Grain Growth in the Heat-Affected Zone of Ti-6Al-4V Welds: Measurements and Three Dimensional Monte Carlo Simulation

The grain size distribution in the heat affected zone (HAZ) of Gas Tungsten Arc (GTA) welded Ti-6Al-4V alloy was measured for various welding conditions. The evolution of grain structure was also calculated using a three dimensional (3D) Monte Carlo (MC) model utilizing thermal cycles computed from a well tested numerical heat transfer and fluid flow model. The computed fusion zone geometry and grain size distribution agreed well with the corresponding experimental results for various heat inputs in the range of 0.55 to 4.33 MJ/m. 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 grain size in the base plate, depending on the heat input. The extent of grain growth was strongly influenced by the heat input. At locations equidistant from the fusion plane, the grains were larger in the central vertical plane as compared to those at the top surface. This size difference originates from the local variations of thermal cycles and indicates that the results from two dimensional (2D) grain growth calculations in the weld HAZ need to be reexamined. The number of edges of grains was proportional to the average grain size for the particular edge class. Both the experimental data and the calculated results indicated that for roughly identical welding conditions, the grains in the HAZ of the Ti-6Al-4V alloy were significantly smaller than those in the commercially pure titanium.