| dc.description.abstract | Laser powder bed fusion is a promising additive manufacturing technique for the fabrication of NiTi shape memory alloy parts with complex geometries that are otherwise difficult to fabricate through traditional processing methods. The technique is particularly attractive for the biomedical applications of NiTi SMAs, such as stents, implants, and dental and surgical devices, where primarily the superelastic effect is exploited. However, few additively manufactured NiTi parts have been reported to exhibit superelasticity under tension in the as-printed condition.
Laser powder bed fusion was utilized to fabricate fully dense, near-equiatomic (Ni₅₀.₁Ti₄₉.₉), Ni-rich NiTi (Ni₅₀.₈Ti₄₉.₂ and Ni₅₁.₂Ti₄₈.₈) shape memory alloy parts which exhibited tensile ductility up to 16%, shape memory strain of 6%, and tensile superelasticity up to 6%, almost twice the maximum reported value in the literature. The selection of optimum processing parameters that yielded fully dense parts was guided by a process optimization framework based on a computationally inexpensive analytical model used to predict the melt pool dimensions based on single-track experiments. This framework allowed for constructing printability maps for the present NiTi shape memory alloys and revealed that fully dense parts could be printed over a wide range of process parameters. By controlling the process parameters, in particular laser power, laser scan speed, and volumetric energy density, in the processing space that result in fully dense parts, it was demonstrated systematically that the composition of the printed parts could be precisely changed by controlling the evaporation of Ni. The flexibility of parameter selection to print defect-free NiTi SMAs and composition control by preferential evaporation of Ni opens the possibility to print functional NiTi parts or devices without post-processing. Crystallographic texture analysis demonstrated that the as-printed NiTi parts had a strong preferential texture for superelasticity, a factor that needs to be carefully considered when complex shaped parts are to be subjected to combined loadings. Transmission electron microscopy investigations revealed the presence of nano-sized oxide particles and Ni-rich precipitates in the as-printed parts of Ni₅₀.₈Ti₄₉.₂ and Ni₅₁.₂Ti₄₈.ׅ₈, which play a role in the improved superelasticity by suppressing inelastic accommodation mechanisms for martensitic transformation. | |
