Drawing method of nickel-titanium alloy ultrafine wire with high surface smoothness

Abstract

The invention discloses a drawing method of a high-surface-smoothness nickel-titanium alloy ultrafine wire. The drawing method comprises the following steps: step 1, carrying out surface pretreatment on a nickel-titanium alloy wire, cleaning a bright wire obtained in the step 1, carrying out heat treatment on the wire obtained in the step 2 by utilizing a tubular heat treatment furnace, and testing the austenite phase transformation ending temperature of the wire obtained in the step 3; the outer wall of the wire obtained in the fourth step is coated with lubricating liquid; the wire obtained in the fifth step is drawn through a wire drawing machine till the diameter and the length of the wire reach preset targets; according to the method, a traditional route of lubricating an oxide layer first and then removing the oxide layer is bypassed, and a high-smoothness surface is directly obtained in the drawing forming process of the superfine nickel-titanium wire, so that various problems caused by secondary treatment are avoided; and the superfine nickel-titanium alloy wire with the high surface smoothness and the diameter smaller than or equal to 0.1 mm can be directly obtained without follow-up polishing treatment.

Description

Technical Field

[0001] This invention belongs to the field of precision metal wire processing, and particularly relates to a method for drawing ultrafine nickel-titanium alloy wires with high surface finish. Background Technology

[0002] Bright nickel-titanium alloy ultrafine wires, namely bright nickel-titanium alloy microwires (diameter ≤0.1mm), are widely used in high-end medical devices (such as neurointerventional guidewires and precision braided scaffolds) and optical devices. They have extremely stringent requirements for surface smoothness (low roughness), no scratches, and no residues.

[0003] Currently, the traditional manufacturing process for this type of bright fiber has inherent defects: First, during the production process, an oxide layer or coating needs to be formed on the surface of the filament to act as a lubricant and prevent sticking and scratching during drawing. After drawing to the target size, this surface coating must be removed through subsequent processing. For filaments with a diameter greater than 0.5 mm, mechanical polishing can be used to obtain a bright surface; however, for microfilaments with a diameter less than 0.5 mm, especially less than 0.1 mm, mechanical polishing is very likely to cause filament breakage, out-of-tolerance ellipticity, or surface damage.

[0004] However, using acid pickling or electrochemical polishing presents many challenges: acid pickling can easily cause surface corrosion pits and hydrogen embrittlement risks; electrochemical polishing has extremely high requirements for equipment precision, electrolyte formulation and process parameters, and for microfilaments, its uniformity and consistency are difficult to control, often resulting in poor surface gloss, spots or residues, which cannot meet the surface quality and cleanliness requirements of interventional medical devices. Summary of the Invention

[0005] The purpose of this invention is to provide a high-surface-finish nickel-titanium alloy ultrafine wire drawing method to solve the problems of wire breakage, excessive ellipticity, surface damage, surface corrosion pits and hydrogen embrittlement risk caused by pickling, and difficulty in controlling the uniformity and consistency of electrolyte formulation and process in existing preparation methods.

[0006] This invention employs the following technical solution: a method for drawing ultrafine nickel-titanium alloy wires with high surface finish, comprising: Step 1: Perform surface pretreatment on the nickel-titanium alloy wire to make its surface roughness Ra ≤ 0.2μm, and obtain bright wire; Step 2: Clean the bright yarn obtained in Step 1 to remove the lubricant and impurities from the surface of the bright yarn; Step 3: Heat treat the wire obtained in Step 2 using a tubular heat treatment furnace to eliminate work hardening and increase the phase transformation temperature of the wire. Step 4: Test the austenitic phase transformation end temperature of the wire obtained in Step 3; Step 5: Apply a lubricating layer to the outer wall of the filament obtained in Step 4; Step 6: The filament obtained in Step 5 is drawn using a wire drawing machine. During the drawing process, the temperature of the wire drawing lubricant in the lubricant storage tank of the wire drawing machine is 30-40°C below the austenitic phase transformation end temperature of the filament. Repeat steps 2-6 until the wire diameter and length reach the predetermined target.

[0007] The beneficial effects of this invention are: This invention bypasses the traditional "oxidation layer lubrication followed by oxide layer removal" route, enabling ultrafine nickel-titanium wires to directly obtain a high-smooth surface during the drawing process, thereby avoiding various problems caused by secondary processing; ultrafine nickel-titanium alloy wires with a diameter ≤0.1mm and high surface smoothness can be obtained directly without subsequent polishing. This invention uses bright filaments for direct drawing, avoiding the polishing process of finished filaments. Gradual diameter filaments are formed through heat treatment, and the phase transformation temperature of the filaments is increased to a certain extent through step heat treatment in step 3. Before drawing, a specific lubricating layer is coated on the surface of the filaments. Finally, a drawing machine is used to perform low-speed, small-deformation precision drawing of bright filaments at a specific temperature, so that the filaments are in a martensitic state or a state in which martensitic phase transformation can be induced by small stress. Combined with specific lubrication and environmental control, the high surface finish of the filaments is directly protected and maintained during the drawing process, eliminating the need for any subsequent polishing process. This invention enables the direct high-gloss forming of ultrafine filaments, fundamentally eliminating the traditional complex route of "coating first and then processing". By performing precision drawing in the martensitic phase or in a state where martensitic phase transformation can be achieved with relatively small stress, ultrafine bright filaments with a surface roughness Ra≤0.1μm are directly produced, eliminating all subsequent mechanical or chemical polishing processes and simplifying the process flow. This invention significantly improves surface quality and consistency. The low elastic modulus of the martensitic phase makes the filament "softer." At the same time, the gradually changing diameter of the filament ensures better adhesion of the lubricating coating on the filament surface. Furthermore, the deformation of the filament gradually increases during the drawing process, and under low deformation rate and excellent lubrication, it has better fit and filling properties to the die, effectively avoiding the generation of surface micro-cracks and scratches. Combined with a clean environment, it ensures that the filament surface is extremely clean, residue-free, and has a uniform gloss. This invention avoids the risks of hydrogen embrittlement and corrosion caused by acid pickling, and also avoids the uneven composition and residue contamination that may occur with electrochemical polishing. It is particularly suitable for interventional medical devices with extremely high requirements for surface integrity. This invention first processes the filament into a "bright blank," and then creates a "low-wear, low-damage" environment (martensitic state or easily achievable martensitic phase transformation, low speed, low deformation, gradually increasing deformation, high-quality lubrication, and a clean environment) in all subsequent diameter reduction drawing processes, so that the bright surface is "maintained" until the finished product size, instead of waiting for damage before repair. Detailed Implementation

[0008] The present invention will now be described in detail with reference to specific embodiments.

[0009] This invention discloses a method for drawing ultrafine nickel-titanium alloy wires with high surface finish, comprising: Step 1: Perform surface pretreatment on nickel-titanium alloy wire with a diameter of 0.2mm~0.6mm to make its surface roughness Ra ≤0.2μm, and obtain bright wire; Step 2: Clean the bright yarn obtained in Step 1 to remove the lubricant and impurities from the surface of the bright yarn; Step 3: Use a tubular heat treatment furnace to perform online heat treatment on the wire obtained in Step 2 to eliminate work hardening and increase the phase transformation temperature of the wire. Step 4: Test the austenitic phase transformation end temperature of the wire obtained in Step 3; Step 5: Apply a lubricating layer to the outer wall of the filament obtained in Step 4; Step 6: The filament obtained in Step 5 is drawn using a wire drawing machine. During the drawing process, the temperature of the wire drawing lubricant in the lubricant storage tank of the wire drawing machine is 30-40°C below the austenitic phase transformation end temperature of the filament. Repeat steps 2-6 until the wire diameter reaches the predetermined target.

[0010] The initial tension at the start of the wire drawing machine is less than the termination tension at the end of annealing, thereby causing the wire to change from a uniform diameter to a gradually decreasing diameter wire.

[0011] In step 6, the single-pass drawing deformation is 30-40%.

[0012] The inner wall of the lubricant storage tank of the wire drawing machine is equipped with a cooling pipe. The cooling pipe is used to contain the coolant, which is a mixture of anhydrous ethanol and liquid nitrogen. The coolant is used to exchange heat with the wire drawing lubricant, thereby keeping the wire drawing lubricant 30-40°C below the austenite phase transformation end temperature.

[0013] The wire drawing speed of the wire drawing machine is 1~20m / min.

[0014] In step 5, the lubricant selected is a paraffin lubricant with a melting point of 30~40℃.

[0015] In step 5, the paraffin lubricant is heated to 40-60°C when it is applied to the surface of the wire.

[0016] In step 3, the tubular heat treatment furnace is a multi-temperature zone heating furnace. The front section of the heating furnace is a high-temperature zone of 700~800℃, and the length of the high-temperature zone accounts for 1 / 3 of the furnace length. The rear section of the heating furnace is a low-temperature zone of 350~550℃, and the length of the low-temperature zone accounts for 2 / 3 of the furnace length. The online heat treatment time for the wire is 60~180s. Inert gas is used for atmosphere protection during the heat treatment process.

[0017] The specific operating procedure is as follows: Place one end of the wire on the wire feeding machine, lead out the other end and pass it through the cleaning device and the tubular heat treatment furnace in sequence, and finally fix the head of the wire on the take-up machine and start the take-up machine to complete the heat treatment. Remove the filament from the take-up machine, take a section of filament, and use DSC to test the phase transformation temperature of the filament to obtain the austenitic phase transformation end temperature A of the filament. f ; The wire is installed on the wire feeding machine of the wire drawing machine, the wire head is led out, passes through the lubrication coating device, and then is introduced into the wire drawing machine. After the wire is reduced in diameter by the die, it is taken on the wire taking machine. The wire drawing machine is started and the wire is drawn. During the drawing, the temperature of the wire drawing lubricant in the lubricant storage tank of the wire drawing machine is 30-40°C below the austenite end temperature of the wire. The filament is unloaded from the take-up machine and rewound once using a rewinding machine; Repeat the cleaning, heat treatment, coating, and drawing processes until the filament is drawn to the finished size.

[0018] The finished wire is annealed to obtain high-gloss nickel-titanium alloy microwires with a diameter ≤0.1 mm and a surface roughness Ra ≤0.1 μm.

[0019] The tension is adjusted by the tension bar on the wire feeding machine, which is also equipped with a diameter gauge and a wire feeding speed meter to measure the diameter and annealing speed of the wire. Before starting the operation, the initial tension of the wire T1 (5~10MPa), the final tension T2 (100~200MPa) when annealing is completed, the length L, and the annealing speed of the wire are input into the computer. The computer confirms the change in tension as the annealing length of the wire increases based on the wire length L. The specific tension is converted based on the diameter measured by the diameter gauge to ensure that the wire increases uniformly from T1 to T2 during the annealing process.

[0020] The cleaning device is a constant temperature cleaning water tank used to remove the lubricating layer from the surface of the unannealed wire material. The cleaning water temperature is 60~80℃.

[0021] The drawing process is carried out at low speeds, controlled between 1 and 20 m / min. A polycrystalline diamond die with a nanoscale surface finish (Ra≤0.05μm) is used, along with a highly adhesive, low-residue synthetic ester lubricant. The bright wire is drawn with small deformation amounts, and annealing is performed after every 30-40% deformation during cold drawing. The deformation per drawing pass is ≤40% to minimize surface damage accumulation.

[0022] The lubrication coating device is located between the drawing machine and the pay-off machine. The device is a small, sealed box containing a heating coupler to heat the lubricant filling it. The lubricant inside is paraffin wax lubricant, with a melting point of 30-40°C. During operation, the paraffin wax lubricant is heated to 40-60°C. As the wire passes through the lubricant, it is coated with it. The lubricant at the wire exit of the device cools and gradually solidifies on the wire surface, forming a lubricating coating.

[0023] Preferably, the lubrication cone angle β of the die of the wire drawing machine is 40~50°, and the die angle α is 6~8°.

[0024] Preferably, the finished product is annealed in a tubular heat treatment furnace at a temperature of 450~550℃ for 30~60s under argon protection.

[0025] Example: Preparation of Φ0.05mm ultra-high smoothness nickel-titanium microwires Raw material: Φ0.20mm Ni-50.8at.%Ti wire, electropolished to Ra 0.12μm.

[0026] First cleaning, heat treatment, coating, drawing: Cleaning: Water temperature is 60℃.

[0027] Heat treatment: Input T1=10MPa, T2=200MPa, L=2056m, and wire annealing rate of 2m / min into the computer. The heat treatment furnace operates at a high-temperature zone of 800℃ and a low-temperature annealing zone of 550℃, with a heat treatment time of 180s. Argon atmosphere is used for protection during the heat treatment process. After heat treatment, a section of wire is taken, and the phase transformation temperature is measured using DSC to obtain the austenite termination temperature A. f =10℃.

[0028] Coating: The melting point of the paraffin lubricant is 30°C. During operation, the paraffin lubricant is heated to 40°C.

[0029] Drawing: Φ0.230mm -Φ0.190mm (deformation 32%). Temperature: -20℃. Speed: 1 m / min. Die: α=8°, β=50°, Ra<0.05μm.

[0030] The parameters for the second cleaning, heat treatment, coating, and drawing are the same as those for the first cleaning, heat treatment, coating, and drawing, except that: (1) During heat treatment, L=2170m was selected, the temperature of the high-temperature zone of the heat treatment furnace was 790℃, the temperature of the low-temperature zone was 520℃, and the austenite-forming temperature A of the wire was obtained. f =11℃.

[0031] (2) Drawing: Φ0.190mm -> Φ0.159mm (deformation 30%). Temperature: -20℃. Speed: 2 m / min. Die: α=8°, β=50°, Ra<0.05μm.

[0032] The parameters for the third cleaning, heat treatment, coating, and drawing are the same as those for the first cleaning, heat treatment, coating, and drawing, except that: (1) During heat treatment, T2=180MPa, L=3099m, the high temperature zone temperature of the heat treatment furnace is 780℃, the low temperature zone temperature is 500℃, and the austenite completion temperature A of the wire is obtained. f =9℃.

[0033] (2) Drawing: Φ0.159mm -> Φ0.133mm (deformation 30%). Temperature: -31℃. Speed: 4 m / min. Die: α=8°, β=50°, Ra<0.05μm.

[0034] The parameters for the fourth cleaning, heat treatment, coating, and drawing are the same as those for the first cleaning, heat treatment, coating, and drawing, except that: (1) During heat treatment, T2=160MPa, L=4429m, annealing speed of wire is 3m / min, high temperature zone temperature of heat treatment furnace is 760℃, low temperature zone temperature is 470℃, and annealing time is 120s to obtain the austenite termination temperature A of wire. f =10℃.

[0035] (2) Drawing: Φ0.133mm -> Φ0.111mm (deformation 30%). Temperature: -30℃. Speed: 6 m / min. Die: α=8°, β=50°, Ra<0.05μm.

[0036] The parameters for the 5th cleaning, heat treatment, coating, and drawing are the same as those for the 1st cleaning, heat treatment, coating, and drawing, except that: (1) During heat treatment, T2=140MPa, L=6359m, and the annealing rate of the wire were selected as 3m / min. The high-temperature zone temperature of the heat treatment furnace was 750℃, the low-temperature zone temperature was 440℃, and the annealing time was 120s to obtain the austenite completion temperature A of the wire. f=9℃.

[0037] (2) Drawing: Φ0.111mm -> Φ0.093mm (deformation 30%). Temperature: -21℃. Speed: 8 m / min. Die: α=6°, β=40°, Ra<0.05μm.

[0038] The parameters for the 6th cleaning, heat treatment, coating, and drawing are the same as those for the 1st cleaning, heat treatment, coating, and drawing, except that: (1) The cleaning water temperature is 80℃.

[0039] (2) During heat treatment, T1=5MPa, T2=120MPa, L=9059m, and the annealing rate of the wire were selected as 6m / min. The high-temperature zone temperature of the heat treatment furnace was 730℃, the low-temperature zone temperature was 410℃, and the annealing time was 60s to obtain the austenite completion temperature A of the wire. f =10℃.

[0040] (3) Drawing: Φ0.093mm -> Φ0.078mm (deformation 30%). Temperature: -20℃. Speed: 10 m / min. Die: α=6°, β=40°, Ra<0.05μm.

[0041] The parameters for the 7th cleaning, heat treatment, coating, and drawing are the same as those for the 1st cleaning, heat treatment, coating, and drawing, except that: (1) The cleaning water temperature is 80℃.

[0042] (2) During heat treatment, T1=5MPa, T2=100MPa, L=12878m, and the annealing rate of the wire were selected as 6m / min. The high-temperature zone temperature of the heat treatment furnace was 720℃, the low-temperature zone temperature was 380℃, and the annealing time was 60s to obtain the austenite completion temperature A of the wire. f =10℃.

[0043] (3) Drawing: Φ0.078mm -> Φ0.065mm (deformation 30%). Temperature: -20℃. Speed: 15 m / min. Die: α=6°, β=40°, Ra<0.05μm.

[0044] The parameters for the 8th cleaning, heat treatment, coating, and drawing are the same as those for the 1st cleaning, heat treatment, coating, and drawing, except that: (1) The cleaning water temperature is 80℃.

[0045] (2) During heat treatment, T1=5MPa, T2=100MPa, and L=18545m were selected. The high-temperature zone of the heat treatment furnace was 700℃, and the low-temperature zone was 350℃, to obtain the austenite completion temperature A of the wire. f=11℃.

[0046] (3) Drawing: Φ0.065mm -> Φ0.05mm (deformation 40%). Temperature: -20℃. Speed: 20 m / min. Die: α=6°, β=40°, Ra<0.05μm.

[0047] Finished product annealing: Argon-protected finished product annealing: 450℃ / 60s.

[0048] Environment: The entire process was completed in a Class 1000 cleanroom.

[0049] Results: The prepared Φ0.05mm ultrafine filament exhibited a uniform mirror-like luster. Atomic force microscopy analysis revealed a surface roughness Ra value of 21–25 nm, fully meeting the requirements for the fabrication of interventional medical devices. Microscopic observation showed no scratches, cracks, or residual contaminants on the surface.

[0050] Comparative example (traditional process) Using wire of the same specification, traditional wire with an oxide layer on the surface was drawn to Φ0.05mm and then electrochemical polishing was attempted.

[0051] Results: The surface gloss was uneven, and fine, spot-like corrosion marks were visible. The roughness Ra value was 30-50 nm, and trace amounts of residual sulfur (from the electrolyte) were present, which was deemed unacceptable.

[0052] 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 drawing ultrafine nickel-titanium alloy wires with high surface finish, characterized in that, include: Step 1: Perform surface pretreatment on the nickel-titanium alloy wire to make its surface roughness Ra ≤ 0.2μm, and obtain bright wire; Step 2: Clean the bright yarn obtained in Step 1 to remove the lubricant and impurities from the surface of the bright yarn; Step 3: Use a tubular heat treatment furnace to perform online heat treatment on the wire obtained in Step 2 to eliminate work hardening and increase the phase transformation temperature of the wire. Step 4: Test the austenitic phase transformation end temperature of the wire obtained in Step 3; Step 5: Apply a lubricating layer to the outer wall of the filament obtained in Step 4; Step 6: The filament obtained in Step 5 is drawn using a wire drawing machine. During the drawing process, the temperature of the wire drawing lubricant in the lubricant storage tank of the wire drawing machine is 30-40°C below the austenitic phase transformation end temperature of the filament. Repeat steps 2-6 until the wire diameter reaches the predetermined target.

2. The method for drawing high surface finish nickel-titanium alloy ultrafine wires according to claim 1, characterized in that, In step 3, the initial tension on the wire during heat treatment is less than the termination tension on the wire at the end of heat treatment, which causes the wire to change from a uniform diameter wire to a gradually tapered diameter wire.

3. The method for drawing high surface finish nickel-titanium alloy ultrafine wires according to claim 1, characterized in that, In step 6, the single-pass drawing deformation is 30-40%.

4. The method for drawing high surface finish nickel-titanium alloy ultrafine wires according to claim 1, characterized in that, The inner wall of the lubricant storage tank of the wire drawing machine is equipped with a cooling pipe. The cooling pipe is used to contain the coolant, which is a mixture of anhydrous ethanol and liquid nitrogen. The coolant is used to exchange heat with the wire drawing lubricant, thereby keeping the wire drawing lubricant 30-40°C below the austenitic phase transformation end temperature.

5. The method for drawing high surface finish nickel-titanium alloy ultrafine wires according to claim 1, characterized in that, The wire drawing speed of the wire drawing machine is 1~20m / min.

6. The method for drawing high surface finish nickel-titanium alloy ultrafine wires according to claim 1, characterized in that, In step 5, a paraffin-based lubricant with a melting point of 30-40℃ is selected for coating.

7. The method for drawing high surface finish nickel-titanium alloy ultrafine wires according to claim 6, characterized in that, The paraffin lubricant described in step 5 is heated to 40~60℃ when it is applied to the surface of the wire.

8. The method for drawing high surface finish nickel-titanium alloy ultrafine wires according to claim 1, characterized in that, In step 3, the tubular heat treatment furnace is a multi-temperature zone heating furnace. The front section of the heating furnace is a high-temperature zone of 700~800℃, and the length of the high-temperature zone accounts for 1 / 3 of the furnace length. The rear section of the heating furnace is a low-temperature zone of 350~550℃, and the length of the low-temperature zone accounts for 2 / 3 of the furnace length. The online heat treatment time for the wire is 60~180s. Inert gas is used for atmosphere protection during the heat treatment process.