Abstract:The isothermal compression experiment of as-rolled Ti-55 alloy was carried out on a Gleeble-3800 thermal simulation test machine at the deformation temperature range of 700-1050 • C and strain rate range of 0.001-1 s −1 . The hot deformation behavior and the microstructure evolution were analyzed during thermal compression. The results show that the apparent activation energy Q in α + β dual-phase region and β single-phase region were calculated to be 453.00 KJ/mol and 279.88 KJ/mol, respectively. The deformation softening mechanism was mainly controlled by dynamic recrystallization of α phase and dynamic recovery of β phase. Discontinuous yielding behavior mainly occurred in β phase region, which weakened gradually with the increase of deformation temperature (>990 • C) and strain rate (0.01-1 s −1 ) in β phase region. The processing map derived from Murty's criterion was more accurate in predicting the hot workability than that derived from Prasad's criterion. The optimized hot working window was 850-975 • C/0.001-1 s −1 , in which sufficient dynamic recrystallization occurred and α + β-transus microstructure was obtained. When deformed at higher temperature (≥1000 • C), coarsened lath-shape β-transus microstructure was formed, while deformed at lower temperature (≤825 • C) and higher strain rate (≥0.1 s −1 ), the dynamic recrystallization was not sufficient, thus flow instability appeared because of shear cracking.
In order to manufacture complex curvilinear generatrix workpieces of high-temperature titanium alloy, the hot tensile behavior of Ti55 alloy sheet was tested and the hot press forming process was investigated using Finite Element Method (FEM) simulation and experiment. The hot tensile experiments of Ti55 rolled sheet were conducted at the temperatures of 800–900 °C with the strain rates of 0.001–0.1 s−1. According to the results of hot tensile tests and microstructure evolution, the proper hot press forming parameters were determined as the temperature of 850 °C and the strain rates of 0.001–0.01 s−1. The wrinkling mechanism in the transition region was analyzed and the initial blank sheet geometry was optimized by FE simulation of hot press forming. The two-step hot press forming process was better to produce the complex sheet workpiece of Ti55 alloy than the one-step hot forming scheme, which could restrain the wrinkling trend and ensure the microstructure and mechanical properties of the hot formed workpieces.
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