Weld metal solidification-1- Grain structure

Weld Metal Solidification I:Weld Metal Solidification I:Grain StructureGrain Structure

Weld Metal Solidification I: Grain StructureWeld Metal Solidification I: Grain Structure

EPITAXIAL GROWTH AT FUSION BOUNDARYEPITAXIAL GROWTH AT FUSION BOUNDARY

Nucleation TheoryNucleation Theory

In fusion welding the existing base-metal In fusion welding the existing base-metal grains at the fusion line act as the grains at the fusion line act as the substrate for nucleation. Since the liquid substrate for nucleation. Since the liquid metal of the weld pool is in intimatemetal of the weld pool is in intimatecontact with these substrate grains and contact with these substrate grains and wets them completely, crystalswets them completely, crystalsnucleate from the liquid metal upon the nucleate from the liquid metal upon the substrate grains without difficulties.substrate grains without difficulties.




When welding without a filler metal, When welding without a filler metal, nucleation occurs by arranging atoms nucleation occurs by arranging atoms from the liquid metal upon the from the liquid metal upon the substrate grains without altering their substrate grains without altering their existing crystallographic orientations. existing crystallographic orientations. altering their existing crystallographic altering their existing crystallographic orientations.orientations.




Such a growth Such a growth initiationinitiationprocess, is called process, is called epitaxial growth epitaxial growth or epitaxial or epitaxial nucleation. nucleation.


NONEPITAXIAL GROWTH AT FUSION BOUNDARYNONEPITAXIAL GROWTH AT FUSION BOUNDARY

When welding with a filler metal When welding with a filler metal (or joining two different materials), (or joining two different materials), the weld metal composition is the weld metal composition is different from the base metal different from the base metal composition, and the weld metal composition, and the weld metal crystal structure can differ from crystal structure can differ from the base metal crystal structure. the base metal crystal structure.


NONEPITAXIAL GROWTH AT FUSION BOUNDARYNONEPITAXIAL GROWTH AT FUSION BOUNDARY

When this occurs, epitaxial growth is no longer possible and new grains will have to nucleate at the fusion boundary.


COMPETITIVE GROWTH IN BULK FUSION ZONECOMPETITIVE GROWTH IN BULK FUSION ZONE

The grain structure near the fusion The grain structure near the fusion line of a weld is dominated either line of a weld is dominated either by epitaxial growth when the base by epitaxial growth when the base metal and the weld metal have the metal and the weld metal have the same crystal structure or by same crystal structure or by nucleation of new grains when nucleation of new grains when they have different crystal they have different crystal structuresstructures



Away from the Away from the fusion line, fusion line, however, the however, the grain structure grain structure is dominated is dominated by a different by a different mechanism mechanism known as known as competitive competitive growthgrowth..



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


EFFECT OF WELDING EFFECT OF WELDING PARAMETERS ON PARAMETERS ON GRAIN STRUCTUREGRAIN STRUCTURE


EFFECT OF WELDING PARAMETERS ON GRAIN STRUCTUREEFFECT OF WELDING PARAMETERS ON GRAIN STRUCTURE

The weld The weld pool pool becomes becomes teardropteardrop shaped shaped at high at high welding speedswelding speeds



The weld The weld

pool becomes pool becomes ellipticalelliptical at at low welding speedslow welding speeds



Since the trailing pool boundary of a Since the trailing pool boundary of a teardrop-shaped weld pool is essentially teardrop-shaped weld pool is essentially straight, the columnar grains are also straight, the columnar grains are also essentially straight in order to grow essentially straight in order to grow perpendicular to the pool boundary.perpendicular to the pool boundary.



On the other hand, since the trailingOn the other hand, since the trailingboundary of an elliptical weld pool is boundary of an elliptical weld pool is curved, the columnar grains are also curved, the columnar grains are also curved in order to grow perpendicular to curved in order to grow perpendicular to the pool boundarythe pool boundary



Gas–tungsten arc welds of 99.96% aluminumGas–tungsten arc welds of 99.96% aluminum

(a(a) 1000 mm/min ) 1000 mm/min welding speedwelding speed

((bb) 250 mm/min ) 250 mm/min welding speedwelding speed



Axial grains can also exist in the Axial grains can also exist in the fusion zone. Axial grains can initiate fusion zone. Axial grains can initiate from the fusion boundary at the from the fusion boundary at the starting point of the weld and continue starting point of the weld and continue along the length of the weld, blocking along the length of the weld, blocking the columnar grains growing inward the columnar grains growing inward from the fusion lines. Like other from the fusion lines. Like other columnar grains, these axial grains columnar grains, these axial grains also tend to grow perpendicular to the also tend to grow perpendicular to the weld pool boundary.weld pool boundary.


With a teardrop-shaped pool, onlyWith a teardrop-shaped pool, onlya short section of the trailing pool a short section of the trailing pool boundary can be perpendicular to the boundary can be perpendicular to the axial direction, and the region of axial axial direction, and the region of axial grains is thus rather narrowgrains is thus rather narrow



With an ellipticalWith an ellipticalweld pool, weld pool, however,however,a significantly a significantly longer section of longer section of the trailing pool the trailing pool boundary can be perpendicular to the boundary can be perpendicular to the axial direction, and the region of axial axial direction, and the region of axial grains can thus be significantly wider,grains can thus be significantly wider,




Axial grains in GTAWAxial grains in GTAW

(a)(a)1100 aluminum at 1100 aluminum at 12.7 mm/s12.7 mm/s welding speed. welding speed.

bb) 2014 aluminum at ) 2014 aluminum at 3.6 mm/s3.6 mm/s welding speed. welding speed.


WELD METAL WELD METAL NUCLEATION NUCLEATION MECHANISMSMECHANISMS


WELD METAL NUCLEATION MECHANISMSWELD METAL NUCLEATION MECHANISMS

Microstructure around the weld pool boundary: (Microstructure around the weld pool boundary: (aa) ) phase diagram; (phase diagram; (bb) thermal cycles; () thermal cycles; (cc) microstructure ) microstructure of solid plus liquid around weld pool.of solid plus liquid around weld pool.

the mushy the mushy zone behind zone behind the trailing the trailing portion of the portion of the pool boundary pool boundary consists of consists of solid dendritessolid dendrites(S) and the (S) and the interdendritic interdendritic liquid (L).liquid (L).




The partially melted The partially melted material around the material around the leading portion of the leading portion of the pool boundary, on pool boundary, on the other hand, the other hand, consists of solid consists of solid grains (S) that are grains (S) that are partially melted and partially melted and the intergranular the intergranular liquid (L).liquid (L).




In summary,In summary, there is a there is a region of region of solid–liquid solid–liquid mixture mixture surrounding surrounding the weld pool the weld pool of an alloy.of an alloy.



three possible three possible mechanisms for mechanisms for new grains to new grains to nucleatenucleateduring welding: during welding: dendrite dendrite fragmentationfragmentation, , grain detachmentgrain detachment, , and and heterogeneousheterogeneousnucleationnucleation



The fourth nucleation mechanism, surfaceThe fourth nucleation mechanism, surfacenucleation.nucleation.



Mechanism 1: Dendrite FragmentationMechanism 1: Dendrite Fragmentation

Weld pool Weld pool convection convection can in can in principle principle cause cause fragmentation fragmentation of dendriteof dendritetips in the tips in the mushy zonemushy zone



Mechanism 1: Dendrite FragmentationMechanism 1: Dendrite Fragmentation

These dendrite These dendrite fragmentsfragmentsare carried into are carried into the bulk weld the bulk weld pool and act as pool and act as nuclei for new nuclei for new grains tograins toform if they form if they survive the weld survive the weld pool temperature.pool temperature.



Mechanism 2: Grain DetachmentMechanism 2: Grain Detachment

Weld pool Weld pool convection can convection can also cause also cause partially melted partially melted grains to detach grains to detach themselvesthemselvesfrom the solid–from the solid–liquid mixture liquid mixture surrounding the surrounding the weld pool,weld pool,



Mechanism 2: Grain DetachmentMechanism 2: Grain Detachment

Like dendrite Like dendrite fragments, these fragments, these partially melted partially melted grains, if theygrains, if theysurvive in the survive in the weld pool, can weld pool, can act as nuclei for act as nuclei for the formation of the formation of new grains innew grains inthe weld metal.the weld metal.



Mechanism 3: Mechanism 3: Heterogeneous NucleationHeterogeneous Nucleation

Foreign particles Foreign particles present in the present in the weld pool upon weld pool upon which atoms in which atoms in the liquid metal the liquid metal can be arranged can be arranged in a crystalline in a crystalline form can act as form can act as heterogeneous heterogeneous nuclei.nuclei.



Heterogeneous nucleation and formation Heterogeneous nucleation and formation of equiaxed grains in weld metal.of equiaxed grains in weld metal.

Mechanism 3: Mechanism 3: Heterogeneous NucleationHeterogeneous Nucleation



The weld pool The weld pool surface can be surface can be undercooled undercooled thermally to induce thermally to induce surface nucleationsurface nucleation

by exposure to a stream of cooling gas or by exposure to a stream of cooling gas or by instantaneous reduction orby instantaneous reduction orremoval of the heat input.removal of the heat input.

Mechanism 4: Mechanism 4: Surface NucleationSurface Nucleation



When this occurs, solid nuclei can form at When this occurs, solid nuclei can form at the weld pool surfacethe weld pool surface

Mechanism 4: Surface NucleationMechanism 4: Surface Nucleation



These solid nuclei These solid nuclei then grow intothen grow intonew grains as new grains as they shower they shower down from the down from the weld pool surface weld pool surface due to theirdue to their

Mechanism 4: Surface NucleationMechanism 4: Surface Nucleation

higher density than the surrounding higher density than the surrounding liquid metal.liquid metal.



Effect of Welding Parameters Effect of Welding Parameters on Heterogeneous Nucleationon Heterogeneous Nucleation



Effect of Welding Parameters on Heterogeneous NucleationEffect of Welding Parameters on Heterogeneous Nucleation

The formation of equaixed The formation of equaixed grains is enhanced by higher grains is enhanced by higher heat inputs and welding heat inputs and welding speeds.speeds.


WELD METAL NUCLEATION MECHANISMSWELD METAL NUCLEATION MECHANISMSEffect of Welding Parameters on Heterogeneous NucleationEffect of Welding Parameters on Heterogeneous Nucleation

Equiaxed grains can form a band along the Equiaxed grains can form a band along the centerline of the weld and block off columnar grains centerline of the weld and block off columnar grains as the heat input and welding speed are increasedas the heat input and welding speed are increased

((aa) 70A - 11V heat input and ) 70A - 11V heat input and 5.1 mm/s welding speed5.1 mm/s welding speed

((bb) 120A - 11V heat input and ) 120A - 11V heat input and 12.7 mm/s welding speed.12.7 mm/s welding speed.



The heat input and the welding speed areThe heat input and the welding speed areincreased, the temperature gradient (increased, the temperature gradient (GG) at the end of ) at the end of the weld pool is reduced.the weld pool is reduced.



Furthermore, as the welding speed is Furthermore, as the welding speed is increased, the solidification rate of the increased, the solidification rate of the weld metal (weld metal (RR) is also increased) is also increased

The ratio [The ratio [G / R G / R ] ] should be decreased and should be decreased and the constitutional supercooling in front of the constitutional supercooling in front of the advancing solid–liquid interface should, the advancing solid–liquid interface should, therefore, be increased.therefore, be increased.



((aa) Low constitutional ) Low constitutional supercooling at low welding supercooling at low welding speed and heat input;speed and heat input;

((bb) heterogeneous) heterogeneousnucleation aided by high nucleation aided by high

constitutional supercooling constitutional supercooling at high welding speed and at high welding speed and

heatheat


GRAIN GRAIN STRUCTURE STRUCTURE

CONTROLCONTROL


GRAIN STRUCTURE CONTROLGRAIN STRUCTURE CONTROL

The weld metal grain structure The weld metal grain structure can affect its mechanical can affect its mechanical properties significantly.properties significantly.



The formation of fine equiaxed grains The formation of fine equiaxed grains in the fusion zone has two main in the fusion zone has two main advantages. advantages. FirstFirst, fine grains help , fine grains help reduce the susceptibility of the weld reduce the susceptibility of the weld metal to solidification cracking during metal to solidification cracking during welding. welding. SecondSecond, fine grains can , fine grains can improve the mechanical properties of improve the mechanical properties of the weld, such as the ductility and the weld, such as the ductility and fracture toughness in the case of fracture toughness in the case of steels and stainless steels.steels and stainless steels.



SecondSecond, , fine grains fine grains can improve the can improve the mechanical mechanical properties of the properties of the weld, such as the weld, such as the ductility and ductility and fracture toughness fracture toughness in the case of steels in the case of steels and stainless and stainless steels.steels.



Therefore, much effort has Therefore, much effort has been made to try to grain been made to try to grain refine the weld fusion zonerefine the weld fusion zone



1. Inoculation1. Inoculation

It involves the addition of It involves the addition of nucleating agents or inoculants to nucleating agents or inoculants to the liquid metal to be solidified. As the liquid metal to be solidified. As a result of inoculation, a result of inoculation, heterogeneous nucleation is heterogeneous nucleation is promoted and the liquid metal promoted and the liquid metal solidifies with very fine equiaxed solidifies with very fine equiaxed grains.grains.



2. External Excitation2. External Excitation

Different dynamic grain refining Different dynamic grain refining techniques, including techniques, including - Weld pool stirring, Weld pool stirring, - Arc oscillation, and Arc oscillation, and - Arc pulsation,Arc pulsation, have been applied to fusion have been applied to fusion welding.welding.



2. External Excitation: 2. External Excitation: A. Weld Pool StirringA. Weld Pool Stirring

Weld pool stirring Weld pool stirring can be achieved by can be achieved by electromagnetic electromagnetic stirring, by stirring, by applying an applying an alternating alternating magnetic field magnetic field parallel to the parallel to the welding electrodewelding electrode



2. External Excitation: 2. External Excitation: B. Arc OscillationB. Arc Oscillation

Arc oscillation, on Arc oscillation, on the other hand, can the other hand, can be produced bybe produced bymagnetically magnetically oscillating the arc oscillating the arc column using a column using a single- or multipole single- or multipole magnetic probe or by magnetic probe or by mechanically mechanically vibrating the welding vibrating the welding torch.torch.



2. External Excitation: 2. External Excitation: C. Arc Pulsation

Arc pulsation can be obtained by pulsating Arc pulsation can be obtained by pulsating the welding current. The liquid metal was the welding current. The liquid metal was undercooled when the heat input was undercooled when the heat input was suddenly reduced during the low-current suddenly reduced during the low-current cycle of pulsed arc welding. This caused cycle of pulsed arc welding. This caused surface nucleation and resulted in grainsurface nucleation and resulted in grainrefinement.refinement.

Weld metal solidification-1- Grain structure

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