Lattice structures have excellent mechanical properties such as high specific strength and high specific rigidity as well as outstanding functional characteristics such as vibration reduction, heat dissipation, sound absorption, and electromagnetic shielding. They are widely used in aerospace, biomedicine, and transportation fields. However, the materials used to form lattice structures are mostly stainless steel, Ti6Al4V and AlSi10Mg, which cannot meet the requirements of some complex intelligent components for shapes, performances and functions that can change over time or space. The NiTi shape memory alloy, a new type of smart materials, has excellent super-elasticity, shape memory effect, excellent corrosion resistance and wear resistance, and other functional characteristics. It can achieve a shape recovery through certain external stimuli after deformation, so that it meets the requirements of controllable deformation and regulable performance. Compared with NiTi bulk materials, NiTi lattice structures have low elastic modulus and density, large deformation ability, and can adjust the mechanical properties by designing the size, shape and distribution of holes. The unique performance of NiTi lattice structures makes them have a wide range of application prospects in the aerospace field. For aerospace components, weight reduction is an eternal theme, but it is not clear how to further improve the lightweight characteristics of NiTi lattice structures. In this paper, the NiTi alloy body-centered tetragonal (BCT) hollow lattice structure is proposed, which possesses the advantages of high load-bearing capacity of the BCT lattice structure and the excellent super-elasticity and shape memory effect of NiTi alloys. That is, keeping the outer diameter of the strut constant and hollowing out the inner part of the strut are used to achieve the purpose of reducing the weight of the structure.

In this paper, NiTi pre-alloyed powder is used as the raw material to prepare the BCT hollow lattice structures by laser powder bed fusion (LPBF). The surface morphologies and microstructures of the formed samples are observed by scanning electron microscope (SEM), the phase transition behavior of BCT-100 is characterized by differential scanning calorimeter (DSC), and the phase compositions of BCT-100 are determined by X-ray diffractometer (XRD). The finite element simulation method and the uniaxial compression experiments are applied to analyze the influence of mass coefficient on the compression performance of structures. Cyclic compression-thermal recovery experiments are carried out to reveal the influence mechanism of mass coefficient on the shape memory effect of NiTi lattice structures.

The lattice structures manufactured by LPBF have high forming accuracy and relative density, and no defects such as cracks and irregular large-size holes are found (Fig. 4). The lattice structure with a mass coefficient of 100% has the best bearing capacity, the first maximum compressive force can reach 191.73 kN, and the corresponding deformation rate is 0.22. When the mass coefficient of the structure is reduced to 75% from 100%, the first maximum compressive force is 89.80 kN, and the bearing capacity is reduced by 53.16%. At that time, the compression deformation capacity is not weakened, and the deformation rate is still up to 0.21 (Fig. 7). Therefore, in the non-primary load-bearing members, the struts of lattice structures can be hollowed to reduce the mass coefficient of structures by 25% while maintaining the deformation capacity of components. However, further reducing structural mass coefficient weakens the bearing and deformation capacity. The shape memory effect of the lattice structure with a mass coefficient of 75% is the best, and the recovery rate can reach 98.92% in the first cycle (Fig. 9).

The forming quality of lattice structures fabricated by LPBF is high, but there are still dimensional deviations caused by solidification shrinkage, powder sticking, and staircase effect. The M_s (Martensite transformation start temperature) and A_f (Austenite transformation finish temperature) of BCT-100 are 11.02 ℃ and 33.72 ℃, respectively. The phases of components are mainly composed of B2 and B19′ at room temperature, and the B2 phase occupies the dominant position. The appearance of B19′ phase in components is related to the phase transition of B2 phase induced by thermal stress produced by LPBF. The experimental compression force-deformation rate curves of the four structures can be divided into five stages: elastic deformation of austenite phase, stress induced transformation of austenite phase into martensite phase, elastic deformation of martensite phase, plastic deformation of martensite phase, and fracture stage. The lattice structure with a mass coefficient of 100% has the best bearing capacity, the first maximum compressive force can reach 191.73 kN, and the corresponding deformation rate is 0.22. When the mass coefficient of the structure is reduced to 75% from 100%, the first maximum compressive force is 89.80 kN, and the bearing capacity is reduced by 53.16%. At that time, the compression deformation capacity is not weakened, and the deformation rate is still up to 0.21. When the mass coefficient is further reduced to 50% (BCT-50), the first maximum compressive load and deformation capacity are reduced by 81.52% and 36.36%, respectively, that are 35.43 kN and 0.14. The shape memory effect of lattice structures formed by LPBF is good. In the first cycle, BCT-75 has the best shape memory effect and the highest recovery rate can reach 98.92%. The recoverable rates of BCT-93 and BCT-100 are slightly low, which are 97.71% and 96.77%, respectively. The shape memory effect of BCT-50 is the worst and the recovery rate is only 94.94%. In the last two cycles, all components can achieve a fully recovery.