A strain-path dependent unified constitutive model of titanium alloy coupling coarse grain subdivision and recrystallization: application to multi-directional hot deformation

Abstract

Primary hot working, represented by multi-directional hot forging and annealing, is a crucial step in microstructure control and plays a decisive role in the ultimate performance of ultra-high strength titanium alloy components. However, the interaction mechanisms of multiple physical processes comprising work hardening, dynamic recovery, dynamic recrystallization and grain fragmentation under complex thermo-mechanical routes are not yet well known, which greatly limits the process optimization and control of primary hot working process. In order to accurately predict the macro-micro behaviors of coarse-grained titanium alloys during multi-directional hot deformation and annealing processes, a strain-path dependent unified constitutive model was established comprehensively considering the intragranular coarse grain subdivision (ICGS) caused by ribbon and transgranular subdivided continuous dynamic recrystallization (CDRX), as well as the boundary-based coarse grain subdivision (BCGS) composed of discontinuous dynamic recrystallization (DDRX) coupled with boundary expand CDRX, and the interaction of various mechanisms under dislocation configuration. Through the combination of large deformation framework and viscoplastic theory, the influence of thermo-mechanical loading path and strain rate on grain refinement efficiency was elucidated. In the present model, the cumulative effects of loading direction changes on the degree of grain fragmentation were well identified by defining a new geometric parameter, viz. the loading axis rotation angle of the passes. The ICGS mechanism was introduced to the grain evolution model for the first time, by establishing a quatitative correlation between shear strain and the volume fraction of grain internal subdivision. In this way, the through-process precision prediction of the refinement degree of characteristic regions under multi-directional deformation paths was finally realized by combining BCGS and ICGS mechanisms, and the evolution of mechanical behaviors and internal variables in the alternating multi-directional hot deformation with heat preservation were simulated. The predictive results of the model were consistent with experiments of the titanium alloy with an average error of 4.93% and the refinement degrees of coarse-grained structures under different strain rates, temperatures and cumulative multi-directional large strains were well captured. Moreover, the applicable grain size range of the present constitutive model within a wide strain range was extended to 4 orders of magnitude (from micrometer to centimeter), and the effectiveness of the model in identifying complex multi-directional loading, multiple annealing and the heredity of internal variables during primary hot deformation were validated.