Study On The Hot Deformation Behavior And Processing Map Of TC21 Alloy

With the continuous development of aviation and spaceflight technology in current world, the demand for material properties, especially plastic property, becomes increasingly higher and the application of titanium also enters a higher stage. As one of the key material of aviation structural components, titanium alloys are gradually developing towards damage-tolerant type alloy, which require the matching of excellent strength, plasticity, fracture toughness and low crack growth rate. On the basis of Ti-6-22-22s alloy, the TC21 alloy in Ti-Al-Sn-Zr-Mo-Cr-Nb system, a new type of high damage-tolerant two-phase alloy with 1100MPa strength, is designed. The superplastic deformation of TC21 has been used extensively in aviation and spaceflight industries.In this work, a series of superpalstic compress experiments for TC21 alloy were carried out on Gleeble-1500 thermal simulating tester. Its microstructure and mechanical properties at elevated temperature compression deformation were obtained. The experimental microstructures were analyzed. At the same time, its characteristics of stress versus strain curves and the effect of strain rate, deformation temperature etc hot deformation parameters on flow stress were discussed and the m values (strain rate sensitivity exponent) and deformation activation energies were calculated and analyzed. On that basis, Arrhenius type constitutive equation was established. The data were trained and predicted by BP artificial neural network (ANN) and processing maps based on dynamic material models were established after calculating the power dissipation factor and flow instability value for ANN trained data. Above research work has important practical significance and theoretical value for the set-down of processing schedule and theoretical simulation of TC21 alloy. Specific research conclusions are reached as follows:(1) The microstructures at different temperatures and at different strain rates were compared by microstructural observation of deformed specimens by optical microscope. During deformation, the softening mechanism of TC21 alloy is dynamic recrystallization and phase transformation of aα�'β. The mechanism of dynamic recrystallization is bulging nucleation of strain induced grain boundary migration. During dynamic recrystallization, with the increase of deformation temperature, grains grow and the grain size tends to be uniform. The slower the strain rate is, the more sufficient the dynamic recrystallization proceeds. The proportion of a-phase decreases and the proportion ofβ-phase increases during phase transformation ofα�'β(2) According to the curves of true stress-strain, strain rate sensitivity exponent m and activation energy Q were calculated.It is found that m is greater than 0.3, indicating that TC21 alloy exhibits good superplasticity. The activation energy Q values are from 190KJ/mol to 345KJ/mol under different deformation temperatures and strain rates.(3) Based on Arrhenius equation, flow-stress model, which shows the relation between flow-stress and its influencing factors, was established by multivariate linear fitting. The model was as follows:lnσ＝-13.9616-0.4318lnε-0.02727(lnε)2＋0.0109(lnε)3＋0.59679 lnε+0.26118(lnε)2+0.01865(lnε)3+23.96941/T+0.97945lnε/T(4) The hot compression deformation data were trained by ANN and the power dissipation factor and flow instability value were calculated to draw processing maps based on dynamic material models. Appropriate processing region and flow instability region can be seen in the constructed processing maps, which provide sufficient theoretical basis for understanding practical behavior of TC21 alloy.