Effect of Temperature and Strain Rate on the Thermomechanical Response and Dynamic Restoration Mechanisms of Ti-6Al-4V Alloy During Forging

Abstract

The thermomechanical behavior of the Ti-6Al-4V (Ti64) alloy under hot deformation is determined by a complex interplay between strain hardening and thermally assisted restoration processes that finally lead to the formation of the resultant microstructure and mechanical properties. In this research, the effects of deformation temperature (700, 800, and 900 °C) and strain rates (low, moderate, and high) on microstructural evolution, hardness, and dislocation density of Ti64 alloy were systematically studied. Deformed samples were prepared by hot forging under specified deformation conditions, followed by characterization using optical microscopy, scanning electron microscopy (SEM), X-ray diffraction spectroscopy (XRD) analysis, and Vickers hardness (HV3) testing. The results demonstrated a gradual change from the stable α + β lamellar microstructure at 700 °C to fragmented lamellae and severe globularization at 900 °C, evidencing the increased significance of dynamic recovery (DRV), dynamic recrystallization (DRX), and globularization with increasing deformation temperature. The hardness distribution was strongly dependent on the deformation location, with the outer regions exhibiting higher hardness than the specimen centers; the maximum hardness of approximately 365 HV was obtained in the outer region at 900 °C under high deformation, whereas the center region remained comparatively softer. This spatial variation was attributed to the competition between deformation-induced dislocation accumulation and thermally activated softening. XRD analysis revealed that dislocation density greatly depends on a synergistic influence of deformation temperature and strain. The maximum dislocation density was achieved at moderate deformations at 700 °C and at high deformations at 800 °C, while a significant decrease took place at 900 °C under severe deformations, proving the dominance of thermally activated processes of recovery and recrystallization. Williamson–Hall analysis further indicated that lattice strain and microstrain did not always follow the same trend, confirming that macroscopic lattice distortion and local crystallographic strain represent distinct aspects of the deformation response. Thus, the comprehensive analysis of the microstructure, mechanical properties, and crystallographic features demonstrates that deformation temperature has a crucial influence on the strain hardening vs. dynamic softening balance. Among the investigated conditions, deformation at 900 °C with a relatively low strain rate provided the most favorable combination of lamellar fragmentation, globularization, dynamic restoration, and a comparatively homogeneous hardness response, indicating its potential for controlled thermomechanical processing of Ti64.