Coupling effects of interlayer holes and α-Ti variants on mechanical properties and anisotropy of additively manufactured titanium alloy

Abstract

The influence mechanism of additive manufacturing characteristic pores on mechanical properties of titanium alloys has not yet been well clarified. This study systematically investigates the influence of interlayer flat holes (IFHs) on anisotropic mechanical behaviors of a high-temperature titanium alloy fabricated via electron beam powder bed fusion (EB-PBF). IFHs have a more pronounced effect on strength and particularly ductility than spherical pores. Samples with IFHs exhibit an elongation-to-fracture (EI) only one-third that of highly dense samples in the build direction, with ultimate tensile strength (UTS) reduced by 100 MPa. IFHs also induce significant mechanical anisotropy: EI in the transverse direction reaches 8.0 %, double the value in the build direction, with UTS being 130 MPa higher. For the first time, mechanical degradation and anisotropy are attributed to a coupling effect of IFHs-induced reduction in effective bearing area, strong anisotropic stress fields, and the diversification of α-Ti variants. Under vertical loading, the anisotropic stress fields activate non-Schmid effects on slipping, promoting strain localization near IFHs through basal, prismatic, and pyramidal slips in α-variants. Consequently, an opening mode of IFHs proceeds since crack initiation at the IFHs' ends, causing premature fracture. These findings provide new insights into the role of pore defects in the anisotropic mechanical behaviors of EB-PBF-built titanium alloys.