Lightweight nickel-titanium shape memory alloy elastic sheet with long fatigue life and preparation method thereof

Abstract

The invention provides a light-weight long-fatigue-life nickel-titanium shape memory alloy elastic piece and a preparation method thereof.The elastic piece is an integrally-formed sheet-shaped component, has a preset arched shape symmetrical in the austenite phase state, can respond to the environment temperature and conduct reversible phase change between the flattening state and the arched state, and serves as a single driving source; at least one independent heat dissipation execution mechanism can be synchronously controlled; the preparation method comprises the steps of solution treatment with mold constraint and unconstrained free aging treatment mechanical training. The elastic piece has the temperature sensing and driving functions, system passivity, light weight and efficient heat management are achieved, and the elastic piece has the fatigue life not smaller than 100 thousand times and the large stroke of 1.00-4.50 mm.

Description

Technical Field

[0001] This invention relates to the field of shape memory alloy materials technology, specifically to a lightweight, high-fatigue-life nickel-titanium shape memory alloy spring and its preparation method. Background Technology

[0002] Nickel-titanium shape memory alloys, due to their unique shape memory effect and superelasticity, show great promise for applications in thermal management. Currently, thermal management solutions for automobiles and electronic devices, such as traditional paraffin thermostats, motor-driven fans, or liquid cooling systems, are mostly passive response modes, suffering from common problems such as slow response, low temperature control accuracy, and high energy consumption. For example, paraffin thermostats undergo slow phase transitions, leading to lag in temperature regulation; while liquid cooling systems rely on complex structures such as water pumps and piping for continuous operation, resulting in low energy efficiency. Especially in the heat dissipation applications of power batteries and high-power electronic devices, there is a lack of low-power or zero-power drive components capable of actively and adaptively adjusting to temperature changes.

[0003] While existing technologies utilize shape memory alloys for temperature control, they often struggle to simultaneously achieve large deformation, long fatigue life, and precise operating temperature control within limited installation space. This results in significant performance degradation and insufficient reliability under long-term cyclic operation. Therefore, there is an urgent need in this field for a nickel-titanium shape memory alloy spring that combines lightweight design, long lifespan, large stroke, and high precision, along with an industrially feasible fabrication method. This would meet the stringent requirements of next-generation intelligent thermal management systems for efficient, compact, and passive drive components. Summary of the Invention

[0004] The present invention aims to provide a lightweight, high-fatigue-life nickel-titanium (NiTi) shape memory alloy spring and its preparation method. The spring combines the characteristics of lightweight, high fatigue life, large deformation and precise operating temperature point, and can be used as a passive intelligent driving element to directly respond to changes in ambient temperature and achieve efficient and adaptive thermal management. In addition, through optimized heat treatment and mechanical training processes, the spring is ensured to obtain stable and excellent comprehensive performance.

[0005] To achieve the above objectives, the present invention proposes the following technical solution:

[0006] A lightweight, high-fatigue-life nickel-titanium shape memory alloy spring, wherein the spring is an integrally formed sheet-like shape memory alloy component, which is given a symmetrical, pre-set arched shape in the austenitic phase.

[0007] The spring is configured to respond to changes in ambient temperature, be flattened by external force when cooled to the martensitic phase, and generate a restoring force when heated to the austenitic phase to actively restore to the preset arched shape.

[0008] Furthermore, the spring sheet, as a single driving source, can synchronously drive at least one independent heat dissipation control element to operate through active deformation.

[0009] As a preferred embodiment of the present invention, the preset arched shape is a folded wing type, an arc type, or a combination type composed of a plane and an arc.

[0010] As a preferred embodiment of the present invention, the austenitic phase transformation end temperature of the spring sheet is 20-60℃.

[0011] As a preferred embodiment of the present invention, the overall outline length of the spring piece is (30±0.05) mm, the width is (20±0.05) mm, and the thickness is 0.1-0.6 mm.

[0012] As a preferred embodiment of the present invention, the phase transition stroke of the spring sheet tested under a constant load of 5 mm is 1.00-4.50 mm.

[0013] As a preferred embodiment of the present invention, the fatigue life of the spring is not less than 100,000 cycles, and after 100,000 fatigue tests, its stroke attenuation rate does not exceed 10%.

[0014] This invention also provides a method for producing lightweight, high-fatigue-life nickel-titanium shape memory alloy springs, comprising the following steps:

[0015] The nickel-titanium alloy billet is subjected to a solution treatment with mold constraint to impart the symmetrical pre-set arched shape and memory properties to it.

[0016] The solution-treated spring sheet is subjected to free aging treatment without mold constraints in order to accurately set its austenitic phase transformation end temperature.

[0017] Mechanical training is performed on the aging-treated shrapnel to optimize its fatigue performance.

[0018] As a preferred embodiment of the present invention, the solution treatment with mold constraint is carried out at 450-900℃ for 10-360 minutes.

[0019] As a preferred embodiment of the present invention, the free aging treatment is carried out at 200-650℃ for 10-2880 min.

[0020] As a preferred embodiment of the present invention, the mechanical training includes performing 3-50 training cycles;

[0021] Each of the training cycles includes:

[0022] In an environment 10-40°C below the martensitic transformation end temperature of the spring, it is bent in the opposite direction to the training height and maintained for 5-120 seconds, wherein the training height is 2-5 times the initial height of the spring in the preset arched shape in the free state.

[0023] Subsequently, in an environment 10-50°C higher than the austenitic phase transformation end temperature of the spring sheet, it is allowed to recover in a free state for 5-120 seconds.

[0024] As can be seen from the above technical solutions, the technical solution of the present invention provides a lightweight, high-fatigue-life nickel-titanium shape memory alloy spring and its preparation method, which has the following advantages compared with the prior art:

[0025] 1. Achieving true temperature-driven integration and high system integration. This invention creatively integrates the functions of a temperature sensor and a mechanical actuator into a single spring element. This spring can directly sense changes in ambient temperature and convert them into precise mechanical movements and significant restoring force without any external power supply or complex control circuitry. This feature fundamentally simplifies the architecture of the thermal management system, significantly reducing system cost, weight, and size, providing a core foundation for miniaturization, lightweighting, and intelligentization of equipment.

[0026] 2. Possessing the superior characteristic of passive operation, it achieves ultimate energy saving and high safety. Since the spring operates entirely based on the inherent shape memory effect of the material, its movement requires no electrical energy, achieving true zero-power actuation. This not only significantly reduces system energy consumption, conforming to the design concept of green energy saving, but also, because it contains no electronic components or circuits, avoids the risk of electrical sparks, short circuits, and other malfunctions that may occur in complex electromagnetic environments or flammable and explosive scenarios, fundamentally improving system safety and reliability.

[0027] 3. Achieving synergistic effects by using a single drive source to implement multiple control functions. The spring contact of this invention acts as a single drive source, capable of simultaneously triggering at least one or more independent control functions within a single operating cycle. For example, it can simultaneously realize the physical opening of the air duct valve and the electrical connection of the cooling fan circuit. This synergistic drive mechanism, where one action controls multiple functions, enables the cooling system to instantly enter a highly efficient forced convection mode, significantly improving heat dissipation efficiency and achieving a synergistic effect greater than the sum of its parts (1+1>2). This is particularly suitable for modern electronic devices with extremely limited space.

[0028] 4. Exhibits rapid and proactive response capabilities and precise temperature control. The rapid phase transformation process of nickel-titanium deformation shape memory alloy enables the spring to respond to changes in ambient temperature in milliseconds, overcoming the lag in response of traditional components such as paraffin thermostats. Through optimized heat treatment processes, the spring's operating temperature (austenite phase transformation end temperature) can be precisely set within the target range of 20-60℃, achieving high temperature control accuracy. This enables more timely and precise overheat protection and thermal management for core components such as lithium batteries and CPUs.

[0029] 5. The spring sheet exhibits comprehensive mechanical properties combining large deformation, long fatigue life, and high operational precision. Through a special spatial configuration design (folding wing type, arc surface type, and combined type) and an optimized solution-aging-training preparation process, the spring sheet of this invention achieves a large stroke (single phase transformation stroke) of 1.00-4.50 mm within a limited size, ensuring sufficient driving displacement. Simultaneously, its fatigue life is no less than 100,000 cycles, and the stroke attenuation rate after each cycle is strictly controlled within 10%, demonstrating excellent durability and performance stability. Furthermore, after being flattened in a low-temperature martensitic state, the planar profile is no greater than 0.05 mm, ensuring excellent sealing and operational consistency.

[0030] 6. The stable and controllable manufacturing process ensures a high degree of consistency in product performance. The manufacturing method provided by this invention, through a coordinated process chain of establishing the configuration with mold solid solution, precise temperature control during free aging, and optimization of lifespan through cyclic training, achieves precise control over key properties such as shape memory characteristics, phase transition temperature, superelasticity, and fatigue life of the spring sheet. This process route is clear, the parameter window is well-defined, the repeatability is good, and the yield is high, laying a solid technical foundation for the large-scale and standardized production of high-performance nickel-titanium shape memory alloy spring sheets.

[0031] It should be understood that all combinations of the foregoing concepts and the additional concepts described in more detail below can be considered part of the inventive subject matter of this disclosure, provided that such concepts do not contradict each other.

[0032] The foregoing and other aspects, embodiments, and features of the teachings of the present invention will be more fully understood from the following description in conjunction with the accompanying drawings. Other additional aspects of the invention, such as features and / or beneficial effects of exemplary embodiments, will become apparent from the following description or may be learned through practice of specific embodiments according to the teachings of the present invention. Attached Figure Description

[0033] The accompanying drawings are not drawn to scale. In the drawings, each identical or nearly identical component shown in the various figures may be denoted by the same reference numeral. For clarity, not every component is labeled in each figure. Embodiments of various aspects of the invention will now be described by way of example and with reference to the accompanying drawings, wherein:

[0034] Figure 1 This is a schematic diagram of the folding wing type spring structure of the present invention;

[0035] Figure 2 This is a schematic diagram of the arc-shaped spring sheet structure of the present invention;

[0036] Figure 3 This is a schematic diagram of the combined spring-loaded structure of the present invention;

[0037] Figure 4 The DMA curve of the spring sheet in Embodiment 1 of the present invention under a constant load of 5N during a single phase transition.

[0038] Figure 5 The comparative example 8 spring sheet of the present invention is the DMA curve of the first phase transition stroke under a constant load of 5N;

[0039] Figure 6 The attenuation of the stroke values ​​after 100,000 fatigue cycles in Embodiment 1 and Comparative Example 22 of the present invention is compared with that at the initial value. Detailed Implementation

[0040] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the described embodiments of the present invention without creative effort are within the scope of protection of the present invention. Unless otherwise defined, the technical or scientific terms used herein should have the ordinary meaning understood by those skilled in the art.

[0041] The terms "first," "second," and similar words used in the specification and claims of this patent application do not indicate any order, quantity, or importance, but are merely used to distinguish different components. Similarly, unless the context clearly indicates otherwise, the singular forms of "an," "a," or "the," etc., do not indicate a quantity limitation, but rather indicate the presence of at least one. Terms such as "comprising" or "including" indicate that the element or object preceding "comprising" encompasses the features, wholes, steps, operations, elements, and / or components listed following "comprising" or "including," and do not exclude the presence or addition of one or more other features, wholes, steps, operations, elements, components, and / or sets thereof. Terms such as "upper," "lower," "left," and "right" are used only to indicate relative positional relationships; when the absolute position of the described object changes, the relative positional relationship may also change accordingly.

[0042] In a first aspect, the present invention provides a lightweight, high-fatigue-life nickel-titanium shape memory alloy spring.

[0043] The spring is made of nickel-titanium shape memory alloy, and its key feature is its specially designed spatial configuration. The spring is a one-piece molded sheet-like shape memory alloy component, endowed with a symmetrical, pre-set arched shape in the austenitic phase. The spring is configured to respond to changes in ambient temperature; when cooled to the martensitic phase, it can be flattened by external force, and when heated back to the austenitic phase, it generates a restoring force to actively return to the pre-set arched shape. Furthermore, the spring acts as a single driving source, capable of synchronously driving at least one independent heat dissipation control element through active deformation. Figure 1-3 As shown, the spring can be selected from folded-wing type, arc-surface type, or a combination type composed of flat and arc surfaces, all of which are symmetrical structures. The combination type has a flat center and arc surfaces on both sides; alternatively, the center can be arc surface and the sides flat. These configurations allow the spring to actively deform within a confined flat space or a space with limited installation height, while simultaneously possessing the combined advantages of large deformation, high control precision, and high fatigue life. The spring can operate stably at operating temperatures of 20-60℃, with a fatigue life of no less than 100,000 cycles. In terms of performance, the spring's stroke can reach 1.00mm-4.50mm; when flattened under a constant external force of 2N after a complete martensitic transformation, its planar profile is no greater than 0.05mm, ensuring flatness and sealing at low temperatures; furthermore, after 100,000 fatigue cycles, its stroke attenuation rate does not exceed 10%, demonstrating excellent durability and performance stability.

[0044] The nickel-titanium shape memory alloy spring provided by this invention can be used in various temperature-sensitive devices that require intelligent thermal management, especially in the heat dissipation control systems of various battery systems such as power battery packs, cell modules, and high-power electronic devices such as laptops and desktop computer hosts.

[0045] This invention limits the operating temperature of the spring contact to 20-60℃, precisely meeting the actual temperature management requirements of core components in the aforementioned application scenarios. For example, the optimal operating temperature for lithium-ion batteries in laptops is approximately 20℃. When the temperature approaches 35℃, charging efficiency may decrease, while temperatures exceeding 40℃ or higher may accelerate battery aging and pose safety hazards. Therefore, setting the austenitic phase transformation end temperature (Af) of the spring contact within this temperature range, such as 35-45℃, enables precise and automatic overheat protection.

[0046] In practical applications, the spring can be fixed to the valve body, air duct, or bracket using conventional methods such as gluing, riveting, or mechanical snap-fit / press-fitting. Its core working mechanism lies in utilizing the inherent shape memory effect of the material to achieve purely physical, passive active temperature control. Crucially, within one operating cycle, the spring of this invention can act as a single drive source, simultaneously triggering two independent control functions, thereby achieving synergistic efficiency in the heat dissipation system. This is a significant advantage in space-constrained devices such as laptops that require efficient heat dissipation.

[0047] In some specific embodiments, a high-temperature resistant epoxy resin adhesive can be selected for bonding, which can be used between 150-250°C.

[0048] The specific working process of the spring clip of the present invention is as follows:

[0049] Normal / Low Temperature Off State: When the device is at its optimal operating temperature or in hibernation mode, such as when the battery temperature of a laptop is below 30°C and the ambient temperature is below the martensitic phase transformation end temperature (Mf) of the spring, the spring is in the martensitic state. The material is relatively soft and the restoring force is small. The force of the return spring in the system is greater than the deformation force of the spring, forcing the spring to remain flat. This drives the valve core to keep the air duct or liquid cooling passage closed, or simultaneously close the main air duct passage and disconnect the power supply circuit of the cooling fan, so that the system is in a low power consumption state, which is beneficial for the device to keep warm and operate quietly.

[0050] High Temperature / Overheating Activation: When the device operates under high load, causing the core temperature to rise to the trigger point (e.g., a laptop CPU / battery temperature exceeding 40°C), and the ambient temperature rises above the austenitic phase transformation end temperature (Af) of the spring, the spring rapidly transforms into a hard austenitic phase, generating a large restoring force. This restoring force is sufficient to overcome the resistance of the return spring, driving the spring from a flat state to an arched or bent state. This active deformation action opens at least one cooling channel or heat dissipation path, such as the valve core of an air-cooled or liquid-cooled path, forcing heat dissipation; preferably, it achieves a dual-drive effect: firstly, the spring body directly pushes open the air duct valve, opening the physical heat dissipation path; secondly, a specific part of the spring simultaneously triggers a microswitch or connects a pair of electrical contacts during the arching process, thereby automatically starting the cooling fan. The synchronous opening of the air duct and the fan forms forced convection, achieving efficient and coordinated heat dissipation.

[0051] Cooling and Reset State: When the cooling system is active and the ambient temperature drops to a safe range (e.g., below 35°C), the spring cools down and transforms back into martensite, reducing its restoring force. The reset spring regains control, pressing the spring back to its flat state. This process closes all previously opened heat dissipation pathways. If the fan power is simultaneously cut off and the air duct is closed, the system returns to its initial state, completing a full intelligent temperature control cycle.

[0052] Through the above methods, the spring of the present invention successfully integrates the functions of a temperature sensor and a mechanical actuator, achieving intelligent and adaptive thermal management without the need for an external power supply and complex control circuits, fully demonstrating its practical value of simplified structure and reduced energy consumption. In particular, the spring of the present invention uses a single temperature-sensitive driving element to replace the traditional solution that may require separate temperature sensors, damper actuators, and fan control circuits, achieving a three-in-one integration of sensing, airflow control, and fan start / stop functions. This greatly simplifies the system architecture, reduces cost and energy consumption, and fully demonstrates its outstanding practical value in space-compact electronic devices.

[0053] To clarify that all three structural designs proposed in this invention can achieve the comprehensive performance desired by this invention, the following parameter ranges are defined, as shown in Table 1.

[0054] Table 1. Spring Design Parameters

[0055] Shrapnel type Length L (mm) Width W (mm) Height H (mm) Thickness δ (mm) Travel distance (mm) Surface profile (mm) Fatigue stroke decay rate after 100,000 cycles Folded-wing type 30±0.05 20±0.05 1.10-3.00 0.1-0.6 1.00-3.00 ≤0.05 ≤10% curved surface 30±0.05 20±0.05 1.10-5.00 0.1-0.6 1.00-4.50 ≤0.05 ≤10% Combined type 30±0.05 20±0.05 1.10-3.00 0.1-0.6 1.00-3.00 ≤0.05 ≤10%

[0056] The stroke and surface profile of the spring sheet as shown in Table 1 were tested under the following conditions:

[0057] (1) The spring travel is the DMA test travel value under a constant load of 5N;

[0058] (2) The surface profile of the spring sheet is the surface profile of the spring sheet after it is flattened under an external force of 2N after the complete martensitic phase transformation.

[0059] In Table 1, although the three types of springs have different spatial configurations, resulting in differences in their initial height (H) and achievable single-phase transition stroke, they all follow a unified precision design specification: their length (L) and width (W) are strictly limited to narrow tolerances of (30±0.05) mm and (20±0.05) mm, respectively, and their thickness (δ) is 0.1-0.6 mm. This ensures consistent installation interfaces and compatibility for the springs during application, and also highlights that the present invention can flexibly obtain large deformation amounts (i.e., strokes) ranging from 1.00 to 4.50 mm within a limited flat space by changing the structural height and configuration.

[0060] More importantly, Table 1 clearly states that the stroke must be obtained under specified and uniform test conditions, with a low-temperature profile ≤0.05mm and a stroke attenuation rate ≤10% after 100,000 fatigue tests. This directly confirms that the high control precision and high fatigue life of the spring sheet of this invention are not specific to a single structure, but rather encompass the universal technical effects of the three protected configurations. High control precision is achieved through excellent face wheel width ensuring sealing and consistent action, while high fatigue life is reflected through an extremely low stroke attenuation rate.

[0061] In this invention, any finite flat space refers to a space defined by constraining its length, width, and thickness.

[0062] Secondly, the present invention provides a method for preparing the above-mentioned nickel-titanium shape memory alloy spring. The preparation method includes the following steps:

[0063] (1) Raw material preparation.

[0064] Select a suitable nickel-titanium shape memory alloy sheet as the blank. Preferably, select an annealed blank with an austenitic phase transformation end temperature Af of 20-60℃ and a thickness of 0.1-0.6mm to ensure that the final spring sheet can work within the target temperature range and has the required deformation capability.

[0065] (2) Solution treatment.

[0066] This step is crucial, aiming to obtain a homogeneous supersaturated solid solution that lays the compositional foundation for subsequent aging precipitation. To achieve this, the initially planar nickel-titanium sheet must be loaded into a mold that matches the target configuration of the spring (folded-wing, curved, or combined type), and a clamping force must be applied to constrain it. Optionally, this operation can be performed at room temperature (22-25°C) to ensure that the spring acquires the preset shape memory characteristics and a large stroke of 1.00-4.50 mm.

[0067] Under the constraint of the mold, the component is placed in a vacuum heat treatment furnace and evacuated to a vacuum level of 0.2 × 10⁻⁶. -3 -0.8×10 -3 Pa was subjected to solution treatment at 450-900℃ for 10-360 min, followed by rapid cooling.

[0068] Optionally, the vacuum heat treatment equipment may be a sliding rail type vacuum tube furnace or a horizontal vacuum box furnace, preferably with an empty furnace room temperature vacuum degree of not less than 6×10⁻⁶. -4 Pa.

[0069] (3) Aging treatment. The core of this stage is to precisely control the phase transition behavior and hyperelasticity of the spring sheet by controlling the morphology and distribution of the precipitated phase.

[0070] Specifically, the spring sheet, after solution treatment and cooling, is removed from the mold, leaving it in a free and unconstrained state. Then, the spring sheet is placed back into the aforementioned vacuum heat treatment equipment and maintained at a temperature range of 200-650°C for 10-2880 minutes, followed by cooling.

[0071] During this process, strengthening phases such as Ni4Ti3 are uniformly and finely precipitated, thereby precisely setting the austenitic phase transformation end temperature Af of the spring sheet within the target range of 20-60℃. It also ensures that after the spring sheet undergoes complete martensitic phase transformation, when it is flattened under a constant external force of 2N, its planar profile is ≤0.05mm and its arch height is ≤0.30mm, so as to meet the flatness and sealing requirements under low temperature conditions.

[0072] (4) Mechanical training.

[0073] This step involves mechanically training the shrapnel to optimize its microstructure and fatigue performance.

[0074] This training process is repeated cyclically, and each training cycle includes:

[0075] In an environment below the martensitic transformation end temperature Mf10-40℃, the spring is bent in the reverse direction to a preset training height H2 and maintained for 5-120s, where the training height is 2-5 times the initial height H1 of the spring; then, in an environment above the austenitic transformation end temperature Af10-50℃, it is left to stand in a free position for 5-120s to complete the phase transformation recovery.

[0076] By repeating this cycle 3-50 times, a stable dislocation structure and a dominant martensitic variant structure can be induced inside the spring. After this optimized training, the spring can not only achieve a phase transformation stroke of 1.00-4.50 mm in one cycle, but also ensure a stroke attenuation of ≤10% after 100,000 cycles, significantly improving its action consistency and fatigue life.

[0077] To verify the rationality and necessity of the preparation method and the range of process parameters of the present invention, the following examples and comparative examples are provided for comparison and explanation, and the underlying microscopic mechanisms are analyzed in depth, as shown in Table 2. Examples 1-3 respectively used three different spatial configurations of springs of the present invention, and strictly followed the preferred process parameters of the present invention; Comparative Examples 1-10 systematically investigated the effect of a single process parameter deviating from the range defined by the present invention on the performance of the spring. The specific process parameters and performance test results are summarized in Table 2.

[0078] Table 2 Relationship between heat treatment conditions and the performance of the shrapnel before training

[0079] Shrapnel type Raw material Af (°C) Solution treatment temperature (°C) Solution treatment time (min) Does solid solution use mold constraint? Aging temperature (°C) Delivery time (min) Does the aging process use mold constraints? Phase transition temperature Af after aging (°C) Phase transition path (mm) Arch height (mm) Example 1 Combined type 42.5 635 43 yes 380 48 no 50.4 1.28 0.08 Example 2 curved surface 42.5 635 43 yes 380 48 no 51.7 1.42 0.12 Example 3 Folded-wing type 42.5 635 43 yes 380 48 no 51.8 1.13 0.09 Example 4 Combined type 42.5 635 43 yes 380 12 no 28.8 1.06 0.07 Example 5 Combined type 42.5 635 43 yes 380 2870 no 58.9 2.12 0.05 Comparative Example 1 Combined type 42.5 440 43 yes 380 48 no 18.7 0.89 0.11 Comparative Example 2 Combined type 42.5 910 43 yes 380 48 no 71.1 0.54 1.68 Comparative Example 3 Combined type 42.5 635 9 yes 380 48 no 16.9 0.7 0.52 Comparative Example 4 Combined type 42.5 635 380 yes 380 48 no 66.7 0.93 0.55 Comparative Example 5 Combined type 42.5 635 43 no 380 48 no 45.2 0.15 0.02 Comparative Example 6 Combined type 42.5 635 43 yes 190 48 no 19.1 0.83 0.05 Comparative Example 7 Combined type 42.5 635 43 yes 660 48 no 38.3 0.24 1.53 Comparative Example 8 Combined type 42.5 635 43 yes 380 5 no 19 0.55 1.32 Comparative Example 9 Combined type 42.5 635 43 yes 380 3000 no 48.9 1.07 0.21 Comparative Example 10 Combined type 42.5 635 43 yes 380 48 yes 50.2 0.28 1.46

[0080] Note: The arch height in Table 2 is the deformation height after being subjected to a 2N load at 10-40℃ below the martensitic phase transformation end temperature Mf.

[0081] Through in-depth analysis of the data in Table 2, the following conclusions can be drawn:

[0082] 1. The decisive influence of solution treatment on microstructure and properties

[0083] Examples 1-3, under optimized solution treatment parameters (635℃, 43 min) and mold constraints, yielded a uniform supersaturated solid solution, laying an ideal compositional and structural foundation for subsequent aging precipitation. The microscopic mechanism lies in the fact that this parameter combination ensures the complete dissolution of the precipitated phases (Ni4Ti3, Ni3Ti2), eliminates microsegregation, and makes the Ni / Ti atomic ratio of the matrix close to the nominal composition. This achieves precise control of the austenitic phase transformation end temperature (Af after aging ≈ 50℃), a large primary phase transformation stroke (≥1.10 mm), and excellent low-temperature flatness (arch height ≤0.12 mm). Figure 4 The figure shows the DMA curve of the spring sheet of Example 1 under a constant load of 5N during a single phase transition.

[0084] Compared to Example 1, the solution temperature of Comparative Example 1 (440℃) was too low, and the solution time of Comparative Example 3 (9 min) was too short, resulting in the incomplete dissolution of the precipitated phases (Ni4Ti3, Ni3Ti2, or TiC) into the matrix, leading to severe microscopic inhomogeneity in the solution composition. The direct consequence is that the actual Ni / Ti ratio in localized regions of the matrix is ​​lower than the nominal alloy composition, such as Ni enrichment in the undissolved second phase. This reduction in Ni content directly results in significantly lower austenite transformation end temperatures (Af), only 18.7℃ and 16.9℃, respectively. Simultaneously, the pinning effect of the undissolved phases on grain boundaries and phase boundaries also restricts the strain of the superelastic body, leading to a substantial decrease in the first-stage transformation path.

[0085] Compared to Example 1, the solution temperature of Comparative Example 2 (910℃) was too high, and the solution time of Comparative Example 4 (380 min) was too long, leading to abnormal grain coarsening and potentially causing selective volatilization or oxidation of Ti, thus disrupting the Ni / Ti atomic ratio of the matrix. The relative depletion of Ti increased the average Ni content in the matrix, thereby driving the austenitic transformation end temperature (Af) to abnormally rise to 71.1℃ and 66.7℃, exceeding the operating range. The coarse grain structure is not conducive to the uniform propagation of stress-induced martensitic transformation, resulting in a significant reduction in recoverable strain (i.e., the first phase transformation path) and recoverable force.

[0086] 2. The irreplaceable role of mold constraint in solution treatment

[0087] Compared to Example 1, Comparative Example 5 did not employ mold constraint during solution treatment. Although the austenitic phase transformation end temperature Af (45.2℃) of Comparative Example 5 was acceptable, the single phase transformation stroke was drastically reduced to 0.15 mm. This mechanistically demonstrates that without mold constraint during solution treatment, the spring sheet is in a free state and cannot form a preset, recoverable stress field and lattice orientation memory. Its microstructure lacks the inherent driving force to generate large deformation during thermal cycling. Therefore, mold constraint is a crucial step in endowing the spring sheet with the core characteristic of large deformation, rather than simply fixing its shape.

[0088] 3. The crucial role of free-posture aging treatment in precise performance control

[0089] Compared to Examples 1, 4, and 5, the aging temperature (190°C) of Comparative Example 6 was too low, and the aging time (5 min) of Comparative Example 8 was too short. This resulted in insufficient atomic diffusion kinetic energy, preventing the formation of a sufficiently dense Ni4Ti3 strengthening precipitate. A large amount of Ni atoms remained dissolved in the matrix, leading to a lower austenite phase transformation termination temperature. Simultaneously, due to the lack of effective pinning of the precipitate, the martensite variant interface was prone to irreversible slippage under stress, resulting in a lower formation temperature and poor control of the low-temperature arch height. Figure 5 As shown, the DMA curve of the spring sheet of Comparative Example 8 under a constant load of 5N during the first phase transition is presented. By comparing the DMA curves of the first phase transition of Example 1 and Comparative Example 8, it can be seen that the phase transition temperature of Comparative Example 8 is generally lower and Af is higher. 时效后 At temperatures below 20℃, the phase transition path is less than 1.00 mm, and the thermal hysteresis is slightly wide, which is not conducive to rapid response in narrow temperature ranges.

[0090] Compared with Examples 1, 4, and 5, the aging temperature of Comparative Example 7 (660°C) was too high, and the aging time of Comparative Example 9 (3000 min) was too long. Both excessively high aging temperatures and excessively long aging times caused the initially formed Ni4Ti3 precipitates, coherent with the parent phase, to rapidly coarsen and grow, losing their coherent relationship with the parent phase and even transforming into incoherent Ni3Ti2 or Ni3Ti stable phases. During this process, these Ni atoms returned to the parent phase, causing the average Ni content of the parent phase to rise again, and the phase transformation temperature to decrease from its peak, but still remaining at 38.3°C for Comparative Example 7 or 48.9°C for Comparative Example 9. More importantly, the coarse incoherent precipitates became stress concentration points, severely hindering the martensitic phase transformation and leading to near-failure of the superelasticity, such as a stroke of only 0.24 mm in Comparative Example 7 and only 1.07 mm in Comparative Example 9. Furthermore, the overall hardening of the material resulted in a poorer low-temperature flattening effect, with arching heights reaching 1.53 mm or 0.21 mm. This indicates that even if an excessively long aging time can achieve a phase transition temperature close to the target, it will still cause irreversible damage to the dynamic properties of the material.

[0091] Compared to Example 1, Comparative Example 10 employed mold constraint during aging treatment. Aging treatment under mold constraint introduced high-density, non-uniform dislocation entanglements and internal stress fields into the spring sheet. These internal stresses strongly pinned the martensite interface, significantly suppressing phase transformation driving capability, resulting in a sharp reduction in stroke (0.28 mm), and severely deteriorating low-temperature flatness (arch height 1.46 mm) due to uneven release of internal stress.

[0092] In summary, the solution-aging process of this invention is a meticulously designed whole: it establishes the structural basis for shape memory and large deformation through "solution under a mold", and then precisely controls the precipitated phase through "free aging" to achieve the target phase transition temperature and superelasticity. Both are indispensable.

[0093] The above analysis demonstrates that the phase transition temperature and fundamental hyperelasticity of the spring sheet can be precisely controlled through optimized solution treatment and aging. However, to ensure the spring sheet maintains consistent action and ultra-high fatigue life during long-term, high-frequency working cycles, its microstructure still requires further optimization and stabilization. Mechanical training is a key step in achieving this goal. By simulating the phase transition cycle of the spring sheet's actual working state, it induces the formation of a stable dislocation structure within the material, thereby significantly improving its durability.

[0094] To reveal the refined impact of various parameters in the training process, such as temperature, time, deformation, and number of cycles, on the final performance of the spring sheet, especially its fatigue life and motion accuracy (surface profile), and to clarify the critical significance of the parameter range defined in this invention, a comparative analysis is conducted below using Example 1 and Comparative Examples 11-22. These analyses each target a specific parameter deviation during the training process; the specific process parameters and performance results are shown in Table 3.

[0095] Table 3. Relationship between training process and performance, fatigue life of shrapnel

[0096] First training temperature (°C) First hold time (s) Second training temperature (°C) Second hold time (s) Training high H2 (mm) Number of training sessions Travel distance (mm) Surface profile (mm) Attenuation rate (%) Example 1 20.0 15 70.0 15 6.0 10 1.28 0.01 3.9 Comparative Example 11 45.0 15 70.0 15 6.0 10 0.74 0.19 20.2 Comparative Example 12 0 15 70.0 15 6.0 10 0.59 0.04 13.4 Comparative Example 13 20.0 1 70.0 15 6.0 10 0.65 0.22 18.2 Comparative Example 14 20.0 150 70.0 15 6.0 10 0.46 0.03 18.8 Comparative Example 15 20.0 15 52.0 15 6.0 10 1.03 0.13 30.7 Comparative Example 16 20.0 15 110 15 6.0 10 1.16 0.07 11.1 Comparative Example 17 20.0 15 70.0 1 6.0 10 1.10 0.20 25.5 Comparative Example 18 20.0 15 70.0 150 6.0 10 1.23 0.07 13.2 Comparative Example 19 20.0 15 70.0 15 2.0 10 1.37 0.77 6.7 Comparative Example 20 20.0 15 70.0 15 11.0 10 1.12 0.04 27.4 Comparative Example 21 20.0 15 70.0 15 6.0 0 1.68 0.81 2.2 Comparative Example 22 20.0 15 70.0 15 6.0 60 1.21 0.02 37.4

[0097] In Table 3, the first training temperature is 10-40℃ below the martensitic transformation end temperature Mf (after aging), at which temperature the spring is deformed in the martensitic state; the first holding time is the duration for which the spring remains in the deformed state at the first training temperature, preferably 5-120s; the second training temperature is 10-50℃ above the austenitic transformation end temperature Af (after aging), at which temperature the spring is treated in the austenitic state; the second holding time is the duration for which the spring remains in the free recovery state at the second training temperature, preferably 5-120s. The attenuation rate is the attenuation rate of the stroke after 100,000 fatigue tests.

[0098] The essence of the training process is to train and optimize the microstructure of the fragment through non-destructive, controllable stress-induced phase transformation cycles. Its core lies in inducing the formation of a stable dislocation array and dominant martensite variants, rather than introducing damage.

[0099] Example 1, trained under optimized parameters, demonstrates a microscopic mechanism that successfully induces the formation of stable dislocation walls and dislocation cells, while simultaneously promoting the formation of a single or a few dominant martensite variants. This highly ordered microstructure allows the martensite to undergo reversible nucleation and disappearance along the same path and interface in each subsequent working cycle. Macroscopically, this results in long stroke, extremely high motion consistency, and excellent fatigue resistance.

[0100] When the parameters deviate from the preferred window of this invention, the optimization of the microstructure will fail or even deteriorate, specifically manifested as follows:

[0101] 1. Regarding the low-temperature training phase (deformation in the martensitic state)

[0102] Compared to Example 1, the first training temperature (45°C) of Comparative Example 11 was too high, resulting in insufficient driving force for the martensitic phase transformation and incomplete or insufficient phase transformation. Some austenitic regions were not triggered to transform during training, and their internal dislocation structures failed to be effectively reorganized. This resulted in the inability to achieve a complete reversible phase transformation between austenite and martensite during use, which macroscopically manifested as a significant reduction in stroke (0.74 mm) and a significant shortening of fatigue life (attenuation rate of 20.2%).

[0103] Compared to Example 1, the first training temperature (0°C) of Comparative Example 12 was too low. Although a complete phase transition was achieved, it introduced an excessive amount of disordered dislocation structures. These excessive dislocations acted as phase transition resistance, strongly pinning the martensite interface and causing the restoring force (i.e., stroke) to decrease significantly to 0.59 mm.

[0104] Compared to Example 1, the first holding time (1s) of Comparative Example 13 was too short, resulting in insufficient time for stress relaxation and dislocation reorganization after martensitic deformation, which prevented the formation of a stable defect structure and led to deterioration in stroke, flatness, and lifespan. The first holding time (150s) of Comparative Example 14 was too long, which was equivalent to over-aging the material at low temperature, causing the introduced dislocations to become entangled or even recover, weakening the training effect and resulting in a severe decrease in stroke (0.46mm).

[0105] 2. Regarding the high-temperature training phase (recovery in the austenitic state)

[0106] Compared to Example 1, the second training temperature (52°C) of Comparative Example 15 was too low, and the second holding time (1s) of Comparative Example 17 was too short, resulting in some martensite failing to transform into austenite. These residual martensites eventually accumulated strain in subsequent training cycles, disrupting the reversibility of the phase transformation. This is the main reason for the sharp increase in fatigue decay rate, such as the high 3.7% in Comparative Example 15, and the deterioration of surface profile.

[0107] Compared to Example 1, the second training temperature (110°C) of Comparative Example 16 was too high, and the second holding time (150s) of Comparative Example 18 was too long, which would destroy the fine Ni4Ti3 precipitate structure obtained by aging treatment, causing it to coarsen or transform. This changes the stress field and composition field of the parent phase. Although the short-term resistance is acceptable, it seriously impairs the material's ability to resist cyclic phase transformation damage, resulting in a significant decrease in fatigue life.

[0108] 3. Regarding training intensity (variation amount and number of cycles)

[0109] Compared to Example 1, the training height (2.0 mm) of Comparative Example 19 and the number of training cycles (0 times) of Comparative Example 21 both indicate severely insufficient training intensity, failing to form effective dislocation walls and dislocation cell structures to guide and constrain the formation of martensite variants. As a result, martensite undergoes random nucleation during cooling or loading, with multiple variants competing for growth. Macroscopically, this manifests as extremely poor motion consistency and severely excessive surface profiles, reaching 0.77 mm and 0.81 mm respectively, despite potentially higher initial travel.

[0110] Compared to Example 1, the training height (11.00) of Comparative Example 20 and the number of training iterations of Comparative Example 22 both indicate excessively high training intensity, leading to a continuous excessive increase in dislocation density and the formation of high-density dislocation tangles, which in turn induces microscopic plastic damage. These damage points provide prerequisites for the initiation of fatigue cracks. Although the initial surface profile is still acceptable, the fatigue life deteriorates sharply, with attenuation rates reaching 27.4% and 37.4%, respectively.

[0111] In summary, any deviation from these parameters, whether excessive or insufficient, will disrupt this balance through various microscopic mechanisms, leading to a significant deterioration in one or more key performance indicators, such as stroke, accuracy, or fatigue life. This strongly demonstrates that the training process parameter range proposed in this invention is not a conventional, easily obtainable setting through limited experiments, but rather a precise process condition determined by a deep understanding of dislocation engineering and phase transition behavior, capable of stably fabricating high-performance spring sheets.

[0112] like Figure 6 As shown, the attenuation of the stroke values ​​of Example 1 and Comparative Example 22 after 100,000 fatigue cycles compared to the initial values ​​is presented.

[0113] To clarify the testing standards for the performance parameters of the spring sheet described in this invention and to ensure data comparability and reproducibility, all performance tests were conducted based on the principles of dynamic thermomechanical analysis (DMA) and geometric measurement methods conventional in the art. Specific testing standards and conditions are as follows:

[0114] 1. Phase transition path test:

[0115] Test Principle and Method: The test was conducted using a DMA device under constant load mode. Specifically, the spring sample was placed in a free posture on the test platform, and a constant load of 5N perpendicular to the main plane of the spring was applied by the device. Subsequently, the spring sample was subjected to controlled heating and cooling in a complete thermal cycle (i.e., the high temperature point was higher than its Af temperature, and the low temperature point was lower than its Mf temperature).

[0116] Record the displacement curve of the spring sample under a constant load of 5N as a function of temperature. The first phase transition stroke is the maximum displacement value generated within the phase transition temperature range in this displacement curve.

[0117] 2. Low-temperature surface profile (arch height) test:

[0118] Test Principle and Method: This test aims to evaluate the flatness of the spring sample in the low-temperature martensitic state. First, the spring sample is placed in a temperature environment that will allow it to completely transform into the martensitic phase, typically 10-40°C below the Mf temperature, and a constant external force of 2N perpendicular to the principal plane is applied to flatten it.

[0119] In the flattened state, a coordinate measuring machine or laser displacement sensor with high-precision displacement sensing function is used to scan the unfolded plane of the spring sample or to take measurements at 1mm intervals.

[0120] The surface profile or camber height is defined as the difference between the highest and lowest points along the Z-axis (height direction) among all measurement points.

[0121] 3. Fatigue performance testing and attenuation assessment:

[0122] Test Principle and Method: The fatigue life of the spring fragment is evaluated through accelerated thermal cycling tests. The spring fragment sample is mounted on a fixture capable of applying a constant load, and is subjected to periodic automatic switching between a high-temperature fluid medium (temperature set 10-50°C above Af) and a low-temperature fluid medium (temperature set 10-40°C below Mf). Each cycle includes sufficient heat preservation in the low-temperature and high-temperature media to ensure that the spring fragment sample completes the phase transition.

[0123] Attenuation assessment method: After a single spring sample completes a preset number of fatigue tests (e.g., 10 times), the spring sample is removed and subjected to a DMA test under a constant load of 5N. The change in the stroke value of the spring sample after every 10 fatigue tests is monitored.

[0124] Attenuation assessment criteria: Based on the above attenuation assessment method, the initial stroke value of the shrapnel sample that has not undergone fatigue testing is recorded as S0, and the stroke value S is recorded once every 10 fatigue cycles. 10 S 20 S 30 ...S 100000 When the stroke decay rate (1-S n*10 When n≥1 and is an integer, the spring sample is considered to have no significant decrease in stroke after n×10 fatigue cycles, and the spring sample has not failed.

[0125] While the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the invention. Those skilled in the art can make various modifications and refinements without departing from the spirit and scope of the invention. Therefore, the scope of protection of the present invention shall be determined by the claims.

Claims

1. A lightweight, high-fatigue-life nickel-titanium shape memory alloy spring, characterized in that, The spring is a one-piece molded sheet-like shape memory alloy component, which is given a symmetrical, pre-set arched shape in the austenitic phase. The spring is configured to respond to changes in ambient temperature, be flattened by external force when cooled to the martensitic phase, and generate a restoring force when heated to the austenitic phase to actively restore to the preset arched shape. Furthermore, the spring sheet, as a single driving source, can synchronously drive at least one independent heat dissipation control element to operate through active deformation.

2. The lightweight, high fatigue life nickel-titanium shape memory alloy spring sheet according to claim 1, characterized in that, The preset arch shape is a folded wing type, an arc type, or a combination of a plane and an arc type.

3. The lightweight, high fatigue life nickel-titanium shape memory alloy spring sheet according to claim 1, characterized in that, The austenitic phase transformation end temperature of the spring sheet is 20-60℃.

4. The lightweight, high fatigue life nickel-titanium shape memory alloy spring sheet according to claim 1, characterized in that, The overall outline length of the spring is (30±0.05) mm, the width is (20±0.05) mm, and the thickness is 0.1-0.6 mm.

5. The lightweight, high fatigue life nickel-titanium shape memory alloy spring according to claim 1, characterized in that, The phase transition stroke of the spring sheet tested under a constant load of 5 mm was 1.00-4.50 mm.

6. The lightweight, high fatigue life nickel-titanium shape memory alloy spring according to claim 1, characterized in that, The fatigue life of the spring is not less than 100,000 cycles, and its stroke attenuation rate does not exceed 10% after 100,000 fatigue tests.

7. A method for preparing a lightweight, high-fatigue-life nickel-titanium shape memory alloy spring as described in any one of claims 1-6, characterized in that, Includes the following steps: The nickel-titanium alloy billet is subjected to solution treatment with mold constraint to give it a symmetrical pre-set arched shape and memory properties. The solution-treated spring sheet is subjected to free aging treatment without mold constraints in order to accurately set its austenitic phase transformation end temperature. Mechanical training is performed on the aging-treated shrapnel to optimize its fatigue performance.

8. The method for producing lightweight, high-fatigue-life nickel-titanium shape memory alloy springs according to claim 7, characterized in that, The solution treatment with mold constraint is carried out at 450-900℃ for 10-360 minutes.

9. The method for producing lightweight, high-fatigue-life nickel-titanium shape memory alloy springs according to claim 7, characterized in that, The free aging treatment is carried out at 200-650℃ for 10-2880 min.

10. The method for producing lightweight, high-fatigue-life nickel-titanium shape memory alloy springs according to claim 1, characterized in that, The mechanical training includes 3-50 training cycles; Each of the training cycles includes: In an environment 10-40°C below the martensitic transformation end temperature of the spring, it is bent in the opposite direction to the training height and maintained for 5-120 seconds, wherein the training height is 2-5 times the initial height of the spring in the preset arched shape in the free state. Subsequently, the piece is allowed to recover in a free state for 5-120 seconds in an environment 10-50°C above the austenitic phase transformation end temperature of the piece.

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