Scale levels of quasi-static and dynamic fracture behavior of Ti-6Al-4V parts built by various additive manufacturing methods

Abstract

Comparison of the microstructure and the phase composition of the Ti-6Al-4V alloy parts built by various additive manufacturing technologies was carried out by optical and transmission scanning electron microscopy, as well as X-ray diffraction analysis. It was shown that the martensitic αʹ phase, formed due to the melt pool fast cooling rate, determined the accommodation mechanisms of shear deformation of the as-built Ti-6Al-4V AM fabricated samples under uniaxial quasi-static tension and impact bending (and, accordingly, their mechanical properties). The effect of microstructure on impact toughness, ultimate impact strength, as well as the crack initiation and propagation energy was investigated by the impact bending tests with recording impact load diagram (force–displacement graph). It was concluded that impact toughness of the Ti-6Al-4V samples built by wire-feed electron beam additive manufacturing was significantly higher than that of ones manufactured by selective laser melting and electron beam melting of a powder. Matching the mechanical properties and SEM micrographs of the fracture surfaces of the as-built Ti-6Al-4V AM fabricated samples enabled to reveal the crack initiation and propagation mechanisms during their quasi-static and dynamic loading. Discussion of the results was carried out using the concept of scale levels of plastic deformation.