Modeling And Simulation Of Microstructure For Hot Gas Bulging Forming Of TA15 Titanium Alloy Based On Globularisation Mechanism

With the wide application of titanium alloy in aerospace field, the demand of its thin wall monolithic components is becoming increasingly urgent. However, the internal high pressure forming process is difficult to meet the requirement of formability for its forming temperature is low, and the traditional superplastic forming process greatly limits the production and application of titanium parts, for its high forming temperature, low forming efficiency, and high demand for grain size. To solve this forming problem, while improving production efficiency, hot gas bulging forming process is proposed. This process uses an induction coil to rapidly heat the titanium material, then, with the high-pressure gas as transmission medium, the material can be rapidly formed at high temperature in the tools. There are many advantages of the titanium components formed using this process, such as small springback, high forming precision, good overall performance, and high reliability. Therefore, the hot gas bulging forming process of titanium alloy is becoming the forefront subject of forming researches of titanium alloy.TA15 is one of the most widely used titanium alloys in aerospace field. In this field, high security performance requirements and special working conditions require the TA15 components to be with excellent mechanical properties. TA15 is a duplex polycrystalline material, the diversity of its microstructures leads to its performance uncertainty. Therefore, the researches of the coordinated deformation mechanism between the two phases and the polycrystalline structures, and the microstructure evolution mechanism, have great scientific and engineering significances for the fully use of the advantages of the material, and the improvement of mechanical properties and formability. In this paper, the microstructures of TA15 under different heat treatment methods were systematically researched, especially the thermal deformation mechanism and the microstructure evolution mechanism of the duplex microstructure after double annealing. Basing on these researches, a unified viscoplastic constitutive model, which can predict microstructure evolution, was established. With the combination of secondary development technology of finite element (FE) software, the process of hot gas bulging and microstructure evolution of TA15 tube can be predicted. The researches of the microstructures of TA15 under different heat treatment methods were conducted using heat treatment experiments. The microstructures, mechanical properties and tensile fracture morphology of TA15 were analyzed in the general annealing, recrystallization annealing and double annealing. The influences of different heat treatment methods on the volume fractions, composition and morphology of the phases were quantitative researched, and good mechanical properties of TA15 sheets can be obtained under vacuum double annealing (950℃/2h/AC+600℃/2h/AC).The hot deformation behavior of TA15 under double annealing was analyzed using high temperature tensile tests. The thermal deformation mechanism was revealed through the calculation of hot deformation activation energy and stress exponent. The analysis of true stress-strain curves of TA15 showed that under high temperature tensile deformation, there are interactions and competitions between work hardening, dynamic softening, and plastic damage. The research of influence of deformation conditions on peak stress, fracture strain and rheological softening provided experimental basis and theoretical guidance for the selection of process parameters of hot gas bulging process.Using microscopic detection techniques, such as scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM), the hot deformation microstructure evolution mechanism of TA15 was analyzed, and the influence of forming conditions on the percentage of flaky α-phase globularisation was quantitatively researched. Crystal morphology and distribution of grain boundaries-subgrain boundaries microstructures under different forming conditions were analyzed through the observation of the microstructure, and influence of deformation conditions on crystal morphology and microstructure distribution was researched. Which revealed that the hot deformation microstructure evolution mechanism of TA15 with duplex microstructure is jointly decided by dislocation migration mechanism, and the mechanism of globularisation of flaky α-phase.Considering the microstructure evolution mechanism, evolution of dislocation density, globularisation law of flaky α-phase, and the coordination deformation characteristics of the two phases, a hot deformation constitutive model of TA15 was established basing on the unified viscoplastic constitutive model. By solving the material constants of the model using genetic algorithm optimization, the prediction of true stress strain curves and microstructure evolution of TA15 was achieved. The model was used to quantitatively describe the intrinsic relationship between microstructures and deformation process parameters (e.g. stress, strain rate, deformation temperature), and can be used to reflect the essential hot deformation rules of TA15.Subroutine VUMAT of the unified viscoplastic constitutive model basing on the globularisation mechanism was programed, which used the stress update algorithm. FE analysis of high temperature tensile was conducted using the VUMAT subroutine. And the subroutine was verified by experimental data to verify the calculated true stress strain curves and the curve of globularisation of flaky alpha phase, which was obtained through FE analysis. Subroutine was used to simulate the hot gas bulging forming of TA15. Stress field, strain field, thickness distribution, and degree of globularisation, were analyzed during the forming process to explore the influence of process parameters on formability and microstructure. Finally, through the hot gas bulging experiments of TA15, forecast results of model about thickness distribution and degree of globularisation were verified. The FE simulations and the related verification experiments proved the accuracy and effectiveness of the unified viscoplastic constitutive model, which was based on the globularisation mechanism, about the hot gas bulging forming and microstructure evolution, indicating that the model can be applied to numerically simulate the forming process and microstructure evolution of titanium components using hot gas bulging process.Using experimental method and numerical simulation method in this paper, the high temperature deformation behavior and microstructure evolution mechanism of the two-phase poly crystalline material TA15 was revealed. The establishment of constitutive model and the numerical simulation of microstructure evolution under the hot gas bulging forming were the focus researches. There is a great scientific and engineering significance for the theoretical researches, and manufactures of TA15 components using hot gas bulging process.