Muthanna Journal of Engineering and Technology
Volume (14), Issue (4), Year (2026), Pages (90-102)
DOI:10.52113/3/eng/mjet/2026-14-04-/90-102
Research Article By:
Akeel Hussein Kadhim
Corresponding author E-mail: aqeel.hussien@qu.edu.iq
ABSTRACT
Niobium causes precipitation strengthening of Alloy 718 but also causes segregation during weld solidification. This research investigates the relationship between the filler-wire Nb content of 4.75–5.50 wt.% with interdendritic second-phase formation and cracking in gas tungsten arc welds. Thermo-Calc Scheil-Gulliver calculations, optical microscopy, image analysis, and Varestraint-type testing were used to compare five experimental filler wires using fixed nominal welding parameters. Increasing the calculated operational solidification interval, which is defined between liquidus and 99% solid, grows from 106 to 119 °C with the incorporation of more filler-wire Nb. The expected Laves fraction was predicted to change from 2.1 to 4.8 vol.%, while the measured interdendritic second-phase fraction increased from 2.5 ± 0.3 to 5.3 ± 0.6 area%. At 2% nominal augmented strain, the dimensionless cracking index typing went up from 0.05 ± 0.02 to 0.32 ± 0.06. The five composition means regressions showed that cracking was strongly associated with measured or predicted phase fractions. These links are consistent with cracking assisted by segregation but do not by themselves prove the attribution. The outcomes uncovered the lower-Nb fillers as competent candidates for follow-up weldability assessments. Before a qualified filler-composition recommendation can be defined, direct dilution measurements, verified phase identification and Post weld mechanical testing.
Keywords:
Alloy 718; filler-metal niobium; Laves phase; solidification cracking; GTAW; Varestraint