The Weld Microstructure Part 1

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The weld microstructureThe weld microstructure
Subjects of Interest
• Objectives/Introduction
• Nucleation and growth in the fusion zone
• Nucleation mechanisms and solidification modes
• Weld pool shape and grain structure
• Grain structure control
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Part I The fusion zone
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The weld microstructureThe weld microstructure
Subjects of Interest
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Part II The partially melted zone
• Formation of the partially melted zone
• Difficulties associated with the partially melted zone
Part III The heat - affected zone
• Recrystallisation and grain growth in the heat-affected zone
• Effect of welding parameters on HAZ
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ObjectivesObjectives
• This chapter provides information on the development of 
grain structure in the fusion zone, partially melted zone and 
heat affected zone. 
• This also includes the background of nucleation and grown 
of grain in the weld pool, the formation of the partially melted
zone and phase transformation of heat affected zone
• Students are required to identify the effect of welding 
parameter on the grain structure in the fusion zone, heat 
affected zone and techniques used for weld microstructure 
improvement.
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Part I: Part I: The fusion zoneThe fusion zone
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• Similar to a casting process, the microstructure in the weld 
zone is expected to significantly change due to remelting and 
solidification of metal at the temperature beyond the effective 
liquidus temperature.
• However fusion welding is much more complex due to 
physical interactions between the heat source and the base metal. 
• Nucleation and growth of the new grains occur at the surface 
of the base metal in welding rather than at the casting mould wall.
Cast structure
Fusion line
Fusion zone
Base metal
Welding structure
www.llnl.gov
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Fusion welding
Effect of welding speed on weld structureEffect of welding speed on weld structure
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GTAW of 99.96% aluminium (a) 1000 mm/min 
and (b) 250 mm/min welding speeds.
Axial grains of GTAW (a) 1100 aluminium 
at 12.7 mm/s welding speed, (b) 2014 
aluminium at 3.6/s welding speed.
1000 mm/min
250 mm/min
Axial grains
Axial grains
Weld 
direction
Columnar grains
Columnar grains
Columnar grains
Columnar grains
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Effect of heat input on weld structureEffect of heat input on weld structure
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Typical macro-
segregation of multipass
welds deposited with 
different heat inputs
0.6 kJ/mm 1.0 kJ/mm
2.2 kJ/mm 4.3 kJ/mm
Heat input
Weld bead size
HAZ size
Weld cross sections
A slight tendency for 
the elements C, Mn, Si
to decrease (in the 
composition of the 
weld) when the heat 
input increases.
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Nucleation and growth in the Nucleation and growth in the 
fusion zonefusion zone
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Nucleation theory
A crystal can nucleate from a liquid on a 
flat substrate if the energy barrier ∆∆∆∆G is 
over come, according to Turnbull’s 
equation.
)coscos32(
)(3
4 2
2
23
θθ
πγ
+−
∆∆
=∆
TH
T
G
m
mLC
where
γγγγLC is the surface energy of the liquid-crystal interface
γγγγLS is the surface energy of the liquid-substrate interface
γγγγCS is the surface energy of the crystal-substrate interface
Tm is the equilibrium melting temperature
∆∆∆∆Hm is the latent heat of melting.
∆∆∆∆T is the undercooling temperature below Tm
θθθθ is the contact angle
Note: If the liquid wets the substrate 
completely, θ θ θ θ = 0 � ∆∆∆∆G=0
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Nucleation and growth at the Nucleation and growth at the 
fusion boundaryfusion boundary
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• In fusion welding, the existing base-metal 
grains at the fusion line act as the 
substrate for nucleation.
• If the liquid metal, which is in intimate 
contact, wets the substrate grains
completely, crystals can nucleate from the 
liquid metal upon the substrate without 
difficulties.
Epitaxial growth of weld metal near 
fusion line.
Note: for FCC and BCC structures, 
columnar dendrites (or cell) grow in the 
<100> direction.
• During weld metal solidification, grains tend 
to grow perpendicular to the pool 
boundary along the maximum heat 
extraction.
Heat 
extraction 
direction
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Epitaxial growth in weldingEpitaxial growth in welding
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Epitaxial growth at the fusion boundary
Fusion boundary
Weld metal
Base metal
Easy growth direction of different alloys
• In autogenous welding, (no filler), new 
crystal nucleates by arranging atoms from 
the base metal grains without altering their 
existing crystallographic orientations.
Epitaxial growth
Crystal structure Easy growth direction Examples
FCC <100> Aluminium alloys
Austenitic stainless steels
HCP <1010> Titanium, magnesium
BCT <110> Tin
BCC <100> Carbon steels, 
ferritic stainless steels
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Grain orientations in base Grain orientations in base 
metal and fusion zonemetal and fusion zone
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[010]
[001]
[111]
0.5 mm
Fusion zone
Base 
metal
Base 
metal
HAZ HAZ
Centreline Fusion lineFusion line
Electron beam welding of beta titanium alloys
Grain orientations in (a) base metal and 
(b) fusion zone obtained from EBSD 
analysis
(a)
(b)
Random orientation
Preferred orientation
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NonNon--epitaxial growth in weldingepitaxial growth in welding
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• Non-epitaxial growth can be observed in 
welding with filler metals or welding with two 
different metals.� new grains will have to 
nucleate on the heterogeneous sites at the 
fusion boundary.
• The fusion boundary exhibits random 
misorientations between base metal grains 
and weld metal grains.
• The weld metal grains may or may not follow 
special orientation relationships with the base 
metal grains they are in contact with.
Non-epitaxial growth at the fusion 
boundary of 409 stainless steel 
(bcc) welded with Monel (70Ni-
30Cu) filler wire (fcc), (a) optical, 
(b) SEM.
Fusion boundary
Weld metal
Base metal
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Epitaxial and non epitaxial growth at the Epitaxial and non epitaxial growth at the 
fusion boundariesfusion boundaries
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Epitaxial growth from the 
fusion boundary of 
autogenous TIG welding of 
ββββ titanium alloy.
ββββ Ti base 
metal
ββββ Ti base 
metal
ββββ Ti alloy
Fusion zone
HAZ HAZ
Non-epitaxial growth from the 
fusion boundary of Ti-679 alloy 
TIG welding with ββββ titanium alloy
as filler metal.
Ti679 
base 
metal
ββββ Ti alloy
Ti679 
base 
metal
HAZ HAZ
Fusion zone
2 mm
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Solidification modesSolidification modes
• As constitutional supercooling
increases, the solidification mode 
changes from planar� cellular�
dendritic.
• The fusion zone microstructure depends on the solidification behaviour of 
the weld pool, which controls the size and shape of the grains, segregation, and 
the distribution of inclusions and porosity.
Supercooling Heterogeneous 
nucleation
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Promotes equiaxed grain formationPlanar
Cellular
Columnar 
dendritic
Equiaxed 
dendritic
Time
Size of 
dendrite
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Growth rate and temperature gradientGrowth rate and temperature gradient
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• The growth rate R is low along the fusion 
line and increases toward the centreline.
• Maximum temperature is in the centre
and then decreases toward the fusion line. 
� since the pool is elongated, temperature 
gradient G is highest at the fusion line and 
less at the centreline.
Weld microstructure varies 
noticeably from the edge to 
the centreline of the weld.
Centreline (CL)
Fusion line (FL)
Weld pool
• Since GCL < GFL, 
and RCL >> RFL
FLCL
R
G
R
G





<<





Variation of temperature gradient G and growth 
rate R along pool boundary.
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Growth rate and temperature gradientGrowth rate and temperature gradient
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• Temperature gradient G and growth rate R dominate the 
solidification microstructure.
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Variations in growth mode across weldVariations in growth mode across weld
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Solidification mode may change 
from planar to cellular, columnar 
dendritic and equiaxed dendritic
across the fusion zone.
The ratio G/R decreases from 
the fusion line toward the 
centreline.
Fusion 
line
Pool 
boundary
• Grains grow in the planar 
mode along the easy growth 
direction <100> of the base 
metal grains.
Variation in solidification mode across the 
fusion zone. Planar to cellular and cellular to 
dendritic transitions in 1100 Al welded 
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Weld metal nucleation mechanismsWeld metal nucleation mechanisms
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There are three possible nucleation 
mechanisms for new grains in welding.
• Dendrite fragmentation
• Grain detachment
• Heterogeneous nucleation
Nucleation mechanisms during 
welding (a) top view, (b) side view.
Weld pool convection causes fragmentation 
of dendrite tips in the mushy zone and then 
carried into the bulk weld pool, acting as 
nucleii for new grains.
Weld pool convection also causes partially 
melted grains to detach themselves from 
the solid-liquid mixture surrounding the 
weld pool � giving nucleii for new grains.
Foreign particles present in the weld pool 
can act as heterogeneous nuclei.
• Surface nucleation
Surface nucleation is induced by applying 
cooling gas or by instantaneous reduction 
or removal of heat input at the weld 
pool surface.
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Heterogeneous nucleationHeterogeneous nucleation
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Heterogeneous nucleation and formation 
of equiaxed grains in weld metal.
Heterogeneous nuclei in 
GTAW of 6061 Al (a) 
optical, (b) EDS analysis, 
(c ) SEM.
TiB2
particle
Ex:
1) In GTAW of aluminium, TiB2
particle is found to act as 
heterogeneous nuclei (grain 
refiner as in casting).
2) In GTAW of ferritic stainless 
steel, TiN particles act as 
heterogeneous nuclei. TiN as heterogeneous 
nuclei in ferritic 
stainless steel.
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Effect of welding parameter on Effect of welding parameter on 
heterogeneous nucleationheterogeneous nucleation
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Amount of 
equiaxed grains
Heat input
Welding speed
(a) 70Ax11V heat input and 5.1 mm/s 
welding speed, (b) 120Ax11V heat 
input and 12.7 mm/s welding speed.
Effect of welding speed and heat input on 
heterogeneous nucleation.
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Weld pool structureWeld pool structure
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S – solid dendrite
L – interdendritic liquid
PMM – partially melted material
• If the weld pool is quenched, 
its microstructures at different 
positions can be revealed, i.e., 
aluminium weld pool structure, 
see fig.
• Microstructure near the fusion 
line consists of partially melted 
materials (PMM) and mushy 
zone (MZ).
(a) Sketch of weld pool, (b) microstructure at 
position 1, (c ) microstructure at position 2.
PMM(S+L)
MZ(S+L)
PMM(S+L)
Quenched pool (L) Quenched pool (L)
Base metal (S) Base metal (S)
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Weld pool structureWeld pool structure
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• The mushy zone
behind the shaded area 
consists of solid 
dendrites (S) and 
interdendritic liquid (L).
• Partially melted 
materials (PMM) 
consists of solid grains 
(S) that are partially 
melted and intergranular 
liquid (L).
Microstructure around the weld pool boundary of aluminium alloy 
(a) phase diagram, (b) thermal cycles, (c ) microstructure of solid 
plus liquid around weld pool.
centreline
Fusion line
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Weld pool shape and grain structureWeld pool shape and grain structure
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• The weld pool becomes teardrop shaped at high welding speeds and 
elliptical at low welding speeds.
• Since the columnar grains tend to 
grow perpendicular to the weld pool 
boundary, therefore the trailing 
boundary of a teardrop shaped weld 
pool is essentially straight whereas 
that of elliptical weld pool is curved.
• Axial grains can also exist in the 
fusion zone, which initiate from the 
fusion boundary and align along the 
length of the weld, blocking the 
columnar grains growing inward 
from the fusion lines.
Note: axial grains has been 
reported in Al alloys, austenitic 
stainless steels and iridium 
alloys.
Effect of welding speed on columnar grain 
structure in weld metal.
Weld direction Top view
High speed
Low speed
Teardrop
Elliptical
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Effect of electrode diameter on weld structureEffect of electrode diameter on weld structure
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Electrode diameter
Weld bead size
HAZ size
Weld cross sections
Amount of weld bead
Increase the electrode diameter will increase the heat input and this also 
increase the cooling time. � coarse microstructure.
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Grain structure controlGrain structure control
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• Inoculation
• Arc oscillation
• Arc pulsation
• Stimulated surface nucleation
• Manipulation of columnar grains
• Gravity
• The weld structure significantly affects mechanical properties. 
Similar to casting, refining and alteration of weld grain structure
are considered to be beneficial.
• There are several techniques used;
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InoculationInoculation
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• Similar to casting, inoculants are added into 
the liquid weld metal to promote 
heterogeneous nucleation, giving very fine 
equiaxed grains. 
Effect of inoculation on grain structure in 
SAW of C-Mn steel (a) without inoculation 
(b) inoculation with titanium.
Weld metal 
structure
Weld metal 
structure
1) Titanium carbide powder and 
ferrotitanium-titanium carbide mixture
used in SAW of mild steels.
2) Titanium used in SAW of C-Mn stainless 
steels and GTAW of Al-Li-Cu alloy.
3) Ti and Zr used in aluminium welds.
4) Aluminium nitride used in Cr-Ni iron 
base alloys.
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Effects of inoculation Effects of inoculation 
on grain structureon grain structure
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Effect of grain size on weld metal 
ductility
• Refining of grain structure of the weld 
helps to improve weld metal ductility.
Effect of inoculantson grain structure in GTAW of 2090 Al-Li-Cu alloy 
(a) 2319 Al-Cu filler metal, (b) 2319 Al-Cu filler metal inoculated with 0.38% Ti.
Note: Heterogeneous nucleation in welding is 
more effective than dendritic fragmentation 
since the liquid pool and the mushy zone are 
quite small in comparison to those of casting.
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Weld pool stirringWeld pool stirring
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•Weld pool stirring can be achieved by 
applying an alternating magnetic field 
parallel to the welding electrode.
Schematic showing application of external 
magnetic field during autogenous GTAW.
• Stirring the weld pool tends to lower the 
weld pool temperature, thus help 
heterogeneous nuclei survive (in 
cooperation with inoculants addition).
Effect of electromagnetic pool stirring on 
grain structure in GTAW of 409 ferritic 
stainless steel (a) without stirring, (b) 
with stirring.
Columnar 
grains
Columnar 
grains
Fine 
equiaxed 
grains
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Arc oscillationArc oscillation
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Arc oscillation can be produced by
1) Magnetically oscillating the arc column 
using a single or multiple magnetic probe.
2) Mechanically vibrating the welding torch.
Arc oscillating
Grain refining is achieved by 
dendrite fragmentation and 
heterogeneous nucleation.
Arc vibration 
amplitude
Grain size
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Manipulation of columnar grainsManipulation of columnar grains
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(a) Transverse arc oscillation
• Orientation of columnar grains can be manipulated through low-
frequency arc oscillation (~ 1 Hz)
(b) Circular arc oscillation
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Arc pulsationArc pulsation
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Arc pulsation is obtained 
by pulsing the weld 
current (using peak and 
base current).
AC pulsed current
• The liquid metal was undercooled
when the heat input was suddenly 
reduced during the low-current 
cycle of pulsed arc welding. 
• Grain refinement is due to 
surface nucleation and/or 
heterogeneous nucleation in 
pulsed welding with the aid of grain 
refiner such as 0.04wt% Ti in 6061 
Al alloy.
Equiaxed grains in pulsed arc weld of 
6061 aluminium.
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Effect of arc oscillation and pulsation on Effect of arc oscillation and pulsation on 
weld microstructureweld microstructure
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(a) No arc pulsing or oscillation, (b) with arc pulsing, (c ) with arc 
oscillation, (d) with both arc pulsing and oscillation.
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Stimulated surface nucleationStimulated surface nucleation
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• A stream of cool argon gas is 
directed on the free surface of molten 
metal to cause thermal undercooling
and induce surface nucleation.
• Small solidification nuclei are 
formed at the free surface and 
showered down into the bulk liquid 
metal.
• These nuclei then grew and became 
small equiaxed grains.
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GravityGravity
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• GTAW of 2195 aluminium under high gravity produced by a centrifuge 
welding system and eliminated the narrow band of nondendritic equiaxed 
grains along the fusion boundary.
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