Effects of processing defects on the dynamic tensile mechanical behavior of laser-solid-formed Ti-6Al-4 V

The initial processing defects, i.e. voids, lack-of-fusion pores and inclusions, are the inevitable and unique microstructural features in the Ti-6Al-4 V (Ti64) alloy manufactured by laser solid forming (LSF). This study was to gain insight into the influences of these processing defects on the tensile thermomechanical response of the material under various strain rates of 0.001/s, 500/s, 1000/s and 3000/s, and under a wide range of temperatures from 298 K to 1273 K. The drastic effects of processing defects on the flow stress were founded to be susceptible to tensile loading and increase with the increasing strain rate. The SEM analysis was carried out to further understand the effect of initial processing defects on the dynamic fracture behavior. These processing defects, such as voids and lack-of-fusion pores, are the most vulnerable position to release impact energy instantaneously and then evolve into the initiation site and propagation path for dynamic tensile crack. The tensile crack under high strain rate and high temperature propagates through the columnar prior-beta grain boundary a colony. It was analyzed that the typical tension-compression asymmetry in strength is positively associated with the strain rate in the Ti64 alloy by LSF. Such asymmetry is due to the differences in the deformation mechanism of processing defects and in the activated slip system under tensile and compressive loading. Additionally, the anisotropy of the mechanical property was discussed by examining the positive strain rate effects in two orientations (i.e., perpendicular and parallel to the crystal growth direction). This anisotropy is caused by different mechanical behaviors of lack-of-fusion defects and beta grain boundary a colony in different orientations.