Selective laser melting of Ti-5Al-2.5Sn alloy with isotropic tensile properties: The combined effect of densification state, microstructural morphology, and crystallographic orientation characteristics

Abstract

In this study, three different levels of laser energy inputs (Ev) were used for selective laser melting of Ti-5Al-2.5Sn alloy and the as-deposited tensile properties along the vertical direction with respect to deposition layers were found to be statistically identical to those along the horizontal direction for all the samples. Densification states, microstructures, and crystallographic orientation characteristics of the different samples, as well as their contributions to the isotropic properties were researched. The results first indicated that near spherical pores would form at the relatively high Ev (208 J/mm3) whereas lack-of-fusion defects would generate at the relatively low Ev (51 J/mm3). When the moderate Ev (167 J/mm3) was used, samples with very few defects could be fabricated. Then, it was seen that samples built at 51 and 167 J/mm3 presented a martensite (α′)-dominated microstructure in which the boundaries of columnar prior-β were decorated by grain-boundary α (αGB). When Ev increased to 208 J/mm3, a duplex microstructure composed of α′ and massive α (αM) would form within prior-β matrix of the as-deposited samples. An overall increasing trend in prior-β’s width with the increase of Ev was also identified. Afterwards, it was proved that there was no strong texture within all the samples and this was one cause of the isotropic properties. Besides, for samples built at 51 J/mm3, the negative influence of lack-of-fusion on vertical properties cooperated with the adverse effect of large quantities of αGB on horizontal properties, promoting the obtainment of isotropic properties. For samples built at 167 J/mm3, not only the high densification degree but also the significantly increase in prior-β’s width and the resultant decrease in αGB’s content helped to eliminate significant anisotropy. For samples built at 208 J/mm3, both the near spherical pores which resulted in near-identical bearing areas on different sections and the absent of αGB contributed to the isotropic properties. This paper can provide some basis for the laser additive manufacturing of titanium products with isotropic properties.