3.1 Phase structure
As shown in Fig. 3a, the DSC graph shows the Ms is 29.28 ℃, Mf is -9.66 ℃ upon cooling, and the As is 27.81 ℃, Af is 63.41 ℃ upon heating. And Fig. 3b shows the XRD pattern of the SLM-NiTi, which demonstrates the sample remains B2 austenite state and B19’ martensite state at room temperature (20℃), mainly B2 austenite state. The results consistent with the DSC curve, which proves that the printed material is in B2 austenite state at room temperature with a small amount of B19’ martensite phase.
The tensile stress-strain curve of SLM-NiTi is shown in Fig. 3C. The tensile yield strain of the sample is 1.13%, and the yield strength is 193.1 MPa. The fracture strain is 10.89% and the fracture strength is 735.18 MPa. It can be seen from the figure that a typical stress platform appears after the yield point due to the martensite orientation deformation during the tensile process. The tensile process of the sample is divided into four stages, which are the initial elastic deformation (stage I), the stress platform caused by martensite orientation deformation (stage II), the elastic deformation of directional martensite (stage III), and the elastic-plastic deformation of directional martensite (stage IV).
3.2 Deformation behaviour
Another key measurable output from the finite element simulation and experimental work is the deformation mode of the two structures. Figure 4 shows the deformation process of the quasi-static compressive loading of the CS and DLS. In the early stage of pressure under the rigid loading plate, the unit structure hardly showed obvious deformation due to the strength of NiTi shape memory alloy. Because the CS is placed in the middle of the experiment bed, it is equivalent to restricting the z-direction displacement of the bottom of the bracket only. The energy is absorbed by the structure in a controllable way through regular deformation patterns of the two structures. The deformation modes observed in the experiment are compared with those calculated by finite element simulation.
3.3 Study on mechanical properties and shape memory recovery characteristics
As shown in Fig. 5, equivalent von Mises stress and deformed configurations obtained from FE analysis for two structures. Figure 6a and Fig. 6c respectively shows the FDT curves obtained from the loading-unloading-heating process of the two structures in the simulation analysis and experiment. The two curves can be divided into three parts: loading, unloading and deformation recovery. Firstly, in the loading and unloading part of the curve, the numerical simulation and the experimental data curve have a high consistency. As can be seen in the force displacement curve is given, the compression process can be divided into two stages, the first stage, the bearing capacity of the two structures increased with the increase of the compression displacement of relationship, a kind of approximate linear increase in overall is a kind of elastic stage, Stress concentration occurs at the upper part of the CS and the middle joint of the DLS (Fig. 5). When the load reaches the value of Pmax (point B and point B1 of Fig. 6), it enters the second stage, and local buckling instability occurs from the upper connection of the CS, resulting in plastic deformation. With the increase of compression displacement, the buckling of the structure gradually diffused to the whole leg. However, before the loading displacement was about 1.1mm (point B1), the deformation of the DLS was mainly dominated by the elastic deformation of the lower layer bracket. The deformation of the structure increased with the increase of the compression displacement, and the load presented a linear increasing trend. Later, due to excessive structural deformation, the lower support appears buckling deformation, and the supporting force of the lower support is dominated by the bending moment of the support (Fig. 5). As the bending Angle of the support increases, the supporting force provided by the support becomes smaller, so the load curve shows a downward trend.
In Fig. 6b, with the reduction of load, the shape of the CS gradually recovered, and elastic recovery rate reached about 84%, This is mainly due to the presence of the B2 austenite phase at room temperature, which gives the material part of the superelasticity, so that it has a certain shape recovery after unloading. It is found that the consistency of the simulation results with the experimental results is close to 90%. The results show that the simulation analysis technique can better simulate the deformation process of the NiTi memory alloy double-layer composite module in the process of static crushing, unloading and spring back, to better predict its bearing capacity. After unloading, the elastic recovery rate of the sample reaches 84%, the sample was put into a beaker with hot water (~ 100℃), the SLM-NiTi sample exhibits shape recovery rate of 99%. The reason for the shape recovery after heating is that part of the B19' martensite has undergone heat-induced martensite transformation. It can be inferred that although the total deformation is 70%, the deformation in some areas is much lower than 70%, and it may have only undergone martensitic transformation redirection without plastic deformation. Therefore, the deformed part can be restored to its original shape after heating.
In Fig. 5 and Fig. 6d, the compression process and experimental results of the DLS were further analyzed. In the process of pressing the rigid plate, the deformation first appears in the lower bracket, the edge of the lower bracket opens to the surrounding, while the upper bracket does not show obvious deformation, indicating that the strength of the upper bracket is obviously greater than that of the lower bracket. When pressed down for 3mm, the lap positions of the upper and lower brackets become brittle, resulting in structural instability and the end of the experiment. In this scheme, the support has a thickness of 0.6mm, a width of 1.5mm and a thickness to width ratio of 2:5. In the future, the thickness to width ratio will be reduced, and the support will be designed to be a structure similar to a thin plate as possible to give full play to its hyperelasticity characteristics and avoid premature brittle fracture of the 3D printed structure. Also, compression loading should be withdrawn after the downforce displacement reaches 3mm. In a word, the deformation order of the structure is basically bottom-up. Therefore, in order to improve the bearing capacity of the whole structure or control the deformation sequence of the structure, a support with the lower strength should be chosen. In this study, the quasi-static compressive loading simulation analysis was carried out on the two-layer module, and the whole deformation process of the module was simulated during the loading stage, providing a basis for optimizing nickel titanium alloy in multivariable and multiparameter modules in the future.
There is a slight difference between the simulation analysis and the experiment during loading. Before the loading displacement was about 1.1mm (point B), the deformation of the double-layer structure was mainly dominated by the elastic deformation of the lower layer bracket. The deformation of the structure increased with the increase of the compression displacement, and the load presented a linear increasing trend. Later, due to excessive structural deformation, the lower support appears buckling deformation, and the supporting force of the lower support is dominated by the bending moment of the support. As the bending Angle of the support increases, the supporting force provided by the support becomes smaller, so the load curve shows a downward trend. The final state after unloading is similar to the CS. As shown in Fig. 6a and Fig. 6c, When the structure produces shape memory recovery under the action of thermal excitation, the structure under the deformation state exhibits different recovery efficiency at different temperatures. In experiment, we observed that the DLS rapidly recovered its shape within a few seconds after being stimulated by heat and achieved a recovery rate of about 98%. Therefore, it can be concluded that the DLS has excellent reusable performance.
3.4. Performance indicators
Different parameters are used to compare the performance of various devices. The indicators used within this study include Peak Force (Pmax); Energy Absorption (EA); Specific Energy Absorption (SEA); and shape recovery rate.
The peak force is the force required to initiate plastic deformation within the tube and hence begin the energy absorption. Total energy absorption through plastic deformation is calculated as the area under the force-displacement curve, using Eq. (1):
where d is the total structural volume, P is static force, x is the instantaneous crush displacement.
Given that mass is a key indicator in any automotive structural design, specific energy absorption provides an indicator of the EA per unit mass, as presented in Eq. (2). A high EA and SEA are highly desirable within crashworthy applications.
where M is the structural mass.
3.5. Post-compression analysis
The experimental percentage crush and corresponding reaction force were generated from the experiment machine. The recorded Performance indicator are listed in Table 3, including the Pmax, EA, SEA, M, Compressible rate and shape recovery rate.
From Table. 3, it may be observed that DLS has excellent mechanical properties, but its compressible deformation is small. On the contrary, although CS has weak mechanical properties, its compressible deformation can reach 66.67%, and its shape recovery rate can reach 99% after thermal excitation.