# Presentation: 303344

## Trends in Weld Solidification Research 

## Weld pool solidification lies in between ingot solidification and rapid solidification.

***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.***

**Weld Pool**
**Solidification**

**Ingot**
**Solidification**

**Rapid**
**Solidification**

**Increasing Growth Rate **

**V=10****-4**** ms****-1**

**V=10****1**** ms****-1**

**V=10****2**** ms****-1**

## 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.***

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

***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.***

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

## 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)***

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

***The experiment was performed at different welding speeds. ***

## Results of the observed weld pool shapes in pure succinonitrile:

***Elongated weld pool shapes were observed.***

## Dendritic microstructure was observed in alloy system.

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

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

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

## 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.

***Future Work:***
***Growth Competition (Vitek, David and Boatner)***

**(001) Surface - [100] weld**

**(110) Surface - [111] weld**

## 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.***

## Modeling solute redistribution must consider undercooling due to capillarity.

***Above equations are for conventional solidification***
***Weld pool solidification (J. A. Brooks)***
***Total dendrite tip undercooling is important.***
*** ************T = ************T******d******(constitutional) + ************T************ (capillarity) + ************T******k ******(kinetics) .***
***Convection effects on solute distribution also needs to be considered.***

**Volume**
**Element**

**Liquid**

**Solid**

## Rapid solidification leads to nonequilibrium solidification.

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

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

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

## 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 T******0****** condition.***