Densification behavior, microstructure evolution, and mechanical performances of selective laser melted Ti-5Al-2.5Sn α titanium alloy: Effect of laser energy input

Abstract

Selective laser melting (SLM) technology was employed for the additive manufacturing of Ti-5Al-2.5Sn α titanium alloy in this work and the influences of laser energy input on densification behavior, microstructure evolution and mechanical performances of the as-deposited products were investigated. It was found that the relatively high volumetric energy densities (VEDs) of 167–417 J/mm3 resulted in large quantities of keyhole-induced spherical pores whereas the relatively low VEDs of 51–83 J/mm3 led to notable lack-of-fusion defects. At the proper VED of 125 J/mm3, highly densified products with internal porosity of merely 0.05% were obtained. At the relatively high VEDs of 167–417 J/mm3, as-built samples presented duplex microstructures composed of acicular α′ martensites and bulk αM massive grains. In contrast, overwhelmingly martensitic microstructures were formed at lower VEDs of 51–125 J/mm3. The martensitic microstructure exhibited a unique hierarchical feature, which could be explained using the theory of athermal martensitic transformation kinetics. With the decrease of VED from 417 J/mm3, the average size of α′ needles increased first, then came to its plateau at the VEDs of 167–208 J/mm3, and decreased again at the VEDs of 51–167 J/mm3. At the VEDs of 167–417 J/mm3, the formation of spherical pores and bulk αM grains resulted in relatively poor tensile properties. At the VEDs of 51–83 J/mm3, the formation of lack-of-fusion defects masked the enhanced fine-grain strengthening effect provided by the much refined α′ needles and the decrease in the fraction of bulk αM, thus also leading to very limited tensile properties. At the proper VED of 125 J/mm3, as-built tensile properties reached to their peak with ultimate tensile strength of 1173 MPa, yield strength of 1061 MPa, and elongation of 7.7%. This can be attributed to the high densification degree, the almost complete vanish of bulk αM grains, and the moderate size of α′ needles.