Approach to Enhance Gas Bulging Formability of TA15 Titanium Alloy Tube Based on Dynamic Recrystallization

In order to further improve the gas bulging formability of laser-welded TA15 titanium alloy tube, studies about the effects of initial microstructures on the formability were carried out at 800 °C. Vacuum annealing procedures were performed to obtain different microstructures. The formability of these TA15 titanium alloy sheets with different microstructures was subsequently investigated by hot tensile test and hot gas free bulging. The results show that after the recrystallization annealing in the two phases field, the initial dislocation density of the material was reduced greatly due to recrystallization and the grains became coarser. After the double annealing at the near β phase field, phase transformation of α to β happened, secondary α formed in the β matrix and the equiaxed α grew coarser. The strength of material at elevated temperature increased and the ductility decreased after the annealing. Under the uniaxial tensile deformation, the material consisting of equiaxed α microstructure with high initial dislocation density exhibited the best formability. During its deformation, lots of dynamic recrystallization (DRX) happened due to its high initial dislocation density at the early stage of deformation, which increased the fraction of high angle grain boundaries, refined the microstructure, promoted the occurrence of grain boundary sliding and enhanced the formability of the material. The bulging results are consistent with the tensile results. The annealed tubes deformed more slowly due to the higher strength at elevated temperature. The tube consisting of equiaxed α microstructure with high initial dislocation density also exhibited the best formability. Therefore increasing the occurrence of DRX at the early stage of deformation by increasing the initial dislocation density in the material is an efficient approach to improve the gas bulging formability.