The multiaxial response of traditional and powder bed fusion with electron beam manufactured Ti-6Al-2Sn-4Zr-2Mo at low and high strain rates

This investigation presents the response of Ti-6Al-2Sn-4Zr-2Mo, manufactured using powder bed fusion with electron beam (PBF-EB), under combined tension-torsion loading. Combined loading experiments were conducted quasi statically, at a strain rate of 10-3 s-1, and dynamically at strain rates ranging between 500 s-1 and 2000 s-1. Quasi static experiments were carried out using a universal electro-mechanical testing machine. Dynamic experiments were performed using a bespoke Hopkinson Tension-Torsion bar apparatus capable of generating stress pulses characterised by different proportions of tensile and torsional loading. The dynamic response of the material was analysed across a wide spectrum of stress states, including uniaxial tension, shear, and various combinations of tension and shear. A novel specimen geometry, consisting of four equally spaced ligaments arranged circumferentially around the axis of the sample is introduced.

The quasi static and dynamic failure envelopes of PBF-EB and traditionally manufactured Ti-6Al-2Sn-4Zr-2Mo are presented in the shear versus direct stress space and compared for the first time. The ultimate stress loci of the PBF-EB processed material are approximated employing the Drucker-Prager and Mohr-Coulomb criteria to evaluate their rate sensitivity and assess the accuracy of existing failure models. The results delineate an evident influence of the strain rate and a moderate tension-compression asymmetry of the failure envelopes. The influence of manufacturing defects, stress state, and loading rate on deformation and failure mechanisms is assessed by analysing the failure surfaces of tested samples by means of scanning electron micrographs.