04 Weld Micro Structure 01

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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

Suranaree University of Technology Sep-Dec 2007

Part I The fusion zone

Tapany Udomphol


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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

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ObjectivesObjectives

This chapter provides information on the development ofgrain 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 meltedzone 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 microstructureimprovement.

Suranaree University of Technology Sep-Dec 2007Tapany Udomphol


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Part I:Part I: The fusion zoneThe fusion zone


Similar to a casting process, the microstructure in the weldzone is expected to significantly change due to remelting and

solidification of metal at the temperature beyond the effective

liquidus temperature.

Howeverfusion welding is much more complexdue tophysical 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


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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

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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 theweld) when the heat

input increases.

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Nucleation and growth in theNucleation and growth in the

fusion zonefusion zone


Nucleation theory

A crystalcan nucleate from a liquidon aflat substrate if the energy barrierGis

over come, according to Turnbulls

equation.

)coscos32()(3

4 22

23

+

=TH

TG

m

mLC

whereLC is the surface energy of the liquid-crystal interface

LS

is the surface energy of the liquid-substrate interface

CS 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

is the contact angle

Note: If the liquid wets the substrate

completely, = 0G=0

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Nucleation and growth at theNucleation 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 metalupon the substrate withoutdifficulties.

Epitaxial growth of weld metal nearfusion line.

Note: forFCCandBCCstructures,

columnar dendrites (or cell) grow in the

direction.

During weld metal solidification, grains tend

to growperpendicular to the pool

boundaryalong the maximum heatextraction.

Heat

extractiondirection

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Grain orientations in baseGrain 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 thefusion 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 theEpitaxial and non epitaxial growth at the

fusion boundariesfusion boundaries


Epitaxial growth from the

fusion boundary of

autogenous TIG weldingof

titanium alloy.

Ti basemetal

Ti basemetal

Ti alloy

Fusion zone

HAZ HAZ

Non-epitaxial growth from the

fusion boundary of Ti-679 alloy

TIG weldingwithtitanium alloy

as filler metal.

Ti679

base

metal

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 fromplanar cellular

dendritic.

The fusion zone microstructure depends on the solidification behaviourof

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

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Growth rate and temperature gradientGrowth rate and temperature gradient


The growth rate Ris 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

gradientGis 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


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Growth rate and temperature gradientGrowth rate and temperature gradient


Temperature gradientGand growth rate Rdominate the

solidification microstructure.Tapany Udomphol


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Variations in growth mode across weldVariations in growth mode across weld


Solidification mode may change

fromplanarto cellular, columnar

dendriticand equiaxed dendritic

across the fusion zone.

The ratio G/Rdecreases from

the fusion line toward the

centreline.

Fusion

line

Pool

boundary

Grains grow in the planar

mode along the easy growth

direction of the base

metal grains.

Variation in solidification mode across the

fusion zone. Planar to cellular and cellular todendritic transitions in 1100 Al welded

with 4047 filler.Tapany Udomphol


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Weld metal nucleation mechanismsWeld metal nucleation mechanisms


There are three possible nucleationmechanisms for new grains in welding.

Dendrite fragmentation

Grain detachment

Heterogeneous nucleation

Nucleation mechanisms duringwelding (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 theweld poolgiving 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


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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.

TiB2particle

Ex:

1) In GTAW ofaluminium, TiB2particle is found to act as

heterogeneous nuclei (grain

refiner as in casting).

2) In GTAW offerritic stainless

steel, TiNparticles act as

heterogeneous nuclei. TiN as heterogeneousnuclei in ferritic

stainless steel.

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Weld pool structureWeld pool structure


S solid dendrite

L interdendritic liquidPMM partially melted material

If the weld poolis quenched,

its microstructures at different

positions can be revealed, i.e.,

aluminium weld pool structure,see fig.

Microstructure near the fusion

line consists ofpartially 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 soliddendrites (S) and

interdendritic liquid (L).

Partially melted

materials (PMM)

consists of solid grains

(S) that are partiallymelted 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


The weld pool becomes teardrop shaped at high welding speeds and

elliptical at low welding speeds.

Since the columnar grains tend to

growperpendicularto the weld poolboundary, therefore the trailing

boundary of a teardrop shaped weld

poolis essentially straightwhereas

that ofelliptical weld poolis 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 directionTop view

High 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


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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;

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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 Zrused in aluminium welds.

4) Aluminium nitride used in Cr-Ni ironbase alloys.

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Effects of inoculationEffects of inoculation

on grain structureon grain structure


Effect of grain size on weld metal

ductility

Refiningof 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 fragmentationsince 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


Weld pool stirringcan 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 helpheterogeneous nucleisurvive (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


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


(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


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-currentcycle ofpulsed arc welding.

Grain refinement is due to

surface nucleation and/or

heterogeneous nucleation inpulsed welding with the aid of grain

refiner such as 0.04wt% Tiin 6061

Al alloy.

Equiaxed grains in pulsed arc weld of

6061 aluminium.

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Effect of arc oscillation and pulsation onEffect 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.

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Stimulated surface nucleationStimulated surface nucleation


A stream ofcool argon gas is

directed on the free surface of molten

metal to cause thermal undercoolingand induce surface nucleation.

Small solidification nucleiare

formed at the free surface and

showered down into the bulk liquidmetal.

These nuclei then grew and became

small equiaxed grains.

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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

04 Weld Micro Structure 01

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