The Deterministic Identification Of Deformation Mechanism In The Parameter Chaos Region And New Pattern Of Fine Crystallization Loading For TC4 Titanium Alloy

In this paper, the relationship between microstructure evolution and process parameters is studied, the fine-grain regions controlled by dynamic recrystallization are identified and obtained optimal loading parameters, which provides the theoretical basis for microstructure control of TC4 titanium alloy. First, the hot compression tests were carried on Gleeble-3500 at the temperature of 1023K?1073K?1123K?1173K?1223K?1273K and 1323 K and the strain rate of 0.01s-1?0.1s-1?1s-1 and 10s-1, and height direction reduction of 60%. Then, elevated temperature flow stress curves of TC4 titanium alloy were obtained. Based on the experimental data,the BP neural network prediction model was developed to predict the flow stress of TC4 titanium alloy, and the volume of strain-stress data can be expanded validly. According to the intensive stress-strain data, the 2-D and 3-D power dissipation maps, instability maps and processing maps were plotted, on the basis of which temperature, strain rate and strain parameters range for steady deformation have been obtained. Finally, the 3-D deformation mechanism map wad established and the fine regions controlled by dynamic recrystallization were obtained. Further, the optimal strain rate loading mode was gained. The main research contents and conclusions are as follows:(1) Isothermal compression tests were conducted at temperature range 1023-1323 K, strain rate range 0.01-10s-1, and the true stress-strain curves of TC4 titanium alloy were obtained, based on which the back-propagation artificial neural network was constructed. This model can not only accurately track non-linear experimental data, but also well predict the stress-strain data beyond experimental data. By this model, the volume of strain-stress data can be expanded validly, which provides data for obtaining accurate processing map and dynamic recrystallization model.(2) Based on the intensive stress-strain data and dynamic materials model theory, 3-D power dissipation maps, 3-D instability maps, 2-D processing map and 3-D deformation mechanism map were plotted, on the basis of which the optimal parameter range for hot forming and the deformation mechanisms were identified. The optimal parameter range of TC4 titanium alloy is temperatures of 1198-1248 K, and strain rates of 0.01-0.032s-1, and temperatures of 1223-1323 K, and strain rates of 0.032-1s-1. Moreover, the fine-grain regions were identified from deformation mechanism maps, which including the region dominated ?-phase dynamic recrystallization, the region dominated ?-phase dynamic recrystallization and ? ?? phase transformation and superplasticity region.(3) Based on the intensive stress-strain data, the critical equation of dynamic recrystallization and the kinetic model of dynamic recrystallization were established for TC4 titanium alloy.(4) Based on the parameter range in the ?-phase dynamic recrystallization region, a series of numerical simulations were conducted by utilizing the finite element simulation software Deform-2D. Through comparing the grain size under different loading conditions, the optimal strain rate loading scheme was selected.