A heat treatment method for improving mechanical properties of a thin-walled nickel-titanium alloy tube and application thereof

Abstract

The application discloses a heat treatment method for improving mechanical properties of a nickel-titanium alloy thin-wall pipe and application, and belongs to the technical field of material heat treatment, and comprises the following steps: S1, establishing a data set about the nickel-titanium alloy thin-wall pipe; the data set comprises physical parameters of the nickel-titanium alloy thin-wall pipe and optimal heat treatment temperatures and heat treatment durations under different pipe parameters obtained through measurement and calculation; S2, constructing a to-be-fitted formula of the heat treatment temperature and the heat treatment duration with a first unknown parameter according to the properties of heat transfer and the physical parameters; S3, fitting the first unknown parameter in the to-be-fitted formula according to the optimal heat treatment temperatures and the heat treatment durations under different pipe parameters, and obtaining an optimal heat treatment formula; and S4, substituting actual pipe parameters into the optimal heat treatment formula to calculate the heat treatment temperature and the heat treatment duration and correspondingly performing heat treatment. The nickel-titanium alloy thin-wall pipe prepared by the application has excellent fatigue performance and bending resistance.

Description

Technical Field

[0001] This invention belongs to the field of heat treatment technology of materials, and particularly relates to a heat treatment method and its application for improving the mechanical properties of thin-walled nickel-titanium alloy tubes. Background Technology

[0002] Near-atomic nickel-titanium alloys possess excellent shape memory effect, superelasticity, fatigue resistance, corrosion resistance, and good biocompatibility, leading to their widespread application in mechanical engineering, military, medical, and chemical fields. Their application primarily utilizes superelasticity and shape memory effects. When a nickel-titanium alloy undergoes plastic deformation at a martensitic-stable temperature, a complete transformation from elastic martensite to austenite occurs once the temperature rises to a certain critical value. Due to the reversibility of this elastic martensitic transformation, the plastic deformation disappears, allowing the alloy to return to its pre-loading shape—a phenomenon known as the "shape memory effect." Furthermore, when a nickel-titanium alloy is loaded at an austenitic-stable temperature, after linear elastic deformation, stress-induced martensite formation and growth occur, superficially resembling both plastic and nonlinear deformation. Upon unloading, the inelastic deformation caused by the stress-induced martensitic transformation recovers, a phenomenon termed "superelasticity."

[0003] The vast majority of nickel-titanium alloy tubing currently manufactured is derived from 50.8 at% nickel alloy composition. This alloy exhibits excellent superelasticity at room temperature and near body temperature, providing resistance to kinking and compression, flexibility, large recoverable deformation, and high energy storage. These properties make nickel-titanium alloys the preferred material for medical devices. Common medical nickel-titanium shape memory alloy products include antennas, tubing connectors, orthodontic archwires, vascular stents, catheters, guidewires, miniature clamps, sensors, and precision instruments.

[0004] Nickel-titanium alloys exhibit excellent tensile and compressive machinability and ductility above 450℃, but poor hot drawing performance. Below 100℃, the material properties are the opposite: good cold drawing performance, but poor tensile and compressive properties. Cold-drawn NiTi alloy thin-walled tubes show severe work hardening and poor material functionality. Therefore, annealing or aging treatment is needed to reduce dislocation density and improve the mechanical properties of NiTi alloy thin-walled tubes. Nickel-titanium alloys are highly sensitive to chemical composition and processing, making the melting and processing control of nickel-titanium alloys extremely challenging, significantly raising the technical threshold for companies entering the nickel-titanium alloy manufacturing field. Taking the cold drawing process of nickel-titanium alloy thin-walled tubes as an example, tube forming generally involves the following steps:

[0005] 1. Assemble the metal core and tubing blanks;

[0006] 2. Apply lubricant to the annealed blank and cold draw it until the metal core size is recovered;

[0007] 3. The pipe is drawn without a mandrel to achieve the designed cold deformation amount;

[0008] 4. Perform shaping or straightening treatments according to the required state;

[0009] 5. Perform surface treatment on the finished pipes.

[0010] In steps 2 and 3, high-density dislocations inevitably occur inside the nickel-titanium alloy during deformation, resulting in work hardening. This phenomenon severely affects the mechanical properties of the finished product and limits its application in the medical field. Therefore, it is urgent to develop a treatment method to solve the above problems. Summary of the Invention

[0011] The purpose of this invention is to meet practical needs by providing a heat treatment method to improve the mechanical properties of nickel-titanium alloy thin-walled tubes, thereby enhancing their fatigue performance and bending resistance.

[0012] In a first aspect, to achieve the aforementioned objectives, the first objective of this invention is to provide a heat treatment method for improving the mechanical properties of a nickel-titanium alloy thin-walled tube. The diameter of the nickel-titanium alloy thin-walled tube is in the range of 0.25 mm to 0.4 mm, the wall thickness is in the range of 0.035 mm to 0.055 mm, and the nickel and titanium content of the tube is both 45 to 55 at%. The nickel-titanium alloy thin-walled tube is heat-treated in a muffle furnace or under an inert gas atmosphere, comprising the following steps:

[0013] S1. Establish a dataset for nickel-titanium alloy thin-walled tubes; the dataset includes the physical parameters of nickel-titanium alloy thin-walled tubes and the optimal heat treatment temperature and heat treatment time for different tube parameters obtained through calculation.

[0014] S2. Construct a fitting formula for heat treatment temperature and heat treatment duration with the first unknown parameter based on the properties and physical parameters of heat transfer;

[0015] S3. Fit the first unknown parameter in the formula to be fitted based on the optimal heat treatment temperature and heat treatment time under different tube parameters to obtain the optimal heat treatment formula.

[0016] S4. Substitute the actual pipe parameters into the optimal heat treatment formula to calculate the heat treatment temperature and heat treatment time, and then perform the heat treatment accordingly.

[0017] In the above-mentioned heat treatment method for improving the mechanical properties of nickel-titanium alloy thin-walled tubes, the physical parameters include: tube outer diameter, tube wall thickness, tube length, groove ratio, and groove curvature.

[0018] In the above-mentioned heat treatment method for improving the mechanical properties of nickel-titanium alloy thin-walled tubes, the optimal heat treatment temperature and heat treatment duration are obtained through the following steps:

[0019] S11. Test the energy loss and strength under different heat treatment temperatures and durations;

[0020] S12. The evaluation index expression is obtained by fitting the heat treatment temperature and heat treatment time; the evaluation index is the parameter value after the loss work and strength are normalized.

[0021] S13. Differentiate the expression of the evaluation index to obtain the extreme points of heat treatment temperature and heat treatment time. The extreme points are the optimal heat treatment temperature and heat treatment time.

[0022] In the above-mentioned heat treatment method for improving the mechanical properties of nickel-titanium alloy thin-walled tubes, the calculation formula for the evaluation index is Z = m * loss work + n * strength, where m + n = 1.

[0023] In the above-mentioned heat treatment method for improving the mechanical properties of nickel-titanium alloy thin-walled tubes, step S12 further includes:

[0024] S121. Select a portion of the evaluation index data under different heat treatment temperatures and heat treatment times as fitting data, and the remaining portion of the evaluation index data under different heat treatment temperatures and heat treatment times as validation data; use the fitting data to draw the profile of the evaluation index data, determine the type of data fitting function, and set up the evaluation index expression with a second unknown parameter.

[0025] S122. Calculate the correlation coefficient and verification accuracy under different parameters, and assign appropriate values ​​to the second unknown parameter based on the correlation coefficient and verification accuracy.

[0026] In the above-mentioned heat treatment method for improving the mechanical properties of nickel-titanium alloy thin-walled tubes, the formula for calculating the verification accuracy is as follows:

[0027]

[0028] The above-mentioned heat treatment method for improving the mechanical properties of nickel-titanium alloy thin-walled tubes also includes using scanning electron microscopy to characterize the surface of the heat-treated nickel-titanium alloy thin-walled tubes.

[0029] The above-mentioned heat treatment method for improving the mechanical properties of nickel-titanium alloy thin-walled tubes also includes conducting bending tests on the heat-treated nickel-titanium alloy thin-walled tubes.

[0030] The above-mentioned heat treatment method for improving the mechanical properties of nickel-titanium alloy thin-walled tubes also includes performing a tensile test on the heat-treated nickel-titanium alloy thin-walled tube.

[0031] Secondly, the present invention provides an application of a heat treatment method for improving the mechanical properties of nickel-titanium alloy thin-walled tubes in the preparation of nickel-titanium alloy thin-walled tubes.

[0032] The positive effects of this application are:

[0033] Based on the above technical solution, without changing its martensitic phase transformation temperature, the internal defects and work hardening caused by cold working of nickel-titanium alloy are eliminated, and the elastic and other mechanical properties of nickel-titanium alloy thin-walled tubes are improved to adapt to their application in various fields such as medical care, and fill the gap in the domestic nickel-titanium alloy thin-walled tube production technology.

[0034] The preparation process is simple and can improve production efficiency;

[0035] The prepared nickel-titanium alloy thin-walled tube has excellent fatigue performance and bending resistance. After heat treatment, the tensile strength of the nickel-titanium alloy thin-walled tube is increased by about 40%, and no by-product small molecules are generated during the treatment process, resulting in less environmental pollution. Attached Figure Description

[0036] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0037] Figure 1 This shows a scanning electron microscope image of the surface of the nickel-titanium alloy thin-walled tube provided in an embodiment of the present invention;

[0038] Figure 2 The image shown is a scanning electron microscope image of the nickel-titanium alloy thin-walled tube provided in the embodiment of the present invention after being treated at 500°C for 1 min;

[0039] Figure 3 The image shown is a scanning electron microscope image of the nickel-titanium alloy thin-walled tube provided in the embodiment of the present invention after being treated at 600°C for 1 min;

[0040] Figure 4 The tensile curve of the nickel-titanium alloy thin-walled tube provided in the embodiment of the present invention after heat treatment is shown;

[0041] Figure 5 The three-point bending curve of the nickel-titanium alloy thin-walled tube provided in the embodiment of the present invention after heat treatment is shown.

[0042] Figure 6 The metallographic structure of the NiTi-650-1 sample provided in this embodiment of the invention after annealing is shown.

[0043] Figure 7 The metallographic structure of the NiTi#-650-1 sample provided in this embodiment of the invention after annealing is shown.

[0044] Figure 8 The DMA test curve of the nickel-titanium alloy thin-walled tube provided in the embodiment of the present invention is shown;

[0045] Figure 9 The DMA test curve of the nickel-titanium alloy thin-walled tube provided in the embodiment of the present invention after being treated at 800K for 300s is shown.

[0046] Figure 10 A flowchart of a heat treatment method for improving the mechanical properties of thin-walled nickel-titanium alloy tubes provided in an embodiment of the present invention is shown. Detailed Implementation

[0047] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0048] All numerical designations, such as temperature, length, flow rate, and range, are approximate values. It's important to understand that while it's not always explicitly stated that all numerical designations are preceded by the term "approximately."

[0049] Relational terms such as “first” and “second” are used merely to distinguish one component from another that has the same name, without necessarily requiring or implying any such actual relationship or order between the components.

[0050] First, the following explanation is provided:

[0051] The hysteresis loop can intuitively reflect the time relationship between stress and displacement. Generally, the curve can be simplified into several straight segments. The intersection of the first and second straight segments is the intersection of the elastic segment and the plastic segment. Based on the elastic segment curve, its elastic modulus can be calculated using the formula, characterizing the stiffness of the material. The hysteresis loop is a closed curve representing the stress versus strain generated by a material during periodic deformation. The energy loss during this period is equal to the area enclosed by its loop, and it depends on the intermolecular interactions of the material.

[0052] Metallographic microscopy can study the microstructure of metals and alloys after different processing and heat treatments. It can determine the quality of metal materials, such as the quantity and distribution of various non-metallic inclusions - oxides, sulfides, etc. in the microstructure, as well as the size of metal grains.

[0053] DMA (Differential Scanning Calorimetry) dynamic analysis curves typically refer to curves measuring the dynamic mechanical properties of a sample as a function of temperature, time, or frequency, as measured in a DMA experiment. DMA curves can show how dynamic mechanical parameters such as elastic modulus, shear modulus, and loss factor of nickel-titanium alloy thin-walled tubes change with temperature or frequency under different conditions. By analyzing DMA dynamic analysis curves, the dynamic mechanical properties of nickel-titanium alloy thin-walled tubes under different conditions can be understood.

[0054] Please see Figures 1 to 10 ;

[0055] First Embodiment

[0056] This invention provides a heat treatment method for improving the mechanical properties of nickel-titanium alloy thin-walled tubes. The diameter of the nickel-titanium alloy thin-walled tube ranges from 0.25 mm to 0.4 mm, the wall thickness ranges from 0.035 mm to 0.055 mm, and the nickel and titanium content is 45-55 at%. The nickel-titanium alloy thin-walled tubes refer to straight tubes, reducing tubes, and thiocyanate tubes, etc. It should be noted that the heat treatment process involves heat treatment in a muffle furnace or inert gas atmosphere, and the tooling for the heat treatment is of our own design.

[0057] like Figure 10 As shown, this application includes the following steps:

[0058] S1. Establish a dataset for nickel-titanium alloy thin-walled tubes; the dataset includes the physical parameters of nickel-titanium alloy thin-walled tubes and the optimal heat treatment temperature and heat treatment time for different tube parameters obtained through calculation.

[0059] In practice, medical nickel-titanium alloy tubing mainly uses two types: reducing tubing and thiourea tubing. Therefore, this application primarily considers both types of tubing.

[0060] (1) Construct the forming formula for variable diameter nickel-titanium alloy thin-walled tubes in cylindrical coordinates (ρ, θ, z):

[0061] Equation F of variable diameter pipe r (ρ, θ, z, n, r1,..., r n ,r1',...,r n ', l1, ..., l n ), where n is an odd number, and i and j are positive integers; ρ has the following range of values:

[0062]

[0063] Where n is the total number of reducer connections, and it is easy to see that (n+1) / 2 is the number of reducer sections; r i r is the inner radius of the pipe at the i-th position;i Let ' be the outer radius of the pipe at position i. In particular, when i is even, we have r. i '=r i ' -1 and r i =r i+1 ;l i Let be the radius of the pipe at position i;

[0064]

[0065] (2) Construct the groove equation for the submersible tube in cylindrical coordinates:

[0066]

[0067]

[0068] Where i and j are positive integers; The characteristic modulus of z is used to simplify the equation; T is the number of groove cycles in the sodium hypochlorite tube; w i di is the width of the groove at the i-th position on the sodium hypochlorite tube; d0 is the axial offset spacing of the grooves on the sodium hypochlorite tube. i (i>0) represents the axial spacing of the i-th groove in the sodium hypochlorite tube, a i Let b be the radian of the starting point of the i-th groove. i Let be the radian of the end point of the i-th groove;

[0069] For sodium hypochlorite tubes, there is usually d i =d1,b i+1 -a i+1 =b i -a i ,

[0070] (3) Combining the equations of the reducing pipe and the sodium hypopipe, we can obtain the equation for the reducing pipe:

[0071] F(ρ,θ,z,n,r1,...,r n ,r1',...r n ',l1,...,l n ,T,w1,...,w T ,d0,...,d T ,a1,...,a T ,b1,...,b T ) = F r ·F w ,

[0072] Where ρ is within the following range:

[0073]

[0074] (4) Based on the above expression, five more practically significant physical parameters are obtained, including: outer diameter of the pipe. Pipe wall thickness pipe chief Groove ratio and groove curvature

[0075] The optimal heat treatment temperature and duration are obtained through the following steps:

[0076] S11. Test the energy loss and strength under different heat treatment temperatures and durations;

[0077] The following explanation uses the pipe parameters in Table 1 as an example:

[0078] Table 1. Pipe Parameters

[0079]

[0080] For the parameters of this tube, the loss of energy and strength of the samples under different heat treatment temperatures Te and heat treatment times Du were tested, as shown in Table 2.

[0081] Table 2. Work loss and intensity at different heat treatment temperatures Te and heat treatment durations Du

[0082]

[0083] S12. The evaluation index expression is obtained by fitting the heat treatment temperature and heat treatment time; the evaluation index is the parameter value after the loss work and strength are normalized.

[0084] Normalizing the energy loss and intensity in Table 2, experiments have shown that the best results are achieved when weights of 0.68 and 0.32 are assigned, i.e., when the evaluation index Z = 0.68 * energy loss + 0.31 * intensity. Thus, the value of the evaluation index Z is obtained. It should be noted that the weights can be selected according to the actual situation, and no specific limitation is made here.

[0085] A portion of the evaluation index data under different heat treatment temperatures and durations was selected as the fitting data, and the remaining portion of the evaluation index data under different heat treatment temperatures and durations was selected as the validation data; here, 90% of the data was selected as the fitting data, and the remaining 10% of the data was selected as the validation data.

[0086] Use the fitted data to draw the data profile of the evaluation index, determine the type of data fitting function, and set the expression of the evaluation index with a second unknown parameter;

[0087] Here, the approximate outline of the fitted data is plotted using MATLAB software. Other software that meets the requirements can also be used; no specific limitation is made here. It can be determined that the data is suitable for fitting a bivariate polynomial function, therefore the evaluation index expression is set as follows:

[0088]

[0089] Calculate the correlation coefficient and validation accuracy under different parameters, and assign appropriate values ​​to the second unknown parameter based on the correlation coefficient and validation accuracy; obviously, the second unknown parameter here refers to n, m, and a. i b i and c ij .

[0090] The formula for calculating the verification accuracy is as follows:

[0091]

[0092] Obviously, based on the actual physical meaning of y1 and y2, y1 and y2 do not contain power product terms, therefore let c ij The constant value is zero; to prevent overfitting, n and m are both less than or equal to 5. Furthermore, by observing the data profile, n and m should be greater than 2, i.e., 3 ≤ n ≤ 5, 3 ≤ m ≤ 5. Therefore, nine different expression forms can be obtained. These nine forms are then used for fitting, yielding the corresponding correlation coefficients R in Table 3. 2 And verify accuracy C;

[0093] Table 3 shows R under 9 different values ​​of (n,m). 2 With the value of C

[0094]

[0095]

[0096] R 2 As an indicator for evaluating the goodness of fit, C is empirically considered to be superior to R². 2 More importantly, the weight of C should be greater than 50%. From Table 3, when the weight of C is greater than 50%, the correlation coefficient and validation accuracy value corresponding to (5, 4) are the best overall results. At this point, the polynomial coefficients obtained from the fitting are shown in Table 4.

[0097] Table 4. Coefficients of the fitted expression for (n, m) = (5, 4)

[0098] 0 -2257.38 -7867.44 1 14.19 400.76 2 0.029 -6.47 3 -0.00024 0.045 4 4.37E-7 -0.00011 5 -2.51E-10

[0099] The evaluation index expression can be obtained from the above steps.

[0100] S13. Differentiate the expression of the evaluation index to obtain the extreme points of heat treatment temperature and heat treatment time. The extreme points are the optimal heat treatment temperature and heat treatment time.

[0101] Taking the top 10% of the largest Z values ​​obtained from the evaluation index expression, the corresponding y1 range is [462, 618], in ℃, and the y2 range is [94, 130], in s. Then, their extreme points can be obtained as y1 = 525 and y2 = 112, that is, 525℃ and 112s are the optimal heat treatment temperature and duration under the tube parameters.

[0102] Based on the above steps, the optimal heat treatment temperature and duration for other tube parameters can be obtained, as shown in Table 5:

[0103] Table 5. Summary of Optimal Heat Treatment Temperature and Duration for Each Pipe Parameter

[0104]

[0105]

[0106] In summary, a dataset of nickel-titanium alloy thin-walled tubes was obtained.

[0107] S2. Based on the properties and physical parameters of heat transfer, construct a fitting formula for the heat treatment temperature and heat treatment duration with the first unknown parameter.

[0108] Based on the properties of heat transfer, the influence of pipe diameter x1 on heat treatment temperature y1 and heat treatment time is low-order nonlinear, so the ln function is used for fitting; while the influence of pipe thickness on heat treatment temperature and heat treatment time is linear, so a linear function is used for fitting; other forms can be obtained similarly, and the specific formulas to be fitted are as follows:

[0109]

[0110] The first positional parameters include a1, a2, a3, a4, a5, c1, c2, c3, c4, c5, b1-b6, and d1-d6.

[0111] S3. Fit the first unknown parameter in the formula to be fitted based on the optimal heat treatment temperature and heat treatment time under different tube parameters.

[0112] Based on the data in Table 5, the optimal heat treatment formula for the reducer can be obtained through software fitting:

[0113]

[0114] Where y1 is the heat treatment temperature in °C; y2 is the heat treatment time in seconds; x1 is the outer diameter of the tube in mm; x2 is the tube thickness in mm; x3 is the tube length in mm; x4 is the groove ratio; and x5 is the groove radius in rad.

[0115] Therefore, this formula can be used to provide different heat treatment temperatures and durations for tubes with different parameters.

[0116] S4. Substitute the actual pipe parameters into the optimal heat treatment formula to calculate the heat treatment temperature and heat treatment time, and then perform the heat treatment accordingly.

[0117] Second Embodiment

[0118] Based on the content of the first embodiment, pipe parameters are set. It is 0.3mm. It is 0.042mm. It is 120mm. It is 0.4. Substituting the π-value of the nickel-titanium tube into the optimal heat treatment formula in the first embodiment, the optimal heat treatment temperature and duration were found to be approximately 500℃ and 2 min, respectively. To verify the accuracy of the above formula, three sets of control experiments were set up.

[0119] Specifically, regarding pipe parameters It is 0.3mm. It is 0.042mm. It is 120mm. It is 0.4. Heat treatment and material property characterization experiments were conducted on π-type nickel-titanium tubes.

[0120] 1. The cold-drawn nickel-titanium alloy thin-walled tube was placed in a specific tooling and annealed in a muffle furnace at 500℃ for 1 min, and named NiTi-500-1. After cooling to room temperature, a standard sample was prepared, and its mechanical properties were tested. The results showed that its tensile strength was approximately 967 MPa, its elastic modulus was approximately 3.06 MPa, and its energy loss was approximately 2.15 kJ.

[0121] 2. The cold-drawn nickel-titanium alloy thin-walled tube was placed in a specific tooling and hot-annealed at 650℃ for 1 min in a muffle furnace. It was named NiTi-650-1. After cooling to room temperature, a standard sample was prepared, and its mechanical properties were tested. The results showed that its tensile strength was approximately 1363 MPa, its elastic modulus was approximately 6.23 MPa, and its energy loss was approximately 2.28 kJ.

[0122] 3. The cold-drawn nickel-titanium alloy thin-walled tube was placed in a specific tooling and annealed in a muffle furnace at 500℃ for 2 minutes, and named NiTi-500-2. After cooling to room temperature, a standard sample was prepared and its mechanical properties were tested. Its tensile strength was approximately 927 MPa, its elastic modulus was approximately 4.16 MPa, and its energy loss was approximately 1.89 kJ.

[0123] It should be noted that the annealing temperature should be between 100℃ and 900℃, and the annealing time should be between 10s and 300s.

[0124] The above heat treatment experimental results were obtained by preparing samples and conducting experiments according to the following requirements:

[0125] 1> Prepare a nickel-titanium alloy thin-walled tube with a length of about 100 mm. Test the tensile properties of the sample according to the standard ASTM F2516-22 to study the effects of cold deformation and annealing temperature on the mechanical properties of the nickel-titanium alloy thin-walled tube.

[0126] 2> Prepare thin-walled nickel-titanium alloy tubes with a length of about 3 mm, and test the bending properties of the samples according to the standard ISO 25539-2:2020 to study the effects of cold deformation and annealing temperature on the superelasticity of NiTi alloy tubes.

[0127] 3> The surface of the heat-treated nickel-titanium alloy thin-walled tube was characterized using optical microscopy and scanning electron microscopy. The effect of heat treatment on the surface morphology of the nickel-titanium alloy thin-walled tube was observed.

[0128] It should be noted that, unless otherwise specified, all materials used in the invention can be purchased on the market.

[0129] Specifically, the performance of nickel-titanium alloy thin-walled tubes is measured by the following parameters:

[0130] (1) Elastic modulus

[0131] The elastic modulus is an indicator that measures how easily a material undergoes elastic deformation. It is represented by E. The smaller the value, the greater the degree of elastic deformation that occurs under a certain stress.

[0132]

[0133] Where E is the elastic modulus, f is the force applied to the sample, l is the distance between the fulcrums, w represents the width of the sample, h represents the thickness of the sample, and d represents the deflection of the sample.

[0134] (2) Elongation

[0135] Elongation refers to the percentage of the original gauge length elongation after the specimen fractures under tension, and is used to characterize the uniform deformation of nickel-titanium alloy thin-walled tubes.

[0136]

[0137] Among them, l k l0 is the gauge length of the cross-sectional area of ​​the nickel-titanium alloy thin-walled tube after it is broken, and l0 is the gauge length of the cross-sectional area of ​​the nickel-titanium alloy thin-walled tube before it is broken.

[0138] Note that the cold deformation range is 5% to 40% to ensure that the thin-walled tube has high and stable mechanical properties.

[0139] Obviously, as Figure 1 , Figure 2 and Figure 3 As shown, the surface morphology of the nickel-titanium alloy thin-walled tube did not change significantly under different experimental conditions.

[0140] In summary, as Figure 4 and Figure 5 As shown, the plasticity and rigidity of nickel-titanium alloy thin-walled tubes under different heat treatment conditions changed significantly. However, the nickel-titanium alloy thin-walled tubes treated at the optimal heat treatment temperature and time exhibited the least energy loss after superelastic behavior. In addition, they also had strong elastic modulus and tensile strength.

[0141] Third Embodiment

[0142] Based on the experimental conditions in the second embodiment, the cold-drawn nickel-titanium alloy plates were subjected to heat treatment, and the treated samples were named NiTi#-650-1. The NiTi-650-1 sample and NiTi#-650-1 were polished with sandpaper, and then etched with an etchant for 10 seconds to obtain the metallographic structure; wherein, the etchant was a mixed solution of hydrofluoric acid, nitric acid, and water. Figure 6 and Figure 7 The image shows the metallographic structure of samples NiTi-650-1 and NiTi#-650-1 after annealing. The grain size of sample NiTi-650-1 is about 3 μm, while that of sample NiTi#-650-1 is about 11 μm, showing a clear difference. This indicates that under the same annealing conditions, nickel-titanium alloy products with the same composition but different morphologies have different microstructures and properties.

[0143] Fourth embodiment

[0144] Based on the aforementioned embodiments, the cold-drawn nickel-titanium alloy wires were subjected to heat treatment, heated to 800K and treated for 300s. The treated samples were named 800K-300s. Dynamic thermomechanical analysis (DMA) was used to test the A content of the nickel-titanium alloy after heat treatment. f A s Temperature change.

[0145] like Figure 8 and 9As shown, the temperature ranges in which the storage modulus E' of the 800K-0s nickel-titanium alloy thin-walled tube and the 800K-300s nickel-titanium alloy thin-walled tube change are 4~16℃ and 3~15℃, respectively. This indicates that after this heat treatment step, its martensitic phase transformation temperature does not change significantly and will not limit its hyperelastic behavior or shape memory effect in the human body, thus possessing practical application capabilities.

[0146] Fifth embodiment

[0147] This invention provides a heat treatment method for improving the mechanical properties of nickel-titanium alloy thin-walled tubes, and its application in the preparation of nickel-titanium alloy thin-walled tubes. For other details, please refer to the foregoing embodiments.

[0148] Based on the above technical solution, the prepared nickel-titanium alloy thin-walled tube has excellent fatigue performance and bending resistance.

[0149] The above description is only a preferred embodiment of the present invention. It should be noted that, for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A heat treatment method for improving the mechanical properties of a nickel-titanium alloy thin-walled tube, wherein the diameter of the nickel-titanium alloy thin-walled tube is in the range of 0.25 mm to 0.4 mm, the wall thickness of the nickel-titanium alloy thin-walled tube is in the range of 0.035 mm to 0.055 mm, the nickel and titanium content of the nickel-titanium alloy thin-walled tube is both 45 to 55 at%, and the nickel-titanium alloy thin-walled tube is heat-treated in a muffle furnace or inert gas, characterized in that... Includes the following steps: S1. Establish a dataset for nickel-titanium alloy thin-walled tubes; the dataset includes the physical parameters of nickel-titanium alloy thin-walled tubes and the optimal heat treatment temperature and heat treatment time for different tube parameters obtained through calculation. S2. Construct a fitting formula for heat treatment temperature and heat treatment duration with the first unknown parameter based on the properties and physical parameters of heat transfer; S3. Fit the first unknown parameter in the formula to be fitted according to the optimal heat treatment temperature and heat treatment time under different tube parameters to obtain the optimal heat treatment formula. S4. Substitute the actual tube parameters into the optimal heat treatment formula to calculate the heat treatment temperature and heat treatment time, and perform heat treatment accordingly. The physical parameters include: outer diameter of the tube, wall thickness of the tube, tube length, groove ratio, and groove curvature. The optimal heat treatment temperature and heat treatment duration in S1 are obtained through the following steps: S11. Test the energy loss and strength under different heat treatment temperatures and durations; S12. The evaluation index expression is obtained by fitting the heat treatment temperature and heat treatment time; the evaluation index is the parameter value after the loss work and strength are normalized. S121. Select a portion of the evaluation index data under different heat treatment temperatures and heat treatment times as fitting data, and the remaining portion of the evaluation index data under different heat treatment temperatures and heat treatment times as validation data; use the fitting data to draw the profile of the evaluation index data, determine the type of data fitting function, and set up the evaluation index expression with a second unknown parameter. S122. Calculate the correlation coefficient and verification accuracy under different parameters, and assign appropriate values ​​to the second unknown parameter based on the correlation coefficient and verification accuracy. S13. Differentiate the expression of the evaluation index to obtain the extreme points of heat treatment temperature and heat treatment time. The extreme points are the optimal heat treatment temperature and heat treatment time. The calculation formula for the evaluation index is Z = m × loss power + n × intensity, where m + n = 1; The formula for calculating the verification accuracy is as follows: 。 2. The heat treatment method for improving the mechanical properties of nickel-titanium alloy thin-walled tubes according to claim 1, characterized in that, It also includes using scanning electron microscopy to characterize the surface of heat-treated nickel-titanium alloy thin-walled tubes.

3. The heat treatment method for improving the mechanical properties of nickel-titanium alloy thin-walled tubes according to claim 1, characterized in that, It also includes tensile tests on heat-treated nickel-titanium alloy thin-walled tubes.

4. The heat treatment method for improving the mechanical properties of nickel-titanium alloy thin-walled tubes according to claim 1, characterized in that, It also includes bending tests on heat-treated nickel-titanium alloy thin-walled tubes.

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