Deformation behaviour of Ti-25Al-10Nb-3V-1Mo (Super α2) during isothermal forging

Samples of plasma-melted Ti-25Al-10Nb-3V-1Mo (Super α2) have been isothermally upset forged in the β-phase field. It has been found that Super α2 has good hot workability and no cracks have been found on billet surfaces even when reduced in height by up to 70%. The maximum flow stress during forging varied from 33 MPa to 80 MPa over the range of conditions used. The forging pressure required for isothermal forging is strongly dependent on forging temperature and strain rate. The experimental results have shown that isothermal forging can be applied satisfactorily to deform Super α2 billets within temperature and strain rate ranges of 1120–1180 °C and 0.005 s−0.5 s−1 to more than 70% height reduction. A strain rate sensitivity exponent (m) of 0.2 and a deformation activation energy of 286 kJ mol−1 are obtained for the forging conditions used and it is considered that the deformation mechanism of Super α2 isothermally forged in the β phase field is dynamic recovery and recrystallization together with a diffusion-dependent process. An empirical forging flow equation has been developed which is valid over the range of temperatures and strain rates investigated. The equation acurately describes the temperature and strain rate dependence of the forging peak flow stress.