Small-pored NiTi thermomechanical working medium and 4D printing method thereof

Abstract

This application discloses a small porous NiTi elastic-thermal working fluid and its 4D printing method. It employs laser powder bed melting technology, utilizes the surface response method to establish a laser powder bed melting process window, designs a porous topology with a high specific surface area, and divides the porous topology into phase transition and non-phase transition domains. Within the process window, a high energy density parameter is used for the phase transition domain, and the scanning speed is synergistically controlled; a low energy density parameter is used for selective melting of the non-phase transition domain. The formed component undergoes direct aging treatment to obtain the porous NiTi elastic-thermal working fluid. The printing method of this application is simple, easy to implement, and can conveniently prepare functionally customized small porous NiTi elastic-thermal working fluids, supporting the development of miniaturized high-performance elastic-thermal cooling systems.

Description

Technical Field

[0001] This invention relates to the field of elastothermal cooling component development, and more particularly to a small porous NiTi elastothermal working fluid and its 4D printing method. Background Technology

[0002] To achieve the goal of reducing carbon emissions, developing next-generation green and efficient refrigeration technologies has become an important direction. Elastic-thermal refrigeration technology, which uses mechanically induced solid-state phase transitions to achieve heat absorption and release, is widely regarded as the next-generation green refrigeration route due to the absence of liquid refrigerant leakage risk, the lack of need for fluorine- or hydrocarbon-based refrigerants, and its large temperature difference and high energy efficiency. Among these technologies, superelastic NiTi alloys are the preferred material due to their high latent heat of phase transition and comprehensive mechanical advantages such as high strength and low brittleness.

[0003] However, existing NiTi-based elastomeric devices mostly employ simple geometric shapes such as rods, tubes, and wires. To improve heat transfer capacity, they are typically combined using methods such as parallel connection of multiple tubes or wire winding. This results in a large prototype size, making it difficult to meet the needs of space-constrained applications such as medical instruments and laser equipment where space utilization is crucial. Furthermore, the service temperature range of NiTi alloys is determined by their phase transformation temperature, which is extremely sensitive to composition and microstructure. For example, a change of only 0.1 at.% in Ni content can cause a phase transformation temperature shift of approximately 20°C. To achieve miniaturization and obtain an adjustable phase transformation temperature, a feasible approach is to integrate porous / channelized structures into the components and spatially customize the composition and microstructure. However, under traditional processing systems, it is difficult to achieve integrated collaborative design and manufacturing of the "composition-microstructure-complex structure."

[0004] Therefore, there is an urgent need to develop a manufacturing process that can flexibly control the composition and microstructure and efficiently form complex structures, so as to provide a technical foundation for the compaction and engineering application of thermo-elastic cooling devices. Summary of the Invention

[0005] To address the current technical bottleneck of achieving both porous structure and service temperature range matching in refrigeration applications of NiTi alloys, this application proposes a method based on laser powder bed melting technology, which combines complex porous forming with spatial customization of microstructure and composition.

[0006] On the one hand, this application proposes a 4D printing method for small porous NiTi elastic thermal working fluid, employing laser powder bed melting technology, including:

[0007] The process window for laser powder bed melting and forming was established using the surface response method.

[0008] The pre-designed porous topology with high specific surface area is divided into two functional domains, which include phase transition domains and non-phase transition domains.

[0009] Within the process window, a high energy density parameter is used for the phase transition domain and the scanning speed is controlled in a coordinated manner, while a low energy density parameter is used for selective melting of the non-phase transition domain to obtain the shaped porous NiTi component.

[0010] The porous NiTi component is subjected to direct aging treatment to induce asynchronous precipitation of precipitates in different functional domains while retaining the heterogeneity of the microstructure, thus obtaining a porous NiTi elastothermal working fluid.

[0011] In an optional embodiment, the porous topology is a three-dimensional porous topology with interconnected channels, wherein the porosity of the three-dimensional porous topology is 40%~70%, and the specific surface area is ≥10mm². -1 .

[0012] In an optional implementation, the low energy density parameter includes:

[0013] The laser power is 140~180W, the scanning speed is 600~900mm / s, the scanning spacing is 80~120μm, and the layer thickness is 20~50μm, or one or more combinations thereof.

[0014] In an optional implementation, the high energy density parameter includes:

[0015] The laser power is 160~250W, the scanning speed is 700~1000mm / s, the scanning spacing is 80~120μm, and the layer thickness is 20~50μm, or one or more combinations thereof.

[0016] In an optional embodiment, the direct aging treatment is performed at a temperature of 250~450°C and a holding time of 0.5~2h.

[0017] In an optional implementation, the functional domains are divided into two categories, including:

[0018] Static analysis of the porous topology is performed using finite element method or equivalent numerical simulation to identify and determine the spatial distribution of the functional domains.

[0019] In an optional embodiment, the process window includes: laser power of 100~250 W, spot diameter of 70 μm, scanning speed of 300~1000 mm / s, scanning spacing of 80~130 μm, and powder layer thickness of 25 μm.

[0020] In an optional embodiment, the laser powder bed melting technology uses gas-atomized pre-alloyed nickel-titanium powder for powder spreading, with a particle size distribution range of 15~53μm.

[0021] In an optional embodiment, the porous NiTi component exhibits a force-to-heat conversion ratio greater than or equal to 14 K·kN under the target operating conditions. -1 And the specific surface area is greater than or equal to 10 mm² -1 .

[0022] On the other hand, a small porous NiTi elasto-thermal working fluid is prepared by any one of the 4D printing methods in the preceding claims.

[0023] The embodiments of this application have the following beneficial effects:

[0024] This application discloses a 4D printing method for small porous NiTi elastic-thermal working fluid. A laser powder bed fusion process window is established using the surface response method. A porous topology with a high specific surface area is designed and divided into two functional domains: a phase transformation domain and a non-phase transformation domain. Within the process window, a high energy density parameter is used for the phase transformation domain, and the scanning speed is synergistically controlled. A low energy density parameter is used for selective melting of the non-phase transformation domain to obtain the formed component. The formed component undergoes direct aging treatment to induce asynchronous precipitation of precipitates in different functional domains while preserving the heterogeneity of the microstructure, resulting in a porous NiTi elastic-thermal working fluid. This printing method defines functional domains in the porous NiTi elastic-thermal working fluid and implements selective melting with differentiated energy densities to customize the microstructure, composition, and precipitation kinetics of the functional domains. Direct aging treatment preserves the heterogeneity of the microstructure and induces asynchronous precipitation of precipitates, causing asynchronous martensitic phase transformations in each functional domain under stress, thereby obtaining a programmable force-thermal coupling response. This 4D printing method is simple and easy to implement, and can conveniently prepare small porous NiTi elastothermal working fluids with customized functions, supporting the development of miniaturized high-performance elastothermal cooling systems. Attached Figure Description

[0025] To more clearly illustrate the technical solution of the present invention, the accompanying drawings required in the embodiments will be briefly described below. It should be understood that the following drawings only show some embodiments of the present invention and therefore should not be regarded as a limitation on the scope of protection of the present invention. In the various drawings, similar components are numbered similarly.

[0026] Figure 1 A schematic diagram of the manufacturing process window established in Embodiment 1 of this application is shown;

[0027] Figure 2 The diagram shows the design parameter correlation of the porous topology structure of Embodiment 1 of this application;

[0028] Figure 3 This is the functional domain diagram identified through static simulation in Example 1;

[0029] Figure 4These are micron-scale characterization diagrams of different functional domains in Example 1;

[0030] Figure 5 These are nanoscale characterization diagrams of different functional domains in Example 1;

[0031] Figure 6 This is the temperature change and mechanical curve shown by infrared thermal imaging in Example 1;

[0032] Figure 7 This is a comparison diagram of phase change temperature and refrigeration performance between Example 2 and Example 1;

[0033] Figure 8 The diagram shows the compressive fracture mechanical properties and refrigeration performance of the porous structure in the comparative example. Detailed Implementation

[0034] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Unless otherwise specified, the experimental methods used in the following embodiments are conventional methods; unless otherwise specified, the reagents and materials used in the following embodiments are commercially available.

[0035] To more clearly demonstrate the implementation steps and advantages of this invention, the specific implementation methods are described below with reference to the illustrations.

[0036] This application proposes a 4D printing method for preparing small porous NiTi elastic-thermal working fluids. This method, based on laser powder bed melting technology, is used for the integrated forming of high-specific-cooling-capacity small porous NiTi elastic-thermal working fluids, achieving a unified "composition-structure-complex structure" and overcoming the technical bottlenecks of conventional processes in terms of complex porous structures and adjustable phase transition temperatures. The printing method of this application enables adjustable phase transition temperatures and gives the porous structure both a high mechanical-thermal conversion ratio and a high specific surface area. This printing method provides a feasible fabrication path for key components of miniaturized high-performance elastic-thermal cooling systems. The specific implementation steps of this printing method are illustrated below:

[0037] By utilizing the surface response method with density and crack length as indicators, the relationship between these parameters and laser power and scanning speed is established. This provides a reasonable selection space for process parameters to achieve functional customization in subsequent 4D printing. In other words, the surface response method is used to quickly establish a laser powder bed fusion forming process window, determining the range of parameters such as laser power, scanning speed, scanning spacing, and layer thickness, in order to achieve high-density, crack-free, and controllable forming of NiTi (nickel-titanium) alloys. Specifically, the process conditions used in the aforementioned process window, i.e., fusion forming, are: laser power 100~250W, spot diameter 70μm, scanning speed 300~1000mm / s, scanning spacing 80~130μm, and powder bed thickness 25μm. The substrate used in laser powder bed melting is a NiTi alloy substrate, and the powder raw material is pre-alloyed NiTi powder obtained by atomic gas atomization. The pre-alloyed NiTi powder can be a Ni-rich NiTi alloy with a nominal composition of Ni 50.8 at.% and Ti 49.2 at.% or equivalent, and its particle diameter distribution is in the range of 15~53μm. During the laser forming process, the oxygen content will be ensured to be ≤300ppm, that is, the oxygen content value displayed by the oxygen sensor in the laser powder bed melting equipment will be ≤0.05%, and rare gases such as argon will be used as protective gases, with the gas pressure maintained at 10~20mbar. The aforementioned surface response method and atomic gas atomization method are well known to those skilled in the art. The laser powder bed melting forming equipment can be any laser selective melting forming equipment known in the prior art, and will not be described in detail here.

[0038] In this application, a porous topology with a high specific surface area will be pre-designed. This porous topology is a three-dimensional porous topology with interconnected channels, and its type is not limited. The porosity of this porous structure can be 40%~70%, and the specific surface area is ≥10 mm². -1Based on static simulation, the pre-designed porous topology is divided into two functional domains. In other words, finite element method (FEM) or equivalent numerical simulation is used to perform static analysis on the porous topology to identify and determine the spatial distribution of the two functional domains. Static analysis is well-known to those skilled in the art and will not be elaborated upon here. The two functional domains refer to the non-phase-change domains and the phase-change domains where the porous structure does not undergo phase change under a given mechanical pressure within the target service temperature range. The target service temperature range is the temperature range that the heat-absorbing end of the obtained porous elastothermal working fluid needs to reach and maintain during normal operation. The phase change temperature of the phase change domain determines the selection of the target service temperature range. For example, if the phase change temperature of the phase change domain is 25°C, then the service temperature range is 25°C to 50°C, i.e., the target service temperature range is the phase change temperature to the phase change temperature + 25°C. When the service temperature is too high, it can activate dislocation activity and initiate slip in the material itself. For example, if the phase change temperature is + 50°C, there is a risk that the material itself will fracture and fail. Therefore, the optimal target service temperature for elastothermic refrigerants is limited by the choice of phase transition temperature.

[0039] Furthermore, in the layering and path planning stages of additive manufacturing, differentiated processes will be applied to the non-phase change domain and the phase change domain. In other words, differentiated process parameters will be imported and melt-formed using independent STL (STereoLithography) files for in-situ customization of microstructures. Within the aforementioned process window, the non-phase change domain will be selectively melted using low energy density parameters, i.e., selectively melted with lower laser power. This can be combined with higher scanning speeds and / or lower laser power to ensure the forming quality of fine features in the non-phase change domain and achieve a high specific surface area. The low energy density parameters include one or more combinations of: laser power 140–180 W, scanning speed 600–900 mm / s, scanning spacing 80–120 μm, and layer thickness 20–50 μm. Within the aforementioned process window, the phase change domain will be treated with high energy density parameters, such as higher laser power, and melt-formed by coordinating with controlled scanning speeds. This will customize the phase change temperature and microstructure of the phase change domain to match the target service temperature range and cooling capacity. The cooling capacity refers to the maximum amount of heat that the porous NiTi thermodynamic medium can absorb and remove from the object being cooled. High energy density parameters include one or more combinations of the following: laser power 160~250W, scanning speed 700~1000mm / s, scanning spacing 80~120μm, and layer thickness 20~50μm.

[0040] Direct aging treatment was applied to the formed porous NiTi components to induce asynchronous precipitation of precipitates in different functional domains while preserving the heterogeneity of the microstructure. This achieved non-in-situ optimization of the microstructure of the two types of functional domains to meet the target service temperature range and cooling capacity of elasto-thermal cooling. The direct aging treatment temperature was 250–450°C, and the holding time was 0.5–2 h, to induce the precipitation of Ni4Ti3 precipitates with differences in size and volume fraction in different functional domains.

[0041] Due to differences in microstructure and precipitate content between the phase transformation domain and the non-phase transformation domain, they exhibit different martensitic phase transformation behaviors and functional responses. Therefore, under applied mechanical stress, the phase transformation domain and the non-phase transformation domain undergo asynchronous martensitic phase transformations. The phase transformations in the formed porous nickel-titanium components occur at different times, thus introducing a time dimension into the three-dimensional structure, demonstrating the 4D printing effect, and forming a 4D programmable force-thermal response.

[0042] Furthermore, by adjusting the energy input and aging regime of the phase transformation domain—that is, by controlling the direct aging treatment—the microstate of the phase transformation domain can be altered, allowing the martensitic phase transformation temperature to remain within the temperature range of 20–60°C, which can be specifically set as needed. The aforementioned porous NiTi elasto-thermal working fluid exhibits a force-to-heat conversion ratio greater than or equal to 14 K·kN under the target operating conditions. -1 And the specific surface area is greater than or equal to 10 mm² -1 The target operating conditions can be adjusted as needed within the range of 20~85°C.

[0043] In this application, the 4D printing method described in the claims can be used to prepare small porous NiTi thermo-elastomeric working fluid.

[0044] Example 1

[0045] In this embodiment 1, laser powder bed melting technology is used, and the powder is gas-atomized pre-alloyed NiTi powder with a nominal composition of Ni. 50.8 Ti 49.2 (Atomic ratio), powder particle size distribution is 15~53μm.

[0046] like Figure 1 As shown, the surface response method is used to establish the laser powder bed melting manufacturing process window, and the laser power and scanning speed are optimized to ensure the dense, crack-free, and controllable forming of NiTi alloy; in this embodiment, the designed forming size is 8×8×8mm. 3The porous topology of the NiTi alloy was analyzed using the surface response method, with density and crack length as indicators, to establish their relationship with laser power and scanning speed. This provides a reasonable selection space for process parameters to achieve customized 4D printing functions. Crack-free forming can be achieved when the laser power is within the range of 175–200W. Combined with a density distribution of ≥99% within the density window, the process parameters determined by the surface response method ensure that the NiTi alloy can achieve dense, crack-free, and controllable forming.

[0047] like Figure 2 As shown in (a), a porous topology with high specific surface area is designed. The horizontal axis represents the parameter t (mm) that controls the volume fraction, and the vertical axis represents the volume V (mm). 3 ) and volume fraction % (mm -1 ), Figure 2 In (a), as the parameter t controlling the volume fraction increases, it can be seen that the volume increases. Combined with... Figure 2 As shown in (b), the specific surface area decreases as the volume fraction increases. Considering the minimum precision and high specific surface area requirements of the L-PBF (laser powder bed fusion) forming equipment, the preferred volume in this embodiment is 20 mm². 3 At this point, t is -0.37. The governing equations for the designed porous topology with high specific surface area are:

[0048]

[0049] Where X = 2π / a·x, Y = 2π / a·y, Z = 2π / a·z, a represents the unit cell size, and x, y, and z represent physical coordinates in a spatial rectangular coordinate system, with units of mm, which can be 2 mm. Based on the formula: , where c represents half the wall thickness.

[0050] The isosurface is offset along the normal / anti-normal direction. The offset distance t, i.e., the parameter t controlling the volume fraction, is used to adjust the volume and volume fraction. In this embodiment, the volume V is taken as 20 mm. 3 Static simulations were performed using Abaqus to identify the phase transition domain and the non-phase transition domain. The phase transition domain and the non-phase transition domain are as follows: Figure 3 As shown, the designed porous topology model is split into two STL files for differentiated process forming.

[0051] Preferably, the non-phase-change domain will be formed using a laser power of 180W and a scanning speed of 750mm / s, while the phase-change domain will be formed using a laser power of 200W and a scanning speed of 900mm / s. The formed phase-change domain and the non-phase-change domain are as follows: Figure 4As shown, the phase transition domain corresponding to lower heat input is beneficial for preserving fine features and obtaining high specific surface area; the phase transition domain corresponding to higher heat input has a larger rod diameter due to the higher melt volume, and the specific surface area will be lower than the model design, but it helps to reduce forming defects, improve mechanical properties, reduce premature fracture failure of the structure under mechanical pressure, and ensure the stable induction of elasto-thermal effect.

[0052] Preferred heat treatment such as Figure 5 As shown, under direct aging conditions of 400°C and 1 h, asynchronous precipitation of Ni4Ti3 nanoparticles was induced. Figure 5 As shown in the upper left and lower left images, the spherical and ellipsoidal bright white particles are nanoprecipitates. Under the same conditions, there are more non-phase change domain nanoprecipitates than phase change domain nanoprecipitates. Figure 5 The top right figure shows the relationship between the size and frequency of non-phase change domain nanoprecipitates. It can be seen that the average size of the non-phase change domain nanoprecipitates is approximately 26 nm. Figure 5 The lower right figure shows the relationship between the size of the nanoprecipitates in the phase transition domain and the frequency. It can be seen that the average size of the nanoprecipitates in the phase transition domain is approximately 10 nm. This indicates that the average size of the Ni4Ti3 precipitate in the phase transition domain is smaller than that in the non-phase transition domain, and the precipitation volume fractions of the two functional domains are different, thus achieving a difference in the phase transition initiation temperature and phase transition range.

[0053] like Figure 6 As shown, based on a phase transition temperature of 50°C, the porous NiTi elastic thermal working fluid was tested at an ambient temperature of 60°C, and the temperature change was measured using infrared thermography. When mechanical stress was applied to the porous NiTi elastic thermal working fluid, the temperature rose in the intermediate phase transition region; when the applied mechanical stress was removed, the temperature dropped in the intermediate phase transition region. Figure 6 As shown in (b), after the porous NiTi elasto-thermal working fluid was subjected to a loading force and then rapidly unloaded, the surface temperature of its core phase transition domain structure decreased from 63°C to 58.3°C, exhibiting a temperature change of 4.7K. Figure 6 As shown in (c), a loading force of 300 N and a compressive strain of 4% were applied. The calculated force-to-heat conversion ratio of the porous NiTi elastomer was 15.7 K·kN. -1 (4.7K / 300N=15.7K·kN) -1 The specific surface area is 12.1 mm². -1 The specific surface area is approximately 38 mm² based on mass testing. 3 Substitute Figure 2 (b) It is concluded that this is one of the best levels in the current field.

[0054] The above results show that by constructing functional domain units within the same three-dimensional structure through 4D printing, high specific surface area and stable cooling effect can be achieved synergistically.

[0055] Example 2

[0056] The difference between this embodiment and Embodiment 1 lies in the laser process and aging temperature. In this embodiment, the non-phase change domain of the porous topology with high specific surface area will be formed by laser melting at a power of 160W and a scanning speed of 600mm / s; the phase change domain will be formed by laser melting at a power of 200W and a scanning speed of 700mm / s, and the direct aging treatment conditions are a temperature of 250°C and a holding time of 1 hour.

[0057] like Figure 7 As shown in (a), compared with Example 1, Example 2 has the same holding time for direct aging treatment but a lower temperature. The phase transition temperature of the porous topology in Example 1 is approximately 50°C; the phase transition temperature of the porous topology in this example is approximately 20°C, making it more suitable for room temperature applications. Tests show that: Figure 7 As shown in (b), under an ambient temperature of approximately 30°C, the corresponding cooling temperature drop under a loading force of 570 N is 8 K, at which point the force-to-heat conversion ratio is 14 K·kN. -1 (8K / 570N=14K·kN) -1 ), with a specific surface area of ​​10 mm -1 The specific surface area is approximately 42 mm² based on the volume measured at this time using mass testing. 3 Substitute Figure 2 (b) It is concluded that its overall structure-function index is comparable to that of Example 1, but the applicable temperature scenario changes from 60°C in Example 1 to 30°C in this example.

[0058] As demonstrated in Examples 1 and 2, when the holding time under direct aging conditions is the same, reducing the direct aging temperature can effectively lower the phase transition temperature. Furthermore, this example shows that simultaneously and moderately increasing the laser heat input in the phase transition domain can suppress forming defects and maintain the intrinsic elastic-thermal response of the material. Specifically, compared to Example 1 with a scanning power of 200W and a scanning speed of 900mm / s, the scanning power of 200W and scanning speed of 700mm / s in this example is lower, the heat input is higher, and the corresponding mechanical properties can reach 570N without fracture. Therefore, this method is simpler to implement than traditional manufacturing methods.

[0059] Comparative Example

[0060] The comparative example differs from Example 1 only in the forming strategy. The comparative example did not perform functional domain splitting on the designed porous topology model, and the entire part was formed by laser melting at a power of 180W and a scanning speed of 750mm / s.

[0061] like Figure 8As shown in (a), the horizontal axis represents the compression ratio (%), and the vertical axis represents the loading force (N). Figure 8 As shown in (a) and (c), the control sample in the comparative example fractured at a compressive strain of 3%, while the sample in Example 1 could withstand a compressive strain of up to 5%. Figure 8 (b) It can be seen that the cooling temperature drop of the comparative example is 2.6K, while that of Example 1 ( Figure 6 The cooling temperature drop was 4.7 K. In contrast, the force-to-heat conversion ratio of the comparison sample was only 11 K·kN. -1 Although the measured specific surface area was relatively high at 13 mm², -1 However, its mechanical properties and force-to-heat conversion ratio are low, making it difficult to meet service requirements.

[0062] The results indicate that relying solely on single microstructure optimization is insufficient to simultaneously improve the overall performance of porous structures; functional domain decomposition and differentiated forming and aging are key to obtaining high-performance elastothermal working fluids.

[0063] In summary, this application achieves spatial customization of microstructure, composition, and precipitation kinetics of functional domains by defining functional domains in a porous NiTi elastothermal working fluid and implementing selective melting with differentiated energy densities. Direct aging treatment preserves the heterogeneity of the microstructure and induces asynchronous precipitation of precipitates, enabling asynchronous martensitic phase transformations in each functional domain under external loads, thus obtaining a programmable force-thermal coupling response. This 4D printing method is simple and easy to implement, with adjustable phase transformation temperature. The resulting working fluid possesses both a high force-thermal conversion ratio and a high specific surface area (measured at 15.7 K·kN⁻¹ and 12.1 mm⁻¹), allowing for the convenient fabrication of functionally customized small porous NiTi elastothermal working fluids, supporting the development of miniaturized high-performance elastothermal cooling systems. Furthermore, the small porous NiTi elastothermal working fluid printed in this application is suitable for space-constrained scenarios in miniaturized high-performance elastothermal cooling systems, including heat dissipation / temperature control modules for medical instruments and laser equipment.

[0064] The above are merely specific embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for 4D printing of small porous NiTi thermomechanical working medium, characterized in that, Laser powder bed melting technology is employed, including: The process window for laser powder bed melting and forming was established using the surface response method. The pre-designed porous topology with high specific surface area is divided into two functional domains, which include phase transition domains and non-phase transition domains. Within the process window, a high energy density parameter is used for the phase transition domain and the scanning speed is controlled in a coordinated manner, while a low energy density parameter is used for selective melting of the non-phase transition domain to obtain the shaped porous NiTi component. The porous NiTi component is subjected to direct aging treatment to induce asynchronous precipitation of precipitates in different functional domains while retaining the heterogeneity of the microstructure, thus obtaining a porous NiTi elastothermal working fluid.

2. The 4D printing method for small porous NiTi elastothermal working fluid according to claim 1, characterized in that, The porous topological structure is a three-dimensional porous topological structure with connected pores, the porosity of the three-dimensional porous topological structure is 40% to 70%, and the specific surface area is greater than or equal to 10 mm -1 .

3. The 4D printing method for small porous NiTi elastothermal working fluid according to claim 1, characterized in that, The low energy density parameters include: The laser power is 140~180W, the scanning speed is 600~900mm / s, the scanning spacing is 80~120μm, and the layer thickness is 20~50μm, or one or more combinations thereof.

4. The 4D printing method for small porous NiTi elastothermal working fluid according to claim 1, characterized in that, The high energy density parameters include: The laser power is 160~250W, the scanning speed is 700~1000mm / s, the scanning spacing is 80~120μm, and the layer thickness is 20~50μm, or one or more combinations thereof.

5. The 4D printing method for small porous NiTi elasto-thermal working fluid according to claim 1, characterized in that, The direct aging treatment is performed at a temperature of 250~450°C for a holding time of 0.5~2h.

6. The 4D printing method for small porous NiTi elastothermal working fluid according to claim 1, characterized in that, It is divided into two functional domains, including: Static analysis of the porous topology is performed using finite element method or equivalent numerical simulation to identify and determine the spatial distribution of the functional domains.

7. The 4D printing method for small porous NiTi elasto-thermal working fluid according to claim 1, characterized in that, The process window includes: laser power of 100~250 W, spot diameter of 70 μm, scanning speed of 300~1000 mm / s, scanning spacing of 80~130 μm, and powder layer thickness of 25 μm.

8. The 4D printing method for small porous NiTi elastothermal working fluid according to claim 1, characterized in that, The laser powder bed melting technology uses gas-atomized pre-alloyed nickel-titanium powder for powder spreading, with a particle size distribution range of 15~53μm.

9. The 4D printing method for small porous NiTi elasto-thermal working fluid according to claim 1, characterized in that, The porous NiTi member has a force-heat conversion ratio greater than or equal to 14 K·kN under a target working condition -1 , and a specific surface area greater than or equal to 10 mm -1 .

10. A small porous NiTi elasto-thermal working fluid, characterized in that, It is prepared by any one of the 4D printing methods of claims 1 to 9.

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