Trends in Weld Solidification Research

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Trends in Weld Solidification Research
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Weld pool solidification lies in between ingot
solidification and rapid solidification.
Increasing Growth Rate
Weld Pool Solidification
Ingot Solidification
Rapid Solidification
V101 ms-1
V10-4 ms-1
V102 ms-1

• Our current knowledge of weld pool solidification
  is an extrapolation of the freezing of castings,
  ingots, and single crystals at lower thermal
  gradients and slower growth rates.
• In recent years theories to understand weld pool
  solidification have been developed.

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Weld solidification microstructure is controlled
by temperature gradient (G) and crystal growth
rate (R).

• Weld pool shape, cooling rate and composition of
  the weld affect the microstructure.

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Variations in weld microstructures as a function
of temperature gradient (G), growth rate (R) and
combinations of these (G.R and G/R).

• Weld pool shape has profound effect
• No nucleation barrier.

• As in castings, solidification parameters namely
  undercooling (?T), growth rate (R), Thermal
  gradient (G) and alloy constitution influence
  weld microstructures.
• Scale of microstructural features are much finer.

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Predicted breakdown from planar to non-planar
solidification growth front can be observed.

• Analytical Modeling and Experiments
• David and Vitek
• H. W. Kerr
• S. Kou
• Cellular Automata Model
• Rappaz et al. (Castings)
• Dress et al. (Weld Solidification)

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Using metal analog system, we can observe the
evolution of solidification microstructure.

• The experiment was performed at different welding
  speeds.

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Results of the observed weld pool shapes in pure
succinonitrile

• Elongated weld pool shapes were observed.

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Dendritic microstructure was observed in alloy
system.

• The dendrite selection based on heat-flow was
  observed.

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At higher cooling rate, instability along
liquid/solid interface was observed.

• This leads to excessive growth of certain
  dendrites into the weld pool.

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Using a geometrical analysis and expressing the
solidification front normal and the
crystallographic growth directions in terms of a
fixed set of reference axes, dendrite growth
selection process can be calculated.
(001) Surface - 100 weld
(110) Surface - 111 weld

• Future Work
• Growth Competition (Vitek, David and Boatner)

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weld region consists of three distinct
dendrite-growth directions and stray crystals.

• This is due to epitaxial solidification from the
  single-crystal base metal.
• Stray crystals were associated with weld cracks.

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Modeling solute redistribution must consider
undercooling due to capillarity.
Volume Element
Solid
Liquid

• Above equations are for conventional
  solidification
• Weld pool solidification (J. A. Brooks)
• Total dendrite tip undercooling is important.
• ?T ?Td(constitutional) ?T? (capillarity)
  ?Tk (kinetics) .
• Convection effects on solute distribution also
  needs to be considered.

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Rapid solidification leads to nonequilibrium
solidification.

• Nonequilibrium solidification leads to
• Nonequilibrium partition coefficient, k.
• formation of nonequilibrium phases.
• changes in general microstructure.

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It is possible to calculate the departure from
equilibrium partition coefficient using
theoretical equations.

• Extension of these equations to multicomponent
  systems needs further work.

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Tutorials

• Evaluate the partition coefficient as a function
  of velocity for Fe-Ni-Cr-C-N alloy system during
  solidification and assume different diffusion
  coefficient for interstitial and substitutional
  atoms.
• Evaluate the conditions at which you can have
  different phase (FCC or BCC) selection in Fe-C-Mn
  welds, use T0 condition.