A high fatigue life nickel-titanium shape memory alloy based on functional motif regulation and a 4D printing preparation method and application thereof

Abstract

The application discloses a kind of high fatigue life nickel-titanium shape memory alloy based on functional unit regulation and its 4D printing preparation method and application.The 4D printing preparation method includes four steps of part model design, functional unit size determination, size pool combination printing process design and 4D printing functional unit regulation, and its core is to generate size pool by high and low laser energy, and the initial functional unit of large pool is regulated in situ by small pool, specifically, the cellular crystal region of large pool is in-situ heat treated by the heat affected zone of small pool, the transition zone and heat affected zone of large pool are remelted by the core of small pool, and the target functional unit of Ti2Ni nanocrystalline dispersion uniform distribution is obtained.The nickel-titanium shape memory alloy prepared by the application has good functional cycle stability and high fatigue life, can better meet the reliability and durability under dynamic load and cyclic strain service, and can be widely applied in medical devices, automobile industry, aerospace and building water heating products and other fields.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of shape memory alloys and additive manufacturing technology, and particularly relates to a high fatigue life nickel-titanium shape memory alloy based on functional unit regulation and a 4D printing preparation method and application thereof. BACKGROUND

[0002] Shape memory alloys can recover large deformation through reversible martensitic transformation under thermal / force driving, and are widely used in medical devices, automotive industry, aerospace and building plumbing, etc. Its application scenarios involve long-term dynamic load and cyclic strain, so it is crucial to focus on its cyclic stability and fatigue performance. Nickel-titanium shape memory alloy is the most widely used memory alloy, which has good structural strength and excellent functional characteristics (memory effect and superelasticity), and can be applied to thermoelectric coolers, vascular stents, artificial heart valves, medical implants, dental burs, dental drills, dental alignment wires, temperature sensing triggers, temperature control devices, temperature control springs, self-deploying trusses, self-deploying satellite antennas, engine heat exchange fan clutches, exhaust automatic adjustment nozzles, flexible wings, etc. However, under the service condition of cyclic loading, the performance of nickel-titanium shape memory alloy will degrade, showing residual strain accumulation, recoverable strain attenuation, and external work reduction, i.e. functional fatigue, and even fracture failure, i.e. structural fatigue.

[0003] 4D printing is based on shape memory alloy 3D printing additive manufacturing, which can overcome the forming and machining difficulties of complex nickel-titanium shape memory alloy parts. Selective laser melting is one of the most commonly used additive manufacturing technologies, which can obtain good forming precision and extremely high production efficiency, and can also be personalized and fine-tuned to customize microstructure and macrostructure. However, 4D printed nickel-titanium alloy faces more severe functional instability than traditional processes. In terms of superelasticity, most existing research on 4D printed nickel-titanium alloy only focuses on its single cycle or 10-20 cycle deformation recovery, and few studies focus on the fatigue limit of 4D printed nickel-titanium alloy (J. Mater. Process. Technol. 252 (2018) 440-453). In constant stress / strain amplitude cyclic loading tests, the deformation recovery ability of 4D printed nickel-titanium alloy is greatly attenuated, with recoverable strain less than 2% (J. Mater. Res. Technol. 13 (2021) 241-250, Acta Mater. 194 (2020) 178-189), and even only after a few cycles, premature fracture occurs (Acta Mater. 229 (2022) 117781).

[0004] The above situation is attributed to two problems: 1. The forming defects of 4D printed nickel-titanium alloy. During the 4D printing process, the metal powder undergoes a complex thermodynamic process. Improper process parameters will affect the flow and cooling solidification of the molten pool, and generate excessive thermal stress, resulting in metallurgical defects such as keyhole and thermal cracking, so that the 4D printed nickel-titanium alloy fails before it exhibits functional effects. When the roughness is large, it will accelerate the initiation of fatigue cracks from the surface defects of the sample, and then cause damage. 2. The unique microstructure of 4D printed nickel-titanium alloy. 4D printing is a process of stacking molten pools one by one. Different parts of the molten pool experience different thermal histories, resulting in three characteristic zones: cellular crystal zone, transition zone, and heat affected zone. The functional unit composed of the three characteristic zones is orderly constructed into the whole part (Virtual Phys. Prototy. 3 (2022) 563-581). In the cellular crystal zone, the cooling rate is extremely high, and the molten pool undergoes non-equilibrium solidification. The long-range diffusion of solute atoms is blocked, resulting in large microsegregation. The preferentially solidified cellular crystal zone contains more Ti, which has a high melting point. Nickel-titanium alloy is highly sensitive to microsegregation, and has a small solid solubility of Ti. Therefore, Ti2Ni precipitates are often observed along the grain boundaries of the cellular crystal zone in 4D printed nickel-titanium alloy. At the same time, the transition zone and the heat affected zone located in the core of the molten pool form columnar crystals due to the slow solidification rate, and the precipitates are dispersedly distributed. The continuous distribution of hard and brittle intermetallic compounds at the grain boundaries causes non-coordinated deformation between the nickel-titanium alloy component and the matrix during loading, resulting in stress concentration, hindering the reversible progress of the martensitic transformation, and easily inducing intergranular fracture of the material, which deteriorates the mechanical properties.

[0005] For problem one, with the development of lasers and additive manufacturing equipment, as well as a large amount of work on the exploration and optimization of 4D printing process parameters, it is currently possible to prepare nickel-titanium alloy samples without cracks and keyholes. For a small amount of micro-pores caused by gas entrainment and Ni element volatilization during rapid solidification, it is difficult to overcome through optimization of a single process parameter (J. Mater. Process. Technol. 303 (2022) 117546). For problem two, the morphology and properties of the functional unit determine the microstructure and service performance of the final part. Therefore, precise control of the functional unit is the basic idea and effective way to improve the 4D printed nickel-titanium shape memory alloy. Currently, methods for improving the function stability of traditional nickel-titanium alloys can be used as a reference: introducing nano-precipitates to strengthen the matrix, reducing the introduction of non-recoverable strain during the martensitic transformation process, ensuring good cycle stability, reducing the localization of deformation and stress concentration, delaying the initiation and propagation of cracks, and improving the fatigue life of the component (Acta Mater. 50 (2002) 4643-4657). SUMMARY

[0006] In order to overcome the deficiencies and shortcomings of the prior art, the primary object of the present application is to provide a 4D printing preparation method of high fatigue life nickel-titanium shape memory alloy based on functional unit regulation, which is based on the idea of regulating functional units by combining large and small melt pools, and regulates the three characteristic zones of cellular crystal zone, transition zone and heat affected zone of the initial functional unit as in-situ heat treatment zone and remelt zone. During the layer-by-layer forming process, first, a large melt pool is generated by high laser energy scanning to melt and form the powder, and an initial functional unit composed of a cellular crystal zone, a transition zone and a heat affected zone is generated; then, a small melt pool is generated by low laser energy scanning, which on the one hand eliminates micro-pores and bonded powder to improve the forming quality, and on the other hand, the small melt pool regulates the initial functional unit of the large melt pool in-situ, the cellular crystal zone of the large melt pool is subjected to in-situ heat treatment in the heat affected zone of the small melt pool, and the transition zone and the heat affected zone of the large melt pool are subjected to remelting in the core of the small melt pool, thereby optimizing the microstructure, changing the distribution and morphology of the grain boundary precipitates, and obtaining high fatigue life 4D printed nickel-titanium shape memory alloy based on functional unit regulation.

[0007] The second object of the present application is to provide high fatigue life 4D printed nickel-titanium shape memory alloy based on functional unit regulation prepared by the above preparation method.

[0008] The third object of the present application is to provide the application of the above 4D printing preparation method of high fatigue life nickel-titanium shape memory alloy based on functional unit regulation.

[0009] The primary object of the present application is achieved by the following technical solutions:

[0010] A 4D printing preparation method of high fatigue life nickel-titanium shape memory alloy based on functional unit regulation, comprising the following steps:

[0011] (1) Part model design: the part model is meshed to construct a finite element model, the deformation behavior of the part in different service environments is simulated according to the load conditions and functional characteristic requirements determined by the part service environment, and the part geometric model is topologically optimized;

[0012] (2) Functional unit size determination: the temperature field of the 4D printed nickel-titanium alloy single-track melt pool and the contour size determined thereby are predicted by the finite volume method, the size relationship and process parameter range of the large and small melt pools corresponding to the functional unit in step (3) are determined, and the size of the functional unit is determined;

[0013] (3) Large and small melt pool combined printing process design: the part geometric model obtained in step (1) is imported into the 4D printing forming system to generate two groups of models placed in overlap, and the two groups of process parameters of the large and small melt pools obtained in step (2) are respectively assigned, so as to realize large and small melt pool combined printing, and the printing file is generated according to the set process parameters;

[0014] (4) 4D printing functional element regulation: Run the print file generated in step (3) to perform 4D printing on nickel-titanium alloy powder; use a combination of large and small molten pools for printing, high laser energy density produces a large molten pool to form an initial functional element, low laser energy density produces a small molten pool, and in situ regulates the initial functional element to be the target functional element, and repeats this process until a high fatigue life nickel-titanium shape memory alloy based on functional element regulation is obtained.

[0015] Preferably, the part model in step (1) is meshed by Hyper Mesh and then imported into ANSYS software for finite element simulation, and the topology optimization method is any one of the variable thickness method, the progressive structure optimization method, and the independent continuous mapping method.

[0016] Preferably, the finite volume method described in step (2) is specifically as follows: using fluent software based on a ray tracing model to simulate the heat transfer process of the molten pool under laser heat source input at a specific power and scanning rate, and using CFD-Post visualization processing software to generate a molten pool temperature field cloud map to determine the contour size of the molten pool.

[0017] Preferably, the sizes of the large and small molten pools in step (2) are determined according to the molten pool profile, including the molten pool width W = 160-180 μm and the molten pool depth H = 70-150 μm. The specific relationship between the molten pool sizes is: W 大 =W 小 , H 小 >H 大 -t, where t is the thickness of the powder layer.

[0018] Preferably, the laser energies of the two sets of process parameters for the large and small molten pools in step (3) are different and satisfy: 2E 低 ≤E 高 ≤4E 低 , specifically: the first group is high laser energy density E 高 =40~65J / mm 3 , laser power P 高 =180W~250W, scanning rate V 高 =800~1400mm / s; the second group is low laser energy density E 低 =9~32J / mm 3 , laser power P 低 =45W~125W, scanning rate V 低 =800~1400mm / s; the scanning paths are: layer-by-layer rotation 67~90°, scanning spacing h=80~120μm, and powder layer thickness t=30~40μm.

[0019] Preferably, the in-situ regulation of the initial functional unit to the target functional unit in step (4) is specifically as follows: the large pool is mutually overlapped to form an initial functional unit containing three characteristic zones of cellular crystal zone, transition zone and heat affected zone; the heat affected zone of the small pool is located in the cellular crystal zone of the large pool, the cellular crystal zone of the large pool is subjected to in-situ heat treatment, the core of the small pool is located in the transition zone and the heat affected zone of the large pool, so that the small pool is remelted, and finally the target functional unit is obtained.

[0020] The second object of the application is achieved by the following technical scheme:

[0021] A high fatigue life nickel-titanium shape memory alloy based on functional unit regulation is prepared by the 4D printing preparation method.

[0022] Preferably, the high fatigue life nickel-titanium shape memory alloy based on functional unit regulation comprises a target functional unit, the target functional unit comprises an in-situ heat treatment zone of the small pool boundary and a remelted zone of the small pool core; the in-situ heat treatment zone is cellular crystal with a grain size of 800-1000 nm, which contains uniformly distributed Ti2Ni nanocrystals with a size of 10-30 nm, and the Ti2Ni nanocrystals have a coherent or semi-coherent relationship with the matrix; the remelted zone is coarsened columnar crystal with a width of 4-8 microns, which dispersively contains Ti2Ni nanocrystals with a size of 10-50 nm.

[0023] The third object of the application is achieved by the following technical scheme:

[0024] The application of a high fatigue life nickel-titanium shape memory alloy based on functional unit regulation is specifically the application of the dynamic load super-elasticity and the cyclic deformation shape memory function of the nickel-titanium shape memory alloy in the fields of medical devices, medical implants, building plumbing products, automotive industry products, aerospace products.

[0025] Preferably, the medical devices are vascular stents, artificial heart valves, dental burs, orthodontic wires, etc.; the medical implants are joint implants, spinal implants, shoulder implants, craniofacial implants, foot and ankle implants, sternum implants, etc.; the building plumbing products are heat activated refrigerators, temperature controlled springs; the automotive industry products are temperature sensing triggers, temperature control devices; the aerospace products are self-deploying trusses, self-deploying satellite antennas, engine heat exchange fan clutches, exhaust automatic regulation nozzles, flexible wings, etc.

[0026] The principle of the application is:

[0027] (1) The melt pool of 4D printing of nickel-titanium shape memory alloy is a basic functional element, and various products are formed by sequentially building up the functional elements row by row and layer by layer. The shape and properties of the characteristic regions in the functional element are adjusted to finally control the functional characteristics of the 4D printed shape memory alloy. The high fatigue life nickel-titanium shape memory alloy designed based on the functional element control in the present application adopts a 4D printing method of printing and controlling the functional element based on the combination of large and small melt pools. The forming strategy is as follows: first, high energy density laser is used to melt metal powder to form a large melt pool, to ensure sufficient overlap between passes and close bonding between layers, and to form an initial functional element containing three characteristic regions of cellular crystal region, transition region and heat affected zone; then, the same path is scanned again using a low energy density laser, and by controlling the size of the small melt pool, the cellular crystal region of the large melt pool is subjected to in-situ heat treatment in the heat affected zone at the boundary of the small melt pool, and the transition region and heat affected zone of the large melt pool are remelted in the core of the small melt pool.

[0028] (2) Different laser energy levels will result in different melt pool solidification conditions. High laser energy density can fully melt the metal powder, improve the flowability and wettability of the melt, and promote the solidification connection between powder particles. However, high laser energy density also leads to two deficiencies. On the one hand, it is easy to cause melt spatter and powder bed oscillation, which increases the surface roughness. On the other hand, the melt pool temperature rises sharply, the non-equilibrium solidification is intensified, and a large amount of micro-segregation is caused. Therefore, the Ti2Ni in the cellular crystal region of the initial functional element is distributed along the grain boundaries, and the transition region and the heat affected zone are columnar crystals and dispersed precipitates. When a low laser energy density is used for re-scanning, on the one hand, the flow of the metal melt fills the small holes caused by gas entrapment or Ni evaporation, eliminating the surface-bonded powder and droplets; on the other hand, the heat affected zone of the small melt pool makes the temperature of the originally cellular crystal region higher than the melting temperature (982℃) of Ti2Ni phase, promoting the dissolution of Ti2Ni at the grain boundaries through reverse peritectic reaction (Ti2Ni + NiTi → L + NiTi), and then, due to the relatively small temperature difference and small temperature gradient between this region and the surrounding solidified deposition layer, the cooling rate is slowed down, which provides more uniform thermodynamic and kinetic conditions for the precipitation of Ti2Ni. Therefore, the Ti2Ni in the in-situ heat treatment zone (i.e. the initial cellular crystal region) of the target functional element is uniformly distributed; due to the reduced solidification rate, the columnar crystals in the remelted zone are somewhat coarsened, and the precipitates are dispersedly distributed.

[0029] (3) Considering the significant remelting of the previous layer during the layer-by-layer forming process of 4D printing, the low laser energy should not be too low. It is known that there must be a powder layer thickness t between the bottoms of the melt pools of adjacent layers, so the depth difference between the small melt pool and the large melt pool should be less than a powder layer thickness, so as to ensure that the small melt pool exceeds the remelted range of the large melt pool of the next layer, thereby retaining the target functional element.

[0030] (4)Under the service condition of dynamic load and cyclic strain, the nickel-titanium shape memory alloy will occur structural fatigue and functional fatigue. The structural fatigue is the low load and low strain fracture caused by the initiation and propagation of fatigue cracks; the functional fatigue is the increase of non-recoverable deformation and the decay of phase transition ability caused by the introduction of slip deformation and the stabilization of martensite. The improvement of micro-holes and surface roughness can avoid the holes and surface defects from serving as the nucleation points of fatigue cracks, delay the fracture and improve the fatigue life of the nickel-titanium alloy parts. The larger coherent stress field generated around the dispersedly distributed nano Ti2Ni precipitated phase can effectively pin the dislocation movement, inhibit the initiation of dislocation source, promote the saturation of dislocation, thereby reducing the non-recoverable deformation, preventing the stabilization of martensite, maintaining the reversibility of stress-induced phase transition and promoting the functional response of the nickel-titanium alloy to tend to be stable.

[0031] Compared with the prior art, the application has the following advantages and beneficial effects:

[0032] (1) The 4D printing preparation method of the high fatigue life nickel-titanium shape memory alloy based on functional unit regulation provided by the application designs a high-low energy combined printing-functional unit regulation 4D printing method, the high laser energy density and the low laser energy density are scanned in sequence, the large molten pool and the small molten pool cooperate with each other, the porosity and the surface roughness are further reduced on the basis of the crack-free and few-hole formed layer. In addition, the in-situ regulation of the initial functional unit by the small molten pool optimizes the size and distribution of the precipitated phase, achieves the effect of heat treatment and provides a new idea for the process optimization and microstructure performance regulation of the 4D printed nickel-titanium shape memory alloy.

[0033] (2) The high fatigue life nickel-titanium shape memory alloy based on functional unit regulation prepared by the application has high functional cycle stability, in the maximum strain constant (3%) tensile cycle test, the non-recoverable strain basically does not increase after only 5 cycles, and the fully recoverable super-elastic strain reaches 2.3%; in the constant amplitude pulsating fatigue test with a strain amplitude of 5%, the fatigue life reaches 583 times, and the fully recoverable super-elastic strain remains 2.2%. In the temperature-driven fatigue test with the maximum constant load (200 MPa), the shape memory response reaches stability after only 6 cycles, the fully recoverable strain is 4.4%, and the fatigue life can reach 497 times.

[0034] (3) The present application realizes the effect of functional unit regulation on the basis of preparing nickel-titanium shape memory alloy by 4D printing, only through simple modeling and parameter setting. Unlike uniform heat treatment, the present application can realize specific treatment of different parts of the part through the construction of functional units, and reduce the process flow, energy consumption and cost. The prepared high fatigue life nickel-titanium shape memory alloy based on functional unit regulation can better meet the reliability and durability of 4D printed nickel-titanium alloy components in service, and further promote the application of 4D printing in actual production. BRIEF DESCRIPTION OF DRAWINGS

[0035] Figure 1 (a) and Figure 1 (b) are microstructure diagrams of the high fatigue life nickel-titanium shape memory alloy based on functional unit regulation obtained in Example 1 and the nickel-titanium alloy obtained in Comparative Example 1, respectively;

[0036] Figure 2 (a) and Figure 2 (b) are superelasticity cycle test curves of 3% constant strain tension of the high fatigue life nickel-titanium shape memory alloy based on functional unit regulation obtained in Example 1 and the nickel-titanium alloy obtained in Comparative Example 1, respectively;

[0037] Figure 3 is the fatigue test curve of strain-controlled constant-amplitude pulsation of the high fatigue life nickel-titanium shape memory alloy based on functional unit regulation obtained in Example 1. DETAILED DESCRIPTION

[0038] The present application will be further described in detail below in combination with examples and drawings, but the embodiments of the present application are not limited thereto.

[0039] Example 1

[0040] (1) Part model design: The porous spinal rod medical implant model is subjected to Hyper Mesh grid division to construct a finite element model. According to the load condition and functional characteristic requirement of the porous spinal rod in service: pre-bending before implantation, recovery temperature close to human body temperature, and bearing bending stress against spinal deformity after implantation. The ANSYS software is used to simulate the deformation behavior of the porous spinal rod in different service environments, and the progressive structure optimization method is used to topologically optimize the pore diameter, rod diameter and porosity of the porous spinal rod geometric model;

[0041] (2) Functional unit size determination: The temperature field of the 4D printed nickel-titanium alloy single-track molten pool and its determined contour size are predicted by the finite volume method. The size relationship of the functional unit corresponding to the large molten pool and the process parameter range in step (3) are determined, and the size of the functional unit is determined, the large molten pool width W 大 = 160 μm, the depth H 大 = 80 μm, the small molten pool width W小 = 160 pm, depth H 小 = 70 pm, the pool size relationship is specifically: W 大 = W 小 , H 小 > H 大 - t (powder layer thickness t, 30 pm);

[0042] (3) Large and small pool combined printing design: the porous spine rod medical implant geometry model obtained in step (1) is imported into the 4D printing forming system to generate two groups of models placed in overlap, and the two groups of process parameters obtained in step (2) are respectively assigned, the energy of the two groups of process parameters is different, and it satisfies: 2E 低 ≤ E 高 ≤ 4E 低 , specifically: the first group is high laser energy density E 高 = 53 J / mm 3 , laser power P 高 = 180 W, scanning speed V 高 = 1400 mm / s; the second group is low laser energy density E 低 = 26 J / mm 3 , laser power P 低 = 90 W, scanning speed V 低 = 1400 mm / s; the scanning path is: layer by layer rotating 90°, scanning spacing h = 80 pm, powder layer thickness t = 30 pm, so as to realize large and small pool combined printing, and generate a printing file according to the set process parameters;

[0043] (4) 4D printing function element regulation: running the printing file generated in step (3) to 4D print the nickel-titanium alloy powder; large and small pool combined printing is adopted, large pool is generated by high laser energy density to form the initial function element, and small pool is generated by low laser energy density to regulate the initial function element in situ to the target function element, the specific process is: the large pool is overlapped to form the initial function element containing cellular crystal zone, transition zone and heat affected zone three characteristic zones; the heat affected zone of the small pool is located in the cellular crystal zone of the large pool to conduct in-situ heat treatment on the cellular crystal zone of the large pool, the core of the small pool is located in the transition zone and the heat affected zone of the large pool to make it remelt, and finally the target function element is obtained, and the process is repeated until the high fatigue life nickel-titanium shape memory alloy part based on function element regulation is obtained.

[0044] The roughness, phase composition, microstructure characterization and cyclic tensile test of the high fatigue life nickel-titanium shape memory alloy based on functional unit regulation prepared by the above steps are carried out by using 3D optical profiler, X-ray diffractometer, transmission electron microscope and Instron 8862 test system. The results show that the surface roughness of the nickel-titanium shape memory alloy sample prepared by the high-low energy combined printing-functional unit regulation 4D printing method in the embodiment is low, S a The in-situ heat treatment zone of the small molten pool boundary and the remelting zone of the small molten pool core are contained in the target functional unit. The in-situ heat treatment zone is cellular crystal with a size of about 300-500 nm, and Ti2Ni precipitated phase is uniformly distributed in the cellular crystal with a size of about 20 nm, which has a coherent orientation relationship with the matrix. The remelting zone is coarsened columnar crystal with a width of 4-6 μm, and Ti2Ni is dispersedly distributed with a size of about 40 nm. In the tensile cycle test with a maximum strain of 3%, the non-recoverable strain basically does not increase after only 5 cycles, and the fully recoverable superelastic strain reaches 2.3%; in the constant amplitude pulsating fatigue test with a strain amplitude of up to 5%, the fatigue life reaches 583 times, and the fully recoverable superelastic strain remains 2.2%. Compared with the related literature reports, such as the recoverable strain of 1.25% after 10 superelastic cycles (Mater. Design 220 (2022) 110886), the recoverable strain of 1.8% after 10 superelastic cycles (J. Mater. Res. Technol. 13 (2021) 241-250), the fatigue life of 430 times at a maximum strain of 4.5% (Int. J. Fatigue 66 (2014) 78-85), the single constant load (200 MPa) temperature driven strain of 4% and the residual strain of 1% (Scripta Mater. 14 (2018) 164-168), the nickel-titanium shape memory alloy prepared by the present application has excellent fatigue life. The porous spinal rod medical implant made of the nickel-titanium shape memory alloy prepared by the present application can be more easily implanted into the pedicle screw of the spinal deformity after pre-bending, and the nickel-titanium alloy spinal rod can be used to correct the scoliosis, and the service life can reach 20 years.

[0045] Example 2

[0046] (1) Part model design: the model of the temperature control spring for building water heating is meshed by Hyper Mesh to construct a finite element model, according to the load condition and functional characteristic requirement of the temperature control spring during service: bearing pressure stress is about 5N, temperature control temperature is 20-60℃, using ANSYS software to simulate its deformation behavior under different service environments, using independent continuous mapping method to optimize the spring wire diameter ratio and effective number of turns of the temperature control spring geometric model;

[0047] (2) Functional unit size determination: The temperature field of a single-track melt pool of 4D printed nickel-titanium alloy and its determined profile size are predicted by the finite volume method. The size relationship of the functional unit corresponding to the large melt pool and the process parameter range in step (3) are determined, the size of the functional unit is determined, the large melt pool width W 大 = 170 μm, the depth H 大 = 140 μm, the small melt pool width W 小 = 170 μm, the depth H 小 = 115 μm, and the melt pool size relationship is specifically: W 大 = W 小 , H 小 > H 大 -t (powder layer thickness t, 30 μm);

[0048] (3) Large and small melt pool combination printing design: The building water heating temperature control spring geometric model obtained in step (1) is imported into the 4D printing forming system to generate two groups of models placed in overlapping positions, and the two groups of process parameters obtained in step (2) are respectively assigned. The energy levels of the two groups of process parameters are different, and satisfy: 2E 低 ≤ E 高 ≤ 4E 低 , specifically: the first group is high laser energy density E 高 = 66 J / mm 3 , laser power P 高 = 100 W, scanning speed V 高 = 1000 mm / s; the second group is low laser energy density E 低 = 26 J / mm 3 , laser power P 低 = 80 W, scanning speed V 低 = 1000 mm / s; the scanning path is: rotating by 67° layer by layer, scanning spacing h = 100 μm, powder layer thickness t = 30 μm, so as to realize large and small melt pool combination printing, and a printing file is generated according to the set process parameters.

[0049] (4) 4D printing functional unit regulation: running the printing file generated in step (3) to perform 4D printing forming on the nickel-titanium alloy powder; large and small melt pool combination printing is adopted, large melt pool is generated by high laser energy density to form the initial functional unit, and small melt pool is generated by low laser energy density to regulate the initial functional unit in situ to the target functional unit, and the specific process is as follows: the large melt pools are overlapped to form the initial functional unit containing three characteristic regions of cellular crystal region, transition region and heat affected zone; the heat affected zone of the small melt pool is located in the cellular crystal region of the large melt pool to conduct in-situ heat treatment on the cellular crystal region of the large melt pool, the core of the small melt pool is located in the transition region and the heat affected zone of the large melt pool to cause remelting, and finally the target functional unit is obtained, and the process is repeated until the high fatigue life nickel-titanium shape memory alloy part based on functional unit regulation is obtained.

[0050] The roughness, phase composition, microstructure characterization and cyclic tensile test of the high fatigue life nickel-titanium shape memory alloy based on functional unit regulation prepared by the above steps are carried out by using 3D optical profiler, X-ray diffractometer, transmission electron microscope and Instron 8862 test system. The results show that the surface roughness of the nickel-titanium shape memory alloy sample prepared by the high-low energy combined printing-functional unit regulation 4D printing method in the embodiment is low, S a is 4.75 μm, and the target functional unit includes an in-situ heat treatment zone with a small pool boundary and a remelting zone with a small pool core. The in-situ heat treatment zone is cellular crystal with a size of 600-800 nm, and Ti2Ni precipitated phase is uniformly distributed in the cellular crystal with a size of about 25 nm, and has a semi-coherent orientation relationship with the matrix. The remelting zone is coarsened columnar crystal with a width of 6-8 μm, and Ti2Ni is dispersedly distributed with a size of 50 nm. In the maximum constant load (200 MPa) temperature driven fatigue test, the shape memory response reaches stability after only 6 cycles, the recoverable strain is 4.4%, and the fatigue life can reach 497 times. Compared with the related literature reports, such as the recoverable strain of 1.25% after 10 super-elasticity cycles (Mater. Design 220 (2022) 110886), the recoverable strain of 1.8% after 10 super-elasticity cycles (J. Mater. Res. Technol. 13 (2021) 241-250), the fatigue life of 430 times at a maximum strain of 4.5% (Int. J. Fatigue 66 (2014) 78-85), and the single constant load (200 MPa) temperature driven strain of 4% with a residual strain of 1% (Scripta Mater. 14 (2018) 164-168), the nickel-titanium shape memory alloy prepared by the present application has excellent fatigue life. The temperature control spring made of the nickel-titanium shape memory alloy prepared by the present application is used for building water heating, can drive deformation adjustment of hot water flow by water temperature, can improve the temperature control sensitivity of constant temperature shower and constant temperature faucet, and the temperature control spring is not easy to age and has a service life of 10 years.

[0051] Example 3

[0052] (1) Part model design: the multi-aerospace self-deploying truss hinge model is meshed by Hyper Mesh to construct a finite element model, according to the load condition and functional characteristic requirement of the truss hinge during service: about 2% of reciprocating shear strain is borne during deployment and reset, the deformation temperature is 0-50℃, the deformation behavior of the truss hinge in different service environments is simulated by using ANSYS software, and the thickness distribution of the aerospace self-deploying truss hinge geometric model is optimized by using the variable thickness method;

[0053] (2) Functional unit size determination: The temperature field of a single-track 4D printing NiTi alloy melt pool and its determined profile size are predicted by the finite volume method. The size relationship between the functional unit corresponding to the large melt pool and the process parameter range in step (3) are determined, the size of the functional unit is determined, the large melt pool width W 大 = 180 μm, the depth H 大 = 150 μm, the small melt pool width W 小 = 180 μm, the depth H 小 = 120 μm, and the melt pool size relationship is specifically: W 大 = W 小 , H 小 > H 大 -t (powder layer thickness t, 40 μm);

[0054] (3) Large and small melt pool combination printing design: The self-unfolding truss geometric model obtained in step (1) is imported into the 4D printing forming system to generate two groups of models placed in overlapping positions, and the two groups of process parameters obtained in step (2) are respectively assigned. The energy levels of the two groups of process parameters are different, and satisfy: 2E 低 ≤ E 高 ≤ 4E 低 , specifically: the first group is high laser energy density E 高 = 65 J / mm 3 , laser power P 高 = 250 W, scanning speed V 高 = 800 mm / s; the second group is low laser energy density E 低 = 18 J / mm 3 , laser power P 低 = 70 W, scanning speed V 低 = 800 mm / s; the scanning path is: rotating 67° layer by layer, scanning spacing h = 120 μm, powder layer thickness t = 40 μm, so as to realize large and small melt pool combination printing, and generate a printing file according to the set process parameters.

[0055] (4) 4D printing functional unit regulation: running the printing file generated in step (3) to perform 4D printing forming on the NiTi alloy powder; large and small melt pool combination printing is adopted, large melt pool is generated by high laser energy density to form the initial functional unit, and small melt pool is generated by low laser energy density to regulate the initial functional unit in situ to the target functional unit, and the specific process is: the large melt pools are overlapped to form the initial functional unit containing three characteristic regions of cellular crystal region, transition region and heat affected zone; the heat affected zone of the small melt pool is located in the cellular crystal region of the large melt pool to conduct in-situ heat treatment on the cellular crystal region of the large melt pool, and the core of the small melt pool is located in the transition region and the heat affected zone of the large melt pool to cause remelting, so as to finally obtain the target functional unit, and the process is repeated until the high fatigue life NiTi shape memory alloy part based on functional unit regulation is obtained.

[0056] The roughness, phase composition, microstructure characterization and cyclic tensile test of the high fatigue life nickel-titanium shape memory alloy based on functional unit regulation prepared by the above steps are carried out by using 3D optical profiler, X-ray diffractometer, transmission electron microscope and Instron 8862 test system. The results show that the surface roughness of the nickel-titanium shape memory alloy sample prepared by the high-low energy combined printing-functional unit regulation 4D printing method in the embodiment is low, S a is 6.75 μm, the matrix is B19' martensite phase at room temperature, the target functional unit contains in-situ heat treatment zone of small molten pool boundary and remelted zone of small molten pool core. The in-situ heat treatment zone is cellular crystal with a size of 400-600 nm; Ti2Ni precipitated phase is uniformly distributed in the cellular crystal with a size of about 30 nm, and has a semi-coherent orientation relationship with the matrix. The remelted zone is coarsened columnar crystal with a width of 4-6 μm, and Ti2Ni is dispersedly distributed with a size of about 50 nm. In the constant load (100 MPa) temperature driven fatigue test, the shape memory response reaches stability after only 8 cycles, the recoverable strain is 2.04%, and the fatigue life can reach 558 times. Compared with the related literature reports, such as the recoverable strain of 1.25% after 10 super-elasticity cycles (Mater. Design 220 (2022) 110886), the recoverable strain of 1.8% after 10 super-elasticity cycles (J. Mater. Res. Technol. 13 (2021) 241-250), the fatigue life of 430 times at the maximum strain of 4.5% (Int. J. Fatigue 66 (2014) 78-85), the single constant load (200 MPa) temperature driven strain of 4% and the residual strain of 1% (Scripta Mater. 14 (2018) 164-168), the nickel-titanium shape memory alloy prepared by the present application has excellent fatigue life. The aerospace self-deploying truss made of the nickel-titanium shape memory alloy prepared by the present application has a fatigue life of 10 max times at the shear strain γ 3 of 2%, and a service life of 4-5 years.

[0057] Comparative Example 1

[0058] (1) Part model design: the porous spine rod medical implant model is meshed by Hyper Mesh to construct a finite element model, according to the load condition and functional characteristic requirement of the porous spine rod during service: pre-bending before implantation, recovery temperature close to human body temperature, and bearing bending stress against spinal deformity after implantation. The deformation behavior of the porous spine rod in different service environments is simulated by using ANSYS software, and the pore diameter, rod diameter and porosity of the porous spine rod geometric model are topologically optimized by using the progressive structure optimization method;

[0059] (2) Single-pool printing design: the porous spinal rod medical implant model obtained in step (1) is imported into the 4D printing forming system, and the model is given process parameters of high laser energy density, energy density E = 53 J / mm 3 , corresponding laser power P = 180 W, scanning speed v = 1400 mm / s, scanning path: strategy rotates 90° layer by layer, scanning spacing h = 80 μm, powder layer thickness t = 30 μm, thus, a printing file is generated according to the set process parameters;

[0060] (3) 4D printing: running the printing file generated in step (2) to perform 4D printing on the nickel-titanium alloy powder; high laser energy density is used for separate printing, high laser energy density generates a large pool, the pool width W = 160 μm, the depth H = 80 μm, forming a functional element, repeating the process until a nickel-titanium shape memory alloy part is obtained.

[0061] As Figures 1 to 2 shown, 3D optical profilometer, X-ray diffractometer, transmission electron microscope, Instron8862 test system are used to characterize the roughness, phase composition, microstructure and cyclic tensile test of the nickel-titanium shape memory alloy prepared by the above steps. The results show that the surface roughness of the nickel-titanium shape memory alloy sample in the comparative example is larger, S a is 22.57 μm, the matrix is B2 austenite phase at room temperature, the grain size of the cellular crystal region is 300-800 nm, Ti2Ni precipitated phase is continuously distributed at the grain boundary, the size is about 100 nm, and Ti2Ni has no definite orientation relationship with the matrix. The heat affected zone is columnar crystal, the columnar crystal width is 0.5-1.5 μm, Ti2Ni is dispersedly distributed, and the size is about 80 nm. The transition zone is the transition of cellular crystal and columnar crystal, and Ti2Ni is dispersedly distributed. In the tensile cycle test with a maximum strain of 3%, the sample breaks in the loading process of the second cycle, and cannot reach the stable superelasticity. The porous spinal rod medical implant made of the nickel-titanium shape memory alloy prepared by the present application cannot meet the service requirements.

[0062] Comparative Example 2

[0063] (1) Part model design: the building water heating temperature control spring model is meshed by Hyper Mesh to construct a finite element model, according to the load condition and functional characteristic requirement of the temperature control spring during service: bearing pressure stress about 5 N, temperature control temperature 20-60℃, using ANSYS software to simulate its deformation behavior under different service environments, using independent continuous mapping method to optimize the spring wire diameter ratio and effective number of the temperature control spring geometric model;

[0064] (2) Functional unit size determination: The temperature field of a single-track 4D printing nickel-titanium alloy melt pool and its determined contour size are predicted by the finite volume method. The size relationship of the functional unit corresponding to the large melt pool and the process parameter range in step (3) are determined, the size of the functional unit is determined, the large melt pool width W 大 = 160 μm, the depth H 大 = 80 μm, the small melt pool width W 小 = 160 μm, and the melt pool size relationship is specifically: the depth H 小 = 40 μm, H 小 < H 大 -t (powder layer thickness t, 30 μm);

[0065] (3) Large and small melt pool combined printing design: The building water heating temperature control spring geometric model obtained in step (1) is imported into the 4D printing forming system, the two groups of models are overlapped and placed, and the two groups of process parameters obtained in step (2) are respectively given. The energy levels of the two groups of process parameters are different, E 高 > 4E 低 , specifically: the first group is high laser energy density E 高 = 53 J / mm 3 , laser power P 高 = 180 W, scanning speed V 高 = 1400 mm / s; the second group is low laser energy density E 低 = 9 J / mm 3 , laser power P 低 = 30 W, scanning speed V 低 = 1400 mm / s; the scanning path is: layer by layer rotation 90°, scanning spacing h = 80 μm, powder layer thickness t = 30 μm, so as to realize large and small melt pool combined printing, and generate a printing file according to the set process parameters.

[0066] (4) 4D printing functional unit regulation: running the printing file generated in step (2) to 4D print nickel-titanium alloy powder; large and small melt pool combined printing is adopted, large melt pool is generated by high laser energy density to form initial functional unit, small melt pool is generated by low laser energy density, the depth of the small melt pool is too small to effectively regulate the initial functional unit, and the next layer of large melt pool will completely remelt and cover the small melt pool, and the process is repeated until the nickel-titanium shape memory alloy part is obtained.

[0067] The roughness, phase composition, microstructure characterization and cyclic tensile test of the nickel-titanium shape memory alloy prepared by the above steps are carried out by using 3D optical profiler, X-ray diffractometer, transmission electron microscope and Instron 8862 test system. The results show that in the comparative example, the low laser energy density is too low, the small molten pool size is too small, the surface roughness of the prepared nickel-titanium shape memory alloy sample is large, Sa is 11.73 μm, the matrix is B2 austenite phase at room temperature, the cellular crystal zone, the grain size is 500-800 nm, most of Ti2Ni is still distributed at the cellular crystal grain boundary, and there is no certain orientation relationship with the matrix, and the size is about 100 nm. The heat affected zone is columnar crystal, the columnar crystal width is 1-2 μm, Ti2Ni is dispersedly distributed, and the size is about 80 nm. The transition zone is the transition of cellular crystal and columnar crystal, Ti2Ni is dispersedly distributed, and in the tensile cycle test with the maximum strain of 3%, the sample is broken in the loading process of the 6th cycle, and the superelasticity is not stable. In the constant load (100 MPa) temperature driving fatigue test, the shape memory effect decays significantly, the recoverable strain is only 1.08%, and the fatigue life is only 156 times. The temperature control spring made of the nickel-titanium shape memory alloy prepared by the present application cannot meet the service requirements.

[0068] The above examples are the preferred embodiments of the present application, but the embodiments of the present application are not limited by the above examples, and any changes, modifications, substitutions, combinations, simplifications made without departing from the spirit and principles of the present application should be equivalent replacement methods, and are included in the protection scope of the present application.

Claims

1. A 4D printing preparation method for high fatigue life nickel-titanium shape memory alloy based on functional element regulation, characterized in that: The steps include: (1) Part model design: Mesh the part model to construct a finite element model. Based on the load conditions and functional characteristics determined by the part's service environment, simulate its deformation behavior under different service environments and topologically optimize the part's geometric model. (2) Determination of the size of the functional primitive: The temperature field of the single-pass molten pool of 4D-printed nickel-titanium alloy and its determined contour size are predicted by the finite volume method, and the size relationship and process parameter range of the large and small molten pools corresponding to the functional primitive in step (3) are determined to determine the size of the functional primitive; (3) Process design for combined printing of large and small molten pools: The part geometry model obtained in step (1) is imported into the 4D printing molding system to generate two sets of overlapping models, and two sets of process parameters of the large and small molten pools obtained in step (2) are assigned respectively, thereby realizing combined printing of the large and small molten pools, and generating a printing file according to the set process parameters; (4) 4D printing functional primitive regulation: Run the print file generated in step (3) to perform 4D printing on nickel-titanium alloy powder; use a combination of large and small molten pools for printing, high laser energy density to generate a large molten pool to form an initial functional primitive, low laser energy density to generate a small molten pool, and in situ regulate the initial functional primitive to be the target functional primitive, repeat this process until a high fatigue life nickel-titanium shape memory alloy based on functional primitive regulation is obtained; The specific steps of in-situ regulating the initial functional primitive to the target functional primitive in step (4) are as follows: the large molten pools overlap each other to form an initial functional primitive containing three characteristic zones: a cellular crystal zone, a transition zone, and a heat-affected zone; the heat-affected zone of the small molten pool is located in the cellular crystal zone of the large molten pool, the cellular crystal zone of the large molten pool is subjected to in-situ heat treatment, and the core of the small molten pool is located in the transition zone and the heat-affected zone of the large molten pool, so that it is remelted, and finally the target functional primitive is obtained; The high fatigue life nickel-titanium shape memory alloy based on functional element regulation includes a target functional element, which includes an in-situ heat treatment zone at the boundary of a small molten pool and a remelting zone in the core of the small molten pool; the in-situ heat treatment zone is a cellular crystal with a grain size of 800-1000 nm, which contains uniformly distributed Ti2Ni nanocrystals with a size of 10-30 nm, and the Ti2Ni nanocrystals have a coherent or semi-coherent relationship with the matrix; the remelting zone is a coarsened columnar crystal with a width of 4-8 μm, in which Ti2Ni nanocrystals with a size of 10-50 nm are dispersed.

2. The 4D printing preparation method of high fatigue life nickel-titanium shape memory alloy based on functional element regulation according to claim 1 is characterized in that: The part model in step (1) is meshed by Hyper Mesh and then imported into ANSYS software for finite element simulation. The topology optimization method is any one of the variable thickness method, the progressive structure optimization method, and the independent continuous mapping method.

3. The 4D printing preparation method of high fatigue life nickel-titanium shape memory alloy based on functional element regulation according to claim 1 is characterized in that: The size of the molten pool in step (2) is determined according to the molten pool profile, including the molten pool width W = 160 ~ 180 μm and the molten pool depth H = 70 ~ 150 μm. The specific relationship between the molten pool size is: W 大 =W 小 , H 小 >H 大 - t, where t is the thickness of the powder layer.

4. The 4D printing preparation method of high fatigue life nickel-titanium shape memory alloy based on functional element regulation according to claim 1, characterized in that: The energy levels of the two sets of process parameters for the large and small molten pools in step (3) are different and satisfy: 2E 低 ≤E 高 ≤4E 低 , specifically: the first group is high laser energy density E 高 =40~65J / mm³, laser power P 高 =180W~250W, scanning rate V 高 =800~1400mm / s; The second group is low laser energy density E 低 =9~32 J / mm³, laser power P 低 =45W~125W, scanning rate V 低 =800~1400mm / s; the scanning path is: 67~90° rotation layer by layer, scanning spacing h=80~120μm, powder layer thickness t=30~40μm.

5. A high fatigue life nickel-titanium shape memory alloy based on functional element regulation, characterized in that: Prepared by the 4D printing preparation method according to any one of claims 1 to 4.

6. An application of the high fatigue life nickel-titanium shape memory alloy based on functional element regulation according to claim 5, characterized in that: The application specifically involves utilizing the dynamic load superelastic properties and cyclic deformation shape memory function of nickel-titanium shape memory alloy in the fields of medical devices, medical implants, building plumbing products, automotive industry products, and aerospace products; the medical devices include vascular stents, artificial heart valves, dental files, and orthodontic wires; and the medical implants include spinal implants, shoulder implants, craniofacial implants, ankle implants, and sternum implants.

7. The application of the high fatigue life nickel-titanium shape memory alloy based on functional element regulation according to claim 6, characterized in that: The building water heating products include elastic heat coolers and temperature control springs; the automotive industry products include temperature sensor triggers and temperature control devices; the aerospace products include self-deployed trusses, self-deployed satellite antennas, engine heat exchange fan clutches, exhaust automatic adjustment nozzles, and flexible wings.

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