Weld Metal Solidification-2-Microstructure Within Grains[1]

Weld Metal Solidification II:Weld Metal Solidification II:Microstructure within GrainsMicrostructure within Grains

Weld Metal Solidification II: Microstructure within GrainsWeld Metal Solidification II: Microstructure within Grains

SOLIDIFICATION MODESSOLIDIFICATION MODESAs As constitutional constitutional supercooling supercooling increases, the increases, the solidification solidification mode changes mode changes from planar to from planar to cellular and cellular and from cellular from cellular to dendriticto dendritic


SOLIDIFICATION MODESSOLIDIFICATION MODES

While the solidification mode can While the solidification mode can vary from one weld to another it vary from one weld to another it can also vary within a single can also vary within a single weld from the fusion line to the weld from the fusion line to the centerlinecenterline



The ratio The ratio GG//R R decreases from decreases from the fusion line the fusion line toward the toward the centerline. This centerline. This suggests that the suggests that the solidification mode solidification mode may change frommay change from

planar to cellular, columnar dendritic, and planar to cellular, columnar dendritic, and equiaxed dendritic across the fusion zone,equiaxed dendritic across the fusion zone,



Planar-to-Planar-to-cellular and cellular and cellular-to-cellular-to-dendritic dendritic transitions in transitions in 1100 Al 1100 Al weldedweldedwith 4047 with 4047 filler.filler.


DENDRITE AND CELL SPACING

The spacing between dendrite The spacing between dendrite arms or cells, just as the arms or cells, just as the solidification mode, can also solidification mode, can also vary across the fusion zone.vary across the fusion zone.



the cooling rate (the cooling rate (G G x x RR) is higher at the ) is higher at the weld centerline and lower at the fusion weld centerline and lower at the fusion line. This suggests that the dendrite line. This suggests that the dendrite arm spacing decreases from the arm spacing decreases from the fusion line to the centerline because fusion line to the centerline because the dendrite arm spacing decreases the dendrite arm spacing decreases with increasing cooling ratewith increasing cooling rate


the cooling time through the solidification the cooling time through the solidification temperature range is shorter at the weld centerline temperature range is shorter at the weld centerline (2¯4/(2¯4/VV) and longer at the fusion line (1¯3/) and longer at the fusion line (1¯3/VV).).



through the solidification temperature range through the solidification temperature range increases and the dendrite arm spacing decreases increases and the dendrite arm spacing decreases from the fusion line to the centerline.from the fusion line to the centerline.


As such, the As such, the cooling ratecooling rate


Transverse cross-section of gas–tungsten arc weld in Transverse cross-section of gas–tungsten arc weld in 6061 aluminum:6061 aluminum:((aa) ) finer microstructure near centerline;finer microstructure near centerline; ( (bb) coarser ) coarser microstructure near fusion line.microstructure near fusion line.



EFFECT OF EFFECT OF WELDING PARAMETERSWELDING PARAMETERS


EFFECT OF WELDING PARAMETERSEFFECT OF WELDING PARAMETERS

1. Solidification Mode1. Solidification Mode

The heat input and the welding The heat input and the welding speed can affect the solidification speed can affect the solidification mode of the weld metal mode of the weld metal significantlysignificantly




The solidification mode changes The solidification mode changes from planar to cellular and from planar to cellular and dendritic as the ratio dendritic as the ratio GG//R R decreasesdecreases




TABLE Effect of Welding Parameters on Weld Metal MicrostructureTABLE Effect of Welding Parameters on Weld Metal Microstructure




The higher the heat input (The higher the heat input (QQ) under the ) under the same welding speed (same welding speed (VV), the lower the ), the lower the temperature gradient temperature gradient G G and hence the and hence the lower the ratio lower the ratio G/RG/R..Therefore, at higher heat inputs Therefore, at higher heat inputs G/R G/R is is lower and dendritic solidification lower and dendritic solidification prevails, while at lower heat inputs prevails, while at lower heat inputs G/R G/R is higher and cellular solidification is higher and cellular solidification prevails.prevails.



2. Dendrite and Cell Spacing2. Dendrite and Cell Spacing

The heat input and the welding The heat input and the welding speed can also affect the spacing speed can also affect the spacing between dendrite arms and cells. between dendrite arms and cells. The dendrite arm spacing or cell The dendrite arm spacing or cell spacing decreases with increasing spacing decreases with increasing cooling ratecooling rate




The cooling rate increases with The cooling rate increases with decreasing heat input–welding speed decreasing heat input–welding speed ratio ratio QQ//VV. This ratio also represents . This ratio also represents the amount of heat per unit length of the amount of heat per unit length of the weld (J/cm or cal/cm). Therefore, the weld (J/cm or cal/cm). Therefore, the dendrite arm spacing or cell the dendrite arm spacing or cell spacing can be expected to increase spacing can be expected to increase with increasing with increasing QQ//V V or amount of heat or amount of heat per unit length of the weld.per unit length of the weld.




Effect of heat Effect of heat input per input per unit length of unit length of weld on weld on dendrite arm dendrite arm spacing, spacing,

For Al–Mg–Mn alloy.For Al–Mg–Mn alloy.




Effect of heat Effect of heat input per input per unit length of unit length of weld on weld on dendrite arm dendrite arm spacing, spacing,

For 2014 Al–Cu alloy.For 2014 Al–Cu alloy.


REFINING REFINING MICROSTRUCTURE MICROSTRUCTURE

WITHIN GRAINSWITHIN GRAINS


REFINING MICROSTRUCTURE WITHIN GRAINSREFINING MICROSTRUCTURE WITHIN GRAINS

1. Arc Oscillation1. Arc Oscillation

The dendrite arm spacing was reduced The dendrite arm spacing was reduced significantly by transverse arc oscillation at significantly by transverse arc oscillation at low frequencieslow frequencies

Microstructures near fusion line of gas-tungsten Microstructures near fusion line of gas-tungsten arc welds of 2014 aluminum:arc welds of 2014 aluminum:

TR

AN

SV

ER

SE

TR

AN

SV

ER

SE

OS

CIL

LA

TIO

NO

SC

ILL

AT

ION

NO

NO

OS

CIL

LA

TIO

NO

SC

ILL

AT

ION




This reduction in This reduction in the dendrite arm the dendrite arm spacing has spacing has contributed to the contributed to the significantsignificantimprovement in improvement in both the strength both the strength and ductility of the and ductility of the weldweld




when arc oscillation is when arc oscillation is applied, the weld poolapplied, the weld poolgains a lateral velocity, gains a lateral velocity, vv, , in addition to its original in addition to its original velocity velocity u u in the weldingin the weldingdirection (16). The direction (16). The magnitude of magnitude of v v can be can be comparable with that of comparable with that of u u depending on the depending on the amplitude and frequency amplitude and frequency of arc oscillation.of arc oscillation.




The resultant velocity The resultant velocity of the weld pool, of the weld pool, w, w, is is greater than that ofgreater than that ofthe un-oscillated weld the un-oscillated weld pool, pool, uu..




Furthermore, the Furthermore, the temperature gradient temperature gradient ahead of the solid–ahead of the solid–liquid interface, liquid interface, GG, , could also be increased could also be increased due to the smaller due to the smaller distance between the distance between the heat source and the heat source and the pool boundary.pool boundary.




Consequently, theConsequently, theproduct product GR GR or or the the cooling ratecooling rate is increased is increased significantly by the significantly by the action of arc oscillation. action of arc oscillation. This explains why the This explains why the microstructure is finer in microstructure is finer in the oscillated arc weld.the oscillated arc weld.



It is interesting to point out It is interesting to point out that the microstructure in that the microstructure in oscillated arc welds is much oscillated arc welds is much more uniform than that in more uniform than that in welds without oscillation. In welds without oscillation. In oscillated arc welds the weld oscillated arc welds the weld centerline is no longer acenterline is no longer alocation where the cooling location where the cooling rate (or rate (or GRGR) is clearly at its ) is clearly at its maximum.maximum.

TRANSVERSETRANSVERSEOSCILLATIONOSCILLATION

NO OSCILLATIONNO OSCILLATION



2. Arc Pulsation2. Arc Pulsation

In pulsed arc welds, the cell spacing varied In pulsed arc welds, the cell spacing varied periodically along the weld, larger where periodically along the weld, larger where solidification took place during the high-solidification took place during the high-current portion of the cycle. Obviously, the current portion of the cycle. Obviously, the cooling rate was significantly lower during the cooling rate was significantly lower during the high-current portion of the cycle. This is high-current portion of the cycle. This is because, the cooling rate can be expected to because, the cooling rate can be expected to decrease with increasing heat Input.decrease with increasing heat Input.


Weld Metal Solidification-2-Microstructure Within Grains[1]

Documents