The weld microstructure - Suranaree University of …eng.sut.ac.th/metal/images/stories/pdf/04_Weld...

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

Suranaree University of Technology Sep-Dec 2007

Part I The fusion zone

Tapany Udomphol

The weld microstructureThe weld microstructureSubjects of Interest


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

Tapany Udomphol

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.

Suranaree University of Technology Sep-Dec 2007Tapany Udomphol

Part I: Part I: The fusion zoneThe fusion zone


• 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

Tapany Udomphol

Fusion welding

Effect of welding speed on weld structureEffect of welding speed on weld structure


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

Tapany Udomphol

Effect of heat input on weld structureEffect of heat input on weld structure


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 sectionsA 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


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

θθπγ

+−∆∆

=∆TH

TG

m

mLC

whereγγγγLC 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 Tmθθθθ is the contact angle

Note: If the liquid wets the substrate

completely, θ θ θ θ = 0 � ∆∆∆∆G=0

Tapany Udomphol

Nucleation and growth at the Nucleation and growth at the fusion boundaryfusion boundary


• 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

Tapany Udomphol

Epitaxial growth in weldingEpitaxial growth in welding


Epitaxial growth at the fusion boundary

Fusion boundaryWeld 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


[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


• 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

Tapany Udomphol

Epitaxial and non epitaxial growth at the Epitaxial and non epitaxial growth at the fusion boundariesfusion boundaries


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 alloyas filler metal.

Ti679

base

metal

ββββ Ti alloy

Ti679

base

metal

HAZ HAZ

Fusion zone

2 mm

Tapany Udomphol

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


Promotes equiaxed grain formation

Planar

Cellular

Columnar

dendritic

Equiaxed

dendritic

Time

Size of

dendrite

Tapany Udomphol

Growth rate and temperature gradientGrowth rate and temperature gradient


• 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

FLCLR

G

R

G

<<

Variation of temperature gradient G and growth

rate R along pool boundary.

Tapany Udomphol

Growth rate and temperature gradientGrowth rate and temperature gradient


• 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


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

with 4047 filler.Tapany Udomphol

Weld metal nucleation mechanismsWeld metal nucleation mechanisms


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 nucleationSurface nucleation is induced by applying

cooling gas or by instantaneous reduction

or removal of heat input at the weld

pool surface.Tapany Udomphol

Heterogeneous nucleationHeterogeneous nucleation


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, TiB2particle 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.

Tapany Udomphol

Effect of welding parameter on Effect of welding parameter on heterogeneous nucleationheterogeneous nucleation


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.

Tapany Udomphol

Weld pool structureWeld pool structure


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


• 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

Tapany Udomphol

Weld pool shape and grain structureWeld pool shape and grain structure


• 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 viewHigh speed

Low speed

Teardrop

Elliptical

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Effect of electrode diameter on weld structureEffect of electrode diameter on weld structure


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.

Tapany Udomphol

Grain structure controlGrain structure control


• 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;

Tapany Udomphol

InoculationInoculation


• 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.

Tapany Udomphol

Effects of inoculation Effects of inoculation on grain structureon grain structure


Effect of grain size on weld metal

ductility

• Refining of grain structure of the weld

helps to improve weld metal ductility.

Effect of inoculants on 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.

Tapany Udomphol

Weld pool stirringWeld pool stirring


•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

Tapany Udomphol

Arc oscillationArc oscillation


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

amplitudeGrain size

Tapany Udomphol

Manipulation of columnar grainsManipulation of columnar grains


(a) Transverse arc oscillation

• Orientation of columnar grains can be manipulated through low-

frequency arc oscillation (~ 1 Hz)

(b) Circular arc oscillation

Tapany Udomphol

Arc pulsationArc pulsation


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


(a) No arc pulsing or oscillation, (b) with arc pulsing, (c ) with arc

oscillation, (d) with both arc pulsing and oscillation.

Tapany Udomphol

Stimulated surface nucleationStimulated surface nucleation


• 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.

Tapany Udomphol

GravityGravity


• 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.

Tapany Udomphol

The weld microstructure - Suranaree University of …eng.sut.ac.th/metal/images/stories/pdf/04_Weld...

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