Effects of Yttrium Microalloying on the Epitaxial Grain Growth in Ti-6Al-4V Weld Fusion Zones

: The fusion zone microstructure in Ti-6Al-4V welds consists of long columnar grains, grown epitaxially from the fusion zone (FZ)/heat affected zone (HAZ) interface. Introducing small amounts (0.01 to 0.1 wt %) of yttrium to the fusion zone by microalloyed filler wire, results in significant grain refinement. However, initial grains still grow epitaxially, suggesting that some time dependent processes are responsible for the observed refinement away from the FZ/HAZ interface. Classical nucleation and growth theories were modified by introducing two competing time concepts; one time to nucleate one grain and the other time for epitaxial grains to grow across the total width of the bead. The theoretical model was verified experimentally by determining the nucleation time and growth rate from the microstructural observation of grain orientation with respect to welding direction and speed. Yttrium microalloying increases the solidification rate and decreases the nucleation time. The later effect is more pronounced and is responsible for the fusion zone grain refinement.