Microstructural mechanisms during hot working of commercial grade Ti–6Al–4V with lamellar starting structure

Abstract

The hot deformation behavior of commercial grade Ti–6Al–4V with a lamellar starting microstructure is studied in the temperature range 750–1100°C and strain rate range 3×10⁻⁴–10 s⁻¹ with a view to model the microstructural evolution. On the basis of flow stress data obtained as a function of temperature and strain rate in compression, a processing map for hot working has been developed. In the ranges 800–975°C and 3×10⁻⁴–10⁻² s⁻¹, globularization of lamellae occurs for which an apparent activation energy of 455 kJ mol⁻¹ has been estimated using the kinetic rate equation. Stress-dependent thermal activation analyses proposed by Schöck and Cocks et al. have shown that the apparent activation energies are in the range 160-245 kJ mol⁻¹ and the normalized activation volumes are in the range 20-80, which suggest that cross-slip is the rate controlling process during globularization. The variation of primary α grain size with Zener-Hollomon parameter (Z) in the globularization region exhibited a linear relationship on a log–log scale. At strain rates slower than 10⁻¹ s⁻¹ and temperatures below 900°C, cracking at the prior β grain boundaries/triple junctions occurs, which sets the lower limits for globularization. At strain rates higher than 10⁻¹ s⁻¹ in the α+β range, the material exhibited flow instabilities manifested as adiabatic shear bands. These bands are intense below 800°C and above 1 s⁻¹ and caused cracking along the bands. In the β phase field, dynamic recrystallization (DRX) occurs at about 1100°C and in the strain rate range 10⁻³–10⁻¹ s⁻¹. The apparent activation energy for DRX of β is about 172 kJ mol⁻¹ which is close to that for self-diffusion in β phase (153 kJ mol⁻¹). The application of these results in the design of bulk metalworking processes for achieving microstructural control is discussed.