The main methods of energy dissipation under impact loading are plastic deformation and fracturing of dot structures prepared from conventional metallic materials, which cannot satisfy the demands of multiple reuses and reversible energy absorption. NiTi shape memory alloy dot matrix structures prepared via selective laser melting not only have excellent mechanical and functional properties but also exhibit reversible energy absorption. When impacted, the B2-phase austenite in the NiTi alloy matrix transforms into B19' martensite, which converts mechanical energy into the internal energy of the triply periodic minimal surface (TPMS) dot matrix structure and causes energy dissipation. The aim of this study is to provide design ideas and theoretical guidance for the potential application of NiTi alloy gyroid lattice structures in energy-absorbing devices by investigating the formation quality, phase-transition behavior, and mechanical properties of different types of NiTi alloy gyroid lattice structures fabricated via laser selective melting (SLM).

In this study, the mathematical models of G-sheet and G-solid lattice structures were constructed using MATLAB. Samples of these two lattice structures were successfully prepared via SLM using near-equiatomic NiTi alloy powder. Subsequently, the forming quality of the samples was systematically analyzed using scanning electron microscope (SEM), three-dimensional (3D) topography, and microfocusing X-ray computed tomography (micro-CT). Additionally, the phase-transition behavior of the samples was analyzed using X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The mechanical and energy absorption properties of the lattice structure samples were comprehensively evaluated by combining the obtained findings with the results of uniaxial compression test and numerical simulation using the Abaqus software.

The results show that the two SLM-formed gyroid lattice structures possess good forming quality, high surface smoothness, and clear metal luster. The profile of the lattice structure shows clear and regular shapes, thus indicating that the internal defects of the lattice structure are well controlled during the manufacturing process (Fig. 11). The two gyroid lattice structures show three different stages on the compression curve: elastic deformation, elastic-plastic deformation, and structural layer-by-layer failure. The compressive modulus, nominal yield strength, and ultimate yield strength of the G-sheet lattice structure are higher than those of the G-solid lattice structure (Fig. 13). This is because the bending section modulus of the G-sheet lattice structure is much smaller than that of the G-solid lattice structure. Under the same compressive load, the internal-stress coefficient of the G-sheet lattice structure is smaller than that of the G-solid lattice structure, and the G-sheet lattice structure has a greater bearing capacity (Fig. 14). Compared with the G-sheet lattice structure, the G-solid lattice structure absorbs less total energy. During the elastic deformation stage, the energy-absorption efficiencies and unit-volume energy absorptions of the two lattice structures are similar. In the elastic-plastic deformation and early layer-by-layer failure stages, the energy-absorption efficiency of the G-solid lattice structure is slightly higher than that of the G-sheet lattice structure. This is because the smaller the real-time compressive stress, the greater is the energy-absorption efficiency. When the compressive stress reaches its valley value, the energy-absorption efficiency reaches its peak value, thus indicating that the energy generated by the external stress on the sample is absorbed. The higher the degree, the greater is the energy absorbed by the gyroid lattice structure through the fracture of the support wall and support layer. The energy absorption per unit volume of the G-sheet lattice structure is much higher than that of the G-solid lattice structure in the elastic-plastic deformation and later layer-by-layer failure stages. This is because compared with the G-solid lattice structure (Fig. 16), the G-sheet lattice structure has a higher elastic modulus, thus resulting in better structural stiffness and energy-absorption performance. The fracture behavior of the two lattice structures is dominated by brittleness, and ductility exists in the local areas (Fig. 17). The stress-distribution cloud diagram obtained from finite-element simulation shows that the stress of the G-sheet lattice structure is concentrated at the junction of the inclined plane, whereas that of the G-solid lattice structure is concentrated in the middle of the inclined rod (Fig. 18).

In this study, homogeneous G-sheet and G-solid lattice structures with volume fractions of 20% were designed. The formation accuracies, phase-transformation behavior, compression performance, and stress distributions of different types of gyroid lattice structures formed via SLM were systematically investigated via experiments and simulations. The overall size of the two SLM-formed gyroid lattice structures satisfies the design requirements, whereas the lattice structure has stickier powder, thus resulting in greater surface roughness. The forming quality of the overhanging section of the G-solid lattice structure is unsatisfactory. The two SLM-formed gyroid lattice structures are composed of an austenite phase (B2 phase) and a martensite phase (B19' phase), and the phase transformation behavior shows B2$\stackrel{}{?}$B19'. The compression curves of the two SLM-formed gyroid lattice structures show three stages: elastic deformation, elastic-plastic deformation, and layer-by-layer failure. The energy-absorption efficiency of the G-solid lattice structure is higher than that of the G-sheet lattice structure in the early stages of elastic deformation, elastic-plastic deformation, and structural layer-by-layer failure. The energy-absorption efficiency of the G-solid lattice structure is lower than that of the G-sheet lattice structure in the later stage of layer-by-layer failure. In the elastic deformation stage, the unit-volume energy-absorption values of the two gyroid lattice structures are basically the same, whereas in the elastic-plastic deformation and structural layer-by-layer failure stages, the energy absorption per unit volume of the G-sheet lattice structure is much higher than that of the G-solid lattice structure. The compressive fracture behavior of the two SLM-formed gyroid lattice structures is brittle fracture. During the compression of the G-sheet lattice structure, the stress of the sample under compression load is primarily concentrated at the connection of the inclined surface, whereas during the compression of the G-solid lattice structure, the stress of the sample under compression load is primarily concentrated in the middle of the inclined rod, and the center of the inclined rod shows a higher yield trend.