatings 1
4 Physics of Welding Arc 1
5 Heat flow in welding 1-2
6 Design of weld joints 2
7. Testing and inspection of weld joints 2-3
8 Metallurgical characteristics of welded joints, Weldability and welding of
various metals and alloys 2
Total 19
1

2. 9/4/2012
Schedule of Lectures (Welding)
Lecture 1 23rd Aug 2012, Thursday, 8.30-9.30 am
Lecture 2 24th Aug 2012, Friday, 11.30 am-12.30 pm
Lecture 3 30 Aug 2012, Thursday, 8.30-9.30 am
Lecture 4 31 Aug 2012, Friday, 11.30 am-12.30 pm
Lecture 5 06 Sept 2012, Thursday, 8.30-9.30 am
Lecture 6 07 Sept 2012, Friday, 11.30 am-12.30 pm
Lecture 7 13 Sept 2012, Thursday, 8.30-9.30 am
Lecture 8 14 Sept 2012, Friday, 11.30 am-12.30 pm
Lecture 9
L 20 S
Sept 2012 Th d
2012, Thursday, 8 30 9 30 am
8.30-9.30
Lecture 10 21 Sept 2012, Friday, 11.30 am-12.30 pm
Mid Semester Exam (30 %)
3
Schedule of Lectures (Welding)
Lecture 11 04 Oct 2012, Thursday, 8.30-9.30 am
Lecture 12 05 Oct 2012, Friday, 11.30 am-12.30 pm
, y, p
Lecture 13 11 Oct 2012, Thursday, 8.30-9.30 am
Lecture 14 12 Oct 2012, Friday, 11.30 am-12.30 pm
Lecture 15 18 Oct 2012, Thursday, 8.30-9.30 am
Lecture 16 19 Oct 2012, Friday, 11.30 am-12.30 pm
Lecture 17 01 Nov 2012, Thursday, 8.30-9.30 am
Lecture 18 02 Nov 2012, Friday, 11.30 am-12.30 pm
Lecture 19 08 Nov 2012, Thursday, 8.30-9.30 am
End Semester Exam (50 %)
4
2

3. 9/4/2012
Lecture 1
23rd Aug 2012, Thursday, 8.30-9.30 am
Introduction to welding
5
Overview of Joining processes
Joining
processes
Mechanical
Brazing
Assembly
Welding Soldering
(e.g., Threaded
Adhesive bonding
fastners, rivets)
6
3

4. 9/4/2012
Joining processes-overview
Riveted Joint
Threaded fastner
Welded Joint
Brazed Joint 7
Some application areas of welding
Ship building
Aircraft industry
Automotive industry 8
4

5. 9/4/2012
Welding: Application areas
• Applications in Air, Underwater & Space
• Automobile industry, aircraft industry
industry industry,
ships and submarines
• Buildings, bridges, pressure vessels,
girders, pipelines, machine tools, offshore
structures, nuclear power plants, etc.
• House hold products, farm, mining, oil
industry, jigs & fixtures, boilers, furnaces,
railways etc.
9
Welding process-Features
• Permanent joining of two materials through
localized coalescence resulting f
l li d l lti from a
suitable combination of Temperature &
Pressure
• Formation of Common metallic crystals at
the joints/interface
• With or Without filler material
10
5

6. 9/4/2012
Welding process-Features
• Continuity: absence of any physical
disruption on an atomic scale
p
• Not necessarily homogeneous but same in
atomic structure, thereby allowing the
formation of chemical bonds
Material Metals Ceramic Polymer
(similar/dissimilar)
Type of bond Metallic Ionic/coval Hydrogen, van der
ent Waals, or other
dipolar bonds
11
Welding Process: Advantages
• Exceptional structural integrity, continuity,
fluid tightness, portable equipments
• Strength of joints can approach or exceed
the strength of the base material(s)
• Wide range of processes & approaches
• Can be performed manually, semi
automatically or completely automatically
• Can be performed remotely in hazardous
environments (e.g., underwater, areas of
radiation, outer space) using robots
12
6

7. 9/4/2012
Welding Process: Disadvantages
• Precludes disassembly
• Requirement for heat in producing many
welds can disrupt the base material
microstructure and degrade properties;
may induce residual stresses
• Requires considerable operator skill
• Capital equipment can be expensive (e.g.,
laser beam, vacuum chambers etc.)
13
Types of joints in welding
Butt joint
Corner joint Lap joint
Tee joint Edge joint
14
7

8. 9/4/2012
Types of welds
1) Fillet weld
Fillet weld Fillet weld
Fillet weld on corner joint
on lap joint on T-joint
15
Types of welds
2) Groove weld
(c) single
(a) square groove weld
weld, ( )
(b) single bevel
g
V-groove
V groove weld
groove weld
(d) single (e) single (f) Double V- groove
U-groove weld J-groove weld weld for thicker
sections
16
8

9. 9/4/2012
Types of welds
3) Plug & slot weld
• Drill hole/slot on the top plate only
• Hole/slot is filled with filler metal
17
Types of welds
4) Spot weld
5) Seam weld
• Fused section between the surfaces of two sheets
• Mostly associated with resistance welding
18
9

10. 9/4/2012
Types of welds
6) Flange weld & Surfacing weld
• Surfacing weld is not for joining parts
• The purpose is to increase the thickness of the plate or to provide a
protective coating on the surface.
19
Lecture 2
24th Aug 2012, Friday, 11.30 am-12.30 pm
Weld Microstructure & Concept
of continuity
y
20
10

11. 9/4/2012
Some material science basics…
Atoms Lattice Grains
• Grain size, Grain boundaries,
• Recrystalization ~0.4-0.6 Tm  Atoms remain in lattice,
but new grains will be formed
• Melting  Atoms displaced from lattice, free to move
21
Some material science basics…
• Metals are crystalline in nature and
consists of irregularly shaped grains of
i t fi l l h d i f
various sizes
• Each grain is made up of an orderly
arrangement of atoms known as lattice
• The orientation of atoms in a grain is
uniform but differ in adjacent grains
22
11

12. 9/4/2012
Basic Classification of welding
(a) Fusion welding (b) solid-state welding
a) Fusion Welding
• Uses heat to melt the base metals
• A filler metal is mostly added to the molten
pool to facilitate the process and provide bulk
and strength to the welded joint.
joint
• e.g., Arc welding, resistance welding, Gas
welding, Laser beam welding, Electron beam
welding
23
Micro-structural zones in Fusion
welding
1) Fusion zone 2) Weld interface/partially melted zone
3) Heat affected zone 4) Unaffected base metal
24
12

13. 9/4/2012
Grain growth in Fusion welding
• Direction solidification in fusion zone Epitaxial
grain growth  Columnar grains
• HAZ  Possible recrystallization/ grain refinement or
phase change
• Shrinkage of fusion zone  Residual stress on the
base metal surrounding HAZ
25
Basic Classification of welding
b) Solid state Welding
• C l
Coalescence results f
lt from application of
li ti f
pressure alone or a combination of heat and
pressure
• If heat is used, the temperature in the process
is below the melting point of the metals being
welded
• No filler metal is used
• e.g., Diffusion welding, friction welding,
ultrasonic welding
26
13

14. 9/4/2012
Micro-structural zones in Solid
state welding
• No Fusion zone
• Little or no HAZ
• Mechanically upset region
• Plastic deformation at the interface
27
Role of Temperature in Fusion/
solid state welding
• Drives off volatile adsorbed layers of gases,
moisture, or organic contaminants
g
• Breaks down the brittle oxide through differential
thermal expansion
• Lowers yield/flow strength of base materials
helps plastic deformation
• Promotes dynamic recrystallization during plastic
deformation (if T > Tr)
• Accelerates the rates of diffusion of atoms
• Melts the substrate materials, so that atoms can
rearrange by fluid flow (if T > Tm) 28
14

15. 9/4/2012
Role of Pressure in solid state
welding
• Disrupts the adsorbed layers of gases/organic
compound or moisture by macro- or
microscopic deformation
• Fractures brittle oxide or tarnish layers to
expose clean base material atoms
• Plastically deform asperities (lattice) to increase
the number of atoms that come into intimate
contact (at equilibrium spacing)
29
Mechanisms for obtaining
material continuity
(1) Solid phase plastic deformation
Solid-phase deformation,
without or with recrystallization
(2) Diffusion, and
(3) Melting and solidification
30
15

16. 9/4/2012
Obtaining continuity
1) Solid-phase plastic deformation
• Atoms are brought together by
plastic deformation
• Sufficiently close to ensure that
bonds are established at their
equilibrium spacing
• Significant lattice deformation
• L tti
Lattices are left in the strained
l ft i th t i d
state (distorted) in cold (a) Cold deformation and
deformation lattice strain
Prevailing mechanism in solid state welding
with out heat 31
Obtaining continuity
1) Solid-phase plastic deformation (with heat)
• In hot state (0.4-0.5 Tm), the
strained lattice recover from the
distorted state
• Atomic rearrangement &
Recrystallization
• Grain growth across original
interface (b) hot deformation and
• Eliminates the original physical dynamic recrystallization
interface
Prevailing mechanism in solid state welding
with heat 32
16

17. 9/4/2012
Obtaining continuity
2) Diffusion
• Transport of mass through atom
movement
• Can occur entirely in solid
phase or with liquid phase
• For dissimilar materials  thin
layer of alloy at the interface
• R t of diffusion  Diff
Rate f diff i Difference in
i a) S lid h
) Solid-phase diff i
diffusion
composition (Fick’s law) across the original interface
(dotted line)
Prevailing mechanism in brazing/soldering
33
Obtaining continuity
3) Melting and solidification
Liquid provided by melting Establishing a bond upon
the parent materials without epitaxial solidification of
or with additional filler this liquid
• Solidifying crystals take up the grain structure &
orientation of substrate/unmelted grains
• Prevailing mechanism in most fusion welding process
34
17

18. 9/4/2012
Lecture 3
30th Aug 2012, Thursday, 8.30 am-9.30 am
Elements of welding set up,
power d
density & h
i heat transfer
f
35
Basic elements of a welding setup
1.
1 Energy source to create union by
pressure/heat
2. Method to remove surface contaminants
3. Protect metal from atmospheric
contamination
4. Control of weld metallurgy
36
18

19. 9/4/2012
1. Energy source
Classification of Fusion welding based on energy source
Energy Types of welding
source
Oxy fuel gas welding, Exothermic welding/ Thermite
Chemical
welding, Reaction brazing/Liquid phase bonding
Radiant Laser beam welding, Electron beam, Infrared welding/
energy brazing, Imaging arc welding, Microwave welding,
Electric-Perm. Gas tungsten arc welding, plasma arc welding, Carbon
electrode arc arc welding, atomic hydrogen welding, Stud arc welding
Electric-
El t i Gas
G metal arc welding, Shi ld d metal arc welding,
t l ldi Shielded t l ldi
Consumable Submerged arc welding, Electrogas welding, Electroslag
electrode welding, Flux cored arc welding
Electric- Resistance spot, resistance seam, projection welding,
Resistance flash/ upset welding, Percussion, Induction welding
37
1. Energy source
Classification of solid state welding based on energy source
Energy Types of welding
source
Cold welding, Hot pressure welding, Forge welding, Roll
welding, Friction welding, Ultrasonic welding, Friction stir
Mechanical
welding, Explosion welding, Deformation diffusion welding,
Creep isostatic pressure welding, Super plastic forming
Chemical + Pressure gas welding, Exothermic pressure welding,
Mechanical Pressure thermit forge welding
Stud arc welding, Magnetically impelled arc butt welding,
Electrical + resistance spot welding, resistance seam welding,
Mechanical projection welding, flash welding, upset welding,
percussion welding, resistance diffusion welding
38
19

20. 9/4/2012
2. Removal of Surface
contaminants
• Surface contaminants may be organic films,
absorbed gases or chemical compounds of the
b b d h i l d f th
base metals (usually oxides)
• Heat when used as source of energy removes
organic films and absorbed gases
• Fluxes are used to clean oxide films and other
contaminants to f
i form slag
l
• Slag floats and solidifies above weld bead
protecting the weld from further oxidation
39
3. Protection from atmospheric
contamination
• Shielding gases are used to protect molten
weld pool from atmospheric contaminants
like O2 & N2 present in air
• Shielding gases could be Ar, He,CO2
• Alternatively, welding could be carried out in
y g
an inert atmosphere.
40
20

21. 9/4/2012
4. Control of weld metallurgy
• Microstructures formed in the weld and
HAZ determines the properties of the weld
• Depends on heating, cooling rates (power,
weld travel speed)
• Can be controlled by preheating/ post heat
treatment
• De-oxidants, alloying elements etc. added
to control weld metal properties
41
Power density
• Defined as the power transferred to work per
unit surface area (W/mm2)
(
• Time to melt the metal is inversely proportional
to power density
Welding Process Approx. Power density
(W/mm2)
Oxy-fuel welding 10
Arc welding 50
Resistance welding 1000
Laser beam welding 9000
Electron beam welding 10,000
42
21

22. 9/4/2012
Heat transfer mechanisms in
Fusion Welding
Heat transf. factor f1= Heat transf. to work / Heat gen. by source
Melting Factor f2 = Heat used for melting / Heat tranf. to work
Useful heat or energy = f1.f2
43
Example 1
The power source in a particular welding setup generates
3500 W that can be transferred to the work surface with
a heat transfer factor f1 = 0 7 The metal to be welded is
0.7.
low carbon steel, whose melting temperature is 1760K.
The melting factor in the operation is 0.5. A continuous
fillet weld is to be made with a cross-sectional area of 20
mm2. Determine the travel speed at which the welding
operation can be accomplished?
Heat
H t capacity of low carbon steel (Cp) 480 J/Kg.K
it f l b t l )=480 J/K K
Latent heat of melting Lm =247 kJ/Kg
Density  = 7860 kg/m3
Initial sample temperature T0 = 300 K
44
22

23. 9/4/2012
Example 1-Solution
Rate of heat input to the weld bead = 3500 × f1 × f2
= 3500 × 0 7 × 0 5 = 1225 J/s
0.7 0.5
Heat input = Energy used for heating to Tm + Energy used
for melting
1225 = [Cp(Tm-T0) + Lm ]  × A × v
1225 = [480(1760 300) + 247 ×103] × 7860 × 20 ×10-6 × v
[480(1760-300) 10 10 6
Travel speed v = 0.0082 m/s = 8.2 mm/s
45
Summary: Lectures 1-3
• Overview of welding, applications,
ad a tages
advantages
• Welded Joint types
• Fusion & Solid state welding
• Elements of weld setup, Heat Balance,
Power density y
• N.B: Characteristics, micro-structural
zones and concept of lattice continuity in
fusion & solid state welding
46
23

24. 9/4/2012
Lecture 4
31th Aug 2012, Friday, 11.30 am-12.30 pm
Welding Processes
1) Oxy-Fuel gas welding
) y g g
47
Welding Processes-
1) Oxy-Fuel gas welding
• Uses oxygen as oxidizer
• Acetylene, H2 or Natural gas, methane,
propane, butane or any hydrocarbon as
fuel
• Fuel + Oxidizer  Energygy
• Acetylene is preferred (high flame
temperature-3500 C)
48
24

25. 9/4/2012
Gases used in Oxy-gas welding
Fuel Peak Heat of
reaction combustion
Temp (C) (MJ/m3)
(MJ/ 3)
Acetylene 3500 54.8
Methylacetylene- 2927 91.7
propadiene (C3H4)
Hydrogen 2660 12.1
Propylene 2900 12.1
Propane 2526 93.1
Natural gas 2538 37.3
49
Oxy-acetylene welding (OAW)
operation
50
25

26. 9/4/2012
Reactions in Oxy-acetylene welding
• Flame in OAW is produced by the chemical
reaction of C2H2 and O2 in two stages
Stage 1 C2H2 + O2  2CO + H2 + heat
Stage 2 2CO + H2 + 1.5O2  2CO2 + H2O + heat
51
Flames in OAW
52
26

27. 9/4/2012
Flames in OAW
Neutral flame is used for most applications 53
Flames in OAW- Reducing flame
• Reducing flame for removing oxides
from metals s ch as al mini m or
metals, such aluminium
magnesium
• Preventing oxidation reactions during
welding
• To prevent decarburization (i.e., C to
CO,) in steels.
• Low carbon, alloy steels, monel metal
(Ni+Cu+…), hard surfacing
54
27

28. 9/4/2012
Flames in OAW-Oxy. flame
•The oxidizing flame causes the metal
being welded to form an o ide
elded oxide.
• Useful for preventing the loss of high
vapor-pressure components, such as
zinc out of brass, through the formation
of an impermeable “oxide skin” (here,
copper oxide)
• Brass, bronze, Cu, Zn & Sn alloys
55
OAW set up
• Pressurized cylinders of
O2 and C2H2
d
• Gas regulators for
controlling pressure and
flow rate
• A torch for mixing the
gases
• Hoses for delivering the
gases from the cylinders to
the torch
56
28

29. 9/4/2012
OAW Torch
57
Example 1 - OAW
An oxyacetylene torch supplies 0.3 m3 of acetylene per
hour and an equal volume rate of oxygen for an OAW
operation on 4.5-mm-thick steel.
Heat generated by combustion is transferred to the work
surface with a heat transfer factor f1 = 0.20. If 75% of
the heat from the flame is concentrated in a circular
area on the work surface that is 9.0 mm in diameter,
find
(a) t f heat liberated during
( ) rate of h t lib t d d i combustion,
b ti
(b) rate of heat transferred to the work surface, and
(c) average power density in the circular area.
(Heat of combustion of Acetylene in O2 = 55×106 J/m3)
58
29

30. 9/4/2012
Example 1 - OAW
(a) The rate of heat generated by the torch is the product
of the volume rate of acetylene times the heat of
combustion: RH = (0 3 m3/hr) (55×106) J/m3 = 16 5×106
(0.3 ) 16.5×10
J/hr or 4583 J/s
(b) With a heat transfer factor f1 = 0.20, the rate of heat
received at the work surface is
f1 × RH = 0.20×4583 = 917 J/s
(c) The area of the circle in which 75% of the heat of the
flame is concentrated is A = Pi. (9)2/4 = 63.6 mm2
The power density in the circle is found by dividing the
available heat by the area of the circle:
Power density = 0.75 × 917/63.6 = 10.8 W/mm2 59
OAW-Advantages
• The OAW process is simple and highly
portable
bl
• Inexpensive equipment
• Control over temperature
• Can be used for Pre-heating, cutting &
welding
ldi
60
30

31. 9/4/2012
OAW-Disadvantages
• Limited energy  welding is slow
• Low protective shielding  welding of reactive
metals (e.g., titanium) is generally impossible
• Low power density, Energy wastage, total heat
input per linear length of weld is high
• Unpleasant welding environment
• Weld lines are much rougher in appearance
than other kinds of welds  Require more
finishing
• Large heat affected zones
61
OAW-Applications
• Preheating/post heat treatment
• C b used f cutting, grooving, or piercing
Can be d for tti i i i
(producing holes), as well as for welding
• Oxyfuel gas processes can also be used for
flame straightening or shaping
• Oxidizing flame for welding Brass, bronze, Cu-
Zn and Tin alloys
• Reducing flame for low carbon & alloy steels
62
31

32. 9/4/2012
Pressure Gas welding
(Special case of OAW)
• Oxyfuel gas used for preheating the weld
interface
63
References
• Principles of Welding, Robert W Messler
• M t ll
Metallurgy of W ldi
f Welding, J F L
J.F. Lancaster
t
• Welding Science and Technology, Md.
Ibrahim Khan
• Welding Technology-O.P. Khanna
• Manufacturing Engineering and
Technology, S. Kalpakjian
64
32