Correlating multi-pass hydrostatic extrusion characteristics with microstructure and mechanical properties of titanium grade 4

Abstract In this work, influence of the variant of the Ti grade 4 plastic deformation with multi-pass hydrostatic extrusion (HE) on its microstructure and mechanical properties was studied. The emphasis was placed on the correlation between the extrusion pressure curves and the microstructure of produced rods after subsequent extrusion passes. The qualitative and quantitative microstructure characterization was performed with SEM/EBSD and orientation imaging in TEM. The mechanical properties were assessed in static tensile test. The results revealed that dynamic recrystallization during three-pass HE allowed to obtain bimodal microstructure consisting of elongated grains and a fraction of ultrafine defect-free equiaxed grains facilitating the plastic deformation. Adding a fourth extrusion pass produced highly deformed microstructure, as the temperature was insufficient to activate recrystallization processes. In this way, the highest mechanical strengthening was obtained inhibiting the extrusion process represented by nonuniform pressure curves. Application of recrystallization annealing to sustain uniform and stable plastic deformation represented by smooth extrusion curves, enabling to obtain relatively high plasticity of the final product. It has been demonstrated that optimization of extrusion path by selection of cross-sectional reduction or application of recrystallization annealing leads to obtaining either large mechanical strengthening or increased plasticity, depending of intended application.