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Effect of enhanced cooling on microstructure evolution of alloy 718 using the gas tungsten arc welding process

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Abstract

The microsegregation in alloy 718 fusion zone during gas tungsten arc welding affected the aging response and the mechanical properties. The average weld cooling rate was enhanced in gas tungsten arc (GTA) welding process by employing liquid nitrogen during the process. The combination of in-process and external cooling methods reduced the microsegregation of Nb in alloy 718 fusion zone. Detailed characterization of the weld microstructures in the as-welded and the direct aged conditions was conducted. The dendritic structure have been refined to a level of 10-20 μm dendrite arm spacing and Laves particle sizes ranging from 0.04 to 1.43 μm due to the enhanced weld cooling rate. Similarly, the average Laves volume fraction was reduced from 45 to 0.1 % when compared to the conventionally cooled GTA welding process. The microsegregation was quantified through the computed solidification time through the DSC analysis and the volume fraction of the eutectic phases. It was found that the solidification time was reduced significantly from 2.499 s to as low as 0.36 s with the liquid nitrogen-cooled GTA welding process. The microhardness survey revealed an improved aging response of the fusion zone.

Cryogenically cooled alloy 718 weldments

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Abbreviations

CCAR:

Constant current argon

CCPAR:

Compound current argon

CCHE:

Constant current helium

CCPHE:

Compound current helium

EBW:

Electron beam welding

AW:

As welded

DA:

Direct aged

STR:

Solidification temperature range

t s :

Solidification time

DTA:

Differential thermal analysis

DSC:

Differential scanning calorimetry

GTAW:

Gas tungsten arc welding

Act:

Actual

Nom:

Nominal

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Authors and Affiliations

  1. Indian Space Research Organization, Trivandrum, India

    SGK. Manikandan & D. Sivakumar

  2. University of Utah, Salt Lake City, UT, USA

    K. Prasad Rao

  3. Indian Institute of Technology Madras, Chennai, India

    M. Kamaraj

Authors
  1. SGK. Manikandan
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  2. D. Sivakumar
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  3. K. Prasad Rao
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  4. M. Kamaraj
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Corresponding author

Correspondence to SGK. Manikandan.

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Highlights

• Interdendritic segregation has been controlled

• Laves phase formation have been studied with liquid nitrogen cooling

• Microfissure-free HAZ is demonstrated at higher cooling rates with LN2 cooling

• Aging response with solid solution filler metal have been demonstrated

Recommended for publication by Commission IX - Behaviour of Metals Subjected to Welding

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Manikandan, S., Sivakumar, D., Rao, K.P. et al. Effect of enhanced cooling on microstructure evolution of alloy 718 using the gas tungsten arc welding process. Weld World 60, 899–914 (2016). https://doi.org/10.1007/s40194-016-0349-1

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  • DOI: https://doi.org/10.1007/s40194-016-0349-1

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