A method for preparing ultrapure nickel-titanium alloy thin-walled tube for heart valves

Abstract

The present invention discloses a method for preparing ultrapure nickel-titanium alloy thin-walled tubes for heart valves, the method comprising: 1. mixing ingredients and then subjecting the ingots to magnetic levitation induction melting to obtain an ingot; 2. gradually hot forging and rolling the ingots to obtain round bars; 3. forming the round bars into tube blanks and honing and polishing them to obtain nickel-titanium tubes; 4. assembling the nickel-titanium tubes into titanium tubes to obtain a composite tube; 5. heat treating the composite tubes; 6. three-roll precision cold rolling; 7. repeating the process to obtain a semi-finished tube; 8. straightening and cylindrical grinding to remove the titanium tubes. The present invention adopts magnetic levitation induction melting to prepare high-purity nickel-titanium alloy ingots, and by introducing an outer layer titanium tube to assemble with an inner layer nickel-titanium tube, utilizing the restraining and protective effect of the titanium tubes, increases the deformation per pass, reduces the number of rolling and heat treatments, and shortens the processing cycle, thereby obtaining ultrapure nickel-titanium alloy thin-walled tubes with high purity, high dimensional accuracy, good surface quality, high wall thickness uniformity, and good comprehensive mechanical properties, which are suitable for the field of heart valves.

Description

Technical Field

[0001] The present invention belongs to the technical field of thin-walled pipe processing, and in particular relates to a method for preparing an ultra-pure nickel-titanium alloy thin-walled pipe for a heart valve. Background Art

[0002] Nickel-titanium alloy tubing has excellent physical properties, biocompatibility, and a unique shape memory effect and superelasticity. It has been widely used in medical devices in various fields, including cardiovascular and peripheral vascular systems. In the field of heart valves in particular, nickel-titanium alloy heart valve stents can be compressed to an extremely small size at room temperature, allowing them to be delivered to the target location via a catheter and then automatically restore their predetermined shape to ensure smooth blood flow. Superelasticity ensures that the stent is not easily deformed or broken when subjected to stress, which greatly reduces the high risks associated with open-chest surgery, especially for elderly patients with multiple comorbidities and underlying diseases.

[0003] However, the high-quality thin-walled nickel-titanium alloy tubes used in heart valve stents remain highly dependent on imports. Besides requiring high-purity nickel-titanium alloy smelting technology, the production of thin-walled tubes involves multiple complex deformation mechanisms and phase transitions, placing stringent manufacturing requirements, high costs, and long lead times, severely hindering the development of high-end medical devices in China.

[0004] Traditional nickel-titanium alloy melting techniques include vacuum consumable melting and induction melting. Vacuum consumable melting is generally used to produce a master alloy, which is then followed by vacuum induction melting. The use of graphite crucibles in induction melting increases the carbon content, as oxide crucibles easily react with titanium, introducing oxygen. Impurity elements can cause changes in the composition of the nickel-titanium alloy, thereby affecting the phase transition temperature and mechanical properties. For nickel-titanium alloys used in heart valves, in addition to their excellent mechanical properties, long-term fatigue performance is also a critical parameter, and fatigue performance is inevitably related to impurity elements.

[0005] The preparation of nickel-titanium alloy tube billets includes hot extrusion, hot rolling and warm rolling. The deformation resistance of nickel-titanium alloy is reduced at a certain temperature, but the dimensional accuracy and surface quality are poor, and titanium is very active and easily reacts with high-temperature graphite lubricant or oxygen in the air, introducing impurities, which brings troubles to subsequent processing and requires pickling to remove, which may cause environmental pollution. For example, Chinese patent CN114850219A discloses a method for preparing high-precision, thin-diameter, thin-walled nickel-titanium alloy tubes, which uses warm rolling to obtain semi-finished tubes, and then draws finished tubes through a moving core head. The key process is that warm rolling improves the plastic processing properties of the material and reduces the number of processing passes. However, the nickel-titanium tube billets use graphite lubrication, and warm processing uses hydrogen heating. The rolling temperature is 500℃~700℃. The rolling process is very easy to absorb hydrogen and increase carbon, causing alloy component contamination, making it difficult to meet the low inclusion element requirements of medical implant-grade nickel-titanium alloys.

[0006] Finished nickel-titanium alloy pipes are processed by hot drawing or cold drawing. The drawing process is simple, and the mold can be easily disassembled and replaced, making it suitable for pipe production, but it has high requirements for lubricants. Chinese patent CN 103394532A discloses a continuous hot drawing forming method for nickel-titanium alloy pipes, which uses induction heating to achieve continuous hot drawing of nickel-titanium alloy pipes (with a mandrel), and then plastically stretches the mandrel to reduce the diameter of the mandrel and remove it to obtain the nickel-titanium alloy pipe. The key process is to use induction heating to achieve continuous hot drawing, which improves production efficiency; however, the induction heating process will cause uneven heating of the pipe, resulting in low pipe precision and surface quality, and may also cause an increase in the content of impurity elements. Chinese patent CN115870365A discloses a processing method for medical nickel-titanium pipes, which uses a nickel-titanium tube blank + a hard mandrel to be cold drawn once or multiple times, annealed as a whole, and hardened to produce a finished medical nickel-titanium pipe. The key process is to use a hard mandrel to improve the inner surface quality of the pipe. However, after drawing, there is a large clamping force between the tube blank and the mandrel, making it difficult to remove the mandrel, and there is a risk of contamination caused by element diffusion at the interface of the mandrel and tube blank when the two materials are heat treated together.

[0007] Cold rolling provides the most suitable stress state for plastic deformation of metal tubes. The resulting tubes have much better internal and external surface quality and dimensional accuracy than cold drawing, making it a precision cold process. However, nickel-titanium alloys harden very quickly during cold working, and the amount of work performed per pass is small. This leads to large deformation of the outer surface of the tube, making it prone to cracking. The deformation during rolling must be strictly controlled between 15% and 20% (see, for example, Hu Jie et al. Research on the Processing Technology of Titanium-Ni Shape Memory Alloy Capillaries [J]. New Technologies and Processes, 2006, 6: 51-52).

[0008] Therefore, it is urgent to develop a full-process preparation process for ultra-pure nickel-titanium alloy thin-walled tubes to improve the deep processing technology level of high-end medical devices. Summary of the Invention

[0009] The technical problem to be solved by the present invention is to address the deficiencies of the above-mentioned prior art and provide a method for preparing ultra-pure nickel-titanium alloy thin-walled tubing for heart valves. This method uses magnetic levitation induction melting to prepare high-purity nickel-titanium alloy ingots and prepare nickel-titanium tubing. The ingots are then assembled into inner and outer titanium tubing layers before being cold-rolled. By introducing the outer titanium tubing, the cold rolling deformation is increased without introducing new impurity elements, shortening the processing cycle. The result is an ultra-pure nickel-titanium alloy thin-walled tubing with high purity, high dimensional accuracy, good surface quality, high wall thickness uniformity, and excellent comprehensive mechanical properties. This method solves the problems of conventional nickel-titanium alloy tubing, such as the introduction of impurity elements during hot (warm) rolling and drawing, low cold rolling efficiency, and poor dimensional accuracy during cold drawing.

[0010] To solve the above technical problems, the technical solution adopted by the present invention is: a method for preparing an ultra-pure nickel-titanium alloy thin-walled tube for a heart valve, characterized in that the method comprises the following steps:

[0011] Step 1: Using grade 0 small-particle titanium sponge and nickel plate with a purity of 99.99%, and then using magnetic levitation induction melting to prepare a nickel-titanium alloy ingot; the magnetic levitation induction melting is repeated 2 to 3 times;

[0012] Step 2: hot-forging the nickel-titanium alloy ingot obtained in step 1 into a square rod, and then rolling it into a nickel-titanium alloy round rod; the nickel-titanium alloy round rod has a diameter of 20 mm to 25 mm;

[0013] Step 3: The nickel-titanium alloy round rod obtained in step 2 is prepared into a tube blank by deep hole drilling, and the inner surface is honed and the outer surface is polished until the inner and outer surface roughness does not exceed 0.4 μm, thereby obtaining a nickel-titanium tube;

[0014] Step 4: Select a titanium tube with an inner and outer surface roughness of no more than 0.4 μm and a processed microstructure, and assemble the nickel-titanium tube obtained in step 3 into the titanium tube so that the inner diameter of the titanium tube and the outer diameter of the nickel-titanium tube are fitted with a clearance fit, and the clearance is 0.05 mm to 0.1 mm, to obtain a combined tube;

[0015] Step 5: heat-treating the composite tube obtained in step 4, so that the outer diameter of the inner nickel-titanium tube in the composite tube increases by 0.15 mm to 0.2 mm due to the memory effect, and an interference fit is achieved with the outer titanium tube; the heat treatment temperature is 650° C. to 700° C., and the holding time is 30 min to 45 min;

[0016] Step 6: The combined pipe after the heat treatment in step 5 is subjected to three-roll precision cold rolling, with a single-pass deformation of 25% to 33%, and an intermediate heat treatment is performed after each cold rolling pass. The intermediate heat treatment temperature is 600°C to 645°C, and the final intermediate heat treatment temperature is 350°C to 400°C;

[0017] Step 7: Repeat the cold rolling and intermediate heat treatment process in step 6 until the target size is obtained to obtain a semi-finished pipe;

[0018] Step 8: Straighten the semi-finished pipe obtained in step 7 to a straightness of no more than 0.3 mm / 300 mm, and then use a centerless grinder to remove the titanium pipe by cylindrical grinding, with a grinding amount of 0.1 mm to 0.2 mm to obtain an ultra-pure nickel-titanium alloy thin-walled pipe.

[0019] The above-mentioned method for preparing ultrapure nickel-titanium alloy thin-walled tubes for heart valves is characterized in that the process parameters of the magnetic levitation induction melting in step 1 are: the vacuum degree does not exceed 2×10 -2 Pa, electromagnetic induction heating frequency of 1800Hz to 2200Hz, holding power of 320kW to 350kW, holding time of 10min to 15min, cooling time of 60min to 90min, and the weight of the nickel-titanium alloy ingot of 10kg to 20kg. By controlling the process parameters of magnetic levitation induction melting and the weight of the nickel-titanium alloy ingot, the melt of the nickel-titanium alloy ingot is suspended, ensuring the smooth progress of magnetic levitation induction melting, while increasing the single processing volume of ultra-pure nickel-titanium alloy thin-walled pipes and ensuring preparation efficiency.

[0020] The aforementioned method for preparing an ultrapure nickel-titanium alloy thin-walled tube for a heart valve is characterized in that the wall thickness of the titanium tube in step 4 is 0.1 mm to 0.2 mm. The present invention uses a titanium tube as a protective layer for the nickel-titanium tube to prevent damage from direct contact with the mold during initial rolling, protecting the nickel-titanium tube from excessive stress, and allowing for convenient removal of the titanium layer through centerless grinding.

[0021] The aforementioned method for preparing an ultrapure nickel-titanium alloy thin-walled tubing for heart valves is characterized in that the composition of the ultrapure nickel-titanium alloy thin-walled tubing in step eight complies with ASTM F2063-2018, "Standard Specification for Wrought Nickel-Titanium Shape Memory Alloy for Medical Devices and Surgical Implants," wherein the weight percentage of impurity elements is: C ≤ 0.005%, H ≤ 0.0005%, N ≤ 0.005%, and O ≤ 0.02%. The weight percentage of impurity elements in the ultrapure nickel-titanium alloy thin-walled tubing prepared by the present invention is lower than the requirements of ASTM F2063-2018, meeting the requirements for metal tubing for heart valves.

[0022] The aforementioned method for preparing an ultrapure nickel-titanium alloy thin-walled tube for heart valves is characterized in that the ultrapure nickel-titanium alloy thin-walled tube in step eight has an outer diameter of 4 mm to 12 mm and a wall thickness of 0.1 mm to 0.5 mm. The dimensions of the ultrapure nickel-titanium alloy thin-walled tube prepared by the present invention meet the requirements for medical metal tubes, particularly metal tubes for heart valves.

[0023] Compared with the prior art, the present invention has the following advantages:

[0024] 1. The present invention first selects high-purity raw materials and prepares high-purity nickel-titanium alloy ingots through magnetic levitation induction melting, then hot forges and rolls them into round bars, and prepares nickel-titanium tubes through deep drilling. The nickel-titanium tubes are then assembled into the interior of the titanium tubes for heat treatment. The shape memory effect of the nickel-titanium alloy is utilized to ensure that the nickel-titanium tubes are tightly bonded to the titanium tubes. A large deformation amount of cold rolling is then performed in combination with intermediate heat treatment to make the tubes close to the finished product size. After straightening, the outer layer of the titanium tube is removed by centerless grinding. In this preparation process, the outer layer of the titanium tube is introduced, that is, no new impurity elements are introduced, and the cold rolling deformation amount of the nickel-titanium alloy tube is increased, the processing cycle is shortened, and the purity and dimensional accuracy of the product nickel-titanium alloy thin-walled tube are ensured.

[0025] 2. The present invention adopts magnetic levitation induction melting to prepare nickel-titanium alloy ingots, and utilizes high-frequency magnetic field heating and melt stirring to improve the composition uniformity and accuracy of the nickel-titanium alloy ingots. In addition, the melting process does not contact the crucible, eliminating crucible contamination, reducing the content of C, H, O, and N impurity elements, and controlling the content of inclusions from the source. At the same time, due to the effect of electromagnetic induction, the melt will roll up and down during the melting process, which plays a stirring role, making the nickel-titanium alloy composition more uniform, which is beneficial to improving the long-term fatigue performance of the heart valve stent produced therefrom.

[0026] 3. In view of the defects that titanium-nickel alloy hardens very quickly during work hardening, is extremely sensitive to deformation, has a small deformation in a single rolling (single deformation of 15% to 20%), requires frequent annealing during cold working, and is very prone to cracking due to the large stress on the outer surface of the pipe during cold rolling, especially the end of the pipe, which is extremely prone to cracking due to the instantaneous stress of contact with the mold. The present invention assembles the nickel-titanium pipe into a titanium pipe with better processing performance and achieves interference fit before cold rolling. During cold rolling, the outer layer of the titanium pipe is subjected to large stress, while the inner layer of the nickel-titanium pipe is relatively slightly deformed, thereby achieving protection for the nickel-titanium pipe. and stress constraints, thereby improving the processability of the outer side of the nickel-titanium tube, so that the nickel-titanium tube is not prone to defects such as cracks and folds during the rolling process. At the same time, the titanium tube has high biocompatibility, does not introduce new elements, and avoids pollution. In addition, due to the above-mentioned effect of the outer titanium tube, the single-pass deformation during cold rolling of the present invention is significantly increased (the single-pass deformation is 25% to 33%, which is about 1.6 times the original nickel-titanium tube cold rolling deformation), and the number of intermediate heat treatments is reduced, thereby effectively shortening the processing cycle, improving the yield rate, and reducing costs.

[0027] 4. The cold rolling process of the present invention does not introduce elements such as C, H, O, and N, thereby ensuring the composition of the ultra-pure nickel-titanium alloy thin-walled tube. At the same time, compared with the traditional drawing method, it eliminates the need for surface lubrication, subsequent core removal, and other processes, thus simplifying the process.

[0028] 5. The ultrapure nickel-titanium alloy thin-walled tube prepared by the present invention has high purity, high dimensional accuracy, good surface quality, high wall thickness uniformity, and good comprehensive mechanical properties, and is suitable for the field of heart valves.

[0029] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 This is a physical picture of the nickel-titanium alloy ingot prepared in Example 1 of the present invention.

[0031] Figure 2 This is a physical picture of the ultra-pure nickel-titanium alloy thin-walled tube prepared in Example 1 of the present invention.

[0032] Figure 3 This is a physical picture of the ultra-pure nickel-titanium alloy thin-walled tube prepared in Example 2 of the present invention. DETAILED DESCRIPTION

[0033] Example 1

[0034] This embodiment includes the following steps:

[0035] Step 1: Use grade 0 small particle titanium sponge and nickel plate with a purity of 99.99% and then use magnetic levitation induction melting to prepare nickel-titanium alloy ingots. Figure 1 As shown; the magnetic levitation induction melting is repeated twice; the process parameters of the magnetic levitation induction melting are: the vacuum degree does not exceed 2×10 -2 Pa, electromagnetic induction heating frequency 1800 Hz, holding power 320 kW, holding time 10 min, cooling time 60 min, the nickel-titanium alloy ingot weight is 10 kg;

[0036] Step 2: hot-forging the nickel-titanium alloy ingot obtained in step 1 into a square rod, and then rolling it into a nickel-titanium alloy round rod with a diameter of 2 mm;

[0037] Step 3: The nickel-titanium alloy round rod obtained in step 2 is prepared into a tube blank by deep hole drilling, and the inner surface is honed and the outer surface is polished until the inner and outer surface roughness does not exceed 0.4 μm, thereby obtaining a nickel-titanium tube with an outer diameter × wall thickness of Φ17.0 mm × 1.2 mm;

[0038] Step 4: Select a titanium tube with an inner and outer surface roughness of no more than 0.4 μm and a processed microstructure, and an outer diameter × wall thickness of Φ17.5 mm × 0.2 mm. Assemble the nickel-titanium tube obtained in step 3 into the titanium tube so that the inner diameter of the titanium tube and the outer diameter of the nickel-titanium tube are fitted with a clearance of 0.05 mm, to obtain a combined tube with an outer diameter × wall thickness of Φ17.5 mm × 1.4 mm.

[0039] Step 5: heat-treating the composite tube obtained in step 4, so that the outer diameter of the inner nickel-titanium tube in the composite tube increases by 0.2 mm due to the memory effect, and an interference fit is achieved with the outer titanium tube; the heat treatment temperature is 700° C., and the holding time is 45 minutes;

[0040] Step 6: The combined pipe after heat treatment in step 5 is subjected to three-roll precision cold rolling. The change of outer diameter × wall thickness during the cold rolling process is: Φ17.5mm×1.4mm→Φ15.8mm×1.1mm→Φ14mm×0.9mm→Φ12.4mm×0.7mm (retaining the processed state), and an intermediate heat treatment is performed after each cold rolling pass. The intermediate heat treatment temperature is 645°C and the time is 15 minutes. The last intermediate heat treatment temperature is 400°C and the time is 35 minutes.

[0041] Step 7: Repeat the cold rolling and intermediate heat treatment process in step 6 until the target size is obtained to obtain a semi-finished pipe;

[0042] Step 8: Straighten the semi-finished pipe obtained in step 7 to a straightness of 0.2mm / 300mm, and then use a centerless grinder to remove the titanium pipe by cylindrical grinding. The grinding amount is 0.2mm to obtain an ultra-pure nickel-titanium alloy thin-walled pipe with an outer diameter × wall thickness of Φ12.0mm×0.5mm. Figure 2 As stated.

[0043] After testing, the composition of the ultra-pure nickel-titanium alloy thin-walled tube prepared in this embodiment complies with ASTM F2063-2018 "Standard Specification for Wrought Nickel-Titanium Shape Memory Alloy for Medical Devices and Surgical Implants", wherein the mass percentage of impurity elements is: C ≤ 0.005%, H ≤ 0.0005%, N ≤ 0.005%, O ≤ 0.02%.

[0044] Example 2

[0045] This embodiment includes the following steps:

[0046] Step 1: Use grade 0 small particle titanium sponge and nickel plate with a purity of 99.99% and then prepare nickel-titanium alloy ingot by magnetic levitation induction melting; repeat the magnetic levitation induction melting three times; the process parameters of the magnetic levitation induction melting are: vacuum degree not exceeding 2×10 -2 Pa, electromagnetic induction heating frequency 2100 Hz, holding power 345 kW, holding time 13 min, cooling time 80 min, the nickel-titanium alloy ingot weight is 16 kg;

[0047] Step 2: hot-forging the nickel-titanium alloy ingot obtained in step 1 into a square rod, and then rolling it into a nickel-titanium alloy round rod with a diameter of 20 mm;

[0048] Step 3: The nickel-titanium alloy round rod obtained in step 2 is prepared into a tube blank by deep hole drilling, and the inner surface is honed and the outer surface is polished until the inner and outer surface roughnesses do not exceed 0.4 μm, thereby obtaining a nickel-titanium tube with an outer diameter × wall thickness of Φ11.2 mm × 0.5 mm;

[0049] Step 4: Select a titanium tube with an inner and outer surface roughness of no more than 0.4 μm and a processed microstructure, and an outer diameter × wall thickness of Φ11.5 mm × 0.1 mm. Assemble the nickel-titanium tube obtained in step 3 into the titanium tube so that the inner diameter of the titanium tube and the outer diameter of the nickel-titanium tube are fitted with a clearance of 0.1 mm, to obtain a combined tube with an outer diameter × wall thickness of Φ11.5 mm × 0.6 mm;

[0050] Step 5: heat-treating the composite tube obtained in step 4, so that the outer diameter of the inner nickel-titanium tube in the composite tube increases by 0.15 mm due to the memory effect, and an interference fit is achieved with the outer titanium tube; the heat treatment temperature is 680° C., and the holding time is 45 minutes;

[0051] Step 6: The combined pipe after heat treatment in step 5 is subjected to three-roll precision cold rolling. The change of outer diameter × wall thickness during the cold rolling process is: Φ11.5mm×0.6mm→Φ10.2mm×0.5mm→Φ9.2mm×0.3mm→Φ8.2mm×0.3mm (retaining the processed state), and an intermediate heat treatment is performed after each cold rolling pass. The intermediate heat treatment temperature is 620°C and the time is 10 minutes. The last intermediate heat treatment temperature is 380°C and the time is 30 minutes.

[0052] Step 7: Repeat the cold rolling and intermediate heat treatment process in step 6 until the target size is obtained to obtain a semi-finished pipe;

[0053] Step 8: Straighten the semi-finished pipe obtained in step 7 to a straightness of 0.1mm / 300mm, and then use a centerless grinder to remove the titanium pipe by cylindrical grinding, with a grinding amount of 0.1mm to obtain an ultra-pure nickel-titanium alloy thin-walled pipe with an outer diameter × wall thickness of Φ8.0mm×0.2mm, as shown in FIG. Figure 3 shown.

[0054] After testing, the composition of the ultra-pure nickel-titanium alloy thin-walled tube prepared in this embodiment complies with ASTM F2063-2018 "Standard Specification for Wrought Nickel-Titanium Shape Memory Alloy for Medical Devices and Surgical Implants", wherein the mass percentage of impurity elements is: C ≤ 0.005%, H ≤ 0.0005%, N ≤ 0.005%, O ≤ 0.02%.

[0055] Example 3

[0056] This embodiment includes the following steps:

[0057] Step 1: Use grade 0 small particle titanium sponge and nickel plate with a purity of 99.99% and then prepare nickel-titanium alloy ingot by magnetic levitation induction melting; repeat the magnetic levitation induction melting three times; the process parameters of the magnetic levitation induction melting are: vacuum degree not exceeding 2×10 -2 Pa, electromagnetic induction heating frequency 2200 Hz, holding power 350 kW, holding time 15 min, cooling time 90 min, the nickel-titanium alloy ingot weight is 20 kg;

[0058] Step 2: hot-forging the nickel-titanium alloy ingot obtained in step 1 into a square rod, and then rolling it into a nickel-titanium alloy round rod with a diameter of 20 mm;

[0059] Step 3: The nickel-titanium alloy round rod obtained in step 2 is prepared into a tube blank by deep hole drilling, and the inner surface is honed and the outer surface is polished until the inner and outer surface roughnesses do not exceed 0.4 μm, thereby obtaining a nickel-titanium tube with an outer diameter × wall thickness of Φ5.95 mm × 0.3 mm;

[0060] Step 4: Select a titanium tube with an inner and outer surface roughness of no more than 0.4 μm and a processed microstructure, and an outer diameter × wall thickness of Φ6.2 mm × 0.1 mm. Assemble the nickel-titanium tube obtained in step 3 into the titanium tube so that the inner diameter of the titanium tube and the outer diameter of the nickel-titanium tube are fitted with a clearance of 0.05 mm, to obtain a combined tube with an outer diameter × wall thickness of Φ6.2 mm × 0.4 mm;

[0061] Step 5: heat-treating the composite tube obtained in step 4, so that the outer diameter of the inner nickel-titanium tube in the composite tube increases by 0.15 mm due to the memory effect, and an interference fit is achieved with the outer titanium tube; the heat treatment temperature is 650° C., and the holding time is 35 minutes;

[0062] Step 6: The combined pipe after heat treatment in step 5 is subjected to three-roll precision cold rolling. The change of outer diameter × wall thickness during the cold rolling process is: Φ6.2mm×0.4mm→Φ5.5mm×0.3mm→Φ4.8mm×0.25mm→Φ4.2mm×0.2mm (retaining the processed state), and an intermediate heat treatment is performed after each cold rolling pass. The intermediate heat treatment temperature is 600°C and the time is 5 minutes. The last intermediate heat treatment temperature is 350°C and the time is 20 minutes.

[0063] Step 7: Repeat the cold rolling and intermediate heat treatment process in step 6 until the target size is obtained to obtain a semi-finished pipe;

[0064] Step 8: The semi-finished pipe obtained in step 7 is straightened to a straightness of 0.1 mm / 300 mm, and then the titanium pipe is removed by cylindrical grinding using a centerless grinder with a grinding amount of 0.1 mm to obtain an ultra-pure nickel-titanium alloy thin-walled pipe with an outer diameter × wall thickness of Φ4.0 mm × 0.1 mm.

[0065] The composition of the ultrapure nickel-titanium alloy thin-walled tube prepared in this embodiment complies with ASTM F2063-2018 "Standard Specification for Wrought Nickel-Titanium Shape Memory Alloy for Medical Devices and Surgical Implants", wherein the mass percentage of impurity elements is: C ≤ 0.005%, H ≤ 0.0005%, N ≤ 0.005%, O ≤ 0.02%.

[0066] The above description is only a preferred embodiment of the present invention and does not limit the present invention in any way. Any simple modification, change and equivalent variation made to the above embodiment based on the essence of the invention technology shall still fall within the scope of protection of the technical solution of the present invention.

Claims

1. A method for preparing an ultrapure nickel-titanium alloy thin-walled tube for a heart valve, characterized in that: The method comprises the following steps: Step 1: Using grade 0 small-particle titanium sponge and nickel plate with a purity of 99.99%, and then using magnetic levitation induction melting to prepare a nickel-titanium alloy ingot; the magnetic levitation induction melting is repeated 2 to 3 times; Step 2: hot-forging the nickel-titanium alloy ingot obtained in step 1 into a square rod, and then rolling it into a nickel-titanium alloy round rod; the nickel-titanium alloy round rod has a diameter of 20 mm to 25 mm; Step 3: The nickel-titanium alloy round rod obtained in step 2 is prepared into a tube blank by deep hole drilling, and the inner surface is honed and the outer surface is polished until the inner and outer surface roughness does not exceed 0.4 μm, thereby obtaining a nickel-titanium tube; Step 4: Select a titanium tube with an inner and outer surface roughness of no more than 0.4 μm and a processed microstructure, and assemble the nickel-titanium tube obtained in step 3 into the titanium tube so that the inner diameter of the titanium tube and the outer diameter of the nickel-titanium tube are fitted with a clearance fit, and the clearance is 0.05 mm to 0.1 mm, to obtain a combined tube; Step 5: heat-treating the composite tube obtained in step 4, so that the outer diameter of the inner nickel-titanium tube in the composite tube increases by 0.15 mm to 0.2 mm due to the memory effect, and an interference fit is achieved with the outer titanium tube; the heat treatment temperature is 650° C. to 700° C., and the holding time is 30 min to 45 min; Step 6: The combined pipe after the heat treatment in step 5 is subjected to three-roll precision cold rolling, with a single-pass deformation of 25% to 33%, and an intermediate heat treatment is performed after each cold rolling pass. The intermediate heat treatment temperature is 600°C to 645°C, and the final intermediate heat treatment temperature is 350°C to 400°C; Step 7: Repeat the cold rolling and intermediate heat treatment process in step 6 until the target size is obtained to obtain a semi-finished pipe; Step 8: Straighten the semi-finished pipe obtained in step 7 to a straightness of no more than 0.3 mm / 300 mm, and then use a centerless grinder to remove the titanium pipe by cylindrical grinding, with a grinding amount of 0.1 mm to 0.2 mm to obtain an ultra-pure nickel-titanium alloy thin-walled pipe.

2. The method for preparing an ultrapure nickel-titanium alloy thin-walled tube for a heart valve according to claim 1, characterized in that: The process parameters of the magnetic levitation induction melting in step 1 are: vacuum degree not exceeding 2×10 -2 Pa, electromagnetic induction heating frequency 1800Hz~2200Hz, holding power 320kW~350kW, holding time 10min~15min, cooling time 60min~90min, the weight of the nickel-titanium alloy ingot is 10kg~20kg.

3. The method for preparing an ultrapure nickel-titanium alloy thin-walled tube for a heart valve according to claim 1, characterized in that: The wall thickness of the titanium tube in step 4 is 0.1 mm to 0.2 mm.

4. The method for preparing an ultrapure nickel-titanium alloy thin-walled tube for a heart valve according to claim 1, characterized in that: The composition of the ultra-pure nickel-titanium alloy thin-walled tube described in step eight complies with ASTM F2063-2018 "Standard Specification for Wrought Nickel-Titanium Shape Memory Alloy for Medical Devices and Surgical Implants", wherein the mass percentage of impurity elements is: C ≤ 0.005%, H ≤ 0.0005%, N ≤ 0.005%, O ≤ 0.02%.

5. The method for preparing an ultrapure nickel-titanium alloy thin-walled tube for a heart valve according to claim 1, characterized in that: The ultrapure nickel-titanium alloy thin-walled tube in step eight has an outer diameter of 4 mm to 12 mm and a wall thickness of 0.1 mm to 0.5 mm.

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