Measurements and Monte Carlo simulation of grain growth in the heat-affected zone of Ti-6Al-4V welds

Grain size and topological class distributions in the heat-affected zone (HAZ) of gas tungsten arc welded Ti-6Al-4V alloy were measured for various heat inputs in the range of 0.55-4.33 MJm^-1. The evolution of grain structure and topological class distributions were also calculated using a three-dimensional Monte Carlo model utilizing thermal cycles computed from a well tested numerical heat transfer and fluid flow model. Both the experimental data and the calculated results showed that the average prior-β grain size near the fusion plane was about four to twelve times larger than the average grain size in the base plate, depending on the heat input. At locations equidistant from the fusion plane, the grains were larger in the mid-section vertical symmetry plane as compared to those at the top surface due to local variations of the thermal cycles. The normalized grain size distributions were unaffected by the local differences in the thermal cycles. It is demonstrated that the presence of a spatial gradient of temperature in the HAZ significantly impeded grain growth due to thermal pinning effect. Furthermore, the steep temperature gradients near the fusion plane did not introduce any significant texture in the grains. Both the experimental data and the calculated results indicated that the grains in the HAZ of the Ti-6Al-4V alloy were significantly smaller than the grains in the commercially pure titanium for identical welding conditions.