In functionally graded materials (FGMs) fabricated using directed energy deposition (DED) additive manufacturing (AM), cracks may form due to interdendritic stress during solidification, the formation of deleterious phases, or the buildup of residual stresses. This study builds on our previously proposed concept of FGM feasibility diagrams to identify gradient pathways that avoid deleterious phases in FGMs by also considering hot cracking. Here, five hot cracking criteria were integrated into the feasibility diagrams, and equilibrium simulations were carried out based on Scheil results (termed hybrid Scheil-equilibrium simulation) to predict phase formation below the solidus temperature considering solidification micro-segregation. The new feasibility diagrams were applied to four previously studied FGMs, and the newly proposed approach predicted high crack susceptibility, detrimental phase formation, or interdendritic BCC phase formation in the experimentally observed cracking region. This demonstrates the utility of the proposed framework for crack prediction in the design of future FGMs gradient pathways.
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Effect of nitrogen on solidification cracking resistance in ERNiCr-3 weld metal
Additions of nitrogen were made to ERNiCr-3 (FM 82)
weld metal to investigate the effect on weld metal microstructure and solidification cracking susceptibility.
Weld samples were prepared with different argon-nitrogen
shielding gas mixtures to produce nitrogen variations in
the weld metal. The cast pin tear test (CPTT) was then
used to evaluate solidification cracking susceptibility as a
function of weld metal nitrogen content. Phase fraction
and composition of constituents in the final solidification
microstructure were characterized via optical and electron
microscopy. Thermodynamic (Scheil) calculations were
performed to determine the phase formation during solidification, associated solidification path, and solidification
temperature range. Solidification cracking susceptibility
was found to increase significantly with increasing nitrogen content. The overall amount of second phase in the
solidified microstructure increased when nitrogen was
added to the weld metal. Small skeletal constituents in
the interdendritic regions, primarily Nb-rich carbides (NbC),
were more frequently observed with increasing weld metal
nitrogen content. Larger cuboidal Ti-rich nitrides (TiN) and
carbonitrides (Ti, Nb)(N, C) were found only when nitrogen
was added to the weld metal. Their location in dendrite
core regions indicates formation during an earlier stage of
the solidification process. Scheil calculations confirmed
the strong effect of nitrogen additions on the amount and
sequence of phase formation during solidification of ERNiCr-3 weld metal. High nitrogen levels ( 100 ppm) facilitate
primary nitride formation prior to the solidification of the
gamma () dendrites, and increase the amount of phase
constituents in the solidification microstructure. The presence of nitrogen also shifts the start of the eutectic
/NbC formation at the end of solidification to lower temperatures, which results in an increase in the solidification
temperature range. This occurs at much lower nitrogen levels ( 25 ppm) and correlates with the observed increase
in solidification cracking susceptibility.
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- Award ID(s):
- 1822144
- NSF-PAR ID:
- 10106163
- Date Published:
- Journal Name:
- Welding journal
- Volume:
- 97
- Issue:
- 8
- ISSN:
- 0372-7203
- Page Range / eLocation ID:
- 3411-3418
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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