Production of Large Scale Billets with Submicrocrystalline Structure out of Ti-6Al-4V Alloy Using the 3D Isothermal Forging

摘要

The effect of deformation temperature between 450 and 800°C at the strain rate of 10~(-3)s~(-1) on microstructure evolution of Ti-6Al-4V alloy was studied. It was shown that at the temperatures below 700°C the Submicrocrystalline structure (SMC) formed. An influence of preform microstructure on formation of SMC structure was investigated. Martensitic type of structure allows to obtain the most homogeneous microstructure. To produce large-scale billets with SMC structure compressive tests of samples successively in three orthogonal directions at 550°C and 10~(-3) s~(-1) were conducted. It was obtained the cumulative true stress — true strain curve that was characterized by a peak flow stress on initial stage of deformation followed by flow softening and then steady-state flow caused by Superplasticity. The alloy microstructure after 3D-deformation is characterized by average grain size of 0.4 μm in the center and inhomogeneity in periphery of sample. To improve uniformity of plastic flow a modeling of forging was made. Large billets (150-mm diameter x 200-mm length) with a homogeneous SMC structure by 3D isothermal forging were produced. The refined grain size in this material led to a substantial increase in the strength of the material without a loss in ductility.