Abstract
The effect of cooling rate and impure gas on microstructure in Ti-6Al-4V weld metals was studied. The width and length of columnar prior-β grain of weld metal definitely depended on both the cooling rate during solidification and the prior-β grain size in HAZ. The microstructures within prior-β grain strongly depended on the cooling rate below β-transus. The weld metal consisted of α', α and β phases on smaller cooling rate, and only α' phase on larger cooling rate. The microsegregations during solidification readily decreased at very early stage of the weld cooling. Consequently, the distribution profiles of solute elements in the weld metal is due to the element partition during transformation from β to α' and α on weld cooling. When some amount of nitrogen or oxygen was added through the shielding gas on GTAW process, it was found that the enrichment of Al, and Fe, and the depletion of N or O took place at dendritic boundaries during solidification. These weld metals exhibited stable lamellar dual-microstructures of α and β phases, where depleted Al, V, and Fe. enriched N or O in α-phase, vice versa in another.
