Effect of recrystallization on hot deformation mechanism of TA15 titanium alloy under uniaxial tension and biaxial gas bulging conditions

To investigate the effect of recrystallization on hot deformation mechanism of TA15 titanium alloy, rolled sheet and laser-welded tubes before and after recrystallization annealing were tested by uniaxial tensile tests and biaxial gas bulging respectively. The results show that both of the initial rolled sheet and the as-welded tube consisted of equiaxed α microstructure with high initial dislocation density. During uniaxial tensile tests, a quasi-steady flow state was achieved at 800 °C, 0.001 s−1, which demonstrated the best formability with a maximum elongation of 536%. Significant recrystallization occurred at the early stage of tensile deformation and the grain size was found to decrease first and then increase. Significant recrystallization also occurred at the early stage of the biaxial gas bulging of the as-welded tube. During the bulging of the annealed tube consisting of equiaxed α with low initial dislocation density, dynamic recrystallization (DRX) was the main microstructure evolution characteristic, but most of the DRX occurred at the middle-late stage. The relatively coarse equiaxed α and increasing fraction of low angle grain boundaries during deformation may restrict the occurrence of grain boundary sliding (GBS), which resulted in the relatively worse formability of the annealed tube. However, recrystallization which happened widely at the early stage, increased the fraction of high angle grain boundaries, refined the microstructure, promoted the occurrence of GBS and enhanced the formability of the initial rolled sheet and as-welded tube. It was also found that the stress state had no effect on the DRX mechanism for TA15 at 800 °C, which was confirmed to be discontinuous dynamic recrystallization under both uniaxial tension and biaxial gas bulging conditions.