Phase Transformation and Grain Growth in the Heat Affected Zone During Welding of Ultra Fine Grain Ti-6Al-4V

Ultra fine grain Ti-6Al-4V alloy newly developed is used extensively in aerospace applications and aircraft because of its excellent strength, toughness and corrosion resistance. In this paper welding and thermal simulation experiments with Gleeble-1500D were used to investigate the evolution of microstructure and grain growth in the coarse grain heat affected zone during gas tungsten arc welding of ultra fine grain Ti-6Al-4V. Especially grain size distributions in the coarse grain heat affected zone were measured for various heat inputs. The thermal simulation experimental data showed that the average prior- grain sizes of coarse grain zone near the fusion plane were about 108 to 227 times larger than the average grain size of the base plate, depending on the cooling rate. Rapid grain growth generated with increasing heat input. However, extreme high heat input resulted in the tendency of grain growth slowing up. It was demonstrated that the presence of larger grain boundary area of ultra fine grain Ti-6Al-4V alloy significantly impelled grain growth due to principle of free energy minimum. Furthermore, the steep temperature gradients near the fusion plane introduced α′ martensitic-type phase transformation, and small crossed α′ clusters transformed into coarse α′ beams through the whole grain in the coarse grain heat-affected zone with increasing heat input. Both the welding experimental data and the simulation results indicated that the grains in the coarse grain heat affected zone of the ultra fine grain Ti-6Al-4V alloy were significantly smaller than that in the conventional Ti-6Al-4V alloy for identical welding conditions.