# Presentation: 136117

## S. A. David & S. S. Babu
Metals and Ceramics Division
Oak Ridge National LaboratoryOak Ridge, TN 37831-6096

Departmental Seminar

***Metals and Ceramics Division***
***Oak Ridge National Laboratory******Oak Ridge, TN 37831-6096***
***Departmental Seminar***

***Nickel-base Superalloy Welding ***

## Outline

***Background***
***Experimental Alloys***
***Weld Cracking***
***Monitoring of weld cracks***
***Welding Process Parameters on Weld Cracking***
***Fusion Zone Grain Development***
***Microstructure Development***
***Eutectic Reactions***
***Solid State Decomposition During Weld Cooling***
***Numerical Modeling***
***Future Directions***
***Summary and Conclusions***

## Background

## Significant breakthroughs have taken place in alloy design and processing of nickel based superalloys to meet the demands on turbine airfoils which operate at high temperatures.

***These include:***
***Directionally solidified alloys and single crystals****** ***
***Single crystals for land-based turbine engines****** ***

## Economical reuse and reclamation of used and failed directionally solidified and single-crystal blades are necessary.

***Poor weldability and strength of nickel based superalloy welds have remained a critical issue.***
***Welding of single crystals: necessary to develop process and process parameters.***

**air craft engine blade**

**land based turbine engine blades**

## Experimental

## Alloys (2- 3 mm single crystal thick sheets)
PWA1480:	Ni - 5.0Al - 10.0Cr - 1.5Ti - 5.0Co - 12.0Ta - 4.0W (wt.%)
CMSX4: 	Ni - 5.5Al -   6.5Cr - 1.1Ti - 7.0Co -   7.0Ta - 6.5W (wt.%)
		(small amounts of Mo and Re)
Welding procedure
Electron Beam Welding (EB):
accelerating voltage: 100-125 kV
beam current: 7.5-10 mA, and 
welding speed: 4.2 x 10-3  to 42 x 10-3  m s- 1 
Pulsed Laser Beam(PL):
average power: 90-240 W
pulse rate: 30 - 100 s-1 
welding speed : 2.1 10-3 (low) to 12.6  10-3 ms-1 (high)
Some of the welds were made with preheat of 500 °C
Characterization
Optical, Transmission electron and Atom-Probe Field-Ion microscopy
Vision-systems crack monitoring and modeling

_***PWA1480***_***:******	******Ni - 5.0Al - 10.0Cr - 1.5Ti - 5.0Co - 12.0Ta - 4.0W (wt.%)***
_***CMSX4***_***: ******	******Ni - 5.5Al -   6.5Cr - 1.1Ti - 7.0Co -   7.0Ta - 6.5W (wt.%)***
***	******	******(small amounts of Mo and Re)***
***Welding procedure***
_***Electron Beam Welding (EB)***_***:***
***accelerating voltage: 100-125 kV***
***beam current: 7.5-10 mA, and ***
***welding speed: 4.2 x 10******-3******  to 42 x 10******-3  ******m s******- 1****** ***
_***Pulsed Laser Beam(PL)***_***:***
***average power: 90-240 W***
***pulse rate: 30 - 100 s******-1****** ***
***welding speed : 2.1 ************10******-3****** (low) to 12.6 ************ 10******-3****** ms******-1****** (high)***
***Some of the welds were made with preheat of 500 °C***
***Characterization***
***Optical, Transmission electron and Atom-Probe Field-Ion microscopy***
***Vision-systems crack monitoring and modeling***

***Experimental***

**e-beam**

**y**

**z**

**x**

## It is possible to monitor the weld cracking in real time.

***“******Vision-Systems” applies stroboscopic light to record the weld surface features.***
***The monitoring location was found out by trial and  error.***

## Vision-system: Weld crack monitoring

***Fixed or moving location can be monitored by this setup.***

## Effect of Welding and Welding Process Parameters on Weld Cracking

## Most of the PWA-1480 single-crystal electron beam welds showed cracks.

***There was no effect of weld orientation on weld cracking.***
***The weld cracking tendency was reduced by applying preheat of 500°C at low speeds.***

**Transverse section**

**Top Surface**

## Preheating reduced the cracking problem at low speeds in both EB welds.

## Structure of PWA-1480 single crystal laser welds

- Top Surface

- Welding Speed: 12.6 mm/s

- Welding Speed: 16.8 mm/s

- Welding Speed: 25.2 mm/s

## Preheating, low welding speeds, and moderate pulse rate reduced the cracking in LB welds.

## Fusion Zone Grain Development

## Using a geometric 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

## 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.***

## Geometric model agrees with the experimentally observed dendrite-growth directions.

***However, modification of the model is necessary to handle the “stray” crystal formation.***
***Stray crystal formation is attributed to constitutional supercooling.***

## Weld Microstructures

## Weld metal microstructure revealed the presence of inter-dendritic phase that has been identified as  ́ eutectic with laves phase.

## TEM revealed  ́ precipitates inside primary  dendrites and eutectic ́ along the dendritic boundaries.

***Size of ************ ́ precipitates varied from ~0.05 to 0.5 μm. ***
***Average volume percentage of ************ ́ was 75%. ***

##  Atom Probe Field Ion Microscopy

***Combination of Field Ion Microscope and Time of Flight Mass Spectrometer.***

## APFIM Data Analysis

***Successive evaporated ions are identified and recorded.***
***The variation of detected number of a particular ion with respect to total number of detected ions gives the concentration profile.***

**1**

**2**

**3**

**Cylinder of**
**Analysis**

**Boundary**

## APFIM concentration profile from PWA1480 welds shows no segregation at - ́ interface.

*** ******Not much difference in the composition of ************ and ************ ́phases compared to published results.***

## In certain cases, the conventional microstructural development can be drastically altered. For example CMSX-4 pulsed laser welds.

***Very little interdendritic eutectic ************ ́ phases were  observed***
***Fine L1******2 ******- ordered irregularly shaped  ************ ́ phases (~75%) were observed.***

## Leads to changes in calculated lattice misfit.
This necessitates further investigation.

***This necessitates further investigation.***

***Extensive concentration gradients were observed inside the ************ phase in CMSX4 welds.***

## Comparison of lattice misfit at - ́ interface in laser welds and in single-crystal base metal.

***Lattice misfit % = 100 ************  (a************ - a************ ́******)/[ (a************ + a************ ́******)/2 ]***
***a******************a************ ́******  are the lattice parameter of ************ and ************ ́ phases.***
***Lattice misfit in laser welds is positive compared to that of negative value in the single-crystal base metal.***
***will influence the microstructural evolution during PWHT and service***

## Numerical Modeling of Single-Crystal Ni-base Superalloy Welds

## Sub-solidus crack formation in a stressed weldment was observed.

***Weld cracks in M738 (~IN738) was transverse in nature.***
***The cracking is attributed to the presence of low-melting eutectics and the presence of longitudinal stress.***

**Welding**
**Direction**

## Complex stress patterns in the welds may lead to longitudinal and transverse cracks.

***The presence of mechanical driving force and weak microstructure will lead to weld cracking.***
***The above work is supported by crack monitoring results.***

***Feng, et. al. 1996***
***Uncoupled Finite Element Model***

## Ni-Al-Cr quasi-binary phase diagram suggests a eutectic reaction during final stages of weld solidification.

***Ni-Al-Cr quasi-binary phase diagram suggests a eutectic reaction during final stages of weld solidification.***

***Calculated with ThermoCalcTM software.***
***The calculations are in agreement with the experimental observations.***

**1 μm**

## Eutectic reaction in Ni-Al-Cr alloy was simulated using DicTra® software.

***Slow cooling leads to higher enrichment at liquid/************ interface.***
***Qualitatively supported by the CMSX-4 results.***
***Must extend to multicomponent phase equilibrium calculations.***

**10 °C / s**

**100 °C / s**

## Microstructure development during continuous cooling

## In this work, the phase transformations and microstructure development were investigated under controlled cooling conditions.

***CM247DS alloy was investigated.***
***Ni - 5.5% Al -   8.0% Cr - 0.8% Ti - 9.0% Co -   3.2% Ta - 9.5% W (wt.%)***
***	***
***Samples were continuously cooled from the solutionizing temperature of 1300°C (5min) at different rates in a Gleeble® thermomechanical simulator.***
***Transmission Electron Microscopy and Atom Probe Field Ion Microscopy were used for microstructure characterization.***
***Thermodynamic and Diffusion Controlled Growth Calculations were used to describe the microstructure evolution during welding.***

## Transformation strains measured with dilatometry showed complex behavior.

***No established method to interpret this data without prior knowledge of thermal expansion and specific volume of the precipitating phases.***

## Microstructure development during slow cooling was similar to that of base metal microstructure.

***Micrographs are taken from same zone axis [001].***
***Coarsening stage was reached in the sample cooled at 0.17°C/s.***
***Samples cooled at 1 •C/s showed some secondary ************ ́ precipitates.***

## Number density of  ́ precipitates increased with an increase in cooling rate from 1300°C.

***Change is more dramatic on water-quenching the sample.***
***The change in ************ ́ shape from cuboidal to irregular with increase in cooling rate was observed.***

## Chromium atom image obtained from atom probe tomography revealed both  matrix and  ́ precipitates. 

***The above data was further analyzed to evaluate the morphology of ************ ́ precipitates and elemental partitioning characteristics.***

****

****** ́**

## Interconnected  ́ and distinct  ́ precipitates were observed based on 15 at.% Cr surfaces.

***Thin ************ regions in between the interconnected ************ ́ were not enriched in chromium and the reasons for this are unclear.***

## Is it model the above precipitation characteristics using computational thermodynamics and kinetics modeling?

***Atom probe tomography from samples cooled at 75 °C/s failed to show any interconnected ************ ́ precipitate morphology***

**Cr**

**Al**

## Thermodynamic calculations allow us to predict the formation of  ́precipitates.

***ThermoCalc*****TM***** with Thermotech Ni-Fe*****TM***** data base was used.***
***To consider the kinetic effects, diffusion controlled growth calculations were used.***

- Equilibrium Diagram

## Diffusion controlled growth calculations predict the kinetics of  ́ formation during cooling at 500°C/s from 1300°C.

***The decomposition of ************ occurs rapidly below 1090°C.***
***Calculations are sensitive to inter-particle spacing and therefore need to be modified for different conditions.***

## Simulated concentration profiles are compared with the measured APFIM concentration profiles.

***The magnitude of the calculated concentration gradients are different than the measured values.***

## Experimental and modeling results are necessary to design welding process parameters and post weld heat treatment conditions.

***Nonequilibrium composition may lead to redistribution during heat treatment and/or service.***
***Partitioning characteristics control the lattice-misfit between ************ and ************ ́ precipitates.***
***Lattice-misfit controls the creep resistance properties during service.***

## Future Directions

***Ultimately, a generalized process model should be based on ******a coupled thermomechanical, and microstructural models.***

## Summary and Conclusions

***Weld cracking in nickel based superalloys were reduced by preheating and low welding speed.***
***Fusion zone structure  development can be predicted using geometric models.***
***Stray crystals were found to be associated with weld cracking and the formation of stray crystals  was related to constitutional supercooling.***
***Eutectic ************ ́ was observed along dendritic boundary.***
***Rapid weld cooling leads to large concentration gradients within the  ************ phase.***
***Numerical modeling needs to incorporate coupled thermomechanical, and metallurgical models.***