Deformation And Microstructure Evolution In Whole Process Of Radial-axial Ring Rolling Of TA15 Titanium Alloy

Radial-axial ring rolling is an advanced technology for manufacturing high-quality titanium alloy rings, and has wide application in many fields such as aviation and aerospace. In order to satisfy the high requirements of service performance, the titanium alloy rings should possess high-precision dimensions and qualified microstructure. Thus it is important to realize the integrated forming of shape and performance for titanium alloy rings. However, radial-axial ring rolling of titanium alloy is dynamic and complex process under the effect of multi-die, multi-parameter, multi-field and time-varying, and this process usually includes complicate temperature and deformation histories such as heating, transferring, rolling and cooling. So the shape and microstructure control is a challenge for manufacturing titanium alloy rings. To this end, a systematic and thorough investigation on the forming and microstructure evolution in the radial-axial ring rolling of TA15 alloy has been carried out using finite element（FE） simulation combined with theoretical analysis and experimental study. A brief introduction to the work and its main achievements obtained are as follows.The rule of microstructure evolution in the whole process of hot deformation of TA15 titanium alloy has been investigated with the assistances of experiment. And the microstructure models are established for each process. In the cooling process, the growth rate of primary α phase increases with cooling rate. However, in the same temperature range, lower cooling rate can lead to more growth time, and the final volume fraction and grain size of primary α phase are larger. In the heating process, the dissolution rate of primary α phase increases with heating rate. However, in the same temperature range, lower heating rate can lead to more dissolution time, and the final volume fraction and grain size of primary α phase are smaller. In the compressing deformation process, the grain refinement of primary α phase enhances with the increase of strain rate. The deforming temperature determines the volume fraction of primary α phase, but the deformation has little influence on volume fraction of primary α phase.The correlation relations among the motions of guide rolls, mandrel and axial rolls and the current outer diameter, height and thickness of the ring are determined. Then the motion control models of the rolls are established. Under the ABAQUS software, the motion control models are integrated into the solver via developing subroutine VUAMP, and the outer diameter, height and thickness of the ring are real-timely measured in process by setting virtual sensors, meanwhile the microstructure evolution models of TA15 alloy at the heating, transferring, rolling and cooling stages are integrated into the solver via developing subroutine USDFLD and VUSDFLD. Thus, the macro-micro and through-process FE simulation of radial-axial ring rolling of titanium alloy based on the feedback control for the motions of rolls is realized. Then the developed FE model is validated by comparing the deformation and microstructure with experiment.The effects of the rolling ratio Ψ and ratio k of axial to radial deformation amount of blank on the deformation and microstructure evolution of TA15 alloy rings have been investigated thoroughly. It is found that the ring roundness error is larger when Ψ being large meanwhile k being small, while the ring roundness is better under other conditions. With the increase of Ψ, both the height and thickness errors increase, the uniformity of primary α grain size becomes worse, while the uniformity of volume fraction of primary α phase gets better first and then worse. With the increase of k, the height error decreases while the thickness error increases. When Ψ being large, with k increasing, the uniformities of both grain size and volume fraction of primary α phase become better first and then worse. When Ψ being small, with k increasing, the uniformity of primary α grain size becomes worse, while the uniformity of volume fraction of primary α phase gets better. Comprehensively considering the roundness, cross-section quality and microstructure of the ring, the preferable blanks in the defined scope of present work are suggested to be: Ψ being about 1.7 and k being 0.3-0.7.Based on the preferable blank, the effects of blank heating temperature T₀, growth rate of ring diameter V_D and rolling velocity V₁ on the deformation and microstructure evolution of TA15 alloy rings have been investigated thoroughly. It is found that the ring roundness and height errors are small under different forming conditions. The thickness error becomes smaller with the decrease of V_D as well as the increase of V₁. T₀ determines the final volume fraction of primary α phase. The uniformities of both grain size and volume fraction of primary α phase get better as the increase of V_D. With the increase of V₁, the uniformity of volume fraction of primary α phase gets better, while the uniformity of primary α grain size becomes worse. Comprehensively considering the roundness, cross-section quality and microstructure of the ring, the preferable forming conditions of the radial-axial ring rolling of TA15 alloy is proposed to be that: T₀ being about 25 ℃below β-transus temperature, and V_D and V₁ should be increased as much as possible if the ring stability can keep better and the roll forces do not exceed the force limit of the mill. In the defined scope of present work, the preferable V_D and V₁ are 40mm/s and 1.38m/s, respectively.