Preparation method of hyperelastic ultra-fine grain nickel-titanium alloy wire

Abstract

The invention discloses a preparation method of a hyperelastic ultra-fine grain nickel-titanium alloy wire, which comprises the following steps: carrying out phase change heat treatment on a nickel-titanium alloy wire blank in an austenite state to enable the wire to be in a martensite or martensite and austenite mixed state, and carrying out cold drawing on the obtained wire until the wire reaches a target size; wherein the heating number deformation is 40%-80%, and the single-pass drawing deformation is larger than 15%; according to the method, an austenite-state wire is converted into a martensite-state or austenite-martensite coexisting-state wire, the nickel-titanium alloy wire with the high grain size grade is prepared through design of the wire deformation amount, the annealing system, the drawing speed, the mold angle and the like, and the raw material wire with excellent performance is provided for application to the high-end field of medical instruments and the like.

Description

Technical Field

[0001] This invention belongs to the field of wire processing, and particularly relates to a method for preparing a superelastic ultrafine crystalline nickel-titanium alloy wire. Background Technology

[0002] In the field of metal wire processing, especially for materials like nickel-titanium alloys with unique physical and chemical properties, the processing is particularly complex and demanding. Nickel-titanium alloys, with their unique superelasticity, shape memory effect, good corrosion resistance, and excellent biocompatibility, show broad application prospects in various fields such as medical devices, aerospace, and petrochemicals. However, achieving these superior properties often depends on every detail of the wire processing.

[0003] With the development of minimally invasive medical technology, the medical field has placed higher demands on nickel-titanium alloy medical devices. This necessitates the use of wires with superior overall performance in the fabrication of these devices. For example, implantable medical devices require higher fatigue life, thus necessitating optimization of the wire material in terms of interstitial elements and grain size. Grain refinement of nickel-titanium alloy wires is primarily achieved through cold drawing, but due to A… f Nickel-titanium alloy wires at temperatures below room temperature exhibit excellent superelasticity. The austenitic phase has a high elastic modulus of about 70-85 GPa and high resistance to deformation, which means that the deformation during the drawing process is generally controlled at about 40%. However, the martensitic phase has an elastic modulus of about 28-41 GPa and lower resistance to deformation during the drawing process, allowing for greater deformation and higher speed drawing, thereby obtaining finer grain sizes. Summary of the Invention

[0004] The purpose of this invention is to provide a method for preparing ultra-elastic ultrafine-grained nickel-titanium alloy wire, which can increase the grain size of the wire to meet the requirements in the preparation or processing of medical devices.

[0005] This invention employs the following technical solution: a method for preparing a superelastic, ultrafine-grained nickel-titanium alloy wire, comprising: The nickel-titanium alloy wire blank in the austenitic state is subjected to phase transformation heat treatment to transform the wire into a martensitic or martensitic-austenitic mixed state, and the resulting wire is then cold drawn. Repeat the above steps until the filament reaches the target size; Among them, the deformation amount per pass is 40% to 80%, and the single-pass drawing deformation is greater than 15%.

[0006] The beneficial effects of this invention are: This invention converts austenitic wire into martensitic or austenitic-martensitic coexisting wire, and through the design of wire deformation, annealing process, drawing speed, die angle, etc., prepares nickel-titanium alloy wire with high grain size, providing high-performance raw material wire for applications in high-end fields such as medical devices. This invention transforms austenite into martensite or a state of martensite and austenite coexistence through heat treatment. Utilizing its low elastic modulus (28~41GPa), the deformation amount in a single pass breaks through the traditional 40% limit, reaching up to 80%. Combined with multi-pass drawing and annealing, it achieves a significant improvement in grain size. This invention can improve production efficiency. The low deformation resistance of the martensitic phase allows the single-pass drawing deformation to be increased to 50%~80%, which is much higher than the 40% upper limit of traditional austenitic drawing. This reduces the number of drawing passes and intermediate annealing times, shortens the production cycle, and reduces energy consumption. This invention can improve product consistency. By combining vacuum or inert gas protected heat treatment with precise temperature-time control, it effectively prevents material oxidation and the introduction of impurities, ensures the uniformity of filament composition and structure, and improves batch stability of finished product yield. Detailed Implementation

[0007] The present invention will now be described in detail with reference to specific embodiments.

[0008] This invention discloses a method for preparing a superelastic ultrafine-grained nickel-titanium alloy wire, comprising: The austenitic nickel-titanium alloy wire blank is subjected to phase transformation heat treatment to transform the wire into a martensitic or martensitic-austenitic mixed state, and the resulting wire is cold drawn; the above steps are repeated until the wire reaches the target size. Among them, the deformation amount per pass is 40% to 80%, and the single-pass drawing deformation is greater than 15%.

[0009] The phase change heat treatment process is as follows: under vacuum, the temperature is first heated to 550~650℃, then heating is stopped, and inert gas is introduced to cool down. When the temperature drops to 400~500℃, the inert gas is stopped, and the temperature is maintained at 400~500℃ for a predetermined time T. After the holding period, inert gas is introduced again to cool down. When the temperature drops to 100℃, the inert gas is stopped, and the phase change heat treatment is completed.

[0010] As a preferred embodiment, the phase change heat treatment process is as follows: The wire to be drawn is coiled on a high-temperature resistant metal or ceramic I-beam and placed in a box-type heat treatment furnace. The heat treatment furnace is then evacuated to a vacuum level of less than 1*10. -3Pa, stop evacuation, heat to 550~650℃, stop heating, and introduce inert gas to cool down at a flow rate of 0.1~0.2V / min; when the temperature drops to 400~500℃, stop introducing inert gas and maintain it at 400~500℃ for a predetermined time T. After the holding period, introduce inert gas again to cool down at a flow rate of 0.2~0.5V / min. When the temperature drops to 100℃, stop introducing inert gas and complete the annealing. Here, V is the volume of the furnace.

[0011] The method for obtaining the predetermined time T is as follows: First, calculate the R value: R = 0.1 * (T2 - T1). Where T1 is the austenitic phase transformation end temperature of the nickel-titanium alloy wire, and T2 is the temperature of the wire drawing lubricant; Then, when 0≤R≤2, T=6R; when R>2, T=60*(R-1).

[0012] The method for obtaining T1 is as follows: A nickel-titanium alloy wire blank (usually hot-rolled or hot-drawn, with a diameter of approximately 2.0 mm) is placed in a box-type heat treatment furnace at 800-850℃ and held for 10-20 minutes; the austenitic phase transformation end temperature M of the wire is measured using a differential scanning calorimeter. f This step is designated as T1. The purpose of this step is to bring the billet to a fully annealed state. This is to eliminate the residual processing stress from the previous process and to test the austenitic transformation end temperature of the wire in a fully annealed state. Only the austenitic transformation end temperature obtained under this condition can represent the intrinsic transformation temperature of the material.

[0013] The billet is placed in a box-type heat treatment furnace at 800-850℃ and held for 10-20 minutes to allow the nickel-titanium alloy wire to fully recrystallize and eliminate pre-work hardening, resulting in a uniform and stable fully annealed microstructure. In this state, the austenitic phase transformation end temperature (Mf), measured by differential scanning calorimetry, accurately reflects the intrinsic phase transformation characteristics of the material under the influence of residual stress. This data serves as a key benchmark for subsequent calculations of the R value and setting the annealing time T, ensuring that the heat treatment process parameters after each drawing precisely match the current microstructure of the material, thereby effectively controlling the phase transformation behavior of the wire and creating suitable phase conditions for subsequent cold drawing.

[0014] The core purpose of heat treatment after each drawing process is to systematically increase the austenitic phase transformation end temperature (A) of the wire through a multi-stage temperature-controlled and inert gas-regulated annealing process. fThis process ensures the wire remains in a martensitic or martensitic / austenitic mixed state during subsequent drawing. Because the martensitic phase has a lower elastic modulus and higher plastic deformation capacity, this state allows the wire to withstand greater single-pass deformation (>15%) and heat-drawn deformation (40%~80%) during cold drawing, effectively preventing crack formation and promoting grain refinement. By cyclically implementing this heat treatment and drawing process, the hyperelastic properties and microstructure of the material can be continuously optimized while gradually reducing the diameter, ultimately achieving the efficient and stable preparation of ultrafine-grained nickel-titanium alloy wires.

[0015] The annealing conditions for the finished wire material are as follows: annealing temperature is 480~580℃, and annealing time is 5~180s.

[0016] During the cold drawing process: When the wire diameter is greater than 1 mm, the deformation after each firing is 40-50%. When the wire diameter is 0.5mm~1mm, the deformation after each firing is 50~60%; When the wire diameter is 0.1mm~0.5mm, the deformation after each firing is 60~70%. When the wire diameter is 0.01mm to 0.1mm, the deformation of the wire after each firing is 70 to 80%.

[0017] During the cold drawing process: When the wire diameter is greater than 1 mm, the wire drawing speed is 1~15 m / min. When the wire diameter is 0.5mm~1.0mm, the wire drawing speed is 15~30m / min. When the wire diameter is 0.1mm~0.5mm, the wire drawing speed is 30~60m / min. When the wire diameter is 0.01mm~0.1mm, the wire drawing speed is 60~120m / min.

[0018] During the drawing process, a commercially available wire drawing machine is used for cold drawing. The lubrication cone angle β of the drawing die is 40~60°, and the die angle α is 9~12°, in order to optimize the lubrication effect and reduce the drawing stress.

[0019] The method for determining the austenitic phase transformation end temperature of wire can refer to the standard methods of GB / T 24627-2023 or YS / T 970-2014.

[0020] This invention controls the phase composition of the wire through heat treatment, placing it in a low elastic modulus state during drawing, thus allowing for cold drawing with a much larger deformation than traditional processes. Through a cyclic process of "large deformation cold drawing + intermediate annealing," a huge amount of strain energy is accumulated, ultimately obtaining an ultrafine-grained microstructure with fine grains.

[0021] The cumulative deformation of this invention is extremely large, and the annealing temperature of the wire is relatively low. During the intermediate annealing process, the grains do not grow significantly, thus providing a basis for grain refinement. After the wire reaches the target size, it undergoes final annealing. This short-time heat treatment aims to eliminate residual stress, set the final superelastic properties, and simultaneously prevent excessive grain growth, preserving a fine-grained structure.

[0022] Example 1: Preparation of ultrafine crystalline nickel-titanium vascular stent wire (finished product Φ0.15 mm) Raw material: Ni-50.8at.%Ti alloy wire with a diameter of Φ1.85 mm was selected. A section of wire was placed in a box-type heat treatment furnace at 850℃ and held for 20 minutes. Then, the phase transformation temperature was tested, and T1 was found to be -19℃. The temperature of the wire drawing lubricating oil was measured using a thermometer, and T2 was found to be 10℃. R was calculated to be 2.9.

[0023] First phase change heat treatment: The wire to be drawn is wound onto a stainless steel I-beam and placed in a box-type heat treatment furnace. The heat treatment furnace is then evacuated to a vacuum level of 8*10. -4 At step 1, stop evacuation, heat to 650℃, then stop heating and introduce argon gas at a flow rate of 12.8 L / min (furnace volume 64 L). When the temperature drops to 500℃, stop introducing the inert gas and maintain it at 500℃ for a predetermined time of 114 min. After the holding period, introduce argon gas again at a flow rate of 32 L / min to cool down. After treatment, the wire completes the phase transformation and is in a mixed martensitic and austenitic state at room temperature.

[0024] First drawing: A polycrystalline diamond die with a lubricated cone angle β=60° and a die angle α=12° was used for drawing. The wire was drawn from Φ1.85 mm to Φ1.37 mm, with a deformation of 45% per drawing (belonging to the large size range), and the drawing speed was 1 m / min.

[0025] Second heat treatment and drawing: Repeat phase change heat treatment to draw the wire from Φ1.37 mm to Φ0.97 mm, with a deformation of 50% in one heat treatment (in the medium size range), and a drawing speed of 15 m / min.

[0026] Third heat treatment and drawing: Phase change heat treatment is performed again to draw the wire from Φ0.97 mm to Φ0.68 mm. The deformation amount of the heat treatment is 50% (which is in the large size range), and the drawing speed is 15 m / min.

[0027] Fourth heat treatment and drawing: Phase change heat treatment is performed to draw the wire from Φ0.68mm to the target size Φ0.43mm. The deformation amount after each heat treatment is 60% (which is within the small size range), and the drawing speed is 30m / min.

[0028] Fifth heat treatment and drawing: Phase change heat treatment is performed to draw the wire from Φ0.43mm to the target size Φ0.27mm. The deformation per heat treatment is 60% (which is within the small size range), and the drawing speed is 30m / min.

[0029] The sixth heat treatment and drawing: Phase change heat treatment is carried out to draw the wire from Φ0.27mm to the target size Φ0.15mm. The deformation amount of the heat treatment is 70% (which is within the small size range), and the drawing speed is 60m / min.

[0030] Finished product annealing: Rapid annealing is performed at 480℃ for 180 seconds.

[0031] Performance results: Φ0.15 mm wire was prepared with an average grain size of ~0.5 μm.

[0032] Comparative Example 1 (Traditional austenitic drawing) Using Φ1.85 mm wire with the same composition as in Example 1, without phase transformation heat treatment, the wire was drawn directly in the austenitic state. The deformation per pass was strictly controlled to be below 40%.

[0033] The drawing passes are as follows: Φ1.85 mm → Φ1.55 mm (30% deformation) → 700℃-120 seconds intermediate annealing → Φ1.30 mm (30% deformation) → 700℃-120 seconds intermediate annealing → Φ1.07 mm (32% deformation) → 700℃-120 seconds intermediate annealing → Φ0.87 mm (34% deformation) → 700℃-120 seconds intermediate annealing → Φ0.7 mm (35% deformation) → 700℃-120 seconds intermediate annealing → Φ0.55 mm (38% deformation) → 700℃-120 seconds intermediate annealing → Φ0.43 mm (40% deformation) → 650℃-120 seconds intermediate annealing → Φ0.33 mm (40% deformation) → 650℃-120 seconds intermediate annealing → Φ0.26 mm (40% deformation) → 650℃ - 120 seconds intermediate annealing → Φ0.20 mm (41% deformation) → 650℃ - 120 seconds intermediate annealing → Φ0.15 mm (42% deformation) Rapid annealing at -480℃ for 180 seconds (total drawing 11 times).

[0034] Performance comparison: The Φ0.15mm wire prepared in Comparative Example 1 requires more annealing cycles, and the grains coarsen further with the increase of annealing cycles, with an average grain size of ~2.3μm.

[0035] Example 2: Preparation of ultrafine crystalline nickel-titanium braided wire (finished product Φ0.08 mm) Raw material: Ni-50.7at.%Ti alloy wire with a diameter of Φ1.0 mm was selected. A section of wire was placed in a box-type heat treatment furnace at 800℃ and held for 10 minutes. Then, the phase transformation temperature was tested, and T1 was found to be -14℃. The temperature of the wire drawing lubricating oil was measured using a thermometer, and T2 was found to be 5℃. R was calculated to be 1.9.

[0036] First phase change heat treatment: The wire to be drawn is wound onto a stainless steel I-beam and placed in a box-type heat treatment furnace. The furnace is then evacuated to a vacuum level of 8.9*10. -4 Pa, stop evacuation, heat to 550℃, stop heating, and introduce argon gas at a flow rate of 2L / min (furnace volume is 20L). When the temperature drops to 400℃, stop introducing inert gas and maintain it at 400℃ for a predetermined time of 11.4min. After the holding period, introduce argon gas again at a flow rate of 4L / min to cool down. After treatment, the wire completes the phase transformation and is in the martensitic state at room temperature.

[0037] Drawing was performed using a polycrystalline diamond die with a lubricating cone angle β=40° and a die angle α=9°.

[0038] Drawing pass 1 (Φ1.0mm → Φ0.68mm): Deformation 54%, speed 15m / min.

[0039] Second phase change heat treatment: Same as the first phase change heat treatment.

[0040] Drawing pass 2 (Φ0.68mm → Φ0.43mm): Deformation 60%, speed 30 m / min.

[0041] Third phase change heat treatment: Same as the first phase change heat treatment.

[0042] Drawing pass 3 (Φ0.43mm → Φ0.27mm): Deformation 60%, speed 30 m / min.

[0043] Fourth phase change heat treatment: Same as the first phase change heat treatment.

[0044] Drawing pass 4 (Φ0.27mm → Φ0.15mm): Deformation 70%, speed 50 m / min.

[0045] Fifth phase change heat treatment: Same as the first phase change heat treatment.

[0046] Drawing pass 5 (Φ0.15mm → Φ0.08mm): Deformation 70%, speed 60 m / min.

[0047] Finished product annealing: 580℃ / 10s.

[0048] Results: Φ0.08 mm ultrafine filaments were obtained with a grain size of about 0.4 μm, a smooth surface without obvious defects, and good softness, making them very suitable for high-density weaving.

[0049] Compared with Comparative Example 1, Example 1 achieved a finer size and superior performance with only 6 drawing operations, while Example 2 only required 5 drawing operations to obtain a finished product with a fineness of Φ0.08 mm.

[0050] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing a superelastic ultrafine-grained nickel-titanium alloy wire, characterized in that, include: The nickel-titanium alloy wire blank in the austenitic state is subjected to phase transformation heat treatment to transform the wire into a martensitic or martensitic-austenitic mixed state, and the resulting wire is then cold drawn. Repeat the above steps until the filament reaches the target size; Among them, the deformation amount per pass is 40% to 80%, and the single-pass drawing deformation is greater than 15%.

2. The method for preparing a superelastic ultrafine-grained nickel-titanium alloy wire according to claim 1, characterized in that, The phase change heat treatment process is as follows: under vacuum, the temperature is first heated to 550~650℃, then heating is stopped, and inert gas is introduced to cool down. When the temperature drops to 400~500℃, the inert gas is stopped, and the temperature is maintained at 400~500℃ for a predetermined time T. After the holding period, inert gas is introduced again to cool down. When the temperature drops to 100℃, the inert gas is stopped, and the phase change heat treatment is completed. The method for obtaining the predetermined time T is as follows: First, calculate the R value: R = 0.1 * (T2 - T1). Where T1 is the austenitic phase transformation end temperature of the nickel-titanium alloy wire, and T2 is the temperature of the wire drawing lubricant; Then, when 0≤R≤2, T=6R; when R>2, T=60*(R-1).

3. The method for preparing a superelastic ultrafine-grained nickel-titanium alloy wire according to claim 1, characterized in that, The annealing conditions for the finished wire are: annealing temperature of 480~580℃ and annealing time of 5~180s.

4. The method for preparing a superelastic ultrafine-grained nickel-titanium alloy wire according to claim 1, characterized in that, During the cold drawing process: When the wire diameter is greater than 1 mm, the deformation after each firing is 40-50%. When the wire diameter is 0.5mm~1mm, the deformation after each firing is 50~60%; When the wire diameter is 0.1mm~0.5mm, the deformation rate of the wire after each firing is 60~70%. When the wire diameter is 0.01mm~0.1mm, the deformation of the wire after each firing is 70~80%.

5. The method for preparing a superelastic ultrafine-grained nickel-titanium alloy wire according to claim 1, characterized in that, During the cold drawing process: When the wire diameter is greater than 1 mm, the wire drawing speed is 1~15 m / min. When the wire diameter is 0.5mm~1.0mm, the wire drawing speed is 15~30m / min. When the wire diameter is 0.1mm~0.5mm, the wire drawing speed is 30~60m / min. When the wire diameter is 0.01mm~0.1mm, the wire drawing speed is 60~120m / min.

6. The method for preparing a superelastic ultrafine-grained nickel-titanium alloy wire according to claim 2, characterized in that, The method for obtaining T1 is as follows: the billet is placed in a box-type heat treatment furnace at 800~850℃ and held for 10~20 minutes; the austenitic phase transformation end temperature M of the wire is measured using a differential scanning calorimeter. f , which is designated as T1.