# Presentation: 900456

## Weld Solidification

## Study of weld solidification requires detailed analysis of crystallography, macro- and micro-segregation, phase selection and nonequilibrium partitioning.

***Crystallography:***
***Epitaxial growth of grains predominant***
***Heterogeneous nucleation on inclusions do occur***
***Macro- and micro-segregation:***
***Fluid flow and cooling rate affect these characteristics***
_***There is a need to predict multicomponent partitioning & Kinetics of solidification with weld cooling.***_
***Phase selection:***
***On large undercooling austenite can outgrow ferrite***
***Nonequilibrium Partitioning:***
***At higher interface velocities, solute trapping occurs and the partitioning coefficient can approach 1.***

## Last stages of solidification, sulfur may enrich in the inter-dendritic regions.

**Solid–Liquid Boundary**

***Sulfur enrichment as the solidification front  moves through control volume.***
***Partitioning of alloying elements in multi component system (Fe,C,Si,Mn,Ni,O,Al,Ti,N & S)***
***Modified Scheil (Clyne and Kurz)***
***Diffusion controlled solidification (DicTraTM)***

## Multi-phase equilibria calculations were done as a function of solidification.

***Calculate multi-component phase equilibria for a given composition level.***
***Remove the solid and amount of inclusions formed.***
***Keep track of volume fraction of oxides and solid formed.***
***Calculate the  multi-component phase equilibria for the residual liquid and allow for diffusion in solid.***
***Continue the iteration until the liquid reaches a small value.***

**Liquid**

**Liquid**

**Liquid**

**Liquid**

**Solid**

**Solid**

**Solid**

**T****1**

**T****2**

**T****3**

**T****4**

**T****1****>T****2****>T****3****>T****4**

## Example calculation:

***Fe-C-Si-Mn-S-Al-Ti-O alloy system was considered.***
***Phases considered***
***Al******2******O******3******, SiO******2******, Si******2******O******4******.Al******6******O******9******, TiO******2******, Ti******3******O******5******,Ti******2******O******3,****** TiO, MnS, ***
*** ******MnO.SiO******2******, MnO.Al******2******O******3******, MnO, Mn******2******O******2******.SiO******2****** ***
***Austenite, Ferrite, and Liquid***
***Governing equation is:***
***The final liquid composition from the inclusion model is used for the above calculations.***

## Effect of solidification partitioning on the inclusion formation was modeled using computational thermodynamics.

***The formation of complex MnO.SiO******2****** oxides affects the formation of manganese sulfide.***

## In a self-shielded flux cored arc weld, we observe the following microstructure.

***The reasons for the formation of skeletal ************-ferrite need to be explained.***

***Weld Composition (wt.%):***
***Fe - 0.234 C - 0.50 Mn - 0.28 Si - 1.71 Al - 0.003 Ti - 0.006 O - 0.064 N (PV532) - High aluminum weld***
***Fe- 0.149 C - 0.64 Mn - 0.30 Si - 0.53 Al - 0.058 Ti - 0.030 O - 0.033 N (PV611) - Low aluminum weld***
***FCAW-S Welding Process Parameters:***
***31.0 V, 590 A, 0.120 inch wire, 225 ipm wire-speed, 98 kJ/in (High-Al)***
***19.5 V, 270 A, 0.078 inch wire, 120 ipm wire-speed, 54-58 kJ/in (Low-Al)***

## Pseudo-binary Fe-C-Si-Mn-Al phase diagrams shows the effect of aluminum on the stability of  ferrite.

***The phase diagram shows the relation between ************ ferrite stability and aluminum concentration and supports the experimental weld microstructure.***

**High-Aluminum Weld**

**Low-Aluminum Weld**

## Thermodynamic calculations do not consider the process effects.  Therefore, we need to calculate the effects of weld cooling rate.

***Diffusion controlled growth calculations were carried out for both high and low-aluminum weld metal compositions.***

## Conditions used for simulating microstructure evolution

***Dendrite arm spacing: ***
***100 μm***
***Cooling rate from 1805 K***
***1 to 10 °C/s***
***Compositions studied (wt.%):***
***Fe - 0.216 C - 0.56 Mn - 1.77 Al ***
***Fe - 0.149 C - 0.64 Mn - 0.53 Al***
***Peritectic reaction***
***Multicomponent diffusion in liquid and FCC phase***
***No diffusion in compounds and no temperature gradient within the system.***

- Liquid

- Austenite(FCC)

-  Ferrite (BCC)

## Calculations show the incomplete austenite formation from  ferrite for high-aluminum welds.

***Cooling rates control the amount of austenite that is formed from ************ ferrite.***

- 10 °C/s

- 1 °C/s

## As cooling rate increases, larger fractions of -ferrite will be retained at room temperature.

***However, the simulations do not consider large undercooling that may occur in high-energy density (EBW and LBW) welding processes.***

## Calculations show the complete austenite formation from  ferrite for low-aluminum welds.

- 1 °C/s

***Calculations show the complete austenite formation from ************ ferrite for low-aluminum welds.***

***The cooling rate just controls the temperature at which the austenite formation is completed.***

## Exercises:

***Reproduce the Fe-C-Mn-Si-(Al) quasi-binary diagram and evaluate a maximum level of aluminum that can be allowed to allow for 100% austenite formation.***
***Later evaluate Mn or Ni on this phase stability to see whether we can sustain more aluminum in the weld.***
***Reproduce the diffusion controlled solidification simulation in Fe-Al-C-Ni system.***