Effects of hydrogen on the hot deformation behaviour of Ti–6Al–4V alloy: Experimental and constitutive model studies

• Hydrogen can be retained in the alloy in air when deformed at 1100 °C.
• Hydrogen addition has a positive effect on the increment of flow stress.
• The strain rate sensitivity shows an increasing trend with hydrogen.
• Hydrogen significantly affects the variables introduced in the constitutive models.

Hot deformation behaviour of Ti–6Al–4V alloy with different hydrogen contents (0, 0.35 and 0.6 wt.%) was investigated over the temperature range from 1050 to 1100 °C and strain rate range from 0.005 to 5 s−1. The effects of hydrogen on the microstructural evolution, flow stress, work hardening, strain energy density and strain rate sensitivity were systematically analysed. Constitutive models for Ti–6Al–4V alloy with different hydrogen contents were established by using a stepwise multiple-linear regression method. The results show that δ hydride with an FCC crystal structure exists in the deformed matrix of the hydrogenated specimens. The size of δ hydride is refined when the deformation temperature is raised from 1050 to 1100 °C. The work hardening rate increases with hydrogen when the strain is lower than 0.01. When the strain is higher than 0.01, however, hydrogen does not show significant effect on the work hardening rate. Both the flow stress and the strain rate sensitivity show an increasing trend with hydrogen, and the strain rate sensitivity increases gradually with strain and temperature. The selected optimum variables for the constitutive equations after stepwise multiple-linear regression are different for the Ti–6Al–4V alloy with different hydrogen contents. The calculated flow stress is in good agreement with the tested value.