Pressure-driven lightweight fastening tool structure and its preparation method based on 4D printing technology

Abstract

The present invention discloses a pressure-driven lightweight fastening tool structure and a preparation method thereof based on 4D printing technology. The pressure-driven lightweight fastening tool structure includes a screw, a threaded fastener and a self-fastening member. The self-fastening member and the threaded fastener are integrated, and the self-fastening member is made of NiTi shape memory alloy as a raw material. At the same time, there is a suitable working gap between the self-fastening member and the screw; the self-fastening member includes an upper ring, a lower ring and a number of connecting rods located between the upper ring and the lower ring. Each connecting rod is obliquely connected between the upper ring and the lower ring and is distributed clockwise or counterclockwise. It can be seen that when the self-fastening member of the present invention is subjected to force, rotational compression will occur, causing the threaded fastener to loosen. At the same time, the self-fastening member will also achieve shape restoration due to temperature change, thereby achieving self-tightening of the fastening tool, thereby solving the problem of loose connection of the fastening part.

Description

Technical Field

[0001] The present invention belongs to the field of laser additive manufacturing, and specifically relates to a pressure-induced lightweight fastening tool structure and a 4D printing method thereof. Background Art

[0002] Fasteners, as one of the most widely used basic mechanical parts, are widely used in daily life and production. Bolts are one of the most important fastener parts, playing an important role in fastening and connecting in the fields of aerospace, shipbuilding, automobiles, machinery, etc. However, various types of mechanical equipment will inevitably experience impacts and vibrations during operation, causing the fastening bolts to loosen and thus leading to serious accidents. On the other hand, the inspection and tightening of fastening bolts used in satellites, space stations and other fields where manual maintenance is inconvenient is also an extremely difficult problem. Therefore, there is an urgent need to adopt a simple and effective measure to solve the problem of loose fastening bolts.

[0003] Achieving self-tightening and self-resetting of bolts by changing external conditions can be a new approach to solving the problem of loosening fasteners. Shape memory alloy is a new functional material with deformation recovery capabilities. After deformation, the material can return to its original shape due to its unique shape memory effect and superelastic effect. The properties of shape memory alloy make it the preferred material for the production of self-tightening and self-resetting bolts. On the other hand, bionic chiral structures are gradually being used in a growing number of industrial applications due to their load-bearing capacity and energy dissipation capabilities. By reducing the impact of impact and vibration, a new bionic chiral structure can be used to improve the shock absorption and impact resistance of fastening bolts and reduce the probability of bolt loosening. Laser additive manufacturing technology achieves part manufacturing by accumulating material layer by layer, which has inherent advantages for forming complex structural parts. It also has the advantages of short production cycles and high part precision, making it particularly suitable for the rapid small-batch production of complex structural components.

[0004] Based on laser additive manufacturing technology and the characteristics of shape memory alloys, combined with the practical problems faced in life and production, the design of a lightweight fastening and anti-loosening bolt has positive practical significance for reducing production costs and reducing the occurrence of serious accidents. Summary of the Invention

[0005] The purpose of the present invention is to provide a pressure-driven lightweight fastening tool structure and a preparation method thereof based on 4D printing technology to address the problem of loosening and failure of fastening bolts that actually exist in daily life and production. The pressure-driven lightweight fastening tool is based on the shape memory effect and superelastic effect peculiar to shape memory alloys and the unique mechanical properties of bionic chiral structures. Through external temperature changes, it causes a thermoelastic martensitic phase transformation inside the material, which causes the bolts that have loosened due to vibration to rotate, thereby achieving self-tightening of the bolts and avoiding the occurrence of major safety accidents. In addition, the pressure-driven lightweight fastening tool adopts a laser additive manufacturing production method, which reduces the materials required to produce bolts and reduces the cost of component forming, in line with the goal of low-carbon intelligent manufacturing.

[0006] In order to achieve the above technical objectives, the present invention will adopt the following technical solutions:

[0007] A lightweight pressure-driven fastening tool structure for fastening workpieces, comprising a screw and a threaded fastener engaged with the screw thread, the screw having a nut at its tail end; and a self-fastening member; the self-fastening member and the threaded fastener are integrally provided, the self-fastening member being made of NiTi shape memory alloy, and having a suitable working gap between the self-fastening member and the screw.

[0008] The self-fastening member includes an upper circular ring, a lower circular ring, and a plurality of connecting rods located between the upper circular ring and the lower circular ring, wherein each connecting rod is obliquely connected between the upper circular ring and the lower circular ring and is distributed clockwise or counterclockwise;

[0009] During installation, after the front end of the screw passes through the workpiece, the self-fastening member and the threaded fastener are sequentially sleeved on the screw, and then the threaded fastener is rotated to push the self-fastening member toward the workpiece until the upper ring is tightly against the workpiece to achieve the fastening of the workpiece. At this time, the self-fastening member is in a state where the lower ring is constrained and the upper ring can rotate freely.

[0010] When the workpiece vibrates during operation, the angle θ between the connecting rod and the central axis of the self-fastener can change, causing the connecting rod to twist clockwise or counterclockwise and drive the upper ring to move toward the lower ring. The self-fastener is compressed and causes the threaded fastener to rotate, thereby causing the threaded fastener to loosen the workpiece. During this process, the self-fastener can undergo temperature changes, causing the self-fastener to undergo a phase change and then produce shape recovery. The force generated by the self-fastener during the shape recovery process can drive the threaded fastener to rotate in the opposite direction, pushing the self-fastener toward the workpiece until the upper ring is tightly pressed against the workpiece, thereby achieving self-fastening of the workpiece.

[0011] Preferably, the self-fastening member is first printed by laser powder bed fusion technology and then manufactured by cyclic compression axial deformation behavior test; in the cyclic compression axial deformation behavior test, the test temperature T is at the austenite transformation temperature of the self-fastening member. The above is carried out, and the test temperature T meets:

[0012]

[0013] Yield strength σ of NiTi shape memory alloy used in self-fastening fasteners y satisfy:

[0014]

[0015] Indicates the temperature at which austenite completes transformation after deformation; (σ0) SIM is the minimum critical stress for stress-induced martensitic transformation of self-fasteners; σ is the stress for inducing martensitic transformation of self-fasteners; △σ is the stress hysteresis in the superelastic stress-strain curve; η d is the thermal hysteresis of the phase change cycle, M s Indicates the temperature at which martensite transformation begins.

[0016] Preferably, the upper circular ring is provided with a plurality of first nodes, and the lower circular ring is provided with a plurality of second nodes corresponding one-to-one to the first nodes, and the corresponding first nodes and second nodes are connected by an inclined connecting rod; there is an angle θ between the connecting rod and the central axis of the self-fastening member, so that each connecting rod staggers the first node on the upper circular ring with the second node on the lower circular ring in a clockwise inclined manner.

[0017] Preferably, there are 12 geometric feature points evenly distributed on the upper ring, named in sequence: N1-N12; there are also 12 geometric feature points evenly distributed on the lower ring, named in sequence: n1-n12; the first node N1 and the second node n1 have overlapping projections on the horizontal plane; the second node n1 corresponds to the first node N4 and is connected through an inclined connecting rod, the second node n2 corresponds to the first node N5 and is connected through another inclined connecting rod, and so on, until all the first nodes and the corresponding second nodes are connected through a connecting rod with an angle of θ relative to the central axis.

[0018] Preferably, based on the micropolar theory, the torsion angle of the self-fastening member The relationship between axial strain ε is:

[0019]

[0020]

[0021] Where: H represents the height between the upper and lower rings when the self-tightening fastener is in a naturally stretched state; d represents the diameter of the connecting rod; D represents the diameter of the upper ring / lower ring; δ z and δ y They represent the displacements in the vertical and horizontal directions during the stress process of the self-tightening fixture.

[0022] Another technical object of the present invention is to provide a method for preparing the self-fastening member in the above-mentioned pressure-driven lightweight fastening tool structure based on 4D printing technology, comprising the following steps:

[0023] Step 1: Determine the forming material and forming parameters of the self-fastening parts: The forming material of the self-fastening parts is Ni-rich pre-alloyed powder Ni 50.6 Ti 49.4 The forming parameters of the self-fastening element are: inclination angle θ = 41°, diameter of the upper ring / lower ring D = 10.70 mm, diameter of the connecting rod d = 0.7 mm, and height H = 8.7 mm between the upper and lower rings when the self-fastening element is in its naturally stretched state.

[0024] Step 2: Analyze the geometric structure of the self-fastening fixture, reduce the dimensionality of the 3D data of the self-fastening fixture to 2D plane slice data, and transmit the data to the industrial computer for storage;

[0025] Step 3: Based on the forming material selected in step 1 and the two-dimensional plane slice data obtained in step 2, the self-fastening component is printed using laser powder bed fusion technology. The laser process parameters are set as follows: laser power of 250W, laser scanning speed of 1200mm / s, powder thickness of 30μm, scanning spacing of 60μm, linear grating scanning, and a 36° deflection of the scanning vector between adjacent layers.

[0026] Step 4: Perform cyclic compression axial deformation behavior test on the self-fastening component printed in step 3 until the relative recovery rate of the self-fastening component gradually stabilizes at 1 and the cumulative irreversible displacement no longer increases; the test temperature T is at the austenite transformation temperature. The above is carried out; the test temperature T meets:

[0027]

[0028] Yield strength σ of NiTi shape memory alloy used in self-fastening fasteners y satisfy

[0029]

[0030] Indicates the temperature at which austenite completes transformation after deformation; (σ0) SIMis the minimum critical stress for stress-induced martensitic transformation of self-fasteners; σ is the stress for inducing martensitic transformation of self-fasteners; △σ is the stress hysteresis in the superelastic stress-strain curve; η d is the thermal hysteresis of the phase change cycle, M s Indicates the temperature at which martensite transformation begins.

[0031] Preferably, the cyclic compression axial deformation behavior test in step 4 is performed under displacement control, with the maximum displacement set to 0.89 mm, and a total of 10 loading and unloading cycles are performed.

[0032] Based on the above technical objectives, the present invention has the following advantages over the prior art:

[0033] 1. The present invention uses shape memory alloy powder as raw material to produce self-fastening parts with bionic chiral structures. Compared with traditional bolts, self-fastening parts can achieve self-resetting and self-tightening during use, which reduces the difficulty of maintenance in industries such as space stations, aerospace, and ships, and greatly reduces the probability of equipment failure.

[0034] Typically, aircraft experience vibrations and turbulence during flight (for example, the flap track fairing on passenger aircraft, also known as the boat-shaped fairing, is a shell-like structure that wraps around the flap track and other flap actuating components. Its function is to organize the airflow under the aircraft and reduce drag. Therefore, this part inevitably experiences vibrations caused by airflow changes, which is prone to bolt loosening and failure). The loosening of conventional fasteners has a significant impact on the safety of the aircraft itself. On the other hand, if bolts and screws that fall off due to vibration are sucked into other aircraft, they can damage the engine, cut the aircraft tires, and cause casualties. The self-fastening member with shape memory effect formed by the present invention can achieve self-fastening of the fastening part during flight by utilizing the heat generated by the friction between the aircraft itself and the air, and the shape memory effect unique to NiTi shape memory alloy. In addition, the self-resetting bolt of the present invention can achieve a maximum compression deformation of 48% during vibration, showing strong load-bearing capacity. At the same time, in subsequent repeated loading and unloading tests, the self-resetting bolt still exhibits good shape recovery ability, which can meet the service life requirements of practical applications. It can be seen that the pressure-driven lightweight fastening tool structure described in the present invention is particularly suitable for replacing conventional fasteners (ordinary bolts or screws and other fasteners) on aircraft, effectively reducing the probability of accidents, and is also a more convenient measure to improve the safety factor of aircraft.

[0035] 2. The present invention uses laser additive manufacturing technology to print and form self-fastening parts. The formed parts have excellent mechanical properties and good surface quality. Compared with traditional bolts, their load-bearing capacity and energy dissipation capacity are greatly improved, which reduces material consumption and production costs, and meets the goal of low-carbon intelligent manufacturing.

[0036] 3. The lightweight fastening structure with shape memory effect described in the present invention has strong flexibility and can change its own structural parameters according to actual needs, such as the angle between the connecting rod and the central axis, the inner diameter of the upper and lower rings, etc., to obtain the mechanical properties required for actual application scenarios.

[0037] 4. The present invention adopts axial compression deformation behavior test to conduct mechanical property test on the formed self-fastener. The relationship between force, axial displacement and angle change obtained can be used as reference data for practical application and has certain practical significance.

[0038] 5. The self-fastening parts formed by 4D printing technology in the present invention are able to achieve targeted changes in shape, performance or function under external stimulation compared to components formed by general 3D printing technology. The self-fastening parts are in an uncompressed and unfastened state in the initial state. During use, collisions are inevitable, causing the fasteners to loosen. When the self-fastening parts are subjected to external stimulation such as temperature changes, their shape will change due to phase change, thereby achieving shape recovery of the self-fastening parts and targeted tightening of bolt fasteners. Compared with traditional fasteners, they have outstanding advantages. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 This is a schematic structural diagram of the pressure-driven lightweight fastening tool structure of the present invention;

[0040] Figure 2 This is a characteristic diagram of the shape memory effect phenomenon of shape memory alloy;

[0041] Figure 3 This is a characteristic diagram of the superelastic effect phenomenon of shape memory alloy;

[0042] Figure 4 This is a structural diagram of a self-fastening member with shape memory effect;

[0043] Figure 5 Simplified structural diagram of self-fastening parts with shape memory effect;

[0044] Figure 6 Force-displacement curve of the compression process of shape memory effect self-fastening parts prepared by laser powder bed fusion technology;

[0045] Figure 7Simulate stress cloud diagram of the initial state of compression deformation of shape memory effect self-fasteners prepared by laser powder bed fusion technology;

[0046] Figure 8 The stress cloud diagram of 13.792% strain of shape memory effect self-fastener prepared by laser powder bed fusion technology;

[0047] Figure 9 The simulated stress cloud diagram of 41.379% strain of shape memory effect self-fastener prepared by laser powder bed fusion technology;

[0048] Reference numerals: 1, screw; 2, workpiece; 3, self-fastening member; 4, threaded fastener;

[0049] 3-1, upper ring; 3-1-1, first node; 3-2, connecting rod; 3-3, lower ring; 3-3-1, second node. DETAILED DESCRIPTION

[0050] The technical solutions in the embodiments of the present invention will be described clearly and completely below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. The following description of at least one exemplary embodiment is actually only illustrative and in no way serves as any limitation on the present invention and its application or use. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative work are within the scope of protection of the present invention. Unless otherwise specified, the relative arrangement of components and steps, expressions and numerical values ​​described in these embodiments do not limit the scope of the present invention. Technologies, methods and equipment known to ordinary technicians in the relevant fields may not be discussed in detail, but where appropriate, the technologies, methods and equipment should be considered part of the specification. In all examples shown and discussed here, any specific values ​​should be interpreted as merely exemplary and not as limitations. Therefore, other examples of the exemplary embodiments may have different values.

[0051] For ease of description, spatially relative terms such as "above", "above", "on the upper surface of", "above", etc. may be used here to describe the spatial positional relationship between a device or feature and other devices or features as shown in the figures. It should be understood that spatially relative terms are intended to include different orientations of the device in use or operation in addition to the orientation described in the figures. For example, if the device in the drawings is inverted, the device described as "above other devices or structures" or "above other devices or structures" will be positioned as "below other devices or structures" or "below other devices or structures". Thus, the exemplary term "above" can include both "above" and "below". The device can also be positioned in other different ways (rotated 90 degrees or in other orientations).

[0052] The present invention discloses a pressure-driven lightweight fastening tool structure, including a screw, a self-fastening member and a threaded fastener; the screw has a nut at the tail end, the self-fastening member and the threaded fastener are integrally arranged, and the self-fastening member is located between the nut and the threaded fastener. The threaded fastener is provided with an internal thread that matches the external thread on the screw, so that the threaded fastener can be connected with the screw thread; the self-fastening member is made of NiTi shape memory alloy powder as raw material and through laser powder bed fusion (LPBF) technology. In the present invention, the self-fastening member has a structure as follows Figure 3As shown, the self-tightening device includes an upper ring 3-1, a lower ring 3-3, and several connecting rods 3-2 located between the upper ring 3-1 and the lower ring 3-3. The upper ring 3-1 is provided with several first nodes 3-1-1, and the lower ring 3-3 is provided with several second nodes 3-3-1 corresponding one-to-one to the first nodes 3-1-1. The corresponding first nodes 3-1-1 and second nodes 3-3-1 are connected by an inclined connecting rod 3-2. That is, the upper end of each connecting rod 3-2 is connected to the first node 3-1-1 provided on the upper ring 3-1, and the lower end is connected to the corresponding second node 3-3-1 provided on the lower ring 3-3. An angle θ is formed between the connecting rod 3-2 and the central axis of the self-tightening device, so that each connecting rod 3-2 staggers the first node 3-1-1 on the upper ring 3-1 with the second node 3-3-1 on the lower ring 3-3 in a clockwise tilted manner. When connecting rod 3-2 is subjected to external force F, the angle between connecting rod 3-2 and the central axis of the self-fastening member changes under the combined action of the horizontal force component Fx and the vertical force component Fy, causing the self-fastening member to rotate about its own axis and undergo axial compression. When tightening a workpiece, after the front end of the screw is passed through the workpiece, the self-fastening member and the threaded fastener are sequentially sleeved on the screw, and then the threaded fastener is rotated to push the self-fastening member toward the workpiece until the self-fastening member and the workpiece are tightly abutted, thereby achieving the tightening of the workpiece. When the workpiece vibrates during operation, it compresses the self-fastening member, causing the threaded fastener to rotate and loosen. During this process, the position of the workpiece will undergo temperature changes due to the heat released during operation, causing the self-fastening member to undergo shape recovery due to phase change. Since the self-fastening member and the threaded fastener are an integral component, the force generated by the self-fastening member during the shape recovery process will drive the threaded fastener to rotate in the opposite direction, thereby tightening the workpiece.

[0053] In the accompanying drawings, 12 geometric feature points are evenly distributed on the upper ring 3-1, sequentially named N1-N12. Twelve geometric feature points are also evenly distributed on the lower ring 3-3, sequentially named n1-n12. The horizontal projections of the first node N1 and the second node n1 coincide. Each first node is connected to a second node using a staggered connection. In this embodiment, the second node n1 is connected to the first node N4 via an inclined connecting rod, the second node n2 is connected to the first node N5 via another inclined connecting rod, and so on, until all first nodes are connected to their corresponding second nodes via a connecting rod with an angle of θ relative to the central axis.

[0054] Since the self-fastening member of the present invention adopts the laser additive manufacturing integrated forming processing method, the connecting rod 3-2 is actually a whole with the upper ring 3-1 and the lower ring 3-3. It is only the connecting rod 3-2 that plays the role of connection and is therefore called a "connecting rod".

[0055] When the self-tightening device is subjected to external force, the angle θ will change, as follows: the change of the angle is mainly due to the application of external force. The angle between the external force F and the horizontal direction is α. After decomposing the external force, the force F along the horizontal direction is x F·cosα, the force F in the vertical direction y The horizontal force F·cosα causes the self-fastener to rotate clockwise around the central axis, and the vertical force F·sinα causes the self-fastener to be compressed in the vertical direction. The combined effect of the two causes the height of the self-fastener to decrease and the angle θ to change. The micropolar theory is used to specifically describe the mechanical behavior of the bolt feature unit, giving each geometric feature point on the bolt 6 degrees of freedom, including 3 rotational degrees of freedom. and 3 planar degrees of freedom μ i , where i = 1, 2, 3; express

[0056] During use, the self-tightening device can be regarded as a linear elastic micropolar medium without residual stress. The relationship between its stress tensor and strain tensor can be calculated according to the following formula:

[0057]

[0058]

[0059] In the above formula, A ijkl and D ijkl Represent the elastic modulus matrix and shear modulus matrix respectively, B ijkl and C ijkl are interplaced with each other, describing the translation-rotation coupling and rotation-translation coupling respectively; the tensor group ζ ij and ε kl Corresponding to the stress field and strain field respectively, the tensor group m ij and Corresponding to the torque field and the rotation field.

[0060] In addition, during the use of the self-fastening device, the upper ring 3-1 can rotate freely, while the lower ring 3-3 can be regarded as being fixedly constrained, that is, u x =u y =0; Based on the micropolar theory, for small deformation of the bolt during use, the change in torsion angle caused by unit axial strain can be calculated according to the following formula:

[0061]

[0062] In the above formula, δ z and δ yrepresent the displacement of the self-tightening member in the vertical and horizontal directions respectively; and for a structure with certain geometric parameters, the displacement δ z and displacement δ y The ratio between them is constant, which can be expressed as:

[0063]

[0064] Furthermore, combining the above two equations, the torsion angle The relationship between axial strain ε can be simplified as:

[0065]

[0066] It can be found that there is a linear relationship between the torsion angle and the axial strain.

[0067] The present invention also provides a method for preparing the aforementioned self-fastening device. Specifically, the method uses NiTi shape memory alloy powder as raw material and employs laser powder bed fusion (LPBF) technology to achieve the integrated formation of complex structural bolts (i.e., the self-fastening device designed for special needs of the present invention). Compression tests and simulated compression tests are also performed on the formed components of the self-fastening device to determine their ultimate deformation. The method specifically comprises the following steps:

[0068] Step 1: Analyze the geometric structure of the self-fastening fixture, reduce the dimensionality of the 3D data of the self-fastening fixture to 2D plane slice data, and transmit the data to the industrial computer for storage;

[0069] Step 2: Set the forming raw materials and forming part parameters. Regular spherical NiTi pre-alloyed powder is used as the raw material, and the nominal composition is Ni 50.6 Ti 49.4 The specific composition is Ni content of 55.73wt.%, C content of 0.01wt.%, N content of 0.015wt.%, H content of 0.0006wt.%, O content of 0.08wt.%, and the balance is Ti. The particle size distribution is 15-53μm; the structure of the self-fastening fastener is as follows: Figure 3 As shown in the figure, the diameters of the upper and lower rings are the same, denoted as D, the height between the upper and lower rings is set to H, the diameter of the connecting rod is d, the length of the connecting rod is L, and the angle between the connecting rod and the central axis of the self-fastening member is θ. The specific parameters are set as follows: θ = 41°, D = 10.70 mm, d = 0.7 mm, H = 8.7 mm;

[0070] Step 3: Laser powder bed fusion (LPBF) technology is used to print the self-fastening parts with the determined raw materials and structure. Before printing, the powder is vacuum dried to ensure good powder fluidity. The drying temperature is set to 120°C and the drying time is 4 hours. The specific laser process parameters are set as follows: laser power of 250W, laser scanning speed of 1200mm / s, powder thickness of 30μm, scanning spacing of 60μm, and a simple linear grating scanning method is used as the scanning strategy, but the scanning vector between adjacent layers is deflected by 36°.

[0071] Step 4: Test the axial compression deformation behavior of the printed self-tightening parts. Use displacement control mode and control the crossbeam movement speed at 2mm / min. Specifically, for the compression failure test, the test is carried out at room temperature. For the cyclic loading and unloading compression behavior, the test is carried out at the austenite transformation temperature. The above is carried out (according to the DSC test of the formed component, the austenite transformation temperature under this parameter is is 90°C), the test temperature of the present invention is set at about 100°C;

[0072] The self-fastening member with shape memory effect of the present invention comprises an upper ring and a lower ring with an inner diameter of 10.70 mm, and twelve connecting rods connecting the upper ring and the lower ring at an angle of 41 degrees (i.e., angle θ) to the central axis.

[0073] In the compression failure test of the upper ring, lower ring and connecting rod, as the load increases, it can be observed that the upper ring and the connected connecting rod undergo obvious torsional deformation around the central axis of the structure;

[0074] Furthermore, the torsional deformation can quantitatively reflect the precise load data of the self-fastener deformation under different displacement conditions through the corresponding force-displacement curve.

[0075] Specifically, according to the evolution law of the force-displacement curve and the structural changes occurring inside the material, the entire compression deformation process can be divided into six stages;

[0076] Specifically, in the linear elastic stage, the strain reaches 3.448% and the cumulative displacement is 0.3mm; in the first quasi-platform stage, the strain reaches 10.345% and the cumulative displacement is 0.9mm; in the hardening stage, the strain can reach 13.793% and the cumulative displacement is 1.2mm; in the second quasi-platform stage, the strain reaches 37.931% and the cumulative displacement is 3.3mm; in the densification stage, the strain reaches 48.276% and the cumulative displacement is 4.2mm; in the crushing stage, the strain exceeds 48.276% and the cumulative displacement exceeds 4.2mm, and the connecting rod breaks.

[0077] Further analysis of the self-fastening parts that broke was conducted, and the fracture location was the connection between the connecting rod and the upper and lower rings. By observing the fracture surface morphology of the broken connecting rod, it can be concluded that when the strain exceeds 48.276%, the self-fastening parts will undergo ductile shear fracture behavior, which is prone to cause safety accidents.

[0078] Furthermore, by numerically simulating the deformation of the compression process, the corresponding stress distribution prediction cloud map under different deformation conditions was obtained;

[0079] Specifically, when the strain reaches 3.448%, the predicted maximum stress value is 604.5MPa, and the higher stress values ​​are mostly distributed at the intersection of the upper and lower rings and the ring-connecting rod, while the stress levels in other parts of the bolt are between 186.9-365.9MPa; when the strain is increased to 6.896%, the average stress level of the bolt is 490.5MPa, and stress-induced martensitic phase transformation occurs (the critical stress of stress-induced martensitic phase transformation fluctuates around 400MPa); the strain is further increased to 10.344%, at which point the overall stress level exceeds the critical stress of martensitic phase transformation, and work hardening occurs; when the strain reaches 13.793%, the intersection of the connecting rod and the upper and lower rings presents a higher stress level, reaching 1203-1713MPa; after the cumulative strain exceeds 34.483%, all connecting rods show obvious torsional deformation, and the predicted maximum stress value increases rapidly;

[0080] Furthermore, based on the analysis of compression deformation characteristics and stress distribution, the torsional behavior of self-fasteners under compression load is analyzed;

[0081] Specifically, according to the previously derived relationship, the torsion angle and the axial compressive strain can be roughly regarded as a linear relationship. At the same time, the torsion angle change and the axial compressive strain obtained from the compression test results and the corresponding numerical simulation results also show a quasi-linear relationship.

[0082] Furthermore, based on the experimental data of this example, under small strain conditions (strain degree <4.6%), the unit axial strain torsion angle λ of the self-tightening device decreases from 2.13° / % to 0.81° / %. According to the previously derived relationship, the λ value continuously decreases with increasing strain. Based on the experimental results and simulation predictions of this example, for large deformation conditions, the λ value first decreases rapidly with increasing strain, and then gradually stabilizes at around 0.6° / %.

[0083] Furthermore, according to the relationship:

[0084]

[0085]

[0086] The quantitative relationship between the connecting rod inclination angle θ and the axial strain in this example can be obtained more intuitively;

[0087] Specifically, it can be concluded from the relationship that when the connecting rod inclination angle θ is in the range of (0, π / 4), the number of compression modulus components decreases monotonically as θ increases, and the number of shear modulus components increases monotonically as θ increases; when the connecting rod inclination angle is in the range of (π / 4, π / 2), the change trend is the opposite;

[0088] Furthermore, for the initial connecting rod inclination angle θ of 41° adopted in this example, an axial strain increment of approximately 13% is required before the value of θ increases to 45°. That is, within the initial axial strain range of 13%, axial compression deformation is relatively easy to carry out, while torsional deformation is relatively difficult.

[0089] Based on the complexity of the actual application and use environment of the designed self-fastening parts with shape memory effect in daily life and production, as well as the reversible martensitic phase transformation in NiTi alloy, this example conducted cyclic compression tests on the printed self-fastening parts to characterize their superelastic effect and corresponding reversible recovery behavior.

[0090] Specifically, based on the process parameter settings of this example, the cyclic compression test experiment was carried out under displacement control, with the maximum displacement set to 0.89 mm (approximately 10.2% axial strain), and a total of 10 loading and unloading cycles were performed;

[0091] Furthermore, in the first loading and unloading cycle, a large residual displacement was obtained, and the recoverable displacement accounted for 0.94%, showing poor recoverability. This is related to the internal dislocation slip and irreversible twin formation caused by the relatively random grain orientation in the bolt.

[0092] Furthermore, as the number of loading and unloading cycles increases, the proportion of recoverable displacement increases significantly. In the fourth cycle, the relative recovery rate begins to gradually stabilize at around 1; the cumulative irreversible displacement no longer increases and gradually stabilizes at 0.065 mm.

[0093] Furthermore, the recovery behavior of the torsion angle is also involved during repeated loading and unloading. Specifically, based on the process parameters adopted in this example and the pressure-induced torsion effect of the structure adopted, the recovery behavior of the torsion angle of the self-fastener is controlled by the axial reversible displacement, and the change patterns of the two are roughly similar.

[0094] The self-fastening devices with shape memory effects developed in this invention address the problems of loosening and tightening difficulties faced by bolts in aerospace, shipbuilding, and machinery applications. Compared to traditional fastening bolts, the self-fastening devices designed in this invention significantly reduce the probability of safety accidents, lower production costs, and reduce subsequent maintenance costs, achieving the goal of low cost and high returns.

Claims

1. A pressure-driven lightweight fastening tool structure for fastening a workpiece, comprising a screw and a threaded fastener connected to the screw thread, wherein the screw has a nut at the tail end; characterized in that: It also includes a self-fastening member; the self-fastening member is integrally provided with the threaded fastener, and the self-fastening member is made of NiTi shape memory alloy as a raw material, and there is a suitable working gap between the self-fastening member and the screw; The self-fastening member includes an upper circular ring, a lower circular ring, and a plurality of connecting rods located between the upper circular ring and the lower circular ring, wherein each connecting rod is obliquely connected between the upper circular ring and the lower circular ring and is distributed clockwise or counterclockwise; During installation, after the front end of the screw passes through the workpiece, the self-fastening member and the threaded fastener are sequentially sleeved on the screw, and then the threaded fastener is rotated to push the self-fastening member toward the workpiece until the upper ring is tightly against the workpiece to achieve the fastening of the workpiece. At this time, the self-fastening member is in a state where the lower ring is constrained and the upper ring can rotate freely. When the workpiece vibrates during operation, the angle θ between the connecting rod and the central axis of the self-fastening member can change, causing the connecting rod to twist clockwise or counterclockwise and drive the upper ring to move toward the lower ring. The self-fastening member is compressed and causes the threaded fastener to rotate, thereby causing the threaded fastener to loosen the workpiece. During this process, the self-fastening member can undergo temperature changes, causing the self-fastening member to undergo a phase change, thereby generating shape recovery. The force generated by the self-fastening member during the shape recovery process can drive the threaded fastener to rotate in the opposite direction, pushing the self-fastening member toward the workpiece until the upper ring and the workpiece are tightly pressed against each other, thereby achieving self-fastening of the workpiece. The self-fastening component is first printed by laser powder bed fusion technology and then manufactured by cyclic compression axial deformation behavior test; in the cyclic compression axial deformation behavior test, the test temperature T is at the austenite transformation temperature of the self-fastening component. The above is carried out, and the test temperature T meets: Yield strength σ of NiTi shape memory alloy used in self-fastening fasteners y satisfy: Indicates the temperature at which austenite completes transformation after deformation; (σ0) SIM is the minimum critical stress for stress-induced martensitic transformation of self-fasteners; σ is the stress for inducing martensitic transformation of self-fasteners; △σ is the stress hysteresis in the superelastic stress-strain curve; η d is the thermal hysteresis of the phase change cycle, M s Indicates the temperature at which martensite begins to transform; Based on the micropolar theory, the torsion angle of the self-tightening fastener The relationship between axial strain ε is: Where: H represents the height between the upper and lower rings when the self-fastening device is in a naturally stretched state; d represents the diameter of the connecting rod; D represents the diameter of the upper ring / lower ring; δ z and δ y They represent the displacements in the vertical and horizontal directions during the stress process of the self-tightening fixture.

2. The pressure-driven lightweight fastening tool structure according to claim 1, characterized in that: The upper circular ring is provided with a plurality of first nodes, and the lower circular ring is provided with a plurality of second nodes corresponding one to one with the first nodes. The corresponding first nodes and second nodes are connected by an inclined connecting rod; there is an angle θ between the connecting rod and the central axis of the self-fastening member, so that each connecting rod staggers the first node on the upper circular ring with the second node on the lower circular ring in a clockwise inclined manner.

3. The pressure-driven lightweight fastening tool structure according to claim 2, characterized in that: There are 12 geometric feature points evenly distributed on the upper ring, named in sequence: N1-N12; there are also 12 geometric feature points evenly distributed on the lower ring, named in sequence: n1-n12; the projections of the first node N1 and the second node n1 on the horizontal plane coincide; the second node n1 corresponds to the first node N4 and is connected by an inclined connecting rod, the second node n2 corresponds to the first node N5 and is connected by another inclined connecting rod, and so on, until all the first nodes and the corresponding second nodes are connected by a connecting rod with an angle of θ relative to the central axis.

4. A method for preparing the self-fastening member in the pressure-driven lightweight fastening tool structure according to claim 1 based on 4D printing technology, characterized in that: The steps include: Step 1: Determine the forming material and forming parameters of the self-fastening parts: The forming material of the self-fastening parts is Ni-rich pre-alloyed powder Ni 50.6 Ti 49.4 The forming parameters of the self-fastening element are: inclination angle θ = 41°, diameter of the upper ring / lower ring D = 10.70 mm, diameter of the connecting rod d = 0.7 mm, and height H = 8.7 mm between the upper and lower rings when the self-fastening element is in its naturally stretched state. Step 2: Analyze the geometric structure of the self-fastening fixture, reduce the dimensionality of the 3D data of the self-fastening fixture to 2D plane slice data, and transmit the data to the industrial computer for storage; Step 3: Based on the forming material selected in step 1 and the two-dimensional plane slice data obtained in step 2, the self-fastening component is printed using laser powder bed fusion technology. The laser process parameters are set as follows: laser power of 250W, laser scanning speed of 1200mm / s, powder thickness of 30μm, scanning spacing of 60μm, linear grating scanning, and a 36° deflection of the scanning vector between adjacent layers. Step 4: Perform cyclic compression axial deformation behavior test on the self-fastening component printed in step 3 until the relative recovery rate of the self-fastening component gradually stabilizes at 1 and the cumulative irreversible displacement no longer increases; the test temperature T is at the austenite transformation temperature. The above is carried out; the test temperature T meets: Yield strength σ of NiTi shape memory alloy used in self-fastening fasteners y satisfy: Indicates the temperature at which austenite completes transformation after deformation; (σ0) SIM is the minimum critical stress for stress-induced martensitic transformation of self-fasteners; σ is the stress for inducing martensitic transformation of self-fasteners; △σ is the stress hysteresis in the superelastic stress-strain curve; η d is the thermal hysteresis of the phase change cycle, M s Indicates the temperature at which martensite transformation begins.

5. The method for preparing the self-fastening member in the pressure-driven lightweight fastening tool structure based on 4D printing technology according to claim 4 is characterized in that: The cyclic compression axial deformation behavior test in step 4 was carried out under displacement control, with the maximum displacement set to 0.89 mm, and a total of 10 loading and unloading cycles were performed.

6. The method for preparing the self-fastening member in the pressure-driven lightweight fastening tool structure based on 4D printing technology according to claim 4, characterized in that: Ni-rich pre-alloyed powder Ni 50.6 Ti 49.4 The Ni content is 55.73wt.%, the C content is 0.01wt.%, the N content is 0.015wt.%, the H content is 0.0006wt.%, the O content is 0.08wt.%, and the balance is Ti. The particle size distribution is 15-53μm.

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