Dependence of microstructure morphology on processing in subtransus isothermal local loading forming of TA15 titanium alloy

Control of microstructure morphology in isothermal local loading of titanium alloys is important to obtaining high performance components. To this end, the effect of thermo-mechanical processing on the microstructure development of TA15 alloy during isothermal local loading was experimentally investigated. It is found that bi-modal or equiaxed microstructure can be achieved when the heating temperature of the last loading step is not lower than that of the previous loading step. The volume fraction of primary alpha phase and the morphology of transformed beta matrix are determined by heating temperature and cooling rate in the last loading step, respectively. Tri-modal structure can be achieved by near-beta forging followed by conventional forging in the last loading pass. The volume fraction of each constituent phase in tri-modal structure is determined by the heating temperatures of the last two loading steps. The emergence of secondary alpha platelets in beta phase is promoted by increasing the heating temperature or cooling rate of the last loading step. Globularization of alpha lamellae can be avoided by thickening the alpha laths or decreasing the deformation degree in the last loading pass.

► Equiaxed, bi-modal and tri-modal structures can be obtained in local loading.
► Increasing temperature or cooling rate promotes formation of secondary α in β phase.
► Heating temperatures of the last two steps determine the fraction of each phase.
► Decreasing deformation in the last step prevents globularization of α lamellae.