This document discusses heat flow and solidification processes during welding. It begins by introducing how heat flow affects phase transformations, microstructure, properties, and residual stresses in the weld and heat affected zone. It then describes various heat sources for welding and analyzes heat flow using Rosenthal's equations. Key welding parameters that influence heat distribution and zone shapes are examined. Metal transfer modes and forces are reviewed. Reactions in the weld pool involving gases like nitrogen, oxygen, and hydrogen are analyzed, along with techniques to prevent these reactions from causing defects.

1. 1
1-
-2 Solidification
2 Solidification Processes
Processes
1

2. INTRODUCTION
INTRODUCTION
• Heat flow during welding can strongly affect
phase transformations during welding.
– The resulting microstructure and properties of the
weld is also affected.
– Weld residual stresses and distortion also can be
– Weld residual stresses and distortion also can be
very significant.
• The knowledge of heat flow assists in
predicting structural changes in the fusion
zone (FZ) and heat affected zone (HAZ).
– This enables control of microstructure changes in
the FZ and distortion and residual stresses in HAZ
2

3. Sources of Energy for Welding
Sources of Energy for Welding
• Welding requires heat, pressure, or both to
create chemical bonds and obtain sufficient
continuity between atoms of the joined
parts.
– The strength of the joint approaches the
strength of the weaker base material or filler.
• Thus two main sources of energy for
welding are:
– heat and
– mechanical energy in the form of pressure.
3

4. Heat
Heat Sources
Sources
• The key feature of any heat source is
– to enable welding to take place by providing
sufficient energy.
• Different heat sources for welding include:
– chemical, electric arc, electrical resistance,
– chemical, electric arc, electrical resistance,
plasmas, electron beams,
– micro- waves, light beams (lasers or focused
IR or imaged arcs),
– mechanical friction, and mechanical plastic
deformation.
4

5. Heat Flow
Heat Flow
• Heat flow on the surface is characterized by a
heat-flux distribution given by:
– where q(x, y) is given in W/mm2 and is directed
onto the surface at z = 0.
– This heat is lost to the surroundings by a
combination of radiation and convection and
combination of radiation and convection and
conduction .
• This general equation can be solved for one,
two, or three dimensions
– depending on the weldment and weld geometry,
including plate thickness and whether there is full
or partial penetration, etc thickness
5

6. Heat Flow …
Heat Flow …
• Heat flow in a workpiece of sufficient
length is steady or quasistationary, with
respect to the moving heat source.
– The temperature distribution and the pool
geometry do not change with time from the
geometry do not change with time from the
point of observation.
• This steady-state assumption was first
applied by Rosenthal to simplify the
mathematical treatment of heat flow during
welding.
6

7. Heat Flow …
Heat Flow …
• Interaction of the heat source with the material,
– leads to variation in the severity of thermal
distribution experienced by the material, resulting
in three distinct regions in the weld
– The fusion zone (FZ),
– The fusion zone (FZ),
– The heat-affected zone (HAZ),
– The base metal (BM)
• It is the FZ that experiences
melting, solidification and
subsequent microstructure
7

8. Heat Flow …
Heat Flow …
• Rosenthal derived
analytical equations for
heat flow during welding
by assuming:
– steady-state heat flow,
– point heat source,
Rosenthal’s Two- Equations Model
– point heat source,
– negligible heat of fusion,
– constant thermal
properties,
– no heat losses from the
work piece surface, and
– no convection in the weld
pool. Rosenthal’s Three-Dimensional Equation Model
Rosenthal’s Two- Equations Model
8

9. Rosenthal’s Two-Dimensional
Equation
Rosenthal’s Three-Dimensional
Equation
Heat Flow …
Heat Flow …
• T = instant temperature
• T0= work piece temperature
before welding
• g = work piece thickness
• k=termal conductivity.
• Q = heat transferred
V = travel speed.
= workpiece thermal diffusivity,
namely, k/C, where  and C are
density and specific heat of the
workpiece, respectively.
K0 =modified Bessel function
R or r = radial distance from origin,
namely, (x2+ y2+ z2)1/2 or (x2+ y2)1/2.
9

10. Heat Flow …
Heat Flow …
• The two Rosenthal’s equations can be used
to calculate the temperature T(x, y) at any
location in the workpiece (x, y) with respect
to the moving heat source.
Temperature
distribution on
thin plates
Temperature
distribution on
thick plates
10

11. Welding Parameters on Heat
Welding Parameters on Heat
Distribution
Distribution
• The size and the shape of the melt region in a
FZ affects:
– the mechanics and kinetics of solidification and,
– therefore, the structure and properties of the
resulting weld is also affected.
resulting weld is also affected.
• The shape and the size of the weld pool,
along with the size and shape of the
surrounding HAZ also affects:
– the induced thermal stresses that act on the weld,
– leading to the formation of defects or residual
stresses or distortion.
11

12. Welding
Welding Parameters
Parameters,
, Heat
Heat Distribution
Distribution …
…
• To a lesser degree, the shape of the HAZ
influences overall performance of weld.
• Understanding the effect of heat flow and
temperature distribution on the size and shape
of the 3 major zones:
of the 3 major zones:
– Is critically important in understanding,
predicting, and, ultimately, controlling the final
weld and weldment properties and performance.
• The shape of the melt, although not necessarily
its physical size,
– is a function of material, welding speed, and
welding power (voltage times current) and can
12

13. Welding Speed On Zone Shape
Welding Speed On Zone Shape
• The effect of welding speed is to change
both the weld pool, FZ and the
surrounding HAZ zone shapes.
13

14. Welding Speed On Zone Shapes…
Welding Speed On Zone Shapes…
• Once the heat source
is moved with
constant velocity,
the weld pool and
the weld pool and
surrounding HAZ
become elongated to
an elliptical plan form
14

15. Metal Transfer
Metal Transfer
• Predominant metal transfer modes in fusion
welding are:
– free flight modes (spray and globular) and short-
circuiting.
• Spray transfer mode:
• Spray transfer mode:
– Characterized by transfer of fine, discrete
molten particles or droplets from the consumable
electrode to the work at rates of several hundred
per second,
• This metal transfer mode is very stable,
directional, and essentially free of spatter.
15

16. Metal Transfer…
Metal Transfer…
• Globular transfer mode:
– Characterized by
releasing large globules
of molten metal at the tip
of the consumable
of the consumable
electrode to the work
piece by gravity.
• The rate of droplet
transfer is slow,
typically around 1 to 10
droplets per second.
16

17. Metal Transfer…
Metal Transfer…
• Short-circuiting mode:
– Welding currents and voltages are kept low
and the slow-forming molten globules at the
end of the consumable electrode periodically
touches the weld puddle;
touches the weld puddle;
– bridging the electrode and work piece gap,
– to cause their release through surface tension
forces.
• This mode occurs at rates in excess of 50
per second, but requires special power
sources.
17

18. Forces Contributing to Molten Metal
Forces Contributing to Molten Metal
Transfer in Welding
Transfer in Welding
• Molten metal transfer should occur with
minimal loss due to spatter.
• A combination two or more mechanisms
have been suggested based on the
have been suggested based on the
following forces:
– 1. Pressure generated by the evolution of gas
at the electrode tip .
– 2. Electrostatic attraction between consumable
electrode and the work piece.
– 3. Gravity.
18

19. Molten Metal Transfer Forces…
Molten Metal Transfer Forces…
– 4. The “pinch effect” caused near the tip of the
consumable electrode by electromagnetic field
forces.
– 5. Explosive evaporation of a necked region formed
between the molten drop and solid portions of the
between the molten drop and solid portions of the
electrode due to very high conducting current
density.
– 6. Electromagnetic action produced by a divergence
of current in the arc plasma around a drop.
– 7. Friction effects of the plasma jet.
– 8. Surface tension effects once the molten drop (or
electrode tip) contacts the molten weld pool.
19

20. Reactions in the Weld Pool
Reactions in the Weld Pool
• Most metals can react with gases, non-
metallic elements, and even other metals.
– Some react at all temperatures (e.g. Ti and Zr),
while all do so in molten state.
while all do so in molten state.
• Molten metal reacts with almost any gas
except the noble or inert gases.
– The basic chemical reactions during fusion
welding include gas–metal reactions and slag–
metal reactions.
20

21. Reactions in the Weld Pool
Reactions in the Weld Pool...
...
• Protection Techniques in Common Welding
Processes (Air and Slag)
21

22. Techniques for Protection from Air
Techniques for Protection from Air
• Shielding (inert) gases:
– Use of inert shielding gases (Ar or He) in GTAW and
GMAW
• Self-shielded arc welding:
– uses strong nitride formers such as Al, Ti, and Zr in
– uses strong nitride formers such as Al, Ti, and Zr in
the electrode wire alone to protect against nitrogen.
• Arc welding:
– uses coated electrodes.
• Oxyfuel processes:
– Provide protection from air through their relatively
nonreactive combustion products of CO, CO2, and H2
22

23. Gas
Gas-
-Metal Reactions
Metal Reactions
• These are chemical reactions at the
interface between the gas phase and the
liquid metal.
– They arise from dissolution of N2, O2, H2 in
liquid metal and evolution of carbon monoxide.
liquid metal and evolution of carbon monoxide.
• The gases come from air, the consumables
such as the shielding gas and flux, or from
the surface of work piece.
• N2, O2, and H2 gases can dissolve in the
weld metal during welding.
23

24. Gas
Gas-
-Metal Reactions...
Metal Reactions...
• The gas-metal reactions can take one of two
forms:
– Dissolution
– Chemical
• These reactions can cause one or more of the
• These reactions can cause one or more of the
following effects:
– Gas can dissolve to a solubility limit and remain
dissolved;
– Gas can try to dissolve beyond a solubility limit and
appear as porosity; and
– Gas can react chemically to form a brittle compound
layer or inclusions.
24

25. Gas
Gas-
-Metal Reactions ...
Metal Reactions ...
• Gases, including N2, O2, and
H2, dissolve in liquids,
including molten metals.
– The gas molecules occupy the
large spaces between atoms of
the metal in liquid form.
 
2
2
1
gas
gas
P
k 
the metal in liquid form.
• The amount of dissolving
gas increases with
temperature of the liquid, and
– It is a function of the partial gas
pressure above the liquid as
expressed by Sievert’s law:
where k is the equilibrium
constant, [gas] is the
concentration (wt%) in the
molten metal, and Pgas, is
the partial pressure of the
particular gas in diatomic
molecular form (N2, O2,
and H2).
25

26. Gas
Gas-
-Metal Reactions ...
Metal Reactions ...
• The rate of gas absorption is controlled by :
– The rate of gas arrival at the molten surface.
– Solubility of the gas.
– Rate of gas mixing with the molten metal.
• Solubility of N2 is far higher in fcc -iron than
• Solubility of N2 is far higher in fcc -iron than
in bcc - or -iron form.
– This is due to differences in the number and
size of interstitial sites available.
– fcc iron has larger interstitial sites than
octrahedral sites in the bcc;
– thus accommodate gas atoms with less strain
energy.
26

27. Gas
Gas-
-Metal Reactions ...
Metal Reactions ...
• The presence of different alloying elements
(solutes) in a molten metal can affect the
solubility of a gas in that metal.
– Some elements increase the solubility, while
others decrease it;
others decrease it;
• As a result of non-equilibrium conditions of
fusion welding,
– the actual amount of a gas in both a molten metal
and solidified welds is almost always different
than that predicted by Sievert's law for equilibrium.
27

28. Gas
Gas-
-Metal Reactions ...
Metal Reactions ...
• N2 and O2 are potent solid solution
strengtheners or hardeners to most metals,
whether ferrous or nonferrous.
– This is because of occupying interstitial sites
between atoms of the host or solvent.
• Despite their small size, they are still too
large to fit into interstices without causing
fairly substantial distortion of bonds and storing
of energy.
• The effectiveness of N2 as a strengthening
addition is comparable to C in Fe.
28

29. Oxygen (O
Oxygen (O2
2)
)
• Sources of oxygen
– From the air, use of excess oxygen in oxyfuel
welding, use of O2 or CO2 containing shielding
gases.
– From SiO2, MnO and FeO in the flux and from the
slag-metal reaction.
slag-metal reaction.
• Effects of oxygen
– O2 can oxidize carbon and other alloying elements
in the liquid metal, modifying their prevailing role,
depressing hardenability, and producing inclusions.
29

30. Effects of O
Effects of O2
2 in a weld
in a weld
• Reduction of strength and ductility
30

31. Nitrogen (N
Nitrogen (N2
2)
)
• Sources of N2
– N2 neither dissolve nor react with Cu and Ni
• It can be used as the shielding gas during welding.
– Metals such as Fe, Ti, Mn, and Cr dissolve N2
or form Nitrides (or both),
or form Nitrides (or both),
• the protection of the weld metal from N2 is
necessary.
– The presence of nitrogen in the welding zone is
usually a result of:
• improper protection against air.
• Intentional addition through the shielding gases
31

32. • Effects of Nitrogen
– N2 is an austenite stabilizer
for austenitic and duplex
stainless steels.
– Increasing the weld metal
nitrogen content can
decrease the ferrite
decrease the ferrite
content and increase the
risk of solidification cracking.
– Can significantly affect the
mechanical properties
• Formation needlelike structure
of a brittle iron nitride (Fe4N) in
ferrite which acts as crack
initiators 32

33. Protection Against N2
• In the self-shielded arc welding process
adding strong nitride formers (Ti,Al, Si, and
Zr) to the filler wire.
– The nitrides formed enter the slag and
– The nitrides formed enter the slag and
nitrogen in the weld metal is thus reduced.
33

34. Hydrogen (H
Hydrogen (H2
2)
)
• Sources of Hydrogen
• H2 in the welding zone can come from
several different sources:
– combustion products in oxyfuel welding
– combustion products in oxyfuel welding
– decomposition products of cellulose-type
electrode coverings in SMAW
– moisture or grease on the surface of the work-
piece or electrode; and
– moisture in the flux, electrode coverings, or
shielding gas.
34

35. Effects of H
Effects of H2
2
• H2 in the welding of high-strength steels can
cause hydrogen cracking.
• H2 can cause porosity in Al welds.
– Oxide films on the surface of the work-piece or
electrode can absorb air moisture and introduce
electrode can absorb air moisture and introduce
hydrogen into molten aluminum during welding.
• H2 can also cause problems in Cu welding
– by reacting with O2 to form steam and thus
causing porosity in the weld metal.
– H2 can also diffuse to the HAZ and react with O2
to form steam along the grain boundaries.
35

36. Hydrogen Reduction Methods
• Avoiding hydrogen-containing shielding gases.
• Drying the electrode covering and flux to remove
moisture and clean the filler wire and work-piece
to remove grease.
to remove grease.
• Adjusting the composition of the consumables if
feasible
36

37. Slag
Slag-
-metal Reactions
metal Reactions
• The principle requirements of slag are:
– Degasification of the weld metal
– Protection of the weld metal
– Refinement of the weld pool
– Refinement of the weld pool
– Arc stabilization
– Alloying of the weld
– Giving good appearance to the weld bead and
good slag detachability
37

38. Thermochemical Reactions
Thermochemical Reactions
• Are reactions at the interface between the
molten slag and the liquid metal.
• Examples
– decomposition of metal oxides in the flux,
– decomposition of metal oxides in the flux,
– oxidation of alloying elements in the liquid
metal by the oxygen dissolved in the liquid
metal, and
– desulfurization of the weld metal
38

39. Thermochemical Reactions ...
Thermochemical Reactions ...
Decomposition of metal
oxides in the flux
Oxidation by Oxygen
in Metal
• Desulfurization of Liquid
Metal
39

40. Electrochemical Reactions
Electrochemical Reactions
• Anodic oxidation reactions:
– occur at the electrode tip–slag interface in the
electrodepositive polarity or the weld pool–slag
interface in the electrode-negative polarity.
– Oxidation losses of alloying elements and pickup of
oxygen are expected at the anode.
40

41. Electrochemical Reactions...
Electrochemical Reactions...
• Cathodic reduction reactions
• The first two reactions are the reduction of
metallic cations from the slag,
• The third reaction is the removal (refining) of O2
from the metal.
41

42. Solidification Processes
Solidification Processes
• Metals in liquid state have atoms in random order.
• When a metal begins to freeze small geometric
groups of atoms (nuclei) begin to form.
– This is called nucleation
– The formed nuclei grow larger to form crystals of the
solid.
solid.
• The resultant solid may not necessarily have
uniform composition.
– The solute atoms in the liquid get redistributed
during solidification.
– The redistribution depends on both
• Thermodynamics (phase diagram), and
• Kinetics (diffusion, undercooling, fluid flow, etc).
42

43. Solidification Processes ...
Solidification Processes ...
• When solute atoms cannot completely
dissolve either substitutionally or interstitially;
– the end up forming atomic groupings
(polycrystals) or form chemical (inter-metallic)
(polycrystals) or form chemical (inter-metallic)
compounds.
• The solidification of weld metal can be likened
to the solidification of a casting.
43

44. Solidification of Weld Metals
Solidification of Weld Metals
• The solidification and microstructures of welds
are controlled by:
– growth rate, temperature gradient, under-cooling,
and alloy composition.
• The solidification involves nucleation and grain
• The solidification involves nucleation and grain
growth.
– The microstructure can be interpreted by considering
classical theories of nucleation and growth.
• The solidification of welds can lead to different
types of grain morphologies, the common are:
– columnar and equiaxed grains.
44
Planar, Cellular, Cellular
dendritic, Columnar dendritic
and equiaxed dendritic

45. Solidification of Weld Metals...
Solidification of Weld Metals...
• The microstructure of the FZ depends on the
solidification behavior of the weld pool.
• We noted that the heat from the source flows
away from the molten metal –usually
perpendicular to the fusion line (boundary)
fusion line
– Nucleation begin on the fusion line from the
existing solid.
– The newly formed crystals grow preferentially in
the direction of the heat flow and also in other
directions.
– Until when the solidification is completed and the
entire FZ has been filled with new grains.
45

46. Microstructure Formation
Microstructure Formation
• The growth rate and
temperature gradient vary
considerably across the weld
pool.
– The welding speed is related
to actual growth rates of the
solid at various locations in the
weld pool
weld pool
• The growth rate along the
fusion line is low while the
temperature gradient is
steepest.
– The growth rate increases
towards the weld centerline
while the temperature gradient
decreases.
46

47. Microstructure Formation...
Microstructure Formation...
• The resulting microstructure varies
noticeably from the edge (Fusion line)
towards the centerline of the weld.
• The schematic diagram (Figure 7.2) shows
epitaxial growth or epitaxial nucleation.
epitaxial growth or epitaxial nucleation.
– The arrow in each grain indicates its <100>
direction.
• Most of the micro-structural features can
be interpreted by considering classical
theories of nucleation and growth.
47

48. Nucleation
Nucleation
• Microstructure development in the FZ is
more complicated due to physical
processes
– such as re-melting, heat and fluid flow,
vaporization, dissolution of gasses,
vaporization, dissolution of gasses,
solidification, subsequent solid-state
transformation, stresses, and distortion.
• These processes and their interactions
profoundly affect weld pool solidification
and microstructure.
48

49. Nucleation…
Nucleation…
• In fusion welding, the base-metal grains
at the fusion line act as the substrate for
nucleation.
• Initial solidification often occurs from
existing crystals at the FZ/HAZ
boundary.
– It is essential in locations far away from the
– It is essential in locations far away from the
boundary.
• For FCC or BCC crystal structure
materials, the trunks of columnar
dendrites grow in the <100> direction.
• As shown, each grain grows without
changing its <100> direction.
• Crystallographic orientation of base
metal “seed crystal” is maintained
(Fig. 7.2)
49

50. Nucleation…
Nucleation…
• Due to intimate contact between the
molten metal and the substrate grains
on the fusion line,
– crystals nucleate from the molten metal
upon the substrate grains without difficulties.
upon the substrate grains without difficulties.
• When welding without a filler metal,
– nucleation occurs by arranging atoms from
the liquid metal upon the substrate grains
without altering their existing crystallographic
orientations.
50

51. Growth
Growth
• As shown previously columnar and equiaxed are
formed during solidification.
• After nucleation in undercooled isothermal
– melt growth is essentially equiaxed (similar to heat
flux) forming eutectic and dendritic morphologies.
• If the heat flux is unidirectional or freezing is
• If the heat flux is unidirectional or freezing is
slow
– Grains tend to be longer and melt growth is columnar
• In equiaxed growth
– the solid-liquid interface is morphologically unstable
leading to single phase solidification to dendrites
– It is common when freezing if fast
51

52. Growth…
Growth…
• In the columnar growth depending on local
growth conditions:
– planar, cellular and dendritic morphologies are
possible
• Solidification controls size and shape of the
grains, segregation, and the distribution of
grains, segregation, and the distribution of
inclusions and porosity.
– The weld pool shape is important in the
development of grain structure and dendrite growth
selection process
• The isothermal pattern and high cooling
rates favours columnar epitaxial growth
– Equiaxed grains also growth at the fusion boundary
52

53. Growth….
Growth….
• During welding, where the
molten pool is moved
through the material,
– the growth rate and
temperature gradient vary
considerably across the weld
pool. 






2
cos

V
Vk
• Geometrical analyses have
related the welding speed, V,
to the actual growth rates,
Vk, of the solid at various
locations in the weld pool.
• At slow welding speed is
unpredictable

 2
2
 Growing crystal axis to
the welding direction
2

53

54. Growth…
Growth…
• Liquid low in solidification temperature phase
tend to accumulate at the growing crystal
preventing further crystal growth unless
dispersed out.
• Under normal conditions growth takes place
• Under normal conditions growth takes place
along <111> due to isothermal movement.
• Flow rate of crystal growth at the fusion line
results in undercooling at the weld pool centre
– This favours nucleation and growth of equiaxed
grains.
54

55. Various grain morphologies
55

56. Epitaxial Growth in Welding
Epitaxial Growth in Welding
• Savage et al.(??) 1st discovered epitaxial growth in
fusion welding and confirmed continuity of
crystallographic orientation across the fusion
boundary.
• If the base metal has a different crystal structure to
that of the weld metal at the solidification
temperature,
temperature,
– nucleation of solid weld metal occurs on heterogeneous
sites on the partially melted base metal at the fusion
boundary.
• The fusion boundary exhibits random
misorientations between base metal grains and weld
metal grains
– as a result of heterogeneous nucleation at the pool
boundary
56

57. Competitive Growth in FZ
Competitive Growth in FZ
• As described earlier, the grain structure near
the fusion line of a weld is dominated either
– by epitaxial growth when the BM and the weld
metal have the same crystal structure, or
– by nucleation of new grains when they have
different crystal structures.
• Away from the fusion line,
– the grain structure is dominated by a different
mechanism known as competitive growth
(Fig. 7.7).
57

58. Competitive Growth…
Competitive Growth…
• During solidification of weld
metal grains tend to grow in
the direction perpendicular
to pool boundary.
– this is the direction of the
maximum temperature
maximum temperature
gradient and hence
maximum heat extraction
• However, columnar
dendrites or cells within
each grain tend to grow in
the easy-growth direction.
58

59. Effect of Welding Parameters On Grain
Effect of Welding Parameters On Grain
Structure
Structure
• It was shown that the weld
pool becomes teardrop at
high welding speeds and
elliptical at low welding
speeds.
– The columnar grains are also
essentially straight in order to
essentially straight in order to
grow perpendicular to the pool
boundary, as shown
schematically in the Figure
• Since the trailing boundary of
an elliptical weld pool is
curved, the columnar grains
– are also curved in order to
grow perpendicular to the
pool boundary (part b)
59

60. Weld Metal Nucleation Mechanisms
• Four (4) possible
mechanisms for new
grains to nucleate
during welding:
– dendrite
– dendrite
fragmentation,
– grain detachment,
and
– heterogeneous
nucleation
– surface nucleation
60

61. 61

62. Dendrite fragmentation mechanism
• Weld pool convection can in principle
cause fragmentation of dendrite tips in the
mushy zone.
– The dendrite fragments are carried into the
– The dendrite fragments are carried into the
bulk weld pool and act as nuclei for new
grains to form if they survive the weld pool
temperature.
62

63. Grain Detachment Mechanism
• Weld pool convection can also cause
partially melted grains to detach
themselves from the solid–liquid mixture
surrounding the weld pool
surrounding the weld pool
– These partially melted grains, if they survive
in the weld pool, can also act as nuclei for the
formation of new grains in the weld metal.
63

64. Heterogeneous Nucleation
Mechanism
• Foreign particles present in the weld pool
upon which atoms in the liquid metal
– can be arranged in a crystalline form can act
as heterogeneous nuclei.
as heterogeneous nuclei.
64

65. Surface Nucleation Mechanism
• The weld pool surface can be undercooled
thermally to induce surface nucleation by
exposure to a stream of cooling gas or by
instantaneous reduction or removal of the
heat input.
heat input.
– Solid nuclei can form at the weld pool surface.
– They grow into new grains as they shower
down from the weld pool surface due to their
higher density than the surrounding liquid
metal.
65

66. Grain Structure Control
Grain Structure Control
• The weld metal grain structure can affect its
mechanical properties significantly.
• The formation of fine equiaxed grains in the
FZ has two main advantages.
– First, fine grains help reduce the susceptibility of
the weld metal to solidification cracking during
welding.
– Second, fine grains can improve ductility and
fracture toughness in the case of steels and
stainless steels.
66

67. Techniques to control grain
structure
• Inoculation
– This technique has been used extensively in
metal casting
– It involves the addition of nucleating agents
– It involves the addition of nucleating agents
or inoculants to the liquid metal to be
solidified.
– As a result of inoculation, heterogeneous
nucleation is promoted and the liquid metal
solidifies with very fine equiaxed grains.
67

68. • External Excitation
– Weld Pool Stirring
• by electromagnetic stirring
– Stirring of the weld pool
– Stirring of the weld pool
tends to lower the weld pool
temperature,
• thus helping heterogeneous
nuclei survive.
– Arc Oscillation
• by pulsating the welding
current
electromagnetic stirring
68

69. • Stimulated Surface Nucleation
– A stream of cool argon gas was directed on the free
surface of molten metal to cause thermal
undercooling and induce surface nucleation.
• Small solidification nuclei formed at the free surface and
showered down into the bulk liquid metal.
• They grew and became small equiaxed grains
• They grew and became small equiaxed grains
• It resulted in the refinement of Al-2.5 Mg.
• Manipulation of Columnar Grains
• Manipulating the orientation of columnar grains
in Al welds by low-frequency arc oscillation led to
– columnar grains grew perpendicular to the trailing
portion of the weld pool, and the weld pool in turn
followed the path of the moving oscillating arc.
69

70. • Gravity
– High gravity produced by a centrifuge welding
system and eliminated the narrow band of
nondendritic equiaxed grains along the fusion
boundary in Al.
boundary in Al.
– buoyancy convection enhanced by high
gravity caused sweeping nuclei near the pull
boundary into the bulk pool.
• thus eliminating formation of equiaxed grains by
heterogeneous nucleation.
70

71. Solidification
Solidification Rate
Rate
• The rate at which a weld metal solidifies has
profound effect on its microstructure, properties,
and response to post-weld heat treatment.
• The solidification time, St, in seconds, is given
by:
net
H
L
S


– where L is the latent heat of fusion (J/mm3), k is the
thermal conductivity of the base material (J/m s-' K-
'), and , C, Tm, and To are as before. St, is the time
(s) elapsed from the beginning to the end of
solidification.
 2
2 o
m
net
t
T
T
kpC
H
L
S





71

72. Solidification Rate...
Solidification Rate...
• The solidification rate helps determine the
nature of the growth mode (with temperature
gradient) and the size of the grains (e.g.,
dendrite arm spacing) or the coarseness or
fineness of the microstructure
fineness of the microstructure
72