Nickel-titanium (NiTi) alloy is considered the most crucial shape memory alloy owing to its excellent superelasticity and shape memory effect. It can widely be used in aerospace, automobile manufacture, and biomedical fields. Compared with traditional metal materials, NiTi shape memory alloy has high damping capability owing to its martensitic transformation characteristics. However, fabricating NiTi alloy into structures with complex geometric configurations is difficult owing to its high wear resistance and superelastic properties. Nonetheless, as an emerging additive manufacturing technology, selective laser melting (SLM) has outstanding advantages in forming complex lattice structures with high geometric accuracy and surface finish. As one of the widely used structures in SLM structure design, the periodic lattice structure is often used as a buffer absorber given its light weight and high strength. Presently, most studies on forming NiTi alloy lattice structure using SLM focus on the elastic-plastic behavior under quasistatic conditions (tensile/compression). However, only few researchers have focused on the dynamic damping behavior of the lattice structure, especially the coupling between the damping characteristics of the material and the structural damping of the lattice structure. Therefore, the influence mechanism of the dynamic damping characteristics on the material-structure coupling must be investigated.
First, body-centered cubic (BCC) lattice structures with different rod diameters were modeled using the UG12.0 modeling software. BCC lattice structures with different rod diameters were fabricated using Ni50.4Ti49.6 shape memory alloy powder by SLM. Moreover, the first six orders of modalities and deformation modes of the BCC lattice structure were predicted using ANSYS finite element simulation. The first-order intrinsic frequencies and damping ratios of the BCC lattice structure with different rod diameters were obtained by shaking the table with sinusoidal sweeping experiments. The influencing factors of damping drop on the BCC lattice structure with a decreasing rod diameter were explored. The phase composition, chemical element content, number of defects, and morphology of the NiTi-BCC lattice structure at different rod diameters were analyzed through differential scanning calorimetry (DSC), oxygen-nitrogen-hydrogen analyzer, and microfocus X-ray computed tomography (Micro-CT).
Simulation and experimental results indicate that the first-order intrinsic frequency of the structure increased linearly as the rod diameter increased from 0.6 to 1.2 mm (Figs. 5 and 6). The enlargement of the rod diameter resulted in increased the volume fraction and elastic modulus of the structure with a certain rod length, thereby increasing the first-order intrinsic frequency of the structure. The decrease in the rod diameter contributed to the deterioration of the structural damping ratio from 0.020 to 0.012 (Fig. 7). To explain the decline in the structural damping ratio as the rod diameter decreases, phase transition temperature and chemical elemental analyses were conducted on samples with different rod diameters (Fig. 8, Table 1). First, the phase transition temperature gradually reduced as the rod diameter decreased. The decline of the laser scanning speed and the deterioration of the rod heat dissipation ability increased the actual laser energy input and absorption. Therefore, the effective oxidation led to the augmentation of the Ti element loss and the reduction of martensite content in the structure. The movable twin interface in the martensite phase is one of the critical damping sources of the NiTi alloy. Thus, the damping property of the SLM-NiTi alloy was reduced by the decreased phase transition temperature. Second, the porosity and number of pores in the BCC structure were characterized and analyzed by using Micro-CT. With the decrease in the rod diameter from 1.2 to 0.6 mm, the type of pores did not considerably change, but the pore number reduced dramatically (Fig. 9). On the one hand, the thermal energy dissipation resulted from the multiaxial stresses around the pores was attenuated by the decrease of pore number, which reduced the damping property. On the other hand, the interface area of NiTi matrix and pores were reduced by the decrease of pores number, thereby reducing the stress-assisted twin grain boundary motion in the martensite. Thus, the material damping was decreased. As mentioned above, the drop in material damping reduces structural damping.
The effects of rod diameter on the first-order intrinsic frequency and structural damping of NiTi alloys prepared by the SLM method were investigated using finite element analysis and experiments. Results show that the rod diameter had an essential effect on the first-order intrinsic frequency and damping of the BCC lattice structure. As the rod diameter increased, the overall stiffness of the structure increased and the first-order mode rose accordingly, which provide a basis to achieve multifrequency damping performance by controlling the rod diameter. The reduction of phase transition temperature and pore number, thereby reducing material damping, was the result of the decreased rod diameter. Therefore, the significant decrease in the structural damping of the NiTi-BCC lattice structure was attributed to the decreased rod diameter.
