Experimental study and numerical simulation of α lamellar globularization for TC21 titanium alloy during multidirectional forging

Evolution of microstructure and the globularization mechanism of alpha lamellar for TC21 titanium alloy during multi-directional forging (MDF) were analyzed with the help of OM, SEM, EDS and TEM techniques, FE simulation was also used to observe the above process. The main conclusions are as follows: With the increase of temperature, forging cycle and single pass strain, the fraction of globularization of alpha lamellar gradually improved. The volume fraction of globularization of alpha lamellar achieves a high value about 85%, the average grain size of TC21 titanium alloy can be refined to 2 mu m through MDFed at 910 degrees C with strain of 0.69 in 3 cycles. Forging cycle is the most key factor to obtain the ideal microstructure. Increasing temperature and MDFed pass can not only promote the fraction of DRX (dynamic recrystallization), but also achieve uniformity of deformation. The error between simulation and experiment is below 14%. DRX is the major globularization mechanism of the alpha lamellar during MDF. In the early stage of MDF process, the globularization mechanism is boundary splitting accompanied by CDRX (continuous dynamic recrystallization) and DDRX (discontinuous dynamic recrystallization), which is formed by the beta phase into alpha lamella along the sub-grain boundaries. In the later stage of the process, the globularization mechanism is termination migration, which is caused by elements diffusion due to variant curvature of different position of alpha lamellar, while a slow process for globularization.