An experimental investigation on the influence of elevated-temperature constrained groove pressing on the microstructure, mechanical properties and hardening behaviour of Ti6Al4V alloy

Due to its high strength-to-weight ratio and remarkable resistance to corrosion, Ti6Al4V alloy finds its use in a wide range of fields such as defence, aerospace and biomedical applications. To further enhance its properties and widen its applications, this paper focuses upon the novel application of constrained groove pressing (CGP) on Ti6Al4V alloy at elevated temperatures as it is hard-to-deform at room temperature due to its HCP structure. Till date, CGP is reported mainly on FCC and BCC alloys which are relatively easier to deform as compared to HCP alloys and metals, especially at room temperature. In this work, CGP process has been successfully performed on Ti6Al4V at 750 degrees C and 550 degrees C. Mechanical and microstructural evolution in CGP processed (CGPed) material is characterized by tensile testing, microhardness testing, optical microscopy, SEM Fractography and XRD. Enhancement in mechanical properties at room temperature such as yield strength, ultimate tensile strength and microhardness is observed more for CGP at 550 degrees C as compared to CGP at 750 degrees C and as-received material. Further analysis of tensile test results revealed that CGPed specimens at 750 degrees C and 550 degrees C follow different hardening behaviour although hardening mechanism for both is governed by Voce hardening model. Optical microscopy revealed fine distribution of beta-phase along the grain boundaries in CGPed specimens. Microstructural analysis showed evolution of grain structure due to dynamic recrystallization in CGPed specimen at 750 degrees C whereas work hardening and grain fragmentation was observed for CGP at 550 degrees C. As-received (AR) and CGPed Ti6Al4V were observed to follow the Hall Petch relationship. SEM Fractography was performed on broken tensile test specimens, revealing the change in mode of failure from ductile mode for AR to mixed ductile-brittle mode for CGPed specimen at 550 degrees C. No precipitates were observed in SEM images which confirmed strain hardening effect of CGP and inherent voids as the main reason of failure. XRD analysis confirmed bulk strain hardening in CGPed Ti6Al4V. Thus, CGP at elevated temperatures may effectively be used for tailoring the microstructure and mechanical properties of Ti6Al4V alloys for extending its application domain.